The basic principle of solar photovoltaic power generation is to use the photovoltaic effect of solar cells to directly convert the sun's radiant energy into electrical energy. The energy converter of solar photovoltaic power generation is the solar cell, also known as a photovoltaic cell. A photovoltaic cell is actually a large area silicon semiconductor device. The positive charge represents the silicon atom and the negative charge represents the four electrons surrounding the silicon atom. When impurities (elements) of boron or phosphorus are doped into the semiconductor silicon crystal, because there are only three electrons around the boron atom and five electrons around the phosphorus atom, a crystal structure with holes and a crystal structure with excess electrons as shown in Figure 1-2 is created, forming a P-type or N-type semiconductor.
Since P-type semiconductors contain more holes and N-type semiconductors contain more electrons, when P-type and N-type semiconductors, a special thin layer will be formed in the intersection region of the two semiconductors, and the P-type side of the thin layer is negatively charged, and when combined together, the type side is positively charged, forming a PN junction.
When the sunlight shines on the PV cell, some of the light is reflected, some is absorbed, and some passes through the cell. The absorbed light energy excites the electrons in the bound high-energy state, producing electron-hole pairs. Under the action of the built-in electric field of the PN junction, electrons and holes move with each other, with holes in the N zone moving toward the P zone and electrons in the P zone moving toward the N zone, so that a large amount of negative charge (electrons) accumulates on the light-receiving side of the solar cell, while a large amount of positive charge (holes) accumulates on the back side of the solar cell. If a load is connected to both ends of the cell, there will be current passing through the load, and when the light keeps shining, there will be a constant current flowing through the load. A single solar cell is a thin semiconductor PN junction with a rated output voltage of 0.5~0.55 V under standard lighting conditions. In order to obtain higher output voltage and larger power capacity, in practical applications, multiple solar cells are often connected together to form a cell assembly, or more cells are used to form a PV array, as shown in Figure 1-5. The output power of solar cells is random, and the output power of the same solar cell is different at different times, different locations, different light intensities, and different installation methods.
The principle and advantages of solar photovoltaic power generation
From what solar energy is to solar energy application related technology explanation, I believe that many people have a general understanding of solar energy, the following talk about solar energy application technology in photovoltaic power generation, the main two aspects: the principle and its advantages.
Ground-based concentrated solar power generation applications
1, solar photovoltaic power generation principle
Photovoltaic effect occurs in both liquid and solid substances, but only solid (especially semiconductor PN junction devices) in the sunlight irradiation photoelectric conversion efficiency is higher. The photovoltaic effect principle is used to make crystalline silicon solar cells, which can convert light energy from the sun directly into electricity. The energy converter of solar photovoltaic power generation is the solar cell, also known as photovoltaic cell, is the basis and core device of solar photovoltaic power generation system. The process of converting solar energy into electrical energy consists of 3 main steps.
(1) solar cells absorb a certain amount of energy photons, the semiconductor produces an electron-hole pair, called "photogenerated carriers", the electrical polarity of the two opposite, electrons are negatively charged, holes are positively charged.
(2) The photogenerated carriers with opposite electrical polarity are separated by the electrostatic field generated by the semiconductor PN junction.
(3) The photogenerated carrier electrons and holes are collected by the positive and negative poles of the solar cell, respectively, and generate current in the external circuit to obtain electrical energy.
The principle of solar photovoltaic power generation is shown in the figure below. When light shines on the surface of the solar cell, part of the photons are absorbed by the silicon material and the energy of the photons is transferred to the silicon atoms, causing the electrons to leap and become free electrons, which cluster on both sides of the PN junction to form a potential difference. When the external circuit is turned on, under the action of this voltage, there will be a current flow through the external circuit to produce a certain output power. The essence of this process is the conversion of photon energy into electrical energy.
In a solar power system, the total efficiency of the system η is determined by the photovoltaic conversion efficiency of the solar cell module, the controller efficiency, the battery efficiency, the inverter efficiency and the efficiency of the load. At present, the photoelectric conversion efficiency of solar cells is only about 17%. Therefore, to improve the photoelectric conversion efficiency of solar cell modules, reduce the cost per unit power of solar photovoltaic power generation system, is the focus and difficulty of the industrialization of solar photovoltaic power generation. Since the introduction of solar cells, crystalline silicon as the main material to maintain the dominant position. At present, the research on the conversion efficiency of silicon solar cells mainly revolves around increasing the energy absorption surface (such as the use of double-sided cells to reduce reflection), the use of absorption technology and passivation process to improve the conversion efficiency of silicon solar cells, cell ultra-thin type and other aspects.
At present, solar photovoltaic power generation systems are mainly used in the following 3 broad areas.
(1) To provide power for non-electricity occasions, mainly for the majority of non-electricity areas to provide power for the residents' living and production, microwave relay stations and cell phone base stations to provide power, etc.
(2) Solar energy daily electronic products, such as various types of solar chargers, solar street lamps and solar lawn lamps, etc.
(3) Grid-connected power generation, that is, access to the national grid. China's grid-connected photovoltaic power generation has been very mature, since 2013, the state has introduced a series of photovoltaic power generation promotion policies to encourage the construction of photovoltaic power generation facilities in areas with conditions for commercial and industrial and household use, public buildings, etc.
2、The advantages of solar power generation
Through the comparative analysis of several common new energy sources such as biomass, water, wind and solar energy, it is clear that solar power generation has the following unique advantages.
(1) Photovoltaic power generation has economic advantages
The economy of solar energy can be seen from two aspects: First, solar energy is inexhaustible, and no "tax" is levied when receiving solar energy, so it can be used anywhere; second, at the current level of technological development, some solar energy use already has economic. With the development of science and technology and human development and use of solar energy technology breakthroughs, the economic use of solar energy will be more obvious. If the 20th century is the century of oil, then the 21st century is the century of renewable energy (the century of solar energy).
From the point of view of solar photovoltaic power station construction cost, with the large-scale application and promotion of solar photovoltaic power generation, especially the upstream crystalline silicon industry and photovoltaic power generation technology is becoming more and more mature, building roofs, exterior walls and other platforms of composite development and utilization, the construction cost of solar photovoltaic power generation per kilowatt is getting lower and lower, compared to other renewable energy sources already have the same economic advantages, and with the implementation of the national policy of price parity Its popularity will become more and more widespread.
(2) Solar energy is an inexhaustible source of renewable energy
According to calculations, the total amount of solar energy reaching the earth's surface in a year is equivalent to a total of about 1.892x10^24 tons of standard coal, which is 10,000 times the current proven reserves of the world's major energy sources. The life span of the sun is at least 4 billion years, compared to the history of mankind, solar energy can be continuously supplied to the earth can be said to be infinite, which determines the development and utilization of solar energy will be the most effective way to solve the scarcity of conventional energy, depletion.
(3) No pollution to the environment
Solar energy, like wind power, tidal energy and other clean energy, its development and utilization of almost no pollution, coupled with its unlimited reserves, is the ideal alternative energy for mankind. As traditional fossil fuels (coal, oil and natural gas) emit a large amount of toxic and harmful substances in the process of use, which can cause serious pollution to water, soil and the atmosphere, forming the greenhouse effect and acid rain, seriously endangering the human living environment and health, there is an urgent need to develop new and relatively clean alternative energy sources, and solar energy as a relatively ideal clean energy, is receiving increasing attention from countries around the world.
From the current carbon emission rate (g/kW-h) of various power generation methods, not counting their upstream links: 275g/kW-h for coal, 204g/kW-h for oil, 181g/kW-h for natural gas, 20g/kW-h for wind power, while solar photovoltaic power generation is close to zero emission. Moreover, there is no waste material, waste water, waste gas, noise, or harmful substances in the process of power generation, and it does not pollute the environment.
No pollution from photovoltaic power generation
(4) Minimal energy conversion links
In terms of energy conversion, solar photovoltaic power generation directly converts solar radiation energy into electrical energy, and among all renewable energy sources, solar photovoltaic power generation has the least conversion links and the most direct utilization. Generally speaking, in the energy flow of the whole ecological environment, as the conversion link increases and the conversion chain lengthens, the energy loss will increase geometrically, and at the same time greatly increase the construction and operation costs and instability of the whole system. At present, the practical level of crystalline silicon solar cells photoelectric conversion efficiency of 15%-20%, the highest level of laboratory has reached 35%.
(5) The most economical and environmentally friendly
From the resource conditions, especially land occupation, biological energy, wind energy is more demanding, while the use of solar energy is very flexible. If solar photovoltaic power generation occupies an area of land 1,wind is 8-10 times more than solar energy, bioenergy is up to 100 times. In the case of hydropower, the construction of a large dam often requires the flooding of tens to hundreds of square kilometers of land. In contrast, solar power does not need to occupy more land, roofs, walls can become solar photovoltaic power generation using the site, but also the use of China's vast deserts, by building solar photovoltaic power generation base on the desert, directly reduce the desert zone direct solar radiation to the surface, effectively reduce the surface temperature, reduce evaporation, and thus make the survival and growth of plants in a considerable degree possible, and stabilize and reduce sand dunes, and in turn claim the clean renewable energy needed from nature.
(6) Free to use and no transportation
Human beings can convert solar energy into heat or electricity through specialized technology and equipment, and use it locally without transport for the benefit of mankind. Moreover, the use of this inexhaustible energy source is free of charge. Although the differences in latitude and climate conditions cause uneven solar radiation, solar energy is universal to most areas of the earth compared to other energy sources and can be used locally. This offers bright prospects for solving energy problems in countries and regions where conventional energy is scarce.
Advantages and disadvantages of solar photovoltaic power generation
Photovoltaic power generation is the process of directly converting light energy into electricity by using the photovoltaic effect generated at the semiconductor interface. The DC energy generated by the generator is converted into AC energy through an inverter, which is then boosted and transmitted to the power grid through a booster transformer station, and the electricity is transmitted to the consumer side through the grid transmission line.
Photovoltaic power plants are mainly divided into two categories: centralized power plants and distributed power plants. Because of the high flexibility of PV power plant installation compared to wind power, compared to traditional large-scale wind power plants, PV power plants can not only be installed centrally on a large scale, but can also be distributed on the surface of buildings, outdoors and many other scenarios.
Centralized photovoltaic power plant is the PV array installed in the mountains, water, desert and other wide areas, sunlight photovoltaic array can produce DC electricity, inverter and then DC electricity into AC electricity, through the booster station to connect to the grid. The scale of centralized photovoltaic power plants is generally larger, generally above 10MW, and the number of very large photovoltaic power plants above 100MW is gradually increasing.
Distributed photovoltaic power plants generally have a small installed capacity and are more flexible in installation. Distributed power plants can be mainly divided into two categories: those combined with buildings and those not combined with buildings, and those combined with buildings can be divided into building integrated photovoltaic (BAPV) and building integrated photovoltaic (BIPV).
Under the background of carbon peak and carbon neutral, PV power generation is developing rapidly as the backbone of China's energy transformation, and its main advantages include.
(1) Green and environmental protection. PV power generation itself does not consume fuel, does not emit anything including greenhouse gases and other exhaust gases, does not pollute the air and does not produce noise.
(2) Improving national energy stability. Through photovoltaic power generation, people can reduce their dependence on fossil fuel power generation and effectively avoid shocks caused by energy crises or unstable fuel markets, thus improving national energy security.
(3) Extensiveness. The sun's rays irradiate the earth's surface and are not limited to any geographical area, whether on land, sea, mountains or flatlands, and can be exploited. Although the irradiation time and intensity vary, they are widely distributed and will not be inaccessible due to geographical or weather reasons.
(4) Infinity and sustainability. Based on the current rate of nuclear energy generated by the sun, it is estimated that there is enough hydrogen in storage to last for tens of billions of years. In today's increasingly serious ecological pollution, solar energy resources are inexhaustible, a truly renewable and clean energy.
(5) Low operation and maintenance costs. Photovoltaic power generation has no mechanical transmission parts, and is stable and reliable. A set of photovoltaic power generation system as long as there are solar modules can generate electricity, coupled with the extensive use of automatic control technology, basically unattended, low maintenance costs.
(6) Flexible installation location. The roof of a building is open, and has the advantages of not being affected by the orientation of the building, receiving light for a long time, and avoiding shadow interference to the greatest extent. Photovoltaic power generation can be installed not only on the roofs of residential facilities, but also in industrial-scale facilities to obtain electricity through solar power generation to meet the electricity demand in buildings. In the field of rural revitalization, the development of rooftop distributed photovoltaic technology can also effectively solve the problem of electricity consumption in county areas.
At the same time, there is no denying that there are some disadvantages of PV power generation at this stage, and better development of the industry in the future should focus on overcoming these disadvantages. The main disadvantages are manifested in.
(1) Wide footprint. Due to the small energy distribution density of sunlight irradiation, this leads to the large-scale application of photovoltaic power generation, the equipment covers a large area relative to other power generation equipment.
(2) Unstable energy supply. Photovoltaic power generation is affected by seasonal changes, weather conditions, alternating day and night, and the intensity of solar radiation. Long-term rainy and snowy days, cloudy days and even changes in cloud cover can affect PV power generation, and when there is no sun it cannot generate electricity or the amount of electricity generated is very small, which can affect the normal use of electricity-using equipment.
(3) Strong geographical dependence. Due to different geographical locations and different climates, sunlight resources vary greatly from region to region. Some areas rich in light energy resources are far away from the load center of electricity consumption, and the local economy is underdeveloped, so the ability to consume is weak, which requires long-distance centralized transmission through the transmission grid, resulting in the inevitable loss of energy generated by photovoltaic power generation.