**Thermal Energy Storage**

**Chapter 1**

**Provisional chapter**

**Heat Exchange Analysis on Latent Heat Thermal Energy**

**Heat Exchange Analysis on Latent Heat Thermal** 

**Energy Storage Systems Using Molten Salts and** 

DOI: 10.5772/intechopen.73672

**Storage Systems Using Molten Salts and Nanoparticles**

The increase of carbon dioxide emissions is the most important contributor to climate change. A better use of produced energy, increasing systems efficiency and using renewable sources, can limit them. A key technological issue is to integrate a thermal energy storage (TES). It consists in stocking thermal energy through the heating/cooling of a storage material for future needs. Among various technologies, latent heat TES (LHTES) provides high energy storage density at constant temperature during melting/solidification of storage media. The bottleneck in the use of typical PCMs is their low thermal conductivity. To improve the heat exchange between heat transfer fluid and PCM, three methods are possible and here experimentally analyzed: conductivity systems enhancements; convective flows promotion in liquid phase; and improvement of PCM thermal properties including small amounts of nanoparticles. CFD models were used to evaluate physical phenomena that are crucial for optimized LHTES systems design. The study of the heat exchange mode allowed some useful indications to achieve an optimized LHTES, taking advantage by convective flows and conductivity promotion systems. The use of NEPCM, to maximize the stored energy density and realize compact systems, makes necessary the improvement of its thermal diffusivity. These will be the future

**Keywords:** heat exchange, latent heat, nanoparticles, phase change material,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**as Phase Change Materials**

Manila Chieruzzi, Elisabetta Veca, Tommaso Crescenzi and Luigi Torre

Tommaso Crescenzi and Luigi Torre

Adio Miliozzi, Raffaele Liberatore,

http://dx.doi.org/10.5772/intechopen.73672

**Abstract**

research topics.

thermal energy storage

Adio Miliozzi, Raffaele Liberatore, Daniele Nicolini,

Daniele Nicolini, Manila Chieruzzi, Elisabetta Veca,

**Nanoparticles as Phase Change Materials**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

#### **Heat Exchange Analysis on Latent Heat Thermal Energy Storage Systems Using Molten Salts and Nanoparticles as Phase Change Materials Heat Exchange Analysis on Latent Heat Thermal Energy Storage Systems Using Molten Salts and Nanoparticles as Phase Change Materials**

DOI: 10.5772/intechopen.73672

Adio Miliozzi, Raffaele Liberatore, Daniele Nicolini, Manila Chieruzzi, Elisabetta Veca, Tommaso Crescenzi and Luigi Torre Adio Miliozzi, Raffaele Liberatore, Daniele Nicolini, Manila Chieruzzi, Elisabetta Veca, Tommaso Crescenzi and Luigi Torre

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73672

#### **Abstract**

The increase of carbon dioxide emissions is the most important contributor to climate change. A better use of produced energy, increasing systems efficiency and using renewable sources, can limit them. A key technological issue is to integrate a thermal energy storage (TES). It consists in stocking thermal energy through the heating/cooling of a storage material for future needs. Among various technologies, latent heat TES (LHTES) provides high energy storage density at constant temperature during melting/solidification of storage media. The bottleneck in the use of typical PCMs is their low thermal conductivity. To improve the heat exchange between heat transfer fluid and PCM, three methods are possible and here experimentally analyzed: conductivity systems enhancements; convective flows promotion in liquid phase; and improvement of PCM thermal properties including small amounts of nanoparticles. CFD models were used to evaluate physical phenomena that are crucial for optimized LHTES systems design. The study of the heat exchange mode allowed some useful indications to achieve an optimized LHTES, taking advantage by convective flows and conductivity promotion systems. The use of NEPCM, to maximize the stored energy density and realize compact systems, makes necessary the improvement of its thermal diffusivity. These will be the future research topics.

**Keywords:** heat exchange, latent heat, nanoparticles, phase change material, thermal energy storage

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **1. Introduction**

The increased world energy demand is the most important contributor to climate change. A better use of the produced energy, by increasing the energy efficiency in industrial and civil applications, as well as the use of renewable sources, such as solar energy, can limit the carbon dioxide emissions and, consequently, the man-made greenhouse effect [1]. European Commission introduced in October 2014 new and ambitious targets for the year 2030 [2, 3]: 27% share for renewable energy penetration, 40% cuts in greenhouse gas (GHG) emissions, and 27% improvement in energy efficiency.

charging times, and low released power. The enhancement of the thermal conductivity of the PCMs is one of the most important topics in LHTES system design. Many development activities, indeed, are focused on this topic with the aim of creating materials with high latent heat,

Heat Exchange Analysis on Latent Heat Thermal Energy Storage Systems Using Molten Salts and…

http://dx.doi.org/10.5772/intechopen.73672

5

To improve the heat exchange between the heat transfer fluid (HTF) and PCM, three methods

**i.** Increase of the heat exchange surface through the introduction of suitable thermal con-

**ii.** Increase of the heat exchange coefficient by exploiting the development of convective flows inside the PCM during the solid-liquid phase change (convective thermal exchange);

**iii.** Increase the thermal conductivity of HSM by altering its properties through the introduction of small amounts of proper nanoparticles (nanoenhanced PCM (NEPCM)).

The effect of these solutions on heat transfer mode should be carefully analyzed to assess the

In the first part of this chapter, the commonly used storage media and the methods of increasing the conductivity will be illustrated. Subsequently, some experimental tests will be described, and the results discussed. Finally, after analyzing the numerical methodologies useful to simulate the highlighted physical phenomena, future steps for the development of

The most common HSMs for medium-high applications are solar salts and their mixtures. They shows good thermal properties and low cost. **Table 1** summarizes some of these HSMs. It was demonstrated that the main thermal properties of PCMs (in particular in the solar salts) can be enhanced with the addition of several kind of nanoparticles. Some of these properties are latent heat, specific heat, and thermal conductivity. In particular in this paragraph, it is reported how the enhancement of thermal properties is strictly related to some factors like the size, the type, and the weight percentage of nanoparticles, as well as synthesis protocol and the parameters used. For example, nanoparticles with a too little diameter may precipitate instead of dispersing in the liquid or a too high weight percentage can be responsible for a

As for the synthesis protocols, the NEPCM can be obtained with several experimental procedures. The most used is the two-step liquid solution method which involves the production of a solution and the use of ultrasound. In particular, in this case, the NEPCMs are prepared by dissolving 200 mg of salt and nanoparticles in solid state (as powder) into 20 ml of distilled water. The dispersion of the nanoparticles is ensured by the ultrasonication of the solution

high specific heat, and high thermal conductivity.

innovative latent heat storage systems will be described.

deserve to be cited [10]:

ductivity promotion systems;

advantages and disadvantages.

**2. Heat storage materials**

difficult nanoparticle dispersion.

followed by water evaporation on hot plate.

A key technological issue to reach these objectives is to integrate in the productive system an efficient and low-cost thermal energy storage (TES) [4, 5]. Thermal energy storage is a technology that consists in stocking thermal energy through the heating and cooling of a storage material. The energy stored in this way can be used for future needs in particular to face the fluctuating energy demand, increasing the efficiency of several systems. The heat can be stored in a heat storage material (HSM) in three different modes: as sensible heat (HSM temperature increasing), as latent heat (HSM phase change, i.e. solid-liquid), or as thermochemical energy (reversible thermochemical reactions).

Among these technologies, latent heat thermal energy storage (LHTES) provides, due to the high absorbed/released energy required by the phase change process of the material, a high-energy storage density at an almost constant temperature during the melting and the solidification of the storage media. These materials are called phase change materials (PCMs). However, in practice, in the place of a single temperature, the system operates in a temperature range, which includes the melting temperature. A PCM can store higher amount of heat if in comparison with a material using only sensible heat, and this leads to a significant decrease of the size and cost of the LHTES systems [6, 7].

PCMs are commonly used in applications for both thermal management and thermal energy storage. For example, LHTES systems can find application [7] in:


Typical HSM for medium-high applications are salts or mixtures thereof [8, 9]. The bottleneck of these PCMs is their low thermal conductivity which, combined with a high thermal capacity, leads to a low thermal diffusivity and, therefore, a low exploitation of the material, high charging times, and low released power. The enhancement of the thermal conductivity of the PCMs is one of the most important topics in LHTES system design. Many development activities, indeed, are focused on this topic with the aim of creating materials with high latent heat, high specific heat, and high thermal conductivity.

To improve the heat exchange between the heat transfer fluid (HTF) and PCM, three methods deserve to be cited [10]:


The effect of these solutions on heat transfer mode should be carefully analyzed to assess the advantages and disadvantages.

In the first part of this chapter, the commonly used storage media and the methods of increasing the conductivity will be illustrated. Subsequently, some experimental tests will be described, and the results discussed. Finally, after analyzing the numerical methodologies useful to simulate the highlighted physical phenomena, future steps for the development of innovative latent heat storage systems will be described.

#### **2. Heat storage materials**

**1. Introduction**

4 Advancements in Energy Storage Technologies

27% improvement in energy efficiency.

cal energy (reversible thermochemical reactions).

decrease of the size and cost of the LHTES systems [6, 7].

storage. For example, LHTES systems can find application [7] in:

plied heat allows a more efficient operation of the gas turbine;

industry (60–220°C), and other industrial sectors (30–180°C).

• in district heating, industrial cooling, or waste heat recovering.

The increased world energy demand is the most important contributor to climate change. A better use of the produced energy, by increasing the energy efficiency in industrial and civil applications, as well as the use of renewable sources, such as solar energy, can limit the carbon dioxide emissions and, consequently, the man-made greenhouse effect [1]. European Commission introduced in October 2014 new and ambitious targets for the year 2030 [2, 3]: 27% share for renewable energy penetration, 40% cuts in greenhouse gas (GHG) emissions, and

A key technological issue to reach these objectives is to integrate in the productive system an efficient and low-cost thermal energy storage (TES) [4, 5]. Thermal energy storage is a technology that consists in stocking thermal energy through the heating and cooling of a storage material. The energy stored in this way can be used for future needs in particular to face the fluctuating energy demand, increasing the efficiency of several systems. The heat can be stored in a heat storage material (HSM) in three different modes: as sensible heat (HSM temperature increasing), as latent heat (HSM phase change, i.e. solid-liquid), or as thermochemi-

Among these technologies, latent heat thermal energy storage (LHTES) provides, due to the high absorbed/released energy required by the phase change process of the material, a high-energy storage density at an almost constant temperature during the melting and the solidification of the storage media. These materials are called phase change materials (PCMs). However, in practice, in the place of a single temperature, the system operates in a temperature range, which includes the melting temperature. A PCM can store higher amount of heat if in comparison with a material using only sensible heat, and this leads to a significant

PCMs are commonly used in applications for both thermal management and thermal energy

• small and large size solar thermal power plants, where the constant temperature of the sup-

• in industrial plants, where the process heat, deriving from renewable sources, is provided at the required temperature allowing a wide range of processes: food (30–120°C), beverages (60–90°C), paper Industry (60–150°C), metal surface treatment (30–80°C), bricks and blocks curing (60–140°C), textile industry (40–180°C), chemical industry (60–260°C), plastic

Typical HSM for medium-high applications are salts or mixtures thereof [8, 9]. The bottleneck of these PCMs is their low thermal conductivity which, combined with a high thermal capacity, leads to a low thermal diffusivity and, therefore, a low exploitation of the material, high The most common HSMs for medium-high applications are solar salts and their mixtures. They shows good thermal properties and low cost. **Table 1** summarizes some of these HSMs.

It was demonstrated that the main thermal properties of PCMs (in particular in the solar salts) can be enhanced with the addition of several kind of nanoparticles. Some of these properties are latent heat, specific heat, and thermal conductivity. In particular in this paragraph, it is reported how the enhancement of thermal properties is strictly related to some factors like the size, the type, and the weight percentage of nanoparticles, as well as synthesis protocol and the parameters used. For example, nanoparticles with a too little diameter may precipitate instead of dispersing in the liquid or a too high weight percentage can be responsible for a difficult nanoparticle dispersion.

As for the synthesis protocols, the NEPCM can be obtained with several experimental procedures. The most used is the two-step liquid solution method which involves the production of a solution and the use of ultrasound. In particular, in this case, the NEPCMs are prepared by dissolving 200 mg of salt and nanoparticles in solid state (as powder) into 20 ml of distilled water. The dispersion of the nanoparticles is ensured by the ultrasonication of the solution followed by water evaporation on hot plate.


nanoparticles into the same molten salt mixture different results can be obtained by changing the protocol parameters [16, 17]. In particular, with 200 min and 60°C, a specific heat increase of 34% in solid phase and 101% in liquid phase can be achieved. The morphology of the NEPCMs is characterized by the formation of needle-like structures of nanoparticles in the salt mixture. These structures could be responsible of the specific heat enhancement. Moreover, slow water evaporation (i.e. at lower temperature) seems to be more effective in increasing the specific heat of the salt due to the reduced presence of agglomerates. Other kind of nanoparticles can also be used. A low amount of graphite nanoparticles (0.1 wt.%)

Heat Exchange Analysis on Latent Heat Thermal Energy Storage Systems Using Molten Salts and…

cific heat of 40 and 57% in solid and liquid phase, respectively [18]. Chloride salts and their eutectic mixtures can also be used as HSM at high temperature. An eutectic mixture of barium

The nanoparticle size can also affect the thermal behavior of molten salts and nanoparticles. It seems that higher specific heat increase is obtained with higher nanoparticle diameter. This trend was shown by studying nanofluids based on the common binary nitrate salt of

in diameter. The possible explanation of this effect is that nanoparticles with little diameter (<10 nm) tend to precipitate or form agglomerates, thus preventing a good dispersion into the

Several experimental studies have been conducted by University of Perugia and ENEA in

Some considerations could be found. First of all, the NEPCMs showed the highest improvement of the specific heat with 1 wt.% of nanoparticles added, while 0.5 and 1.5 wt.% were not

+57% in solid and liquid state). Since no substructure was formed in this case, the observed enhancement of heat capacity was attributed to the formation of a solid-like nanolayer on the

In a recent study, another HSM was developed through water solution method by using KNO3

as nanoparticles [23]. The HSM has an increased specific heat with silica and silica-alumina nanoparticles (+9.5% and +4.7%, respectively). In any case, the two-step method involves the use of high amount of water and the evaporation of the water can be expensive and time consuming. In other words, in industrial scale, it should be tried to produce NEPCMs with easier methods. Few studies report the production of nanofluids based on molten salt and

considered effective. Second, the type of nanoparticle played an important role: TiO2

 Al<sup>2</sup> O3

molten salt, which is a fundamental step to achieve thermal properties enhancement.

described above with 100 min of dispersion and water evaporation at 200°C.

(60:40) having a melting temperature of about 225°C by using the two-step

O3

nanoparticles (82–86% silica, 14–18% alumina) were added to the salt mixture

(60:40). The NEPCMs were obtained by the two-step liquid solution method

, Al2 O3

62:38) may produce an increase of the spe-

http://dx.doi.org/10.5772/intechopen.73672

nanoparticles with 13 and 90 nm diam-

nanoparticles (1 wt.%) with 5, 10, 30, and 60 nm

, Al2 O3 , TiO2

showed the greatest enhancement (+22 and

, and a mixture of SiO2

:NaCl:CaCl2

:LiCl

7

nanopar-

, and a mixture

did not


CO3 : K2 CO3

chloride, sodium chloride, calcium chloride, and lithium chloride (BaCl2

34.59:12.52:40:12.89) has a melting point of 378°C. The addition of 1 wt.% of SiO2

ticles to this mixture showed an increase of the specific heat of about 14.5% [19].

added into the same eutectic salt (Li2

liquid solution method. Lu and Huang [20] used Al<sup>2</sup>

order to develop NEPCM as HSMs. In one of these [22], SiO2

eter, while Dudda and Shin [21] used SiO2

increase the Cp of nitrate salts, while SiO2

as molten salt (melting point of 334°C) and 1 wt.% of SiO2

NaNO3

of SiO2

NaNO3



surface of the nanoparticle.

:KNO3

**Table 1.** Main properties for some HSMs used as PCMs [11–14].

In general, a sonication is made by using sound energy to disperse particles into a material. When the ultrasonic frequencies are used, the process is called ultra-sonication. The aim of this method is to obtain a uniform dispersion of the nanoparticles, thus avoiding the formation of agglomerates (i.e., clusters of hundreds of single nanoparticles).

Different sonication times (100 and 200 min) and evaporation temperatures (60, 100, and 200°C) can be used in this procedure [15–17].

The typical salts used as PCMs in LHTES are nitrates, at medium-high temperatures (due to their melting temperature which is between 200 and 400°C), and carbonates and chlorides, at high temperatures (since their melting temperature is above 400°C). For example, the dispersion of 1 wt.% of SiO2 nanoparticles into a mixture of Li2 CO3 and K<sup>2</sup> CO3 (62:38) with a melting point of 488°C was done under sonication for 100 min followed by water evaporation at 200°C [15]. Above the melting point (in the range 525–555°C), the specific heat of the NEPCM is enhanced by 19–24%. The enhancement was attributed to the formation of a substructure close to the nanoparticles and to the high specific surface energy of the nanoparticles. By increasing the nanoparticles, percentage (1.5 wt.% instead of 1 wt.%) of the same nanoparticles into the same molten salt mixture different results can be obtained by changing the protocol parameters [16, 17]. In particular, with 200 min and 60°C, a specific heat increase of 34% in solid phase and 101% in liquid phase can be achieved. The morphology of the NEPCMs is characterized by the formation of needle-like structures of nanoparticles in the salt mixture. These structures could be responsible of the specific heat enhancement. Moreover, slow water evaporation (i.e. at lower temperature) seems to be more effective in increasing the specific heat of the salt due to the reduced presence of agglomerates. Other kind of nanoparticles can also be used. A low amount of graphite nanoparticles (0.1 wt.%) added into the same eutectic salt (Li2 CO3 : K2 CO3 62:38) may produce an increase of the specific heat of 40 and 57% in solid and liquid phase, respectively [18]. Chloride salts and their eutectic mixtures can also be used as HSM at high temperature. An eutectic mixture of barium chloride, sodium chloride, calcium chloride, and lithium chloride (BaCl2 :NaCl:CaCl2 :LiCl 34.59:12.52:40:12.89) has a melting point of 378°C. The addition of 1 wt.% of SiO2 nanoparticles to this mixture showed an increase of the specific heat of about 14.5% [19].

The nanoparticle size can also affect the thermal behavior of molten salts and nanoparticles. It seems that higher specific heat increase is obtained with higher nanoparticle diameter. This trend was shown by studying nanofluids based on the common binary nitrate salt of NaNO3 :KNO3 (60:40) having a melting temperature of about 225°C by using the two-step liquid solution method. Lu and Huang [20] used Al<sup>2</sup> O3 nanoparticles with 13 and 90 nm diameter, while Dudda and Shin [21] used SiO2 nanoparticles (1 wt.%) with 5, 10, 30, and 60 nm in diameter. The possible explanation of this effect is that nanoparticles with little diameter (<10 nm) tend to precipitate or form agglomerates, thus preventing a good dispersion into the molten salt, which is a fundamental step to achieve thermal properties enhancement.

Several experimental studies have been conducted by University of Perugia and ENEA in order to develop NEPCM as HSMs. In one of these [22], SiO2 , Al2 O3 , TiO2 , and a mixture of SiO2 -Al<sup>2</sup> O3 nanoparticles (82–86% silica, 14–18% alumina) were added to the salt mixture NaNO3 -KNO3 (60:40). The NEPCMs were obtained by the two-step liquid solution method described above with 100 min of dispersion and water evaporation at 200°C.

In general, a sonication is made by using sound energy to disperse particles into a material. When the ultrasonic frequencies are used, the process is called ultra-sonication. The aim of this method is to obtain a uniform dispersion of the nanoparticles, thus avoiding the forma-

Different sonication times (100 and 200 min) and evaporation temperatures (60, 100, and 200°C)

The typical salts used as PCMs in LHTES are nitrates, at medium-high temperatures (due to their melting temperature which is between 200 and 400°C), and carbonates and chlorides, at high temperatures (since their melting temperature is above 400°C). For example, the

nanoparticles into a mixture of Li2

melting point of 488°C was done under sonication for 100 min followed by water evaporation at 200°C [15]. Above the melting point (in the range 525–555°C), the specific heat of the NEPCM is enhanced by 19–24%. The enhancement was attributed to the formation of a substructure close to the nanoparticles and to the high specific surface energy of the nanoparticles. By increasing the nanoparticles, percentage (1.5 wt.% instead of 1 wt.%) of the same

CO3

and K<sup>2</sup>

CO3

(62:38) with a

tion of agglomerates (i.e., clusters of hundreds of single nanoparticles).

can be used in this procedure [15–17].

dispersion of 1 wt.% of SiO2

**Composition Melting** 

6 Advancements in Energy Storage Technologies

:NaNO3

LiNO3

KNO3

53:40:7 <sup>a</sup>

LiNO3

LiNO3

NaNO3

NaNO2

LiNO3

NaNO3

NaNO3

a wt.%. b mol.%.

LiCl:Ca(NO3

)2

NaOH:NaNO2

NaOH:NaNO3

:KNO3

:NaNO2

:NaNO3

:KNO3

**temp, °C**

**Latent heat, J/g**

33:67 <sup>a</sup> 0133 170 NaOH:NaCl:Na2

54:46 <sup>a</sup> 222 110 MgCl2

:NaOH 80:20 <sup>b</sup> 232 252 KCl:MgCl2

73:27 <sup>b</sup> 237 272 Li2

:NaCl 93.6:6.4 <sup>a</sup> 255 354 Li2

:NaOH 41:59 <sup>b</sup> 266 278 Na<sup>2</sup>


LiOH:LiCl:KCl 62:36.5:1.5 <sup>b</sup> 282 300 Li2

**Table 1.** Main properties for some HSMs used as PCMs [11–14].

59:41 <sup>b</sup> 270 167 NaCl:CaCl2

LiCl:LiOH 37:63 <sup>a</sup> 262 485 CaCl2

**Composition Melting** 

CO3

142 80 NaNO3 307 183

49:51 <sup>a</sup> 194 265 KNO3 337 100

CO3 :K2 CO3 :Na2 CO3

CO3 :K2 CO3

CO3 :Li2 CO3

CO3

62:17:21 <sup>a</sup>

CO3 :Na2 CO3 :K2 CO3

:NaF:KCl

28:72 <sup>b</sup> 246 225 LiF:LiOH 80:20 <sup>b</sup> 430 528

:NaCl 87:13 <sup>a</sup> 208 369 LiCl:KCl 58:42 <sup>b</sup> 348 170

LiNO3 253 363 Li:NaF:KF 29:12:59 <sup>a</sup> 463 442

**temp, °C**

86:8:6 <sup>b</sup> 298 286

:NaCl:KCl 63:23:24 <sup>a</sup> 385 461

:NaCl 20:50:30 <sup>b</sup> 396 291

47:53 <sup>a</sup> 488 342

56:44 <sup>a</sup> 496 368

22:16:62 <sup>a</sup> 580 288

520 274

33:67 <sup>a</sup> 500 281

:NaCl 55:45 <sup>b</sup> 495 236

31:35:34 <sup>a</sup> 397 275

**Latent heat, J/g**

> Some considerations could be found. First of all, the NEPCMs showed the highest improvement of the specific heat with 1 wt.% of nanoparticles added, while 0.5 and 1.5 wt.% were not considered effective. Second, the type of nanoparticle played an important role: TiO2 did not increase the Cp of nitrate salts, while SiO2 Al<sup>2</sup> O3 showed the greatest enhancement (+22 and +57% in solid and liquid state). Since no substructure was formed in this case, the observed enhancement of heat capacity was attributed to the formation of a solid-like nanolayer on the surface of the nanoparticle.

> In a recent study, another HSM was developed through water solution method by using KNO3 as molten salt (melting point of 334°C) and 1 wt.% of SiO2 , Al2 O3 , and a mixture of SiO2 -Al<sup>2</sup> O3 as nanoparticles [23]. The HSM has an increased specific heat with silica and silica-alumina nanoparticles (+9.5% and +4.7%, respectively). In any case, the two-step method involves the use of high amount of water and the evaporation of the water can be expensive and time consuming. In other words, in industrial scale, it should be tried to produce NEPCMs with easier methods. Few studies report the production of nanofluids based on molten salt and

nanoparticles without water by mixing them with the ball milling procedure (i.e., directly in solid state). In this study [24], a ball-mill with 9 mm stainless steel bearing was used to mix the powder salts and the nanoparticles (CuO and TiO2 ). With this procedure and materials, the NEPCMs obtained showed higher latent heat (+2.4 and +3.8%) and specific heat.

Another fundamental property of heat storage materials is their thermal conductivity. However, for many PCMs (especially molten salts), it needs to be increased. Thus, the main idea to increase this property is to combine these PCMs with highly conductive nanomaterials [26]. The research on other nanofluids is wide, but only few studies were performed in particular on molten salts with nanoparticles to enhance the thermal conductivity of the pure materials. One

Heat Exchange Analysis on Latent Heat Thermal Energy Storage Systems Using Molten Salts and…

respect to the base salt and the thermal diffusivity by 28 and 25% at 150 and 300°C, respectively [27]. The formation of a percolation network was considered responsible of the enhancement of the thermal conductivity and diffusivity of the nanocomposite (as previously reported about the specific heat enhancement). Recently, University of Perugia [28] produced NEPCMs starting

(82–86% silica, 14–18% alumina) were dispersed into the salt mixture in a concentrated water solution (from 100 to 500 g/l). The salts and the nanoparticles were mixed by a mechanical stirrer, and the NEPCMs produced showed a higher thermal conductivity value in comparison to the base salt mixture (up to +25%). Moreover, it increased by increasing the aqueous solution

The improvement of thermal exchange between heat transfer fluid and PCM is one of the main themes in development of LHTES systems [29]. Commonly, used PCMs show a low conductivity of 0.5–1 W/m K. Therefore, the design of the heat exchanger is dominated by the task to identify effective solutions to increase the equivalent thermal conductivity within the heat storage material [30]. In the previous paragraph, the possibility to act on the material itself by altering its thermal properties by adding small amounts of nanoparticles (NEPCM) was evaluated. In this section, however, two other methods to increase the apparent conductivity of the thermal storage medium were analyzed: introducing a system to promote the conduc-

Different solutions can be applied to promote thermal conductivity or to improve heat

• Increasing of heat transfer area: the contact area of the heat exchanger between HTF and PCM is enlarged to reduce the average distance for heat diffusion within the PCM. Possible

• Composite material with increased thermal conductivity: a material showing a high thermal conductivity is added to the PCM. The PCM can be infiltrated in a porous matrix made up of the additional material, or the two components can be mixed as powders, fibers, or small particles;

CO3 :K2 CO3

:KNO3

concentration (from 100 to 500 g/l), as well as thermal diffusivity (up to +47%).

tivity and favoring a convective heat exchange in the liquid phase of PCM.

implementations of this approach can use either finned tubes or capsules;

**3. Enhancement of heat exchange in LHTES**

nanoparticles may enhance the thermal conductivity of

60:40). Only 1.0 wt.% of silica-alumina nanoparticles

62:38) by 47% at 150°C and 37% at 300°C with

http://dx.doi.org/10.5772/intechopen.73672

9

of these showed that only 1 wt.% of SiO2

the eutectic mixture of carbonates (Li2

from the nitrate mixture (NaNO3

**3.1. Promoting thermal conductivity**

exchange and increase power level [11, 31]:

Recently, a new mixing methodology was developed by University of Perugia and ENEA [25]. This technique does not involve the use of water, since salts and nanoparticles are mixed together directly at high temperature (i.e., in liquid state). NaNO3 :KNO3 (60:40) and 1.0 wt% of nanoparticles (silica, alumina and a mixture of silica/alumina as above) were mixed together in powder and then heated above the melting temperature (at 300°C) by using a twin screw microcompounder (**Figure 1**). The presence of a recirculating channel ensures the recycling of the NEPCM and the good dispersion of nanoparticles. The materials were mixed at screw speed of 100 and 200 rpm, for 15 and 30 min. After this heating, the mixture is then cooled down at room temperature and ground to powder. The thermal properties of the NEPCMs produced in this way depend on the type of nanoparticle and the time and speed used to mix them. A good HSM was obtained with silica/alumina nanoparticles mixed for 30 min at 200 rpm, reaching a high increase of Cp (+52.1% in solid phase and +18.6% in liquid phase) and good increase of the heat of fusion (+4.7%). The thermal storage capability of molten salts and nanoparticles can be also calculated and evaluated as the integration of the heat flow curve between the minimum and maximum working temperatures. The stored heat was found to enhance by 13.5%.

Another important property of the HSM is their latent heat. The advantage of the increase of latent heat is the increased storage heat capability of the material per unit volume. About the latent heat enhancement, there is lack of information. However, some studies performed by University of Perugia reported an increase of 15% mainly with silica-alumina nanoparticles in nitrate mixture [22] and also in KNO3 as molten salt having a melting point of 334°C (the latent heat increased by 12%). In both studies, the NEPCMs were produced in water solution [23]. It was shown that the stored heat of molten salts as NaNO3 -KNO3 [22] and KNO3 [23] is increased with the addition of 1.0 wt.% of SiO2 -Al<sup>2</sup> O3 and SiO2 .

**Figure 1.** Mixing of nanoparticles and the salt mixture at high temperature in a twin screw microcompounder [21].

Another fundamental property of heat storage materials is their thermal conductivity. However, for many PCMs (especially molten salts), it needs to be increased. Thus, the main idea to increase this property is to combine these PCMs with highly conductive nanomaterials [26]. The research on other nanofluids is wide, but only few studies were performed in particular on molten salts with nanoparticles to enhance the thermal conductivity of the pure materials. One of these showed that only 1 wt.% of SiO2 nanoparticles may enhance the thermal conductivity of the eutectic mixture of carbonates (Li2 CO3 :K2 CO3 62:38) by 47% at 150°C and 37% at 300°C with respect to the base salt and the thermal diffusivity by 28 and 25% at 150 and 300°C, respectively [27]. The formation of a percolation network was considered responsible of the enhancement of the thermal conductivity and diffusivity of the nanocomposite (as previously reported about the specific heat enhancement). Recently, University of Perugia [28] produced NEPCMs starting from the nitrate mixture (NaNO3 :KNO3 60:40). Only 1.0 wt.% of silica-alumina nanoparticles (82–86% silica, 14–18% alumina) were dispersed into the salt mixture in a concentrated water solution (from 100 to 500 g/l). The salts and the nanoparticles were mixed by a mechanical stirrer, and the NEPCMs produced showed a higher thermal conductivity value in comparison to the base salt mixture (up to +25%). Moreover, it increased by increasing the aqueous solution concentration (from 100 to 500 g/l), as well as thermal diffusivity (up to +47%).

#### **3. Enhancement of heat exchange in LHTES**

The improvement of thermal exchange between heat transfer fluid and PCM is one of the main themes in development of LHTES systems [29]. Commonly, used PCMs show a low conductivity of 0.5–1 W/m K. Therefore, the design of the heat exchanger is dominated by the task to identify effective solutions to increase the equivalent thermal conductivity within the heat storage material [30]. In the previous paragraph, the possibility to act on the material itself by altering its thermal properties by adding small amounts of nanoparticles (NEPCM) was evaluated. In this section, however, two other methods to increase the apparent conductivity of the thermal storage medium were analyzed: introducing a system to promote the conductivity and favoring a convective heat exchange in the liquid phase of PCM.

#### **3.1. Promoting thermal conductivity**

nanoparticles without water by mixing them with the ball milling procedure (i.e., directly in solid state). In this study [24], a ball-mill with 9 mm stainless steel bearing was used to mix the

Recently, a new mixing methodology was developed by University of Perugia and ENEA [25]. This technique does not involve the use of water, since salts and nanoparticles are mixed

of nanoparticles (silica, alumina and a mixture of silica/alumina as above) were mixed together in powder and then heated above the melting temperature (at 300°C) by using a twin screw microcompounder (**Figure 1**). The presence of a recirculating channel ensures the recycling of the NEPCM and the good dispersion of nanoparticles. The materials were mixed at screw speed of 100 and 200 rpm, for 15 and 30 min. After this heating, the mixture is then cooled down at room temperature and ground to powder. The thermal properties of the NEPCMs produced in this way depend on the type of nanoparticle and the time and speed used to mix them. A good HSM was obtained with silica/alumina nanoparticles mixed for 30 min at 200 rpm, reaching a high increase of Cp (+52.1% in solid phase and +18.6% in liquid phase) and good increase of the heat of fusion (+4.7%). The thermal storage capability of molten salts and nanoparticles can be also calculated and evaluated as the integration of the heat flow curve between the minimum and maximum working temperatures. The stored

Another important property of the HSM is their latent heat. The advantage of the increase of latent heat is the increased storage heat capability of the material per unit volume. About the latent heat enhancement, there is lack of information. However, some studies performed by University of Perugia reported an increase of 15% mainly with silica-alumina nanoparticles

latent heat increased by 12%). In both studies, the NEPCMs were produced in water solution

**Figure 1.** Mixing of nanoparticles and the salt mixture at high temperature in a twin screw microcompounder [21].


and SiO2

.

NEPCMs obtained showed higher latent heat (+2.4 and +3.8%) and specific heat.

together directly at high temperature (i.e., in liquid state). NaNO3

). With this procedure and materials, the

(60:40) and 1.0 wt%

:KNO3

as molten salt having a melting point of 334°C (the


[22] and KNO3

[23] is

powder salts and the nanoparticles (CuO and TiO2

8 Advancements in Energy Storage Technologies

heat was found to enhance by 13.5%.

in nitrate mixture [22] and also in KNO3

increased with the addition of 1.0 wt.% of SiO2

[23]. It was shown that the stored heat of molten salts as NaNO3

Different solutions can be applied to promote thermal conductivity or to improve heat exchange and increase power level [11, 31]:


• Intermediate heat transfer medium liquid/gaseous: the PCM and a heat exchanger are arranged in a container filled with a medium that transfers the energy between these two components. The heat transport involves the phase change of the heat transfer medium.

A **sandwich structure** is an approach to increase the effective thermal conductivity of the PCM, integrating highly conductive materials layers inside the PCM. The layers are arranged in the heat transport direction (**Figure 2a**). The application of tubes with wings embedded in a PCM is described as a sandwich concept. This concept has been developed, since it seems to be the most promising option for making efficient and low-cost latent heat storage systems [11, 12, 32].

The choice of steel would be a simple solution as well as finned steel tubes are standard components for heat exchangers. Instead, graphite or aluminum sheets are chosen as materials for wings due to their high thermal conductivity. To obtain the same heat transfer performance in comparison with the graphite or aluminum fins, the steel fins require a higher volume and, therefore, significantly higher costs, also due to the higher density (**Table 2**). The costs are proportional to the c/λ factor, where λ is the thermal conductivity and c is the volume specific cost of the fins material.

A second solution is to pack the PCM into capsules in order to reduce the maximum distance for heat transfer or to increase the heat exchange area: this concept is called **macroencapsulation** of the storage medium [11]. **Figure 3** shows an example of LHTES with macrocapsules used in a laboratory scale experiment. In this case, the cylindrical capsules have a length of 0.5 m and a

) 1000 2700 7800 7800 8800

) 66.7 35.0 1000 500 114.3

Heat Exchange Analysis on Latent Heat Thermal Energy Storage Systems Using Molten Salts and…

**Graphite foil**

Thermal conductivity λ (W/m K) 150 200 20 30 350

are arranged in parallel and integrated inside a tank. Due to PCM specific volume variations that can reach up to 10% during phase change, the tubes are not fully filled. A volumetric gas fraction of about 20% is required within the rigid capsules to limit the increase in pressure during PCM

Because of the corrosive behavior of some PCM salts, it is necessary to provide a certain minimum thickness of the wall and avoid a flexible encapsulation. However, this concept is not considered a promising solution due, in particular, to some economic aspects and the fol-

• The amount of material required for pressure capsules is significant, if steel is used: the

• HTF contamination with PCM should be avoided due to leakage from the capsules: this

Thermal conductivity can also be improved by using **composite materials** in which the properties of a high latent heat of a PCM are combined with that of a good thermal conductivity of an additive. Said composite materials are manufactured in blocks and subsequently

If the operational temperature range is 120–300°C, nitrates or nitrites and various types of graphite are currently being used. Graphite was chosen because of its high thermal conductivity and chemical stability. The objective is generally to obtain a composite material with an effective thermal conductivity in the range of 5–15 W/m K using a small amount of graphite to obtain a high capacity and a low cost of the LHTES system. The fraction of graphite mass significantly influences the effective thermal conductivity. The composite material is


) 10,000 7000 20,000 15,000 40,000

**Aluminum Stainless steel Carbon steel Copper**

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eutectic mixture. The capsules

diameter of 15–25 mm and they are filled with a NaNO3

**Table 2.** Main properties for materials used in LHTES sandwich systems [31, 32].

steel mass is almost equivalent to the mass of PCM;

assembled together with the tube assembly (**Figure 4**).

• The volume fraction of PCM in the pressure tank may be less than 40%;

requires high standards of quality with consequent further increase in costs.

• The filling and sealing procedure with molten PCM are complex;

melting.

Density ρ (kg/m3

Estimated *c/λ* (€K/W m2

Estimated volume specific costs *c* (€/m3

lowing disadvantages [11]:

Mounting the fins on the pipes is a key issue for the sandwich concept. If the material of the fins is different from the material of the pipes two other phenomena can occur: a different thermal expansion between the used materials and a galvanic corrosion (the damage induced when two dissimilar materials are coupled in a corrosive electrolyte). In addition, the application of fins made of expanded graphite offers several advantages beyond a good thermal conductivity. Expanded graphite has good chemical stability with nitrates and nitrites at temperatures up to 250°C, and galvanic corrosion does not occur when it is in contact with steel tubes. Because graphite laminae exhibit a high degree of flexibility and they are often used as a sealing material, a tight contact between the tubes and the fins can easily be achieved [32]. The highest specific price, compared to stainless or carbon steel, is largely compensated by its low density and high thermal conductivity. However, graphite is stable with nitrates only below 250°C. For higher temperature applications, the use of metallic fins is necessary [11]. **Figure 2b** shows semiindustrial applications of this concept.

**Figure 2.** Functional scheme and test module of the LHTES sandwich concept [29, 31, 32].


**Table 2.** Main properties for materials used in LHTES sandwich systems [31, 32].

• Intermediate heat transfer medium liquid/gaseous: the PCM and a heat exchanger are arranged in a container filled with a medium that transfers the energy between these two components. The heat transport involves the phase change of the heat transfer medium.

10 Advancements in Energy Storage Technologies

A **sandwich structure** is an approach to increase the effective thermal conductivity of the PCM, integrating highly conductive materials layers inside the PCM. The layers are arranged in the heat transport direction (**Figure 2a**). The application of tubes with wings embedded in a PCM is described as a sandwich concept. This concept has been developed, since it seems to be the most promising option for making efficient and low-cost latent heat storage systems [11, 12, 32]. The choice of steel would be a simple solution as well as finned steel tubes are standard components for heat exchangers. Instead, graphite or aluminum sheets are chosen as materials for wings due to their high thermal conductivity. To obtain the same heat transfer performance in comparison with the graphite or aluminum fins, the steel fins require a higher volume and, therefore, significantly higher costs, also due to the higher density (**Table 2**). The costs are proportional to the c/λ factor, where λ is the thermal conductivity and c is the volume specific cost of the fins material. Mounting the fins on the pipes is a key issue for the sandwich concept. If the material of the fins is different from the material of the pipes two other phenomena can occur: a different thermal expansion between the used materials and a galvanic corrosion (the damage induced when two dissimilar materials are coupled in a corrosive electrolyte). In addition, the application of fins made of expanded graphite offers several advantages beyond a good thermal conductivity. Expanded graphite has good chemical stability with nitrates and nitrites at temperatures up to 250°C, and galvanic corrosion does not occur when it is in contact with steel tubes. Because graphite laminae exhibit a high degree of flexibility and they are often used as a sealing material, a tight contact between the tubes and the fins can easily be achieved [32]. The highest specific price, compared to stainless or carbon steel, is largely compensated by its low density and high thermal conductivity. However, graphite is stable with nitrates only below 250°C. For higher temperature applications, the use of metallic fins is necessary [11].

**Figure 2b** shows semiindustrial applications of this concept.

**Figure 2.** Functional scheme and test module of the LHTES sandwich concept [29, 31, 32].

A second solution is to pack the PCM into capsules in order to reduce the maximum distance for heat transfer or to increase the heat exchange area: this concept is called **macroencapsulation** of the storage medium [11]. **Figure 3** shows an example of LHTES with macrocapsules used in a laboratory scale experiment. In this case, the cylindrical capsules have a length of 0.5 m and a diameter of 15–25 mm and they are filled with a NaNO3 -KNO3 eutectic mixture. The capsules are arranged in parallel and integrated inside a tank. Due to PCM specific volume variations that can reach up to 10% during phase change, the tubes are not fully filled. A volumetric gas fraction of about 20% is required within the rigid capsules to limit the increase in pressure during PCM melting.

Because of the corrosive behavior of some PCM salts, it is necessary to provide a certain minimum thickness of the wall and avoid a flexible encapsulation. However, this concept is not considered a promising solution due, in particular, to some economic aspects and the following disadvantages [11]:


Thermal conductivity can also be improved by using **composite materials** in which the properties of a high latent heat of a PCM are combined with that of a good thermal conductivity of an additive. Said composite materials are manufactured in blocks and subsequently assembled together with the tube assembly (**Figure 4**).

If the operational temperature range is 120–300°C, nitrates or nitrites and various types of graphite are currently being used. Graphite was chosen because of its high thermal conductivity and chemical stability. The objective is generally to obtain a composite material with an effective thermal conductivity in the range of 5–15 W/m K using a small amount of graphite to obtain a high capacity and a low cost of the LHTES system. The fraction of graphite mass significantly influences the effective thermal conductivity. The composite material is

phases. During melting, the PCM has to be heated by the HTF (charge process), the heat flows toward the PCM by conduction and later by natural convection. The solid PCM near the heat exchanger surface heats up and then starts to melt, and the thickness of the liquid region increases over time at fluid-solid interfaces. As the thermal conductivity of liquid PCM is less than that of solid PCM, the heat transfer by conduction becomes almost negligible as the melting process moves forward, and the movement of the fluid in terms of convection must

Heat Exchange Analysis on Latent Heat Thermal Energy Storage Systems Using Molten Salts and…

Convective flows are a result of the varying density of the PCM in function of temperature. The whole mass of fluid is subject to a downward gravitational pull. Consequently, lighter portions of fluid will be subject to Archimedes' upward buoyant force. The nonhomogeneity in temperature causes the same in density [10]. The heat transfer to the PCM followed three regimes [33]: conduction; mixed conduction and convection, where conduction domination was gradually replaced by convection when a sufficiently large amount of liquid had formed; and convection. Regarding solidification, the HTF must be heated. A solid layer was produced on the surface of the heat exchanger on the PCM side that affected the heat transfer by conduction. This is the reason why it is important to enhance the thermal conductivity of the LHTES system. The transition from a conductive to a convective regime produced a significant increase in thermal exchange with a reduction in charging and discharging times and consequent increase in available thermal power and system efficiency. The conditions necessary to establish these convective motions have been extensively studied both experimentally [30, 34–36] and numerically [37–41]. The last goal is to promote this sort of heat exchange

A preliminary experimental test of PCM melting was performed in ENEA in a small tubular reactor in a temperature range between 180 and 300°C. The experiment also included the cooling down of the system. Only the time period of the fusion phase was taken into account.

dimensions of the AISI 316 reactor were 66 mm internal diameter, 70 mm external diameter, and 310 mm high. It contained about 2 kg of molten salts, and the tubular reactor was equipped with an external heating element, made in kanthal, placed along the wall. Instrumentation for

There are 18 thermocouples for temperature measurement inside the reactor: they are placed on six planes. Each plane contains three thermocouple tubes positioned at 120 degrees from

For the melting test, the reactor was loaded with 2 kg of PCM in order to have a uniform com-

• The heating from 180 to 300°C was set without programming any temperature ramp on the

• During the heating phase, the salts started to melt in the temperature range of 220–240°C. In **Figure 5**, the blue dotted lines correspond to the melting temperature interval, and the blue

controller. This is the reason why it overshoots the desired temperature value.


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(60:40%wt), the

within the LHTES to achieve an optimized and efficient system.

The PCM was a mixture of nitrate salts, whose composition is NaNO3

the acquisition of the temperature and pressure was also present.

position. The PCM was heated up to 300°C and then cooled down.

each other at three different radial distances.

line is the time of the melting process.

also be taken into consideration.

**Figure 3.** PCM capsules array and external containers [12, 31].

**Figure 4.** Example of composite material (expanded graphite) with PCM in a laboratory scale element [29, 31].

produced by compression of PCM powders and expanded graphite. The development of an efficient HSM requires the limitation of graphite content because of material costs (ratio of specific expanded graphite/PCM costs is about 20:1) and the reduction in volumetric storage capacity. An important aspect is also the cyclic behavior of composite material. The separation of the components must not be the result of repeated charge/discharge. Indeed, during some experiments, described in literature [11], significant PCM loss occurred, about 40% of nitrate salt separated from graphite. There are several possible causes for the salt loss of the storage module. These causes include the requirement for an empty volume for salt expansion, degassing caused by impurities and moisture in the salts, and poor testability of alkaline metal nitrate salts on graphite and their good wettability on metallic surfaces (tendency to the sliding). Probably, the salt leakage was caused by a combination of these critical phenomena.

#### **3.2. Convective heat exchange**

The heat transfer efficiency between the HTF and PCM strongly affects the performance of the charging/discharging cycles of an LHTES system, especially during melting/solidification phases. During melting, the PCM has to be heated by the HTF (charge process), the heat flows toward the PCM by conduction and later by natural convection. The solid PCM near the heat exchanger surface heats up and then starts to melt, and the thickness of the liquid region increases over time at fluid-solid interfaces. As the thermal conductivity of liquid PCM is less than that of solid PCM, the heat transfer by conduction becomes almost negligible as the melting process moves forward, and the movement of the fluid in terms of convection must also be taken into consideration.

Convective flows are a result of the varying density of the PCM in function of temperature. The whole mass of fluid is subject to a downward gravitational pull. Consequently, lighter portions of fluid will be subject to Archimedes' upward buoyant force. The nonhomogeneity in temperature causes the same in density [10]. The heat transfer to the PCM followed three regimes [33]: conduction; mixed conduction and convection, where conduction domination was gradually replaced by convection when a sufficiently large amount of liquid had formed; and convection. Regarding solidification, the HTF must be heated. A solid layer was produced on the surface of the heat exchanger on the PCM side that affected the heat transfer by conduction. This is the reason why it is important to enhance the thermal conductivity of the LHTES system. The transition from a conductive to a convective regime produced a significant increase in thermal exchange with a reduction in charging and discharging times and consequent increase in available thermal power and system efficiency. The conditions necessary to establish these convective motions have been extensively studied both experimentally [30, 34–36] and numerically [37–41]. The last goal is to promote this sort of heat exchange within the LHTES to achieve an optimized and efficient system.

A preliminary experimental test of PCM melting was performed in ENEA in a small tubular reactor in a temperature range between 180 and 300°C. The experiment also included the cooling down of the system. Only the time period of the fusion phase was taken into account. The PCM was a mixture of nitrate salts, whose composition is NaNO3 -KNO3 (60:40%wt), the dimensions of the AISI 316 reactor were 66 mm internal diameter, 70 mm external diameter, and 310 mm high. It contained about 2 kg of molten salts, and the tubular reactor was equipped with an external heating element, made in kanthal, placed along the wall. Instrumentation for the acquisition of the temperature and pressure was also present.

produced by compression of PCM powders and expanded graphite. The development of an efficient HSM requires the limitation of graphite content because of material costs (ratio of specific expanded graphite/PCM costs is about 20:1) and the reduction in volumetric storage capacity. An important aspect is also the cyclic behavior of composite material. The separation of the components must not be the result of repeated charge/discharge. Indeed, during some experiments, described in literature [11], significant PCM loss occurred, about 40% of nitrate salt separated from graphite. There are several possible causes for the salt loss of the storage module. These causes include the requirement for an empty volume for salt expansion, degassing caused by impurities and moisture in the salts, and poor testability of alkaline metal nitrate salts on graphite and their good wettability on metallic surfaces (tendency to the sliding). Probably, the salt leakage was caused by a combination of these critical phenomena.

**Figure 4.** Example of composite material (expanded graphite) with PCM in a laboratory scale element [29, 31].

The heat transfer efficiency between the HTF and PCM strongly affects the performance of the charging/discharging cycles of an LHTES system, especially during melting/solidification

**3.2. Convective heat exchange**

**Figure 3.** PCM capsules array and external containers [12, 31].

12 Advancements in Energy Storage Technologies

There are 18 thermocouples for temperature measurement inside the reactor: they are placed on six planes. Each plane contains three thermocouple tubes positioned at 120 degrees from each other at three different radial distances.

For the melting test, the reactor was loaded with 2 kg of PCM in order to have a uniform composition. The PCM was heated up to 300°C and then cooled down.


• Isotherm at 300°C for 3 h 20 min.

within the same range of temperature.

orthogonal to the temperature contour lines.

aluminum sheet (**Figure 8c**).

cylinders in series of same type:

**1.** PCM in the cylinder with plain exchanger tube inside; **2.** PCM in the cylinder with finned exchanger tube inside;

**4. Experimental tests on simple LHTES**

• The cooling down of the system from 300 to 180°C was by natural convection at room temperature (after approximately 5 h). During the cooling down phase, the PCM solidified

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Only the melting process was taken into consideration, the temperature contour lines inside the reactor are described in **Figure 6**. The melting process started after 1 h 46 min, and all of the experimental temperature data are reported in function of d and z for six different times as the fusion took place. At 1 h 38 min, the PCM was solid, and shortly thereafter, it started to melt; at 1 h 45 min, the solid phase started to collapse. Five minutes later, the solid phase was bound at the bottom right. After 2 h, it was completely liquid and homogenization by convection started, which was in turn induced by internal temperature gradients. At 2 h 15 min, the PCM homogenized by simple conduction with the presence of a convective cell at the top of the reactor. Moreover, the liquid phase pushed the solid downward, and the heat flux is

An experimental system (**Figure 7a**), whose operating temperature varies in the range 150– 300°C, was installed in ENEA CR Casaccia to carry out a cognitive survey of the involved phenomena. It consists of a series of stainless steel cylinders (**Figure 8a**) able to contain about 2.5 l of material, an heating/cooling circulator to handle thermal oil used as transfer fluid to charge and discharge it, four electro-valves to allow different oil circulations, a flowmeter, as well as 32 thermocouples to detect the temperature evolutions. The thermocouples inside the cylinder (**Figure 7b**) were equally spaced both axially and radially in such a way that they have a homogeneous temperature record in order to properly analyze their developments within the material. Eventually, a SW interface, based on LabView®, permits to control the system also in remote. The pipe, where the oil (Therminol 66) flows, together with the cylinder containing it, is a shell and tube type exchanger. In order to study the effect of the exchange surface, the external area of the tube in the cylinder can be plain or finned (**Figure 8b**). These cylinders were then insulated with 10 cm of Rockwool and covered by an

A thermocryostat Julabo® HT30M1CU ensures the heating and cooling, as well as the handle of the oil thanks to an integrated centrifugal pump, which allows a volume flow rate up to 11 l/min, ensuring a turbulent flow. This equipment, with a 3 kW heating power and a 15 kW cooling power, is able to heat a thermal fluid up to 400°C, but, in our case, the oil was wormed up to 300°C to avoid its cracking. Four kinds of tests were carried out, each one using three

**Figure 5.** (A) All of the 18 thermocouples over experimental test time; (B) The black line is the mean temperature value; and the blue line is the standard deviation ± σ.

**Figure 6.** Temperature contour lines inside the reactor for 6 different times during the melting process. d(mm) is the distance from the wall heated by electrical heater and z(mm) is the height.


Only the melting process was taken into consideration, the temperature contour lines inside the reactor are described in **Figure 6**. The melting process started after 1 h 46 min, and all of the experimental temperature data are reported in function of d and z for six different times as the fusion took place. At 1 h 38 min, the PCM was solid, and shortly thereafter, it started to melt; at 1 h 45 min, the solid phase started to collapse. Five minutes later, the solid phase was bound at the bottom right. After 2 h, it was completely liquid and homogenization by convection started, which was in turn induced by internal temperature gradients. At 2 h 15 min, the PCM homogenized by simple conduction with the presence of a convective cell at the top of the reactor. Moreover, the liquid phase pushed the solid downward, and the heat flux is orthogonal to the temperature contour lines.

#### **4. Experimental tests on simple LHTES**

An experimental system (**Figure 7a**), whose operating temperature varies in the range 150– 300°C, was installed in ENEA CR Casaccia to carry out a cognitive survey of the involved phenomena. It consists of a series of stainless steel cylinders (**Figure 8a**) able to contain about 2.5 l of material, an heating/cooling circulator to handle thermal oil used as transfer fluid to charge and discharge it, four electro-valves to allow different oil circulations, a flowmeter, as well as 32 thermocouples to detect the temperature evolutions. The thermocouples inside the cylinder (**Figure 7b**) were equally spaced both axially and radially in such a way that they have a homogeneous temperature record in order to properly analyze their developments within the material. Eventually, a SW interface, based on LabView®, permits to control the system also in remote. The pipe, where the oil (Therminol 66) flows, together with the cylinder containing it, is a shell and tube type exchanger. In order to study the effect of the exchange surface, the external area of the tube in the cylinder can be plain or finned (**Figure 8b**). These cylinders were then insulated with 10 cm of Rockwool and covered by an aluminum sheet (**Figure 8c**).

A thermocryostat Julabo® HT30M1CU ensures the heating and cooling, as well as the handle of the oil thanks to an integrated centrifugal pump, which allows a volume flow rate up to 11 l/min, ensuring a turbulent flow. This equipment, with a 3 kW heating power and a 15 kW cooling power, is able to heat a thermal fluid up to 400°C, but, in our case, the oil was wormed up to 300°C to avoid its cracking. Four kinds of tests were carried out, each one using three cylinders in series of same type:

**1.** PCM in the cylinder with plain exchanger tube inside;

**Figure 6.** Temperature contour lines inside the reactor for 6 different times during the melting process. d(mm) is the

**Figure 5.** (A) All of the 18 thermocouples over experimental test time; (B) The black line is the mean temperature value;

distance from the wall heated by electrical heater and z(mm) is the height.

and the blue line is the standard deviation ± σ.

14 Advancements in Energy Storage Technologies

**2.** PCM in the cylinder with finned exchanger tube inside;


Every test has been conducted 2–3 times to verify the repeatability of the results. During the tests, the thermocryostat was programmed in order to allow the oil to ensure the following temperature trend, where seven distinct phases can be clearly identified: (a) heating phase from room temperature to 200°C (1 h), (b) maintaining temperature at 200°C (10 h), (c) heating phase from 200 to 280°C (1 h), (d) charging phase at 280°C (8 h), (e) cooling phase from 280 to 150°C, (f) system discharging phase at 150°C (8 h), and finally, (g) cooling phase (4 h). **Figure 9** shows the temperature evolutions in the middle section of a cylinder (**Figure 7b**). In case of test with more cycles, the phases c-d-e-f were replicated (**Figure 10**).

A comparison between tests 1 and 2 points out that, in the test using finned tube as exchanger, the temperature trend of the HTF in the zones near the exchanger (blue line) is closer to the oil in the tube. Thus, it shows a better thermal exchange, surely due to the kind of exchange surface;

**Figure 7.** ATES plant (a) and thermocouple positioning inside a reference cylinder (b).

nevertheless, the temperature gradient increases, as it goes to the peripheral areas, which means a worse heat absorption within the medium. This phenomenon could be explained also by the Agyenim observations, in fact he claims [9]: "*Phase change problems, first treated as pure conduction controlled, has in recent times moved to a different level of complexity with added convection in the* 

**Figure 10.** (a) Input (A1) and output (A6) temperature evolution and temperature drop in storage system and (b)

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**Figure 9.** HSM temperatures evolution in the middle section of the first TES [42].

temperature in-out HTF and their difference.

**Figure 8.** LHTES shell and tube configuration: (a) elementary systems; (b) finned tube; (c) complete systems.

Heat Exchange Analysis on Latent Heat Thermal Energy Storage Systems Using Molten Salts and… http://dx.doi.org/10.5772/intechopen.73672 17

**Figure 9.** HSM temperatures evolution in the middle section of the first TES [42].

**3.** NEPCM (PCM doped with 1%wt of nanoparticles, 20–200 nm, of SiO2

Every test has been conducted 2–3 times to verify the repeatability of the results. During the tests, the thermocryostat was programmed in order to allow the oil to ensure the following temperature trend, where seven distinct phases can be clearly identified: (a) heating phase from room temperature to 200°C (1 h), (b) maintaining temperature at 200°C (10 h), (c) heating phase from 200 to 280°C (1 h), (d) charging phase at 280°C (8 h), (e) cooling phase from 280 to 150°C, (f) system discharging phase at 150°C (8 h), and finally, (g) cooling phase (4 h). **Figure 9** shows the temperature evolutions in the middle section of a cylinder (**Figure 7b**). In case of

A comparison between tests 1 and 2 points out that, in the test using finned tube as exchanger, the temperature trend of the HTF in the zones near the exchanger (blue line) is closer to the oil in the tube. Thus, it shows a better thermal exchange, surely due to the kind of exchange surface;

**4.** NEPCM in the cylinder with finned exchanger tube inside;

test with more cycles, the phases c-d-e-f were replicated (**Figure 10**).

**Figure 7.** ATES plant (a) and thermocouple positioning inside a reference cylinder (b).

**Figure 8.** LHTES shell and tube configuration: (a) elementary systems; (b) finned tube; (c) complete systems.

der with plain exchanger tube inside;

16 Advancements in Energy Storage Technologies


) in the cylin-

**Figure 10.** (a) Input (A1) and output (A6) temperature evolution and temperature drop in storage system and (b) temperature in-out HTF and their difference.

nevertheless, the temperature gradient increases, as it goes to the peripheral areas, which means a worse heat absorption within the medium. This phenomenon could be explained also by the Agyenim observations, in fact he claims [9]: "*Phase change problems, first treated as pure conduction controlled, has in recent times moved to a different level of complexity with added convection in the*  *melt being accounted for*". This phenomenon was not detected, in general, if finned tubes were used (test 2 and 4), probably for the physical limitations of the fins. These convective flows, however, were not found even in the NEPCM with plain exchanger tube, as it can be easily seen by observing test 3, where the green line is more distant from the others than in test 1. In this case, the presence of nanoparticles significantly increases the viscosity [9, 43, 44] of the fluid and probably inhibits the starting of the abovementioned flows [40]. Thus, we can affirm that, when the PCM is in liquid phase and there are no physical limitations (i.e. with plain tubes), the low thermal conductivity (λ) and diffusivity (α = λ(ρcp) −1) are counterbalanced by the start of convective flows thus improving the heat transfer. In NEPCM, despite a substantial invariance of the thermal diffusivity, the thermal capacity and the correlated thermal effusivity, e = (λρcp) 1/2, are increased. So, in this case, the storage material better exchanges thermal energy with its surroundings (e.g., exchanger) but not inside itself (depending on the diffusivity).The use of finned tubes highlights the promotion of the thermal conductivity, and so the charging and discharging times are lower and substantially independent from the storage medium. It is worth to notice that finned tubes make discharge rate faster because the insulation effect, due to the salt solidification on the wall of the exchanger tube, is compensated by the action of the fins. They in fact improve the thermal exchange for the increased surface. In any case, the TES system with NEPCM and finned tubes through the greater heat capacity, coupled with the lower discharge times, allows the system to deliver a higher average project power.

dynamic behavior of PCM is the most important issue, only this material has been simulated.

The PCM was modeled as a constant-density fluid, and the buoyancy force was simulated

density, g is the gravity acceleration, and α is the thermal expansion coefficient at the refer-

liquid fraction (β), defined as: β = 0 for T ≤ Tsol, β = (T−Tsol)/(Tliq−Tsol) for Tsol < T < Tliq and β = 1 for T ≥ Tliq; where Tsol is the temperature at which the solid material begins the liquefaction process, and Tliq is the temperature at which the material has completed the liquefaction process. The fluid-dynamic behavior of phase change is simulated by inserting the dissipative

very small number (0.001), introduced in the equation to avoid division for zero as β = 0, Amush

thermodynamic behavior associated with absorption and release of latent heat (Lfus) has been simulated by adding, in the transition zone between Tsol and Tliq, to the specific heat of PCM the term Lfus/(Tliq−Tsol). The external walls have been simulated as adiabatic surfaces, while the wall corresponding to the surface of the steel tube has been considered as a temperature-controlled wall in both models. The temperature evolution on this wall provides a first linear rise step, from the initial minimum temperature of 200°C to the maximum temperature of 250°C in 1 h and a subsequent step at the maximum temperature for 3 h. Then, it follows a linear descent ramp from the maximum value to the minimum temperature value in 1 h and a step at the minimum temperature for 3 h. The total time of the simulated test is 8 h. **Figure 12** shows the distributions of liquid fraction of PCM, for both models, at time t = 3900 s, and **Figure 13** shows the velocity in the complete models and in their upper part always at t = 3900 s.

. The phase change condition has been simulated through the use of the

Heat Exchange Analysis on Latent Heat Thermal Energy Storage Systems Using Molten Salts and…

/(β3 + ε)]·Amush·u in the equation of conservation of momentum; where ε is a

), where ρ is the

19

s), and u is the velocity of the fluid. The

http://dx.doi.org/10.5772/intechopen.73672

by inserting the vertical component of a volume force equal to ρ·g·α·(T−T0

**Figure 11** shows the two models with a mesh detail.

is the "mushy zone constant", equal to 100,000 kg/(m3

ence temperature T<sup>0</sup>

term F = [(1−β)2

**Figure 11.** CFD models with a mesh detail.

Cyclability tests (**Figure 10a**) performed on the NEPCM showed the substantial invariance of the storage medium behavior along the cycles. **Figure 10b**, in particular, emphasizes the perfect overlap of the temperature differences between inlets and outlets at 10th and 30th cycle. This makes it well to hope that nanostructured material keeps its features in time.

#### **5. Numerical analysis on LHTES thermal behavior**

The correct evaluation of the physical phenomena, that are at the base of the LHTES systems, is crucial for the design of optimized LHTES systems. Since computational fluid dynamics (CFD) models to simulate the thermofluid dynamics behavior of the LHTES system is essential for a good design of these systems, in this section, two models are briefly described. They were developed, by the use of COMSOL Multiphysics® software, ver. 5.2, to simulate the heat storage process for two different geometrical configurations.

Two different geometrical configurations were considered for the LHTES system. In the first one (Model 1), the heat exchange between the heat transfer fluid, oil, and the PCM, "solar salts" (NaNO3 -KNO3 ), occurs through tubes, whose outer wall is smooth. In the second geometry (Model 2), the tubes have a series of transversal fins on the outer surface, so as to improve the thermal exchange with the PCM. In particular, it was considered a section of steel tube of external diameter of 16 mm, thickness 1 mm, and length 500 mm, surrounded by a tubular crown of PCM of external diameter 70 mm. In the finned configuration, a series of transversal fins of 1 mm thickness and 10 mm height were inserted on the outer surface of the tube and placed at constant interval of 50 mm. Both systems have axial symmetry, and so two 2-D axial symmetry models have been made. Furthermore, since the comparison of the thermofluid dynamic behavior of PCM is the most important issue, only this material has been simulated. **Figure 11** shows the two models with a mesh detail.

The PCM was modeled as a constant-density fluid, and the buoyancy force was simulated by inserting the vertical component of a volume force equal to ρ·g·α·(T−T0 ), where ρ is the density, g is the gravity acceleration, and α is the thermal expansion coefficient at the reference temperature T<sup>0</sup> . The phase change condition has been simulated through the use of the liquid fraction (β), defined as: β = 0 for T ≤ Tsol, β = (T−Tsol)/(Tliq−Tsol) for Tsol < T < Tliq and β = 1 for T ≥ Tliq; where Tsol is the temperature at which the solid material begins the liquefaction process, and Tliq is the temperature at which the material has completed the liquefaction process. The fluid-dynamic behavior of phase change is simulated by inserting the dissipative term F = [(1−β)2 /(β3 + ε)]·Amush·u in the equation of conservation of momentum; where ε is a very small number (0.001), introduced in the equation to avoid division for zero as β = 0, Amush is the "mushy zone constant", equal to 100,000 kg/(m3 s), and u is the velocity of the fluid. The thermodynamic behavior associated with absorption and release of latent heat (Lfus) has been simulated by adding, in the transition zone between Tsol and Tliq, to the specific heat of PCM the term Lfus/(Tliq−Tsol). The external walls have been simulated as adiabatic surfaces, while the wall corresponding to the surface of the steel tube has been considered as a temperature-controlled wall in both models. The temperature evolution on this wall provides a first linear rise step, from the initial minimum temperature of 200°C to the maximum temperature of 250°C in 1 h and a subsequent step at the maximum temperature for 3 h. Then, it follows a linear descent ramp from the maximum value to the minimum temperature value in 1 h and a step at the minimum temperature for 3 h. The total time of the simulated test is 8 h. **Figure 12** shows the distributions of liquid fraction of PCM, for both models, at time t = 3900 s, and **Figure 13** shows the velocity in the complete models and in their upper part always at t = 3900 s.

**Figure 11.** CFD models with a mesh detail.

*melt being accounted for*". This phenomenon was not detected, in general, if finned tubes were used (test 2 and 4), probably for the physical limitations of the fins. These convective flows, however, were not found even in the NEPCM with plain exchanger tube, as it can be easily seen by observing test 3, where the green line is more distant from the others than in test 1. In this case, the presence of nanoparticles significantly increases the viscosity [9, 43, 44] of the fluid and probably inhibits the starting of the abovementioned flows [40]. Thus, we can affirm that, when the PCM is in liquid phase and there are no physical limitations (i.e. with plain tubes), the low

vective flows thus improving the heat transfer. In NEPCM, despite a substantial invariance of the thermal diffusivity, the thermal capacity and the correlated thermal effusivity, e = (λρcp)

are increased. So, in this case, the storage material better exchanges thermal energy with its surroundings (e.g., exchanger) but not inside itself (depending on the diffusivity).The use of finned tubes highlights the promotion of the thermal conductivity, and so the charging and discharging times are lower and substantially independent from the storage medium. It is worth to notice that finned tubes make discharge rate faster because the insulation effect, due to the salt solidification on the wall of the exchanger tube, is compensated by the action of the fins. They in fact improve the thermal exchange for the increased surface. In any case, the TES system with NEPCM and finned tubes through the greater heat capacity, coupled with the lower discharge

Cyclability tests (**Figure 10a**) performed on the NEPCM showed the substantial invariance of the storage medium behavior along the cycles. **Figure 10b**, in particular, emphasizes the perfect overlap of the temperature differences between inlets and outlets at 10th and 30th cycle.

The correct evaluation of the physical phenomena, that are at the base of the LHTES systems, is crucial for the design of optimized LHTES systems. Since computational fluid dynamics (CFD) models to simulate the thermofluid dynamics behavior of the LHTES system is essential for a good design of these systems, in this section, two models are briefly described. They were developed, by the use of COMSOL Multiphysics® software, ver. 5.2, to simulate the heat

Two different geometrical configurations were considered for the LHTES system. In the first one (Model 1), the heat exchange between the heat transfer fluid, oil, and the PCM, "solar

etry (Model 2), the tubes have a series of transversal fins on the outer surface, so as to improve the thermal exchange with the PCM. In particular, it was considered a section of steel tube of external diameter of 16 mm, thickness 1 mm, and length 500 mm, surrounded by a tubular crown of PCM of external diameter 70 mm. In the finned configuration, a series of transversal fins of 1 mm thickness and 10 mm height were inserted on the outer surface of the tube and placed at constant interval of 50 mm. Both systems have axial symmetry, and so two 2-D axial symmetry models have been made. Furthermore, since the comparison of the thermofluid

), occurs through tubes, whose outer wall is smooth. In the second geom-

This makes it well to hope that nanostructured material keeps its features in time.

−1) are counterbalanced by the start of con-

1/2,

thermal conductivity (λ) and diffusivity (α = λ(ρcp)

18 Advancements in Energy Storage Technologies

times, allows the system to deliver a higher average project power.

**5. Numerical analysis on LHTES thermal behavior**

storage process for two different geometrical configurations.

salts" (NaNO3


**Figure 12.** PCM liquid fraction at t = 3900 s.

**Figure 13.** Velocity at t = 3900 s.

**Figure 14** shows the comparison between the time evolutions of the melted PCM in the two simulations during the entire charge-discharge cycle. **Figure 15** shows the comparison between the time evolutions of the energy stored by the system in the two models. For comparison, the energy is reported as a percentage of the total energy that can be stored in each of the two systems.

**6. Conclusions and future developments**

**Figure 15.** Time evolution of stored energy.

**Figure 14.** Time evolution of melted PCM.

Thermal energy storage is a key technological issue to have an efficient use of the energy and reduce the carbon dioxide emissions and greenhouse effect. Latent heat storage offers

Heat Exchange Analysis on Latent Heat Thermal Energy Storage Systems Using Molten Salts and…

http://dx.doi.org/10.5772/intechopen.73672

21

Comparing the results obtained by the two CFD simulations, it is possible to see how the presence of the fins on the outer surface of the steel tube strongly increases the efficiency of the system.

Heat Exchange Analysis on Latent Heat Thermal Energy Storage Systems Using Molten Salts and… http://dx.doi.org/10.5772/intechopen.73672 21

**Figure 14.** Time evolution of melted PCM.

**Figure 15.** Time evolution of stored energy.

**Figure 14** shows the comparison between the time evolutions of the melted PCM in the two simulations during the entire charge-discharge cycle. **Figure 15** shows the comparison between the time evolutions of the energy stored by the system in the two models. For comparison, the energy is reported as a percentage of the total energy that can be stored in each of the two systems. Comparing the results obtained by the two CFD simulations, it is possible to see how the presence of the fins on the outer surface of the steel tube strongly increases the efficiency of

the system.

**Figure 13.** Velocity at t = 3900 s.

**Figure 12.** PCM liquid fraction at t = 3900 s.

20 Advancements in Energy Storage Technologies

#### **6. Conclusions and future developments**

Thermal energy storage is a key technological issue to have an efficient use of the energy and reduce the carbon dioxide emissions and greenhouse effect. Latent heat storage offers the possibility of designing much smaller TES systems and thus decreasing the cost of stored energy. However, to develop an efficient LHTES system is necessary to select or synthetize an appropriate heat storage material (PCM) with high thermal properties and to increase the HTF-PCM heat exchange, actually limited by the PCM low thermal conductivity/diffusivity. In particular, it is worth to pay attention to the selection of the materials in order to have good thermal properties for storing a large density of energy. For this purpose, a proper synthesis of new PCMs with increased thermal properties by adding little amount of nanoparticles (NEPCMs) can be convenient. In addition, the heat exchange surface plays an important role through the introduction of suitable thermal conductivity promotion systems, and so its design must be well evaluated and optimized, even in the light of the possible exploitation of the development of convective flows inside the PCM during the solid-liquid phase change. Both experimental and theoretical analyses to deepen these phenomena are necessary, and at this purpose, an experimental facility called ATES was developed by ENEA to take into account these phenomena and produce data for elaborating numerical analysis to carefully analyze the advantages and disadvantages of the various solutions and design innovative LHTES systems.

**References**

[1] Miliozzi A, Liberatore R, Giannuzzi GM, Veca E, Nicolini D, Lanchi M, Chieruzzi M. ENEA research and innovation on Thermal Energy Storage for CSP plants. In: 16th CIRIAF National Congress, Sustainable Development, Human Health and Environ-

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23

[2] EU Commission. Climate strategies & targets. 2030 Climate & energy framework. Available from: https://ec.europa.eu/clima/policies/strategies/2030\_en [Accessed: Feb 2017]

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[5] Salomoni VA, Majorana CE, Giannuzzi GM, Miliozzi A, Nicolini D. New trends in designing parabolic trough solar concentrators and heat storage concrete Systems in Solar Power Plants. In: Rugescu RD, editor. Solar Energy. InTech; 2010. p. 267-292. ISBN:

[6] Chieruzzi M, Veca E Miliozzi A, Torre L. Phase change materials for latent heat storage: research and future trend. In: 16th CIRIAF National Congress, Sustainable Development, Human Health and Environmental Protection; April 7-9, 2016; Assisi (Italy). 2016

[7] Fleischer AS, editor. Thermal Energy Storage Using Phase Change Materials - Fundamentals and Applications. SpringerBriefs in Applied Sciences and Technology. New York, Dordrecht, London: Springer Heidelberg; 2015. 94 p. DOI: 10.1007/978-3-319-20922-7 [8] Liu M, Saman W, Bruno F. Review on storage materials and thermal performance enhancement techniques for high temperature phase change thermal storage systems.

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978-953-307-052-0

The study on the heat storage materials and heat exchange mode has allowed to obtain some useful indications on the LHTES design and optimization. It should take advantage by the presence of convective flows and conductivity promotion systems to facilitate the heat exchange. Instead, the use of NEPCM as a storage medium, useful to maximize the stored energy density and realize compact systems, makes necessary to improve its thermal diffusivity: this could be done by adding carbon-based nanoparticles to the PCM because they show a high thermal conductivity. Among these, carbon nanotubes (CNTs) and graphene nanoplatelets (GNP) can be used.

These will be the main research topics for the development of new concepts of LHTES.

#### **Acknowledgements**

The authors would like to acknowledge the 2014 Annual Research Plan of the Electric System Research Program (RSE) of the Italian Ministry of Economic Development and the EU through the 7th FP in the frame of the STAGE-STE Project (Ctr. Nr. 609837) for the financial support of this work.

#### **Author details**

Adio Miliozzi<sup>1</sup> \*, Raffaele Liberatore<sup>1</sup> , Daniele Nicolini<sup>1</sup> , Manila Chieruzzi2 , Elisabetta Veca<sup>1</sup> , Tommaso Crescenzi<sup>1</sup> and Luigi Torre2

\*Address all correspondence to: adio.miliozzi@enea.it

1 ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Casaccia Research Centre, Rome, Italy

2 Civil and Environmental Engineering Department, University of Perugia, Terni, Italy

#### **References**

the possibility of designing much smaller TES systems and thus decreasing the cost of stored energy. However, to develop an efficient LHTES system is necessary to select or synthetize an appropriate heat storage material (PCM) with high thermal properties and to increase the HTF-PCM heat exchange, actually limited by the PCM low thermal conductivity/diffusivity. In particular, it is worth to pay attention to the selection of the materials in order to have good thermal properties for storing a large density of energy. For this purpose, a proper synthesis of new PCMs with increased thermal properties by adding little amount of nanoparticles (NEPCMs) can be convenient. In addition, the heat exchange surface plays an important role through the introduction of suitable thermal conductivity promotion systems, and so its design must be well evaluated and optimized, even in the light of the possible exploitation of the development of convective flows inside the PCM during the solid-liquid phase change. Both experimental and theoretical analyses to deepen these phenomena are necessary, and at this purpose, an experimental facility called ATES was developed by ENEA to take into account these phenomena and produce data for elaborating numerical analysis to carefully analyze the advantages and disadvantages of the various solutions and design innovative LHTES systems. The study on the heat storage materials and heat exchange mode has allowed to obtain some useful indications on the LHTES design and optimization. It should take advantage by the presence of convective flows and conductivity promotion systems to facilitate the heat exchange. Instead, the use of NEPCM as a storage medium, useful to maximize the stored energy density and realize compact systems, makes necessary to improve its thermal diffusivity: this could be done by adding carbon-based nanoparticles to the PCM because they show a high thermal conductivity. Among these, carbon nanotubes (CNTs) and graphene

These will be the main research topics for the development of new concepts of LHTES.

The authors would like to acknowledge the 2014 Annual Research Plan of the Electric System Research Program (RSE) of the Italian Ministry of Economic Development and the EU through the 7th FP in the frame of the STAGE-STE Project (Ctr. Nr. 609837) for the financial support

, Daniele Nicolini<sup>1</sup>

1 ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic

2 Civil and Environmental Engineering Department, University of Perugia, Terni, Italy

, Manila Chieruzzi2

, Elisabetta Veca<sup>1</sup>

,

nanoplatelets (GNP) can be used.

22 Advancements in Energy Storage Technologies

**Acknowledgements**

of this work.

**Author details**

Tommaso Crescenzi<sup>1</sup>

\*, Raffaele Liberatore<sup>1</sup>

\*Address all correspondence to: adio.miliozzi@enea.it

Development, Casaccia Research Centre, Rome, Italy

and Luigi Torre2

Adio Miliozzi<sup>1</sup>


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**Chapter 2**

**Provisional chapter**

**High Temperature Energy Storage (HiTES) with Pebble**

**High Temperature Energy Storage (HiTES) with Pebble** 

In modern power systems with high penetration of renewable energy generation, the energy storage is very important, not just for the load control for quite different time periods, but even in the frequency control. If it is missing, the anomalies occur, like the

tailments of wind-mills and/or negative market prices for electricity. The new technology is a high temperature thermal electric energy storage. It is based on the combination of three state-of-the-art technologies: pebble-heater, radial gas-turbine and electric resistive heating. Due to very high temperature (1100°C), low exergy losses during the heat transfer and water injection in the gas-turbine process, the round-trip efficiency is high even with nowadays available components. With some moderate improvements of the gas-turbine it could be increased towards 60%, even at 2MW low generator capacity. The discharge time is 10 h; due to the modular design, it may increase to 20 or even 30 h. The analysis of LCOES (levelized cost of electricity storage) shows that even today that system could be used in a viable way in countries with high insolation or on sites where an autarchic power

**Keywords:** energy storage, high temperature, pebble-heater, radial gas turbine, hot air

Energy storage is used to store an overproduction of electricity and to use it again in periods of higher power demand. The pumped hydro storage is one of the oldest systems, especially for mass storage, which has been in use for many years. Previously it was used only for load control, that is, for smoothening the electricity consumption, mostly between day and night,

emission, export of the overproduction under unfavourable conditions, cur-

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.75093

**Heater Technology and Gas Turbine**

**Heater Technology and Gas Turbine**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

supply may replace expensive market electricity.

turbine, resistive heating, power-to-heat-to-power, LCOES

http://dx.doi.org/10.5772/intechopen.75093

Dragan Stevanovic

Dragan Stevanovic

**Abstract**

stagnant CO2

**1. Introduction**

#### **High Temperature Energy Storage (HiTES) with Pebble Heater Technology and Gas Turbine High Temperature Energy Storage (HiTES) with Pebble Heater Technology and Gas Turbine**

DOI: 10.5772/intechopen.75093

Dragan Stevanovic Dragan Stevanovic

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.75093

#### **Abstract**

In modern power systems with high penetration of renewable energy generation, the energy storage is very important, not just for the load control for quite different time periods, but even in the frequency control. If it is missing, the anomalies occur, like the stagnant CO2 emission, export of the overproduction under unfavourable conditions, curtailments of wind-mills and/or negative market prices for electricity. The new technology is a high temperature thermal electric energy storage. It is based on the combination of three state-of-the-art technologies: pebble-heater, radial gas-turbine and electric resistive heating. Due to very high temperature (1100°C), low exergy losses during the heat transfer and water injection in the gas-turbine process, the round-trip efficiency is high even with nowadays available components. With some moderate improvements of the gas-turbine it could be increased towards 60%, even at 2MW low generator capacity. The discharge time is 10 h; due to the modular design, it may increase to 20 or even 30 h. The analysis of LCOES (levelized cost of electricity storage) shows that even today that system could be used in a viable way in countries with high insolation or on sites where an autarchic power supply may replace expensive market electricity.

**Keywords:** energy storage, high temperature, pebble-heater, radial gas turbine, hot air turbine, resistive heating, power-to-heat-to-power, LCOES

#### **1. Introduction**

Energy storage is used to store an overproduction of electricity and to use it again in periods of higher power demand. The pumped hydro storage is one of the oldest systems, especially for mass storage, which has been in use for many years. Previously it was used only for load control, that is, for smoothening the electricity consumption, mostly between day and night,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

in systems with higher capacities of base-load plants, like coal and nuclear power plants. Instead of reducing their output and entering into the zone of lower process efficiency, their overproduction was stored, always enabling the optimal operation parameters. Those were well-defined periods of time, well planned and without rush load changes.

California is the US state with the highest penetration of wind and solar power generation. Contrary to Germany, they have started thinking about and analysing the potential problems of intermittent generation much earlier. The "Duck Chart" was created by the California Independent System Operator to show that increasing solar generation paired with conventional base-load plants that cannot be turned off (e.g. nuclear and less flexible natural gas) can cause over-generation in the afternoons during certain months [4]. The chart shows that the shape of the net load curve begins to shift dramatically in 2015 due to increasing solar generation and there is potential for over-generation during the afternoons beginning in 2018. An especially big problem is a very fast ramping between 17 and 19 h (13.5 GW in 2 h!). To solve those potential problems, several measures were planned, like demand response, import/export, curtailment and energy storage. Being unfavourable measures, import/export and especially curtailment were minimised. Nowadays California is the area with the most

High Temperature Energy Storage (HiTES) with Pebble Heater Technology and Gas Turbine

http://dx.doi.org/10.5772/intechopen.75093

29

In some areas of Chile, there is a locally very high penetration of the solar power generation, based on photovoltaic systems. As the insolation is very high, the price of that electricity is low (<3 ₵ /kWh). However, every day at around noon, the market price goes into the negative area, as there are not enough consumers when the generation reaches its daily maximum. Therefore, there are some projects for energy storage facilities and some new generation facilities but based on concentrated solar power (CSP) with integrated molten

Many countries with high insolation will be very soon in the same situation. In fact, many of them are waiting on a suitable solution for the energy storage in order to start a wide range of

There are many technologies for energy storage. Some are suitable for long-term storage, the other for short-term and some of them even for the frequency control, with very fast response time. They are all using different principles, and therefore they have different advantages and disadvantages. Roughly, there are mechanical, chemical, electrical and thermal storage

Thermal energy storage is mostly famous for the molten salt facilities. They are used almost exclusively with the concentrated solar power (CSP) systems, where solar heat is stored and later used for power generation through the steam turbine cycle. The efficiency is about 32–36%, and the investment cost is still high. Due to the storage capacity of up to 10 h (some special cases), they are attractive, as that is the only available technology nowadays to store solar power for a longer period. There is also development to transform the electricity into heat and store it underground in some rocks or gravels (see the web presentation of Siemens [5]). Afterwards, that heat is used to generate electricity, again over a steam turbine cycle. In that case, the temperature is limited to 600°C. The HiTES system [6] has a considerably higher temperature: up to 1100°C. The leading idea is that the exergy of stored heat is much higher at elevated temperatures. The quality of heat at 600°C is about 65%, while at 1100°C it is already 80% [7]. That makes it possible to reach a higher round-trip efficiency, even with heat storage

installed energy storage systems, and several new projects are planned.

usage of solar power, which is a very competitive solution there.

systems. That system is presented in more details in the next chapters.

salt storage.

technologies.

**1.3. Thermal energy storage**

#### **1.1. Influence of renewable power generation**

In the modern systems with high penetration of renewable energy generation, like from wind and solar, the situation has been drastically changed. Not the consumption, but the generation is now what has to be smoothened. Those changes may be very fast and may last very long, even several days or weeks. It means the modern power storage devices have to participate in the load control for quite different time periods but additionally even in the frequency control.

The problems with the intermittent generation do not start immediately after the installation of the first unit. Big power generation systems may absorb easily small disturbances in the system. It depends on many factors, but the experience shows that with about 20% penetration of the intermittent power generation, big problems occur. Then there is a strong need for higher usage of energy storage systems, together with other measures, like new grids, demand response, etc. Otherwise, curtailment or export under unfavourable conditions has to take place. However, it is important to understand the difference between the effects of new additional grids and the energy storage systems: With a grid, it is possible to transport a local power overproduction to some other areas with higher demand at that moment; however, a time shift, like with storage systems, is not possible. Moreover, new big grids implicitly lead to a more centralised generation, which was not the idea with the introduction of renewable power generation. Therefore, the best long-term solutions are energy storage systems that support distributed power generation.

#### **1.2. Examples for the need of energy storage**

In Germany, which is one of the leaders in the renewable power generation with some 33% in 2015, the opinion [1] was that in the next 10–20 years, there is no need for energy storage. That will change first when a very high share of renewable power generation (even 90%!) is reached. Meanwhile, in the last years, many anomalies that appeared on the market demonstrated that this is not the case. The electricity price on the stock market is falling, as the share of renewables is increasing. On the other hand, prices for industry and households increase with a rate of more than 5% per year. The net export is steadily growing but brings ever smaller income. The most important and the most absurd fact is that, in spite of all efforts with increasing the usage of renewable generation, the emission of CO2 is more or less stagnant! That infliences fast changing in the previous opinion: it is now recognised that energy storage, together with new grids expansion, is the inevitable component of the German "Energiewende". That case of Germany was described in detail in [2], showing that 40% of intermittent renewable electricity cannot be used domestically and has to be exported. In 2016 that trend has continued, as presented in [3]. Although the yearly increase of intermittent electricity was small (only 4.0 TWh), all of that increase had to be exported! On the other hand, the curtailment of many wind mills occurs very often, as the last option to get rid of the electricity overproduction. The need for energy storage in Germany is obvious.

California is the US state with the highest penetration of wind and solar power generation. Contrary to Germany, they have started thinking about and analysing the potential problems of intermittent generation much earlier. The "Duck Chart" was created by the California Independent System Operator to show that increasing solar generation paired with conventional base-load plants that cannot be turned off (e.g. nuclear and less flexible natural gas) can cause over-generation in the afternoons during certain months [4]. The chart shows that the shape of the net load curve begins to shift dramatically in 2015 due to increasing solar generation and there is potential for over-generation during the afternoons beginning in 2018. An especially big problem is a very fast ramping between 17 and 19 h (13.5 GW in 2 h!). To solve those potential problems, several measures were planned, like demand response, import/export, curtailment and energy storage. Being unfavourable measures, import/export and especially curtailment were minimised. Nowadays California is the area with the most installed energy storage systems, and several new projects are planned.

In some areas of Chile, there is a locally very high penetration of the solar power generation, based on photovoltaic systems. As the insolation is very high, the price of that electricity is low (<3 ₵ /kWh). However, every day at around noon, the market price goes into the negative area, as there are not enough consumers when the generation reaches its daily maximum. Therefore, there are some projects for energy storage facilities and some new generation facilities but based on concentrated solar power (CSP) with integrated molten salt storage.

Many countries with high insolation will be very soon in the same situation. In fact, many of them are waiting on a suitable solution for the energy storage in order to start a wide range of usage of solar power, which is a very competitive solution there.

#### **1.3. Thermal energy storage**

in systems with higher capacities of base-load plants, like coal and nuclear power plants. Instead of reducing their output and entering into the zone of lower process efficiency, their overproduction was stored, always enabling the optimal operation parameters. Those were

In the modern systems with high penetration of renewable energy generation, like from wind and solar, the situation has been drastically changed. Not the consumption, but the generation is now what has to be smoothened. Those changes may be very fast and may last very long, even several days or weeks. It means the modern power storage devices have to participate in the load control for quite different time periods but additionally even in the frequency control. The problems with the intermittent generation do not start immediately after the installation of the first unit. Big power generation systems may absorb easily small disturbances in the system. It depends on many factors, but the experience shows that with about 20% penetration of the intermittent power generation, big problems occur. Then there is a strong need for higher usage of energy storage systems, together with other measures, like new grids, demand response, etc. Otherwise, curtailment or export under unfavourable conditions has to take place. However, it is important to understand the difference between the effects of new additional grids and the energy storage systems: With a grid, it is possible to transport a local power overproduction to some other areas with higher demand at that moment; however, a time shift, like with storage systems, is not possible. Moreover, new big grids implicitly lead to a more centralised generation, which was not the idea with the introduction of renewable power generation. Therefore, the best long-term solutions are energy storage systems that

In Germany, which is one of the leaders in the renewable power generation with some 33% in 2015, the opinion [1] was that in the next 10–20 years, there is no need for energy storage. That will change first when a very high share of renewable power generation (even 90%!) is reached. Meanwhile, in the last years, many anomalies that appeared on the market demonstrated that this is not the case. The electricity price on the stock market is falling, as the share of renewables is increasing. On the other hand, prices for industry and households increase with a rate of more than 5% per year. The net export is steadily growing but brings ever smaller income. The most important and the most absurd fact is that, in spite of all efforts with increasing the

fast changing in the previous opinion: it is now recognised that energy storage, together with new grids expansion, is the inevitable component of the German "Energiewende". That case of Germany was described in detail in [2], showing that 40% of intermittent renewable electricity cannot be used domestically and has to be exported. In 2016 that trend has continued, as presented in [3]. Although the yearly increase of intermittent electricity was small (only 4.0 TWh), all of that increase had to be exported! On the other hand, the curtailment of many wind mills occurs very often, as the last option to get rid of the electricity overproduction. The

is more or less stagnant! That infliences

well-defined periods of time, well planned and without rush load changes.

**1.1. Influence of renewable power generation**

28 Advancements in Energy Storage Technologies

support distributed power generation.

**1.2. Examples for the need of energy storage**

usage of renewable generation, the emission of CO2

need for energy storage in Germany is obvious.

There are many technologies for energy storage. Some are suitable for long-term storage, the other for short-term and some of them even for the frequency control, with very fast response time. They are all using different principles, and therefore they have different advantages and disadvantages. Roughly, there are mechanical, chemical, electrical and thermal storage technologies.

Thermal energy storage is mostly famous for the molten salt facilities. They are used almost exclusively with the concentrated solar power (CSP) systems, where solar heat is stored and later used for power generation through the steam turbine cycle. The efficiency is about 32–36%, and the investment cost is still high. Due to the storage capacity of up to 10 h (some special cases), they are attractive, as that is the only available technology nowadays to store solar power for a longer period. There is also development to transform the electricity into heat and store it underground in some rocks or gravels (see the web presentation of Siemens [5]). Afterwards, that heat is used to generate electricity, again over a steam turbine cycle. In that case, the temperature is limited to 600°C. The HiTES system [6] has a considerably higher temperature: up to 1100°C. The leading idea is that the exergy of stored heat is much higher at elevated temperatures. The quality of heat at 600°C is about 65%, while at 1100°C it is already 80% [7]. That makes it possible to reach a higher round-trip efficiency, even with heat storage systems. That system is presented in more details in the next chapters.

#### **2. Components of the HiTES system**

**Hi**gh **T**emperature **E**nergy **S**torage (or shortly HiTES) is a new technology for energy storage based on three technologies that are state of the art:

Material is the limiting factor—is not easy to realise, for example, 2000°C. In the case of HiTES, it is 1100°C: all available components are made of material that can withstand it, and the qual-

High Temperature Energy Storage (HiTES) with Pebble Heater Technology and Gas Turbine

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31

The pebble heater technology has been selected as it is very suitable for high temperatures, and due to high heat exchange surface, it produces very low exergy losses. Such an example is presented in **Figure 2**. A heating gas enters the bed with 1350°C and has 160°C at the outlet. During the next phase, a gas which has to be heated (air in this case) has 90°C at the inlet and leaves the bed with 1280°C. The temperature difference between those two gases is only 70 K on both sides of the pebble bed. That gives an exergy efficiency of 95.2%. It is even less than 50 K in many applications. The recorded minimum was 15 K, leading to the exergy efficiency

Those characteristics make the pebble heater technology very efficient for the applications like thermal oxidizers (recuperation efficiency above 98% [9]), hot gas supply at temperatures

chemical reactions, steel converters [10, 11], blast furnaces [12], regenerative burners [13], etc. For HiTES technology, the pebble heater is a component which is crucial for reaching high

That type of gas turbine sets has been selected due to its extraordinary reliability recorded even in extreme conditions on oil rigs and gas fields, from sea platforms till Siberia. Despite small capacity (approx. 2 MW electric) and its simplicity, a modern design leads to

process efficiency. For more details about that technology, see Chapter 3.

**Figure 2.** Entropy increase during the heat transfer in a pebble heater.

has been preheated), steam superheating (1200°C) for some special

ity factor is still very high (0.8).

of above 98%.

above 1400°C (even H2

**2.3. Why radial gas turbine?**

**2.2. Why pebble heater technology?**


Those three technologies are combined in a new system, which suits well for medium-term storage, from several minutes up to several days. Electricity is used to heat up the heat storage material (pebbles) in a high temperature pebble heater by electric resistance heaters, during periods with electricity overproduction. If there is a need for additional electricity, a gas turbine coupled with a generator will produce it from the stored high temperature heat.

#### **2.1. Why high temperature?**

It is a common truth that heat is the lowest form of energy. It means the electricity may be transformed into heat with high efficiency, but transforming heat into electricity will be coupled with high losses. However, temperature defines the quality of heat, as shown in **Figure 1** [8]. It is not the same if one transforms electricity to heat at 2000°C or at 100°C. Moreover, heat available at 10,000°C has higher exergy than the natural gas, for example. Therefore, the common opinion mentioned here at the beginning is not generally right, but depends strongly on temperature. That is the reason why the electricity transformation into high temperature heat has been selected in this storage process.

**Figure 1.** The thermodynamic quality factor of heat, indicating the fraction of exergy in the amount of energy (adapted from Klimstra [8]).

Material is the limiting factor—is not easy to realise, for example, 2000°C. In the case of HiTES, it is 1100°C: all available components are made of material that can withstand it, and the quality factor is still very high (0.8).

#### **2.2. Why pebble heater technology?**

**2. Components of the HiTES system**

• Pebble heater technology

30 Advancements in Energy Storage Technologies

• Electric resistive heating

**2.1. Why high temperature?**

from Klimstra [8]).

has been selected in this storage process.

• Radial gas turbine

based on three technologies that are state of the art:

**Hi**gh **T**emperature **E**nergy **S**torage (or shortly HiTES) is a new technology for energy storage

Those three technologies are combined in a new system, which suits well for medium-term storage, from several minutes up to several days. Electricity is used to heat up the heat storage material (pebbles) in a high temperature pebble heater by electric resistance heaters, during periods with electricity overproduction. If there is a need for additional electricity, a gas tur-

It is a common truth that heat is the lowest form of energy. It means the electricity may be transformed into heat with high efficiency, but transforming heat into electricity will be coupled with high losses. However, temperature defines the quality of heat, as shown in **Figure 1** [8]. It is not the same if one transforms electricity to heat at 2000°C or at 100°C. Moreover, heat available at 10,000°C has higher exergy than the natural gas, for example. Therefore, the common opinion mentioned here at the beginning is not generally right, but depends strongly on temperature. That is the reason why the electricity transformation into high temperature heat

**Figure 1.** The thermodynamic quality factor of heat, indicating the fraction of exergy in the amount of energy (adapted

bine coupled with a generator will produce it from the stored high temperature heat.

The pebble heater technology has been selected as it is very suitable for high temperatures, and due to high heat exchange surface, it produces very low exergy losses. Such an example is presented in **Figure 2**. A heating gas enters the bed with 1350°C and has 160°C at the outlet. During the next phase, a gas which has to be heated (air in this case) has 90°C at the inlet and leaves the bed with 1280°C. The temperature difference between those two gases is only 70 K on both sides of the pebble bed. That gives an exergy efficiency of 95.2%. It is even less than 50 K in many applications. The recorded minimum was 15 K, leading to the exergy efficiency of above 98%.

Those characteristics make the pebble heater technology very efficient for the applications like thermal oxidizers (recuperation efficiency above 98% [9]), hot gas supply at temperatures above 1400°C (even H2 has been preheated), steam superheating (1200°C) for some special chemical reactions, steel converters [10, 11], blast furnaces [12], regenerative burners [13], etc. For HiTES technology, the pebble heater is a component which is crucial for reaching high process efficiency. For more details about that technology, see Chapter 3.

#### **2.3. Why radial gas turbine?**

That type of gas turbine sets has been selected due to its extraordinary reliability recorded even in extreme conditions on oil rigs and gas fields, from sea platforms till Siberia. Despite small capacity (approx. 2 MW electric) and its simplicity, a modern design leads to

**Figure 2.** Entropy increase during the heat transfer in a pebble heater.

**3.1. Physical description**

some special applications.

**Figure 4.** Pebble heater with radial design [12].

Al2 O3

Outside, the pebble heater is a cylindrical vessel. Inside, there are two permeable grids with a pebble bed fixed in between. The inner grid (hot grid) is always at high temperature. It is composed of high-quality ceramic bricks. Each brick has a hole that ends with a honeycomb segment, in order to prevent the pebble fluidization. Those bricks are tested up to 1500°C, first in an industrial scale pilot facility and later in tenths of industrial facilities. The improved quality of the honeycomb segments will enable higher temperatures, up to 1700°C, as required for

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33

The outer grid is on the cold side and therefore referred as the cold grid. It is constructed of a perforated steel plate. There is no possibility for pebbles to fluidize or circulate, as they are fixed between those two grids. The cold grid temperature is usually held under 250°C. In such cases, the material for the outer vessel may be a conventional steel. Moreover, the outer insulation is not required, only some touch protection in some cases. Indeed, as presented in blue in **Figure 4**, some fire-clay insulation is only required at the hot gas inlet/outlet (bottom)

In the case of applications with very high temperatures, the bed of alumina pebbles (>99%

) is the best choice. They are very resistant to the thermal shocks, so that the temperature

and for the so-called dome, which closes the hot grid on the top.

**Figure 3.** Rotor of a radial gas turbine with expander (left) and compressor (right) [14].

the efficiency of even 25% in a simple open cycle. They are very robust, have single shaft with cold end drive, have easy maintenance, have low lube oil consumption and have long inspection intervals. The models present on the market (produced by OPRA from the Netherlands and Dresser-Rand with its production facilities in Norway) have proven its abilities in much more than 1000 units built. The design for external firing, which is required for the HiTES system, is now also available. **Figure 3** [14] presents a modern allradial design (expander, compressor and shaft).

#### **3. Pebble heater**

The name "pebble heater" is already known in the field of regenerative heat exchangers. It may operate at high temperatures for heating and cooling gaseous media by means of bulk material consisted of spherical balls, called pebbles. That is common for the previous and the new design of the pebble heater.

The most important difference is the flow direction: the bulk material is fixed between two vertical, concentric and permeable cylinders (hot and cold grid), so that the fluid flows radially. At the first sight, small difference results in further extraordinary advantages. As there is no danger of fluidization, the flow velocity may be increased, and smaller pebbles may be used. That improves dramatically the heat transfer, especially through very high ratio of surface to volume, that is, specific surface. With those characteristic it is easy to reach a thermal recuperation efficiency of 95%; even 98% has been achieved with a unit in operation. As a result, the exergy losses are small, and the temperature difference between two gases (heating and cooling) is as little as 20 K—all that at temperatures up to 1500°C!

That intensive heat transfer results in a temperature gradient as high as 2000 K/m. Therefore the bed of pebbles is thin, and the pressure drop stays low or acceptable. The units are compact, which lead to low investment cost—the most important fact for investors.

#### **3.1. Physical description**

the efficiency of even 25% in a simple open cycle. They are very robust, have single shaft with cold end drive, have easy maintenance, have low lube oil consumption and have long inspection intervals. The models present on the market (produced by OPRA from the Netherlands and Dresser-Rand with its production facilities in Norway) have proven its abilities in much more than 1000 units built. The design for external firing, which is required for the HiTES system, is now also available. **Figure 3** [14] presents a modern all-

The name "pebble heater" is already known in the field of regenerative heat exchangers. It may operate at high temperatures for heating and cooling gaseous media by means of bulk material consisted of spherical balls, called pebbles. That is common for the previous and the

The most important difference is the flow direction: the bulk material is fixed between two vertical, concentric and permeable cylinders (hot and cold grid), so that the fluid flows radially. At the first sight, small difference results in further extraordinary advantages. As there is no danger of fluidization, the flow velocity may be increased, and smaller pebbles may be used. That improves dramatically the heat transfer, especially through very high ratio of surface to volume, that is, specific surface. With those characteristic it is easy to reach a thermal recuperation efficiency of 95%; even 98% has been achieved with a unit in operation. As a result, the exergy losses are small, and the temperature difference between two gases (heating

That intensive heat transfer results in a temperature gradient as high as 2000 K/m. Therefore the bed of pebbles is thin, and the pressure drop stays low or acceptable. The units are com-

and cooling) is as little as 20 K—all that at temperatures up to 1500°C!

pact, which lead to low investment cost—the most important fact for investors.

radial design (expander, compressor and shaft).

**Figure 3.** Rotor of a radial gas turbine with expander (left) and compressor (right) [14].

**3. Pebble heater**

new design of the pebble heater.

32 Advancements in Energy Storage Technologies

Outside, the pebble heater is a cylindrical vessel. Inside, there are two permeable grids with a pebble bed fixed in between. The inner grid (hot grid) is always at high temperature. It is composed of high-quality ceramic bricks. Each brick has a hole that ends with a honeycomb segment, in order to prevent the pebble fluidization. Those bricks are tested up to 1500°C, first in an industrial scale pilot facility and later in tenths of industrial facilities. The improved quality of the honeycomb segments will enable higher temperatures, up to 1700°C, as required for some special applications.

The outer grid is on the cold side and therefore referred as the cold grid. It is constructed of a perforated steel plate. There is no possibility for pebbles to fluidize or circulate, as they are fixed between those two grids. The cold grid temperature is usually held under 250°C. In such cases, the material for the outer vessel may be a conventional steel. Moreover, the outer insulation is not required, only some touch protection in some cases. Indeed, as presented in blue in **Figure 4**, some fire-clay insulation is only required at the hot gas inlet/outlet (bottom) and for the so-called dome, which closes the hot grid on the top.

In the case of applications with very high temperatures, the bed of alumina pebbles (>99% Al2 O3 ) is the best choice. They are very resistant to the thermal shocks, so that the temperature

**Figure 4.** Pebble heater with radial design [12].

cycling of even over 400 K cannot cause any damage. In the case of less demanding applications, bulk materials like fire-clay balls or even river gravels are much cheaper solution.

All physical properties, including the "effective" heat conductivity, are functions of the temperature and indirectly of the radial coordinate, as temperature changes significantly with the radial position. Due to the change of the flow cross section with the radial coordinate, the mass flux of gas is also a function of the radial position. The "effective" heat conductivity in

ductivity in the usual *Fourier* equation as it includes the effects of convection and radiation, besides the heat conductivity of fluid and solid. One can find several correlations for those terms in the literature. The correlations given by Bauer [16] and Till [17], which are valid up

Eq. (1) is a partial differential equation of the second order. To solve it, one initial and two boundary conditions are required. The initial condition is some given temperature distribution over the bed radius. The boundary conditions are heat fluxes on the hot and cold end (i.e.

As the "effective" heat conductivity and gas flux are not constant, there is not an analytical solution for Eq. (1). That type of *Fourier* equation may be effectively solved by using the Crank–Nicolson numerical method, as shown in [18]. That method is implicit, which is its main advantage, because it enables long time steps with good stability of the calculation process. Based on that method, a numerical code for simulation of the pebble heater operation has been developed, with typical results of the characteristic temperature profile through the

), which is introduced in this model, differs from the "classical" heat con-

High Temperature Energy Storage (HiTES) with Pebble Heater Technology and Gas Turbine

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35

radial direction (Λ*<sup>r</sup>*

bed given in **Figure 5**.

to a *Péclet* number of *Pe =* 30, are used here.

hot and cold grid), which have to be defined.

**Figure 5.** Typical temperature distribution inside the pebble heater.

The thermal expansion of the hot grid does not cause any sealing or stress problem, as it may expand together with the "floating" dome upwards freely; see **Figure 4**.

#### **3.2. Operation of pebble heater**

Being a regenerative heat exchanger, at least two units of pebble heater are required for a continuous operation. One unit is producing hot gas (blast), while one or more units are reheated. After a certain time, the reheated unit will switch to the blast phase, and the other unit will switch to the reheating phase. On that way a continuous supply of hot gas is secured.

The operation of pebble heater during those two phases is seen in **Figure 4**. In the case that the heating gas is a combustion product, the combustion takes place mainly in the chamber inside the hot grid. Flue gases enter the bed through the hot grid. Flowing radially through the bed, flue gases leave their heat to it and have a low temperature at the end of the bed. Cooled gases pass upwards, through the gap between the cold grid and the outer wall of the vessel, towards the exit.

When the bed is fully reheated, the burner stops, and the vessel is pressurised at the cold blast pressure. Then it enters and flows in opposite direction: first it distributes in the gap around the pebble bed and then passes through it. Heat is now transferred in reverse direction, from the pebbles to the gas. In the chamber inside the hot grid, the hot blast is collected and flows out through the hot blast main.

In some cases an existing waste hot gas may be used for reheating phase. Then there is no need for a burner, as the pebble heater recuperates the waste heat from that gas and uses it for preheating air or some other gas.

#### **3.3. Mathematical modelling of pebble heater**

To simulate the operation of such pebble heater with radial flow, the mathematical model has been developed, based on Crank-Nicholson numerical method [18].

The heater is axial-symmetric, and the upper and bottom walls are adiabatic, so the heat is transferred just in radial direction. Due to a very high specific surface available for the heat transfer (usually between 500 and 1000 m2 /m3 ), the difference between gas and pebble temperature is almost negligible when compared to the temperature change during each phase. Vortmeyer and Schäfer have presented a so-called "homogeneous" model [15], which uses only one energy balance equation and gives good results in such cases. Originally, the equation of Vortmeyer and Schäfer describes a cylindrical pebble bed with axial flow. For the radial geometry, it was rewritten as follows:

$$\frac{1}{r}\frac{\partial}{\partial r}\left(r\,\Lambda\_r\frac{\partial T}{\partial r}\right) = \frac{\partial}{\partial r}(m\_s\,c\_{p\uparrow}T) + \left[\left(1-\psi\right)\,\rho\_s\,c\_s + \psi\rho\_f\,c\_{p\uparrow}\right]\frac{\partial T}{\partial t} \tag{1}$$

with the following notation: *r* is the radial coordinate; *t* – time; Λ*<sup>r</sup>* – "effective" heat conductivity; *T* – temperature; *mo* - mass flux of gas; *Ψψ* - void fraction (i.e. bed porosity); *cs* , *cPf* – specific heat (solid and gas phase, respectively); *ρ<sup>s</sup>* ,*ρf* – density (solid and gas phase, respectively).

All physical properties, including the "effective" heat conductivity, are functions of the temperature and indirectly of the radial coordinate, as temperature changes significantly with the radial position. Due to the change of the flow cross section with the radial coordinate, the mass flux of gas is also a function of the radial position. The "effective" heat conductivity in radial direction (Λ*<sup>r</sup>* ), which is introduced in this model, differs from the "classical" heat conductivity in the usual *Fourier* equation as it includes the effects of convection and radiation, besides the heat conductivity of fluid and solid. One can find several correlations for those terms in the literature. The correlations given by Bauer [16] and Till [17], which are valid up to a *Péclet* number of *Pe =* 30, are used here.

cycling of even over 400 K cannot cause any damage. In the case of less demanding applications, bulk materials like fire-clay balls or even river gravels are much cheaper solution.

The thermal expansion of the hot grid does not cause any sealing or stress problem, as it may

Being a regenerative heat exchanger, at least two units of pebble heater are required for a continuous operation. One unit is producing hot gas (blast), while one or more units are reheated. After a certain time, the reheated unit will switch to the blast phase, and the other unit will

The operation of pebble heater during those two phases is seen in **Figure 4**. In the case that the heating gas is a combustion product, the combustion takes place mainly in the chamber inside the hot grid. Flue gases enter the bed through the hot grid. Flowing radially through the bed, flue gases leave their heat to it and have a low temperature at the end of the bed. Cooled gases pass upwards, through the gap between the cold grid and the outer wall of the

When the bed is fully reheated, the burner stops, and the vessel is pressurised at the cold blast pressure. Then it enters and flows in opposite direction: first it distributes in the gap around the pebble bed and then passes through it. Heat is now transferred in reverse direction, from the pebbles to the gas. In the chamber inside the hot grid, the hot blast is collected and flows

In some cases an existing waste hot gas may be used for reheating phase. Then there is no need for a burner, as the pebble heater recuperates the waste heat from that gas and uses it for

To simulate the operation of such pebble heater with radial flow, the mathematical model has

The heater is axial-symmetric, and the upper and bottom walls are adiabatic, so the heat is transferred just in radial direction. Due to a very high specific surface available for the heat

perature is almost negligible when compared to the temperature change during each phase. Vortmeyer and Schäfer have presented a so-called "homogeneous" model [15], which uses only one energy balance equation and gives good results in such cases. Originally, the equation of Vortmeyer and Schäfer describes a cylindrical pebble bed with axial flow. For the

<sup>∂</sup>*r*(*mo cPf <sup>T</sup>*) <sup>+</sup> [(1 <sup>−</sup> *<sup>ψ</sup>*) *<sup>ρ</sup><sup>s</sup> cs* <sup>+</sup> *ψρ<sup>f</sup> cPf*]


), the difference between gas and pebble tem-

\_\_\_ ∂*T*

– density (solid and gas phase, respectively).

<sup>∂</sup>*<sup>t</sup>* (1)

, *cPf* – specific

– "effective" heat conductiv-

/m3

,*ρf*

switch to the reheating phase. On that way a continuous supply of hot gas is secured.

expand together with the "floating" dome upwards freely; see **Figure 4**.

**3.2. Operation of pebble heater**

34 Advancements in Energy Storage Technologies

vessel, towards the exit.

out through the hot blast main.

preheating air or some other gas.

**3.3. Mathematical modelling of pebble heater**

transfer (usually between 500 and 1000 m2

radial geometry, it was rewritten as follows:

\_\_\_ ∂*T* <sup>∂</sup>*r*) <sup>=</sup> \_\_<sup>∂</sup>

with the following notation: *r* is the radial coordinate; *t* – time; Λ*<sup>r</sup>*

*r* \_\_\_\_\_\_ ∂ <sup>∂</sup>*r*(*<sup>r</sup>* <sup>Λ</sup>*<sup>r</sup>*

heat (solid and gas phase, respectively); *ρ<sup>s</sup>*

\_\_1

ity; *T* – temperature; *mo*

been developed, based on Crank-Nicholson numerical method [18].

Eq. (1) is a partial differential equation of the second order. To solve it, one initial and two boundary conditions are required. The initial condition is some given temperature distribution over the bed radius. The boundary conditions are heat fluxes on the hot and cold end (i.e. hot and cold grid), which have to be defined.

As the "effective" heat conductivity and gas flux are not constant, there is not an analytical solution for Eq. (1). That type of *Fourier* equation may be effectively solved by using the Crank–Nicolson numerical method, as shown in [18]. That method is implicit, which is its main advantage, because it enables long time steps with good stability of the calculation process. Based on that method, a numerical code for simulation of the pebble heater operation has been developed, with typical results of the characteristic temperature profile through the bed given in **Figure 5**.

**Figure 5.** Typical temperature distribution inside the pebble heater.

Solving the energy Eq. (1) runs side by side with calculating the pressure drop through the pebble bed, through integration of the following equation:

$$\frac{dp}{dr} = \frac{1}{\psi^2} \mu \sharp \frac{\rho}{2} \frac{w\_o^2}{D\_e} \tag{2}$$

**3.5. Application for energy storage**

tion, with time sequences of about 30 min.

compressed air for the turbine drive.

**4. Operation of HiTES**

resulting with stable outlet temperature for a long time.

The extraordinary characteristics of the pebble heater technology are of decisive importance for the energy storage concept presented in this article. Each storage module consists of four smaller pebble heaters for the gas turbine recuperation (temperature range up to 550°C) and one more than 12 m tall electrically heated pebble heater. Small pebble heaters are in operation during gas turbine operation, and they are intermittent in charging-discharging opera-

High Temperature Energy Storage (HiTES) with Pebble Heater Technology and Gas Turbine

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37

**Figure 6.** Typical temperature distribution inside the storage tower during 10 h discharging.

The big pebble heater (or storage tower) is electrically charged when the gas turbine is out of operation. During that time (up to 10 h per tower), the temperature of pebbles rises from 550°C towards 1100°C. When the gas turbine is in operation, it delivers the high temperature

**Figure 6** gives the typical temperature distribution inside such storage tower with a column of 12 m filled with 12 mm pebbles, during a discharging phase of 10 h. At the beginning, all pebbles are at the highest temperature of 1100°C. Then the compressed air with 550°C enters from the bottom (the right side of **Figure 6**) and flows upwards (to the left side). It is heated to 1100°C and the pebbles are cooled down. The S-type temperature profile is established,

The principles of HiTES operation are based on the patent document [6] and given in [7]. When the system is charging, only one pebble heater (PH-E in **Figure 7**) is in use. Electrical heaters

with following parameters: *p* – pressure; *μξ* – friction and path factor; *wo* – gas velocity; *De* – equivalent pebble diameter.

The usual way for calculating the friction and path factor is the famous Ergun equation [19]. The comparison with measured values has shown that the correlation of Kast [20] is more accurate. Based on own measurements, the new correlations have been defined, which give even better results. Especially in the case of irregular shape of pebbles, those advantages were distinct.

#### **3.4. Performance of pebble heater**

**Figure 5** presents the typical, S-shaped temperature profile inside the bed, which is an extraordinary characteristic of the pebble heater technology. It arises from the intensive heat transfer and resulting low temperature difference between gas and solid phases. That S-shape enables a temperature change of more than 400 K in the middle of the bed, and correspondingly high storage capacity, while the hot grid (and hot blast at the exit) exhibits a very moderate temperature drop.

Therefore the hot blast temperature stays almost constant at the first two thirds of the blast phase. Only in the course of the last third of the blast phase, there is a more intensive drop of the blast temperature (ΔT = 30–100 K, in different designs and operation mode. The same is with the temperature changes on the cold), usually in the range ΔT = 100–150 K. That is the reason for low mean value of the flue gas temperature and resulting low exit loss.

An extensive test series on the pilot unit PH 104 (10,000 mi.N. 3 /h) has proven the extraordinary characteristics of this new concept of heat regenerators:


In that test series, all main operational parameters (like blast rate, cycling time, flame temperature, etc.) were modified from test to test, in order to cover any possible operating condition. Those tests have proved that the recuperation efficiency of the pebble heater overpasses by far the efficiency of the modern stoves, even with recuperative heat exchangers for preheating combustion air and/or fuel gas.

In the meantime, since 1996 more than 20 facilities have been built. Most of them were used as thermal oxidizers; however, the biggest units are for hot blast supply for iron and steel industry. In the next years, even bigger units are expected to replace the old Cowper technology for blast furnaces.

High Temperature Energy Storage (HiTES) with Pebble Heater Technology and Gas Turbine http://dx.doi.org/10.5772/intechopen.75093 37

**Figure 6.** Typical temperature distribution inside the storage tower during 10 h discharging.

#### **3.5. Application for energy storage**

Solving the energy Eq. (1) runs side by side with calculating the pressure drop through the

The usual way for calculating the friction and path factor is the famous Ergun equation [19]. The comparison with measured values has shown that the correlation of Kast [20] is more accurate. Based on own measurements, the new correlations have been defined, which give even better results. Especially in the case of irregular shape of pebbles, those advantages were distinct.

**Figure 5** presents the typical, S-shaped temperature profile inside the bed, which is an extraordinary characteristic of the pebble heater technology. It arises from the intensive heat transfer and resulting low temperature difference between gas and solid phases. That S-shape enables a temperature change of more than 400 K in the middle of the bed, and correspondingly high storage capacity, while the hot grid (and hot blast at the exit) exhibits a very moderate tem-

Therefore the hot blast temperature stays almost constant at the first two thirds of the blast phase. Only in the course of the last third of the blast phase, there is a more intensive drop of the blast temperature (ΔT = 30–100 K, in different designs and operation mode. The same is with the temperature changes on the cold), usually in the range ΔT = 100–150 K. That is the

In that test series, all main operational parameters (like blast rate, cycling time, flame temperature, etc.) were modified from test to test, in order to cover any possible operating condition. Those tests have proved that the recuperation efficiency of the pebble heater overpasses by far the efficiency of the modern stoves, even with recuperative heat exchangers for preheating

In the meantime, since 1996 more than 20 facilities have been built. Most of them were used as thermal oxidizers; however, the biggest units are for hot blast supply for iron and steel industry. In the next years, even bigger units are expected to replace the old Cowper technol-

3

/h) has proven the extraordinary

reason for low mean value of the flue gas temperature and resulting low exit loss.

An extensive test series on the pilot unit PH 104 (10,000 mi.N.

characteristics of this new concept of heat regenerators:

• Between 92 and 95% of thermal recuperation efficiency

• Ability to sustain high temperature operation

• Stability of the hot blast temperature

combustion air and/or fuel gas.

ogy for blast furnaces.

*<sup>ψ</sup>*<sup>2</sup> *<sup>ρ</sup> wo*

2 \_\_\_\_ 2 *De*

(2)

–

– gas velocity; *De*

*dr* <sup>=</sup> \_\_1

with following parameters: *p* – pressure; *μξ* – friction and path factor; *wo*

pebble bed, through integration of the following equation:

*dp*\_\_\_

36 Advancements in Energy Storage Technologies

equivalent pebble diameter.

**3.4. Performance of pebble heater**

perature drop.

The extraordinary characteristics of the pebble heater technology are of decisive importance for the energy storage concept presented in this article. Each storage module consists of four smaller pebble heaters for the gas turbine recuperation (temperature range up to 550°C) and one more than 12 m tall electrically heated pebble heater. Small pebble heaters are in operation during gas turbine operation, and they are intermittent in charging-discharging operation, with time sequences of about 30 min.

The big pebble heater (or storage tower) is electrically charged when the gas turbine is out of operation. During that time (up to 10 h per tower), the temperature of pebbles rises from 550°C towards 1100°C. When the gas turbine is in operation, it delivers the high temperature compressed air for the turbine drive.

**Figure 6** gives the typical temperature distribution inside such storage tower with a column of 12 m filled with 12 mm pebbles, during a discharging phase of 10 h. At the beginning, all pebbles are at the highest temperature of 1100°C. Then the compressed air with 550°C enters from the bottom (the right side of **Figure 6**) and flows upwards (to the left side). It is heated to 1100°C and the pebbles are cooled down. The S-type temperature profile is established, resulting with stable outlet temperature for a long time.

#### **4. Operation of HiTES**

The principles of HiTES operation are based on the patent document [6] and given in [7]. When the system is charging, only one pebble heater (PH-E in **Figure 7**) is in use. Electrical heaters

When the system is discharging, the ambient air at 15°C, 1.013 bar absolute and a relative humidity of 60% enters into the compressor. In front of compressor, that air is cooled down by fogging. On one hand that decreases the compression work, and on the other hand, the fogging increases the mass flow by increasing the density. At the compressor outlet, the air has 7.3 bar and 275°C. The subsequent heat exchanger takes out some amount of heat from the compressed air, and by water injection, the further cooling is achieved. It is possible to inject 0.3 kg/s of water, before reaching the dew point temperature. Again, that leads to the higher mass flow rate and lower temperature of the compressed air. That injected water can be considered as a replacement for the fuel mass flow missing in such application of the gas turbine. In that way, the compressor and the expander work very close to the design point. The cooling of air entering the pebble heaters PH1…PH4 is important for the efficient heat transfer, as it leads to lower outlet temperatures during the exhaust air phase, meaning lower heat losses. Such preconditioned compressed air passes through one of the pebble heaters (PH1…PH4). Due to previously described advantages of the pebble heater technology, the air is preheated almost to the turbine exhaust temperature, leading to the high recuperation of the gas turbine cycle. After the low temperature PH, the compressed air flows through the high temperature pebble heater (PH-E), where it is heated further to 1100°C. In front of the expander, the air temperature is adjusted to the required inlet parameters (970°C), by controlling the bypass valve. That air is mixed with the turbine cooling air, preventing the overheating of the turbine guide blades. In that way, the turbine inlet temperature (TIT) of 940°C is reached. At the turbine outlet, the expanded air is at about ambient pressure and 540°C. It flows through the remaining three pebble heaters and heats up the pebbles. Thus it is cooled down to about 110°C and with that temperature leaves the

High Temperature Energy Storage (HiTES) with Pebble Heater Technology and Gas Turbine

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39

Starting from the nominal turbine output of 2 MW and the round-trip efficiency of 40%, the suitable high temperature storage capacity of pebble heater PH-E is 50 MWh (i.e. 10 h heating with 5 MW of input power). In that case the discharging phase may deliver maximum 20 MWh of electricity (again 10 h with 2 MW electrical power). In a modular concept of the facility, two or three identical PH-E may be coupled with one gas turbine set. Respectively,

With the refractory inside the PH-E made of high-quality fibre modules, the storage time of the high temperature heat may be from several minutes up to several days. **Figure 8** presents the temperature drop through such refractory over 5 days. Storing times longer than 7 days are not feasible, due to considerable efficiency reduction, as approx. 1% of stored heat would be lost per storage day. It means that after 5 days, some 5% of heat would be lost, as presented in **Figure 9**, leading to the reduction of the round-trip efficiency from 40 to 38%. The results presented in both **Figures 8** and **9** are based on the heat transfer calculations for a cylindrical structure with inside refractory made of silica-based fibre modules. According to the product sheet of a famous refractory supplier, those modules have an excellent heat conductivity coefficient. Although it increases with the temperature, at 1100°C it is only 0.25 W/mK (at 200°C 0.05 W/mK). That temperature dependence is taken into account in

the output of 2 MW power may be delivered for 20 or even 30 h.

system through the chimney.

the calculations.

**Figure 7.** Flow diagram and nominal process parameters of HiTES [7].

heat up the storage material from 550 to 1100°C. That is very important for achieving good round-trip efficiency, as the charging electricity is stored only in form of high temperature heat.

When the system is discharging, all system components presented in **Figure 7** are in use. First, the low temperature pebble heaters (PH1…PH4) preheat compressed air to 550°C, and then it enters the high temperature PH-E where it is preheated to the end temperature of 1100°C. Hot compressed air enters the gas turbine and expands there, releasing mechanical work for compressor and generator drive. Expanded exhaust air heats up again the low temperature storage PH1… PH4. That heat is used later for preheating the compressed air, by activating the set of presented valves. In that way, always one pebble heater is in compressed air loop, and the remaining three are in the exhaust air phase. When a certain time (e.g. 20 min) has elapsed, another PH changes over to the compressed air loop, and the previous PH goes to the exhaust air loop and so on.

In the first step, it is preferred to use a radial gas turbine existing on the market, instead of developing a new one. Even the existing gas turbine (2 MW output power) in the HiTES system will reach approx. 40% of the round-trip efficiency that enables the profitability of the first units. After the maturity of this technology is proved in the industrial application, a new or modified gas turbine will be used, leading to higher efficiency and increased profitability of the system.

Some additional components have to be introduced with the existing gas turbine model in order to reach that high round-trip efficiency. Those are the fogging of the inlet air and compressed air cooling with water injection. The following description gives more precise process parameters (based on the ISO conditions 15°C, 1.013 bar abs).

When the system is discharging, the ambient air at 15°C, 1.013 bar absolute and a relative humidity of 60% enters into the compressor. In front of compressor, that air is cooled down by fogging. On one hand that decreases the compression work, and on the other hand, the fogging increases the mass flow by increasing the density. At the compressor outlet, the air has 7.3 bar and 275°C. The subsequent heat exchanger takes out some amount of heat from the compressed air, and by water injection, the further cooling is achieved. It is possible to inject 0.3 kg/s of water, before reaching the dew point temperature. Again, that leads to the higher mass flow rate and lower temperature of the compressed air. That injected water can be considered as a replacement for the fuel mass flow missing in such application of the gas turbine. In that way, the compressor and the expander work very close to the design point. The cooling of air entering the pebble heaters PH1…PH4 is important for the efficient heat transfer, as it leads to lower outlet temperatures during the exhaust air phase, meaning lower heat losses. Such preconditioned compressed air passes through one of the pebble heaters (PH1…PH4). Due to previously described advantages of the pebble heater technology, the air is preheated almost to the turbine exhaust temperature, leading to the high recuperation of the gas turbine cycle. After the low temperature PH, the compressed air flows through the high temperature pebble heater (PH-E), where it is heated further to 1100°C. In front of the expander, the air temperature is adjusted to the required inlet parameters (970°C), by controlling the bypass valve. That air is mixed with the turbine cooling air, preventing the overheating of the turbine guide blades. In that way, the turbine inlet temperature (TIT) of 940°C is reached. At the turbine outlet, the expanded air is at about ambient pressure and 540°C. It flows through the remaining three pebble heaters and heats up the pebbles. Thus it is cooled down to about 110°C and with that temperature leaves the system through the chimney.

Starting from the nominal turbine output of 2 MW and the round-trip efficiency of 40%, the suitable high temperature storage capacity of pebble heater PH-E is 50 MWh (i.e. 10 h heating with 5 MW of input power). In that case the discharging phase may deliver maximum 20 MWh of electricity (again 10 h with 2 MW electrical power). In a modular concept of the facility, two or three identical PH-E may be coupled with one gas turbine set. Respectively, the output of 2 MW power may be delivered for 20 or even 30 h.

**Figure 7.** Flow diagram and nominal process parameters of HiTES [7].

38 Advancements in Energy Storage Technologies

heat up the storage material from 550 to 1100°C. That is very important for achieving good round-trip efficiency, as the charging electricity is stored only in form of high temperature heat. When the system is discharging, all system components presented in **Figure 7** are in use. First, the low temperature pebble heaters (PH1…PH4) preheat compressed air to 550°C, and then it enters the high temperature PH-E where it is preheated to the end temperature of 1100°C. Hot compressed air enters the gas turbine and expands there, releasing mechanical work for compressor and generator drive. Expanded exhaust air heats up again the low temperature storage PH1… PH4. That heat is used later for preheating the compressed air, by activating the set of presented valves. In that way, always one pebble heater is in compressed air loop, and the remaining three are in the exhaust air phase. When a certain time (e.g. 20 min) has elapsed, another PH changes over to the compressed air loop, and the previous PH goes to the exhaust air loop and so on.

In the first step, it is preferred to use a radial gas turbine existing on the market, instead of developing a new one. Even the existing gas turbine (2 MW output power) in the HiTES system will reach approx. 40% of the round-trip efficiency that enables the profitability of the first units. After the maturity of this technology is proved in the industrial application, a new or modified gas turbine will be used, leading to higher efficiency and increased profitability of the system. Some additional components have to be introduced with the existing gas turbine model in order to reach that high round-trip efficiency. Those are the fogging of the inlet air and compressed air cooling with water injection. The following description gives more precise

process parameters (based on the ISO conditions 15°C, 1.013 bar abs).

With the refractory inside the PH-E made of high-quality fibre modules, the storage time of the high temperature heat may be from several minutes up to several days. **Figure 8** presents the temperature drop through such refractory over 5 days. Storing times longer than 7 days are not feasible, due to considerable efficiency reduction, as approx. 1% of stored heat would be lost per storage day. It means that after 5 days, some 5% of heat would be lost, as presented in **Figure 9**, leading to the reduction of the round-trip efficiency from 40 to 38%. The results presented in both **Figures 8** and **9** are based on the heat transfer calculations for a cylindrical structure with inside refractory made of silica-based fibre modules. According to the product sheet of a famous refractory supplier, those modules have an excellent heat conductivity coefficient. Although it increases with the temperature, at 1100°C it is only 0.25 W/mK (at 200°C 0.05 W/mK). That temperature dependence is taken into account in the calculations.

**5. System improvements**

**5.1. Fogging or evaporative cooling**

**5.2. Water injection**

power releases, too.

**5.4. Pressure ratio**

**5.3. Turbine inlet temperature, TIT**

energy drops and the turbine efficiency increases.

output, leading to the higher power generation efficiency.

forward compared to 970°C in the nowadays available gas turbine.

As mentioned previously, some well-known measures may improve the actual round-trip efficiency of the existing process. Those measures and their effects, including the turbine optimization, have been analysed in [21]. In some cases those improvements may be done with the existing gas turbine; in other words it would be required to make more or less complex optimizations or design changes. In the case of the existing design, the pressure ratio was 7.27 and the isentropic compressor efficiency 80.89%. The turbine mass flow was 9.663 kg/s and the isentropic turbine efficiency 83.96%. For the improved optimised designs, the same characteristics were assumed, except that in the case of "pressure ratio", it was reduced to 3.5. Here follows a short overview of those measures, and the summary at the end gives the effect of all measures that could be applied simultaneously on the existing or on the related improved designs.

High Temperature Energy Storage (HiTES) with Pebble Heater Technology and Gas Turbine

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41

Fogging is a measure well analysed in theory [22–24] and successfully used nowadays. It brings the biggest effects in climates with hot and dry ambient air. The compressor inlet air is saturated with water, which causes its temperature to drop, as the water evaporation takes the energy from it. The wet bulb temperature is the resulting temperature, causing the mass flow to rise due to the density increase. For example, for ambient air at 15°C and the relative humidity of 60%, the wet bulb temperature comes to 10.8°C. Respectively, the compression

Water injection into the compressed air makes the same effect as the evaporative cooling. Due to water evaporation, the temperature drops. Although that evaporation energy is lost for the external heat usage, the mass flow rate and the heat capacity increase and so the turbine

The compressor work stays constant, and the generator gets the whole increase of the turbine

An increased TIT leads to a higher inlet enthalpy and increases the specific turbine work. The robustness of materials is improving continuously with the progress in material science. Presently, some expander rotors operate with the turbine inlet temperature of 1050°C, without blade cooling. Therefore it seems just a matter of time until 1100°C will be reached, maybe with some additional measures, like blade coating. That would be another significant step

It is well known that in a simple cycle, there is always an optimum pair of the pressure ratio and the inlet gas turbine temperature. However, that optimum is not the same for a recuperated gas

**Figure 8.** Temperature drop through the refractory of high-quality fibre modules, over 5 days.

**Figure 9.** Decrease of stored heat due to heat losses and the drop of temperature inside the storage tower, during 5 days.

### **5. System improvements**

As mentioned previously, some well-known measures may improve the actual round-trip efficiency of the existing process. Those measures and their effects, including the turbine optimization, have been analysed in [21]. In some cases those improvements may be done with the existing gas turbine; in other words it would be required to make more or less complex optimizations or design changes. In the case of the existing design, the pressure ratio was 7.27 and the isentropic compressor efficiency 80.89%. The turbine mass flow was 9.663 kg/s and the isentropic turbine efficiency 83.96%. For the improved optimised designs, the same characteristics were assumed, except that in the case of "pressure ratio", it was reduced to 3.5. Here follows a short overview of those measures, and the summary at the end gives the effect of all measures that could be applied simultaneously on the existing or on the related improved designs.

#### **5.1. Fogging or evaporative cooling**

Fogging is a measure well analysed in theory [22–24] and successfully used nowadays. It brings the biggest effects in climates with hot and dry ambient air. The compressor inlet air is saturated with water, which causes its temperature to drop, as the water evaporation takes the energy from it. The wet bulb temperature is the resulting temperature, causing the mass flow to rise due to the density increase. For example, for ambient air at 15°C and the relative humidity of 60%, the wet bulb temperature comes to 10.8°C. Respectively, the compression energy drops and the turbine efficiency increases.

#### **5.2. Water injection**

Water injection into the compressed air makes the same effect as the evaporative cooling. Due to water evaporation, the temperature drops. Although that evaporation energy is lost for the external heat usage, the mass flow rate and the heat capacity increase and so the turbine power releases, too.

The compressor work stays constant, and the generator gets the whole increase of the turbine output, leading to the higher power generation efficiency.

#### **5.3. Turbine inlet temperature, TIT**

An increased TIT leads to a higher inlet enthalpy and increases the specific turbine work. The robustness of materials is improving continuously with the progress in material science. Presently, some expander rotors operate with the turbine inlet temperature of 1050°C, without blade cooling. Therefore it seems just a matter of time until 1100°C will be reached, maybe with some additional measures, like blade coating. That would be another significant step forward compared to 970°C in the nowadays available gas turbine.

#### **5.4. Pressure ratio**

**Figure 9.** Decrease of stored heat due to heat losses and the drop of temperature inside the storage tower, during 5 days.

**Figure 8.** Temperature drop through the refractory of high-quality fibre modules, over 5 days.

40 Advancements in Energy Storage Technologies

It is well known that in a simple cycle, there is always an optimum pair of the pressure ratio and the inlet gas turbine temperature. However, that optimum is not the same for a recuperated gas cycle. There, the optimum pressure ratio depends on the recuperation efficiency and is much lower for the same inlet temperature. The gas turbine in consideration here is optimised for a simple cycle operation, and the selected pressure ratio of 7.27 is too high for the recuperated cycle. If it would be reduced to, e.g. 3.5, the efficiency would rise by 8.8% points.

#### **5.5. Wet compression**

A further improvement at the compressor inlet may be reached by wet compression. In that case water is sprayed above the saturation point into the inlet air. It is oversaturated with small droplets, which evaporate during the compression and therefore cool the air inside the compressor. The effects are similar to those of fogging, i.e. water injection, as the higher density and increased flow rate improve the efficiency.

As presented in several literature references [22–26], the droplet size is the most important factor for the impact of the water injection, as it defines the evaporation rate. A maximum of 3% water can be sprayed, when a pressure ratio is about 7 [23]. In case of the HiTES cycle, that is about 0.3 kg/s. The increase of electric power is about 320 kW, while the power generation efficiency climbs by 6.2% points.

#### **5.6. Intercooling**

The intercooling is an old and well-known method for improving the gas turbine cycle. The compressor is divided in two units, and between them there is a heat exchanger where the air is cooled. Due to the intercooling, the compression work is reduced in the second compressor by the higher density, as the air temperature is lower. The available shaft power for the generator increases due to the higher difference between the expander and the compressor power. In the case of a recuperative cycle, there is not a negative impact of the lower compressor outlet temperature. The electric efficiency and the generator power increase drastically.

The scenario "Future III" includes the effects of intercooling together with reduced pressure ratio. The total pressure ratio is achieved in two stages with equal pressure ratios of 1.87 but reduced again to 3.5. The compressed air cools down to 37°C by water injection between two compressor units. The power output is above 2.5 MW (+35% compared to the actual case), and

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The advantage is that all those improvements are not a special technological challenge and the robustness and simple design of the original gas turbine can be preserved. Eventual higher costs (e.g. expander blades for higher inlet temperature) will be overwhelmed by the increase

The round-trip efficiency is usually the first criterion for comparing different energy storage technologies. The concern is to keep the electricity losses induced by the storage at a minimum. However, in order to achieve the economic viability of the storage system, the investment cost has to be acceptable, too. Therefore some authors use the specific investment costs per kWh of stored electricity, and the others prefer the specific investment costs per 1 kW of the output (or input) capacity. The problem is that comparisons based on such different crite-

the round-trip efficiency rises to 54.5%.

**Figure 10.** Potential improvements of HiTES cycle efficiency [21].

in the power output, so that the specific costs will not rise.

**6. Levelized cost of electricity storage**

ria give quite different results.

#### **5.7. Summary of all improvem.5ents**

The summary of all those improvements is presented in **Figure 10** [21]. In the "uprate" scenario, all measures that may be done with the existing gas turbine are collected. As the roundtrip efficiency would rise to 44.6%, it is obvious that the effect is not negligible.

Contrary to those measures, considerable design improvements and/or optimizations are required for the scenarios "Future I", "Future II" and "Future III". In all three scenarios, TIT is raised to 1100°C, and therefore at least some improved materials are required, together with an optimised expander design. The pressure ratio is reduced to 3.5 in the "Future I" scenario, increasing the efficiency to 53.7%.

Only in the scenario "Future II" the wet compression is used. The pressure ratio is preserved at the old value of 7.3, and therefore the efficiency with 51.9% is lower than in the previous case. The advantage is that the power output jumps to above 2.8 MW (+50% compared to the actual case). At least a new design of the gear box and power generator would be needed.

High Temperature Energy Storage (HiTES) with Pebble Heater Technology and Gas Turbine http://dx.doi.org/10.5772/intechopen.75093 43

**Figure 10.** Potential improvements of HiTES cycle efficiency [21].

cycle. There, the optimum pressure ratio depends on the recuperation efficiency and is much lower for the same inlet temperature. The gas turbine in consideration here is optimised for a simple cycle operation, and the selected pressure ratio of 7.27 is too high for the recuperated

A further improvement at the compressor inlet may be reached by wet compression. In that case water is sprayed above the saturation point into the inlet air. It is oversaturated with small droplets, which evaporate during the compression and therefore cool the air inside the compressor. The effects are similar to those of fogging, i.e. water injection, as the higher den-

As presented in several literature references [22–26], the droplet size is the most important factor for the impact of the water injection, as it defines the evaporation rate. A maximum of 3% water can be sprayed, when a pressure ratio is about 7 [23]. In case of the HiTES cycle, that is about 0.3 kg/s. The increase of electric power is about 320 kW, while the power generation

The intercooling is an old and well-known method for improving the gas turbine cycle. The compressor is divided in two units, and between them there is a heat exchanger where the air is cooled. Due to the intercooling, the compression work is reduced in the second compressor by the higher density, as the air temperature is lower. The available shaft power for the generator increases due to the higher difference between the expander and the compressor power. In the case of a recuperative cycle, there is not a negative impact of the lower compressor outlet temperature. The electric efficiency and the generator power increase drastically.

The summary of all those improvements is presented in **Figure 10** [21]. In the "uprate" scenario, all measures that may be done with the existing gas turbine are collected. As the round-

Contrary to those measures, considerable design improvements and/or optimizations are required for the scenarios "Future I", "Future II" and "Future III". In all three scenarios, TIT is raised to 1100°C, and therefore at least some improved materials are required, together with an optimised expander design. The pressure ratio is reduced to 3.5 in the "Future I" scenario,

Only in the scenario "Future II" the wet compression is used. The pressure ratio is preserved at the old value of 7.3, and therefore the efficiency with 51.9% is lower than in the previous case. The advantage is that the power output jumps to above 2.8 MW (+50% compared to the actual case). At least a new design of the gear box and power generator would be needed.

trip efficiency would rise to 44.6%, it is obvious that the effect is not negligible.

cycle. If it would be reduced to, e.g. 3.5, the efficiency would rise by 8.8% points.

**5.5. Wet compression**

42 Advancements in Energy Storage Technologies

efficiency climbs by 6.2% points.

**5.7. Summary of all improvem.5ents**

increasing the efficiency to 53.7%.

**5.6. Intercooling**

sity and increased flow rate improve the efficiency.

The scenario "Future III" includes the effects of intercooling together with reduced pressure ratio. The total pressure ratio is achieved in two stages with equal pressure ratios of 1.87 but reduced again to 3.5. The compressed air cools down to 37°C by water injection between two compressor units. The power output is above 2.5 MW (+35% compared to the actual case), and the round-trip efficiency rises to 54.5%.

The advantage is that all those improvements are not a special technological challenge and the robustness and simple design of the original gas turbine can be preserved. Eventual higher costs (e.g. expander blades for higher inlet temperature) will be overwhelmed by the increase in the power output, so that the specific costs will not rise.

#### **6. Levelized cost of electricity storage**

The round-trip efficiency is usually the first criterion for comparing different energy storage technologies. The concern is to keep the electricity losses induced by the storage at a minimum. However, in order to achieve the economic viability of the storage system, the investment cost has to be acceptable, too. Therefore some authors use the specific investment costs per kWh of stored electricity, and the others prefer the specific investment costs per 1 kW of the output (or input) capacity. The problem is that comparisons based on such different criteria give quite different results.

Using the levelized cost of electricity (LCOE) is the most correct approach for that comparison. It is similar to the model used for costs of electricity from power plants. It includes all relevant parameters: capital expenditure (*CAPEX*), annual operational expenditure (*OPEX*), energy output *Wel*, interest rate *i* and the lifespan *n* in years [7]. Due to some differences compared to the electricity production costs, the LCOE has to be extended with the characteristics of energy storage systems: costs of the input electricity *σ* and the round-trip efficiency *ηel.* The resulting formula for the levelized cost of electricity storage (LCOES) is given in Eq. (3):

$$\text{LCOES} = \frac{\text{CAPEX}}{\text{W}\_{\text{el}}} \frac{\text{i} \cdot (\text{1} + \text{i})^{n}}{(\text{1} + \text{i})^{n} - 1} + \frac{\text{OPEX}\_{\text{s}}}{\text{W}\_{\text{el}}} \frac{\sigma}{\eta\_{\text{el}}} \tag{3}$$

investment cost and the efficiency, are subjected to steady improvements. Therefore, it is difficult to calculate reliable value of LCOES for the related technology and to compare them. As an example, **Figure 11** gives such a comparison, based on data from [27]. The following

High Temperature Energy Storage (HiTES) with Pebble Heater Technology and Gas Turbine

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45

For the simplicity of presentation, those figures do not include the price of the input electricity, nor the eventual cost for personnel operating the storage plant. The minimal (blue) and maximal (red) costs are presented, illustrating the wide range of different input data. Therefore, in order to point out the most important parameters, in **Figure 12** only two technologies are compared, taking into account the costs of the input electricity (i.e. the influence

• One high specific investment cost of 1600 €/kWh and with 85% round-trip; the cost may be

• The other system has a considerably lower round-trip efficiency of 40% but also lower the specific investment cost of 250 €/kWh (e.g. HiTES with 20 h discharge time); the round-trip

The change of storage cost LCOES (€/MWh) is given with presented curves, as a function of the input electricity cost (also €/MWh). The development of the solar and wind generation technologies in the last years has resulted in a tendency of steady price reduction of generated electricity. The lowest recorded prices from a photovoltaic system are 25 €/MWh in Chile and 20.7 €/MWh in Abu Dhabi, and just recently 15.3 €/MWh have been bided in Saudi Arabia [29]. Moreover, due to high penetration of intermittent renewable power generation in some energy systems, it happens more and more often that the stock market prices are negative.

• The specific investment costs are more important than the efficiency; the improvements in the investment cost are more important than the improvements in the efficiency; of course

• With the falling prices of renewable generation, the above effect becomes more and more important: the efficiency is not as important as the investment cost. (In the case of negative

of the round-trip efficiency) as well. Those two energy storage systems are compared:

efficiency may be improved to 50 and 60%, retaining the same specific cost.

parameters are taken the same for all technologies:

• Life time 25 years (if lower, related investment cost increased)

reduced to 800 €/kWh due to the further development.

Therefore, the negative input electricity prices are plotted, as well.

Two important conclusions may be drawn from the graph in **Figure 12** [28]:

cost of the input electricity, lower efficiency gives lower LCOES.)

that is limited to the values which are common for the contemporary systems.

• Power output 2 MW

• Time on power generation 4150 h/a

• Loan cost 6.5% interest rate, 10 years long

• Power generation 8300 MWh/a

The above formula is the most objective way for comparing the energy storage technologies. However, all technology details have to be known and well analysed for a correct comparison. There is a difference between the capacity which is achievable in praxis and the nominal discharge capacity of some technologies. In such cases there is a drastic reduction in the lifespan if the full discharge cycles would performed regularly. For example, it is well known that in the case of chemical batteries, there is a significant difference in the capacity at the beginning and at the end of their lifespan. Therefore, a realistic pair of capacity and lifespan must be selected in such cases.

With LCOES it is possible to analyse the whole complexity of the energy storage. In the available literature, there are data about many available storage technologies that vary for more than a factor 2 in some cases. On the other hand, some important parameters, like the specific

**Figure 11.** Comparison of LCOES for different storage technologies, based on data from [27].

investment cost and the efficiency, are subjected to steady improvements. Therefore, it is difficult to calculate reliable value of LCOES for the related technology and to compare them. As an example, **Figure 11** gives such a comparison, based on data from [27]. The following parameters are taken the same for all technologies:

• Power output 2 MW

**Figure 11.** Comparison of LCOES for different storage technologies, based on data from [27].

Using the levelized cost of electricity (LCOE) is the most correct approach for that comparison. It is similar to the model used for costs of electricity from power plants. It includes all relevant parameters: capital expenditure (*CAPEX*), annual operational expenditure (*OPEX*), energy output *Wel*, interest rate *i* and the lifespan *n* in years [7]. Due to some differences compared to the electricity production costs, the LCOE has to be extended with the characteristics of energy storage systems: costs of the input electricity *σ* and the round-trip efficiency *ηel.* The resulting formula for the levelized cost of electricity storage (LCOES) is given in Eq. (3):

> <sup>∙</sup> <sup>i</sup> <sup>∙</sup> (1 <sup>+</sup> i)n \_\_\_\_\_\_\_ (1 <sup>+</sup> i)n <sup>−</sup> <sup>1</sup> <sup>+</sup> \_\_\_\_\_

The above formula is the most objective way for comparing the energy storage technologies. However, all technology details have to be known and well analysed for a correct comparison. There is a difference between the capacity which is achievable in praxis and the nominal discharge capacity of some technologies. In such cases there is a drastic reduction in the lifespan if the full discharge cycles would performed regularly. For example, it is well known that in the case of chemical batteries, there is a significant difference in the capacity at the beginning and at the end of their lifespan. Therefore, a realistic pair of capacity and lifespan must be selected in such cases.

With LCOES it is possible to analyse the whole complexity of the energy storage. In the available literature, there are data about many available storage technologies that vary for more than a factor 2 in some cases. On the other hand, some important parameters, like the specific

OPEX Wel

+ \_\_\_<sup>σ</sup> ηel

(3)

CAPEX Wel

LCOES = \_\_\_\_\_\_

44 Advancements in Energy Storage Technologies


For the simplicity of presentation, those figures do not include the price of the input electricity, nor the eventual cost for personnel operating the storage plant. The minimal (blue) and maximal (red) costs are presented, illustrating the wide range of different input data. Therefore, in order to point out the most important parameters, in **Figure 12** only two technologies are compared, taking into account the costs of the input electricity (i.e. the influence of the round-trip efficiency) as well. Those two energy storage systems are compared:


The change of storage cost LCOES (€/MWh) is given with presented curves, as a function of the input electricity cost (also €/MWh). The development of the solar and wind generation technologies in the last years has resulted in a tendency of steady price reduction of generated electricity. The lowest recorded prices from a photovoltaic system are 25 €/MWh in Chile and 20.7 €/MWh in Abu Dhabi, and just recently 15.3 €/MWh have been bided in Saudi Arabia [29]. Moreover, due to high penetration of intermittent renewable power generation in some energy systems, it happens more and more often that the stock market prices are negative. Therefore, the negative input electricity prices are plotted, as well.

Two important conclusions may be drawn from the graph in **Figure 12** [28]:


The specific investment cost is considerably more important than the round-trip efficiency. With the steadily falling prices of the renewable generation, that effect will become more and

High Temperature Energy Storage (HiTES) with Pebble Heater Technology and Gas Turbine

http://dx.doi.org/10.5772/intechopen.75093

47

Engineering and Consulting/Energy and Environment, Sulzbach-Rosenberg, Germany

[1] Fürstenwerth D, Waldmann L. Stromspeicher in der Energiewende. Study of Agora

[2] Stevanovic D. Energiewende – German energy transition – Integration of Renewable Electricity. Industrial Energy and Environmental Protection in South Eastern Europe,

[3] Graichen P, Marthe Kleiner M, Podewils Ch. Die Energiewende im Stromsektor: Stand

[4] Loutan C. Overcoming grid intermittancy. CAISO California Independent System

[6] Stevanovic D, Brotzmann K. "Stromspeicherung über thermische Speicher und Luftturbine", Patent application DE102013017010A1. German Patent and Trade Mark Office, Munich 2013 (also: "Storing energy using a thermal storage unit and an air turbine", US

[7] Stevanovic D, Rembold T. High temperature energy storage based on hot air turbine and pebble-heater technology. 4th International Symposium on Environment Friendly

[8] Klimstra J. Energy, exergy or economy? Cogeneration & On-Site Power Production;

[9] Stevanovic D, Faßbinder H-G. Regenerative thermal oxidizers based on the pebbleheater technology. 5th European Conference on Industrial Furnaces and Boilers – INFUB

[10] Wimmer G, Pastucha K, Kluge J, Fleischanderl A, Spiess J. Jet process for highest scrap and DRI rates in converter. 1st ESTAD & 31st JSI; 7-8 April 2014; Paris, France. 2014

Energies and Applications EFEA-2016; 14-16 September 2016; Belgrade. 2016

Operator, CA, CSP Today; 26-27 June 2013; Mandalay Bay, Las Vegas. 2013

[5] SIEMENS. 2016. web-site: http://www.siemens.com/press/WindEnergy2016

more pronounced.

**Author details**

Dragan Stevanovic

**References**

Address all correspondence to: office@pebble-heater.com

Energiewende, Berlin; September 2014

IEEP-2015; 24-27 June 2015; Zlatibor, Serbia. 2015

patent application US2016/0237892A1, 2016); 2013

July–August 2014; PennWell. 2014. pp. 22-27

5; 11-14 April 2000; Porto, Portugal. 2000

der Dinge 2016. Berlin: Agora Energiewende; January 2017

**Figure 12.** Influence of specific investment, efficiency and price of input electricity on LCOES [28].

The further improvements of HiTES should go in direction indicated in the presented analysis: reduction of investment costs before the efficiency improvements. It is not allowed to pay the efficiency improvements with increased investment cost. The main potential for the reduction of the specific investment costs is an increased number of installed units.

#### **7. Concluding remarks**

Without energy storage, it is impossible to implement the generation of renewable electricity based on intermittent sources like wind and solar. Although there are many different technologies available nowadays, they are still not widely used, as they are still very expensive and not suitable for distributed power generation. One possibility with a huge potential is the HiTES technology, which is attractive because of its relative low specific investment cost, its long discharge time (10, 20 or even 30 h) and its potential to improve its efficiency easily towards 60%. Thus, its round-trip efficiency is considerably higher than the efficiency of some other long discharge systems, like power-to-gas, or molten salt. With its relatively low capacity compared to CAES and PHES, it is especially suitable for distributed generation. That shows how the previously developed technology for biomass CHP [30, 31] may be adjusted for new tasks, preserving its simplicity and improving the efficiency.

The analysis of LCOES, which is the best comparison criteria, shows that those systems are more favourable than the battery storage. Even today, that system could be used in a viable manner in countries with high insolation. In combination with photovoltaic plants, it gives lower cost of the electricity supply than concentrated solar plants (CSP).

The specific investment cost is considerably more important than the round-trip efficiency. With the steadily falling prices of the renewable generation, that effect will become more and more pronounced.

#### **Author details**

Dragan Stevanovic

Address all correspondence to: office@pebble-heater.com

Engineering and Consulting/Energy and Environment, Sulzbach-Rosenberg, Germany

#### **References**

The further improvements of HiTES should go in direction indicated in the presented analysis: reduction of investment costs before the efficiency improvements. It is not allowed to pay the efficiency improvements with increased investment cost. The main potential for the

Without energy storage, it is impossible to implement the generation of renewable electricity based on intermittent sources like wind and solar. Although there are many different technologies available nowadays, they are still not widely used, as they are still very expensive and not suitable for distributed power generation. One possibility with a huge potential is the HiTES technology, which is attractive because of its relative low specific investment cost, its long discharge time (10, 20 or even 30 h) and its potential to improve its efficiency easily towards 60%. Thus, its round-trip efficiency is considerably higher than the efficiency of some other long discharge systems, like power-to-gas, or molten salt. With its relatively low capacity compared to CAES and PHES, it is especially suitable for distributed generation. That shows how the previously developed technology for biomass CHP [30, 31] may be adjusted

The analysis of LCOES, which is the best comparison criteria, shows that those systems are more favourable than the battery storage. Even today, that system could be used in a viable manner in countries with high insolation. In combination with photovoltaic plants, it gives

reduction of the specific investment costs is an increased number of installed units.

**Figure 12.** Influence of specific investment, efficiency and price of input electricity on LCOES [28].

for new tasks, preserving its simplicity and improving the efficiency.

lower cost of the electricity supply than concentrated solar plants (CSP).

**7. Concluding remarks**

46 Advancements in Energy Storage Technologies


[11] Günther C et al. Increased scrap rate to the BOF process by application of hot air post combustion – PS-BOP project. 1st ESTAD & 31st JSI; 7-8 April 2014; Paris, France. 2014

[26] Zheng Q, Li M, Sun Y. Thermodynamic performance of wet compression and regenerative (WCR) gas turbine. In: ASME Turbo EXPO 2003, Collocated with the 2003

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[28] Stevanovic D. Energy storage for integration of renewable electricity—Case of HiTES. In: Industrial Energy and Environmental Protection in South Eastern Europe, IEEP-2017;

[29] Dipaola A. Saudi Arabia gets cheapest bids for solar power in auction. Bloomberg Markets. October 3, 2017. https://www.bloomberg.com/news/articles/2017-10-03/saudi-

[30] Stevanovic D. Innovative biomass power plant based on pebble-heater technology and

[31] Stevanovic D. Method and device for converting thermal energy from biomass into mechanical work. United States Patent Application US2012/0137701A1. June 7, 2012

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[16] Bauer R. Wärmeleitung in durchströmten Schüttungen. Chapter Mh. In: VDI-Wär-

[17] Till K. Modellierung und Berechnung turbulenter Strömungen mit chemischer Reaktion in Festbettreaktoren [Dr.-Ing. dissertation]. Merseburg, Germany: Fakultät für Tech-

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**48**:89-94


**Chapter 3**

**Provisional chapter**

**Technologies for Seasonal Solar Energy Storage in**

**Technologies for Seasonal Solar Energy Storage in** 

DOI: 10.5772/intechopen.74404

Thermochemical heat storage is a very promising technology that enables us to save the excess heat produced during summer time for the needs in the winter, when we have higher heating needs. Thermochemical heat storage bases and an overview of thermochemical materials (TCMs), suitable for the solar energy storage, are given. Choosing a suitable adsorbent and adsorbate is very important. The most important properties of the substance are high energy density for high thermal storage, low charging temperature for low energy consumption, high uptake of sorbate kg(sorbate)/kg(sorbent) and environmental safety and easy to handle‑nonpoisonous. The paper also presents the differences between the closed and the open sorption system. The biggest difference between those two systems is the importance of sorbent, which in case of open systems means that sorbent must be environmentally friendly. Also, various closed and open systems

**Keywords:** thermochemical heat storage, adsorption, solar energy, open and closed

In 2030 Energy Strategy, European Union (EU) set a goal to achieve the following 3 major

• To reduce at least 30% of greenhouse gas (GHG) emissions, compared to 1990 levels • 27% increase of the share of renewable energy sources in final energy consumption

• To achieve at least 27% increase of energy efficiency

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Buildings**

**Abstract**

are presented.

systems

**1. Introduction**

targets by 2030:

**Buildings**

Uroš Stritih and Urška Mlakar

Uroš Stritih and Urška Mlakar

http://dx.doi.org/10.5772/intechopen.74404

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

#### **Technologies for Seasonal Solar Energy Storage in Buildings Technologies for Seasonal Solar Energy Storage in Buildings**

DOI: 10.5772/intechopen.74404

#### Uroš Stritih and Urška Mlakar Uroš Stritih and Urška Mlakar

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.74404

**Abstract**

Thermochemical heat storage is a very promising technology that enables us to save the excess heat produced during summer time for the needs in the winter, when we have higher heating needs. Thermochemical heat storage bases and an overview of thermochemical materials (TCMs), suitable for the solar energy storage, are given. Choosing a suitable adsorbent and adsorbate is very important. The most important properties of the substance are high energy density for high thermal storage, low charging temperature for low energy consumption, high uptake of sorbate kg(sorbate)/kg(sorbent) and environmental safety and easy to handle‑nonpoisonous. The paper also presents the differences between the closed and the open sorption system. The biggest difference between those two systems is the importance of sorbent, which in case of open systems means that sorbent must be environmentally friendly. Also, various closed and open systems are presented.

**Keywords:** thermochemical heat storage, adsorption, solar energy, open and closed systems

#### **1. Introduction**

In 2030 Energy Strategy, European Union (EU) set a goal to achieve the following 3 major targets by 2030:


© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The key to achieve these goals is improving the building energy performance, because buildings are responsible for 36% of CO2 emissions and 40% of energy consumption in the EU [1, 2]. Since solar energy has a lot of potential, it would be smart to use more of it. Because of the mismatch between the availability of the source and the energy needs of the building, the use of heat storage technologies is needed to realize its potential.

In this paper, a review of thermochemical heat storage technologies and systems with solar energy utilization in buildings, regarding TCMs with a charging temperature below 140°C, is presented [3]. The paper is organized as follows: Section 2 sums up the fundamentals of thermochemical heat storage and contains an overview of TCMs suitable for solar energy storage. Section 3 presents possible system configurations for thermochemical heat storage and evaluates applications appropriate for reducing the energy needs of buildings. An overview of models for predicting and optimizing the performance of thermochemical storage systems

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Thermochemical heat storage is generally classified under chemical heat storage processes (**Figure 2**). Under the term thermochemical heat storage, we usually summarize sorption heat storage processes. Some authors (e.g., [4]) also mention thermochemical storage without sorption but with no exact definition of the latter. Sorption can be defined as a phenomenon of fixation of a gas by a substance in solid or liquid phase [5]. We differ between absorption and adsorption. Absorption is used when we have substance in gas phase and substance in liquid (usually) or solid phase. The substance in gas phase enters a liquid or solid and in the process changes the composition of the liquid or solid [6], while adsorption is defined as a gaseous substance that binds to the surface of a solid or porous material [4]. Further, adsorption is divided into chemical adsorption or chemisorption and physical adsorption or physisorption. The attraction between substances is caused by different forces, in physisorption by Van der Waals forces and in case of chemisorption by valence forces, which create stronger bonds. Because of stronger bonds, the chemisorption processes are able to reach higher thermal energy densities than physisorption. Chemisorption processes are also reversible, which makes them suitable for heat storage applications. In the following section, only reversible

Reversible sorption heat storage processes can be written in the following way:

AB + Q ↔ A + B (1)

is included in Section 4. Section 5 concludes the paper.

**2. Thermochemical heat storage**

sorption processes are presented.

**Figure 2.** Classification of chemical heat storage.

**2.1. Basics**

Heat storage can be achieved by different chemical or physical processes. Depending on how heat is stored, we distinguish two basic methods: sensible and latent physical heat storage. Sensible storage is achieved by the temperature change of the storage material. The amount of energy stored depends on the temperature rise and specific heat of the storage material (stored energy per unit volume or mass). Latent heat storage depends on heat interactions associated with phase change of the material (at constant temperature), usually from liquid to solid, and vice versa. Due to higher thermal energy change during phase change, compared to temperature rise of storage material in sensible heat storage, latent heat storage allows higher heat densities. Nevertheless, in practice, the use of sensible heat storage systems predominates due to better and higher thermal stability and cheaper storage materials, compared to phase change materials (PCMs).

In addition to these two storage methods, we also know thermochemical heat storage systems, which are not yet available on the market. In thermochemical heat storages are involved reversible chemical reactions. During the charging stage, heat is usually from the collector supplied to the storage material, which triggers desorption of the water vapor, which is endothermic reaction. As long as the products of endothermic reaction are separated, the supplied heat can be stored for an arbitrary time (almost) without losses. This and a several times higher stored thermal energy density (**Figure 1**) make thermochemical materials (TCMs) a promising option for mid- and long-term heat storage.

**Figure 1.** Energy density of thermal storage technologies [4].

In this paper, a review of thermochemical heat storage technologies and systems with solar energy utilization in buildings, regarding TCMs with a charging temperature below 140°C, is presented [3]. The paper is organized as follows: Section 2 sums up the fundamentals of thermochemical heat storage and contains an overview of TCMs suitable for solar energy storage. Section 3 presents possible system configurations for thermochemical heat storage and evaluates applications appropriate for reducing the energy needs of buildings. An overview of models for predicting and optimizing the performance of thermochemical storage systems is included in Section 4. Section 5 concludes the paper.

#### **2. Thermochemical heat storage**

#### **2.1. Basics**

The key to achieve these goals is improving the building energy performance, because build-

2]. Since solar energy has a lot of potential, it would be smart to use more of it. Because of the mismatch between the availability of the source and the energy needs of the building, the use

Heat storage can be achieved by different chemical or physical processes. Depending on how heat is stored, we distinguish two basic methods: sensible and latent physical heat storage. Sensible storage is achieved by the temperature change of the storage material. The amount of energy stored depends on the temperature rise and specific heat of the storage material (stored energy per unit volume or mass). Latent heat storage depends on heat interactions associated with phase change of the material (at constant temperature), usually from liquid to solid, and vice versa. Due to higher thermal energy change during phase change, compared to temperature rise of storage material in sensible heat storage, latent heat storage allows higher heat densities. Nevertheless, in practice, the use of sensible heat storage systems predominates due to better and higher thermal stability and cheaper storage materials, compared to

In addition to these two storage methods, we also know thermochemical heat storage systems, which are not yet available on the market. In thermochemical heat storages are involved reversible chemical reactions. During the charging stage, heat is usually from the collector supplied to the storage material, which triggers desorption of the water vapor, which is endothermic reaction. As long as the products of endothermic reaction are separated, the supplied heat can be stored for an arbitrary time (almost) without losses. This and a several times higher stored thermal energy density (**Figure 1**) make thermochemical materials (TCMs) a

emissions and 40% of energy consumption in the EU [1,

ings are responsible for 36% of CO2

52 Advancements in Energy Storage Technologies

phase change materials (PCMs).

of heat storage technologies is needed to realize its potential.

promising option for mid- and long-term heat storage.

**Figure 1.** Energy density of thermal storage technologies [4].

Thermochemical heat storage is generally classified under chemical heat storage processes (**Figure 2**). Under the term thermochemical heat storage, we usually summarize sorption heat storage processes. Some authors (e.g., [4]) also mention thermochemical storage without sorption but with no exact definition of the latter. Sorption can be defined as a phenomenon of fixation of a gas by a substance in solid or liquid phase [5]. We differ between absorption and adsorption. Absorption is used when we have substance in gas phase and substance in liquid (usually) or solid phase. The substance in gas phase enters a liquid or solid and in the process changes the composition of the liquid or solid [6], while adsorption is defined as a gaseous substance that binds to the surface of a solid or porous material [4]. Further, adsorption is divided into chemical adsorption or chemisorption and physical adsorption or physisorption. The attraction between substances is caused by different forces, in physisorption by Van der Waals forces and in case of chemisorption by valence forces, which create stronger bonds. Because of stronger bonds, the chemisorption processes are able to reach higher thermal energy densities than physisorption. Chemisorption processes are also reversible, which makes them suitable for heat storage applications. In the following section, only reversible sorption processes are presented.

Reversible sorption heat storage processes can be written in the following way:

$$\mathbf{A}\mathbf{B} + \mathbf{Q} \leftrightarrow \mathbf{A} + \mathbf{B} \tag{1}$$

**Figure 2.** Classification of chemical heat storage.

**Figure 3.** Thermochemical heat storage cycle has three stages: charging stage, storage stage and discharging stage.

where, AB is a compound of components A and B. When AB is split into A and B with energy input (Q)-this is called a "charging stage." Then, A and B are stored separately (storage state). At a discharge when A and B are in contact, they form AB with energy released (Q)-this is called "discharging state" (**Figure 3**). Storage materials consist of component A also called adsorbent and component B also called adsorbate. In charging stage, adsorbate is desorbed from adsorbent, and then in discharging stage, adsorbate is adsorbed on the surface of adsorbent. For adsorbate in desorbed state, term adsorptive is also used. In heat storage applications, mainly water (vapor) is used as adsorbate because of its availability (i.e., cheap) and nontoxicity.

The heat needed for desorption can be divided into three parts [5]:

$$Q = Q\_{sms} + Q\_{cond} + Q\_{bind} \tag{2}$$

storage applications. The sorbents with the highest potential in sorption storage systems are crystalline and amorphous materials and their composites with hygroscopic inorganic salt hydrates. But the abovementioned materials do not meet the requirements for large-scale applications yet [3]. Through the literature [4, 7–11], the following requirements for materials

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• High energy density for high thermal storage-thermal energy density at the operating tem-

• High uptake of sorbate kg(sorbate)/kg(sorbent)-if water is the sorbate, a high selectivity

have been exposed:

for water

perature (kWh/m3

, Wh/kg)

• Thermal and chemical stability-no deterioration • High thermal conductivity for high heat transfer

• Moderate operating pressure range

• Low cost-low price per kWh heat stored

• Low regeneration time

• Noncorrosive

• Low charging temperature for low energy consumption

**Figure 4.** Break down of the required heat for desorption for zeolite as an example [5].

• High heat and mass transfer of the working fluid to the sorbent

• Environmental safety and easy to handle-nonpoisonous

here, *Qsens* represents the sensible heat needed to heat the absorbent to the temperature needed for desorption. *Qcond* is the heat needed to vaporize the adsorbate, while *Qbind* is the heat required to overcome the sorption forces. The latter is therefore usually termed as binding heat. As seen from **Figure 4**, *Qbind* decreases with the increase of sorbate concentration. This means that more is needed at the beginning of the charging process when the sorbate concentration is low. For heat storage applications, the contribution of *Qbind* must be as low as possible compared to *Qcond* since it results in lower temperatures required for desorption [7].

#### **2.2. Reactants**

Knowledge on materials is a prerequisite to design thermal storage systems and their components. Different sorption working pairs or reactants have been studied for thermal energy

**Figure 4.** Break down of the required heat for desorption for zeolite as an example [5].

storage applications. The sorbents with the highest potential in sorption storage systems are crystalline and amorphous materials and their composites with hygroscopic inorganic salt hydrates. But the abovementioned materials do not meet the requirements for large-scale applications yet [3]. Through the literature [4, 7–11], the following requirements for materials have been exposed:


where, AB is a compound of components A and B. When AB is split into A and B with energy input (Q)-this is called a "charging stage." Then, A and B are stored separately (storage state). At a discharge when A and B are in contact, they form AB with energy released (Q)-this is called "discharging state" (**Figure 3**). Storage materials consist of component A also called adsorbent and component B also called adsorbate. In charging stage, adsorbate is desorbed from adsorbent, and then in discharging stage, adsorbate is adsorbed on the surface of adsorbent. For adsorbate in desorbed state, term adsorptive is also used. In heat storage applications, mainly water (vapor) is used as adsorbate because of its availability (i.e., cheap) and

**Figure 3.** Thermochemical heat storage cycle has three stages: charging stage, storage stage and discharging stage.

*Q* = *Qsens* + *Qcond* + *Qbind* (2)

here, *Qsens* represents the sensible heat needed to heat the absorbent to the temperature needed for desorption. *Qcond* is the heat needed to vaporize the adsorbate, while *Qbind* is the heat required to overcome the sorption forces. The latter is therefore usually termed as binding heat. As seen from **Figure 4**, *Qbind* decreases with the increase of sorbate concentration. This means that more is needed at the beginning of the charging process when the sorbate concentration is low. For heat storage applications, the contribution of *Qbind* must be as low as possible compared to *Qcond* since it results in lower temperatures required for

Knowledge on materials is a prerequisite to design thermal storage systems and their components. Different sorption working pairs or reactants have been studied for thermal energy

The heat needed for desorption can be divided into three parts [5]:

nontoxicity.

54 Advancements in Energy Storage Technologies

desorption [7].

**2.2. Reactants**


The abovementioned material requirements of sorption properties represent the foundation of selecting appropriate TCM for application. But complete evaluation of sorption material properties demands precise measurements of sorption isotherms, isobars and isosteres under a wide range of pressures and temperatures [9]. The energy storage densities and charging/discharging temperatures of some materials suitable for thermochemical heat storage are listed in **Table 1**. The most promising materials have low charging temperatures and high energy storage densities.

Because of good properties, water is the most used sorbate for seasonal solar energy storage in buildings. Water is environmentally friendly and cheap, which satisfies most conditions. Hence, hydrophilic materials such as silica gels are appropriate for the counterpart reactant or the sorbent. Silica gels have high affinity to water vapor, large water sorption capacity at low humidity, easy regeneration and low cost, but they provide low material energy densities, because of the low hydrophilic characteristic within the working window [10]. Therefore, the application prospect of silica gels in solar energy storage is obscure. Because of the strong interaction between electrostatically charged framework and the water molecules, zeolites are more hydrophilic than silica gels [12]. Because of that, their desorption or charging temperature needs to be higher, which can be altered with dealumination, ion exchange or the variation of the aluminum-silicon ratio [12–14]. Impregnated mesoporous silicates with hygroscopic salts are another option [15–19] to increase performance of the sorption reaction and enhance heat and mass transfer. Nevertheless, these composite materials have some disadvantages such as they suffer from leakage of salt species and are also corrosive due to the contained salts [20]. Some authors [3, 13, 21–23] favor microporous aluminophosphates (APO-n) and their modified analogs (SAPO‑n and MeAPO‑n) to modified zeolites, because of lower discharging temperatures and higher energy densities. The main focus in research of materials suitable for heat storage applications has been to increase the uptake of sorbate (water) with incorporating silicon or metal cations in aluminophosphates [3, 24, 25]. The performance of the latter substances usually degrades after a few charging/discharging cycles because of framework structure degradation and dislodgement of incorporated cations from the framework [13, 21, 24, 25]. However, the main limitation of aluminophosphates compared to zeolites (and silica

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As we can see in **Table 1**, mostly used sorbents in absorption heat storage studies are cal-

(NaOH) and almost all use water as sorbate. Of the listed substances, NaOH is a base while the other compounds are hygroscopic salts and are thus applied in the form of solutions. Advantages of strong bases and acids are higher water uptake and low cost, but they also

In contrast to sorption processes, chemical reactions are characterized by a change in the molecular configuration of the compound involved during the reactions. Based on the material, chemical reactions have great storage potential. But their performance in applications does not perform well, and after several initial cycles, the reaction is very difficult to continue due to swelling, deliquescence and agglomeration. Thus, the performance of material in applications needs further investigation [7]. Furthermore, because chemical reactions induce

Sorption processes are more suitable for low temperature applications such as seasonal solar energy storage, then chemical reactions, because they require lower activation energy. In the subsequent sections, only heat storage applications utilizing solid-gas adsorption processes are discussed, because liquid-gas absorption systems have limitations with corrosion and

have disadvantages like corrosiveness, need of higher charging temperature, etc.

volume modification of the solid, an obvious hysteresis may exist.

), lithium chloride (LiCl), lithium bromide (LiBr) and sodium hydroxide

gels) is their high synthesis cost [10, 12].

cium chloride (CaCl2

crystallization issues.


**Table 1.** Materials used in thermochemical heat storage studies.

Because of good properties, water is the most used sorbate for seasonal solar energy storage in buildings. Water is environmentally friendly and cheap, which satisfies most conditions. Hence, hydrophilic materials such as silica gels are appropriate for the counterpart reactant or the sorbent. Silica gels have high affinity to water vapor, large water sorption capacity at low humidity, easy regeneration and low cost, but they provide low material energy densities, because of the low hydrophilic characteristic within the working window [10]. Therefore, the application prospect of silica gels in solar energy storage is obscure. Because of the strong interaction between electrostatically charged framework and the water molecules, zeolites are more hydrophilic than silica gels [12]. Because of that, their desorption or charging temperature needs to be higher, which can be altered with dealumination, ion exchange or the variation of the aluminum-silicon ratio [12–14]. Impregnated mesoporous silicates with hygroscopic salts are another option [15–19] to increase performance of the sorption reaction and enhance heat and mass transfer. Nevertheless, these composite materials have some disadvantages such as they suffer from leakage of salt species and are also corrosive due to the contained salts [20].

The abovementioned material requirements of sorption properties represent the foundation of selecting appropriate TCM for application. But complete evaluation of sorption material properties demands precise measurements of sorption isotherms, isobars and isosteres under a wide range of pressures and temperatures [9]. The energy storage densities and charging/discharging temperatures of some materials suitable for thermochemical heat storage are listed in **Table 1**. The most promising materials have low charging temperatures and high energy storage densities.

**temperature (°C)**

O 88 32 50–125

O 160–180 20–40 97–160.5

O 180 65 130–148

O 80–120 20–30 83

O 230 154

O 95–140 40 240

O 95–140 40 —

O 95–140 40 —

O 45–138 21 120–381

O 66–87 30 253–400

O 40–90 30 252–313

O 50–95 70 154–250

O 80 — 60–321

O — 89 390

O 92 — 575

O 103 — 255

O 80–95 80–110 780

O 130–150 30–50 556–695

O 122–150 120 420–924

BaCl2 NH3 56–70 40 787 CaCl2 NH3 95–99 — 673

MnCl2 NH3 152 — 624

O 46–87 30 253

**Discharging temperature (°C)** **Energy density (kWh/m3 )**

**Phenomena Sorbent Sorbate Charging** 

silica gel H2

56 Advancements in Energy Storage Technologies

zeolite 13X H2

zeolite 4A H2

zeolite 5A H2

zeolite MSX H2

APO-n H2

SAPO-n H2

MeAPO-n H2

CaCl2 H2

LiCl H2

LiCl2 H2

LiBr H2

NaOH H2

SrBr2 H2

CaSO4 H2

CuSO4 H2

MgCl2 H2

MgSO4 H2

S H2

**Table 1.** Materials used in thermochemical heat storage studies.

SO4 H2

Li2

Na2

Adsorption

Absorption

Chem. react.

Some authors [3, 13, 21–23] favor microporous aluminophosphates (APO-n) and their modified analogs (SAPO‑n and MeAPO‑n) to modified zeolites, because of lower discharging temperatures and higher energy densities. The main focus in research of materials suitable for heat storage applications has been to increase the uptake of sorbate (water) with incorporating silicon or metal cations in aluminophosphates [3, 24, 25]. The performance of the latter substances usually degrades after a few charging/discharging cycles because of framework structure degradation and dislodgement of incorporated cations from the framework [13, 21, 24, 25]. However, the main limitation of aluminophosphates compared to zeolites (and silica gels) is their high synthesis cost [10, 12].

As we can see in **Table 1**, mostly used sorbents in absorption heat storage studies are calcium chloride (CaCl2 ), lithium chloride (LiCl), lithium bromide (LiBr) and sodium hydroxide (NaOH) and almost all use water as sorbate. Of the listed substances, NaOH is a base while the other compounds are hygroscopic salts and are thus applied in the form of solutions. Advantages of strong bases and acids are higher water uptake and low cost, but they also have disadvantages like corrosiveness, need of higher charging temperature, etc.

In contrast to sorption processes, chemical reactions are characterized by a change in the molecular configuration of the compound involved during the reactions. Based on the material, chemical reactions have great storage potential. But their performance in applications does not perform well, and after several initial cycles, the reaction is very difficult to continue due to swelling, deliquescence and agglomeration. Thus, the performance of material in applications needs further investigation [7]. Furthermore, because chemical reactions induce volume modification of the solid, an obvious hysteresis may exist.

Sorption processes are more suitable for low temperature applications such as seasonal solar energy storage, then chemical reactions, because they require lower activation energy. In the subsequent sections, only heat storage applications utilizing solid-gas adsorption processes are discussed, because liquid-gas absorption systems have limitations with corrosion and crystallization issues.

#### **3. Sorption storage systems**

According to the system design, sorption thermal storage systems can be divided into open and closed systems. The biggest difference between those two systems is the importance of sorbent, which is usually in gaseous phase and interacts with atmospheric environment, so it is not isolated, which means it is important that the sorbate is environmentally friendly. Meanwhile, closed system is isolated and we need storage for the storage material (sorbate). In contrast, the working fluid vapor is released to the environment in open systems, which means that only water (vapor) can be used as the sorbate.

by the sorbent. The air temperature increases, due to the released sorption heat. Weather conditions limit the operation of open systems. For a good discharging rate, the ambient air humidity must be sufficient. Thus, the air must be additionally humidified when the ambient moisture content is insufficient. Compared to closed system, the open system has many advantages, such as the design is simpler, which in the end means cheaper, because they do not require the use of condensers, evaporators and working fluid storage reservoirs. However, the main advantage of using the open system configuration is the better heat and mass transfer conditions, because the heat transfer fluid (air) is in direct contact with the solid reactant, while closed systems require a separate heat transfer loop and hence a heat exchanger in the reactor (heat transferred mainly by conduction). On the other side, open systems may suffer from high energy consumption for overcoming pressure losses through the reactor, which is

The main objective of this section is to review the available equipment used for sorption heat

term storage of low‑temperature heat was developed within the HYDES project [26–28]. The developed storage system consists of multiple storage units combined with solar collectors (**Figure 6**). Each storage unit includes an absorber with an integrated heat exchanger, which is connected through a valve to a combined evaporator/condenser unit. Both the high-temperature heat for desorption and the low-temperature heat for evaporation heat are provided by the solar collectors. To study the performance of the proposed design, a prototype with

heating and domestic hot water production. Two heat sink options for condensation were

tion) and water from the solar plant loop (as in evaporation) for asynchronous condensation. The experimentally determined storage density of silica gel was around 20% lower from the

The follow‑up project of HYDES was called MODESTORE [26–29]. Within this project, a second-generation storage module prototype was developed (**Figure 7**). To improve the performance, the reactor and the condenser/evaporator were combined in a single casing, thereby achieving a significantly more compact design. The reactor contains a spiral heat exchanger containing the silica gel, whereby a channel in the center is left free for vapor diffusion. In contrast to the HYDES prototype, where the evaporator was submerged at the bottom of the reservoir for the entire water for adsorption, only a small amount of water was pumped into the evaporator area at the bottom of the storage module, which significantly improved the heat transfer. The volume of laboratory-tested prototype was approximately 350 L, while the reactor contained around 200 kg of silica gel. The experimental performance of the prototype was unsatisfactory, since the energy storage density of silica gel on the prototype scale was

O as the working pair suitable for the long-

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(1.1 m3

rainwater reservoir for simultaneous condensation (with desorp-

compared to 150 kWh/m3

was installed in a low energy house to provide

).

). The discrepancy between the material and

of silica gel) connected

reflected in higher fan power thereby in higher electrical energy usage.

**3.1. Closed systems**

A closed adsorption system with silica gel-H2

to solar collectors with an area of 20.4 m2

theoretically expected value (i.e., 123 kWh/m3

below that of water sensible storage (50 kWh/m3

provided, namely, a 10 m3

two sorption storage units with a combined volume of 1.25 m3

storage, especially concerning gas-solid reactors for solar energy utilization.

Closed system generally consists of two vessels that are connected (**Figure 5**), namely, a condenser/evaporator where liquid water is collected and a reactor that consists of reactive sorbent. Desorption in the reactor and a phase change in the condenser take place in the charging process. Sorbate exits from the sorbent when heat source with high temperature (e.g., solar energy from solar collector) is supplied to the reactor. Low temperature level in a condenser causes liquefying of vapor where the condensation heat is released into the heat sink. The flow from reactor and the condenser is prevented with the valve after charging process is finished and in such a way the heat is stored. When head demand occurs, the flow from condenser/ evaporator to the reactor is established again by opening the valve. The discharging process works in a reverse direction, whereby an additional low-temperature heat source is needed for water evaporation. Closed systems allow adjusting the operating pressure of the working fluid and are able to reach higher output temperatures for heating applications compared to open systems [7, 11]. However, the regeneration of closed systems usually requires a higher temperature level. Therefore, closed systems are appropriate especially for small-scale applications.

In open systems (**Figure 5**), a dry air stream is guided into a reactor filled with sorbent during the charging process. Water adsorbed/absorbed by the sorbent is extracted by the hot air and exits the reactor bed. Hereby, the air is adiabatically cooled. During discharging, cold humid air stream enters into the (desorbed) reactor. Part of the water vapor in the air is attracted

**Figure 5.** Operation principle of sorption storage systems: (left) closed, (right) open [11].

by the sorbent. The air temperature increases, due to the released sorption heat. Weather conditions limit the operation of open systems. For a good discharging rate, the ambient air humidity must be sufficient. Thus, the air must be additionally humidified when the ambient moisture content is insufficient. Compared to closed system, the open system has many advantages, such as the design is simpler, which in the end means cheaper, because they do not require the use of condensers, evaporators and working fluid storage reservoirs. However, the main advantage of using the open system configuration is the better heat and mass transfer conditions, because the heat transfer fluid (air) is in direct contact with the solid reactant, while closed systems require a separate heat transfer loop and hence a heat exchanger in the reactor (heat transferred mainly by conduction). On the other side, open systems may suffer from high energy consumption for overcoming pressure losses through the reactor, which is reflected in higher fan power thereby in higher electrical energy usage.

The main objective of this section is to review the available equipment used for sorption heat storage, especially concerning gas-solid reactors for solar energy utilization.

#### **3.1. Closed systems**

**3. Sorption storage systems**

58 Advancements in Energy Storage Technologies

means that only water (vapor) can be used as the sorbate.

**Figure 5.** Operation principle of sorption storage systems: (left) closed, (right) open [11].

According to the system design, sorption thermal storage systems can be divided into open and closed systems. The biggest difference between those two systems is the importance of sorbent, which is usually in gaseous phase and interacts with atmospheric environment, so it is not isolated, which means it is important that the sorbate is environmentally friendly. Meanwhile, closed system is isolated and we need storage for the storage material (sorbate). In contrast, the working fluid vapor is released to the environment in open systems, which

Closed system generally consists of two vessels that are connected (**Figure 5**), namely, a condenser/evaporator where liquid water is collected and a reactor that consists of reactive sorbent. Desorption in the reactor and a phase change in the condenser take place in the charging process. Sorbate exits from the sorbent when heat source with high temperature (e.g., solar energy from solar collector) is supplied to the reactor. Low temperature level in a condenser causes liquefying of vapor where the condensation heat is released into the heat sink. The flow from reactor and the condenser is prevented with the valve after charging process is finished and in such a way the heat is stored. When head demand occurs, the flow from condenser/ evaporator to the reactor is established again by opening the valve. The discharging process works in a reverse direction, whereby an additional low-temperature heat source is needed for water evaporation. Closed systems allow adjusting the operating pressure of the working fluid and are able to reach higher output temperatures for heating applications compared to open systems [7, 11]. However, the regeneration of closed systems usually requires a higher temperature level. Therefore, closed systems are appropriate especially for small-scale applications. In open systems (**Figure 5**), a dry air stream is guided into a reactor filled with sorbent during the charging process. Water adsorbed/absorbed by the sorbent is extracted by the hot air and exits the reactor bed. Hereby, the air is adiabatically cooled. During discharging, cold humid air stream enters into the (desorbed) reactor. Part of the water vapor in the air is attracted

A closed adsorption system with silica gel-H2 O as the working pair suitable for the longterm storage of low‑temperature heat was developed within the HYDES project [26–28]. The developed storage system consists of multiple storage units combined with solar collectors (**Figure 6**). Each storage unit includes an absorber with an integrated heat exchanger, which is connected through a valve to a combined evaporator/condenser unit. Both the high-temperature heat for desorption and the low-temperature heat for evaporation heat are provided by the solar collectors. To study the performance of the proposed design, a prototype with two sorption storage units with a combined volume of 1.25 m3 (1.1 m3 of silica gel) connected to solar collectors with an area of 20.4 m2 was installed in a low energy house to provide heating and domestic hot water production. Two heat sink options for condensation were provided, namely, a 10 m3 rainwater reservoir for simultaneous condensation (with desorption) and water from the solar plant loop (as in evaporation) for asynchronous condensation. The experimentally determined storage density of silica gel was around 20% lower from the theoretically expected value (i.e., 123 kWh/m3 compared to 150 kWh/m3 ).

The follow‑up project of HYDES was called MODESTORE [26–29]. Within this project, a second-generation storage module prototype was developed (**Figure 7**). To improve the performance, the reactor and the condenser/evaporator were combined in a single casing, thereby achieving a significantly more compact design. The reactor contains a spiral heat exchanger containing the silica gel, whereby a channel in the center is left free for vapor diffusion. In contrast to the HYDES prototype, where the evaporator was submerged at the bottom of the reservoir for the entire water for adsorption, only a small amount of water was pumped into the evaporator area at the bottom of the storage module, which significantly improved the heat transfer. The volume of laboratory-tested prototype was approximately 350 L, while the reactor contained around 200 kg of silica gel. The experimental performance of the prototype was unsatisfactory, since the energy storage density of silica gel on the prototype scale was below that of water sensible storage (50 kWh/m3 ). The discrepancy between the material and

Schreiber et al. [31] developed a laboratory-scale closed heat storage unit with zeolite 13X and water as the adsorption pair suitable for cogeneration in industrial batch processes (i.e., brewery). The design of the storage unit is similar to that of the MODESTORE prototype, since the absorber and evaporator/condenser unit are integrated into a single container (**Figure 8**) without valves in between. In contrast to the MODESTORE module, a lamellae heat exchanger is used in the reactor bed, while thermal oil was used as the heat transfer fluid to allow temperatures higher than 100°C. For the evaporator/condenser heat exchanger, water was used as the heat transfer fluid. The absorber contained 20 kg of zeolite. In the experiment, the heat supply was provided via an electric heater, while the heat demand was emulated using a water reservoir, which was heated during the discharging process. Measurements were conducted with constant power of the electric heater. The temperature was 120°C during adsorption, while the charging temperature was up to 200°C (i.e., too high for solar energy storage). Three temperature profiles for evaporation/condensation were tested, namely, 60/90°C, 90/60°C and 90/90°C. The results of the study showed a strong dependence between the storage unit performance and the evaporation/condensation temperatures, whereby a low evaporation tempera-

Lu et al. [32] developed a closed adsorption cold storage system also using zeolite 13X and water as a working pair. The system has been installed in an internal combustion engine locomotive for producing chilled water for air conditioning the driver's cab. In contrast to

and condenser are separate units, since the evaporator is used to absorb heat from the cabin (**Figure 9**). During charging, the adsorbent bed is heated by the locomotive's internal combustion engine exhaust gasses, while ambient air is used to cool the condenser and the adsorber

O heat storage unit [31].

O working pair [31], the evaporator

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ture proved to be crucial.

**Figure 8.** Scheme of the closed zeolite-H2

the previously mentioned closed system with zeolite-H2

**Figure 6.** HYDES seasonal storage system concept [27].

**Figure 7.** Scheme of the MODESTORE storage module prototype [29].

system scale energy densities was due to the adsorption conditions required to guarantee desorption under the temperature provided by the solar collectors and the heat sinks (i.e., silica gel water content between 3 and 13%). The authors therefore declared the working pair silica gel-H2 O as unsuitable for solar energy storage systems for building applications. Stritih and Bombač [30] came to the same conclusion with a similarly designed closed storage unit prototype with silica gel-H2 O working pair, but on a smaller scale (5.7 kg of silica gel).

Schreiber et al. [31] developed a laboratory-scale closed heat storage unit with zeolite 13X and water as the adsorption pair suitable for cogeneration in industrial batch processes (i.e., brewery). The design of the storage unit is similar to that of the MODESTORE prototype, since the absorber and evaporator/condenser unit are integrated into a single container (**Figure 8**) without valves in between. In contrast to the MODESTORE module, a lamellae heat exchanger is used in the reactor bed, while thermal oil was used as the heat transfer fluid to allow temperatures higher than 100°C. For the evaporator/condenser heat exchanger, water was used as the heat transfer fluid. The absorber contained 20 kg of zeolite. In the experiment, the heat supply was provided via an electric heater, while the heat demand was emulated using a water reservoir, which was heated during the discharging process. Measurements were conducted with constant power of the electric heater. The temperature was 120°C during adsorption, while the charging temperature was up to 200°C (i.e., too high for solar energy storage). Three temperature profiles for evaporation/condensation were tested, namely, 60/90°C, 90/60°C and 90/90°C. The results of the study showed a strong dependence between the storage unit performance and the evaporation/condensation temperatures, whereby a low evaporation temperature proved to be crucial.

Lu et al. [32] developed a closed adsorption cold storage system also using zeolite 13X and water as a working pair. The system has been installed in an internal combustion engine locomotive for producing chilled water for air conditioning the driver's cab. In contrast to the previously mentioned closed system with zeolite-H2 O working pair [31], the evaporator and condenser are separate units, since the evaporator is used to absorb heat from the cabin (**Figure 9**). During charging, the adsorbent bed is heated by the locomotive's internal combustion engine exhaust gasses, while ambient air is used to cool the condenser and the adsorber

**Figure 8.** Scheme of the closed zeolite-H2 O heat storage unit [31].

system scale energy densities was due to the adsorption conditions required to guarantee desorption under the temperature provided by the solar collectors and the heat sinks (i.e., silica gel water content between 3 and 13%). The authors therefore declared the working pair

and Bombač [30] came to the same conclusion with a similarly designed closed storage unit

O as unsuitable for solar energy storage systems for building applications. Stritih

O working pair, but on a smaller scale (5.7 kg of silica gel).

silica gel-H2

prototype with silica gel-H2

**Figure 7.** Scheme of the MODESTORE storage module prototype [29].

**Figure 6.** HYDES seasonal storage system concept [27].

60 Advancements in Energy Storage Technologies

during discharging. The prototype system was filled with 140 kg of zeolite grains and 185 kg of water. The system reached an average cooling power of 4.1 kW, while a maximum storage capacity of 5.5 kWh was obtained at an adsorption bed temperature of 125°C. Since the expected maximal storage capacity was 23.3 kWh, the authors concluded that heat and mass transfer of the adsorber need to be improved for better performance.

Lass-Seyoum et al. [33, 34] developed a large-scale adsorption storage system (volume 750 L) with water and an unspecified porous material as the adsorption pair. The storage systems consist of two subsystems connected by a valve, i.e., the storage reactor and the evaporator/condenser unit (**Figure 10**). The reactor contains a copper matrix heat exchanger, which enables to reach a relatively uniform temperature distribution in the reactor (**Figure 10**). Solar thermal heat pipes with a capacity of 4 kW served as the high temperature source. Several dynamic performance tests were carried out. The maximum charging temperature varied between 100 and 120°C, while the maximum discharging temperatures lied in the interval 65–70°C. The daily average heat output ranged between 2.5 and 3 kW. The achieved material energy storage density was 30–40% lower than the values expected from laboratory tests due to the significantly lower charging temperatures (laboratory 220–250°C). For this reason, the authors intend to use a storage material more suitable for the available charging temperature range in the developed storage system.

> TNO [35–37] developed and built modular seasonal storage system, whereby zeolite 5A and water were chosen as the reactants. The system basically consists of two separate cylindrical vessels, i.e., heat storage (reactor) and evaporator/condenser (single) unit, connected to a high‑ and low‑temperature heat source. The reactor is built of parallel arranged and finned heat exchangers packed with zeolite (**Figure 11**), placed in a stainless steel vessel. The evaporator/condenser unit consists of a combination of a copper fin connected on one side to a copper spiral and a capillary working material on the other side with a heat exchange area of

> **Figure 10.** 750 L closed adsorption heat storage prototype: (left) scheme [33]; (right) temperature distribution inside the

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 (**Figure 11**). A prototype filled with 41 kg of zeolite corresponding to a storage capacity of 3 kWh was constructed, consisting of one reactor and one evaporator/condenser unit. A 12 kW electrical heater served as the high temperature source, while thermostatic water bats were used as a low-temperature heat source (evaporator) and/or heat sink (condensation, discharging). Although the test results were in accordance with expectations, i.e., maximal heat storage 4 kWh and heat output between 0.7 and 1.6 kW, the system energy density

expect that an approximately 60% performance increase can be achieved by optimizing the

Within the project MONOSORP [38, 39], an open adsorption heat storage with zeolite 4A and water as adsorption pair was developed. The storage system was designed for the inclusion in a building ventilation system with heat recovery. During discharging, the exhaust air flow is blown through the sorption material, which leads to a rise in temperature and dehumidification. The leaving air stream is then guided into an air to air heat exchanger (**Figure 12**) where it releases heat to the fresh air stream. Because the air that exits the sorption material is not appropriate for direct ventilation of the building, the heat recovery with fresh air through the

). The authors

was around 73% lower than the material energy density (22 versus 83 kWh/m3

1.4 m2

reactor [34].

system.

**3.2. Open systems**

**Figure 9.** Closed adsorption cold storage system [32].

**Figure 10.** 750 L closed adsorption heat storage prototype: (left) scheme [33]; (right) temperature distribution inside the reactor [34].

TNO [35–37] developed and built modular seasonal storage system, whereby zeolite 5A and water were chosen as the reactants. The system basically consists of two separate cylindrical vessels, i.e., heat storage (reactor) and evaporator/condenser (single) unit, connected to a high‑ and low‑temperature heat source. The reactor is built of parallel arranged and finned heat exchangers packed with zeolite (**Figure 11**), placed in a stainless steel vessel. The evaporator/condenser unit consists of a combination of a copper fin connected on one side to a copper spiral and a capillary working material on the other side with a heat exchange area of 1.4 m2 (**Figure 11**). A prototype filled with 41 kg of zeolite corresponding to a storage capacity of 3 kWh was constructed, consisting of one reactor and one evaporator/condenser unit. A 12 kW electrical heater served as the high temperature source, while thermostatic water bats were used as a low-temperature heat source (evaporator) and/or heat sink (condensation, discharging). Although the test results were in accordance with expectations, i.e., maximal heat storage 4 kWh and heat output between 0.7 and 1.6 kW, the system energy density was around 73% lower than the material energy density (22 versus 83 kWh/m3 ). The authors expect that an approximately 60% performance increase can be achieved by optimizing the system.

#### **3.2. Open systems**

during discharging. The prototype system was filled with 140 kg of zeolite grains and 185 kg of water. The system reached an average cooling power of 4.1 kW, while a maximum storage capacity of 5.5 kWh was obtained at an adsorption bed temperature of 125°C. Since the expected maximal storage capacity was 23.3 kWh, the authors concluded that heat and mass

Lass-Seyoum et al. [33, 34] developed a large-scale adsorption storage system (volume 750 L) with water and an unspecified porous material as the adsorption pair. The storage systems consist of two subsystems connected by a valve, i.e., the storage reactor and the evaporator/condenser unit (**Figure 10**). The reactor contains a copper matrix heat exchanger, which enables to reach a relatively uniform temperature distribution in the reactor (**Figure 10**). Solar thermal heat pipes with a capacity of 4 kW served as the high temperature source. Several dynamic performance tests were carried out. The maximum charging temperature varied between 100 and 120°C, while the maximum discharging temperatures lied in the interval 65–70°C. The daily average heat output ranged between 2.5 and 3 kW. The achieved material energy storage density was 30–40% lower than the values expected from laboratory tests due to the significantly lower charging temperatures (laboratory 220–250°C). For this reason, the authors intend to use a storage material more suitable for the available charging temperature

transfer of the adsorber need to be improved for better performance.

range in the developed storage system.

62 Advancements in Energy Storage Technologies

**Figure 9.** Closed adsorption cold storage system [32].

Within the project MONOSORP [38, 39], an open adsorption heat storage with zeolite 4A and water as adsorption pair was developed. The storage system was designed for the inclusion in a building ventilation system with heat recovery. During discharging, the exhaust air flow is blown through the sorption material, which leads to a rise in temperature and dehumidification. The leaving air stream is then guided into an air to air heat exchanger (**Figure 12**) where it releases heat to the fresh air stream. Because the air that exits the sorption material is not appropriate for direct ventilation of the building, the heat recovery with fresh air through the

**Figure 11.** TNO adsorption storage module: (left) adsorber/desorber unit; (right) evaporator/condenser unit [37].

heat exchanger is taking place. To achieve a good heat transfer between air flow and sorption material, honeycomb monoliths with a numerous straight, small channels (large contact area) were developed and made by extrusion of zeolite 4A powder (**Figure 12**). Apart from the good heat transfer, the main advantage of these structures is the low pressure loss. The only required component aside from the monolith container is a water to air heat exchanger (**Figure 12**), which is connected to solar collectors (i.e., high-temperature heat source). Since a desorption temperature above 160°C was required, only evacuated tube collectors were suitable. The laboratory prototype (**Figure 12**) with storage volume of 100 L (62 kg of zeolite) was built and connected to 4.4 m2 of collectors via a finned tube heat exchanger. For air to air heat recovery, a plate heat exchanger was used. Both heat exchangers were commercially available, not specifically designed for adsorption heat storage. The prototype preformed satisfactory with an energy density of 130 kWh/m3 (without sensible heat) and a heat output between 1 and 1.5 kW, whereby a maximal temperature lift of 22°C was achieved. The charging rate on the other side ranged from 2 to 2.5 kW. The discharging rate could be increased with additional humidification of the exhaust air. Apart from this, the major drawbacks of the proposed systems are the high desorption temperature and the high material and production costs.

ratio. The built storage unit has a total volume of 8 m3

**Figure 12.** MONOSORP prototype: (left) laboratory setup; (right) zeolite monolith [39].

heat transfer and no dead volume of sorption material.

and is filled with 4.3 m<sup>3</sup>

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(diameter 2 mm) with a heat storage capacity of around 700 kWh. The unit is divided into four quadrants according to the top view and each quadrant itself is subdivided into six segments as is shown in cross-section on right side of **Figure 13**. Each of the two stacked segments form a pair (12 pairs total). The selection of the segment pairs is realized through the opening and closure of apertures in the outlet ducts by a slider mechanism, which is the only moving part within the reactor. Airflow inlet is at the top of the inlet duct, which is placed at the center of the unit. The airflow exits the inlet duct through one of the four ducts on the vertical edges. Airflow can pass through each segment pair individually so the segment pairs can be adsorbed and desorbed separately. On the right side of **Figure 13**, the airflow through the unit is shown for a middle segment pair, which is colored blue. On an exemplary (discharging) operation day, the discharging rate varied between 565 and 790 W, while a temperature lift between 22 and 28°C was achieved. The experiments also showed that the flow through two segments of a segment pair is quite homogeneous, meaning that the charging and discharging process takes place simultaneously in both segments. The authors plan to further monitor the developed system also during the discharging period. Same as for the MONOSORP prototype, the heat output could be increased with humidifying the exhaust air stream before it enters the storage unit. Dividing the unit into segments gives lower pressure drop of air through the unit, greater

**Figure 13.** SolSpaces project: (left) heating system scheme; (right) vertical cut through the storage systems [40].

of zeolite grains

The follow-up project of MONOSORP is called SolSpaces [40, 41]. Within SolSpaces, a new solar heating system, including adsorption storage for seasonal energy storage with binderless zeolite 13X as adsorbent, has been developed. The system concept is similar to the MONOSORP project with the difference that air solar collectors were used (**Figure 13**), therewith eliminating the need for a water to air heat exchanger. The projects also differ in the reactor design. In contrast to MONOSORP, a packed bed of zeolite spheres is used instead of honeycomb monoliths. The storage has been further subdivided into smaller segments (**Figure 13**) to improve the thermal performance (i.e., reduced heat capacities and heat losses). The developed system has been built up in full scale in a research building with vacuum tube air collectors for testing and demonstration. Although the building has an area of only 43 m2 , it is comparable to larger buildings, since it has a relatively high specific heat demand due to a large surface‑to‑volume

**Figure 12.** MONOSORP prototype: (left) laboratory setup; (right) zeolite monolith [39].

heat exchanger is taking place. To achieve a good heat transfer between air flow and sorption material, honeycomb monoliths with a numerous straight, small channels (large contact area) were developed and made by extrusion of zeolite 4A powder (**Figure 12**). Apart from the good heat transfer, the main advantage of these structures is the low pressure loss. The only required component aside from the monolith container is a water to air heat exchanger (**Figure 12**), which is connected to solar collectors (i.e., high-temperature heat source). Since a desorption temperature above 160°C was required, only evacuated tube collectors were suitable. The laboratory prototype (**Figure 12**) with storage volume of 100 L (62 kg of zeolite) was

**Figure 11.** TNO adsorption storage module: (left) adsorber/desorber unit; (right) evaporator/condenser unit [37].

recovery, a plate heat exchanger was used. Both heat exchangers were commercially available, not specifically designed for adsorption heat storage. The prototype preformed satisfac-

1 and 1.5 kW, whereby a maximal temperature lift of 22°C was achieved. The charging rate on the other side ranged from 2 to 2.5 kW. The discharging rate could be increased with additional humidification of the exhaust air. Apart from this, the major drawbacks of the proposed systems are the high desorption temperature and the high material and production costs.

The follow-up project of MONOSORP is called SolSpaces [40, 41]. Within SolSpaces, a new solar heating system, including adsorption storage for seasonal energy storage with binderless zeolite 13X as adsorbent, has been developed. The system concept is similar to the MONOSORP project with the difference that air solar collectors were used (**Figure 13**), therewith eliminating the need for a water to air heat exchanger. The projects also differ in the reactor design. In contrast to MONOSORP, a packed bed of zeolite spheres is used instead of honeycomb monoliths. The storage has been further subdivided into smaller segments (**Figure 13**) to improve the thermal performance (i.e., reduced heat capacities and heat losses). The developed system has been built up in full scale in a research building with vacuum tube air collectors for testing

buildings, since it has a relatively high specific heat demand due to a large surface‑to‑volume

and demonstration. Although the building has an area of only 43 m2

of collectors via a finned tube heat exchanger. For air to air heat

(without sensible heat) and a heat output between

, it is comparable to larger

built and connected to 4.4 m2

64 Advancements in Energy Storage Technologies

tory with an energy density of 130 kWh/m3

**Figure 13.** SolSpaces project: (left) heating system scheme; (right) vertical cut through the storage systems [40].

ratio. The built storage unit has a total volume of 8 m3 and is filled with 4.3 m<sup>3</sup> of zeolite grains (diameter 2 mm) with a heat storage capacity of around 700 kWh. The unit is divided into four quadrants according to the top view and each quadrant itself is subdivided into six segments as is shown in cross-section on right side of **Figure 13**. Each of the two stacked segments form a pair (12 pairs total). The selection of the segment pairs is realized through the opening and closure of apertures in the outlet ducts by a slider mechanism, which is the only moving part within the reactor. Airflow inlet is at the top of the inlet duct, which is placed at the center of the unit. The airflow exits the inlet duct through one of the four ducts on the vertical edges. Airflow can pass through each segment pair individually so the segment pairs can be adsorbed and desorbed separately. On the right side of **Figure 13**, the airflow through the unit is shown for a middle segment pair, which is colored blue. On an exemplary (discharging) operation day, the discharging rate varied between 565 and 790 W, while a temperature lift between 22 and 28°C was achieved. The experiments also showed that the flow through two segments of a segment pair is quite homogeneous, meaning that the charging and discharging process takes place simultaneously in both segments. The authors plan to further monitor the developed system also during the discharging period. Same as for the MONOSORP prototype, the heat output could be increased with humidifying the exhaust air stream before it enters the storage unit. Dividing the unit into segments gives lower pressure drop of air through the unit, greater heat transfer and no dead volume of sorption material.

Within the CWS [42–46] project, a seasonal solar storage system for a composite material of zeolite and salt was developed. The system was designed for integration in a solar combisystem, i.e., solar collectors alternatively heat the combined storage tank or supply the heat required for desorbing the storage material. The main difference from the previously mentioned projects lies in the design of the sorption storage unit. In contrast to the MONOSORP and SolSpaces units, where sorption reactions occur within the storage vessel, the CWS storage unit consists of an external reactor in which the adsorption/desorption takes place and a separate storage vessel for hydrated and dehydrated storage material as well as a material transport system (**Figure 14**). The reactor consists of two chambers: one chamber to load the material and one chamber to regenerate it. Both chambers are separated by an air to HTF (e.g., water, oil) heat exchanger (**Figure 14**). During the reaction, the material is filled into the reactor from the top and emptied through the outlet at the bottom, driven only by gravity. The air enters the reactor from the side. The laboratory prototype reactor with a storage volume of 20 L was built with the flow cross‑section area of 0.25 m<sup>2</sup> and length of 80 mm. A thermostat connected to an air to oil heat exchanger in the reactor was used alternately as heat source or heat sink. The experiment was carried out with zeolite 13X as the adsorbent. Although the prototype suffered from significant heat losses (37% of released heat) due to the uninsulated reactor, the prototype achieved a satisfactory heat output of 750 W and a 30°C temperature lift.

system of the school. The storage is charged by the district heating system during off‑peak periods (desorption temperatures between 130 and 180°C), while during peak hours, the building heating system can be powered only by the energy stored in the zeolite, thereby reducing the peak power demand of the district heating system. During monitoring, the system reached a

use the storage system as a desiccant cooling device for the mentioned jazz club, the system was additionally upgraded with a heat recovery device (exhaust air) to cool the dried supply air exiting the zeolite modules and a supply air humidifier to adiabatically cool the air afterward. The heat recovery device consisted of an exhaust air humidifier with an integrated heat exchanger and the supply air heat exchanger, which were connected by a fluid circuit. Three desorption temperatures were tested, namely 130, 100 and 80°C. The corresponding achieved energy densi-

ties were achieved at higher desorption temperatures, the best overall system performance (COP) was achieved at the lowest tested desorption temperature (i.e., 0.87). The rough economic analysis showed that the payback time of the installed adsorption storage system was estimated to be 7–8 years and is dependent on the price reduction for the off‑peak thermal energy, the investment costs and the number of storage cycles. The authors concluded that the system performance could further be enhanced with an improvement and simplification of the operation control strategies. Zettl et al. developed a revolving drum reactor for open adsorption heat storage systems [48, 49]. The reactor was designed as a slowly rotating cylindrical drum to enable a steady mixing of the storage material in granular form in order to reach homogeneous temperatures and to avoid overhydration of the storage material. Air is supplied/extracted from the reactor through pipe-in-pipe air inlet/outlet construction, whereby the supply air is blown through the outer part while the extracted air leaves the reactor through the central pipe (**Figure 16**). The main advantage of this design is that in contrast to fixed bed reactors no reaction front is formed, since adsorption takes place throughout the whole storage material volume due to bed rotation. The laboratory prototype reactor with a maximum design heat output of 1.5 kW was built (**Figure 16**). It was filled with 70 L of a granular storage material, i.e., alternately zeolite 4A (53 kg) and zeolite MSX (50 kg), covering a volume fraction of about 80% of the interior.

. In order to

67

. Although higher energy densi-

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19% lower energy density than the theoretical maximum value, namely 124 kWh/m3

ties (in the same order) are as follows: 168, 105 and 100 kWh/m3

**Figure 15.** Large-scale open adsorption storage system connected to district heating system [47].

ZAE Bayern designed and installed an adsorption storage system with zeolite 13X as adsorbent for providing space heating and cooling to a school and jazz club connected to district heating system [27, 47]. The built storage system contains 7000 kg of zeolite and consists of three cylindrical storage modules connected in series: a humidifier, water reservoir (for humidification) and a control unit (**Figure 15**). The storage units are connected to the district heating system via a heat exchanger on the supply side and a combined air/radiator/floor heating system on the demand side. The storage system is used as a buffer between the district heating system and space heating

**Figure 14.** Separate adsorption storage: (left) unit concept; (right) reactor design [45].

system of the school. The storage is charged by the district heating system during off‑peak periods (desorption temperatures between 130 and 180°C), while during peak hours, the building heating system can be powered only by the energy stored in the zeolite, thereby reducing the peak power demand of the district heating system. During monitoring, the system reached a 19% lower energy density than the theoretical maximum value, namely 124 kWh/m3 . In order to use the storage system as a desiccant cooling device for the mentioned jazz club, the system was additionally upgraded with a heat recovery device (exhaust air) to cool the dried supply air exiting the zeolite modules and a supply air humidifier to adiabatically cool the air afterward. The heat recovery device consisted of an exhaust air humidifier with an integrated heat exchanger and the supply air heat exchanger, which were connected by a fluid circuit. Three desorption temperatures were tested, namely 130, 100 and 80°C. The corresponding achieved energy densities (in the same order) are as follows: 168, 105 and 100 kWh/m3 . Although higher energy densities were achieved at higher desorption temperatures, the best overall system performance (COP) was achieved at the lowest tested desorption temperature (i.e., 0.87). The rough economic analysis showed that the payback time of the installed adsorption storage system was estimated to be 7–8 years and is dependent on the price reduction for the off‑peak thermal energy, the investment costs and the number of storage cycles. The authors concluded that the system performance could further be enhanced with an improvement and simplification of the operation control strategies.

Within the CWS [42–46] project, a seasonal solar storage system for a composite material of zeolite and salt was developed. The system was designed for integration in a solar combisystem, i.e., solar collectors alternatively heat the combined storage tank or supply the heat required for desorbing the storage material. The main difference from the previously mentioned projects lies in the design of the sorption storage unit. In contrast to the MONOSORP and SolSpaces units, where sorption reactions occur within the storage vessel, the CWS storage unit consists of an external reactor in which the adsorption/desorption takes place and a separate storage vessel for hydrated and dehydrated storage material as well as a material transport system (**Figure 14**). The reactor consists of two chambers: one chamber to load the material and one chamber to regenerate it. Both chambers are separated by an air to HTF (e.g., water, oil) heat exchanger (**Figure 14**). During the reaction, the material is filled into the reactor from the top and emptied through the outlet at the bottom, driven only by gravity. The air enters the reactor from the side. The laboratory prototype reactor with a storage volume of

connected to an air to oil heat exchanger in the reactor was used alternately as heat source or heat sink. The experiment was carried out with zeolite 13X as the adsorbent. Although the prototype suffered from significant heat losses (37% of released heat) due to the uninsulated reactor, the prototype achieved a satisfactory heat output of 750 W and a 30°C temperature lift. ZAE Bayern designed and installed an adsorption storage system with zeolite 13X as adsorbent for providing space heating and cooling to a school and jazz club connected to district heating system [27, 47]. The built storage system contains 7000 kg of zeolite and consists of three cylindrical storage modules connected in series: a humidifier, water reservoir (for humidification) and a control unit (**Figure 15**). The storage units are connected to the district heating system via a heat exchanger on the supply side and a combined air/radiator/floor heating system on the demand side. The storage system is used as a buffer between the district heating system and space heating

and length of 80 mm. A thermostat

20 L was built with the flow cross‑section area of 0.25 m<sup>2</sup>

66 Advancements in Energy Storage Technologies

**Figure 14.** Separate adsorption storage: (left) unit concept; (right) reactor design [45].

Zettl et al. developed a revolving drum reactor for open adsorption heat storage systems [48, 49]. The reactor was designed as a slowly rotating cylindrical drum to enable a steady mixing of the storage material in granular form in order to reach homogeneous temperatures and to avoid overhydration of the storage material. Air is supplied/extracted from the reactor through pipe-in-pipe air inlet/outlet construction, whereby the supply air is blown through the outer part while the extracted air leaves the reactor through the central pipe (**Figure 16**). The main advantage of this design is that in contrast to fixed bed reactors no reaction front is formed, since adsorption takes place throughout the whole storage material volume due to bed rotation. The laboratory prototype reactor with a maximum design heat output of 1.5 kW was built (**Figure 16**). It was filled with 70 L of a granular storage material, i.e., alternately zeolite 4A (53 kg) and zeolite MSX (50 kg), covering a volume fraction of about 80% of the interior.

**Figure 15.** Large-scale open adsorption storage system connected to district heating system [47].

to achieve energy savings of 80%. But for realistic evaluation, other aspects of investigation

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Johannes et al. [50] designed a high-powered energy-dense zeolite heat storage system with the intention to shave the electricity peak loads in a house by reducing the heating part of the demand. A packed bed reactor system was built containing 80 kg of zeolite 13X, which was split into two equal‑sized reactors, in order to test serial and parallel configurations of reactors. The whole system consists of two reactors and ducts to drive the airflow into the reactors (**Figure 17**). An air treatment system was used to prepare (i.e., heat and/or humidify) the airflow during the experiments. Several tests have been carried out both during discharging and charging at various flow rates, relative humidity and temperatures of hydration. The experiments revealed that serial configuration of reactors is unsuitable because the thermal mass of the second reactor draws heat from the first one in serial configuration, which results in a unstable maximum heat output during discharging. For parallel configuration, the results show that the reactor is able to supply a constant power of 2.25 kW for more than 2 hours, while the COP varied between 1.7 and 6.8 depending on the air temperature during charging and air humidity during discharging. For the next step, the authors plan to validate a numerical model of the system to further optimize the developed storage system as well as to

Within the FlowTCS project [51], an open adsorption storage system with an external reactor configuration has been developed, whereby zeolite and salt‑impregnated zeolite were used as the sorbent. The storage system consists of a reactor with approximately 30 L of zeolite and the adsorbent storage reservoir with a volume of 200 L, thereby achieving a high flexibility regarding both storage capacity and heat output. The reactor is designed as a quasicontinuous cross‑flow reactor, i.e., the adsorbent flows down through the reactor led by gravity and controlled by a rotary valve through which it is discharged out of the reactor (**Figure 18**).

needs to take place like efficient desorption and building integration.

numerically asses the performance when coupled to a building.

**Figure 18.** External reactor concept during discharging [51].

**Figure 16.** Revolving drum reactor: (left) laboratory setup; (right) cross-section sketch [49].

Special care was taken in the design of the reactor interior to avoid abrasion of the storage material (e.g., no sharp edges). Only adsorption was monitored in the reactor, desorption took place in a conventional drying oven prior to the adsorption tests. During the adsorption tests, the inlet air was adiabatically humidified. Both materials reached a comparable maximum temperature lifts of 36°C. The test with zeolite 4A generated 10.5 kWh of heat and 11.9 kWh was reached with zeolite MSX, which correspond to a stored energy density of 148 kWh/m3 for zeolite 4A and 154 kWh/m3 for zeolite MSX. The average heat output during the tests was 1.2 kW, while the combined electric energy use for drum rotation and fan operation was around 100 W. The authors plan to upscale the developed reactor to a realistic size, with which airflow velocity, pressure drop of airflow and specific fan power will be reduced. Special care will be taken on the containment material to guarantee loss-free storage. With upscaling of the storage unit, 16 m3 storage volume filled with zeolite and material dehydration at 180°C would be required

**Figure 17.** Experimental setup of an open adsorption storage system with reactors in parallel configuration [50].

to achieve energy savings of 80%. But for realistic evaluation, other aspects of investigation needs to take place like efficient desorption and building integration.

Johannes et al. [50] designed a high-powered energy-dense zeolite heat storage system with the intention to shave the electricity peak loads in a house by reducing the heating part of the demand. A packed bed reactor system was built containing 80 kg of zeolite 13X, which was split into two equal‑sized reactors, in order to test serial and parallel configurations of reactors. The whole system consists of two reactors and ducts to drive the airflow into the reactors (**Figure 17**). An air treatment system was used to prepare (i.e., heat and/or humidify) the airflow during the experiments. Several tests have been carried out both during discharging and charging at various flow rates, relative humidity and temperatures of hydration. The experiments revealed that serial configuration of reactors is unsuitable because the thermal mass of the second reactor draws heat from the first one in serial configuration, which results in a unstable maximum heat output during discharging. For parallel configuration, the results show that the reactor is able to supply a constant power of 2.25 kW for more than 2 hours, while the COP varied between 1.7 and 6.8 depending on the air temperature during charging and air humidity during discharging. For the next step, the authors plan to validate a numerical model of the system to further optimize the developed storage system as well as to numerically asses the performance when coupled to a building.

Within the FlowTCS project [51], an open adsorption storage system with an external reactor configuration has been developed, whereby zeolite and salt‑impregnated zeolite were used as the sorbent. The storage system consists of a reactor with approximately 30 L of zeolite and the adsorbent storage reservoir with a volume of 200 L, thereby achieving a high flexibility regarding both storage capacity and heat output. The reactor is designed as a quasicontinuous cross‑flow reactor, i.e., the adsorbent flows down through the reactor led by gravity and controlled by a rotary valve through which it is discharged out of the reactor (**Figure 18**).

**Figure 18.** External reactor concept during discharging [51].

**Figure 17.** Experimental setup of an open adsorption storage system with reactors in parallel configuration [50].

Special care was taken in the design of the reactor interior to avoid abrasion of the storage material (e.g., no sharp edges). Only adsorption was monitored in the reactor, desorption took place in a conventional drying oven prior to the adsorption tests. During the adsorption tests, the inlet air was adiabatically humidified. Both materials reached a comparable maximum temperature lifts of 36°C. The test with zeolite 4A generated 10.5 kWh of heat and 11.9 kWh was

the combined electric energy use for drum rotation and fan operation was around 100 W. The authors plan to upscale the developed reactor to a realistic size, with which airflow velocity, pressure drop of airflow and specific fan power will be reduced. Special care will be taken on the containment material to guarantee loss-free storage. With upscaling of the storage unit,

storage volume filled with zeolite and material dehydration at 180°C would be required

for zeolite MSX. The average heat output during the tests was 1.2 kW, while

for zeolite

reached with zeolite MSX, which correspond to a stored energy density of 148 kWh/m3

**Figure 16.** Revolving drum reactor: (left) laboratory setup; (right) cross-section sketch [49].

4A and 154 kWh/m3

68 Advancements in Energy Storage Technologies

16 m3

The heat released during discharging is transported to the heating system/buffer store via an air to water heat exchanger. During charging, the air is heated up in the air to water heat exchanger. For heat recovery, an air to air heat exchanger was additionally integrated into the reactor unit. The storage system was experimentally tested by varying the air humidity and the heating demand. The system performance was in compliance with the theoretically expected thermal power and temperature lift based on the heat storage density of the adsorbent. The developers further plan to increase the system compactness, reduce heat losses and lower the charging temperature and also to test the concept in an in situ setup.

Since seasonal storage requires a steady and continuous heat output when discharging, the reactor bed must be optimized in such a way that it guarantees a constant flow rate to enable stable heat output during discharging. In order to achieve this without sacrificing the heat storage unit, compactness numerical modeling techniques have to be employed in the reactor design phase. The review of literature indicates that in the majority of solid-gas adsorption processes modeling efforts, the Dubinin‑Polanyi theory is applied for describing adsorption equilibria, while Darcy's law and LDF model are used to predict the pressure gradient inside

Technologies for Seasonal Solar Energy Storage in Buildings

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71

To conclude, the storage materials represent a bottleneck for the development of thermochemical heat storage systems. Therefore, in order to achieve the commercial breakthrough of thermochemical heat storage systems, a bottom‑up approach of storage material engineering is needed, i.e., during material development, the required material characteristics have to be redefined according to the dynamics of the thermochemical process in the particular storage design, thereby obtaining a better understanding of the relations between material synthesis procedures, structural properties and system‑level properties. In addition, a significant performance increase can also be expected from the optimization of the storage system's control strategies.

the adsorbent bed and the adsorption rate.

**Author details**

**References**

Uroš Stritih\* and Urška Mlakar

adfm.201102734

rser.2009.05.008

\*Address all correspondence to: uros.stritih@fs.uni-lj.si

Faculty of Mechanical Engineering, University of Ljubljana, Ljubljana, Slovenija

the European Union. 2002:65‑71. DOI: 10.1039/ap9842100196

2010:13‑35. DOI: 10.3000/17252555.L\_2010.153.eng

[1] European Commision. Directive 2002/91/EC of the European Parliament and of the council of 16 December 2002 on the energy performance of buildings. Official Journal of

[2] EU. Directive 2010/31/EU of the European Parliament and of the council of 19 may 2010 on the energy performance of buildings (recast). Official Journal of the European Union.

[3] Ristić A, Logar NZ, Henninger SK, Kaučič V. The performance of small‑pore microporous aluminophosphates in low-temperature solar energy storage: The structureproperty relationship. Advanced Functional Materials. 2012;**22**:1952‑1957. DOI: 10.1002/

[4] N'Tsoukpoe KE, Liu H, Le Pierres N, Luo L. A review on long-term sorption solar energy storage. Renewable and Sustainable Energy Reviews. 2009;**13**:2385‑2396. DOI: 10.1016/j.

[5] Hauer A. Sorption theory for thermal energy storage. Thermal Energy Storage for Sustainable Energy Consumption. 2007:393‑408. DOI: 10.1007/978‑1‑4020‑5290‑3\_24

#### **4. Conclusions**

Thermochemical heat storage is considered as the only storage concept with a potential for long-term low-temperature heat storage of high enough storage density to be also economically attractive. In this paper, thermochemical heat storage technologies and systems were reviewed. The studies were reviewed based on used storage materials, system configuration as well as models to predict and optimize system performance. Emphasis was placed on systems suitable for solar energy utilization in buildings.

In the paper, an overview of working pairs studied for thermochemical heat storage and transformation applications was given, but none of the presented materials meet the requirements for large-scale low-temperature heat storage applications due to unsuitable operating conditions (i.e., too high charging temperature), too low energy density and discharging temperature, corrosiveness, thermal/chemical instability, environmentally unfriendly production or high cost. The most promising are solid materials that participate in reversible chemical and physical sorption processes with water vapor as sorbate. The focus of material research has been on zeolites and their composites with hygroscopic inorganic salt hydrates and on microporous aluminophosphates. Nevertheless, one issue is common to all sorption storage materials, i.e., the discrepancy between the material and system energy storage density.

One of the main reasons that the prototypes do not achieve the storage capacity expected based on the material energy storage density is insufficient heat and mass transfer inside the reactor. In this regard, open reactor concepts have an advantage over closed reactor configuration, since the heat transfer fluid is in direct contact with the solid reactant, while closed systems require a separate heat transfer loop and hence a heat exchanger in the reactor. Additionally, the design of open systems is much simpler and consequently cheaper compared to closed systems, because they do not require the use of condensers, evaporators and working fluid storage reservoirs.

However, weather conditions are limiting the operation of open systems, i.e., supply air must be humidified when the ambient moisture content is insufficient (e.g., during winter). Another issue limiting the performance of sorption storage systems is the sensible heat loss during charging and discharging as a consequence of heating up the sorbent material and consequently the reactor to the charging/discharging temperature. Therefore, modular, moving beds and fluidized bed reactors are favorable. Also, attention should be paid on building reactors from materials with lower thermal mass, yet with comparable thermal conductivity, than the usually used steel alloys.

Since seasonal storage requires a steady and continuous heat output when discharging, the reactor bed must be optimized in such a way that it guarantees a constant flow rate to enable stable heat output during discharging. In order to achieve this without sacrificing the heat storage unit, compactness numerical modeling techniques have to be employed in the reactor design phase. The review of literature indicates that in the majority of solid-gas adsorption processes modeling efforts, the Dubinin‑Polanyi theory is applied for describing adsorption equilibria, while Darcy's law and LDF model are used to predict the pressure gradient inside the adsorbent bed and the adsorption rate.

To conclude, the storage materials represent a bottleneck for the development of thermochemical heat storage systems. Therefore, in order to achieve the commercial breakthrough of thermochemical heat storage systems, a bottom‑up approach of storage material engineering is needed, i.e., during material development, the required material characteristics have to be redefined according to the dynamics of the thermochemical process in the particular storage design, thereby obtaining a better understanding of the relations between material synthesis procedures, structural properties and system‑level properties. In addition, a significant performance increase can also be expected from the optimization of the storage system's control strategies.

### **Author details**

The heat released during discharging is transported to the heating system/buffer store via an air to water heat exchanger. During charging, the air is heated up in the air to water heat exchanger. For heat recovery, an air to air heat exchanger was additionally integrated into the reactor unit. The storage system was experimentally tested by varying the air humidity and the heating demand. The system performance was in compliance with the theoretically expected thermal power and temperature lift based on the heat storage density of the adsorbent. The developers further plan to increase the system compactness, reduce heat losses and

Thermochemical heat storage is considered as the only storage concept with a potential for long-term low-temperature heat storage of high enough storage density to be also economically attractive. In this paper, thermochemical heat storage technologies and systems were reviewed. The studies were reviewed based on used storage materials, system configuration as well as models to predict and optimize system performance. Emphasis was placed on sys-

In the paper, an overview of working pairs studied for thermochemical heat storage and transformation applications was given, but none of the presented materials meet the requirements for large-scale low-temperature heat storage applications due to unsuitable operating conditions (i.e., too high charging temperature), too low energy density and discharging temperature, corrosiveness, thermal/chemical instability, environmentally unfriendly production or high cost. The most promising are solid materials that participate in reversible chemical and physical sorption processes with water vapor as sorbate. The focus of material research has been on zeolites and their composites with hygroscopic inorganic salt hydrates and on microporous aluminophosphates. Nevertheless, one issue is common to all sorption storage materials, i.e., the discrepancy between the material and system energy storage density.

One of the main reasons that the prototypes do not achieve the storage capacity expected based on the material energy storage density is insufficient heat and mass transfer inside the reactor. In this regard, open reactor concepts have an advantage over closed reactor configuration, since the heat transfer fluid is in direct contact with the solid reactant, while closed systems require a separate heat transfer loop and hence a heat exchanger in the reactor. Additionally, the design of open systems is much simpler and consequently cheaper compared to closed systems, because they do not require the use of condensers, evaporators and

However, weather conditions are limiting the operation of open systems, i.e., supply air must be humidified when the ambient moisture content is insufficient (e.g., during winter). Another issue limiting the performance of sorption storage systems is the sensible heat loss during charging and discharging as a consequence of heating up the sorbent material and consequently the reactor to the charging/discharging temperature. Therefore, modular, moving beds and fluidized bed reactors are favorable. Also, attention should be paid on building reactors from materials with lower thermal mass, yet with comparable thermal conductivity,

lower the charging temperature and also to test the concept in an in situ setup.

tems suitable for solar energy utilization in buildings.

working fluid storage reservoirs.

than the usually used steel alloys.

**4. Conclusions**

70 Advancements in Energy Storage Technologies

Uroš Stritih\* and Urška Mlakar

\*Address all correspondence to: uros.stritih@fs.uni-lj.si

Faculty of Mechanical Engineering, University of Ljubljana, Ljubljana, Slovenija

#### **References**


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74 Advancements in Energy Storage Technologies


**Section 2**

**Electrical Energy Storage**
