2.1.3 Metal-halide system

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

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.

The existing thermochemical cycle for hydrogen production mainly includes metal oxide system thermochemical cycle, sulfur-containing system, sulfuric acid decomposition method, metal-halide system, and reformed methane hydrogen production. All of the thermochemical cycles could be classified as multi-step ther-

There are four main types of hydrogen production in sulfur-containing systems:

SO2 þ I2 þ 2H2O ! 2HI þ H2SO4 (10)

2HI ! H2 þ I2 (12)

O2 (11)

O2 (13)

1 2

1 2

iodine-sulfur cycle, H2SO4-H2S cycle, sulfuric acid-methanol cycle, and sulfate cycle. Among them, the iodine-sulfur cycle is the most famous. It was invented by the United States GA company in the 1970s, so it is also called the GA cycle. The

H2SO4 ! H2O þ SO2 þ

H2O ! H2 þ

GA company found [19] that the excess I2 exists, and HI and H2SO4 can be separated into two liquid phases, which is the basis for the development of the IS cycle. The advantages of the IS cycle are using of thermal energy below 1000°C for hydrogen generation, closed circuit, only water being needed to be added in the circulation process, and the expected efficiency which can reach 52%. The disadvantages are concentrated sulfuric acid being highly corrosive when heated at high temperature; the equilibrium decomposition ratio of HI being low (20%); and the reaction intermediate products sulfur dioxide and iodine being easy to cause pollu-

This type of method is best known as the Westinghouse cycle [20], and its main process is shown in Figure 9. The highest temperature in the process needs to be above 800°C, and the efficiency of the cycle can reach 40%. If multi-stage electrolysis is used, it can reach 46%. However, the disadvantage is that concentrated

process is shown in Figure 8. The main reaction process is as follows:

mochemical cycles and two-step thermochemical cycles.

2.1 Multi-step thermochemical cycles

tion and liable to have side reactions.

2.1.2 Sulfuric acid decomposition method

148

2.1.1 Hydrogen generation system containing sulfur

work is shown in Figure 7.

Wind Solar Hybrid Renewable Energy System

2. Thermochemical cycle

The most famous in this system is the UT-3 cycle proposed by the University of Tokyo. The main process is as follows:

$$\text{CaBr}\_2 + \text{H}\_2\text{O} \xrightarrow{1033\text{K}} \text{CaO} + 2\text{HBr} \tag{14}$$

$$\text{CaO} + \text{Br}\_2 \xrightarrow{845\text{K}} \text{CaBr}\_2 + \frac{1}{2}\text{O}\_2\tag{15}$$

$$\mathrm{Fe\_3O\_4} + \mathrm{8HBr} \xrightarrow{493\mathrm{K}} \mathrm{3FeBr\_2} + 4\mathrm{H\_2O} + \mathrm{Br\_2} \tag{16}$$

$$\text{\#FeBr}\_2 + 4\text{H}\_2\text{O} \xrightarrow{833\text{K}} \text{Fe}\_3\text{O}\_4 + \text{\#HBr} + \text{H}\_2\tag{17}$$

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

Figure 9. Westinghouse cycle diagram.

Figure 10. Metal oxide thermochemical cycle for hydrogen production.

HBr by electrolytic method or "cold" plasma method. This reaction has the following advantages: its expected thermal efficiency is 35–40%, and if the power is generated at the same time, the overall efficiency can be improved by 10%; the two-step key reaction is a gas-solid reaction, which significantly simplifies the separation of products and reactants; the elements used are cheap and readily available; the process involves only solid and gaseous reactants and products. However, the separation of intermediate products in the reaction process is also a problem and challenge in the process.
