Three types of solar energy applications
In 1839, Edmund Becquerel, a 19 year old French boy, discovered that electricity was present in liquid electrolytes under illumination. In the excitement of the moment, he did not realize that nearly 200 years later, his discovery would be a "Noah's Ark" to save human beings when fossil energy was about to dry up.
The modern application of solar energy is divided into three types: one is to heat, heat up substances such as water, and to provide heat for human beings. The second is to generate electricity directly. When the sun is exposed to a device, it produces electricity directly in it. This technology is called solar photovoltaic technology, which is called the solar cell. The third is to gather the solar energy to make the heated material at a high temperature of hundreds of degrees centigrade. The heated substance can be a kind of oil or other kind of material. Through the exchange of energy, the high temperature state can vaporize water, produce high pressure steam, and promote the gas turbine to generate electricity.
The form of thermal utilization of solar energy has been widely applied in the world. The water heaters that can be seen in many parts of China belong to this kind of technology. However, because solar energy can not generate electricity, its utilization range is small. The conversion of solar energy into electrical energy by solar photovoltaic technology is much more widely used. In 2012, 32 gigawatts of PV modules were installed in the world, with a total installed capacity of 100 gigawatts. If we look back to 2002 10 years ago, we found that the global solar cell installed at that time was only 0.43 gigawatts.
At present, solar photovoltaic power generation is just beginning, and the real large-scale application has not yet started, because the cost of photovoltaic power generation is still high. Despite the joint efforts of the scientific and industrial sectors, the price of solar cell components has dropped from 25 yuan in 2008 to 4 yuan in 2012, but the cost of photovoltaic power generation is still higher than the cost of China's thermal power generation. Efforts are being made to continuously improve the efficiency of solar cells and gradually reduce costs.
Crystal silicon solar cells are the mainstream
At present, the main product of solar cells is crystalline silicon solar cells. Of the solar cell products produced in 2012, 90% belonged to crystalline silicon solar cells. The so-called crystal silicon solar cell is a silicon material which is made of silicon material into 180 micron thick thin slices of crystal, on which the diode structure similar to the semiconductor device is prepared by the diffusion method.
The crystalline silicon solar cell can be divided into monocrystalline silicon cell and polysilicon battery. The whole silicon chip of the monocrystalline silicon cell is a grain, and there are many grains in the polysilicon battery chip. Because of the defects in the grain and the defects in the grain boundaries, the charge carrier movement is hindered, so the efficiency of the polysilicon battery is always lower than that of the monocrystalline silicon solar cell. At present, the efficiency of large scale production of monocrystalline silicon cells is 18.5% to 19%, while the efficiency of polysilicon batteries is about 17.5%, and the efficiency of the two batteries is 16% and 15%, respectively.
The maximum efficiency of the monocrystalline silicon battery has reached 25% in the laboratory. There is still a big gap between the efficiency of the large-scale production of the solar cell and the highest efficiency in the laboratory. However, there are many technical routes in the industry to further improve the efficiency.
Other types of solar cells
In addition to crystal silicon solar cells, scientists have developed many other kinds of solar cells. There are two targets to develop these batteries, one to reduce the cost and the other to improve the efficiency. From the point of view of cost reduction, the main batteries are thin film batteries, including amorphous silicon thin film solar cells, cadmium telluride thin film solar cells, copper indium gallium selenium thin film solar cells, dye sensitized solar cells, all of which are aimed at reducing the use of materials.
The thickness of the material used to absorb sunlight is only 0.5 to 3 microns, which is much lower than that of 180 microns of crystalline silicon. Although the material is low, the efficiency of most thin film cells is low due to the limitation of the preparation process. However, the efficiency of laminated thin film batteries containing III-V elements is very high. For example, three layers of solar cells, made of gallium arsenide, gallium arsenide, and germanium, have an efficiency of 34.1% under non spotlight conditions and 43.5% at 418 times of spotlight, and researchers are confident that the efficiency of this kind of spotlight can be increased to 50% in the near future. However, the material and process cost of this kind of battery are very high, it is difficult to apply in large scale power plant. It can only reduce the cost of power generation by combining with the light and tracking system, and still has many difficulties in the application. So far, thin film batteries are still at a disadvantage in the competition with crystalline silicon solar cells.
The principle of dye-sensitized solar cells is to use the dye to absorb the sunlight, making the electrons in the low energy state transition to the high energy state, and transport the electrons in the high-energy state to the guide band of the titanium dioxide material and transfer them to the outer electrode, and the vacancy left in the low energy state is transported by the iodine ion in the liquid electrolyte. The electrons are filled, and the iodide ions exchange electrons on the back metal electrode, such as platinum, to form a closed loop. At present, the maximum efficiency of the battery in the laboratory is 11.8%, and there is no mass production. The main reasons are: (1) the liquid electrolyte is included in the device, and it is easy to leak in the long term use; (2) the platinum back electrode is more expensive; (3) the long term service life of various materials is still not long; (4) efficiency is still not high.
New energy makes the future more peaceful
In the next 10 years, crystal silicon will still be mass light.