The following article comes from Digital Beijing Science Center, the author Beijing Science Center
Seeing the picture below, many people can say that this is a solar power generation device, but the principle of solar power generation and the reason why it has not been popularized, can you also come one by one? Today we will find out.
1. How does solar power generate electricity?
Solar power generation is the process of converting energy from celestial bodies outside the earth (mainly solar energy) into electric energy that can be used by humans. Solar power generation mainly includes two forms: solar thermal power generation and photovoltaic power generation.
As for solar thermal power generation, everyone may be familiar with it because of the popularity of solar water heaters. The process of solar thermal power generation can be summarized as the conversion process of light-heat-electricity.
A large number of reflectors or transmission mirrors are used to gather sunlight and heat the working fluid (medium), then the high-temperature working fluid passes through the heat exchange system to heat the water into high-temperature and high-pressure steam, and finally, the high-parameter steam enters the turbo-generator unit to perform work and output electricity.
Because the power generation process requires sufficient direct light and sufficient water sources, the construction of the solar thermal system has relatively strict requirements on the terrain and the natural resources of the construction area. However, its power generation stability and sustainability advantages are not available in photovoltaic power generation.
Compared with solar thermal power generation, photovoltaic power generation does not require high natural resources. In addition, it can make full use of the total amount of solar radiation and is compatible with the terrain. Therefore, it has considerable advantages and is the main way to use sunlight.
The photovoltaic power generation system is mainly composed of four parts: inverter (converting direct current into alternating current), solar cell (converting light energy into electric energy), controller (controlling charging and discharging), and storage battery (saving or providing electric energy).
Among them, the solar cell (photovoltaic array) is a key part of the photovoltaic power generation system, and its quality and cost directly determine the quality and cost of the entire power generation system.
2. What are the common solar cells?
Since the invention of solar cells in 1954, the types of solar cells have gradually been enriched, and the structures have been diverse, and the conversion efficiency has been significantly improved. According to different production materials, solar cells on the market can be divided into three types: crystalline silicon solar cells, thin-film solar cells, and photoelectrochemical solar cells.
Crystalline silicon solar cells started the earliest and have the highest market share. Crystalline silicon solar cells are based on semiconductor P-N junctions, which are exposed to sunlight to produce a “photovoltaic effect”, which directly converts light energy into electrical energy.
The P-N junction refers to the area near the junction of the P-type semiconductor and the N-type semiconductor. The characteristics of the P-N junction are determined by the characteristics of the carriers in the semiconductor.
There are a large number of movable hole carriers in P-type semiconductors, while there are almost no holes in N-type semiconductors. The difference in concentration causes holes to diffuse from the P region to the N region.
Similarly, there are a lot of free electron carriers in N-type semiconductors, but there are almost no P-type semiconductors. The difference in free-electron concentration between the two causes electrons to diffuse from the N region to the P region.
Holes and electrons meet and recombine near the junction of the N area and the P area, resulting in a lack of carriers for a certain distance near the interface, that is, a space charge area is formed.
The space charge on one side of the P-type semiconductor is negative ions, and the space charge on the side of the N-type semiconductor is positive ions. The positive and negative ions generate an internal electric field near the junction. This electric field prevents the carriers from further diffusing but forces some carriers to drift in the direction opposite to the diffusion movement.
When drift and diffusion reach an equilibrium state, a barrier layer is formed. The barrier layer is the P-N junction.
When sunlight shines on the P-N junction of the solar cell, the semiconductor material absorbs photons intrinsically to generate photo-generated hole-electron pairs, and under the action of the internal electric field, the photo-generated electrons move to the N end, and the photo-generated holes move to the P end. As the photogenerated electrons accumulate at the boundary of the N zone, the photogenerated holes accumulate in the P zone, and a potential difference is formed between the P zone and the N zone.
When the two ends of the PN are connected into a loop, a current from the N end to the P end is generated in the circuit. The more hole-electron pairs produced by light, the greater the potential difference and the stronger the current.
Compared with crystalline silicon solar cells, thin-film solar cells and photochemical solar cells started late, and the market acceptance is not as good as that of crystalline silicon solar cells.
Thin-film solar cells are solar cells manufactured by using semiconductor thin films as substrates, which consume less energy, and the thickness of the thin film that can generate voltage is only a few microns, which is suitable for photovoltaic building integration.
The thin-film solar cells currently on the market mainly include three types of silicon-based, cadmium telluride, and copper indium gallium selenium thin-film solar cells. The power generation principle is similar to that of crystalline silicon solar cells.
Common photoelectrochemical solar cells include dye-sensitized nanocrystalline solar cells. Dye-sensitized nanocrystalline solar cells are composed of a photoanode, dye sensitizer, a counter electrode, and redox electrolyte. The principle is derived from the imitation of plant photosynthesis by humans, and the excellent light absorption properties of dye sensitizers are used to convert solar energy into electrical energy.
In addition, perovskite solar cells and quantum well semiconductor solar cells have good prospects as rising stars.
The power generation principle and structure of perovskite solar cells are similar to those of dye-sensitized cells. The perovskite (CH3NH3PbX3 (X=Br, I)) is used as the light absorption layer. Type semiconductor contacts form a PIN structure (or NIP inversion structure).
Quantum well semiconductor solar cells are an effective way to develop high-efficiency III-V composite semiconductor solar cells, as well as a means to improve the conversion efficiency of tandem cells. Scientists adjust the content of different elements in the quantum well semiconductor solar cell in the battery to obtain higher photoelectric conversion efficiency.
With the vigorous development of solar cells, the aviation industry has also been promoted. Compared with ordinary solar cells, solar cells used in space stations have the advantages of long-term continuous power generation, high efficiency, zero emissions, and the ability to withstand various impacts.
At present, solar photovoltaic cells used in space stations in my country are mainly crystalline silicon solar cells that are economical and stable.
At the same time, single-junction gallium arsenide solar cells and multi-junction cascaded gallium arsenide solar cells have also been used in space stations, and their development is relatively rapid. However, because solar cells of different compositions have different advantages and disadvantages, various types of solar cells have different advantages and disadvantages. The positions of solar cells in space are also different.
3. Why is solar power not popular?
After the introduction, you may be very puzzled: solar energy has been developed for more than a hundred years, and so many results have been obtained, so why has solar power not been applied to civil power generation on a large scale by 2020?
The reasons are mainly divided into two categories: environmental factors and cost factors. As far as the environment is concerned, solar power generation requires sufficient direct sunlight and light duration, so solar power generation is relatively restricted by climate. Not all regions are suitable for installing solar power generation equipment, and not all seasons are suitable for solar power generation.
In terms of cost, the power generation efficiency of photovoltaic power generation solar cells is extremely low. Therefore, to increase the efficiency, it is necessary to increase the daylighting area. Obviously, this not only leads to very high power generation construction costs but also if it is used for civilian power generation, the electricity bill is expensive and cost-effective. not tall.
If you want to reduce the cost of solar power generation, you can make technical improvements from every step of the whole process of photovoltaic power generation. However, although there is room for cost reduction at every step, there are still many challenges. Materials, equipment, batteries, battery components, megawatt-level photovoltaic power plants, and other technologies are not fully mature.
Therefore, the popularization of solar power generation also depends on technological innovation to obtain solar cells with high efficiency, low cost, and easy manufacturing.
