1.2 Advantages of solar thermochemical fuel generation

Solar energy is a huge amount of clean energy. It is of great significance to develop and utilize solar energy reasonably and efficiently. However, the efficient use of solar energy also faces limitations, such as the low energy density of solar energy, the unstable energy supply, the discontinuous time and spatial distribution of solar radiation, and the difficulty of direct storage [2, 3]. Therefore, solar energy is converted into chemical energy stored in fuels, which is generally considered to be an effective solution to make up for solar defects [2–6].

There are mainly four approaches for converting solar energy into chemical energy to generate solar fuel, which is illustrated in Figure 1. The photobiological process is limited by the low energy conversion efficiency now, and it is still at a very early stage of the development [7]. The other three methods have their own properties and have attracted lots of attention. Photo-electrolysis approach is most convenient, but it is also limited by the conversion rate, and researchers are seeking for the catalysts which have better performance. The electrolysis using photovoltaic (PV) materials and electrolyzer is the most mature approach for producing solar fuel. However, the PV materials can only utilize the light with a certain range of wavelength (usually short wavelength light), and the other part of sunlight absorbed is converted into thermal energy, which is wasted as residual heat, leading to a limited PV cell efficiency (the commercial PV cell efficiency is about 15%; the highest multiple-junction PV cell efficiency in lab is higher than 40% with high cost). The total energy efficiency from solar energy to chemical energy is the product of solar power efficiency (e.g., PV cell efficiency) and electrolysis efficiency, so the total efficiency has potential to be further improved. Compared with electrolysis, solar fuel generation by thermochemistry can utilize the sunlight with whole solar spectrum, which has a high theoretical energy efficiency. So the solar thermochemical fuel generation is a promising method and will be discussed in this chapter in details.

Figure 2 is a schematic diagram of the solar thermochemical energy conversion process. Solar energy with lower energy density is received by solar collectors and converted into solar thermal energy. Solar thermal energy enters the absorber through heat transfer and drives the chemical reaction, so that low-energy-density solar energy is stored in the form of solar fuel as chemical energy with high energy density, which is relatively easy for storage and transportation. The sustainable and stable use of solar energy is achieved by transporting solar fuel to remote and needed places for power generation and chemical processes, etc., and solving the

discontinuity of solar distributed in time and space by means of chemical energy

There are many researches about the reaction and system for solar thermochemical fuel generation published, and some of the significant parts have been classified in Figure 3. The two main fuels from solar energy are hydrogen and carbon monoxide, which both have great higher heating values and are potential to be utilized in the future, especially hydrogen, as hydrogen has the following char-

1.Rich hydrogen energy reserves. On the earth, hydrogen mainly exists in the form of hydrocarbons and water, and more than 70% of the earth's surface is covered by water. Therefore, the earth contains a huge amount of hydrogen

2.The energy density of hydrogen is large. The higher heating value of hydrogen is much higher than that of hydrocarbons and alcohol compounds, and the

consumption of hydrogen energy is increasing every year.

and has great potential for development.

Illustration of solar thermochemical energy conversion process.

Solar Thermochemical Fuel Generation DOI: http://dx.doi.org/10.5772/intechopen.90767

Classification of solar thermochemical fuel generation.

storage.

Figure 3.

Figure 2.

acteristics:

143

Figure 1. Illustration of solar fuel via various approaches.

Solar Thermochemical Fuel Generation DOI: http://dx.doi.org/10.5772/intechopen.90767

1.2 Advantages of solar thermochemical fuel generation

Wind Solar Hybrid Renewable Energy System

be an effective solution to make up for solar defects [2–6].

chapter in details.

Figure 1.

142

Illustration of solar fuel via various approaches.

Solar energy is a huge amount of clean energy. It is of great significance to develop and utilize solar energy reasonably and efficiently. However, the efficient use of solar energy also faces limitations, such as the low energy density of solar energy, the unstable energy supply, the discontinuous time and spatial distribution of solar radiation, and the difficulty of direct storage [2, 3]. Therefore, solar energy is converted into chemical energy stored in fuels, which is generally considered to

There are mainly four approaches for converting solar energy into chemical energy to generate solar fuel, which is illustrated in Figure 1. The photobiological process is limited by the low energy conversion efficiency now, and it is still at a very early stage of the development [7]. The other three methods have their own properties and have attracted lots of attention. Photo-electrolysis approach is most convenient, but it is also limited by the conversion rate, and researchers are seeking for the catalysts which have better performance. The electrolysis using photovoltaic (PV) materials and electrolyzer is the most mature approach for producing solar fuel. However, the PV materials can only utilize the light with a certain range of wavelength (usually short wavelength light), and the other part of sunlight

absorbed is converted into thermal energy, which is wasted as residual heat, leading to a limited PV cell efficiency (the commercial PV cell efficiency is about 15%; the highest multiple-junction PV cell efficiency in lab is higher than 40% with high cost). The total energy efficiency from solar energy to chemical energy is the product of solar power efficiency (e.g., PV cell efficiency) and electrolysis efficiency, so the total efficiency has potential to be further improved. Compared with electrolysis, solar fuel generation by thermochemistry can utilize the sunlight with whole solar spectrum, which has a high theoretical energy efficiency. So the solar thermochemical fuel generation is a promising method and will be discussed in this

Figure 2 is a schematic diagram of the solar thermochemical energy conversion process. Solar energy with lower energy density is received by solar collectors and converted into solar thermal energy. Solar thermal energy enters the absorber through heat transfer and drives the chemical reaction, so that low-energy-density solar energy is stored in the form of solar fuel as chemical energy with high energy density, which is relatively easy for storage and transportation. The sustainable and stable use of solar energy is achieved by transporting solar fuel to remote and needed places for power generation and chemical processes, etc., and solving the

Figure 2. Illustration of solar thermochemical energy conversion process.

Figure 3.

Classification of solar thermochemical fuel generation.

discontinuity of solar distributed in time and space by means of chemical energy storage.

There are many researches about the reaction and system for solar thermochemical fuel generation published, and some of the significant parts have been classified in Figure 3. The two main fuels from solar energy are hydrogen and carbon monoxide, which both have great higher heating values and are potential to be utilized in the future, especially hydrogen, as hydrogen has the following characteristics:


#### 1.3 Thermodynamic of solar thermochemical process

Different chemical reaction processes require different temperatures, so it is necessary to match the solar thermal energy temperature with the chemical reaction temperature for efficient energy utilization. Different solar thermal temperatures need to be achieved with different forms of solar collectors for matching various chemical reactions.

The heat collection temperature of solar collectors depends on many factors, but the most important factor is the concentration ratio, which is the ratio of the total area of the opening of the collector mirror field to the spot area on the focal plane. Concentration ratio is an important parameter for designing concentrating solar thermal utilization. Under the same conditions, the higher the concentration ratio, the higher the heat collection temperature. In a unit of time, the energy emitted by a black body per unit area is proportional to the fourth power of its temperature, and solar energy is close to a 6000 K black body, so the radiant energy it emits is:

$$Q\_s = 4\pi r^2 \sigma T\_s^4 \tag{1}$$

According to the definition of the concentration ratio, it is:

the collector into the absorption cavity, given as [18]:

tracking, etc.

145

Figure 4.

Illustration of solar radiation trajectory.

Solar Thermochemical Fuel Generation DOI: http://dx.doi.org/10.5772/intechopen.90767

<sup>C</sup> <sup>¼</sup> <sup>A</sup>

Among them, θ is the opening angle of the sun, and the value is 32<sup>0</sup>

<sup>a</sup> <sup>¼</sup> <sup>R</sup><sup>2</sup>

theoretical limiting concentration ratio is 45,000. In practical applications, the light ratio is much lower than the theoretical condenser ratio, due to manufacturing errors (misfocus, specular errors, etc.), structural disturbances, unsatisfactory optical properties (specular reflectance, glass absorptivity, etc.), shadows and sun

In the actual application process, there are two types of common solar concentrating forms: linear focusing and point focusing. Among them, linear focusing solar collectors include parabolic trough solar collectors and linear Fresnel solar collectors. Because the collectors have different heat collection methods, they also have different light collection ratios and heat collection temperatures. Because pointfocused solar collector focuses in two dimensions and line-focused solar collector focuses in one-dimensional directions, point-focused solar collectors usually have a larger concentration ratio, which could approach a greater temperature. However, high temperature usually means higher requirements for materials and processing industries, higher radiation losses, and heat costs of the collector. Table 1 lists the typical solar thermal power generation mirror field parameters. In thermal power plants, a higher temperature for power generation will allow the Rankine cycle to have a higher Carnot efficiency, leading to a greater power generation efficiency. In the process of collecting solar energy by using a solar collector, energy is dissipated due to radiation. The absorption efficiency is defined as the ratio of the solar energy absorbed by the absorption cavity to the total solar energy projected by

<sup>η</sup>abs <sup>¼</sup> IAηA<sup>α</sup> � <sup>a</sup>εσT<sup>4</sup>

ηabs is the absorption efficiency; I is the solar radiation intensity; A is the area of the condenser; η<sup>A</sup> is the optical efficiency; α and ε are the absorptance and emissivity of the absorption cavity; a is the area of the absorber; σ is the Stefan-Boltzmann

IA (6)

<sup>r</sup><sup>2</sup> <sup>¼</sup> <sup>1</sup> sin <sup>2</sup> <sup>θ</sup> 2

(5)

, so the

Among them,T<sup>s</sup> is the absolute surface temperature of the sun and σ is the Stefan-Boltzmann constant. If the orbit of the earth is regarded as a circle with a radius R, as shown in Figure 4, the energy that Q<sup>s</sup> throws on the absorber of area A is:

$$Q\_{\rm S\to A} = A \cdot \frac{Q\_s}{4\pi R^2} \tag{2}$$

After the absorber absorbs energy, the temperature will rise. Assuming the temperature rises to Ta, if the conduction and convection losses are ignored, the absorber will radiate energy, given as:

$$Q\_{\mathfrak{a}} = a \sigma T\_{\mathfrak{a}}^4 \tag{3}$$

According to the second law of thermodynamics, heat can only be transferred spontaneously from a high-temperature object to a low-temperature object, so the temperature T<sup>a</sup> of the absorber is always less than or equal to the solar surface temperature. In the limit, the two temperatures are equal, that is,T<sup>a</sup> = Ts, and the amount of heat absorbed by the device is equal to the amount of radiation:

$$Q\_{S \to A} - Q\_{\text{a}} = A \cdot \frac{Q\_s}{4\pi R^2} - a\sigma T\_a{}^4 = 0\tag{4}$$

3.Hydrogen is renewable. Hydrogen can be obtained from water, and the oxidation of hydrogen produces water. Therefore, the hydrogen combustion

4.Hydrogen energy is clean energy. Whether hydrogen is consumed by direct combustion or fuel cell power generation, the only product is water, without

5.Hydrogen is relatively easy to be converted and stored. Compared with other energy sources such as solar energy, wind energy, electrical energy, and thermal energy, hydrogen is a chemical raw material and is easily to be converted into hydrocarbons for storage, thereby expanding the scope of

Different chemical reaction processes require different temperatures, so it is necessary to match the solar thermal energy temperature with the chemical reaction temperature for efficient energy utilization. Different solar thermal temperatures need to be achieved with different forms of solar collectors for matching various

The heat collection temperature of solar collectors depends on many factors, but the most important factor is the concentration ratio, which is the ratio of the total area of the opening of the collector mirror field to the spot area on the focal plane. Concentration ratio is an important parameter for designing concentrating solar thermal utilization. Under the same conditions, the higher the concentration ratio, the higher the heat collection temperature. In a unit of time, the energy emitted by a black body per unit area is proportional to the fourth power of its temperature, and solar energy is close to a 6000 K black body, so the radiant energy it emits is:

Q<sup>s</sup> ¼ 4πr

shown in Figure 4, the energy that Q<sup>s</sup> throws on the absorber of area A is:

2 σT<sup>s</sup>

Among them,T<sup>s</sup> is the absolute surface temperature of the sun and σ is the Stefan-Boltzmann constant. If the orbit of the earth is regarded as a circle with a radius R, as

<sup>Q</sup><sup>S</sup>!<sup>A</sup> <sup>¼</sup> <sup>A</sup> � <sup>Q</sup><sup>s</sup>

After the absorber absorbs energy, the temperature will rise. Assuming the temperature rises to Ta, if the conduction and convection losses are ignored, the

Q<sup>a</sup> ¼ aσT<sup>a</sup>

<sup>Q</sup><sup>S</sup>!<sup>A</sup> � <sup>Q</sup><sup>a</sup> <sup>¼</sup> <sup>A</sup> � Qs

According to the second law of thermodynamics, heat can only be transferred spontaneously from a high-temperature object to a low-temperature object, so the temperature T<sup>a</sup> of the absorber is always less than or equal to the solar surface temperature. In the limit, the two temperatures are equal, that is,T<sup>a</sup> = Ts, and the amount of heat absorbed by the device is equal to the amount of radiation:

<sup>4</sup>πR<sup>2</sup> � <sup>a</sup>σTa

<sup>4</sup> (1)

<sup>4</sup>πR<sup>2</sup> (2)

<sup>4</sup> (3)

<sup>4</sup> <sup>¼</sup> <sup>0</sup> (4)

and energy release cycle does not consume other substances.

hydrogen energy in time and space.

Wind Solar Hybrid Renewable Energy System

absorber will radiate energy, given as:

144

chemical reactions.

1.3 Thermodynamic of solar thermochemical process

any waste pollution, which is clean and environmentally friendly.

Figure 4. Illustration of solar radiation trajectory.

According to the definition of the concentration ratio, it is:

$$C = \frac{A}{a} = \frac{R^2}{r^2} = \frac{1}{\sin^2 \frac{\theta}{2}} \tag{5}$$

Among them, θ is the opening angle of the sun, and the value is 32<sup>0</sup> , so the theoretical limiting concentration ratio is 45,000. In practical applications, the light ratio is much lower than the theoretical condenser ratio, due to manufacturing errors (misfocus, specular errors, etc.), structural disturbances, unsatisfactory optical properties (specular reflectance, glass absorptivity, etc.), shadows and sun tracking, etc.

In the actual application process, there are two types of common solar concentrating forms: linear focusing and point focusing. Among them, linear focusing solar collectors include parabolic trough solar collectors and linear Fresnel solar collectors. Because the collectors have different heat collection methods, they also have different light collection ratios and heat collection temperatures. Because pointfocused solar collector focuses in two dimensions and line-focused solar collector focuses in one-dimensional directions, point-focused solar collectors usually have a larger concentration ratio, which could approach a greater temperature. However, high temperature usually means higher requirements for materials and processing industries, higher radiation losses, and heat costs of the collector. Table 1 lists the typical solar thermal power generation mirror field parameters. In thermal power plants, a higher temperature for power generation will allow the Rankine cycle to have a higher Carnot efficiency, leading to a greater power generation efficiency.

In the process of collecting solar energy by using a solar collector, energy is dissipated due to radiation. The absorption efficiency is defined as the ratio of the solar energy absorbed by the absorption cavity to the total solar energy projected by the collector into the absorption cavity, given as [18]:

$$\eta\_{abs} = \frac{IA\eta\_{\rm A}a - a\varepsilon\sigma T^4}{IA} \tag{6}$$

ηabs is the absorption efficiency; I is the solar radiation intensity; A is the area of the condenser; η<sup>A</sup> is the optical efficiency; α and ε are the absorptance and emissivity of the absorption cavity; a is the area of the absorber; σ is the Stefan-Boltzmann


#### Table 1.

Performance parameters of typical solar collector fields [8–17].

#### Figure 5.

The relationship among the ideal absorption efficiency of the collector, the concentration ratio, and the heat collection temperature.

constant (5:<sup>67</sup> � <sup>10</sup>�8W<sup>=</sup> <sup>m</sup><sup>2</sup> � <sup>K</sup><sup>4</sup> ); and <sup>T</sup> is the set thermal temperature. If it is assumed that the absorption cavity is black body, then ηA, α, and ε are all 1, the above formula can be simplified as:

$$\eta\_{abs} = 1 - \frac{\sigma T^4}{IC} \tag{7}$$

where T<sup>H</sup> is the heat collection temperature of the solar collector and T<sup>0</sup> is the

The relationship among the maximum efficiency from solar thermal energy to work, the concentration ratio,

When decomposing water to produce hydrogen without relying on fossil energy,

According to Eq. (8), when the concentration ratio C is given, the first-order

2

By maintaining Eq. (9) equal to 0, the optimal heat collection temperature can

Variation of maximum theoretical efficiency from solar energy to work and optimal thermal energy collection

ð Þ 3T<sup>0</sup> � 4T<sup>H</sup>

IC (9)

<sup>2</sup> <sup>þ</sup> <sup>σ</sup>T<sup>H</sup>

be obtained at a given concentration ratio, and the optimal heat collection

the temperature required for thermochemical reactions is about 1300–1800°C. According to Figures 5 and 6, it can be seen that a tower or dish collector with a concentration ratio of 5000 should be selected. When using fossil fuel (e.g., methane) to split water for hydrogen generation, the reaction temperature could be decreased to 700–1000°C. A tower or dish solar concentrator with a concentration ratio of 1000 should be used. The reaction temperature of the novel solar hydrogen permeation membrane alternating cycle methane reforming system introduced later in this chapter is about 350–400°C, and a trough solar concentrator with a concentration ratio of 80–100 is enough for it, which has a much lower cost com-

ambient temperature. The calculation results are shown in Figure 6.

Figure 6.

Figure 7.

147

temperature with concentration ratio.

and the heat collection temperature.

Solar Thermochemical Fuel Generation DOI: http://dx.doi.org/10.5772/intechopen.90767

pared with tower or the dish-type solar concentrating collector.

derivative function of can be obtained, shown as Eq. (9):

dη<sup>s</sup> <sup>d</sup><sup>T</sup> <sup>¼</sup> <sup>T</sup><sup>0</sup> T<sup>H</sup>

where <sup>C</sup> is the concentration ratio. If <sup>I</sup> <sup>¼</sup> 1000W=m2, the relationship curve between the ideal absorption efficiency of the collector, the concentration ratio, and heat collection temperature can be obtained through calculation, as shown in Figure 5. When the heat collection temperature is fixed, the heat collection efficiency increases with the increase of the concentration ratio; when the concentration ratio is determined, the heat collection efficiency is a decreasing function of the heat collection temperature, mainly because as the heat collection temperature increases, the temperature difference between reactor and environment rises up, leading to an increase in radiation loss, which reduces the efficiency of heat collection.

With multiplying the obtained absorption efficiency by the Carnot cycle efficiency, the system efficiency can be obtained, which is the maximum theoretical conversion efficiency from the solar thermal energy obtained to work or electricity [18]:

$$\eta\_{\rm s} = \left(\frac{T\_{\rm H} - T\_0}{T\_{\rm H}}\right) \left(1 - \frac{\sigma T\_{\rm H}}{IC}\right) \tag{8}$$

Solar Thermochemical Fuel Generation DOI: http://dx.doi.org/10.5772/intechopen.90767

#### Figure 6.

constant (5:<sup>67</sup> � <sup>10</sup>�8W<sup>=</sup> <sup>m</sup><sup>2</sup> � <sup>K</sup><sup>4</sup> ); and <sup>T</sup> is the set thermal temperature. If it is assumed that the absorption cavity is black body, then ηA, α, and ε are all 1, the

The relationship among the ideal absorption efficiency of the collector, the concentration ratio, and the heat

<sup>η</sup>abs <sup>¼</sup> <sup>1</sup> � <sup>σ</sup>T<sup>4</sup>

where <sup>C</sup> is the concentration ratio. If <sup>I</sup> <sup>¼</sup> 1000W=m2, the relationship curve between the ideal absorption efficiency of the collector, the concentration ratio, and heat collection temperature can be obtained through calculation, as shown in Figure 5. When the heat collection temperature is fixed, the heat collection efficiency increases with the increase of the concentration ratio; when the concentration ratio is determined, the heat collection efficiency is a decreasing function of the heat collection temperature, mainly because as the heat collection temperature increases, the temperature difference between reactor and environment rises up, leading to an

With multiplying the obtained absorption efficiency by the Carnot cycle efficiency, the system efficiency can be obtained, which is the maximum theoretical conversion

<sup>1</sup> � <sup>σ</sup>T<sup>H</sup>

4 IC 

increase in radiation loss, which reduces the efficiency of heat collection.

efficiency from the solar thermal energy obtained to work or electricity [18]:

<sup>η</sup><sup>s</sup> <sup>¼</sup> <sup>T</sup><sup>H</sup> � <sup>T</sup><sup>0</sup> T<sup>H</sup>  IC (7)

(8)

above formula can be simplified as:

Type Annual power

Wind Solar Hybrid Renewable Energy System

Performance parameters of typical solar collector fields [8–17].

Parabolic trough collector power plant

Linear Fresnel collector power plant

plant

plant

Table 1.

Figure 5.

146

collection temperature.

Disc collector power

Tower collector power

generation efficiency (%)

Peak efficiency (%)

Operating temperature (°C)

14 25 400 30–100

13 18 300–400 30

20 32 550–750 1000–10,000

16 22 400–600 500–5000

Concentration ratio

The relationship among the maximum efficiency from solar thermal energy to work, the concentration ratio, and the heat collection temperature.

where T<sup>H</sup> is the heat collection temperature of the solar collector and T<sup>0</sup> is the ambient temperature. The calculation results are shown in Figure 6.

When decomposing water to produce hydrogen without relying on fossil energy, the temperature required for thermochemical reactions is about 1300–1800°C. According to Figures 5 and 6, it can be seen that a tower or dish collector with a concentration ratio of 5000 should be selected. When using fossil fuel (e.g., methane) to split water for hydrogen generation, the reaction temperature could be decreased to 700–1000°C. A tower or dish solar concentrator with a concentration ratio of 1000 should be used. The reaction temperature of the novel solar hydrogen permeation membrane alternating cycle methane reforming system introduced later in this chapter is about 350–400°C, and a trough solar concentrator with a concentration ratio of 80–100 is enough for it, which has a much lower cost compared with tower or the dish-type solar concentrating collector.

According to Eq. (8), when the concentration ratio C is given, the first-order derivative function of can be obtained, shown as Eq. (9):

$$\frac{\text{d}\eta\_s}{\text{d}T} = \frac{T\_0}{T\_\text{H}^2} + \frac{\sigma T\_\text{H}^2 (3T\_0 - 4T\_\text{H})}{IC} \tag{9}$$

By maintaining Eq. (9) equal to 0, the optimal heat collection temperature can be obtained at a given concentration ratio, and the optimal heat collection

Figure 7.

Variation of maximum theoretical efficiency from solar energy to work and optimal thermal energy collection temperature with concentration ratio.

temperature can be substituted into Eq. (8) to obtain the sun at the best heat collection temperature. The maximum theoretical efficiency from solar energy to work is shown in Figure 7.

sulfuric acid is highly corrosive at high temperatures and has high requirements for

The most famous in this system is the UT-3 cycle proposed by the University of

CaO <sup>þ</sup> Br2 !845K CaBr2 <sup>þ</sup>

Sakurai [21] found that the hydrolysis of calcium bromide was the slowest during this cycle, because the calcium oxide agglomerated, reducing the reaction interface area. The addition of lauric acid as a foaming agent for dispersing the calcium oxide aggregates can improve the performance of the reaction. The Argonne National Laboratory in the United States has also researched and developed this process [22]. Its main feature is the decomposition or formation of

CaBr2 <sup>þ</sup> H2O !1033K CaO <sup>þ</sup> 2HBr (14)

O2 (15)

1 2

Fe3O4 <sup>þ</sup> 8HBr !493K 3FeBr2 <sup>þ</sup> 4H2O <sup>þ</sup> Br2 (16)

3FeBr2 <sup>þ</sup> 4H2O !833K Fe3O4 <sup>þ</sup> 6HBr <sup>þ</sup> H2 (17)

material selection.

Figure 9.

149

Figure 8.

Illustration of iodine-sulfur cycle.

Westinghouse cycle diagram.

2.1.3 Metal-halide system

Tokyo. The main process is as follows:

Solar Thermochemical Fuel Generation DOI: http://dx.doi.org/10.5772/intechopen.90767

From Figure 7, as the concentration ratio increases, the intensity of radiation received per unit area of the collector increases, so both the optimal heat collection temperature and the maximum theoretical efficiency increase. Because the solar collector has a fixed concentration ratio in practical applications, Figure 7 has guiding significance for determining the optimal heat-collecting temperature for a solar heat collector with a specific concentration ratio. The solar thermal energy of the system has the maximum work efficiency at the best concentration ratio.
