As we introduced earlier, solar energy in a broad sense refers to many energy sources on the earth, such as wind energy, chemical energy, potential energy of water, and other forms of energy that are caused or converted by solar energy. We collectively call it solar energy.
We divide the basic forms of solar energy utilization into five main methods: solar-thermal energy conversion, solar-electric energy conversion, solar-hydrogen energy conversion, solar-biomass energy conversion, and solar-mechanical energy conversion.
1. Solar energy-thermal energy conversion
Solar-thermal energy conversion, that is, the photothermal conversion of solar energy, is also called photothermal utilization.
The basic principle of solar-thermal energy conversion is to collect solar radiant energy and convert it into heat energy through interaction with matter for use.
Among the substances that collect solar radiant energy, the black absorbing surface material generally has better performance in absorbing solar radiation and can convert solar energy into heat energy well, but its radiant heat loss is also large, so the black absorbing surface is not ideal Solar absorption surface. The absorption surface can be selected to have a high solar absorption ratio, but its emission ratio to solar radiant energy is relatively low. This material has good performance in absorbing solar radiation and has low radiant heat loss. It is an ideal solar absorption surface material . The material of this absorbing surface is made of selective absorbing material, referred to as selective coating for short. The selective coating was put forward in the 1940s and reached practical requirements in 1955. After the 1970s, many new selective coatings were developed and mass-produced and promoted and applied. At present, hundreds of selective coatings have been developed. China began to develop selective coatings in the 1970s, and achieved many results, and it has been widely used in solar collectors with remarkable results.
Currently, the most commonly used solar collectors include three types: flat-plate collectors, evacuated tube collectors, and focused collectors. Generally, solar thermal utilization is divided into low-temperature utilization (<200℃), medium-temperature utilization (200-800℃) and high-temperature utilization (>800℃) according to the different temperatures and uses that can be reached. At present, low-temperature applications mainly include solar water heaters, solar dryers, solar distillers, solar houses, solar greenhouses, solar air-conditioning refrigeration systems, etc., medium-temperature applications mainly include solar cookers, solar thermal power generation concentrating heat collection devices, etc., high-temperature applications mainly include high temperature Sun furnace and so on.
The use of light and heat has the advantages of low cost, convenience, and high utilization efficiency, but it is not conducive to energy transmission. Generally, it can only be used on-site, and the output energy form does not have universality.
2. Solar energy-electric energy conversion
Solar energy-electric energy conversion, that is, solar power generation. There are many conversion methods for solar power generation, including direct photoelectric conversion, that is, direct photovoltaic power generation, such as photovoltaic power generation, optical dipole power generation; and indirect conversion of photothermal power, such as photothermal power generation, photothermal ion power generation, and thermal photovoltaic power generation. , Photothermal thermoelectric power generation, indirect photochemical power generation, photobio cells, etc.
Electricity is a kind of high-grade energy, and it is more convenient to use, transmit and distribute. Converting solar energy into electrical energy is an important technical basis for large-scale utilization of solar energy, and countries all over the world attach great importance to it. The large-scale utilization of solar energy in the future is mainly used for power generation. There are many ways to use solar power to generate electricity. At present, the two conversion methods described above are mainly practical.
① Light-heat-electric conversion. That is to use the heat generated by solar radiation to generate electricity. Generally, a solar collector is used to convert the absorbed heat energy into steam as a working fluid, and then the steam drives a gas turbine to drive a generator to generate electricity. The former process is light-heat conversion, and the latter process is heat-electricity conversion.
②Optical-electrical conversion. Its basic principle is to use the light effect to directly convert solar radiation energy into electrical energy, and its basic device is a solar cell.
Solar-electric energy conversion takes electric energy as the final form of expression, has the characteristics of extremely convenient transmission, and has great advantages in versatility and storability. In addition, the abundant reserves of silicon, the raw material of solar cells, the continuous improvement of solar cell conversion efficiency, and the continuous decline of production costs have promoted solar-electricity conversion power generation to occupy an important position in the future development of energy, environment and human society.
3. Solar energy-hydrogen energy conversion
Solar-hydrogen energy conversion is also called photochemical utilization. Solar energy can be converted into hydrogen energy by splitting water or other ways, that is, solar hydrogen production. This is a photo-chemical conversion method that uses solar radiation energy to directly split water or produce hydrogen by other means. Photochemical conversion is widespread in nature in the form of photosynthesis, but currently it is not well utilized by humans. The main methods are as follows:
(1) Hydrogen production by solar water electrolysis. Hydrogen production by electrolysis of water is a widely used and mature method at present, with high efficiency (75%-85%), but it consumes a lot of electricity, and the use of conventional electricity to produce hydrogen is more than a loss in terms of energy utilization. Therefore, only when the cost of solar power generation drops significantly, can large-scale hydrogen production by electrolyzing water be realized.
(2) Solar thermal decomposition of water to produce hydrogen. The hydrogen and oxygen in the water can be decomposed by heating the water or steam to more than 3000K. This method has high hydrogen production efficiency, but requires a high-power concentrator to obtain such a high temperature. Generally, this method is not used to produce hydrogen.
(3) Hydrogen production by solar thermal chemical cycle. In order to reduce the high temperature required for the direct thermal decomposition of water by solar energy to produce hydrogen, a thermochemical cycle of hydrogen production method has been developed, that is, adding one or several intermediates to the water, and then heating it to a lower temperature and going through different reaction stages. Finally, the water is decomposed into hydrogen and oxygen, and the intermediates are not consumed and can be recycled. The decomposition temperature of the thermochemical cycle is roughly 900-1200K, which is the temperature easily reached by the ordinary rotating parabolic mirror condenser, and its water decomposition efficiency is 17.5%~75.5%. The main problem is the reduction of intermediates. Even if it is reduced at 99.9%-99.99%, it must be supplemented by 0.1% to 0.01%. This will affect the price of hydrogen and cause environmental pollution.
(4) Solar energy photochemical decomposition of water to produce hydrogen. This hydrogen production process is similar to the above-mentioned thermochemical cycle hydrogen production. A photosensitive substance is added to the water as a catalyst to increase the absorption of long-wave light energy from sunlight, and the photochemical reaction is used to produce hydrogen. Some people in Japan used the sensitivity of iodine to light to design a comprehensive hydrogen production process including photochemical and thermoelectric reactions, which can produce 97 liters of hydrogen per hour with an efficiency of about 10%.
(5) The solar photoelectrochemical cell splits water to produce hydrogen. In 1972, Kenichi Honda and others used the n-type titanium dioxide semiconductor electrode as the anode and the black pin as the cathode to make a solar photoelectrochemical cell. Under sunlight, the cathode produces hydrogen gas and the anode produces oxygen. The two electrodes use wires. Electric current flows through the connection, that is, the photoelectrochemical cell realizes the decomposition of water to produce hydrogen, oxygen, and electricity at the same time under the irradiation of sunlight. This experimental result has attracted great attention from scientists all over the world, and it is regarded as a breakthrough in solar energy technology. However, the hydrogen production efficiency of photoelectrochemical cells is very low, only 0.4%, and can only absorb ultraviolet and near-ultraviolet light in sunlight, and the electrodes are susceptible to corrosion and performance is unstable, so it has not yet reached practical requirements.
(6) Solar complex catalyzes the decomposition of water to produce hydrogen. Since 1972, scientists have discovered that the excited state of the triplex pyridine nail complex has the ability to transfer electrons, and from the complexation to catalyze the charge transfer reaction, they proposed to use this process for photolysis of water to produce hydrogen. This complex is a kind of catalyst, its role is to absorb light energy, produce charge separation, charge transfer and aggregation, and through a series of coupling processes, and finally decompose water into hydrogen and oxygen. Complexation catalytic decomposition of water to produce hydrogen is not yet mature, and research work is continuing.
(7) Hydrogen production by biological photosynthesis. More than 40 years ago, it was discovered that under anaerobic conditions, green algae can emit hydrogen when exposed to sunlight; more than ten years ago, it was discovered that many algae such as blue-green algae adapt to a period of time in an oxygen-free environment, and under certain conditions, they have photosynthetic release. Hydrogen effect. At present, due to insufficient understanding of the mechanism of photosynthesis and algae hydrogen release, the efficiency of algae hydrogen release is very low, and there is still a long way to go to achieve engineered hydrogen production. It is estimated that if the hydrogen production efficiency of algae photosynthesis is increased to 10%, the algae can produce 9 grams of hydrogen per square meter per day. The solar energy received by 50,000 square kilometers can meet all the fuel needs of the United States through the photosynthetic hydrogen release project.
4. Solar energy-biomass energy conversion
Solar-biomass energy conversion, also known as photobiological utilization, is the process of converting solar energy into biomass through photosynthesis of plants.
Biomass energy mainly includes agricultural and forestry products, production and processing waste, industrial waste water, and urban household garbage. The development of biomass energy is the use of various biomass, various gas, liquid and solid energy developed and produced by advanced technology, as well as electricity and heat without destroying the environment and ecology. Currently, it mainly includes biogas, bioethanol, and biological energy. Diesel and bio-waste power generation, fast-growing plants (such as firewood), oil crops and giant seaweeds.
Biomass energy is converted through photosynthesis of plants. Solar energy combines carbon dioxide and water into organic matter (biomass energy) and releases oxygen. Photosynthesis is the largest conversion of solar energy on the earth. The fuel used by modern humans is the solar energy that has been fixed by photosynthesis in ancient times and today. At present, the mechanism of photosynthesis is not fully understood, and the energy conversion efficiency is generally only a few percent. The research on the mechanism will have great theoretical and practical significance in the future.
5. Solar energy-mechanical energy conversion
At the beginning of the 20th century, Russian physicists experimented to prove that light has pressure. In the 1920s, physicists in the former Soviet Union proposed that the huge solar sails in space could be used to propel spacecraft forward under the pressure of sunlight and convert solar energy directly into mechanical energy. Scientists estimate that in the next 10-20 years, the solar sail vision can be realized. Generally, the conversion of solar energy into mechanical energy requires indirect conversion through intermediate processes.