Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle with High-Temperature TES Upstream the Gas Turbine

*Fritz Zaversky, Iñigo Les, Marcelino Sánchez, Benoît Valentin, Jean-Florian Brau, Frédéric Siros, Jonathon McGuire and Flavien Berard*

## **Abstract**

This work presents a techno-economic parametric study of an innovative central receiver solar thermal power plant layout that applies the combined cycle (CC) as thermodynamic power cycle and a multi-tower solar field configuration together with open volumetric air receivers (OVARs). The topping gas turbine (GT) is powered by an air–air heat exchanger (two heat exchanger trains in the case of reheat). In order to provide dispatchability, a high-temperature thermocline TES system is placed upstream the gas turbine. The aim is threefold, (i) investigating whether the multi-tower concept has a techno-economic advantage with respect to conventional single-tower central receiver plants, (ii) indicating the technoeconomic optimum power plant configuration, and (iii) benchmarking the technoeconomic optimum of the CC plant against that of a conventional single-cycle Rankine steam plant with the same receiver and TES technology. It is concluded that the multi-tower configuration has a techno-economic advantage with respect to the conventional single-tower arrangement above a total nominal solar power level of about 150 MW. However, the benchmarking of the CC against a Rankine single-cycle power plant layout shows that the CC configuration has despite its higher solar-to-electric conversion efficiency a higher LCOE. The gain in electricity yield is not enough to outweigh the higher investment costs of the more complex CC plant layout.

**Keywords:** concentrated solar power, solar combined cycle, multi-tower central receiver, open volumetric air receiver (OVAR)

## **1. Introduction**

Solar thermal power, also known as concentrated solar power (CSP) or solar thermal electricity (STE), can be considered as a highly promising technology when it comes to dispatchable and thus grid-friendly supply of renewable electricity. This is due to the possibility of thermal energy storage (TES), the key advantage over other renewable technologies (such as wind or photovoltaic), which enables the decoupling between solar energy collection and electricity production. Given the abundant amount of solar power available for terrestrial solar collectors (85 PW) [1], which exceeds the current world's power demand (15 TW) several thousand times [1], CSP is a highly promising and flexible alternative to conventional fossilfuel technologies, setting new standards in terms of environmental impact, sustainability, safety, and thus quality of life.

since the solar receiver has to provide heat to a pressurized air stream coming from the gas turbine's compressor. Several previous research projects have already endeavored to design such a demanding component, which has to operate under

*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle…*

addition at pressures over 6 bar. The first developments started in the 1980's with metallic and ceramic tubular designs [14]. This approach showed however durability issues and also low efficiencies because of the low heat transfer coefficient of air. Therefore, pressurized volumetric receivers [15] appeared to be a promising alternative as they increased the heat transfer area. However, durability issues and size limitations of the needed quartz glass window have hindered their commercial application so far. For this reason, the idea of pressurized tubular or opaque-heatexchanger-type receivers was revisited by several research groups. For example, Grange et al. [16] investigated a modular metallic absorber located at the back of a cavity. The maximum air outlet temperature was reported to be 750°C. Korzynietz et al. [17] developed a pre-commercial scale metallic tubular cavity receiver achieving thermal efficiencies between 71.3 and 78.1% at the maximum air outlet temper-

However, although dispatchability is the key advantage of CSP (due to costeffective thermal energy storage) and its best argument to justify higher costs than

The aim of this work is therefore to present an innovative plant layout (as shown

PV or wind energy, only a few works have covered the integration of hightemperature TES upstream the solar combined cycle. Since only pressurized receivers have been applied so far in the context of solar-powered combined cycles, previous works have proposed the application of pressurized regenerative TES systems [16, 18], which have clear limitations regarding cost-effective large-scale

in **Figure 1**) that not only avoids the design challenges related to pressurized receivers but also allows the integration of an atmospheric air-based hightemperature TES system upstream the combined cycle. In particular, this work proposes the application of the open volumetric air receiver technology [15], which

has already been demonstrated successfully at pre-commercial scale [19], in

*Solar-powered combined cycle scheme with open volumetric air receiver and high-temperature TES (without reheat in the Brayton cycle). The low-temperature TES enables regenerative use of return air heat.*

), at high temperatures (> 900°C), and in

very high solar flux (≈ 0.5–1 MW/m<sup>2</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.90410*

ature of 800°C.

deployment.

**Figure 1.**

**23**

Until recently, the cost of electricity generation for CSP (≈ 14 c€/kWh [2]) has been clearly above conventional technology and other renewables (wind and photovoltaic reach 6 c€/kWh on average [2]). Nevertheless, considering latest bids, cost targets as low as 7 c\$/kWh [3] seem to be realistic for mature CSP technology. However, rather than comparing the pure cost, one should compare the true value of CSP for grid operation and capacity [2] when considering an increasing fraction of not-dispatchable renewables. It is important to note that CSP should not be seen as a competing technology with photovoltaic or wind power. It should rather be seen as an enabling technology of not-dispatchable renewable power generation, as a key feature of CSP is its cheap thermal energy storage, guaranteeing dispatchability.

This work focuses on central receiver CSP plants [4] analyzing the economic competitiveness of the combined cycle (topping GT plus bottoming steam Rankine) with respect to conventional single-cycle Rankine steam technology. The combined cycle technology [5–7] is well known from conventional fossil-fired power generation, reaching cycle efficiencies exceeding 60% on a lower heating value basis [8]. However, these best of class efficiencies are obtained with latest fossil-fired gas turbine (GT) technology, achieving turbine inlet temperatures (TITs) of up to ≈1500°C. It is clear that such high TITs can only be achieved with (i) internal combustion and (ii) turbine blade cooling and single-crystal superalloy turbine blades, furthermore coated with low conductivity ceramics. For the application of solar combined cycles, the TIT has to be significantly lower for two reasons: (i) the optimum receiver operating temperature (≈ TIT) that optimizes solar-to-electric conversion efficiency is a function of the receiver's thermal efficiency and tends to be at around 1000°C [9, 10] depending on the receiver technology; (ii) for solar combined cycles, the concept of externally heated gas turbines [11] has to be exploited, which limits the maximum achievable TIT to lower values (≈ 900–1000°C) in any case. Additionally, cheaper designs for the turbine should be used in order to keep costs down, i.e., uncooled turbine blades, which should be achievable with expected optimum receiver working temperatures of ≈ 1000°C [9]. Furthermore, advanced gas turbine architectures [12] such as reheat will be necessary to achieve good GT efficiencies, despite low TITs. Reheated gas turbines have already been treated in previous works. The main motivations are (i) to keep the average temperature of heat supply high and (ii) to introduce an additional flexibility regarding turbine exit temperature (TET) (the heat recovery steam generator inlet temperature), despite high compressor pressure ratios [13]. In particular, the expansion ratio of the second turbine stage can be specifically designed, so that the resulting TET optimizes the overall combined cycle performance. Note that the higher the heat recovery steam generator's inlet temperature is, the higher are the conversion efficiency and power output of the bottoming cycle and vice versa.

It is clear that the solar receiver unit is the key component of a solar-powered combined cycle plant, since it is of upmost importance to achieve very good solar receiver efficiencies at highest operating temperatures (≈ 1000°C). So far, pressurized air receivers have been the design principle for solar-powered gas turbines,

### *Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle… DOI: http://dx.doi.org/10.5772/intechopen.90410*

since the solar receiver has to provide heat to a pressurized air stream coming from the gas turbine's compressor. Several previous research projects have already endeavored to design such a demanding component, which has to operate under very high solar flux (≈ 0.5–1 MW/m<sup>2</sup> ), at high temperatures (> 900°C), and in addition at pressures over 6 bar. The first developments started in the 1980's with metallic and ceramic tubular designs [14]. This approach showed however durability issues and also low efficiencies because of the low heat transfer coefficient of air. Therefore, pressurized volumetric receivers [15] appeared to be a promising alternative as they increased the heat transfer area. However, durability issues and size limitations of the needed quartz glass window have hindered their commercial application so far. For this reason, the idea of pressurized tubular or opaque-heatexchanger-type receivers was revisited by several research groups. For example, Grange et al. [16] investigated a modular metallic absorber located at the back of a cavity. The maximum air outlet temperature was reported to be 750°C. Korzynietz et al. [17] developed a pre-commercial scale metallic tubular cavity receiver achieving thermal efficiencies between 71.3 and 78.1% at the maximum air outlet temperature of 800°C.

However, although dispatchability is the key advantage of CSP (due to costeffective thermal energy storage) and its best argument to justify higher costs than PV or wind energy, only a few works have covered the integration of hightemperature TES upstream the solar combined cycle. Since only pressurized receivers have been applied so far in the context of solar-powered combined cycles, previous works have proposed the application of pressurized regenerative TES systems [16, 18], which have clear limitations regarding cost-effective large-scale deployment.

The aim of this work is therefore to present an innovative plant layout (as shown in **Figure 1**) that not only avoids the design challenges related to pressurized receivers but also allows the integration of an atmospheric air-based hightemperature TES system upstream the combined cycle. In particular, this work proposes the application of the open volumetric air receiver technology [15], which has already been demonstrated successfully at pre-commercial scale [19], in

### **Figure 1.**

*Solar-powered combined cycle scheme with open volumetric air receiver and high-temperature TES (without reheat in the Brayton cycle). The low-temperature TES enables regenerative use of return air heat.*

it comes to dispatchable and thus grid-friendly supply of renewable electricity. This is due to the possibility of thermal energy storage (TES), the key advantage over other renewable technologies (such as wind or photovoltaic), which enables the decoupling between solar energy collection and electricity production. Given the abundant amount of solar power available for terrestrial solar collectors (85 PW) [1], which exceeds the current world's power demand (15 TW) several thousand times [1], CSP is a highly promising and flexible alternative to conventional fossilfuel technologies, setting new standards in terms of environmental impact, sustain-

Until recently, the cost of electricity generation for CSP (≈ 14 c€/kWh [2]) has been clearly above conventional technology and other renewables (wind and photovoltaic reach 6 c€/kWh on average [2]). Nevertheless, considering latest bids, cost targets as low as 7 c\$/kWh [3] seem to be realistic for mature CSP technology. However, rather than comparing the pure cost, one should compare the true value of CSP for grid operation and capacity [2] when considering an increasing fraction of not-dispatchable renewables. It is important to note that CSP should not be seen as a competing technology with photovoltaic or wind power. It should rather be seen as an enabling technology of not-dispatchable renewable power generation, as

This work focuses on central receiver CSP plants [4] analyzing the economic competitiveness of the combined cycle (topping GT plus bottoming steam Rankine) with respect to conventional single-cycle Rankine steam technology. The combined cycle technology [5–7] is well known from conventional fossil-fired power generation, reaching cycle efficiencies exceeding 60% on a lower heating value basis [8]. However, these best of class efficiencies are obtained with latest fossil-fired gas turbine (GT) technology, achieving turbine inlet temperatures (TITs) of up to ≈1500°C. It is clear that such high TITs can only be achieved with (i) internal combustion and (ii) turbine blade cooling and single-crystal superalloy turbine blades, furthermore coated with low conductivity ceramics. For the application of solar combined cycles, the TIT has to be significantly lower for two reasons: (i) the optimum receiver operating temperature (≈ TIT) that optimizes solar-to-electric conversion efficiency is a function of the receiver's thermal efficiency and tends to be at around 1000°C [9, 10] depending on the receiver technology; (ii) for solar combined cycles, the concept of externally heated gas turbines [11] has to be exploited, which limits the maximum achievable TIT to lower values (≈ 900–1000°C) in any case. Additionally, cheaper designs for the turbine should be used in order to keep costs down, i.e., uncooled turbine blades, which should be achievable with expected optimum receiver working temperatures of ≈ 1000°C [9]. Furthermore, advanced gas turbine architectures [12] such as reheat will be necessary to achieve good GT efficiencies, despite low TITs. Reheated gas turbines have already been treated in previous works. The main motivations are (i) to keep the average temperature of heat supply high and (ii) to introduce an additional flexibility regarding turbine exit temperature (TET) (the heat recovery steam generator inlet temperature), despite high compressor pressure ratios [13]. In particular, the expansion ratio of the second turbine stage can be specifically designed, so that the resulting TET optimizes the overall combined cycle performance. Note that the higher the heat recovery steam generator's inlet temperature is, the higher are the conversion

a key feature of CSP is its cheap thermal energy storage, guaranteeing

efficiency and power output of the bottoming cycle and vice versa.

It is clear that the solar receiver unit is the key component of a solar-powered combined cycle plant, since it is of upmost importance to achieve very good solar receiver efficiencies at highest operating temperatures (≈ 1000°C). So far, pressurized air receivers have been the design principle for solar-powered gas turbines,

ability, safety, and thus quality of life.

*Green Energy and Environment*

dispatchability.

**22**

**Figure 2.** *Innovative coupling of open volumetric air receiver and Brayton cycle.*

combination with a regenerative system working in alternating modes (atmospheric heating, pressurized cooling; see **Figure 2**) in order to power a solar-only combined cycle. This approach decouples the high-temperature and the high heat flux part (solar receiver) from the high pressure part (compressed air stream of the Brayton cycle) via an air–air regenerative heat exchanger. Thus, upstream the combined cycle, the well-proven and relatively cheap regenerator-type heat storage known from the so-called Cowper Stoves [7, 20, 21] can be used. The regenerative matrix consists of refractory bricks with channels in between where the gas is flowing, transferring heat to the bricks or vice versa [20]. This type of regenerative heat storage has already been demonstrated successfully at pilot plants [22–24] for the application of CSP. The big advantages of this technology are (i) a simple design with very low technological risk and (ii) low costs (≈17 €/kWhth [25]).

of molten salts or thermal oil. For this reason, no expensive heat-tracing equipment is needed, and furthermore the application of air as HTF allows the implementation of advanced power cycles such as the combined cycle (topping gas turbine and bottoming Rankine cycle), which requires temperatures of cycle heat input in the range between 800 and 1100°C (the optimum gas turbine inlet temperature

*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle…*

The second advantage is that air is freely available and obviously nontoxic to human kind and nature. Thus, no investment or cost of replacement during power plant life time must be considered. However, it may be necessary to thoroughly clean (filter) the air after recirculation in the heat transport circuit before its release to the ambient, as particulate matter coming from piping material, especially from high-temperature insulation fibers, must not be released to the atmosphere. If

The third advantage is that very cost-effective thermal energy storage technology is available when using atmospheric air as HTF. Here, the well-proven and relatively cheap regenerator-type heat storage known from the so-called Cowper stoves [20, 21], which are applied together with blast furnaces, can be used. These high-temperature regenerators work at temperatures of up to 1250°C [20] providing hot gas at constant temperature to the furnace. The outlet temperature of the regenerators is typically controlled by adding a variable flow of cold air at the outlet [21]. The regenerative matrix consists of refractory bricks with channels in between where the gas is flowing, transferring heat to the bricks or vice versa [20]. This type of regenerative heat storage has already been demonstrated successfully at pilot plants [22, 23] for the application of CSP. The big advantages of this technology are (i) a simple design with very low technological risk and (ii) low costs [25]. Possible storage vessel core geometries are packed beds, consisting of spheres or broken particles, stacks of plates, perforated bricks, or extruded shapes [26]. In packed beds, the efficiency of thermal energy storage depends on the heat transfer between the air and the filler material, as well as on the reached stratification or thermocline. Good heat transfer and limited heat transport within the solid storage media that enhances thermal stratification is reached by porous structures [27]. A truncated conical shape of the container, coupled with subterranean location, may alleviate problems such as rock fracture and tank deformation. The inclined walls reduce the mechanical constraints by guiding the rocks upwards during

depends on the receiver efficiency).

*DOI: http://dx.doi.org/10.5772/intechopen.90410*

**Figure 3.**

thermal expansion [24].

**25**

released it would represent an important health hazard.

*Single cycle Rankine scheme—Optimum receiver outlet temperature is 800°C.*

The temperature level of the "cold" return air stream leaving the air–air heat exchange system is a function of compressor outlet temperature (i.e., compressor pressure ratio and ambient temperature) and the exit temperature of the first turbine stage, in the case of reheat. The resulting air-return temperature level is too high for efficient blower operation and the recirculation to the high-temperature TES or the receiver is thus not feasible. A low-temperature air/rock thermocline TES is thus proposed in order to reuse the return air heat in regenerative manner. In order to keep air transport parasitic power consumption acceptable, the operating temperature of the blower should be kept at ambient temperature level.

A related work [10] has shown that the thermodynamic performance of this power plant layout, having an effective mean solar flux concentration ratio of 500, optimizes at a receiver outlet temperature (≈TIT) of about 1050°C and a Brayton cycle pressure ratio of 14 (reheat ratio *K*=0.75), resulting in about 29.6% peak solarto-electric conversion efficiency. The present work continues the research, focusing on the techno-economic optimization and benchmarking of the proposed power plant layout. The aim is threefold, (i) investigating whether the multi-tower concept has a techno-economic advantage with respect to conventional single-tower central receiver plants, (ii) indicating the techno-economic optimum of the power plant size (the number of towers, solar field size, solar multiple, and hours of TES), and (iii) benchmarking the techno-economic optimum of the solar-powered combined cycle plant against that of a conventional single-cycle Rankine steam plant [22] with the same receiver (but lower operating temperature, ≈ 800°C) and TES technology (see **Figure 3**).

### **1.1 Advantages and limitations of air as heat transfer fluid (HTF)**

Probably the most important advantage of air as HTF is that it has no temperature limit. Thus, overheating and freezing are no issue, in contrast to the application *Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle… DOI: http://dx.doi.org/10.5772/intechopen.90410*

**Figure 3.** *Single cycle Rankine scheme—Optimum receiver outlet temperature is 800°C.*

of molten salts or thermal oil. For this reason, no expensive heat-tracing equipment is needed, and furthermore the application of air as HTF allows the implementation of advanced power cycles such as the combined cycle (topping gas turbine and bottoming Rankine cycle), which requires temperatures of cycle heat input in the range between 800 and 1100°C (the optimum gas turbine inlet temperature depends on the receiver efficiency).

The second advantage is that air is freely available and obviously nontoxic to human kind and nature. Thus, no investment or cost of replacement during power plant life time must be considered. However, it may be necessary to thoroughly clean (filter) the air after recirculation in the heat transport circuit before its release to the ambient, as particulate matter coming from piping material, especially from high-temperature insulation fibers, must not be released to the atmosphere. If released it would represent an important health hazard.

The third advantage is that very cost-effective thermal energy storage technology is available when using atmospheric air as HTF. Here, the well-proven and relatively cheap regenerator-type heat storage known from the so-called Cowper stoves [20, 21], which are applied together with blast furnaces, can be used. These high-temperature regenerators work at temperatures of up to 1250°C [20] providing hot gas at constant temperature to the furnace. The outlet temperature of the regenerators is typically controlled by adding a variable flow of cold air at the outlet [21]. The regenerative matrix consists of refractory bricks with channels in between where the gas is flowing, transferring heat to the bricks or vice versa [20]. This type of regenerative heat storage has already been demonstrated successfully at pilot plants [22, 23] for the application of CSP. The big advantages of this technology are (i) a simple design with very low technological risk and (ii) low costs [25]. Possible storage vessel core geometries are packed beds, consisting of spheres or broken particles, stacks of plates, perforated bricks, or extruded shapes [26]. In packed beds, the efficiency of thermal energy storage depends on the heat transfer between the air and the filler material, as well as on the reached stratification or thermocline. Good heat transfer and limited heat transport within the solid storage media that enhances thermal stratification is reached by porous structures [27]. A truncated conical shape of the container, coupled with subterranean location, may alleviate problems such as rock fracture and tank deformation. The inclined walls reduce the mechanical constraints by guiding the rocks upwards during thermal expansion [24].

combination with a regenerative system working in alternating modes (atmospheric heating, pressurized cooling; see **Figure 2**) in order to power a solar-only combined cycle. This approach decouples the high-temperature and the high heat flux part (solar receiver) from the high pressure part (compressed air stream of the Brayton cycle) via an air–air regenerative heat exchanger. Thus, upstream the combined cycle, the well-proven and relatively cheap regenerator-type heat storage known from the so-called Cowper Stoves [7, 20, 21] can be used. The regenerative matrix consists of refractory bricks with channels in between where the gas is flowing, transferring heat to the bricks or vice versa [20]. This type of regenerative heat storage has already been demonstrated successfully at pilot plants [22–24] for the application of CSP. The big advantages of this technology are (i) a simple design

The temperature level of the "cold" return air stream leaving the air–air heat exchange system is a function of compressor outlet temperature (i.e., compressor pressure ratio and ambient temperature) and the exit temperature of the first turbine stage, in the case of reheat. The resulting air-return temperature level is too high for efficient blower operation and the recirculation to the high-temperature TES or the receiver is thus not feasible. A low-temperature air/rock thermocline TES is thus proposed in order to reuse the return air heat in regenerative manner. In order to keep air transport parasitic power consumption acceptable, the operating

A related work [10] has shown that the thermodynamic performance of this power plant layout, having an effective mean solar flux concentration ratio of 500, optimizes at a receiver outlet temperature (≈TIT) of about 1050°C and a Brayton cycle pressure ratio of 14 (reheat ratio *K*=0.75), resulting in about 29.6% peak solarto-electric conversion efficiency. The present work continues the research, focusing on the techno-economic optimization and benchmarking of the proposed power plant layout. The aim is threefold, (i) investigating whether the multi-tower concept has a techno-economic advantage with respect to conventional single-tower central receiver plants, (ii) indicating the techno-economic optimum of the power plant size (the number of towers, solar field size, solar multiple, and hours of TES), and (iii) benchmarking the techno-economic optimum of the solar-powered combined cycle plant against that of a conventional single-cycle Rankine steam plant [22] with the same receiver (but lower operating temperature, ≈ 800°C) and TES

with very low technological risk and (ii) low costs (≈17 €/kWhth [25]).

*Innovative coupling of open volumetric air receiver and Brayton cycle.*

temperature of the blower should be kept at ambient temperature level.

**1.1 Advantages and limitations of air as heat transfer fluid (HTF)**

Probably the most important advantage of air as HTF is that it has no temperature limit. Thus, overheating and freezing are no issue, in contrast to the application

technology (see **Figure 3**).

**24**

**Figure 2.**

*Green Energy and Environment*

As indicated above, air as HTF has important advantages, which however are limited by the fact that air is not very suitable when it comes to heat transport over longer distances and heat transfer, especially at atmospheric pressure. The heat transport issue of atmospheric air circuits is caused by the very low density of air at high temperatures (≈ 0.28 kg/m<sup>3</sup> at 1000°C), which leads to high flow velocity and thus elevated pressure drops (high blower power consumption). Therefore, the air piping system must provide sufficiently large flow cross sections in order to keep flow velocity in the order of magnitude of 15–30 m/s. This requirement leads to large diameter piping that needs to be insulated internally as high-temperature alloys are too expensive.

However, not only the associated pressure drop (parasitic power consumption) is an issue when applying air as HTF. Also thermal losses and thermal inertia effects are important design aspects for air-based CSP plants.

All the abovementioned disadvantages have to be properly addressed for the scale-up to commercial size of atmospheric air-based CSP plants. As described later on, the upscaling of CSP plants is important, as specific power cycle costs (USD/kWe) are significantly lower when moving to higher nominal power output. Therefore, CSP plants have increased in size recently. When upscaling a CSP plant to several hundreds of MWe, several heliostat fields and receivers may be needed to provide heat to the same power cycle: in such a case, the applied HTF needs to transport heat over very long distances (1–2 km). However, when applying air as HTF, thermal losses as well as thermal inertia effects (because of cooldown at night) are a major hurdle to its large-scale commercial implementation. In the following, the performance of an 800 m high-temperature air duct will be given, transporting the thermal power delivered by a 51 MWth heliostat field (Case C, **Table 1**).

Assuming a receiver outlet temperature of 1000°C and a corresponding receiver efficiency of 0.754 results in a nominal air mass flow of about 36 kg/s. For the design of the needed piping system, several techno-economic considerations need to be taken into account. The piping inner diameter defines the flow cross section and the outer circumferential surface area which needs to be thermally insulated. The ratio of flow cross area to the outer surface area (that defines piping and insulation material mass) becomes higher for larger inner diameters. Therefore, the inner diameter should be chosen as big as possible, i.e., considering manufacturing and structural strength limitations. In principle, circular duct geometry is the better choice as outer surface area per square meter of flow cross section is lower than in the case of rectangular cross section.

For the specific example, an inner piping diameter of 3 m has been chosen. The default thickness and thermal conductivity of the thermal insulation are 0.6 m and 0.035 W/(m K), respectively. The heat transfer coefficient between ambient air and outer pipe surface is assumed to be ≈ 10 W/(m<sup>2</sup> K). For the default case (Case 1 in **Table 2**), the given settings result in a flow velocity of 19 m/s and a total pressure drop [28] of about 0.22 kPa (800 m total piping length). The thermal loss to the ambient air (at 25°C) causes a temperature reduction of about 12°C, down to 988°C. As **Table 2** further indicates, when increasing the mass flow rate, the temperature drop reduces; however pressure drop more than doubles. Thus, one measure to reduce the temperature reduction is to increase mass flow rate, however, with the cost of higher parasitic power consumption. Obviously, since the thermal losses remain practically constant (the same temperature difference to ambient and overall heat transfer coefficient remains almost constant), a higher mass flow rate translates to lower temperature difference in the air flow. The second measure to reduce temperature drop is to increase insulation thickness (see **Table 3**), which however is very expensive.

This analysis shows that a temperature drop of ≈10°C is a reasonable assumption

**Solar field power classes ABCDE**

> 51 MW (North)

**(CC)) (c\$/kWh (RC))**

**1 C** (CC = 13.8) (RC = 13.1)

**2 C** (CC = 13.6) (RC = 13)

**3 C** (CC = 13.4) (RC = 12.5)

(CC = 13.2) (RC = 11.8)

(CC = 12.6) (RC = 11.5)

(CC = 12.9) (RC = 12.3)

153 MW (Surround)

———

———

**1 D** (CC = 13.8) (RC = 12.6)

**2 D** (CC = 14.6) (RC = 13.7)

**3 D** (CC = 14.4) (RC = 14)

(CC = 14.5) (RC = 14.1)

— —

— —

— —

—

—

**1 E** (CC = 14.7) (RC = 14.2)

—

459 MW (Surround)

Last but not the least, the second issue, as mentioned above, is related to low

exchangers need to have a very large area of heat transfer in order to counterbalance this drawback. This is the reason why heat recovery steam generators of combined cycle plants are very bulky and represent an important share of the power block's CAPEX. The same holds for the air–air heat exchanger that is needed for the CAPTure power plant layout (see **Figure 1**). Here, the motivation is to reduce cost

heat transfer performance when applying air as HTF. For this reason, heat

for the multi-tower concepts analyzed later on.

**Table 1.**

**27**

Nominal solar power (MW) 5.666 MW

*DOI: http://dx.doi.org/10.5772/intechopen.90410*

17 **3 A**

34 **6 A**

51 **9 A**

102 — **6 B**

153 — **9 B**

Mirror area (m2

(North)

(CC = 19) (RC = 20.4)

(CC = 17.7) (RC = 18.2)

(CC = 17) (RC = 16.9)

204 — — **4 C**

306 — — **6 C**

459 — — **9 C**

*Solar field base modules and multi-/single-tower configurations analyzed.*

612 ——— **4 D**

17 MW (North)

*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle…*

Annual optical efficiency () 0.7782 0.7644 0.7368 0.6827 0.6486 Field diameter (m) 240 380 750 1300 2400 Tower height (m) 50 80 100 150 200 Nominal solar power (MW) **Tower configurations analyzed and corresponding LCOE (c\$/kWh**

> **1 B** (CC = 16.7) (RC = 17.4)

> **2 B** (CC = 15.6) (RC = 15.7)

> **3 B** (CC = 15.1) (RC = 14.7)

> (CC = 14.1) (RC = 13.6)

> (CC = 13.6) (RC = 12.5)

) 7248 21,960 68,560 221,640 710,240

*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle… DOI: http://dx.doi.org/10.5772/intechopen.90410*


### **Table 1.**

As indicated above, air as HTF has important advantages, which however are limited by the fact that air is not very suitable when it comes to heat transport over longer distances and heat transfer, especially at atmospheric pressure. The heat transport issue of atmospheric air circuits is caused by the very low density of air at high temperatures (≈ 0.28 kg/m<sup>3</sup> at 1000°C), which leads to high flow velocity and thus elevated pressure drops (high blower power consumption). Therefore, the air piping system must provide sufficiently large flow cross sections in order to keep flow velocity in the order of magnitude of 15–30 m/s. This requirement leads to large diameter piping that needs to be insulated internally as high-temperature

However, not only the associated pressure drop (parasitic power consumption) is an issue when applying air as HTF. Also thermal losses and thermal inertia effects

All the abovementioned disadvantages have to be properly addressed for the scale-up to commercial size of atmospheric air-based CSP plants. As described later on, the upscaling of CSP plants is important, as specific power cycle costs (USD/kWe) are significantly lower when moving to higher nominal power output. Therefore, CSP plants have increased in size recently. When upscaling a CSP plant to several hundreds of MWe, several heliostat fields and receivers may be needed to provide heat to the same power cycle: in such a case, the applied HTF needs to transport heat over very long distances (1–2 km). However, when applying air as HTF, thermal losses as well as thermal inertia effects (because of cooldown at night) are a major hurdle to its large-scale commercial implementation. In the following, the performance of an 800 m high-temperature air duct will be given, transporting the thermal power delivered by a 51 MWth heliostat field (Case C,

Assuming a receiver outlet temperature of 1000°C and a corresponding receiver efficiency of 0.754 results in a nominal air mass flow of about 36 kg/s. For the design of the needed piping system, several techno-economic considerations need to be taken into account. The piping inner diameter defines the flow cross section and the outer circumferential surface area which needs to be thermally insulated. The ratio of flow cross area to the outer surface area (that defines piping and insulation material mass) becomes higher for larger inner diameters. Therefore, the inner diameter should be chosen as big as possible, i.e., considering manufacturing and structural strength limitations. In principle, circular duct geometry is the better choice as outer surface area per square meter of flow cross section is lower than in

For the specific example, an inner piping diameter of 3 m has been chosen. The default thickness and thermal conductivity of the thermal insulation are 0.6 m and 0.035 W/(m K), respectively. The heat transfer coefficient between ambient air and outer pipe surface is assumed to be ≈ 10 W/(m<sup>2</sup> K). For the default case (Case 1 in **Table 2**), the given settings result in a flow velocity of 19 m/s and a total pressure drop [28] of about 0.22 kPa (800 m total piping length). The thermal loss to the ambient air (at 25°C) causes a temperature reduction of about 12°C, down to 988°C. As **Table 2** further indicates, when increasing the mass flow rate, the temperature drop reduces; however pressure drop more than doubles. Thus, one measure to reduce the temperature reduction is to increase mass flow rate, however, with the cost of higher parasitic power consumption. Obviously, since the thermal losses remain practically constant (the same temperature difference to ambient and overall heat transfer coefficient remains almost constant), a higher mass flow rate translates to lower temperature difference in the air flow. The second measure to reduce temperature drop is to increase insulation thickness (see **Table 3**), which

alloys are too expensive.

*Green Energy and Environment*

**Table 1**).

the case of rectangular cross section.

however is very expensive.

**26**

are important design aspects for air-based CSP plants.

*Solar field base modules and multi-/single-tower configurations analyzed.*

This analysis shows that a temperature drop of ≈10°C is a reasonable assumption for the multi-tower concepts analyzed later on.

Last but not the least, the second issue, as mentioned above, is related to low heat transfer performance when applying air as HTF. For this reason, heat exchangers need to have a very large area of heat transfer in order to counterbalance this drawback. This is the reason why heat recovery steam generators of combined cycle plants are very bulky and represent an important share of the power block's CAPEX. The same holds for the air–air heat exchanger that is needed for the CAPTure power plant layout (see **Figure 1**). Here, the motivation is to reduce cost


receiver outlet temperature, if an efficient and cost-effective receiver design was

*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle…*

the HTF recirculation, for the case of atmospheric air and open volumetric

However, in contrast to tubular receivers as used with molten salts, for example,

receivers, is not trivial. Previous research projects [29, 30] have targeted the design of open volumetric air receivers with air recirculation, however with limited success since only about 50% of the return air could be successfully recirculated [29]. Therefore, from a thermodynamic point of view, it would be more efficient to store the low-temperature heat of the return air for later use. Also, considering advanced power cycles with elevated HTF return temperatures (see the above), it would be too detrimental for the global performance, if relatively hot air was blown back to the receiver(s), especially when considering a multi-tower concept. It is clear that the operating temperature of air blowers must be as close as possible to ambient

Therefore, the present work considers the use of the return air heat in regenerative manner, applying a low-temperature air/rock thermocline TES (see **Figure 1**). The basic TES subunit consists of three packed beds, one high-temperature and two low-temperature units. Two low-temperature TES units are required because a packed bed cannot be charged and discharged at the same time. Hence, while discharging the TES system, the first low-temperature unit preheats ambient air until the nominal power cycle HTF return temperature is reached. Then, the preheated air enters the high-temperature storage unit where the air is heated to the nominal power cycle inlet temperature. Next, the HTF enters the power cycle, delivers part of its heat to the cycle, and finally charges the second low-temperature storage unit before being rejected to the atmosphere at a temperature very close to ambient conditions. During TES charging, only the high-temperature thermocline unit is being charged, that is, once the nominal receiver outlet temperature is approached at the bottom part of the high-temperature (HT) bed (cutoff condition), the TES system is fully charged. This also means that the low-temperature (LT) TES units need to be designed with larger bed heights (at same diameter and filling material characteristics) so that the blowers always operate close to ambient temperature and no heat is lost to the ambient. When upscaling the thermal storage capacity, several of such three bed subunits (1 HT TES + 2 LT TES) need to be applied. Additionally, in order to keep the system balanced, so that always one LT TES system is empty before the storage system is being discharged, one of the LT TES units needs to be discharged by boosting/powering the bottoming Rankine cycle. Note that the (dispatchable) boost operation is preferred over the direct boost during diurnal charging as the air mass flow is constant and does not depend on current solar irradiance (receiver temperature control). Therefore, the Rankine cycle can be run at constant load. Also, in order to keep the thermocline in the LT TES system balanced, its operation needs to be switched from time to time, so that not always the same LT TES unit is being charged by the HTF return stream. Having separate HT and LT TES units allows the application of temperature-specific materials for the packed bed as well as for the internal insulation of the tank, which

available for the case of atmospheric air.

*DOI: http://dx.doi.org/10.5772/intechopen.90410*

temperature, in order to keep a reasonable density.

allows to design the LT units much more economically.

Refs. [23, 24].

**29**

The design of the packed beds would be equivalent to the system proposed in

Last but not the least, one remark needs to be given on how to operate the plant during the day. On the one hand, there is the possibility to only operate the power block during the night or only at times of high electricity prices, powering it via the TES only. In this case, direct solar operation is not implemented. On the other hand, if direct solar operation during the day is wanted in order to increase the capacity factor of the power block, two possibilities exist: (i) only the Rankine cycle is

### **Table 2.**

*Variation of mass flow—The same piping diameter and insulation.*


### **Table 3.**

*Variation of insulation thickness—The same mass flow and diameter.*

and heat exchanger size by applying a regenerative heat exchange system (atmospheric heating, pressurized cooling).

## **1.2 The high-temperature thermocline thermal energy storage system upstream the gas turbine**

The challenges of employing air as HTF are not only related to the HTF transport itself but also to the solar receiver design. Advanced power cycles are typically highly recuperative, since the average temperature of heat input to the cycle must be maintained high. This necessity implies that the HTF temperature interval in which heat is supplied to the cycle is typically small. In the case of the combined cycle, the temperature difference of the HTF is determined by the turbine inlet temperature and the compressor exit temperature (CET), which is a function of Brayton cycle pressure ratio. In the case of reheat, the HTF temperature difference is also determined by the reheat pressure level, i.e., the turbine exit temperature (TET) of the first turbine stage. Ideally, these two temperature levels (CET and TET) should be similar, in order to reduce losses when mixing the two streams (effective HTF return temperature). The effective HTF return temperature is the temperature at which the HTF leaves the power block after all parallel mass flow streams are merged (mixing temperature). At this temperature level, the HTF would then be recirculated to the solar receiver and again heated to the nominal

### *Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle… DOI: http://dx.doi.org/10.5772/intechopen.90410*

receiver outlet temperature, if an efficient and cost-effective receiver design was available for the case of atmospheric air.

However, in contrast to tubular receivers as used with molten salts, for example, the HTF recirculation, for the case of atmospheric air and open volumetric receivers, is not trivial. Previous research projects [29, 30] have targeted the design of open volumetric air receivers with air recirculation, however with limited success since only about 50% of the return air could be successfully recirculated [29]. Therefore, from a thermodynamic point of view, it would be more efficient to store the low-temperature heat of the return air for later use. Also, considering advanced power cycles with elevated HTF return temperatures (see the above), it would be too detrimental for the global performance, if relatively hot air was blown back to the receiver(s), especially when considering a multi-tower concept. It is clear that the operating temperature of air blowers must be as close as possible to ambient temperature, in order to keep a reasonable density.

Therefore, the present work considers the use of the return air heat in regenerative manner, applying a low-temperature air/rock thermocline TES (see **Figure 1**). The basic TES subunit consists of three packed beds, one high-temperature and two low-temperature units. Two low-temperature TES units are required because a packed bed cannot be charged and discharged at the same time. Hence, while discharging the TES system, the first low-temperature unit preheats ambient air until the nominal power cycle HTF return temperature is reached. Then, the preheated air enters the high-temperature storage unit where the air is heated to the nominal power cycle inlet temperature. Next, the HTF enters the power cycle, delivers part of its heat to the cycle, and finally charges the second low-temperature storage unit before being rejected to the atmosphere at a temperature very close to ambient conditions. During TES charging, only the high-temperature thermocline unit is being charged, that is, once the nominal receiver outlet temperature is approached at the bottom part of the high-temperature (HT) bed (cutoff condition), the TES system is fully charged. This also means that the low-temperature (LT) TES units need to be designed with larger bed heights (at same diameter and filling material characteristics) so that the blowers always operate close to ambient temperature and no heat is lost to the ambient. When upscaling the thermal storage capacity, several of such three bed subunits (1 HT TES + 2 LT TES) need to be applied. Additionally, in order to keep the system balanced, so that always one LT TES system is empty before the storage system is being discharged, one of the LT TES units needs to be discharged by boosting/powering the bottoming Rankine cycle. Note that the (dispatchable) boost operation is preferred over the direct boost during diurnal charging as the air mass flow is constant and does not depend on current solar irradiance (receiver temperature control). Therefore, the Rankine cycle can be run at constant load. Also, in order to keep the thermocline in the LT TES system balanced, its operation needs to be switched from time to time, so that not always the same LT TES unit is being charged by the HTF return stream. Having separate HT and LT TES units allows the application of temperature-specific materials for the packed bed as well as for the internal insulation of the tank, which allows to design the LT units much more economically.

The design of the packed beds would be equivalent to the system proposed in Refs. [23, 24].

Last but not the least, one remark needs to be given on how to operate the plant during the day. On the one hand, there is the possibility to only operate the power block during the night or only at times of high electricity prices, powering it via the TES only. In this case, direct solar operation is not implemented. On the other hand, if direct solar operation during the day is wanted in order to increase the capacity factor of the power block, two possibilities exist: (i) only the Rankine cycle is

and heat exchanger size by applying a regenerative heat exchange system

**1.2 The high-temperature thermocline thermal energy storage system**

The challenges of employing air as HTF are not only related to the HTF transport

**Case 1 Case 2 Case 3 Case 4 Case 5 Case 6**

K) 12.5 13 14 15 16 18

**Case 1 Case 2a Case 3a Case 4a Case 5a**

Roughness (m) 0.001 0.001 0.001 0.001 0.001 0.001 Inner diameter (m) 3 3 3 3 3 3 Insulation thickness (m) 0.6 0.6 0.6 0.6 0.6 0.6 Air flow (kg/s) 36 39 42.4 46 49.5 56.6 Inlet temperature (°C) 1000 1000 1000 1000 1000 1000 Outlet temperature (°C) 988 989 990 991 992 992.5 Pressure drop (kPa) 0.22 0.25 0.3 0.35 0.4 0.52 Velocity (m/s) 19 20 22 24 26 29

Roughness (m) 0.001 0.001 0.001 0.001 0.001 Inner diameter (m) 3 3 3 3 3 Insulation thickness (m) 0.6 0.8 1 1.2 1.4 Air flow (kg/s) 36 36 36 36 36 Pressure drop (kPa) 0.22 0.22 0.22 0.22 0.22 Inlet temperature (°C) 1000 1000 1000 1000 1000 Outlet temperature (°C) 988 990.5 992 992.7 993

itself but also to the solar receiver design. Advanced power cycles are typically highly recuperative, since the average temperature of heat input to the cycle must be maintained high. This necessity implies that the HTF temperature interval in which heat is supplied to the cycle is typically small. In the case of the combined cycle, the temperature difference of the HTF is determined by the turbine inlet temperature and the compressor exit temperature (CET), which is a function of Brayton cycle pressure ratio. In the case of reheat, the HTF temperature difference is also determined by the reheat pressure level, i.e., the turbine exit temperature (TET) of the first turbine stage. Ideally, these two temperature levels (CET and TET) should be similar, in order to reduce losses when mixing the two streams (effective HTF return temperature). The effective HTF return temperature is the temperature at which the HTF leaves the power block after all parallel mass flow streams are merged (mixing temperature). At this temperature level, the HTF would then be recirculated to the solar receiver and again heated to the nominal

(atmospheric heating, pressurized cooling).

*Variation of insulation thickness—The same mass flow and diameter.*

Inner wall heat transfer coefficient (W/m2

*Green Energy and Environment*

*Variation of mass flow—The same piping diameter and insulation.*

**Table 2.**

**Table 3.**

**28**

**upstream the gas turbine**

### *Green Energy and Environment*

operated during the day, which eliminates the need for a regenerative LT TES during the day, and also allows a reduced receiver operating temperature during the day, once the HT TES is full; (ii) the combined cycle is operated during the day, channeling the HTF return stream to the bottoming Rankine cycle, which significantly increases Rankine cycle output (i.e., its nominal power) and leads to reduced overall conversion efficiency, since it is not a true combined cycle. The operating principle of choice will depend on the specific electricity market.

iii. Having higher pressure ratios in the first turbine stage means lower TET at the first stage and thus corresponds to a lower return temperature of the TES medium (thus higher ΔT for the TES). Thus, higher pressure ratios in the first turbine stage are not only preferred in terms of Rankine cycle performance (see point (i)) but also regarding integration with thermal energy storage

*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle…*

In summary, a reheated Brayton cycle is absolutely interesting for the application of CSP as it allows fair conversion efficiencies despite low TITs. Reheat may increase solar-to-electric performance by up to about 2.7 percentage points [10]. Another argument for reheat is that the optimum cycle pressure ratio is much higher than in the case of a simple Brayton cycle. This means that the compressor exit temperature is higher, which allows for a higher effective HTF return temperature (low-temperature TES inlet temperature) and thus a more efficient boost of the bottoming Rankine cycle (see **Figure 1** dashed lines indicating periodical

Basically, there are two options for multi-tower arrangements, (i) a simple multiple placement of identical solar field tower-receiver units, where each heliostat field concentrates solar radiation onto its corresponding receiver (tower) only or (ii) a special arrangement of multiple towers and heliostat fields, where heliostats of one field may point on different receivers (towers) as function of current solar position in order to optimize the overall optical efficiency. We propose to refer to the latter option "multi-tower multi-aiming" configuration, and to the first option "multi-tower assigned-aiming" configuration. The "multi-tower assigned-aiming"

Multi-tower arrangements with compact heliostat fields have significant advantages regarding solar field efficiency, atmospheric attenuation, and solar flux control at the receivers. This is because heliostats placed closer to the tower have higher optical efficiencies than those placed at the peripheral areas of the field [33]. Longer slant ranges, which may reach 1.5 km and more at large-capacity single-tower concepts [34, 35], already cause losses of up to 10% [36] purely considering atmospheric attenuation losses, not to mention spillage losses and increasingly challenging solar flux control. Thus, when upscaling a central receiver plant, there is a point where the heliostat field becomes simply too large. For this reason, a multi-tower approach is a promising way when going for very high capacity power tower plants, where Rankine cycle power blocks become more efficient and also cheaper per

Additionally, the ongoing transformation from centralized conventional power generation to decentralized power supply with a high share of renewables calls for smaller modular units that can be easily adapted to the specific power demand. The general trend is expected to go towards more but lower capacity power generation units with lower capital risk and lower amount of initial investment [37]. However, so far in the case of CSP, current cost reduction trends are mainly driven by the increase of the nominal size of the main components (especially the power block) in order to reduce the cost of electricity production, as specific power cycle costs (\$/ kWe) significantly reduce for large power ratings. This trend would change if we paid attention to the needs of the consumers and the changing, more and more decentralized electricity grid. Also, a modular design may reduce the perceived risk

of the technology, giving the technology a better access to financing.

(second heat exchanger train).

*DOI: http://dx.doi.org/10.5772/intechopen.90410*

configuration is considered in this work.

installed MW.

**31**

Rankine cycle boost in order to discharge the LT TES).

**1.4 Motivations for multi-tower CSP plant arrangements**

## **1.3 The motivation for a reheated Brayton cycle and its application in the context of combined cycle power generation**

As it is well known according to the principles of thermodynamics, reheat increases the average temperature of heat supply and thus increases the conversion efficiency from heat to mechanical work (Carnot). However, since a second heat input adds additional pressure losses, higher compressor pressure ratios are required to offset the performance penalty. This requirement is even more relevant for a solar-powered thus an externally heated Brayton cycle. Reheated gas turbines have been already treated in previous works. The main motivations are (i) to keep the average temperature of heat supply high and (ii) to introduce an additional flexibility regarding turbine exit temperature (the heat recovery steam generator inlet temperature), despite high compressor pressure ratios [13, 31]. In particular, the expansion ratio of the second turbine stage can be specifically designed, so that the resulting TET optimizes the overall combined cycle performance. Note that the higher the heat recovery steam generator's inlet temperature is, the higher is the conversion efficiency of the bottoming cycle and vice versa. As proposed by Siros and Fernández-Campos [12], the reheat pressure level will be defined by a dimensionless parameter *K* (reheat ratio), which determines the ratio of pressure ratios of both turbine stages:

$$K = \frac{\text{pressure ratio of first stage}}{\text{pressure ratio of second stage}} = \frac{\frac{p\_{t1}}{p\_{t1}}}{\frac{p\_{t2}}{p\_{t2}}} = \frac{p\_{t1}}{p\_{t1}} \cdot \frac{p\_{t2}}{p\_{t2}} \tag{1}$$

Here, three considerations must be kept in mind:


*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle… DOI: http://dx.doi.org/10.5772/intechopen.90410*

iii. Having higher pressure ratios in the first turbine stage means lower TET at the first stage and thus corresponds to a lower return temperature of the TES medium (thus higher ΔT for the TES). Thus, higher pressure ratios in the first turbine stage are not only preferred in terms of Rankine cycle performance (see point (i)) but also regarding integration with thermal energy storage (second heat exchanger train).

In summary, a reheated Brayton cycle is absolutely interesting for the application of CSP as it allows fair conversion efficiencies despite low TITs. Reheat may increase solar-to-electric performance by up to about 2.7 percentage points [10]. Another argument for reheat is that the optimum cycle pressure ratio is much higher than in the case of a simple Brayton cycle. This means that the compressor exit temperature is higher, which allows for a higher effective HTF return temperature (low-temperature TES inlet temperature) and thus a more efficient boost of the bottoming Rankine cycle (see **Figure 1** dashed lines indicating periodical Rankine cycle boost in order to discharge the LT TES).

### **1.4 Motivations for multi-tower CSP plant arrangements**

Basically, there are two options for multi-tower arrangements, (i) a simple multiple placement of identical solar field tower-receiver units, where each heliostat field concentrates solar radiation onto its corresponding receiver (tower) only or (ii) a special arrangement of multiple towers and heliostat fields, where heliostats of one field may point on different receivers (towers) as function of current solar position in order to optimize the overall optical efficiency. We propose to refer to the latter option "multi-tower multi-aiming" configuration, and to the first option "multi-tower assigned-aiming" configuration. The "multi-tower assigned-aiming" configuration is considered in this work.

Multi-tower arrangements with compact heliostat fields have significant advantages regarding solar field efficiency, atmospheric attenuation, and solar flux control at the receivers. This is because heliostats placed closer to the tower have higher optical efficiencies than those placed at the peripheral areas of the field [33]. Longer slant ranges, which may reach 1.5 km and more at large-capacity single-tower concepts [34, 35], already cause losses of up to 10% [36] purely considering atmospheric attenuation losses, not to mention spillage losses and increasingly challenging solar flux control. Thus, when upscaling a central receiver plant, there is a point where the heliostat field becomes simply too large. For this reason, a multi-tower approach is a promising way when going for very high capacity power tower plants, where Rankine cycle power blocks become more efficient and also cheaper per installed MW.

Additionally, the ongoing transformation from centralized conventional power generation to decentralized power supply with a high share of renewables calls for smaller modular units that can be easily adapted to the specific power demand. The general trend is expected to go towards more but lower capacity power generation units with lower capital risk and lower amount of initial investment [37]. However, so far in the case of CSP, current cost reduction trends are mainly driven by the increase of the nominal size of the main components (especially the power block) in order to reduce the cost of electricity production, as specific power cycle costs (\$/ kWe) significantly reduce for large power ratings. This trend would change if we paid attention to the needs of the consumers and the changing, more and more decentralized electricity grid. Also, a modular design may reduce the perceived risk of the technology, giving the technology a better access to financing.

operated during the day, which eliminates the need for a regenerative LT TES during the day, and also allows a reduced receiver operating temperature during the day, once the HT TES is full; (ii) the combined cycle is operated during the day, channeling the HTF return stream to the bottoming Rankine cycle, which significantly increases Rankine cycle output (i.e., its nominal power) and leads to reduced overall conversion efficiency, since it is not a true combined cycle. The operating

**1.3 The motivation for a reheated Brayton cycle and its application in the**

As it is well known according to the principles of thermodynamics, reheat increases the average temperature of heat supply and thus increases the conversion efficiency from heat to mechanical work (Carnot). However, since a second heat input adds additional pressure losses, higher compressor pressure ratios are

required to offset the performance penalty. This requirement is even more relevant for a solar-powered thus an externally heated Brayton cycle. Reheated gas turbines have been already treated in previous works. The main motivations are (i) to keep the average temperature of heat supply high and (ii) to introduce an additional flexibility regarding turbine exit temperature (the heat recovery steam generator inlet temperature), despite high compressor pressure ratios [13, 31]. In particular, the expansion ratio of the second turbine stage can be specifically designed, so that the resulting TET optimizes the overall combined cycle performance. Note that the higher the heat recovery steam generator's inlet temperature is, the higher is the conversion efficiency of the bottoming cycle and vice versa. As proposed by Siros and Fernández-Campos [12], the reheat pressure level will be defined by a dimensionless parameter *K* (reheat ratio), which determines the ratio of pressure ratios of

i. The reheat ratio *K* is a key parameter concerning Brayton cycle performance (on its own) as well as combined cycle performance; nevertheless, it has different optimums for the single cycle and the combined cycle. The lower the pressure ratio of the second turbine stage is, the higher the turbine exit temperature, i.e., HRSG inlet temperature, and thus the higher the efficiency of the bottoming Rankine cycle, but the lower the Brayton cycle performance. As shown in Ref. [10], solar combined cycle performance optimizes in the interval of 0.5 < *K* < 1.25. The optimum value of *K* depends on concentration ratio, the corresponding optimum TIT, and HRSG efficiency. Note that Brayton single-cycle performance optimizes for values of *K* lower than in the

ii. Furthermore, the lower the pressure ratio of the second turbine stage is, the lower is the reheat pressure level and thus the bulkier and more expensive the second HTF-to-working-fluid heat exchanger will be. And the pressure drop would increase. Thus, there is clearly a lower practical limit for the second

*pt*<sup>1</sup> *<sup>i</sup> pt*<sup>1</sup> *<sup>o</sup> pt*<sup>2</sup> *<sup>i</sup> pt*<sup>2</sup> *<sup>o</sup>*

<sup>¼</sup> *pt*<sup>1</sup> *<sup>i</sup> pt*<sup>1</sup> *<sup>o</sup>* � *pt*<sup>2</sup> *<sup>o</sup> pt*<sup>2</sup> *<sup>i</sup>*

(1)

principle of choice will depend on the specific electricity market.

*<sup>K</sup>* <sup>¼</sup> *pressure ratio of first stage pressure ratio of second stage* ¼

Here, three considerations must be kept in mind:

case of combined cycle (see Ref. [32]).

turbine stage's pressure ratio.

**30**

**context of combined cycle power generation**

*Green Energy and Environment*

both turbine stages:

The principal problem in this context is that there is no power cycle available so far that is also cost-effective and efficient in smaller power classes. Typically, specific costs (\$/kWe) of gas turbines and Rankine cycles increase significantly for small power classes, and conversion efficiencies also decrease. If there was a cheap and efficient power cycle available in the power class below or around 10 MWe, solar power towers would be very compact plants, as the optical efficiency and consequently the solar-to-thermal efficiency are best for small solar fields. The only way in order to combine (i) good solar-to-thermal efficiency and (ii) an efficient power cycle (i.e., to maximize solar-to-electric energy conversion) is the application of multi-tower power plant concepts.

**2.2 Gas turbine costs**

*DOI: http://dx.doi.org/10.5772/intechopen.90410*

as follows:

**33**

The costs of the turbo machinery (compressor, turbine and turbo generator) are estimated as function of electric power output based on cost figures available in the open literature. Here it is important to capture the cost dependency on turbomachinery size, as smaller engines have higher specific costs (USD/kWe) than larger ones. Gas turbine costs are issued on a yearly basis by Gas Turbine World [40]. They propose a best fit curve, mentioning a � 10% accuracy for gas turbine ratings ranging from 1 to 500 MWe. The investment cost of the turbo machinery *ICGT* in USD (2018) is given as function of electric output power *Pe GT* in kW in Eq. (2):

*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle…*

*eGT* � � � *PeGT* <sup>¼</sup> <sup>9650</sup> � *<sup>P</sup>*0*:*<sup>7</sup>

In order to obtain a reliable cost relationship for the heat recovery steam generator, the cost correlations published by Roosen et al. [41] and Silveira and Tuna [42] have been compared and agree very well once inflation-adjusted. Based on the correlation presented in Ref. [42], the following cost equation has been developed (in USD 2018), which only depends on heat duty *Q*\_ (kW), air inlet temperature

*ln T*ð Þ *air in* � *Tair out* !<sup>0</sup>*:*<sup>8</sup>

Also steam turbine cost relationships published by Roosen et al. [41] and Silveira and Tuna [42] agree very well and, once inflation-adjusted, are also consistent with recent quotes requested by the authors. The adapted correlation (from Ref. [42]) is

*ICST* <sup>¼</sup> <sup>8220</sup> � *<sup>P</sup>*<sup>0</sup>*:*<sup>7</sup>

Heliostat field costs are taken from Pfahl et al. [44], assuming 75 USD/m2

cost figure seems to be a realistic engineering target for heliostat designs that are optimized for mass production. The total heliostat field investment cost is obtained

For the optimization process of the proposed multi-tower plant concept, smallto medium-sized fields are interesting, having tower heights in the range between 50 and 150 m. These tower heights are typical for wind turbines, and cost estimates for wind turbine towers should be applicable for solar power towers too, however considering larger tower diameters depending on needed piping diameter and receiver aperture size. Possible construction types for solar thermal power towers

According to recent quotes, the cost estimate given by Eq. (4) may also include remaining Rankine cycle components, such as dry air-cooled condenser, feed water pumps, and deaerator. Finally, the above presented cost relationships have addi-

*eGT* (2)

3

5 (3)

. This

þ 2195 � *m*\_ *air*

*eST* (4)

*ICGT* <sup>¼</sup> <sup>9650</sup> � *<sup>P</sup>*�0*:*<sup>3</sup>

*Tair in*, stack temperature *Tair out*, and air mass flow *m*\_ *air* (kg/s):

**2.4 Steam turbine and remaining Rankine cycle component costs**

tionally been checked for consistency against Refs. [39, 43].

are either concrete type or metallic lattice type.

**2.5 Heliostat field, tower, solar receiver, TES, and piping costs**

by multiplying the specific cost by the total solar field reflective area.

**2.3 Heat recovery steam generator (HRSG) costs**

*ICHRSG* <sup>¼</sup> <sup>1</sup>*:*<sup>37</sup> � <sup>4745</sup> � *<sup>Q</sup>*\_

2 4

### **2. Cost review of power plant components**

In order to perform a serious techno-economic study, the fundamental step is to collect realistic cost estimates for all power plant components. Therefore, a detailed literature search has been conducted collecting available cost data and also comparing them in order to guarantee consistency. All cost data given in this section has been converted into USD 2018 (inflation-adjusted).

### **2.1 High-temperature heat exchanger costs for powering the topping Brayton cycle externally**

The most critical component of the proposed power plant concept (**Figure 1**) is the needed high-temperature gas–gas heat exchanger in order to power the topping Brayton cycle externally. As the coefficient of heat transfer on the atmospheric air side is very limited, the design is expected to be very bulky, since a large area of heat transfer is needed. In principal, a shell-and-tube heat exchanger design [11] is expected, having the pressurized air stream coming from the Brayton cycle's compressor on the tube side and the heating air stream at ambient pressure (coming from the TES) on the shell side. This type of heat exchanger will be similar to a heat recovery steam generator. Alternatively, and subject of this work, a regenerative heat exchange system working under atmospheric charging and pressurized discharging conditions can be applied [38] (see **Figure 2**), providing better heat exchange effectiveness. Clearly, the vessel size of this regenerative heat exchange system is limited due to the pressurization process, which requires several twovessel subunits (such as shown in **Figure 2**) in parallel depending on the power rating. The second reason for several two-vessel subunits in parallel is the requirement for continuous thermal power transfer (while one system is pressurized/ depressurized, the parallel systems need to take over). Thus, one disadvantage with respect to conventional heat exchangers is the higher complexity, as besides several parallel systems, high-temperature valves and piping are required for managing the pressurization/depressurization process. Furthermore, the pressurization process requires a certain amount of work, i.e., represents an additional parasitic consumption. This disadvantage needs to be offset by higher heat exchange effectiveness and reduced heat exchanger size (with respect to the conventional shell-and-tube layout). It is clear that this innovative regenerative system must have costs that are in the same order of magnitude as conventional heat exchanger designs in order to remain cost competitive. Here, cost figures published by Ilett and Lawn [39] are used. Calculating the cost difference between conventional combined cycle plants and externally fired ones results in a specific heat exchanger cost target of 64 kUSD per kg/s of air flow (topping Brayton cycle compressor air flow).

*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle… DOI: http://dx.doi.org/10.5772/intechopen.90410*

### **2.2 Gas turbine costs**

The principal problem in this context is that there is no power cycle available so

In order to perform a serious techno-economic study, the fundamental step is to collect realistic cost estimates for all power plant components. Therefore, a detailed literature search has been conducted collecting available cost data and also comparing them in order to guarantee consistency. All cost data given in this section has

The most critical component of the proposed power plant concept (**Figure 1**) is the needed high-temperature gas–gas heat exchanger in order to power the topping Brayton cycle externally. As the coefficient of heat transfer on the atmospheric air side is very limited, the design is expected to be very bulky, since a large area of heat transfer is needed. In principal, a shell-and-tube heat exchanger design [11] is expected, having the pressurized air stream coming from the Brayton cycle's compressor on the tube side and the heating air stream at ambient pressure (coming from the TES) on the shell side. This type of heat exchanger will be similar to a heat recovery steam generator. Alternatively, and subject of this work, a regenerative heat exchange system working under atmospheric charging and pressurized discharging conditions can be applied [38] (see **Figure 2**), providing better heat exchange effectiveness. Clearly, the vessel size of this regenerative heat exchange system is limited due to the pressurization process, which requires several twovessel subunits (such as shown in **Figure 2**) in parallel depending on the power rating. The second reason for several two-vessel subunits in parallel is the requirement for continuous thermal power transfer (while one system is pressurized/ depressurized, the parallel systems need to take over). Thus, one disadvantage with respect to conventional heat exchangers is the higher complexity, as besides several parallel systems, high-temperature valves and piping are required for managing the pressurization/depressurization process. Furthermore, the pressurization process requires a certain amount of work, i.e., represents an additional parasitic consumption. This disadvantage needs to be offset by higher heat exchange effectiveness and reduced heat exchanger size (with respect to the conventional shell-and-tube layout). It is clear that this innovative regenerative system must have costs that are in the same order of magnitude as conventional heat exchanger designs in order to remain cost competitive. Here, cost figures published by Ilett and Lawn [39] are used. Calculating the cost difference between conventional combined cycle plants and externally fired ones results in a specific heat exchanger cost target of 64 kUSD

**2.1 High-temperature heat exchanger costs for powering the topping**

per kg/s of air flow (topping Brayton cycle compressor air flow).

**32**

far that is also cost-effective and efficient in smaller power classes. Typically, specific costs (\$/kWe) of gas turbines and Rankine cycles increase significantly for small power classes, and conversion efficiencies also decrease. If there was a cheap and efficient power cycle available in the power class below or around 10 MWe, solar power towers would be very compact plants, as the optical efficiency and consequently the solar-to-thermal efficiency are best for small solar fields. The only way in order to combine (i) good solar-to-thermal efficiency and (ii) an efficient power cycle (i.e., to maximize solar-to-electric energy conversion) is the application

of multi-tower power plant concepts.

*Green Energy and Environment*

**Brayton cycle externally**

**2. Cost review of power plant components**

been converted into USD 2018 (inflation-adjusted).

The costs of the turbo machinery (compressor, turbine and turbo generator) are estimated as function of electric power output based on cost figures available in the open literature. Here it is important to capture the cost dependency on turbomachinery size, as smaller engines have higher specific costs (USD/kWe) than larger ones. Gas turbine costs are issued on a yearly basis by Gas Turbine World [40]. They propose a best fit curve, mentioning a � 10% accuracy for gas turbine ratings ranging from 1 to 500 MWe. The investment cost of the turbo machinery *ICGT* in USD (2018) is given as function of electric output power *Pe GT* in kW in Eq. (2):

$$IC\_{GT} = \left(\mathfrak{G}\mathfrak{G}\mathfrak{D} \cdot P\_{\mathfrak{e}GT}^{-0.3}\right) \cdot P\_{\mathfrak{e}GT} = \mathfrak{G}\mathfrak{G}\mathfrak{D} \cdot P\_{\mathfrak{e}GT}^{0.7} \tag{2}$$

### **2.3 Heat recovery steam generator (HRSG) costs**

In order to obtain a reliable cost relationship for the heat recovery steam generator, the cost correlations published by Roosen et al. [41] and Silveira and Tuna [42] have been compared and agree very well once inflation-adjusted. Based on the correlation presented in Ref. [42], the following cost equation has been developed (in USD 2018), which only depends on heat duty *Q*\_ (kW), air inlet temperature *Tair in*, stack temperature *Tair out*, and air mass flow *m*\_ *air* (kg/s):

$$\text{IC}\_{\text{HRSG}} = \mathbf{1.37} \cdot \left[ \mathbf{4745} \cdot \left( \frac{\dot{Q}}{\ln \left( T\_{air \text{ in}} - T\_{air \text{ out}} \right)} \right)^{0.8} + \mathbf{2195} \cdot \dot{m}\_{air} \right] \tag{3}$$

### **2.4 Steam turbine and remaining Rankine cycle component costs**

Also steam turbine cost relationships published by Roosen et al. [41] and Silveira and Tuna [42] agree very well and, once inflation-adjusted, are also consistent with recent quotes requested by the authors. The adapted correlation (from Ref. [42]) is as follows:

$$IC\_{ST} = \textbf{8220} \cdot P\_{eST}^{0.7} \tag{4}$$

According to recent quotes, the cost estimate given by Eq. (4) may also include remaining Rankine cycle components, such as dry air-cooled condenser, feed water pumps, and deaerator. Finally, the above presented cost relationships have additionally been checked for consistency against Refs. [39, 43].

### **2.5 Heliostat field, tower, solar receiver, TES, and piping costs**

Heliostat field costs are taken from Pfahl et al. [44], assuming 75 USD/m2 . This cost figure seems to be a realistic engineering target for heliostat designs that are optimized for mass production. The total heliostat field investment cost is obtained by multiplying the specific cost by the total solar field reflective area.

For the optimization process of the proposed multi-tower plant concept, smallto medium-sized fields are interesting, having tower heights in the range between 50 and 150 m. These tower heights are typical for wind turbines, and cost estimates for wind turbine towers should be applicable for solar power towers too, however considering larger tower diameters depending on needed piping diameter and receiver aperture size. Possible construction types for solar thermal power towers are either concrete type or metallic lattice type.

### *Green Energy and Environment*

The following tower cost correlation has been established based on data given in Ref. [45] as function of tower height *htower* (m). The result, *ICtower*, is the complete investment cost of the tower construction plus foundations and transport in M USD (2018), taking into account larger diameter towers providing enough space for the needed hot air piping as well as the receiver. Note that the valid height range for Eq. (5) is from 50 to 200 m:

$$\text{IC}\_{\text{tower}} = \text{1.50227} - \text{0.00879597} \cdot h\_{\text{tower}} + \text{0.000189709} \cdot h\_{\text{tower}}^2 \tag{5}$$

year, i.e., their net present value (NPV) is taken into account [47]. Thus, according to Ref. [47], the LCOE can be calculated as given by Eq. (6). Note that *Cn* is the incurred cost in period n (engineering, construction, operation, maintenance, cost of capital), *Qn* is the energy output in year *n*, *d* is the discount rate, and *N* is the

*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle…*

P*<sup>N</sup> n*¼0

P*<sup>N</sup> n*¼1

Also note that the applied discount rate should be the "real" discount rate, taking into account the inflation rate. A real discount rate of 3% is used in this work. The cost of capital for financing a CSP project is assumed to be 5% p.a. Power plant operating time is assumed to be 30 years (SEGS plants in the USA are in operation

The power plant performance modeling is done as outlined in Ref. [10]. In particular, the solar receiver performance is estimated according to Ref. [49], using the detailed 1-D model to establish a receiver performance table as function of receiver operating temperature and incident solar flux. The topping Brayton cycle is modeled applying the isentropic relationships for air as ideal gas and choosing power classdependent isentropic efficiencies. The bottoming Rankine cycle performance has been estimated applying state-of-the-art power cycle simulation software [43] and generating performance tables as function of HRSG inlet temperature and ambient temperature [10], suitable for annual yield simulations. The annual plant performance parameters (i.e., electricity yield, annual solar-to-electric efficiency) have been obtained running annual energy yield simulations using a typical meteorological year for Seville, Spain. The operating strategy is chosen such that the power block always operates under rated conditions (corresponding TES system charging/

*Cn* ð Þ <sup>1</sup>þ*<sup>d</sup> <sup>n</sup>*

*Qn* ð Þ <sup>1</sup>þ*<sup>d</sup> <sup>n</sup>* (6)

*LCOE* ¼

total analysis period in years (power plant lifetime):

*DOI: http://dx.doi.org/10.5772/intechopen.90410*

**3. Power plant performance modeling**

discharging) apart from start-up and shutdown periods.

cept specifications and optimization."

**3.1 Turbo machinery isentropic efficiencies as function of electric power**

As commonly known, the efficiency of turbomachinery is a function of power rating, i.e., the higher the output power, the higher is also the efficiency. Conversely, smaller engines have lower efficiencies. This is principally due to sizespecific impacts of aerodynamic losses. For example, the turbine blade tip clearance (i.e., the radial distance between the blade tip of an axial compressor or turbine and the containment structure) is a major contributing factor to gas path sealing and can significantly affect engine efficiency [50]. The tip-leakage flow contributes negatively to the turbine performance and accounts for approximately one third of the total aerodynamic loss [51]. The bigger the engine, the smaller is the tip clearance with respect to the overall blade length and thus the higher is the efficiency.

It is clear that the size-dependent relationship of the turbomachinery's efficiency needs to be taken into account in the techno-economic optimization. In order to do so, relationships and performance tables have been established that consider efficiency as a function of output power, for both the topping Brayton cycle and for the bottoming Rankine cycle. For detailed information, the interested reader is referred to the corresponding public CAPTure project [38] deliverable D1.4 "CAPTure con-

since the 1980s).

**output**

**35**

The cost estimate of the solar receiver has been based on the CAPTure receiver prototype (≈ 300 kWth) costs, taking into account possible cost reductions when manufacturing the receiver up-scaled and in higher numbers commercially. The costs have been calculated per square meter of aperture area and result in 30 kUSD/ m<sup>2</sup> for receiver aperture areas below 130 m<sup>2</sup> , 50 kUSD/m<sup>2</sup> for receiver aperture areas until 400 m<sup>2</sup> , and 100 kUSD/m<sup>2</sup> for bigger receiver aperture areas. The total aperture size of the receiver is an important factor as the receiver base structure (metallic + insulation) that supports modular ceramic absorber structures (cups + foams) as well as the air duct system becomes more complex and expensive the bigger the receiver is.

The costs of air/rock or air/ceramic thermocline packed-bed thermal energy storage can be assumed to be 20 USD/kWhth [25]. However, in the case of the specific plant arrangement shown in **Figure 1**, the TES costs are higher since two low-temperature TES units are required for each high-temperature TES unit (regenerative use of return air heat). This approach is assumed to double the specific cost per kWhth, resulting in 40 USD/kWhth for the combined cycle option, only. Note that in the case of the Rankine single-cycle (benchmarking) configuration, the low-temperature TES units are not needed and the lower cost assumption applies.

The cost of internally insulated high-temperature piping has been assumed to be 800 USD per meter piping and per square meter flow cross section. It must be noted that the air speed in the air piping system must be kept reasonably low (≈ 20 m/s) in order to achieve acceptable pressure drop and thus blower power consumption.

The cost of blowers for circulation of air in the atmospheric circuit (blowers operate at ambient temperature) is assumed to be 3 kUSD per air volume flow (m<sup>3</sup> /s). This cost assumption is based on several quotes requested by the authors.

Last but not the least, the yearly operations and maintenance (O&M) costs are assumed to be 1.5% of the total plant investment cost [3].

### **2.6 Levelized cost of electricity (LCOE) calculation**

In the literature, the LCOE is an established figure for evaluating purely the economic lifetime energy production and its related lifetime costs, without taking into account revenues [46]. As revenues, i.e., feed-in tariffs, depend strongly on the country, the LCEO is therefore a relatively market-neutral figure and also allows to compare alternative technologies with different scales of investment or operating time [47]. Nevertheless, the LCOE depends on country-dependent parameters such as available solar resource, capital cost, and O&M costs, which must be taken into account in a serious technology benchmarking process. The general understanding of the LCOE in the literature [46, 48] is the total lifetime cost of the plant (engineering + construction + operation + maintenance + capital costs) divided by the lifetime electricity generation (total electric energy produced). Its unit is therefore cost per energy, i.e., USD/kWh. A particular point in the definition of the LCOE is that all costs incurred during the project lifetime are discounted back to the base

*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle… DOI: http://dx.doi.org/10.5772/intechopen.90410*

year, i.e., their net present value (NPV) is taken into account [47]. Thus, according to Ref. [47], the LCOE can be calculated as given by Eq. (6). Note that *Cn* is the incurred cost in period n (engineering, construction, operation, maintenance, cost of capital), *Qn* is the energy output in year *n*, *d* is the discount rate, and *N* is the total analysis period in years (power plant lifetime):

$$LCOE = \frac{\sum\_{n=0}^{N} \frac{C\_n}{\left(1+d\right)^n}}{\sum\_{n=1}^{N} \frac{Q\_n}{\left(1+d\right)^n}}\tag{6}$$

Also note that the applied discount rate should be the "real" discount rate, taking into account the inflation rate. A real discount rate of 3% is used in this work. The cost of capital for financing a CSP project is assumed to be 5% p.a. Power plant operating time is assumed to be 30 years (SEGS plants in the USA are in operation since the 1980s).

## **3. Power plant performance modeling**

The following tower cost correlation has been established based on data given in Ref. [45] as function of tower height *htower* (m). The result, *ICtower*, is the complete investment cost of the tower construction plus foundations and transport in M USD (2018), taking into account larger diameter towers providing enough space for the needed hot air piping as well as the receiver. Note that the valid height range for

*ICtower* <sup>¼</sup> <sup>1</sup>*:*<sup>50227</sup> � <sup>0</sup>*:*<sup>00879597</sup> � *htower* <sup>þ</sup> <sup>0</sup>*:*<sup>000189709</sup> � *<sup>h</sup>*<sup>2</sup>

aperture size of the receiver is an important factor as the receiver base structure (metallic + insulation) that supports modular ceramic absorber structures (cups + foams) as well as the air duct system becomes more complex and expensive the

The costs of air/rock or air/ceramic thermocline packed-bed thermal energy storage can be assumed to be 20 USD/kWhth [25]. However, in the case of the specific plant arrangement shown in **Figure 1**, the TES costs are higher since two low-temperature TES units are required for each high-temperature TES unit (regenerative use of return air heat). This approach is assumed to double the specific cost per kWhth, resulting in 40 USD/kWhth for the combined cycle option, only. Note that in the case of the Rankine single-cycle (benchmarking) configuration, the low-temperature TES units are not needed and the lower cost assumption

The cost of internally insulated high-temperature piping has been assumed to be 800 USD per meter piping and per square meter flow cross section. It must be noted that the air speed in the air piping system must be kept reasonably low (≈ 20 m/s) in order to achieve acceptable pressure drop and thus blower power consumption. The cost of blowers for circulation of air in the atmospheric circuit (blowers operate at ambient temperature) is assumed to be 3 kUSD per air volume flow

/s). This cost assumption is based on several quotes requested by the authors. Last but not the least, the yearly operations and maintenance (O&M) costs are

In the literature, the LCOE is an established figure for evaluating purely the economic lifetime energy production and its related lifetime costs, without taking into account revenues [46]. As revenues, i.e., feed-in tariffs, depend strongly on the country, the LCEO is therefore a relatively market-neutral figure and also allows to compare alternative technologies with different scales of investment or operating time [47]. Nevertheless, the LCOE depends on country-dependent parameters such as available solar resource, capital cost, and O&M costs, which must be taken into account in a serious technology benchmarking process. The general understanding of the LCOE in the literature [46, 48] is the total lifetime cost of the plant (engineering + construction + operation + maintenance + capital costs) divided by the lifetime electricity generation (total electric energy produced). Its unit is therefore cost per energy, i.e., USD/kWh. A particular point in the definition of the LCOE is that all costs incurred during the project lifetime are discounted back to the base

assumed to be 1.5% of the total plant investment cost [3].

**2.6 Levelized cost of electricity (LCOE) calculation**

The cost estimate of the solar receiver has been based on the CAPTure receiver prototype (≈ 300 kWth) costs, taking into account possible cost reductions when manufacturing the receiver up-scaled and in higher numbers commercially. The costs have been calculated per square meter of aperture area and result in 30 kUSD/

, and 100 kUSD/m<sup>2</sup> for bigger receiver aperture areas. The total

*tower* (5)

, 50 kUSD/m<sup>2</sup> for receiver aperture

Eq. (5) is from 50 to 200 m:

*Green Energy and Environment*

areas until 400 m<sup>2</sup>

bigger the receiver is.

applies.

(m<sup>3</sup>

**34**

m<sup>2</sup> for receiver aperture areas below 130 m<sup>2</sup>

The power plant performance modeling is done as outlined in Ref. [10]. In particular, the solar receiver performance is estimated according to Ref. [49], using the detailed 1-D model to establish a receiver performance table as function of receiver operating temperature and incident solar flux. The topping Brayton cycle is modeled applying the isentropic relationships for air as ideal gas and choosing power classdependent isentropic efficiencies. The bottoming Rankine cycle performance has been estimated applying state-of-the-art power cycle simulation software [43] and generating performance tables as function of HRSG inlet temperature and ambient temperature [10], suitable for annual yield simulations. The annual plant performance parameters (i.e., electricity yield, annual solar-to-electric efficiency) have been obtained running annual energy yield simulations using a typical meteorological year for Seville, Spain. The operating strategy is chosen such that the power block always operates under rated conditions (corresponding TES system charging/ discharging) apart from start-up and shutdown periods.

### **3.1 Turbo machinery isentropic efficiencies as function of electric power output**

As commonly known, the efficiency of turbomachinery is a function of power rating, i.e., the higher the output power, the higher is also the efficiency. Conversely, smaller engines have lower efficiencies. This is principally due to sizespecific impacts of aerodynamic losses. For example, the turbine blade tip clearance (i.e., the radial distance between the blade tip of an axial compressor or turbine and the containment structure) is a major contributing factor to gas path sealing and can significantly affect engine efficiency [50]. The tip-leakage flow contributes negatively to the turbine performance and accounts for approximately one third of the total aerodynamic loss [51]. The bigger the engine, the smaller is the tip clearance with respect to the overall blade length and thus the higher is the efficiency.

It is clear that the size-dependent relationship of the turbomachinery's efficiency needs to be taken into account in the techno-economic optimization. In order to do so, relationships and performance tables have been established that consider efficiency as a function of output power, for both the topping Brayton cycle and for the bottoming Rankine cycle. For detailed information, the interested reader is referred to the corresponding public CAPTure project [38] deliverable D1.4 "CAPTure concept specifications and optimization."

## **4. Parametric benchmarking of power plant configurations: Combined cycle (CC) vs. Rankine single cycle (RC)**

field base modules) is that when moving to a higher solar power class, the LCOE decreases. This is principally due to the fact that when moving to higher nominal power of the power block, the turbomachinery becomes more efficient and also the specific power block costs (\$/kWe) reduce. This is the reason why commercial CSP projects have increased in size recently. However, when increasing the nominal solar power of a multi-tower arrangement (i.e., increasing the number of towers) above a certain threshold, the needed piping for HTF transport becomes an issue (investment, thermal losses, and pumping power); hence LCOE increases again (see configurations 9 C, 3 D, and 4 D). The second important trend that can be observed is that the single-tower configuration is only more competitive than a multi-tower configuration (of the same nominal solar power) below about 153 MW total nominal solar power. Above this threshold, the increase in investment (more towers, longer piping) and additional HTF transport power consumption are offset by the positive effect of higher optical efficiency of the more compact solar field

*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle…*

*DOI: http://dx.doi.org/10.5772/intechopen.90410*

base modules and smaller receiver aperture areas (cost advantage).

**Figure 6.**

**37**

**Figure 5.**

*(right-hand side).*

The most competitive (lowest LCOE) combined cycle power plant configuration is 6 C with a LCOE of 12.6 c\$/kWh. However, the most competitive Rankine singlecycle plant configuration (also of type 6 C) achieves an LCOE of 11.5 c\$/kWh. Thus, it is concluded that the combined cycle plant is despite its higher solar-to-electric conversion efficiency more expensive than the much simpler but less efficient single-cycle Rankine option. However, when observing **Table 1**, the difference in LCOE becomes smaller for smaller power classes, and the CC plant achieves better performance at 34 MW (and lower) total solar power. This is an effect of different power cycle efficiency decrease at small power classes, i.e., the combined cycle stays relatively more efficient than the Rankine single cycle configuration, which pays off for very small plant configurations. For this reason, the combined cycle seems to be

*LCOE as function of nominal solar power incident at receiver(s) and plant configuration (see also* **Table 1***).*

*Solar field base module types D and E (solar field dimensions given in meters). Multi-tower configuration 9 A*

The principal objective of this section is to benchmark the techno-economic optimum of the CC plant against that of a conventional single-cycle Rankine steam plant with the same receiver and TES technology (see **Figure 3**). This will allow a fair assessment of the solar-powered combined cycle performance.

In order to analyze the impact of different solar field sizes and number of towersolar-field modules, five solar field base modules (A, B, C, D, and E) have been selected (see **Table 1, Figures 4** and **5**). The applied solar field layout pattern is DELSOL [52], and solar field efficiency matrices can be obtained from CAPTure project deliverable D1.4. The base modules have been chosen such that different multiples achieve the same solar power class. For example, 9 B modules have the same nominal solar power as 3 C modules or 1 D module, i.e., 153 MW solar at the receiver(s). In this way, a direct comparison of conventional single-tower and multi-tower configurations can be achieved, giving also emphasis on the impact of total electric power of the plant. The general expected trends are that:


For each of the 19 configurations as indicated in **Table 1** (3 A to 9 A, 1 B to 9 B, 1 C to 9 C, 1 D to 4 D, and 1 E), the power plant performance models (combined cycle and single-cycle Rankine) have been run estimating the yearly energy yield and in consequence the resulting LCOE. The results are indicated in **Table 1** as well as in **Figure 6**. Note that the solar multiple (SM) and the TES capacity (hours of storage) have been optimized, i.e., obtaining the minimum LCOE at a solar multiple of about 2.3 and 10 full load hours of TES. The optimum solar multiple and TES capacity are typically only functions of geographic location and solar resource. When looking at **Table 1**, the first important trend that can be observed across all columns (all solar

**Figure 4.** *Solar field base module types A, B, and C (solar field dimensions given in meters).*

### *Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle… DOI: http://dx.doi.org/10.5772/intechopen.90410*

field base modules) is that when moving to a higher solar power class, the LCOE decreases. This is principally due to the fact that when moving to higher nominal power of the power block, the turbomachinery becomes more efficient and also the specific power block costs (\$/kWe) reduce. This is the reason why commercial CSP projects have increased in size recently. However, when increasing the nominal solar power of a multi-tower arrangement (i.e., increasing the number of towers) above a certain threshold, the needed piping for HTF transport becomes an issue (investment, thermal losses, and pumping power); hence LCOE increases again (see configurations 9 C, 3 D, and 4 D). The second important trend that can be observed is that the single-tower configuration is only more competitive than a multi-tower configuration (of the same nominal solar power) below about 153 MW total nominal solar power. Above this threshold, the increase in investment (more towers, longer piping) and additional HTF transport power consumption are offset by the positive effect of higher optical efficiency of the more compact solar field base modules and smaller receiver aperture areas (cost advantage).

The most competitive (lowest LCOE) combined cycle power plant configuration is 6 C with a LCOE of 12.6 c\$/kWh. However, the most competitive Rankine singlecycle plant configuration (also of type 6 C) achieves an LCOE of 11.5 c\$/kWh. Thus, it is concluded that the combined cycle plant is despite its higher solar-to-electric conversion efficiency more expensive than the much simpler but less efficient single-cycle Rankine option. However, when observing **Table 1**, the difference in LCOE becomes smaller for smaller power classes, and the CC plant achieves better performance at 34 MW (and lower) total solar power. This is an effect of different power cycle efficiency decrease at small power classes, i.e., the combined cycle stays relatively more efficient than the Rankine single cycle configuration, which pays off for very small plant configurations. For this reason, the combined cycle seems to be

**Figure 5.**

**4. Parametric benchmarking of power plant configurations: Combined**

The principal objective of this section is to benchmark the techno-economic optimum of the CC plant against that of a conventional single-cycle Rankine steam plant with the same receiver and TES technology (see **Figure 3**). This will allow a

In order to analyze the impact of different solar field sizes and number of towersolar-field modules, five solar field base modules (A, B, C, D, and E) have been selected (see **Table 1, Figures 4** and **5**). The applied solar field layout pattern is DELSOL [52], and solar field efficiency matrices can be obtained from CAPTure project deliverable D1.4. The base modules have been chosen such that different multiples achieve the same solar power class. For example, 9 B modules have the same nominal solar power as 3 C modules or 1 D module, i.e., 153 MW solar at the receiver(s). In this way, a direct comparison of conventional single-tower and multi-tower configurations can be achieved, giving also emphasis on the impact of

ii. By arranging multiple solar field units as array, the optical efficiency for a given total solar power is improved; however, there is a point where HTF

detrimental and the global performance is not better than that of a single-

arrangements, the considerable decrease in conversion efficiency of small power cycles, as well as elevated specific costs, generally makes small CSP

For each of the 19 configurations as indicated in **Table 1** (3 A to 9 A, 1 B to 9 B, 1 C to 9 C, 1 D to 4 D, and 1 E), the power plant performance models (combined cycle and single-cycle Rankine) have been run estimating the yearly energy yield and in consequence the resulting LCOE. The results are indicated in **Table 1** as well as in **Figure 6**. Note that the solar multiple (SM) and the TES capacity (hours of storage) have been optimized, i.e., obtaining the minimum LCOE at a solar multiple of about 2.3 and 10 full load hours of TES. The optimum solar multiple and TES capacity are typically only functions of geographic location and solar resource. When looking at **Table 1**, the first important trend that can be observed across all columns (all solar

transport and additional tower and piping investment become too

iii. Despite of much better optical efficiency of compact multi-tower

**cycle (CC) vs. Rankine single cycle (RC)**

*Green Energy and Environment*

fair assessment of the solar-powered combined cycle performance.

total electric power of the plant. The general expected trends are that:

i. Smaller solar fields have higher optical efficiency.

*Solar field base module types A, B, and C (solar field dimensions given in meters).*

tower arrangement.

**Figure 4.**

**36**

plants economically unfeasible.

*Solar field base module types D and E (solar field dimensions given in meters). Multi-tower configuration 9 A (right-hand side).*


not enough to outweigh the higher investment costs of the more complex CC plant layout. The CC configuration seems to be competitive only at smaller power classes. It must be said that all cost assumptions have inherent uncertainty, which makes a final conclusion regarding the best power plant layout very difficult. It is however clear that compact power plant arrangements (A, B, C options) are the preferred choice for the CAPTure power plant concept that applies atmospheric air as HTF, as large diameter piping (low air speeds are mandatory) becomes an issue at higher power classes, not only in terms of investment but also in terms of thermal inertia and losses. Therefore, it is very likely that in practical terms, a single-tower plant configuration will be the best choice when applying atmospheric air as HTF, as differences in LCOE are small. Furthermore, compact power tower plants have clear advantages regarding solar flux control, and also concerning total investment

*Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle…*

Finally, in order to make the CC attractive for CSP plants, the following challenges remain: (i) the efficiency of the solar receiver at relevant operating temper-

economically competitive solar receiver designs are sought that allow long-term operation (≈ 25–30 years) at very high solar flux densities, i.e. high concentration ratios; (ii) with regard to the investigated power plant layout, i.e. when using an open volumetric air receiver and atmospheric air as HTF, it is crucial to design a very economical high-temperature air–air heat exchanger train for powering the

This work has received funding from the European Union's Horizon 2020 research and innovation program under the grant agreement No 640905.

, Marcelino Sánchez<sup>1</sup>

1 National Renewable Energy Center (CENER), Solar Thermal Energy Department,

© 2019 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,

, Benoît Valentin<sup>2</sup>

, Jonathon McGuire<sup>3</sup> and Flavien Berard<sup>3</sup>

,

atures (≈ 1000°C) must be increased, and in particular innovative and

as financing is usually easier to obtain for smaller projects.

*DOI: http://dx.doi.org/10.5772/intechopen.90410*

topping gas turbine externally.

**Acknowledgements**

**Author details**

Fritz Zaversky<sup>1</sup>

Navarra, Spain

**39**

Jean-Florian Brau2

2 EDF – R&D, Chatou, France

\*, Iñigo Les1

, Frédéric Siros2

3 Bluebox Energy Ltd., Hants, United Kingdom

provided the original work is properly cited.

\*Address all correspondence to: fzaversky@cener.com

### **Table 4.**

*Power plant specifications of plant type 6 C and 2 B.*

only attractive for very small power tower plants (below 5 MWe). Although gas turbines can be scaled down quite well having reasonable performance at small power classes, this is not the case for Rankine steam cycles. Hence, when thinking of very small (i.e., "micro") combined cycles, the application of the organic Rankine cycle (ORC) as bottoming power cycle should be considered. This concept could be attractive for small and modular CSP central receiver plants for "electricity islands," i.e., small remote grids, where electricity price is very high.

**Table 4** shows the most important parameters of power plant configurations 6 C and 2 B. For plant configuration 6 C, it can be seen that although the combined cycle option achieves a higher solar-to-electric conversion efficiency, the increased plant complexity and thus its higher investment are not compensated by the increase in electricity yield. The combined cycle becomes cost competitive only at smaller power classes (see results for plant configuration 2 B in **Table 4**).

### **5. Conclusions**

The parametric study shows that the multi-tower configuration has a technoeconomic advantage with respect to the conventional single-tower arrangement above a total nominal solar power level of about 150 MW. The most competitive power plant configuration is of type 6 C. The combined cycle plant configuration reaches an LCOE of 12.6 c\$/kWh, whereas the Rankine single-cycle power plant layout achieves 11.5 c\$/kWh. Hence, the CC configuration has despite its higher solar-to-electric conversion efficiency a higher LCOE. The gain in electricity yield is *Techno-Economic Optimization and Benchmarking of a Solar-Only Powered Combined Cycle… DOI: http://dx.doi.org/10.5772/intechopen.90410*

not enough to outweigh the higher investment costs of the more complex CC plant layout. The CC configuration seems to be competitive only at smaller power classes. It must be said that all cost assumptions have inherent uncertainty, which makes a final conclusion regarding the best power plant layout very difficult. It is however clear that compact power plant arrangements (A, B, C options) are the preferred choice for the CAPTure power plant concept that applies atmospheric air as HTF, as large diameter piping (low air speeds are mandatory) becomes an issue at higher power classes, not only in terms of investment but also in terms of thermal inertia and losses. Therefore, it is very likely that in practical terms, a single-tower plant configuration will be the best choice when applying atmospheric air as HTF, as differences in LCOE are small. Furthermore, compact power tower plants have clear advantages regarding solar flux control, and also concerning total investment as financing is usually easier to obtain for smaller projects.

Finally, in order to make the CC attractive for CSP plants, the following challenges remain: (i) the efficiency of the solar receiver at relevant operating temperatures (≈ 1000°C) must be increased, and in particular innovative and economically competitive solar receiver designs are sought that allow long-term operation (≈ 25–30 years) at very high solar flux densities, i.e. high concentration ratios; (ii) with regard to the investigated power plant layout, i.e. when using an open volumetric air receiver and atmospheric air as HTF, it is crucial to design a very economical high-temperature air–air heat exchanger train for powering the topping gas turbine externally.

## **Acknowledgements**

This work has received funding from the European Union's Horizon 2020 research and innovation program under the grant agreement No 640905.

### **Author details**

only attractive for very small power tower plants (below 5 MWe). Although gas turbines can be scaled down quite well having reasonable performance at small power classes, this is not the case for Rankine steam cycles. Hence, when thinking of very small (i.e., "micro") combined cycles, the application of the organic Rankine cycle (ORC) as bottoming power cycle should be considered. This concept could be attractive for small and modular CSP central receiver plants for "electricity islands,"

LCOE (c\$/kWh) 12.6 11.5 15.6 15.7

**Parameter (unit) 6C CC 6C RC 2B CC 2B RC** Number of towers () 6 622 Nominal solar power per tower (MW) 51 51 17 17 Total nominal solar power (MW) 306 306 34 34

Solar-to-electric peak efficiency () 0.296 0.25 0.27 0.209 Solar-to-electric annual mean efficiency () 0.202 0.195 0.182 0.152 Solar multiple () 2.3 2.3 2.3 2.3 Power cycle nominal power (MWe) 50 45 4.9 3.7 Reheated GT nominal power (MWe) 28 — 3 — Rankine cycle nominal power (MWe) 22 45 1.9 3.7

TES thermal capacity (MWh) 981 1194 109 131.3 Yearly electricity yield (GWh) 161.6 156.8 15.6 13 Total plant cost (M USD) 175.35 154.55 20.93 17.51 Specific plant costs (USD/kWe) 3507 3434 4271 4732

0.75/1050 0.81/

0.496/0.288 / 0.355

800

—/—/ 0.388

849 678 1473 1423

0.75/1050 0.81/

0.434/0.268/ 0.277

800

—/—/ 0.295

Receiver thermal efficiency ()/operating

Power cycle annual mean conversion efficiency:

*Power plant specifications of plant type 6 C and 2 B.*

temperature (°C)

*Green Energy and Environment*

CC/GT/RC ()

(USD/kWe)

**Table 4.**

Specific power cycle costs

**Table 4** shows the most important parameters of power plant configurations 6 C and 2 B. For plant configuration 6 C, it can be seen that although the combined cycle option achieves a higher solar-to-electric conversion efficiency, the increased plant complexity and thus its higher investment are not compensated by the increase in electricity yield. The combined cycle becomes cost competitive only at smaller

The parametric study shows that the multi-tower configuration has a technoeconomic advantage with respect to the conventional single-tower arrangement above a total nominal solar power level of about 150 MW. The most competitive power plant configuration is of type 6 C. The combined cycle plant configuration reaches an LCOE of 12.6 c\$/kWh, whereas the Rankine single-cycle power plant layout achieves 11.5 c\$/kWh. Hence, the CC configuration has despite its higher solar-to-electric conversion efficiency a higher LCOE. The gain in electricity yield is

i.e., small remote grids, where electricity price is very high.

power classes (see results for plant configuration 2 B in **Table 4**).

**5. Conclusions**

**38**

Fritz Zaversky<sup>1</sup> \*, Iñigo Les1 , Marcelino Sánchez<sup>1</sup> , Benoît Valentin<sup>2</sup> , Jean-Florian Brau2 , Frédéric Siros2 , Jonathon McGuire<sup>3</sup> and Flavien Berard<sup>3</sup>

1 National Renewable Energy Center (CENER), Solar Thermal Energy Department, Navarra, Spain

2 EDF – R&D, Chatou, France

3 Bluebox Energy Ltd., Hants, United Kingdom

\*Address all correspondence to: fzaversky@cener.com

© 2019 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.

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Receiver Systems from DNI

Energy. 1999;**67**:249-264

[52] Kistler BL. A User's Manual for DELSOL3: A Computer Code for Calculating the Optical Performance and Optimal System Design for Solar thermal Central Receiver Plants, Sandia National Laboratories, Albuquerque, New Mexico and Livermore, California; 1986

**45**

**Chapter 3**

**Abstract**

The Emerging of Hydrovoltaic

*Jiale Xie, Liuliu Wang, Xiaoying Chen, Pingping Yang,* 

A Case Study for China

*Fengkai Wu and Yuelong Huang*

emerging technology, especially in China.

hydrovoltaic device, potential applications

station, which is a main form of electricity.

**1. Introduction**

Materials as a Future Technology:

Water contains tremendous energy in various forms, but very little of this energy has yet been harvested. Nanostructured materials can generate electricity by water-nanomaterial interaction, a phenomenon referred to as hydrovoltaic effect, which potentially extends the technical capability of water energy harvesting. In this chapter, starting by describing the fundamental principle of hydrovoltaic effect, including water-carbon interactions and fundamental mechanisms of harvesting water energy with nanostructured materials, experimental advances in generating electricity from water flows, waves, natural evaporation, and moisture are then reviewed. We further discuss potential applications of hydrovoltaic technologies, analyze main challenges in improving the energy conversion efficiency and scaling up the output power, and suggest prospects for developments of the

**Keywords:** hydrovoltaic effect, carbon nanomaterial, electrokinetic effect,

Water covers over 70% of the Earth's surface, which means it is abundant and widely available. Water contains enormous energy (35% of the solar energy received by the Earth, 1015 W) in a variety of forms, such as chemical, thermal, and kinetic energy [1]. The chemical energy is harnessed through water splitting under the assistance of electricity or photocatalysts [2]. The thermal energy is exploited for salinity power generation [3]. Kinetic energy is widely utilized by hydroelectric

As the progress of nanomaterials and nanotechnology, a new strategy based on hydrovoltaic effect (HV) has been developed in recent years [1]. In solar cells, the electron-hole pairs are generated by the absorption of photons with higher energy than the bandgap of semiconductor [4]. With the help of the built-in field at the interface of p-n junction, the electron-hole pairs are separated and then accumulated at the terminal of solar cells, generating photovoltaic voltage (**Figure 1a**). HV effect is analogous to the photovoltaic effect described as above. For HV effect, the potential is generated through the interaction between nanostructured materials and water molecules [1]. The form of water can be liquid, droplet, moisture, and

## **Chapter 3**

## The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China

*Jiale Xie, Liuliu Wang, Xiaoying Chen, Pingping Yang, Fengkai Wu and Yuelong Huang*

## **Abstract**

Water contains tremendous energy in various forms, but very little of this energy has yet been harvested. Nanostructured materials can generate electricity by water-nanomaterial interaction, a phenomenon referred to as hydrovoltaic effect, which potentially extends the technical capability of water energy harvesting. In this chapter, starting by describing the fundamental principle of hydrovoltaic effect, including water-carbon interactions and fundamental mechanisms of harvesting water energy with nanostructured materials, experimental advances in generating electricity from water flows, waves, natural evaporation, and moisture are then reviewed. We further discuss potential applications of hydrovoltaic technologies, analyze main challenges in improving the energy conversion efficiency and scaling up the output power, and suggest prospects for developments of the emerging technology, especially in China.

**Keywords:** hydrovoltaic effect, carbon nanomaterial, electrokinetic effect, hydrovoltaic device, potential applications

### **1. Introduction**

Water covers over 70% of the Earth's surface, which means it is abundant and widely available. Water contains enormous energy (35% of the solar energy received by the Earth, 1015 W) in a variety of forms, such as chemical, thermal, and kinetic energy [1]. The chemical energy is harnessed through water splitting under the assistance of electricity or photocatalysts [2]. The thermal energy is exploited for salinity power generation [3]. Kinetic energy is widely utilized by hydroelectric station, which is a main form of electricity.

As the progress of nanomaterials and nanotechnology, a new strategy based on hydrovoltaic effect (HV) has been developed in recent years [1]. In solar cells, the electron-hole pairs are generated by the absorption of photons with higher energy than the bandgap of semiconductor [4]. With the help of the built-in field at the interface of p-n junction, the electron-hole pairs are separated and then accumulated at the terminal of solar cells, generating photovoltaic voltage (**Figure 1a**). HV effect is analogous to the photovoltaic effect described as above. For HV effect, the potential is generated through the interaction between nanostructured materials and water molecules [1]. The form of water can be liquid, droplet, moisture, and

### **Figure 1.**

*(a) Photovoltaic effect with p-n junction. The basis of photovoltaic effect is the asymmetry of structural electronics. (b) Schematic of EDL forms at the solid surface with negative charges (not shown). There are two charge layers near the surface of solid, which are stern and diffusion layers. The stern layer is formed due to the chemical interaction between solid and absorbed ions. The diffusion layer electrically screen the stern layer through coulomb interaction. The blue line is the electrical potential curve around the solid surface [1]. (c) Illustration of induced potential by drawing a droplet on graphene. An electric current is formed in graphene by two moving boundaries of the EDL at the front and rear of the running droplet, respectively [1].*

evaporation [5]. In brief, the basis of photovoltaic effect is the asymmetry of structural electronics (e.g., p-n junction). The nonuniformity of charge distribution at solid-liquid interface is the origin of HV effect. **Figure 1b** shows the electric double layer (EDL) at solid–liquid interface and the potential gradient as the distance increasing from solid surface into solution. **Figure 1c** illustrates the hydrovoltaic current is created in the graphene layer with the opposite orientation of the droplet flowing direction.

Since the HV phenomenon was discovered with carbon nanotubes (CNTs) in 2003, carbon nanomaterials were extensively investigated and considered as the most promising candidates for HV generators [6]. So far plenty of carbon nanomaterials exhibit HV effect with no need of a pressure gradient, including 0D graphene quantum dots (GQDs), 1D CNTs, 2D graphene or graphene oxide (GO), 3D graphene foam, and so on [7, 8]. Yet, unlike photovoltaic effect, research on the HV effect is in its infancy and calls for continued efforts to materialize its great potential. In this chapter, starting by describing fundamental principle of hydrovoltaic effect, including water-carbon interactions and basic mechanisms of harvesting water energy with nanostructured materials, experimental advances in generating electricity from water flows, waves, natural evaporation, and moisture are then reviewed. We further discuss potential device applications of hydrovoltaic technologies, analyze main challenges in improving the energy conversion efficiency and scaling up the output power, and suggest prospects for developments of the emerging technology.

**47**

**Figure 2.**

*from raindrops [1].*

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China*

As in **Figure 2a**, in a nanochannel, the EDL layers form on the interface of solid– liquid and overlap each other due to the small size of nanochannel. Under pressure gradient, a steady current will be generated along with the ion transport from high to low pressure side. The voltage will prohibit the transport of more ions. Therefore the steady current named the streaming current is generated [9]. There is positive correlation between the flow rate, pressure gradient, channel height, and the streaming current. When a nanomaterial (e.g., graphene) is inserted into the liquid level, the EDL layer will be created and changed as the immersed area of nanomaterials changes. This means, if the immersed area increases, the EDL layer will be charged and a voltage will be generated in the nanomaterials (**Figure 2b**). On the contrary, an inverse voltage can be observed due to the discharging of EDL layer. This waveinduced voltage was called waving potential [10]. The voltage and current are proportional to the velocity of graphene and can be scaled up by series and parallel

When the water contained ions is a droplet on graphene, the EDL is emerged only in the region of droplet followed the EDL theory. When the droplet is moving under the external force such as gravity, the EDL region will move accordingly. During this moving process, there is a charging state at the front of the droplet, while a discharging process at the rear of the droplet (**Figure 1c**). Therefore an electrical voltage called as drawing potential can be generated [11]. The voltage and current will increase as the velocity and number of droplets increase. The drawing

When moisture were adsorbed by nanomaterials with oxygen-containing

containing functional groups recombined, resulting in an reverse voltage [12].

*(a) Schematic of electrokinetic effect in the nanochannel. Blue line illustrates the profile of flow velocity through the nanochannel [1]. (b) Illustration of waving potential induced in graphene by one moving boundary of EDL across a graphene sheet on a dielectric substrate [1]. (c) Schematic illustration of harvesting energy* 

can form because of the local salvation effects,

concentration

and oxygen-

vanishes. When the ingress of

bonds. Due to the H<sup>+</sup>

will migrate along the reverse gradient direction. Then a voltage

potential can be developed to harvest raindrop energy (**Figure 2c**).

moisture stopped, the number of migratory ions decreased as free H<sup>+</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.90377*

**2.1 Mechanisms of hydrovoltaic effect**

connections of multiple graphene devices.

*2.1.2 Ion diffusion-induced mechanism*

functional groups, a gradient of H<sup>+</sup>

difference, the H+

which can lead to the breakage of Oδ−-H<sup>δ</sup><sup>+</sup>

would increase continuously until the gradient of H+

**2. Fundamentals of hydrovoltaic effect**

*2.1.1 Electric double layer and pseudocapacitance mechanism*

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China DOI: http://dx.doi.org/10.5772/intechopen.90377*

## **2. Fundamentals of hydrovoltaic effect**

## **2.1 Mechanisms of hydrovoltaic effect**

*Green Energy and Environment*

evaporation [5]. In brief, the basis of photovoltaic effect is the asymmetry of structural electronics (e.g., p-n junction). The nonuniformity of charge distribution at solid-liquid interface is the origin of HV effect. **Figure 1b** shows the electric double layer (EDL) at solid–liquid interface and the potential gradient as the distance increasing from solid surface into solution. **Figure 1c** illustrates the hydrovoltaic current is created in the graphene layer with the opposite orientation of the droplet

*(a) Photovoltaic effect with p-n junction. The basis of photovoltaic effect is the asymmetry of structural electronics. (b) Schematic of EDL forms at the solid surface with negative charges (not shown). There are two charge layers near the surface of solid, which are stern and diffusion layers. The stern layer is formed due to the chemical interaction between solid and absorbed ions. The diffusion layer electrically screen the stern layer through coulomb interaction. The blue line is the electrical potential curve around the solid surface [1]. (c) Illustration of induced potential by drawing a droplet on graphene. An electric current is formed in graphene by two moving boundaries of the EDL at the front and rear of the running droplet, respectively [1].*

Since the HV phenomenon was discovered with carbon nanotubes (CNTs) in 2003, carbon nanomaterials were extensively investigated and considered as the most promising candidates for HV generators [6]. So far plenty of carbon nanomaterials exhibit HV effect with no need of a pressure gradient, including 0D graphene quantum dots (GQDs), 1D CNTs, 2D graphene or graphene oxide (GO), 3D graphene foam, and so on [7, 8]. Yet, unlike photovoltaic effect, research on the HV effect is in its infancy and calls for continued efforts to materialize its great potential. In this chapter, starting by describing fundamental principle of hydrovoltaic effect, including water-carbon interactions and basic mechanisms of harvesting water energy with nanostructured materials, experimental advances in generating electricity from water flows, waves, natural evaporation, and moisture are then reviewed. We further discuss potential device applications of hydrovoltaic technologies, analyze main challenges in improving the energy conversion efficiency and scaling up the output power, and suggest prospects for developments of

**46**

flowing direction.

**Figure 1.**

the emerging technology.

### *2.1.1 Electric double layer and pseudocapacitance mechanism*

As in **Figure 2a**, in a nanochannel, the EDL layers form on the interface of solid– liquid and overlap each other due to the small size of nanochannel. Under pressure gradient, a steady current will be generated along with the ion transport from high to low pressure side. The voltage will prohibit the transport of more ions. Therefore the steady current named the streaming current is generated [9]. There is positive correlation between the flow rate, pressure gradient, channel height, and the streaming current.

When a nanomaterial (e.g., graphene) is inserted into the liquid level, the EDL layer will be created and changed as the immersed area of nanomaterials changes. This means, if the immersed area increases, the EDL layer will be charged and a voltage will be generated in the nanomaterials (**Figure 2b**). On the contrary, an inverse voltage can be observed due to the discharging of EDL layer. This waveinduced voltage was called waving potential [10]. The voltage and current are proportional to the velocity of graphene and can be scaled up by series and parallel connections of multiple graphene devices.

When the water contained ions is a droplet on graphene, the EDL is emerged only in the region of droplet followed the EDL theory. When the droplet is moving under the external force such as gravity, the EDL region will move accordingly. During this moving process, there is a charging state at the front of the droplet, while a discharging process at the rear of the droplet (**Figure 1c**). Therefore an electrical voltage called as drawing potential can be generated [11]. The voltage and current will increase as the velocity and number of droplets increase. The drawing potential can be developed to harvest raindrop energy (**Figure 2c**).

### *2.1.2 Ion diffusion-induced mechanism*

When moisture were adsorbed by nanomaterials with oxygen-containing functional groups, a gradient of H<sup>+</sup> can form because of the local salvation effects, which can lead to the breakage of Oδ−-H<sup>δ</sup><sup>+</sup> bonds. Due to the H<sup>+</sup> concentration difference, the H+ will migrate along the reverse gradient direction. Then a voltage would increase continuously until the gradient of H+ vanishes. When the ingress of moisture stopped, the number of migratory ions decreased as free H<sup>+</sup> and oxygencontaining functional groups recombined, resulting in an reverse voltage [12].

### **Figure 2.**

*(a) Schematic of electrokinetic effect in the nanochannel. Blue line illustrates the profile of flow velocity through the nanochannel [1]. (b) Illustration of waving potential induced in graphene by one moving boundary of EDL across a graphene sheet on a dielectric substrate [1]. (c) Schematic illustration of harvesting energy from raindrops [1].*

**Figure 3.** *Characteristics of a good hydrovoltaic material.*

## **2.2 Rules for hydrovoltaic material**

Based on the above discussion of mechanisms, an excellent hydrovoltaic material should possess the following characteristics: strong water-carbon interaction, rational pore and microstructure for the transport of water molecules, large interaction area for water adsorption and charge storage, sufficient electric conductivity for charge transportation, and abundant/gradient surface functional groups (oxygen functional groups in particular). Nowadays, the most reported hydrovoltaic materials are carbon nanomaterials [7, 8]. However, the hydrovoltaic effect is not limited to carbon nanomaterials but can be generic to other materials as long as they meet the above characteristics (**Figure 3**).

## **3. Hydrovoltaic devices and performance**

### **3.1 Moisture-induced electricity generation**

Moisture is one important form of water in the nature. GQDs have unique properties due to quantum confinements and edge effects [13]. GQDs as the chemically active material have fabricated a moisture-triggered generator [14]. The size of GQDs is 2–5 nm. The GQDs contain an amount of oxygen-containing functional groups. To create a gradient of functional groups, GQDs are treated via electrochemical polarization. The GQD generator achieves a high voltage of 0.27 V, when the variation of relative humidity (RH) is 70%. After the optimization of the

**49**

cm2

**Figure 4.**

steady power output.

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China*

*DOI: http://dx.doi.org/10.5772/intechopen.90377*

load resistor, a power density obtained is 1.86 mW/cm<sup>2</sup>

framework exhibits a high power density of ca. 1 mW/cm2

containing functional groups is the reason of electricity generation with moisture. Similarly, the porous carbon black, and GO framework with the functional group gradient, can also exhibit excellent HV performance under moisture [15, 16]. For example, a superhydrophilic 3D assembly of graphene oxide (g-3D-GO) with open

*(a) Voltage and (b) current output cycle of HV device based on g-3D-GO that is sandwiched by aluminum electrodes in response to the RH variation (ΔRH = 75%) [16]. (c) Schematic illustration of a HV devicebased power source system [16]. (d) Schematic illustration of the mechanism of humidity-driven electricity generation [17]. (e) Condensation and evaporation of ionic liquids under different humidity [17]. (f and g) Voltage generated with wrinkle graphene/salt crystal nanogenerator under a sudden change in humidity [17].*

sion efficiency of ca. 52% [16]. With an RH variation of 75%, the g-3D-GO-based HV device could provide a voltage and current output of ca. 0.26 V and ca. 3.2 mA/

 within 2 s (**Figure 4a** and **b**). As in **Figure 4c**, a power source system consists of four HV cells in series which was fabricated to demonstrate the practical application. This system was attached onto the pendulum bob. The pendulum bob can swing between moist region (RH = 80%) and dry region (RH = 5%). When the system moves to the moist region, the moisture-induced positive voltage is applied on the light emitting diode (LED), and it lights up. Upon traveling to the dry region, the LED will switch off. As a consequence, this power source system could provide a

Recently, Zhen et al. prepared a nanogenerator using the wrinkled graphene, which followed an unusual mechanism of HV effect [17]. In this work, a new cation-π interaction utilization strategy was developed. In other words, electricity is generated through water adsorption and desorption of salt crystals along with the humidity variation. The key of this nanogenerator is to deposit salt crystals onto the wrinkled graphene by manipulating the formation of ionic liquid microdroplets.

. The gradient of oxygen-

and an energy conver-

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China DOI: http://dx.doi.org/10.5772/intechopen.90377*

### **Figure 4.**

*Green Energy and Environment*

**2.2 Rules for hydrovoltaic material**

*Characteristics of a good hydrovoltaic material.*

**Figure 3.**

the above characteristics (**Figure 3**).

**3. Hydrovoltaic devices and performance**

**3.1 Moisture-induced electricity generation**

Based on the above discussion of mechanisms, an excellent hydrovoltaic material should possess the following characteristics: strong water-carbon interaction, rational pore and microstructure for the transport of water molecules, large interaction area for water adsorption and charge storage, sufficient electric conductivity for charge transportation, and abundant/gradient surface functional groups (oxygen functional groups in particular). Nowadays, the most reported hydrovoltaic materials are carbon nanomaterials [7, 8]. However, the hydrovoltaic effect is not limited to carbon nanomaterials but can be generic to other materials as long as they meet

Moisture is one important form of water in the nature. GQDs have unique properties due to quantum confinements and edge effects [13]. GQDs as the chemically active material have fabricated a moisture-triggered generator [14]. The size of GQDs is 2–5 nm. The GQDs contain an amount of oxygen-containing functional groups. To create a gradient of functional groups, GQDs are treated via electrochemical polarization. The GQD generator achieves a high voltage of 0.27 V, when the variation of relative humidity (RH) is 70%. After the optimization of the

**48**

*(a) Voltage and (b) current output cycle of HV device based on g-3D-GO that is sandwiched by aluminum electrodes in response to the RH variation (ΔRH = 75%) [16]. (c) Schematic illustration of a HV devicebased power source system [16]. (d) Schematic illustration of the mechanism of humidity-driven electricity generation [17]. (e) Condensation and evaporation of ionic liquids under different humidity [17]. (f and g) Voltage generated with wrinkle graphene/salt crystal nanogenerator under a sudden change in humidity [17].*

load resistor, a power density obtained is 1.86 mW/cm<sup>2</sup> . The gradient of oxygencontaining functional groups is the reason of electricity generation with moisture. Similarly, the porous carbon black, and GO framework with the functional group gradient, can also exhibit excellent HV performance under moisture [15, 16]. For example, a superhydrophilic 3D assembly of graphene oxide (g-3D-GO) with open framework exhibits a high power density of ca. 1 mW/cm2 and an energy conversion efficiency of ca. 52% [16]. With an RH variation of 75%, the g-3D-GO-based HV device could provide a voltage and current output of ca. 0.26 V and ca. 3.2 mA/ cm2 within 2 s (**Figure 4a** and **b**). As in **Figure 4c**, a power source system consists of four HV cells in series which was fabricated to demonstrate the practical application. This system was attached onto the pendulum bob. The pendulum bob can swing between moist region (RH = 80%) and dry region (RH = 5%). When the system moves to the moist region, the moisture-induced positive voltage is applied on the light emitting diode (LED), and it lights up. Upon traveling to the dry region, the LED will switch off. As a consequence, this power source system could provide a steady power output.

Recently, Zhen et al. prepared a nanogenerator using the wrinkled graphene, which followed an unusual mechanism of HV effect [17]. In this work, a new cation-π interaction utilization strategy was developed. In other words, electricity is generated through water adsorption and desorption of salt crystals along with the humidity variation. The key of this nanogenerator is to deposit salt crystals onto the wrinkled graphene by manipulating the formation of ionic liquid microdroplets.

The wrinkled graphene has many defects and uniform wrinkles, facilitating the ultrafast water evaporation, preventing excessive water accumulation and deposition of well-distributed salt crystals. **Figure 4d** and **e** schematically illustrates the mechanism of electricity generation. As the sudden change of humidity (25–75–25%), two inversed voltage peaks were observed sequentially (**Figure 4f**). This is attributed to the water vapor adsorption and desorption on the salt crystals. The sharper negative peak is due to the strong water adsorption ability of the salt crystals, while the broad positive peak is from the slow desorption process (**Figure 4g**). The voltage of 18 mV with the current of 37 nA was achieved with a 1 × 6 cm2 generator. Among various salts, NaCl exhibits the best performance due to its complete crystallization after each cycle.

As far as we know, the nanomaterials for moisture-induced electricity generation include carbon/graphene quantum dots, carbon black, GO film, and 3D GO frameworks. The main origin of electricity generation is similar to each other. The potential generation is dependent on the water adsorption difference due to the gradient of oxygen-containing functional groups and induced the concentration difference of charge carriers. Interestingly, the porous carbon black film treated partially by plasma could generate continuous electricity, which is totally different from other carbon nanomaterials. This discrepancy may be from the difference of the structure and/or the introducing manner of functional groups.

### **3.2 Electricity generation induced by droplet movement**

In 2014, Yin et al. firstly reported the electricity generation induced by droplet movement on monolayer graphene [11]. When a droplet of 0.6 M NaCl is drawn on graphene at a constant velocity of 2.25 cm/s, a voltage of 0.15 mV is generated. When the direction of droplet movement is opposite, the direction of potential is also reversed. When the movement is stopped, no potential is produced. Rain is one of important existent forms of water in nature. However, the energy in the rain is not yet utilized efficiently in the long term. There are amount of cations (such as Na<sup>+</sup> , NH4 + , Ca2+, Mg2+) and anions (such as Cl<sup>−</sup>, NO3 <sup>−</sup>, SO4 <sup>2</sup><sup>−</sup>). Therefore harvesting energy from rain using HV effects is a promising approach [18, 19].

As previously reported, the low generated voltage of around 0.15 mV, and the external pressure needed, would limit the application of HV effect. Recently, Li et al. reported the electricity generation from water droplets on porous carbon film through capillary infiltrating [20]. **Figure 5a** illustrates the structure of the porous carbon film (PCF) device. When a droplet of 1 μL was dropped onto PCF, a sustainable voltage of 0.3 V was generated (**Figure 5b**). However, electricity generation by droplet movement on graphene or aligned single-walled nanotubes is pulse-like. The retention of voltage depends on the volume of water droplets as shown in **Figure 5c**, but the generated voltage value is nearly identical. More interesting, the dropping position of water droplet would influence the induced voltage (**Figure 5d**). Experimental results reveal the following key characteristics: (i) the merely directional water infiltration can induce the voltage, (ii) no direct correlation between the HV voltage and the position of droplets, and (iii) the direction of water infiltration influences the voltage sign. At last, the authors demonstrated a scale-up application with three devices in series (**Figure 5e**). Twelve 5 μL water droplets can generate a voltage up to 5.2 V and illuminate a liquid crystal display. This work powerfully demonstrate that a hydrophilic porous carbon film with water droplets could realize the energy harvested from rain and a practical application. However, there is no experimental results in real raining environment. More information on the droplet-induced electricity generation can refer one recent review paper [21].

**51**

**Figure 5.**

*dimensions of 50 × 7 mm<sup>2</sup>*

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China*

*DOI: http://dx.doi.org/10.5772/intechopen.90377*

**3.3 Flow-induced electricity generation**

Ocean wave energy is a main form of ocean energy, which is considered as inexhaustible energy. In 2007, Liu and Dai reported that the flow-induced voltage can be greatly improved by aligning the nanotubes along the flow direction [23]. In 2017, Xu et al. fabricated a fluidic nanogenerator fiber with the aligned multiwalled carbon nanotube sheet (inset of **Figure 6b**) [22]. The device shows a power conversion efficiency of 23.3% and an excellent stability over 1,000,000 cycles. The flow direction, the flow distance, the flow velocity, and the NaCl concentration are positive correlation with the induced voltage (**Figure 6a**–**6c**). The authors also discovered that the ordered mesoporous carbon (OMC) can significantly enhance the flow-induced voltage (**Figure 6d**). After OMC introduction, the sustained voltage for over 1 h can be achieved. The maximum voltage output can reach up to 341 mV when the content of OMC is 5.1 μg/cm. Impressively, the stable performance of this

*at the PFDTS@PCF/PCF interface under ambient conditions (~ 23.5°C and RH ~ 71.7%) [20]. (c) Measured Voc vs. time of the device when water droplets with various volumes were dropped onto the PFDTS@PCF/PCF interface [20]. (d) Wetting dependence of the induced voltage. Inset is schematic of the Voc measurement and the water-droplet position [20]. (e) Application demonstration of the water-droplet-induced voltage [20].*

 *[20]. (b) Open-circuit voltage obtained by repeatedly dropping 1 μL water droplets* 

*(a) Schematic of the porous carbon film device with two ends modified with 1H,1H,2H,2Hperfluorodecyltriethoxysilane (PFDTS). The right inset shows a photograph of a typical device with*  *The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China DOI: http://dx.doi.org/10.5772/intechopen.90377*

#### **Figure 5.**

*Green Energy and Environment*

its complete crystallization after each cycle.

1 × 6 cm2

The wrinkled graphene has many defects and uniform wrinkles, facilitating the ultrafast water evaporation, preventing excessive water accumulation and deposition of well-distributed salt crystals. **Figure 4d** and **e** schematically illustrates the mechanism of electricity generation. As the sudden change of humidity (25–75–25%), two inversed voltage peaks were observed sequentially (**Figure 4f**). This is attributed to the water vapor adsorption and desorption on the salt crystals. The sharper negative peak is due to the strong water adsorption ability of the salt crystals, while the broad positive peak is from the slow desorption process (**Figure 4g**). The voltage of 18 mV with the current of 37 nA was achieved with a

generator. Among various salts, NaCl exhibits the best performance due to

As far as we know, the nanomaterials for moisture-induced electricity generation include carbon/graphene quantum dots, carbon black, GO film, and 3D GO frameworks. The main origin of electricity generation is similar to each other. The potential generation is dependent on the water adsorption difference due to the gradient of oxygen-containing functional groups and induced the concentration difference of charge carriers. Interestingly, the porous carbon black film treated partially by plasma could generate continuous electricity, which is totally different from other carbon nanomaterials. This discrepancy may be from the difference of

In 2014, Yin et al. firstly reported the electricity generation induced by droplet movement on monolayer graphene [11]. When a droplet of 0.6 M NaCl is drawn on graphene at a constant velocity of 2.25 cm/s, a voltage of 0.15 mV is generated. When the direction of droplet movement is opposite, the direction of potential is also reversed. When the movement is stopped, no potential is produced. Rain is one of important existent forms of water in nature. However, the energy in the rain is not yet utilized efficiently in the long term. There are amount of cations (such as

As previously reported, the low generated voltage of around 0.15 mV, and the external pressure needed, would limit the application of HV effect. Recently, Li et al. reported the electricity generation from water droplets on porous carbon film through capillary infiltrating [20]. **Figure 5a** illustrates the structure of the porous carbon film (PCF) device. When a droplet of 1 μL was dropped onto PCF, a sustainable voltage of 0.3 V was generated (**Figure 5b**). However, electricity generation by droplet movement on graphene or aligned single-walled nanotubes is pulse-like. The retention of voltage depends on the volume of water droplets as shown in **Figure 5c**, but the generated voltage value is nearly identical. More interesting, the dropping position of water droplet would influence the induced voltage (**Figure 5d**). Experimental results reveal the following key characteristics: (i) the merely directional water infiltration can induce the voltage, (ii) no direct correlation between the HV voltage and the position of droplets, and (iii) the direction of water infiltration influences the voltage sign. At last, the authors demonstrated a scale-up application with three devices in series (**Figure 5e**). Twelve 5 μL water droplets can generate a voltage up to 5.2 V and illuminate a liquid crystal display. This work powerfully demonstrate that a hydrophilic porous carbon film with water droplets could realize the energy harvested from rain and a practical application. However, there is no experimental results in real raining environment. More information on the droplet-induced electricity generation can

<sup>−</sup>, SO4

<sup>2</sup><sup>−</sup>). Therefore harvesting

the structure and/or the introducing manner of functional groups.

**3.2 Electricity generation induced by droplet movement**

, Ca2+, Mg2+) and anions (such as Cl<sup>−</sup>, NO3

energy from rain using HV effects is a promising approach [18, 19].

**50**

refer one recent review paper [21].

Na<sup>+</sup> , NH4 +

*(a) Schematic of the porous carbon film device with two ends modified with 1H,1H,2H,2Hperfluorodecyltriethoxysilane (PFDTS). The right inset shows a photograph of a typical device with dimensions of 50 × 7 mm<sup>2</sup> [20]. (b) Open-circuit voltage obtained by repeatedly dropping 1 μL water droplets at the PFDTS@PCF/PCF interface under ambient conditions (~ 23.5°C and RH ~ 71.7%) [20]. (c) Measured Voc vs. time of the device when water droplets with various volumes were dropped onto the PFDTS@PCF/PCF interface [20]. (d) Wetting dependence of the induced voltage. Inset is schematic of the Voc measurement and the water-droplet position [20]. (e) Application demonstration of the water-droplet-induced voltage [20].*

### **3.3 Flow-induced electricity generation**

Ocean wave energy is a main form of ocean energy, which is considered as inexhaustible energy. In 2007, Liu and Dai reported that the flow-induced voltage can be greatly improved by aligning the nanotubes along the flow direction [23]. In 2017, Xu et al. fabricated a fluidic nanogenerator fiber with the aligned multiwalled carbon nanotube sheet (inset of **Figure 6b**) [22]. The device shows a power conversion efficiency of 23.3% and an excellent stability over 1,000,000 cycles. The flow direction, the flow distance, the flow velocity, and the NaCl concentration are positive correlation with the induced voltage (**Figure 6a**–**6c**). The authors also discovered that the ordered mesoporous carbon (OMC) can significantly enhance the flow-induced voltage (**Figure 6d**). After OMC introduction, the sustained voltage for over 1 h can be achieved. The maximum voltage output can reach up to 341 mV when the content of OMC is 5.1 μg/cm. Impressively, the stable performance of this

### **Figure 6.**

*(a) Voltage curve induced by a saturated NaCl flow at the velocity of 1.2 cm/s [22]. (b) Relationship between the voltage and the flow velocity. Solution is 0.6M NaCl [22]. (c) The voltage vs. current relationship as the concentration variation of NaCl solution (flow velocity: 12.9 cm/s) [22]. (d) Dependence of the voltage on the OMC content in a saturated NaCl solution (flowing velocity: 20 cm/s) [22]. (e) Voltage generated by repeatedly dipping an OMC-incorporated device into a NaCl solution with an increasing number of bending cycles. The inserted graphs show the voltages after 200,000; 600,000; and 1,000,000 bending cycles in the NaCl solution [22].*

device can be maintained even after over 1,000,000 bending cycles (**Figure 6e**). Moreover, the fiber nanogenerator is flexible and stretchable, indicating it can be woven into fabrics for large-scale applications.

Two-dimensional materials or devices have more advantages for ocean wave energy harvesting, which can well float on the surface of the ocean [1, 25]. Recently, Fei et al. achieved volt leveled waving potential using a pair of graphene sheets [24]. **Figure 7a** illustrates the device setup, where a pair of graphene-PET sheets with size of 2.5 × 1.5 cm<sup>2</sup> is immersed in NaCl solution. As one of the graphene sheets moves through the liquid surface, electricity is generated (**Figure 7b**). The peak voltage can be around 60–120 mV. However, no voltage is observed either moving graphene underneath or parallel to the liquid level, indicating the waving potential is due to the dynamic EDL boundary. In this setup, the moving graphene is served as driving force for ion movement, while another graphene is as a reference electrode.

**53**

**Figure 7.**

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China*

**Figure 7c** shows the relationship between the peak voltage and the moving velocity of graphene. The peak voltage exhibits linear relations with velocity at low moving speeds and saturates to certain values at high speeds. This saturation may be due to the limit from the speed of ion adsorption/desorption. During the pulling process, when the load resistance is 0.6 MΩ, the largest power density of 1.6 mW/m2

*(a) Schematic of experimental setup with two graphene-PET sheets immersed vertically in an electrolyte container. GrL and GrR represent graphene samples on the left and right side, respectively [24]. (b) Generated voltage signals at resistor of 0.9 MΩ when separately moving GrL or GrR across NaCl solution [24]. (c) Peak voltage values collected as a function of velocity. The perpendicular moving distance of graphene is kept as 2 cm* 

obtained (**Figure 7d**). Moreover, the ion species can also influence the voltage value. The peak voltage values follow an order of LiCl > NaCl > KCl > BaCl2, indicating the

Apart from the dynamic EDL boundary mechanism, the water-carbon interaction can also generate electricity [26]. In this case, the water does not need the cations and anions for the formation of EDL capacitance. More importantly, the electric signals are continuous, but the inherent mechanism is not very clear yet. More information on the liquid flow-induced electricity in carbon nanomaterials

Water evaporation is a crucial step in the natural water circulation, releasing a huge amount of water energy. In 2017, Xue et al. reported that the water evaporation from centimeter-sized carbon black sheets can reliably generate sustained voltages of up to 1 V for 8 days under ambient conditions [29]. The annealing and plasma treatment introduced functional groups are essential for the electricity generation.

smaller ions are better for higher voltage generation.

*[24]. (d) Calculated output power per unit area of graphene [24].*

can refer the recent review papers [27, 28].

**3.4 Evaporation-induced electricity generation**

can be

*DOI: http://dx.doi.org/10.5772/intechopen.90377*

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China DOI: http://dx.doi.org/10.5772/intechopen.90377*

### **Figure 7.**

*Green Energy and Environment*

**52**

of 2.5 × 1.5 cm<sup>2</sup>

**Figure 6.**

device can be maintained even after over 1,000,000 bending cycles (**Figure 6e**). Moreover, the fiber nanogenerator is flexible and stretchable, indicating it can be

*(a) Voltage curve induced by a saturated NaCl flow at the velocity of 1.2 cm/s [22]. (b) Relationship between the voltage and the flow velocity. Solution is 0.6M NaCl [22]. (c) The voltage vs. current relationship as the concentration variation of NaCl solution (flow velocity: 12.9 cm/s) [22]. (d) Dependence of the voltage on the OMC content in a saturated NaCl solution (flowing velocity: 20 cm/s) [22]. (e) Voltage generated by repeatedly dipping an OMC-incorporated device into a NaCl solution with an increasing number of bending cycles. The inserted graphs show the voltages after 200,000; 600,000; and 1,000,000 bending cycles in the NaCl solution [22].*

Two-dimensional materials or devices have more advantages for ocean wave energy harvesting, which can well float on the surface of the ocean [1, 25]. Recently, Fei et al. achieved volt leveled waving potential using a pair of graphene sheets [24]. **Figure 7a** illustrates the device setup, where a pair of graphene-PET sheets with size

through the liquid surface, electricity is generated (**Figure 7b**). The peak voltage can be around 60–120 mV. However, no voltage is observed either moving graphene underneath or parallel to the liquid level, indicating the waving potential is due to the dynamic EDL boundary. In this setup, the moving graphene is served as driving force for ion movement, while another graphene is as a reference electrode.

is immersed in NaCl solution. As one of the graphene sheets moves

woven into fabrics for large-scale applications.

*(a) Schematic of experimental setup with two graphene-PET sheets immersed vertically in an electrolyte container. GrL and GrR represent graphene samples on the left and right side, respectively [24]. (b) Generated voltage signals at resistor of 0.9 MΩ when separately moving GrL or GrR across NaCl solution [24]. (c) Peak voltage values collected as a function of velocity. The perpendicular moving distance of graphene is kept as 2 cm [24]. (d) Calculated output power per unit area of graphene [24].*

**Figure 7c** shows the relationship between the peak voltage and the moving velocity of graphene. The peak voltage exhibits linear relations with velocity at low moving speeds and saturates to certain values at high speeds. This saturation may be due to the limit from the speed of ion adsorption/desorption. During the pulling process, when the load resistance is 0.6 MΩ, the largest power density of 1.6 mW/m2 can be obtained (**Figure 7d**). Moreover, the ion species can also influence the voltage value. The peak voltage values follow an order of LiCl > NaCl > KCl > BaCl2, indicating the smaller ions are better for higher voltage generation.

Apart from the dynamic EDL boundary mechanism, the water-carbon interaction can also generate electricity [26]. In this case, the water does not need the cations and anions for the formation of EDL capacitance. More importantly, the electric signals are continuous, but the inherent mechanism is not very clear yet. More information on the liquid flow-induced electricity in carbon nanomaterials can refer the recent review papers [27, 28].

### **3.4 Evaporation-induced electricity generation**

Water evaporation is a crucial step in the natural water circulation, releasing a huge amount of water energy. In 2017, Xue et al. reported that the water evaporation from centimeter-sized carbon black sheets can reliably generate sustained voltages of up to 1 V for 8 days under ambient conditions [29]. The annealing and plasma treatment introduced functional groups are essential for the electricity generation.

### **Figure 8.**

*(a) Schematic of the chemical modification with different molecules on carbon nanoparticles [30]. (b) Voc and zeta potentials of the pristine and modified GCFs [30]. (c) Schematic of the ion-selective transport mechanism in the nanochannels with (i) positively and (ii) negatively charged surface [30]. (d) Dependence of Voc of the device on the PEI concentration. The inset shows the evolution of Voc when a GCF was repeatedly modified by DADMAC(A) and PSS(B) [30]. (e) Voltage-time curve of three supercapacitors (SCs) connected in series in charging by the GCFs at ambient condition. Insets show the circuit diagram (top) and photos of the blue LED [30].*

Recently, Li et al. fabricated an evaporation-driven nanogenerator (1 cm × 5 cm) with a high open-circuit voltage of 3 V [30]. The film device is fabricated using carbon black and glass fiber. As in **Figure 8a**, the surface of the hybrid film was modified with several polymer molecules, such as polyethylene imine (PEI); 1,2,3,4-butane tetracarboxylic acid (BTCA); polydimethyl diallyl ammonium chloride (PDADMAC); and poly sodium-p-styrenesulfonate (PSS). The voltage of the glass-fiber-carbon-nanoparticle film (GCF) can vary from −3 V to 3 V, which rely on the difference of the surface functional groups (surface charges) as in **Figure 8b**. PEI- and PDADMAC-modified GCF have positive surface charges and thus positive

**55**

*and E2 + –E2*

*<sup>−</sup> as shown in the inset of c [11].*

**Figure 9.**

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China*

zeta potentials. The ion-selective transport mechanism is schematically shown in **Figure 8c**. The concentration of polymer solution is one of critically experimental conditions, which can significantly affect the voltage from +1 V to −3.2 V by using 0–0.5 wt% PEI solution (**Figure 8d**). There is an optimal concentration of PEI solution (0.05 wt%), which should be attributed to the low conductivity of polymer and/or the partial channel blocking. A hybrid film device using two GCFs with opposite surface charges was also prepared. Therefore the generated voltage was enhanced to around 5 V (5 × 5 cm). They also connected the hybrid film device with supercapacitor. The supercapacitor can store the electric energy from GCFs and provide a high current output. The supercapacitor can be charged up to 2.8 V by the output voltage of GCFs, and then a blue LED can be lighted up for about 10 s without any auxiliary. This work shows the great potential of evaporation-induced

–10 W/m<sup>2</sup>

choice to develop HV effect-based self-powered devices, such as self-powered liquid sensors. The factors which can affect the HV signal could be used for sensing applications, including flow rate, fluid volume, solution component, fluid movement, and so on. Yin et al. developed a monolayer graphene-based HV device in 2013 and demonstrated the self-powered liquid sensor application [11]. The configuration of HV devices is shown in **Figure 9a**. A droplet of 0.6 M NaCl was moved with a SiO2/ Si wafer on the graphene film. During the moving of the droplet, the advancing and receding contact angles are ~91.9° and ~60.2°, respectively. As in **Figure 9b**, the

*(a) A liquid droplet is sandwiched between graphene and a SiO2/Si wafer and drawn by the wafer at specific velocities. Inset: A droplet of 0.6 M NaCl on a graphene surface [11]. (b) Dependence of the output voltage on the volume of a droplet of 0.6 M NaCl [11]. (c) Dependence of the output voltage on the concentration of the solution (three droplets of NaCl solution). Inset: Photograph of handwriting with a Chinese brush on graphene [11]. (d) Voltage induced by moving one, two, and three droplets of 0.6 M NaCl. Dashed lines are curves linearly fitted to the measured data [11]. (e) Voltage for various ionic solutions (three droplets) on monolayer graphene [11]. (f) Sensing the stroke directions (arrows) by the drawing potentials between electrodes E1*

) of HV device, [1] it is a good

*+ –E1 −*

*DOI: http://dx.doi.org/10.5772/intechopen.90377*

electricity generation in the field of portable electronics.

**4. Potential applications**

**4.1 Self-powered liquid sensors**

Because of the low power density (10<sup>−</sup><sup>3</sup>

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China DOI: http://dx.doi.org/10.5772/intechopen.90377*

zeta potentials. The ion-selective transport mechanism is schematically shown in **Figure 8c**. The concentration of polymer solution is one of critically experimental conditions, which can significantly affect the voltage from +1 V to −3.2 V by using 0–0.5 wt% PEI solution (**Figure 8d**). There is an optimal concentration of PEI solution (0.05 wt%), which should be attributed to the low conductivity of polymer and/or the partial channel blocking. A hybrid film device using two GCFs with opposite surface charges was also prepared. Therefore the generated voltage was enhanced to around 5 V (5 × 5 cm). They also connected the hybrid film device with supercapacitor. The supercapacitor can store the electric energy from GCFs and provide a high current output. The supercapacitor can be charged up to 2.8 V by the output voltage of GCFs, and then a blue LED can be lighted up for about 10 s without any auxiliary. This work shows the great potential of evaporation-induced electricity generation in the field of portable electronics.

## **4. Potential applications**

*Green Energy and Environment*

**54**

**Figure 8.**

*the blue LED [30].*

Recently, Li et al. fabricated an evaporation-driven nanogenerator (1 cm × 5 cm) with a high open-circuit voltage of 3 V [30]. The film device is fabricated using carbon black and glass fiber. As in **Figure 8a**, the surface of the hybrid film was modified with several polymer molecules, such as polyethylene imine (PEI); 1,2,3,4-butane tetracarboxylic acid (BTCA); polydimethyl diallyl ammonium chloride (PDADMAC); and poly sodium-p-styrenesulfonate (PSS). The voltage of the glass-fiber-carbon-nanoparticle film (GCF) can vary from −3 V to 3 V, which rely on the difference of the surface functional groups (surface charges) as in **Figure 8b**. PEI- and PDADMAC-modified GCF have positive surface charges and thus positive

*(a) Schematic of the chemical modification with different molecules on carbon nanoparticles [30]. (b) Voc and zeta potentials of the pristine and modified GCFs [30]. (c) Schematic of the ion-selective transport mechanism in the nanochannels with (i) positively and (ii) negatively charged surface [30]. (d) Dependence of Voc of the device on the PEI concentration. The inset shows the evolution of Voc when a GCF was repeatedly modified by DADMAC(A) and PSS(B) [30]. (e) Voltage-time curve of three supercapacitors (SCs) connected in series in charging by the GCFs at ambient condition. Insets show the circuit diagram (top) and photos of* 

### **4.1 Self-powered liquid sensors**

Because of the low power density (10<sup>−</sup><sup>3</sup> –10 W/m<sup>2</sup> ) of HV device, [1] it is a good choice to develop HV effect-based self-powered devices, such as self-powered liquid sensors. The factors which can affect the HV signal could be used for sensing applications, including flow rate, fluid volume, solution component, fluid movement, and so on. Yin et al. developed a monolayer graphene-based HV device in 2013 and demonstrated the self-powered liquid sensor application [11]. The configuration of HV devices is shown in **Figure 9a**. A droplet of 0.6 M NaCl was moved with a SiO2/ Si wafer on the graphene film. During the moving of the droplet, the advancing and receding contact angles are ~91.9° and ~60.2°, respectively. As in **Figure 9b**, the

### **Figure 9.**

*(a) A liquid droplet is sandwiched between graphene and a SiO2/Si wafer and drawn by the wafer at specific velocities. Inset: A droplet of 0.6 M NaCl on a graphene surface [11]. (b) Dependence of the output voltage on the volume of a droplet of 0.6 M NaCl [11]. (c) Dependence of the output voltage on the concentration of the solution (three droplets of NaCl solution). Inset: Photograph of handwriting with a Chinese brush on graphene [11]. (d) Voltage induced by moving one, two, and three droplets of 0.6 M NaCl. Dashed lines are curves linearly fitted to the measured data [11]. (e) Voltage for various ionic solutions (three droplets) on monolayer graphene [11]. (f) Sensing the stroke directions (arrows) by the drawing potentials between electrodes E1 + –E1 − and E2 + –E2 <sup>−</sup> as shown in the inset of c [11].*

voltage of device is linearly proportional to the velocity of droplet. The larger the size of the droplet would induce the larger voltage. The concentration of NaCl solution is also a critical factor, but the trend is not monotonic. The best concentration of NaCl solution is around 0.01 M. The voltage output is less than 0.5 mV. However, in carbon black, the potential is maximum ~1.0 V when deionized (DI) water is used for evaporation HV generator [29]. It is interesting that the voltage can be multiplied by drawing multiple droplets simultaneously as shown in **Figure 9c**. Experimental results indicate the voltage is near zero when two droplets are moving in the opposite directions. The voltage of device is closely related to the ion species, such as MgCl2, HCl, and NH3·H2O. However, there is no response for DI water. This indicates the drawing potential comes from the ion-induced EDL capacitance. The voltage induced with HCl solution is negative. The authors think that there is a H3O+ layer on the surface of graphene. Therefore the positively charged graphene would attract the negative Cl<sup>−</sup> anions, which dominate the electric double layer. At last the authors show a handwriting sensor with a Chinese brush and 0.01 M NaCl. Two pairs of electrodes, E1 + –E1 <sup>−</sup> (right–left) and E2 + –E2 <sup>−</sup> (bottom-top), were patterned perpendicularly on the four sides of the graphene to distinguish the handwriting direction (inset of **Figure 9c**). The drawing direction related to voltage signal can be well detected as in **Figure 9f**. Moreover, the force and speed of the handwriting can also be monitored.

Graphene oxide framework with lots of pores can facilitate the diffusion of water molecules. Along with the asymmetrical oxygen-containing groups, Zhao et al. observed that the transport of the ionic charge carriers is accelerated due to the ionic gradient [16]. When the RH variation is 75%, the potential can increase up to 260 mV in 2 s. The concentration gradient of Li ions in 3D PPy framework can also show good sensitivity for moisture [31]. The potential is 60 mV under the RH variation of 85%. More importantly, the stability of this material is stable during the several hundred cycles.

Besides the HV potential, other signals can also be utilized for sensing, such as the resistance of materials, the length of fibers, and the volume of materials [32–36]. Of course, the change of other physical fields, such as temperature and wind speed, can be detected by HV devices directly or indirectly [29, 37].

### **4.2 All-weather power generation**

As we all know, the common solar cells only work on sunny days, but do not work in the night and on rainy days. Combining PV and HV effects, a hybrid cell for all-weather power generation was developed by integrating a solar cell with a HV device. Tang et al. fabricated a flexible solar cell made of a transparent graphene electrode and a dye-sensitized solar cell [38]. The hybrid cell can be excited by solar light on sunny days and raindrops on rainy days, yielding a solar-to-electric conversion efficiency of 6.53% under AM 1.5 irradiation and power in the range of 5.12–54.19 pW by simulated raindrops (**Figure 10b**). However, the output of this hybrid cell is still far lower than the actual requirement. Then Tang et al. changed the graphene to graphene/carbon black/polytetrafluorethylene (PTFE) for the hybrid cell fabrication [18]. But the voltage and current did not show significant improvement under simulated raindrops. Zhong et al. developed a 2D hybrid nanogenerator based on graphene and silicon for all-weather electricity generation [39]. In this hybrid cell, the graphene and silicon form the van der Waals Schottky diode (**Figure 10c**). This hybrid device delivers a maximum output power of 49.3 μW under light illumination. When the DI water flows on the graphene under light from Au electrode to Ag electrode, an additional potential of 2.54 mV can be generated (**Figure 10d**). However, there is no response in the dark, indicating no interaction

**57**

m2

**Figure 10.**

difference of Na+

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China*

exists between water and hybrid cell during the water flowing process. The authors think that the potential response should arise from the interaction between water and graphene/Si Schottky diode (the doping and dedoping at the front and rear side of water droplet, respectively) instead of the water-graphene interaction or the water-electrode interaction. Moreover, the negatively additional potential can be observed when the water flows toward Au electrode. This phenomenon should be

*(a) The quasi-all-weather solar cell that can produce electricity from rain and sun [38]. (b) Power signals produced by dropping 0.6 M NaCl droplets on rGO film [38]. (c) Schematic structure of the 2D hybrid nanogenerator of graphene/SiO2/Si [θ = 30°, PDMS: Poly(dimethysiloxane)] [18]. (d) Voltage responses to the flow of DI water over graphene/Si Schottky diode in the dark and under room light illumination [18].* 

Due to the intermittency and randomness of raining, it is wise to harvest energy from the ocean wave energy. The ocean wave energy is renewable and inexhaustible because 70% of the Earth's surface is covered by the ocean. The ocean wave energy is also called as blue energy [40]. Tan et al. fabricated a film type wave energy generator with graphene, carbon black, and polyurethane [25]. A voltage of > 20 mV

devices are sustainably stable upon persistent attack by waving ocean. The floating devices on the sea can be packaged into the wave energy stations by series and parallel connections. Tan et al. further proposed a photo-induced charge boosting liquid-solid electrokinetic generator with a structure of polyurethane/graphene oxide-carbon black-multi-walled carbon nanotube/carbon quantum dots/copper (PU/GO-CB-MWCNT/CQDs/Cu) [41]. Under AM1.5 illumination, the voltage, current, and power density achieved by this device are 0.1 V, 0.39 mA, and 26.6 mW/

, respectively. The working mechanism is described in **Figure 11a**-**c**. Due to the

as the principle of waving potential as above, the potential signal would change along with the seawater moving and the capacity change. **Figure 11d** presents an enlarged one-cycle electrical signal generation. When the seawater moves to the

and Cl<sup>−</sup> ions on adsorption energy, the EDL can be formed. Same


attributed to the asymmetric potential profile of the graphene channel.

*(e) Voltage responses to the flow of DI water toward different directions [18].*

**4.3 Harvesting Ocean wave energy**

and a current of > 10 μA are produced in a 15 cm2

*DOI: http://dx.doi.org/10.5772/intechopen.90377*

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China DOI: http://dx.doi.org/10.5772/intechopen.90377*

### **Figure 10.**

*Green Energy and Environment*

pairs of electrodes, E1

can also be monitored.

several hundred cycles.

**4.2 All-weather power generation**

+ –E1

voltage of device is linearly proportional to the velocity of droplet. The larger the size of the droplet would induce the larger voltage. The concentration of NaCl solution is also a critical factor, but the trend is not monotonic. The best concentration of NaCl solution is around 0.01 M. The voltage output is less than 0.5 mV. However, in carbon black, the potential is maximum ~1.0 V when deionized (DI) water is used for evaporation HV generator [29]. It is interesting that the voltage can be multiplied by drawing multiple droplets simultaneously as shown in **Figure 9c**. Experimental results indicate the voltage is near zero when two droplets are moving in the opposite directions. The voltage of device is closely related to the ion species, such as MgCl2, HCl, and NH3·H2O. However, there is no response for DI water. This indicates the drawing potential comes from the ion-induced EDL capacitance. The voltage induced with HCl solution is negative. The authors think that there is a H3O+ layer on the surface of graphene. Therefore the positively charged graphene would attract the negative Cl<sup>−</sup> anions, which dominate the electric double layer. At last the authors show a handwriting sensor with a Chinese brush and 0.01 M NaCl. Two

<sup>−</sup> (right–left) and E2

perpendicularly on the four sides of the graphene to distinguish the handwriting direction (inset of **Figure 9c**). The drawing direction related to voltage signal can be well detected as in **Figure 9f**. Moreover, the force and speed of the handwriting

Graphene oxide framework with lots of pores can facilitate the diffusion of water molecules. Along with the asymmetrical oxygen-containing groups, Zhao et al. observed that the transport of the ionic charge carriers is accelerated due to the ionic gradient [16]. When the RH variation is 75%, the potential can increase up to 260 mV in 2 s. The concentration gradient of Li ions in 3D PPy framework can also show good sensitivity for moisture [31]. The potential is 60 mV under the RH variation of 85%. More importantly, the stability of this material is stable during the

Besides the HV potential, other signals can also be utilized for sensing, such as the resistance of materials, the length of fibers, and the volume of materials [32–36]. Of course, the change of other physical fields, such as temperature and wind speed, can be detected by HV devices directly or indirectly [29, 37].

As we all know, the common solar cells only work on sunny days, but do not work in the night and on rainy days. Combining PV and HV effects, a hybrid cell for all-weather power generation was developed by integrating a solar cell with a HV device. Tang et al. fabricated a flexible solar cell made of a transparent graphene electrode and a dye-sensitized solar cell [38]. The hybrid cell can be excited by solar light on sunny days and raindrops on rainy days, yielding a solar-to-electric conversion efficiency of 6.53% under AM 1.5 irradiation and power in the range of 5.12–54.19 pW by simulated raindrops (**Figure 10b**). However, the output of this hybrid cell is still far lower than the actual requirement. Then Tang et al. changed the graphene to graphene/carbon black/polytetrafluorethylene (PTFE) for the hybrid cell fabrication [18]. But the voltage and current did not show significant improvement under simulated raindrops. Zhong et al. developed a 2D hybrid nanogenerator based on graphene and silicon for all-weather electricity generation [39]. In this hybrid cell, the graphene and silicon form the van der Waals Schottky diode (**Figure 10c**). This hybrid device delivers a maximum output power of 49.3 μW under light illumination. When the DI water flows on the graphene under light from Au electrode to Ag electrode, an additional potential of 2.54 mV can be generated (**Figure 10d**). However, there is no response in the dark, indicating no interaction

+ –E2

<sup>−</sup> (bottom-top), were patterned

**56**

*(a) The quasi-all-weather solar cell that can produce electricity from rain and sun [38]. (b) Power signals produced by dropping 0.6 M NaCl droplets on rGO film [38]. (c) Schematic structure of the 2D hybrid nanogenerator of graphene/SiO2/Si [θ = 30°, PDMS: Poly(dimethysiloxane)] [18]. (d) Voltage responses to the flow of DI water over graphene/Si Schottky diode in the dark and under room light illumination [18]. (e) Voltage responses to the flow of DI water toward different directions [18].*

exists between water and hybrid cell during the water flowing process. The authors think that the potential response should arise from the interaction between water and graphene/Si Schottky diode (the doping and dedoping at the front and rear side of water droplet, respectively) instead of the water-graphene interaction or the water-electrode interaction. Moreover, the negatively additional potential can be observed when the water flows toward Au electrode. This phenomenon should be attributed to the asymmetric potential profile of the graphene channel.

### **4.3 Harvesting Ocean wave energy**

Due to the intermittency and randomness of raining, it is wise to harvest energy from the ocean wave energy. The ocean wave energy is renewable and inexhaustible because 70% of the Earth's surface is covered by the ocean. The ocean wave energy is also called as blue energy [40]. Tan et al. fabricated a film type wave energy generator with graphene, carbon black, and polyurethane [25]. A voltage of > 20 mV and a current of > 10 μA are produced in a 15 cm2 -sized generator. Moreover, the devices are sustainably stable upon persistent attack by waving ocean. The floating devices on the sea can be packaged into the wave energy stations by series and parallel connections. Tan et al. further proposed a photo-induced charge boosting liquid-solid electrokinetic generator with a structure of polyurethane/graphene oxide-carbon black-multi-walled carbon nanotube/carbon quantum dots/copper (PU/GO-CB-MWCNT/CQDs/Cu) [41]. Under AM1.5 illumination, the voltage, current, and power density achieved by this device are 0.1 V, 0.39 mA, and 26.6 mW/ m2 , respectively. The working mechanism is described in **Figure 11a**-**c**. Due to the difference of Na+ and Cl<sup>−</sup> ions on adsorption energy, the EDL can be formed. Same as the principle of waving potential as above, the potential signal would change along with the seawater moving and the capacity change. **Figure 11d** presents an enlarged one-cycle electrical signal generation. When the seawater moves to the

### **Figure 11.**

*(a) When the ocean wave meets with the PU/GO-CB-MWCNT/CQDs film, an EDL is generated due to the formation of Na+ cation layer and electron layer [25]. (b) When the ocean wave reaches the top of the film, a highest voltage can be observed due to the charge of EDL [25]. (c) When the ocean wave is falling downward, a decreasing voltage is obtained because of the discharging of EDL [25]. (d) the voltage and current change of HV cell in one cycle of ocean wave change [25]. (e) under illumination, the electron density is enhanced with CQDs through light excitation [25]. (f) Illustration of the HV networks assembled with HV cells in series and/ or parallel. This network can float on the ocean and harvest wave energy and solar energy [25].*

highest position, the maximum peak electricity signal is obtained. In this work, CQDs are used for visible light absorption in the range of 330–490 nm. Then more electrons can be excited, the surface electron density increased as in **Figure 11e**. The authors lastly proposed a circuit design for scaling up the power output in large-scale networks (**Figure 11f**). However, a cost-effective, stable, and promising scalable approach for efficiently harvesting ocean wave energy is an open question.

## **5. Challenges and perspectives**

As shown in **Table 1**, the performance of reported HV devices based on the carbon materials is summarized. Even though the great progress has been achieved in recent years, there are several challenges to overcome in the future. The challenges

**59**

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China*

hydrovoltaic materials should be synthesized or constructed.

as a new 2D carbon material has unique sp. and sp2

urgent needed in the following decade years.

device into a viable and broad industry technology.

**6. Conclusions**

include the following: (i) the power is far from the need of practical applications. (ii) The stability and durability in real environment is not clear. (iii) It is difficult to achieve large-scale integrated applications. (iv) More advanced experimental technologies should be developed to reveal the unclear mechanism. (v) Non-carbon

Nowadays, the research of hydrovoltaic materials and technologies is still in its infancy. To solve the above issues, we may try to follow the following approaches. (i) Understand the interaction mechanism between water and carbon for the further improvement of hydrovoltaic device performance. This requires to controllably modify the electronic structure of carbon and manipulate the molecules/ions in flows. For example, heteroatom doping can be adopted to tune the electric properties. (ii) The composition and nanostructure of carbon materials can significantly affect the capability of electricity generation. Therefore the nanostructure should be optimized to enhance the effective surface area. Moreover, the composition tailoring can enhance the conductivity, reducing the loss during charge transport. (iii) To improve the output of hydrovoltaic devices, the carbon materials can be combined with other functional materials, such as photovoltaic materials, ferroelectric material, and piezoelectric materials. This route can additionally convert the solar energy and mechanical energy for higher voltage and current outputs. (iv) Develop new nanomaterials with hydrovoltaic properties. For example, graphdiyne

high theoretical conductivity, good chemical activity, good physical stability, and the intrinsic bandgap of ~0.5 eV. Moreover, the synthesis temperature is usually below 100°C. We believe this new carbon material will exhibit unique hydrovoltaic properties. (v) Some in situ and in operando technologies should be used to characterize the interface between water and solid at atom/nano level, such as atomic force microscopy, infrared/Raman spectroscopy, AC impedance spectroscopy, and so on. Nevertheless, hydrovoltaic materials and technologies are very promising for harvesting energy in water. More research efforts should be devoted to realize the practical applications in the near future. In hydrovoltaic field, China stands in the forefront of the world. To realize the practical applications, the multidisciplinary collaboration in research, the government support, and the market promotion are

In summary, we introduced the water-carbon interactions and the popular mechanisms of hydrovoltaic effects and reviewed the recent progress of hydrovoltaic devices. This field is in its infancy but is a promising direction in the future. Great achievements in moisture, droplet, flow, and evaporation-induced electricity generation have been gained. Since water evaporation is uninterrupted and available under any conditions, the hydrovoltaic devices have great advantages over other energy conversion devices if the power could meet the daily usage. As in **Table 1**, it is exciting that the optimized hydrovoltaic devices now can generate voltage of 3 V. We believe that the rapid growth will bring this emerging hydrovoltaic

In China, the water resource is around 6% of the Earth's water resource. However, 80% of the water resource is distributed in the South of China. Therefore the energy harvested from water through hydrovoltaic effect, not the conventional hydropower station, is an attracting and alternative approach in China because the hydrovoltaic effect can generate electricity from not only the water flow, but also the water moisture, droplet, and evaporation. This future technology is a very

hybridized electronic structure,

*DOI: http://dx.doi.org/10.5772/intechopen.90377*

### *The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China DOI: http://dx.doi.org/10.5772/intechopen.90377*

include the following: (i) the power is far from the need of practical applications. (ii) The stability and durability in real environment is not clear. (iii) It is difficult to achieve large-scale integrated applications. (iv) More advanced experimental technologies should be developed to reveal the unclear mechanism. (v) Non-carbon hydrovoltaic materials should be synthesized or constructed.

Nowadays, the research of hydrovoltaic materials and technologies is still in its infancy. To solve the above issues, we may try to follow the following approaches. (i) Understand the interaction mechanism between water and carbon for the further improvement of hydrovoltaic device performance. This requires to controllably modify the electronic structure of carbon and manipulate the molecules/ions in flows. For example, heteroatom doping can be adopted to tune the electric properties. (ii) The composition and nanostructure of carbon materials can significantly affect the capability of electricity generation. Therefore the nanostructure should be optimized to enhance the effective surface area. Moreover, the composition tailoring can enhance the conductivity, reducing the loss during charge transport. (iii) To improve the output of hydrovoltaic devices, the carbon materials can be combined with other functional materials, such as photovoltaic materials, ferroelectric material, and piezoelectric materials. This route can additionally convert the solar energy and mechanical energy for higher voltage and current outputs. (iv) Develop new nanomaterials with hydrovoltaic properties. For example, graphdiyne as a new 2D carbon material has unique sp. and sp2 hybridized electronic structure, high theoretical conductivity, good chemical activity, good physical stability, and the intrinsic bandgap of ~0.5 eV. Moreover, the synthesis temperature is usually below 100°C. We believe this new carbon material will exhibit unique hydrovoltaic properties. (v) Some in situ and in operando technologies should be used to characterize the interface between water and solid at atom/nano level, such as atomic force microscopy, infrared/Raman spectroscopy, AC impedance spectroscopy, and so on. Nevertheless, hydrovoltaic materials and technologies are very promising for harvesting energy in water. More research efforts should be devoted to realize the practical applications in the near future. In hydrovoltaic field, China stands in the forefront of the world. To realize the practical applications, the multidisciplinary collaboration in research, the government support, and the market promotion are urgent needed in the following decade years.

## **6. Conclusions**

*Green Energy and Environment*

**58**

**Figure 11.**

*formation of Na<sup>+</sup>*

**5. Challenges and perspectives**

highest position, the maximum peak electricity signal is obtained. In this work, CQDs are used for visible light absorption in the range of 330–490 nm. Then more electrons can be excited, the surface electron density increased as in **Figure 11e**. The authors lastly proposed a circuit design for scaling up the power output in large-scale networks (**Figure 11f**). However, a cost-effective, stable, and promising scalable approach for efficiently harvesting ocean wave energy is an open question.

*or parallel. This network can float on the ocean and harvest wave energy and solar energy [25].*

*(a) When the ocean wave meets with the PU/GO-CB-MWCNT/CQDs film, an EDL is generated due to the* 

*highest voltage can be observed due to the charge of EDL [25]. (c) When the ocean wave is falling downward, a decreasing voltage is obtained because of the discharging of EDL [25]. (d) the voltage and current change of HV cell in one cycle of ocean wave change [25]. (e) under illumination, the electron density is enhanced with CQDs through light excitation [25]. (f) Illustration of the HV networks assembled with HV cells in series and/*

 *cation layer and electron layer [25]. (b) When the ocean wave reaches the top of the film, a* 

As shown in **Table 1**, the performance of reported HV devices based on the carbon materials is summarized. Even though the great progress has been achieved in recent years, there are several challenges to overcome in the future. The challenges

In summary, we introduced the water-carbon interactions and the popular mechanisms of hydrovoltaic effects and reviewed the recent progress of hydrovoltaic devices. This field is in its infancy but is a promising direction in the future. Great achievements in moisture, droplet, flow, and evaporation-induced electricity generation have been gained. Since water evaporation is uninterrupted and available under any conditions, the hydrovoltaic devices have great advantages over other energy conversion devices if the power could meet the daily usage. As in **Table 1**, it is exciting that the optimized hydrovoltaic devices now can generate voltage of 3 V. We believe that the rapid growth will bring this emerging hydrovoltaic device into a viable and broad industry technology.

In China, the water resource is around 6% of the Earth's water resource. However, 80% of the water resource is distributed in the South of China. Therefore the energy harvested from water through hydrovoltaic effect, not the conventional hydropower station, is an attracting and alternative approach in China because the hydrovoltaic effect can generate electricity from not only the water flow, but also the water moisture, droplet, and evaporation. This future technology is a very


*PMMA, polymethyl methacrylate; PVDF, polyvinylidene fluoride; ITO, indium tin oxide; FTO, fluorine-doped tin oxide; PDMS, polydimethylsiloxane.*

**61**

**Author details**

and Yuelong Huang

People's Republic of China

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China*

can harvest ocean wave energy for island power supply in Chinese waters.

Jiale Xie\*, Liuliu Wang, Xiaoying Chen, Pingping Yang, Fengkai Wu

Institute of Photovoltaics, Southwest Petroleum University, Chengdu,

© 2019 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,

\*Address all correspondence to: jialexie@swpu.edu.cn

provided the original work is properly cited.

promising solution in the North of China. More importantly, this future technology

The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant 21703150), the China Postdoctoral Science Foundation (Grant 2015 M582495), and the Sichuan Science and Technology

*DOI: http://dx.doi.org/10.5772/intechopen.90377*

**Acknowledgements**

**Conflict of interest**

Program (Grant 2018JY0015).

The authors declare no conflict of interest.

### **Table 1.**

*Performance summary of reported hydrovoltaic devices based on the carbon materials.*

*The Emerging of Hydrovoltaic Materials as a Future Technology: A Case Study for China DOI: http://dx.doi.org/10.5772/intechopen.90377*

promising solution in the North of China. More importantly, this future technology can harvest ocean wave energy for island power supply in Chinese waters.

## **Acknowledgements**

*Green Energy and Environment*

Graphene/carbon black/PTFE

Nitrogen-doped graphene

Graphene hydrogel membrane

Carbon black-glass fiber hybrid film

Partially reduced GO

*oxide; PDMS, polydimethylsiloxane.*

sponge

**Table 1.**

Monolayer reduced GO ITO/PET Simulated

Graphene/carbon black Glass, etc. Simulated

**Materials Substrate Solution Flow type Potential** 

GO film / DI water Moisture 700 25 [42] GO framework / DI water Moisture 260 / [16] GO film / DI water Moisture 1500 136 [43] GO film / DI water Moisture 700 0.2 [44] GO / DI water Moisture 340 ~1 [45] GO nanoribbon / DI water Moisture 40 300 [46] GQDs PET DI water Moisture 270 / [14] Wrinkled graphene SiO2/Si NaCl Moisture 20 0.045 [17]

raindrops

raindrops

Monolayer graphene SiO2/Si DI water Droplet 28.14 1800 [39] Monolayer graphene PVDF DI water Droplet 100 / [47] Monolayer graphene SiO2/Si DI water Droplet 10 0.5 [48]

Graphene grid PDMS NaCl Droplet 0.1 / [50] Monolayer graphene SiO2/Si NaCl Droplet 0.15 / [11] Monolayer graphene PET etc. NaCl Droplet 500 / [51] Graphene foam / DI water Flow 0.001 100 [52] Reduced GO Paper-pencil DI water/MgCl2 Flow 280 812.5 [53]

waving

Monolayer graphene PET NaCl Flow 100 11 [10] Carbon black Quartz DI water Flow 1000 0.15 [29]

Few-layer graphene SiO2/Si HCl Flow 25 340 [55] Few-layer graphene SiO2/Si HCl Flow 120 / [56] Pair of graphene sheets / NaCl Flow 1000 2 [24] Aligned CNT fiber / NaCl Flow 341 / [22] Carbon black film DI water Evaporation 1000 / [29] Carbon Film Al2O3 DI water Evaporation 1000 0.6 [57] Graphene/carbon cloth / NaCl Evaporation 370 / [58]

*PMMA, polymethyl methacrylate; PVDF, polyvinylidene fluoride; ITO, indium tin oxide; FTO, fluorine-doped tin* 

*Performance summary of reported hydrovoltaic devices based on the carbon materials.*

SiO2/Si DI water Droplet 380 / [49]

/ NaCl Flow / 0.002 [54]

/ DI water Evaporation 3000 / [30]

/ DI water Evaporation 630 ~100 [59]

ITO/FTO Simulated

**(mV)**

Droplet 0.1 0.49 [38]

Droplet 0.228 5.97 [18]

Flow 11.14 3 [25]

**Current (μA)**

**Refs.**

**60**

The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant 21703150), the China Postdoctoral Science Foundation (Grant 2015 M582495), and the Sichuan Science and Technology Program (Grant 2018JY0015).

## **Conflict of interest**

The authors declare no conflict of interest.

## **Author details**

Jiale Xie\*, Liuliu Wang, Xiaoying Chen, Pingping Yang, Fengkai Wu and Yuelong Huang Institute of Photovoltaics, Southwest Petroleum University, Chengdu, People's Republic of China

\*Address all correspondence to: jialexie@swpu.edu.cn

© 2019 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.

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**Chapter 4**

**Abstract**

energy storage, Li-ion battery

**1. Introduction**

**67**

A Circular Economy of

Electrochemical Energy Storage

SOH/RUL Estimation Methods

*Simon Montoya-Bedoya, Laura A. Sabogal-Moncada,*

*Esteban Garcia-Tamayo and Hader V. Martínez-Tejada*

Humanity is facing a gloomy scenario due to global warming, which is increasing

at unprecedented rates. Energy generation with renewable sources and electric mobility (EM) are considered two of the main strategies to cut down emissions of greenhouse gasses. These paradigm shifts will only be possible with efficient energy storage systems such as Li-ion batteries (LIBs). However, among other factors, some raw materials used on LIB production, such as cobalt and lithium, have geopolitical and environmental issues. Thus, in a context of a circular economy, the reuse of LIBs from EM for other applications (i.e., second-life batteries, SLBs) could be a way to overcome this problem, considering that they reach their end of life (EoL) when they get to a state of health (SOH) of 70–80% and still have energy storage capabilities that could last several years. The aim of this chapter is to make a review of the estimation methods employed in the diagnosis of LIB, such as SOH and remaining useful life (RUL). The correct characterization of these variables is crucial for the reassembly of SLBs and to extend the LIBs operational lifetime.

**Keywords:** second-life batteries, RUL/SOH estimation, circular economy,

The Sustainable Development Goals (SDGs) are a call to action against global issues in the twenty-first century [1] such as climate change, geopolitical topics, overgrowing population, increasing energy demand, and resource scarcity, among others [2]. According to the International Energy Agency (IEA) statistics, the electricity and heat producers and transport sector are the largest greenhouse gas emitters, with at least 90% of the total CO2 emissions [3, 4]. In 2018, above 26% of electric energy was generated from renewable sources (RSs) [5]. However, this percentage is still low in order to maintain global warming below the 2°C increment threshold stated in the 2015 Paris Agreement [6]. Taking this into account, its clear

Systems: Critical Review of

for Second-Life Batteries

## **Chapter 4**
