Photovoltaic power generation

Introduction to 1. Principle
The main principle of photovoltaic power generation is the photoelectric effect of semiconductors. When a photon irradiates a metal, its energy can be absorbed by all electrons in the metal. The energy absorbed by the electrons is large enough to overcome the internal gravity of the metal and escape from the metal surface to become a photoelectron. Silicon atoms have four outer electrons. If pure silicon is doped with atoms with five outer electrons such as phosphorus atoms, it becomes an N-type semiconductor; if pure silicon is doped with atoms with three outer electrons such as boron atoms, a P-type semiconductor is formed. When the P-type and N-type are combined, the contact surface will form a potential difference and become a solar cell. When the sun shines on the P-N junction, the current flows from the P-type side to the N-type side, forming a current.
The photoelectric effect is the phenomenon that light causes a potential difference between different parts of an uneven semiconductor or a semiconductor and a metal combination. First of all, it is the process of converting photons (light waves) into electrons and light energy into electrical energy; secondly, it is the process of forming voltage.
After ingot casting, ingot breaking, slicing and other procedures, polysilicon is made into silicon wafers to be processed. P-N junctions are formed by doping and diffusing trace amounts of boron, phosphorus, etc. on a silicon wafer. Then using screen printing, the fine silver paste is printed on the silicon wafer to make the grid line, after sintering, at the same time to make the back electrode, and a layer of anti-reflection coating on the surface of the grid line, the battery sheet is thus made. The battery pieces are arranged and combined into battery components to form a large circuit board. Generally, aluminum frames are wrapped around the components, with glass on the front and electrodes installed on the back. With battery components and other auxiliary equipment, a power generation system can be composed. In order to convert direct current to alternating current, a current converter needs to be installed. After power generation, it can be stored in a cell and imported into the public grid. Battery components account for about 50% of the cost of power generation systems, while current converters, installation costs, other auxiliary components and other costs account for another 50%.
2. related a bit
Whether from the world or from China, regular energy is very limited. China's one-time energy reserves are far below the world average, only about 10% of the world's total reserves. Solar energy is an inexhaustible renewable energy source for human beings, which is fully clean. The advantages of absolute safety, relative universality, real long life and maintenance-free, resource adequacy and potential economy play an important role in the long-term energy strategy.
Compared with the commonly used thermal power generation system, the advantages of photovoltaic power generation are mainly reflected in:
① No danger of exhaustion;
② safe and reliable, no noise, no pollution emissions, absolutely clean (pollution-free);
③ not limited by the geographical distribution of resources, the advantages of building roofs can be used; for example, areas without electricity, and areas with complex terrain;
④ No need to consume fuel and erect transmission lines to generate power locally;
High energy quality;
Easily accepted by the user;
⑦ The construction period is short, and the time it takes to obtain energy is short.
3. conversion rate
monocrystalline silicon
Mass production conversion rate: 19.8-21%; Most of them are at 17.5. Technological breakthroughs that increase efficiency by more than 30% are less likely.
Polysilicon
Large-scale production conversion rate: 18-18.5%: mostly at 16%. Like monocrystalline silicon, it is less likely to achieve a conversion rate of more than 30% due to material physical property limitations.
gallium arsenide
The conversion rate of gallium arsenide solar cells is relatively high, about 23%. However, it is expensive and is mostly used in important places such as aerospace. There is basically no practical value of large-scale industrialization.
Film
Thin film photovoltaic cells have the advantages of light weight, light weight and good flexibility, and have a wide range of applications, especially suitable for photovoltaic building integration. If the efficiency of thin-film battery components is similar to that of crystalline silicon batteries, its cost performance will be incomparable. Thin film batteries prepared on flexible substrates have the advantages of being able to roll and fold, not afraid of falling, light weight, and good weak light performance. The future application prospects will be broader.
Amorphous silicon thin film conversion rate of about 9%. The conversion rate of amorphous silicon is expected to increase even higher.
efficiency attenuation
After the installation of crystalline silicon photovoltaic modules, exposure to the sun for 50-100 days, the efficiency attenuation is about 2-3%, after which the attenuation amplitude is greatly slowed down and stabilized with an annual attenuation of 0.5-0.8, 20 years of attenuation of about 20%. The single crystal component decays less than the polycrystalline component. The attenuation of amorphous optical components is about lower than that of crystalline silicon.
Therefore, improving the conversion rate and reducing the cost per watt will still be the two major themes of the future development of photovoltaic. Either way, if large-scale applications can increase the conversion rate to 30% at a cost of less than 5,000 yuan per kilowatt (comparable to hydropower), then human beings will get the most extensive, cleanest and cheapest almost unlimited reliable new energy sources before the success of nuclear fusion power generation research.
4. System Classification
Independent photovoltaic power generation
Independent photovoltaic power generation is also called off-grid photovoltaic power generation. It is mainly composed of solar cell module, controller and battery. If you want to supply power for AC load, you need to configure AC inverter. Independent photovoltaic power stations include village power supply systems in remote areas, solar household power supply systems, communication signal power supplies, cathodic protection, solar street lights and other photovoltaic power generation systems that can operate independently with batteries.
grid-connected photovoltaic power generation
Grid-connected photovoltaic power generation is that the direct current generated by solar modules is directly connected to the public grid after being converted into alternating current that meets the requirements of the grid-connected inverter. It can be divided into grid-connected power generation systems with and without batteries.
The grid-connected power generation system with storage battery is dispatchable, can be merged into or out of the power grid as needed, and also has the function of backup power supply, which can supply power urgently when the power grid is cut off for some reason. Large grid-connected power generation systems with storage and flow are often installed in residential buildings; grid-connected power generation systems without batteries are generally installed on larger systems without dispatchability and backup power supply functions.
Grid-connected photovoltaic power generation has centralized large-scale grid-connected photovoltaic power stations are generally national power stations, the main feature is that the generated energy is directly transmitted to the grid, and the grid is uniformly allocated to supply power to users. However, this kind of power station has large investment, long construction period and large area, and has not developed much. The decentralized small-scale grid-connected photovoltaic, especially photovoltaic building integrated photovoltaic power generation, is the mainstream of grid-connected photovoltaic power generation due to the advantages of small investment, fast construction, small footprint, and large policy support.
distributed photovoltaic power generation
Distributed photovoltaic power generation system, also known as distributed power generation or distributed energy supply, refers to the user site or near the site configuration of a smaller photovoltaic power supply system to meet the needs of specific users, to support the economic operation of the existing distribution network, or to meet the requirements of both.
The basic equipment of distributed photovoltaic power generation system includes photovoltaic cell module, photovoltaic square array bracket, DC combiner box, DC distribution cabinet, grid-connected inverter, AC distribution cabinet and other equipment, as well as power supply system monitoring device and environmental monitoring device. Its operation mode is that under the condition of solar radiation, the solar cell module array of the photovoltaic power generation system converts the solar energy into the output electric energy, which is concentrated and sent to the DC power distribution cabinet through the DC combiner box, and the grid-connected inverter is inverted into AC power to supply the building's own load, and the excess or insufficient power is adjusted by connecting the power grid.
5. structure composition
Photovoltaic power generation system is composed of solar cell array, battery pack, charge and discharge controller, inverter, AC power distribution cabinet, solar tracking control system and other equipment. The role of some of its equipment is:
Battery Phalanx
In the case of light (whether it is sunlight or light generated by other luminous bodies), the battery absorbs light energy, and the accumulation of different charges appears at both ends of the battery, which produces "photo-generated voltage", which is the "photovoltaic effect". Under the action of the photovoltaic effect, the solar pool at both ends of the electromotive force, the light energy into electrical energy, is the energy conversion device, solar cells are generally silicon cells, divided into monocrystalline silicon solar cells, polycrystalline silicon solar cells and amorphous silicon solar cells three.
battery pack
Its function is to store the electric energy emitted by the solar cell array when it is exposed to light and can supply power to the load at any time. The basic requirements of solar cell power generation for the battery pack used are: A low self-discharge rate; B. long service life c. strong deep discharge capability; D. high charging efficiency; E. less maintenance or maintenance-free; F. wide working temperature range; Q. low price.
Controller
It is a device that can automatically prevent the battery from overcharging and overdischarging. Since the number of cycles of battery charge and discharge and the depth of discharge are important factors that determine the service life of the battery, the charge and discharge controller that can control the overcharge or overdischarge of the battery pack is an essential device.
Inverter
It is a device that converts direct current into alternating current. Since solar cells and batteries are DC power sources, and the load is an AC load, the inverter is essential. According to the operation mode, the inverter can be divided into independent operation inverter and grid connected inverter.
The independent operation inverter is used for the independent operation of the solar cell power generation system to supply power to independent loads. Grid-connected inverter is used for grid-connected solar cell power generation system.
Inverter according to the output wave can be divided into square wave inverter and sine wave inverter. Square wave inverter circuit is simple, low cost, but the harmonic component is large, generally used for hundreds of watts and less demanding harmonic system. The sine wave inverter has high cost, but can be applied to various loads.
tracking system
Because relative to a fixed location of the solar photovoltaic power generation system, a year of spring, summer, autumn and winter seasons, every day sunrise and sunset, the sun's illumination angle is changing all the time, if the solar panels can always face the sun, the power generation efficiency will reach the best state.
The world's universal solar tracking control system needs to calculate the angle of the sun at different times of each day of the year according to the longitude and latitude of the placement point, and store the sun's position at each time of the year to PLC, microcontroller or computer software, that is, by calculating the sun's position to achieve tracking. The computer data theory is adopted, which requires the data and settings of the longitude and latitude regions of the earth. Once installed, it is inconvenient to move or disassemble. After each movement, the data must be reset and various parameters must be adjusted. The principle, circuit, technology and equipment are complicated, and non-professionals cannot operate casually. The solar power generation system equipped with the intelligent solar eye tracker is installed on high-speed cars, trains, communication emergency vehicles, special military vehicles, warships or ships. No matter where the system travels, how to turn around or turn, the intelligent solar tracker can ensure that the required tracking position of the equipment is facing the sun.
6. generation cost
In the past five years, the cost of photovoltaic power generation has dropped by 1/3. In South America and other countries, photovoltaic power generation has been equal to or even lower than retail electricity prices. In the future, the cost of photovoltaic power generation will be further highlighted. Secondly, thermal power generation will bring extremely high environmental governance costs. The 20th Paris Climate Summit is to guide countries to actively start the carbon trading market pricing mechanism. The cost increase caused by this to high energy-consuming enterprises is obvious. From this perspective, the cost of coal power generation will be higher than that of photovoltaic power generation.
The investment cost fell below 8 yuan/watt and the electricity cost fell to 0.6-0.9 yuan/kWh.
7. Application Areas
1, users of solar power;(1) Small power supplies ranging from 10 to 100W are used for military and civilian domestic electricity in remote areas without electricity, such as plateaus, islands, pastoral areas, border posts, etc., such as lighting, televisions, tape recorders, etc.;(2)3-5KW household roof grid-connected power generation system;(3) Photovoltaic water pump: to solve the problem of drinking and irrigation in deep water wells in areas without electricity.
2. Traffic areas such as beacon lights, traffic/railway signal lights, traffic warning/sign lights, Yuxiang street lights, high-altitude obstacle lights, highway/railway wireless telephone enjoyment, unattended road shift power supply, etc.
3. Communication/communication field: solar unattended microwave relay station, optical cable maintenance station, broadcasting/communication/paging power supply system; Rural carrier telephone photovoltaic system, small communication machine, soldier GPS power supply, etc.
4, oil, marine, meteorological field: oil pipeline and reservoir gate cathodic protection solar power supply system, oil drilling platform life and emergency power supply, marine testing equipment, meteorological/hydrological observation equipment, etc.
5, household lighting power: such as garden lights, street lights, portable lights, camping lights, mountaineering lights, fishing lights, black light, rubber cutting lights, energy-saving lamps.
6, photovoltaic power station: 10KW-50MW independent photovoltaic power station, scenery (wood) complementary power station, a variety of large parking plant charging stations.
Solar building combines solar power generation with building materials, so that large buildings in the future can achieve power self-sufficiency, which is a major development direction in the future.
8. Other fields include: (1) matching with automobiles: solar cars/electric vehicles, battery charging equipment, automobile air conditioners, ventilation fans, cold drinking boxes, etc.;(2) renewable power generation systems for solar hydrogen production and fuel cells;(3) power supply for seawater desalination equipment;(4) satellites, spacecraft, space solar power stations, etc.