In 1887, the German scientist Hertz noticed that under the irradiation of light, electrons in an object will escape, which is the “photoelectric effect.” At the beginning of the 20th century, Einstein proposed the theory of photons and established a mathematical model of the “photoelectric effect”. In 1954, American scientists Chapin and Pilsson made practical monocrystalline silicon solar cells for the first time at Bell Laboratories in the United States, and the practical photovoltaic power generation technology that converts solar energy into electrical energy was born. Solar photovoltaic power generation refers to the process of converting the radiant energy of sunlight into electrical energy through a photovoltaic power generation system. The basic principle is as follows: solar cells are a key component to realize photoelectric conversion and are a kind of semiconductor device. When sunlight shines on a pn junction composed of two different conductivity types of homogenous semiconductor materials, p-type and n-type, under certain conditions, light energy is absorbed by the semiconductor, generating unbalanced carriers—electrons and holes—in the conduction band and valence band. They form concentration gradients in the p and n regions respectively, and do a diffusion movement toward the pn junction, when reaching the boundary of the junction region, the strong built-in electric field in the pn junction barrier region pushes holes to the p region and electrons to the n region; The non-equilibrium carriers in the barrier region are also under the action of the built-in electric field, each moves in the opposite direction and leaves the barrier area, resulting in an increase in the potential of the p-zone and a decrease in the potential of the n-zone, forming a photo-generated electromotive force at both ends of the pn junction; when the load is connected to both ends of the battery, a photovoltaic current can be formed to realize photovoltaic power generation (Figure 1).
It is worth mentioning that the electricity generated by photovoltaic cells is different from wind turbines in that it is direct current. If it is to be integrated into the AC grid for power supply, an inverter is also required for DC-AC conversion. Figure 2 shows a schematic diagram of a solar photovoltaic power generation system.
Solar photovoltaic power generation technology is a kind of clean power generation technology and one of the most sustainable development characteristics of renewable energy power generation technology. It has the following main characteristics:
1. The total amount of available resources is huge
Solar energy resources are currently one of the largest energy resources on the planet. According to a related research report by the US Department of Energy, the total global solar energy resources reached 12,000 TW (1 TW = 109 kW), of which the total economic development reached 600 TW, which was much higher than the reserves of renewable energy resources such as wind power (4 TW) and geothermal (12 TW). On the other hand, the silicon material used to manufacture solar photovoltaic cells is rich in elements in the earth (the content in the earth’s crust is 26.3%, ranking second), and is non-toxic and stable in performance.
2. The energy conversion process is simple, and in theory the energy utilization efficiency is high
Solar photovoltaic power generation is a direct conversion from photons to electrons. There is no intermediate process (such as the conversion of thermal energy into mechanical energy, mechanical energy into electromagnetic energy, etc.) and mechanical motion, and there is no mechanical wear. According to the second law of thermodynamics, photovoltaic power generation has a high theoretical power generation efficiency, and there is a huge potential for technological development. However, in the current practical application process, due to the photovoltaic cell material itself, light absorption, carrier transport, carrier collection and other reasons have greatly reduced the energy efficiency of solar photovoltaic power generation, and the technology needs to be improved.
3. Environmentally friendly and clean
Solar photovoltaic power generation itself does not use fuel, does not emit any substances including greenhouse gases and other exhaust gases, does not pollute the air, does not produce noise, and is environmentally friendly.
4. Strong regional and spatial adaptability
The photovoltaic power generation system can be used as long as there is light, and it is not restricted by geographical, altitude and other factors. Photovoltaic power generation can also be easily combined with buildings to form a photovoltaic building integrated power generation system. It does not need to occupy a separate land and can save valuable land and space resources.
5. Short construction period
The solar cell module is simple in structure, small in size, small in mass, easy to transport and install, and can be large or small according to the power load capacity, convenient and flexible, and easy to combine and expand.
6. Easy maintenance
Solar photovoltaic power generation has no mechanical transmission parts, simple operation and maintenance, and stable and reliable operation. A set of photovoltaic power generation system can generate electricity as long as it has solar cell components, and with the widespread use of automatic control technology, it can basically realize unattended operation and low maintenance cost.