**2.2 PCM selection criteria**

Melting point becomes the main criteria for the selection of the phase change material which in turn depends on the range of operating temperatures of the HTF. However heat transfer losses and enhanced effectiveness of the system are also necessary. The design criteria properties are differentiated into thermodynamic, chemical, economic and nucleation properties. The significant properties are listed as follows:


The material degrading should be avoided during the process of phase change. Considering the other important criteria required for a suitable design of PCM material are high heat density, high thermal conductivity, non-corrosiveness and economic price. In addition, the range of melting point and the availability of the PCM have to be probed.

#### **2.3 Heat transfer fluid selection criteria**

The selection of heat transfer fluid will be based on the following criteria:


### **2.4 Heat exchanger selection criteria**

Selection of design criteria for the heat exchanger type is mainly prioritised as

Performance efficiency is an important criterion and it studies the affects of heat exchanger factors and parameters.

The availability and the compatibility of the components of the heat exchanger without corrosion or contamination along with the cost-effectiveness are the important criteria.

Implementation complexity criteria detect the implementation of the system complexities and handling issues.

#### **2.5 Criteria for heat exchanger components material**

In general, commercial availability, compatibility, stability and cost-effectiveness of the components investigation are few important common criteria considered for all the components and their material.


#### **2.6 Criterion for insulating material**

• As mentioned earlier, the criteria such as commercial availability, least thermal conductivity and withstand maximum operating temperature along with the cost-effectiveness for the insulating material help to appropriately choose the insulating material.

For an efficient thermal energy storage system, the selection of suitable thermal energy storage media based on different criteria and systems is essential. Basically, both sensible and latent heat storage systems are feasible. The heat transfer media for sensible thermal energy storage are selected based on the temperature at which the heat has to be supplied, for example: thermal oils, molten salts, liquid metals, concrete and sand. For latent thermal energy storage systems, water/steam and phase change materials were considered. Liquid metals as heat transfer fluid have been eliminated because of their high density and therefore their bad pump ability. Thermal oils are problematic due to their flammability. If the temperature is less than 100°C, water can be used as a heat transfer fluid.

#### **3. Heat exchanger design**

There are many types of heat exchangers that can be implemented as a TES system with PCMs but few types have been tested such as screw heat exchanger, shell and tube heat exchanger and module PCM heat exchanger. Thus the three types of heat exchangers have been discussed in the next section.

#### **3.1 Screw heat exchanger**

Screw heat exchangers (SHE) are mostly used for industrial purposes as shown in **Figure 5**. In general, it consists of a drum and rotating hollow shafts (screws) arranged

**Figure 5.** *Screw heat exchanger schematic.*

parallel to each other, and fluid passes through them. A prototype of SHE with PCM was implemented and tested to be integrated into the CSP plant as a TES system. The PCM is placed inside SHE; the phase change takes place when HTF flows through the hollow shaft during the charging/discharging process. In this prototype, the storage system type is two storage tanks, where one is for molten PCM and the other is for solid PCM. The continuous rotation of the hollow shafts will decrease the solidification of the PCM during the discharge process. However, during the discharging process, the PCM will be solidified on the shaft, then it will be crushed due to the shaft rotation. Furthermore, its size will be large to be reused for the charging process. Therefore, a granulate crusher is required in order to decrease the size of the collected PCM, and SHE is compatible with the usage of PCMs and HTF. While the implementation is kind of complex due to the two-tank storage system, PCM granulates and the requirement of a granulate crusher. From a cost-effectiveness point of view, SHE cost depends on the size of a small prototype. The benefit of it is the alternative usage in the charging/discharging process, self-shaft cleaning due to the continuous rotation and the less solidification of PCM effects on heat transfer. However, the cost of transportation of SHE, supplied power, two-tank storage system and the crushed granular is much higher than the obtained benefits. Finally, there are no data mentioned about the efficiency of using SHE with PCMs but in general heat exchangers have a storage density of 95% of volume.

### **3.2 Shell and tube heat exchanger**

Shell and tube heat exchanger (STHE) generally consists of one or more round tubes mounted in a parallel configuration to a cylindrical shell shown in **Figure 6**. It is the most investigated type of heat exchanger with PCM. Also, it has a simple structure and can be easily manufactured beside the variety of configurations that can be made. Additionally, PCM can be easily replaced at the end of its lifetime. STHE exchanges heat through the pipe walls between flowing HTF in pipes and the PCM contained in the shell. As HTF passes through the pipes, phase change in PCMs will occur during the charging/discharging process. In discharging, the solidification of PCM will start at the pipe walls so it will act as a thermal resistor.

However, the thermal energy storage system with shell and tube heat exchangers is one of the most promising and cost-effective heat exchangers for latent heat storage. Moreover, its performance was investigated in different heat transfer enhancement techniques such as fins and cascaded PCM. Therefore, available data can be used. Additionally, this design has been recognised to be the promising configuration of a latent storage system acquiring high efficiency and minimum volume. STHE can be manufactured based on design requirements. Moreover, it is compatible with the usage of HTF and PCMs as well as being integrated as a TES system. Furthermore, it has a simple structure that can be easily implemented and handled. Its cost is a function of the outer surface area. In terms of cost-effectiveness, STHE can be alternatively used for charging/discharging, heat transfer enhancement techniques can be integrated with it, and it stands as a TES system. Therefore, an extra storage tank is not needed as the space required is designed properly, and the benefit of thermal stratification can be taken. The only additional cost required is for a pumping system for fluid circulation. Finally, from performance efficiency perspective heat exchangers when used with PCM has a storage density of 95% of volume.

#### **3.3 Heat exchanger module (HEM)**

This type is similar to any heat exchanger in construction. The only difference is that Macro encapsulated PCMs are placed in modules (cylindrical or spherical or any container) where the HTF flows over the PCM modules through the tank as shown in **Figure 7**. This type of design solves issues related to volume change, heat transfer and material compatibility besides availing flexibility and high package density since it can be fabricated in different sizes and shapes. Also, modules are easily handled and shipped. The encapsulation salt particles are more effective than heat exchangers with lower possibilities of success. However, this technology is researched a lot with great potential of energy storage for the purpose of high and medium-temperature storage

systems. Additionally, the coating material of the encapsulation may cause contamination of HTF. In most of the cases, the replacement of PCM at the end of the life cycle is not possible, HEM is not commercial yet, therefore, they have to be individually designed. Moreover, it is not fully compatible with HTF, since the coating material may contaminate it and cause degradation. Implementation complexity is not a big issue since it is a similar and common type of heat exchanger.

In terms of cost-effectiveness, there are no common available data that estimate the cost of it but it is more cost-effective compared to heat exchangers. From a performance efficiency perspective, it has a storage density of 74%. However, heat transfer enhancement methods are being researched.

#### *3.3.1 Design considerations for stratified HEM tanks*

The following design criteria help in the design of an efficient TES tank with enhanced thermal stratification [1].

**Geometrical considerations:** A deep water-storage container is desirable to improve thermal stratification. The water inlet and outlet should be installed to produce a consistent water flow to evade mixing. The location of inlet and outlet openings should be placed as close as possible to the top and bottom respectively of the stored volume to minimise the dead water volume and the surface area in contact with the water.

**Operating considerations:** The difference of temperature linking the top and bottom parts of the tank should at the least be 5°C to 10°C. Controls can be used to maintain fixed water temperatures in the tank's upper and lower parts if desired. The velocity of the water flowing into and out of the tank should be low.

**Other considerations:** The insulating and water-proofing characteristics of the tank should be designed to meet appropriate specifications.

For proper installation and control, using state-of-the-art equipment requires less energy for heating and cooling with substantial potential to substitute with low initial costs. More sophisticated thermal-design calculations are required to get the best design for better operating conditions.

#### *3.3.2 Degradation of thermal stratification*

Degradation of thermal stratification or mixing in a thermal storage tank can be essentially attributed to the following physical mechanisms:


The medium used for heat transfer interchanges heat through direct and indirect contact with the medium used for storage which forms a thermocline in HTF. The dual medium concept has a temperature drop as a drawback when compared to the single medium hot or cold tank while charging and discharging. The single medium maintains a constant outlet temperature while charging and discharging till the heat is removed in the tank, however, the HTF outlet temperature increases as it is discharged and decreases the more it is discharged for a dual medium storage system which leads to unfeasible storage capacity.

During the charging and discharging periods, mixing forms the main cause of stratification loss in general and major mixing losses occur during the lengthy storage periods in general. Enhancing the stratification significantly increases the efficiency of TES compared to a thermally mixed-storage tank. Hence the TES tank has to be evaluated quantitatively and comprehensively to clearly analyse the effects of stratification on the performance of a TES system.

#### *3.3.3 Parameters to measure the stratification*

Different methods for the stratification characterisation in TES system are being recommended over the years. A density method and a temperature approach are two fundamentally distinct approaches to stratification. In general, only temperature-based techniques are presented. Furthermore, it is critical to distinguish between elements that effect stratification (e.g., Richardson number, H/D ratio, etc.) and figures that evaluate the degree at different time intervals. Furthermore, the thermocline thickness, which divides the hot and cold zones inside the storage, may be used to analyse indicators of the degree of stratification.

Numerical figures given in terms of efficiency or ratio, on the other hand, are typically used for comparing the experimental storage process to a hypothetical storage process and thus include information about the history of the storage process. The most commonly used dimensionless numbers, such as MIX number, Richardson number, ratio H/D, discharging efficiency ratio, Peclet number and Reynolds number, to characterise stratification in water tanks and studied their suitability.

According to their findings, Peclet and Reynolds values do not explain stratification inside the tank, and it is unclear if discharging efficiency can define stratification inside the storage tank at low flow rates.

Richardson number is used to evaluate the stratification by comparison, for example, between PCM-filled storage against a reference case without PCM. The Richardson number is a popular way to describe stratification. It is a ratio of buoyancy forces to mixing forces; a low Richardson number implies a mixed storage tank, whereas a high Richardson number indicates a stratified storage tank.

The MIX number is used to analyse stratification in a water storage tank at a specific time, with a figure ranging from 0 to 1 indicating the degree of stratification; nevertheless, it is sensitive to slight variations in the temperature profile. Because the stratification degree is of considerable importance in this work, several stratification efficiencies such as energy efficiency (first law of thermodynamics), exergy efficiency (second law of thermodynamics), stratification efficiency and MIX number were utilised. The MIX number ranges from 0 to 1, with MIX = 0 representing a perfectly stratified tank and MIX = 1 for a fully mixed tank.

Stratification efficiencies based on the first law of thermodynamics most commonly compute a proportion of energy recovered with a certain charging and discharging operation with a set intake temperature and mass flow. The charging energy efficiency is defined by Ref. [4] and is based on the first rule of thermodynamics.

Methods for evaluating exergy efficiency based on the second law of thermodynamics are especially relevant when the energy will be utilised to do work, but they are not the only ones. The exergy stored in this example represents the thermodynamic limit of the work that may be generated. To calculate values based on exergy, a reference' dead state' that corresponds to the thermodynamic condition of the environment must be determined. Exergy is always greater in storage with a more significant temperature gradient and better stratification than in storage with an identical energy content but less pronounced stratification. As a result, numbers based on the second law of thermodynamics may be utilised to provide information regarding stratification efficiency.

Oro et al. [6] and Cabeza et al. [7] reported that the various parameters such as degree of stratification first law efficiencies, second law efficiencies, mix number, stratification number and Richardson number were used to characterise the storage system and they reported that energy and exergy efficiencies have no relationship with thermal stratification.

#### *3.3.4 Advantages of stratification*

Several studies have illustrated that the stratified STES system will give better results in comparison to a mixed tank of STES. In a STES system when connected to a solar panel, the heat extracted from a stratified charged tank is greater and for a longer duration in contrast to a fully mixed STES system. This stratification causes the low temperature at the bottom of the fluid of the tank returning to the solar collector and this creates a higher temperature difference between the fluid and the collector. It is concluded that the use of stratification in thermal storage designs should be considered as it increases the exergy storage capacity of thermal storage.

Developments in Stratification Analysis Stratified TES tanks often exhibit superior performance to non-stratified tanks, especially in low HTF flow rate heating systems. The level of TES stratification is decreased by HTF mixing, due to natural or forced convection, or by heat diffusion by conduction. Haller et al. [3], reviewed and compared methods to determine the stratification efficiency of a TES during a complete thermal cycle. Only one method considers the entropy generation, which provides a

more meaningful interpretation of efficiency than energy analysis. Some of the advantages of stratified thermal storage systems in construction are as follows:


Research activities on TES are ongoing at various national laboratories, universities, and research centres worldwide. The TES system can utilise thermal stratification. Such stratification separates lower-density warm water above higher-density cool water and can be applied in a single economic storage tank to provide demandside management of cooling or heating loads. Stratified thermal storage is expected to minimise the tank's costs, insulation, system integration, piping, and controls. Consider the example of cool supply water being withdrawn from the storage tank's bottom during peak cooling periods and used directly in the cooling loop and then sent back to the top of the tank. However in the off-peak periods, warm water is taken from the tank's top and cooled with the help of low-cost off-peak energy, and sent back to the tank's bottom.

#### **4. Experimentation on the heat exchange module**

An experimental project was conducted using a HEM and its experimental setup [8, 9], along with all the design data is being discussed in the following sections.

#### **4.1 Experimental set-up**

The experiment represented in **Figure 8** has thermal energy storage with a heat exchanger module or storage tank, a circulating hot water bath, a electric heater to simulate a solar heat situation, components of a heat transfer loop along with a data acquisition system. A cylindrical storage tank was fabricated with a height of 300 mm and a diameter of 320 mm as shown in **Figure 8a**. A 50 mm space on the top and a 25 mm space on the bottom is the working space left free and in between these two regions a packed bed of PCM balls is arranged in four layers and each layer was distinguished using a steel mesh and had 11 PCM balls in each layer which were dispersed uniformly within the layers of the storage HEM module. The PCM HS89 was a phase change storage medium and is a hydrated salt having a melting temperature of 89 °C which was procured from Pluss advanced technologies Pvt. Ltd., New Delhi. The properties such as specific heat, phase transition temperature, liquid and solid densities and the latent heat were got from the technical data sheets of the company and are presented in **Table 3**. The 2 mm thick stainless steel encapsulated spherical ball has an average outer diameter of 80 mm. A 40 mm thick insulation of polyurethane foam for the storage tank was used to minimise the heat loss.

#### **Figure 8.**

*Charging and discharging of a HEM and its circuit illustration (a) charging experiment with storage tank (b) discharging experiment.*

The 30 liter HTF hot water bath is simulated with two immersion heaters of 1.5 kW capacities. The temperature of the inlet water to the storage tank from the heater gradually increases until it reaches 95°C, and this temperature was controlled with the help of a thermostat. To observe the water level as lot of steam evaporates the water in the heater tank, a glass tube was parallelly attached to the HEM as shown in **Figure 8**. The steam produced while heating the HTF or water reduces the level of the water and as the level reduces below a required liquid level, makeup water is added. A quarter-watt centrifugal

*Technology in Design of Heat Exchangers for Thermal Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.108462*


**Table 3.**

*Technical specification for HS (Hydrated Salts) 89 PCM.*

pump is used to circulate the HTF from the storage tank to the hot water bath and if the water level was increased beyond the brim of the hot water bath it would flow to the HEM through gravity. A rotameter is placed in the reverse flow from the storage tank to the hot water bath to measure the flow rate and maintain it to 1 liter per minute using a valve placed at the bottom of the HEM tank. The inlet and outlet pipes are placed separately in HEM and used to circulate the water based on the charging or discharging applications as illustrated in **Figure 8**. The temperature in the PCM encapsulation in each layer was measured using a thermocouples located inside any one of the PCM balls itself. The thermocouples were inserted in the head of the PCM balls by drilling a hole in the nut of the encapsulation, inserting the thermocouple, and then applying an adhesive (m-sealcuring epoxy compound) to avoid leakage. Four thermocouples are placed equally among the layers of water to measure and record the layer temperatures. Four thermocouples are also placed in the encapsulated PCM balls to record the PCM temperature inside the ball to identify whether the PCM has melted on heating with HTF. A data acquisition system NI 9213 was used to connect the thermocouples to the laptop using LABVIEW interface. The data is recorded in MS-EXCELL sheets for further analysis.

#### **4.2 Experimental procedure**

In the present experiment, initially after checking all the connections of the circuit charging of the HEM was conducted by allowing the heated water to flow from the hot water bath to the HEM across all the balls so that the PCM capsules get heated and reach the temperature of 89°C. As the temperature of the water increased beyond 90° C, the PCM in the spherical capsules is melted, and while melting it is simultaneously ensured that the hot water bath did not go beyond 95°C using thermostat. Using a centrifugal pump the water from the bottom of the HEM is pumped back at a rate of 1 L/min to flow to the hot water bath, so the centrifugal pump was switched on. After reaching the melting temperature, the temperature of the balls remains constant due to phase change taking place in the HS89 salts in the PCM balls. The data acquisition system NI 9213 is used to monitor and record continuously the increase in temperatures with respect to time. As the temperature of the balls reaches 89°C and remains constant, salts in the PCM balls will melt. This constant temperature indicates the charging of the balls and the HEM is completely charged. After recording the temperatures of balls and the layers of the charging experiment using thermocouples and DAQ as shown in **Figure 8a**, the data is stored and the charging experiment is complete. After ensuring the finish of charging of the experiment as the complete

phase change takes place, the discharging experiment was initiated. In the discharging experiment, the centrifugal pump was stopped and the hot water supply was cut off. Room temperature water at 30°C is circulated from the cold water inlet at a rate of 1 L/min into the hot-charged HEM. As the thermocouples placed inside the balls and within the layers start recording the temperatures at various positions while discharging. The outlet temperature of the water coming out from the storage tank also was recorded separately during this process of discharging. Instantaneous and cumulative heat transfer, charging and discharging efficiencies and the analysis of stratification characteristics for the storage tank were deduced from the transient temperature variation obtained in the PCM and the HTF of the HEM.

#### **4.3 Data analysis**

The various equations used to evaluate the performance of the storage tank such as Q\_ins, Q\_cumm, Q\_loss, Q\_stored, charging efficiency, stratification number and Richardson number are presented in this section. The analysis of the experiments can be done by deriving these parameters which help us to identify the different aspects of the heat flow and the stratification behaviour across the storage tanks. The analysis is drawn based on the values of these parameters also helps to conclude which storage tank is better for which purpose.

#### *4.3.1 Overall heat loss coefficient (Uoverall)*

This parameter is evaluated from the drop in temperature of the water in the storage tank over a long duration of time when the storage tank is under idle condition (without the PCM balls) using Eqs. (3) to (6). This parameter helps to calculate the heat loss and becomes important to understand the loss of heat while charging and discharging experiments are conducted:

$$Q = U\_{overall} \mathcal{A} . T\_{LMTD} \tag{3}$$

$$U\_{overall} = \frac{Q}{A.T\_{LMTD}}\tag{4}$$

$$\mathbf{Q} = m\mathbf{C}\_p \left( T\_{ini} - T\_{final} \right) \tag{5}$$

$$T\_{LMTD} = \frac{(T\_{ini} - T\_{\infty}) - \left(T\_{final} - T\_{\infty}\right)}{\ln \ln \frac{(T\_{in} - T\_{\infty})}{\left(T\_{final} - T\_{\infty}\right)}}\tag{6}$$

*Q* = Total heat lost to the ambient, W

*Tini* = Initial temperature in the tank, °C

*Tfinal* = Final temperature in the tank, °C

*T*<sup>∞</sup> = Ambient Temperature, °C

*Cp* = Specific heat of water, kJ/kgK

*m* = mass of water in the idle tank, kg

*A* = outside area of the idle tank, m<sup>2</sup>

The overall heat loss coefficient (*Uoverall*) is evaluated for all three storage tanks and the average value is considered for further analysis.

Separate experiment was conducted under idle conditions to evaluate the average heat loss coefficient. The temperature of the water in the storage tank was averaged

out for all the layers of water with respect to time. The initial and final temperatures of the layers while discharging experiments under idle conditions were taken to evaluate the LMTD as given in Eq. 6. The overall heat loss coefficient was calculated with Eq. 4 and an average value of 5.056 W/m2 K was being evaluated for the storage tank and used for calculations of both charging and discharging of the storage tank with PCM balls

### *4.3.2 Instantaneous heat transfer (Qi)*

The instantaneous heat transfer represents the amount of heat transferred into the storage tank at any instant of time during the charging process as given in Eq. (7). It shows the effect of heat based on the inlet and outlet temperature and is directly proportional to the stratification of the tank.

$$\mathbf{Q}\_i = \dot{m}\mathbf{C}\_p (T\_{H-in} - T\_{H-out}), \mathbf{W} \tag{7}$$

*TH*�*in* = Inlet HTF temperature to the tank, °C *TH*�*out* = Outgoing HTF temperature from the tank, °C *m*\_ = Mass flow rate of the HTF, kg/s
