Solar Energy-Hydrogen Energy Conversion

Hydrogen energy belongs to high-grade energy. The solar energy can be converted into the hydrogen energy through water decomposing or other approaching. The methods include:

1)        Hydrogen Production by Electrolysis of Water

Electrolysis of water is the method that has been commonly applied and comparably ripe in technologies, with higher efficiency (75%-85%), heavy consumption in power. The hydrogen production by conventional electrolysis can obtain fewer gains in the respect of utilization of energy. Therefore, only if the cost of solar power generation can be reduced greatly, does the mass-scale hydrogen production by electrolysis be realized.

2)        Hydrogen Production by Thermolysis of Water

When water or steam is heated to 3000K, hydrogen and oxygen in the water can be decomposed. This method can produce hydrogen in high efficiency of which high temperature is obtained by concentrator. Normally this method is not applied to producing the hydrogen.

3)        Hydrogen Production by Thermochemical Cycles

In order to mitigate the high temperature required by direct thermolysis, thermochemical cycles are developed. This method is to input one or several intermediate into the water, which is heated to lower temperature. After experiencing variable reaction stage, the water is finally decomposed to hydrogen and oxygen with the consumption of intermediate for recycle use. The temperature for thermochemical cycles is ranging from 900 to 1200K, which is easily obtained by normal rotating parabolic concentrator. The decomposing efficiency is ranging from 17.5% to 75.5%. The existing main problem lies in the deoxidizing of the intermediate, even if deoxidizing by 99.9% to 99.99%, 0.1% to 0.01% complement should be made which may affect the price of hydrogen and result in environmental pollution.

4)        Hydrogen Production by Photochemical Decomposing of Water

The process of hydrogen production is similar to that of thermochemical cycles, which is to add a kind of photosensitive matter as activator to increase the absorption on wave energy in sunlight. The hydrogen is produced by use of photochemical reaction. In Japan, a set of comprehensive hydrogen production flow including photochemical and thermoelectricity reaction was once designed by use of iodine¡¯s sensitiveness upon light. It can produce hydrogen 97L per hour and in an efficiency of 10%.

5)        Hydrogen Production by Photoelectrical Chemical Cells

In 1972, photoelectrical chemical cells were developed in Japan by use of electrode of n-model titanium dioxide semiconductor as anode and platinum black as cathode. Under the radiation of sunlight, the cathode produced hydrogen while the anode produced oxygen. If two end of electrode were connected by conducting wire, current could be passing through which resulted that the hydrogen and oxygen production and electricity energy obtained can be simultaneously realized by photoelectrical chemical cells under the radiation of sunlight. This experiment result aroused high attention in the scientists in each country, and was regarded as a breakthrough in solar energy technologies. But the efficiency was very lower, only 0.4%. It can only absorb the ultraviolet light among the sunlight, and the electrode is subject to be corrosive with unsteady performance. Therefore, the photoelectrical chemical cells have not yet satisfied the application needs.

6)        Hydrogen Production by Complication Catalysis

Since 1972, scientists have found that the excited state of tri link pyridine ruthenium complex compound is capable to transfer the electron, catalyze the transfer of electric charge. They proposed to utilize such process to produce hydrogen. This complex compound was a kind of catalyzer, functioned as absorption of light energy, producing the separation of electric charge, transfer and concentrating of electric charge. Through a serial of process, the water will finally be decomposed to hydrogen and oxygen. This technology has not been ripe and the research is undergoing.

7)        Hydrogen Production by Photosynthesis

40 years ago, it was found that green algae could emit hydrogen without oxygen under the radiation of sunlight. 10 years ago, it was found that many algae can produce hydrogen functioned by photosynthesis under a certain condition that it was applied in an environment of no oxygen.

Right now, due to the insufficient understanding on photosynthesis and hydrogen production theory, and its lower efficiency in hydrogen production, it has a long way to realize its industrialized production. According to estimate, if the production efficiency can be raised to 10%, thus the production can reach 9-gram molecule per square meter per day. If 50,000 square kilometer area can receive the solar energy, then the photosynthesis project can satisfy the total fuel requirement for the whole U.S.