Photovoltaic power generation is a technology that directly converts light energy into electrical energy by utilizing the photovoltaic effect of the semiconductor interface. The key element of this technology is the solar cell. After the solar cells are connected in series, they can be packaged and protected to form a large-area solar cell module, and then combined with power controllers and other components to form a photovoltaic power generation device.
1 Photovoltaic effect
If light strikes a solar cell and the light is absorbed at the interface layer, photons with sufficient energy can excite electrons from covalent bonds in both P-type and N-type silicon, resulting in electron-hole pairs. The electrons and holes near the interface layer will be separated from each other by the electric field effect of space charges before recombination. Electrons move toward the positively charged N region and holes toward the negatively charged P region. Charge separation through the interface layer will generate an outwardly measurable voltage between the P and N regions. At this time, electrodes can be added to both sides of the silicon wafer and connected to a voltmeter. For crystalline silicon solar cells, the typical value of the open circuit voltage is 0.5 to 0.6V. The more electron-hole pairs generated by light on the interface layer, the greater the current flow. The more light energy absorbed by the interface layer, the larger the interface layer, ie the cell area, and the greater the current formed in the solar cell.
2. Principle
Sunlight shines on the semiconductor p-n junction to form a new hole-electron pair. Under the action of the p-n junction electric field, the holes flow from the n region to the p region, and the electrons flow from the p region to the n region. After the circuit is turned on, a current is formed. This is how photoelectric effect solar cells work.
There are two ways of solar power generation, one is light-heat-electricity conversion, and the other is light-electricity direct conversion.
(1) The light-heat-electric conversion method generates electricity by using the thermal energy generated by solar radiation. Generally, the solar collector converts the absorbed thermal energy into the steam of the working medium, and then drives the steam turbine to generate electricity. The former process is a light-to-heat conversion process; the latter process is a heat-to-electricity conversion process, which is the same as ordinary thermal power generation. The disadvantage of solar thermal power generation is that the efficiency is very low and the cost is high. It is estimated that its investment is at least higher than that of ordinary thermal power generation. Power stations are 5 to 10 times more expensive.
(2) Light-to-electricity direct conversion method This method uses the photoelectric effect to directly convert solar radiation energy into electrical energy. The basic device for light-to-electricity conversion is solar cells. A solar cell is a device that directly converts sunlight energy into electrical energy due to the photovoltaic effect. It is a semiconductor photodiode. When the sun shines on the photodiode, the photodiode will convert the sun's light energy into electrical energy and generate electricity. current. When many cells are connected in series or in parallel, it can become a solar cell array with relatively large output power. Solar cells are a promising new type of power source with three major advantages: permanence, cleanliness and flexibility. Solar cells have a long service life. As long as the sun exists, solar cells can be used for a long time with one investment; and thermal power, nuclear power generation. In contrast, solar cells do not cause environmental pollution.
3. System composition
The photovoltaic power generation system is composed of solar cell arrays, battery packs, charge and discharge controllers, inverters, AC power distribution cabinets, sun tracking control systems and other equipment. Some of its equipment functions are:
battery array
When there is light (whether it is sunlight or light generated by other illuminants), the battery absorbs light energy, and the accumulation of opposite-signal charges occurs at both ends of the battery, that is, a "photogenerated voltage" is generated, which is the "photovoltaic effect". Under the action of the photovoltaic effect, the two ends of the solar cell generate electromotive force, which converts light energy into electrical energy, which is an energy conversion device. Solar cells are generally silicon cells, which are divided into three types: monocrystalline silicon solar cells, polycrystalline silicon solar cells and amorphous silicon solar cells.
Battery pack
Its function is to store the electric energy emitted by the solar cell array when it is illuminated and to supply power to the load at any time. The basic requirements for the battery pack used in solar cell power generation 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. working temperature Wide range; g. low price.
Controller
It is a device that can automatically prevent the battery from overcharging and overdischarging. Since the number of cycles of charge and discharge and the depth of discharge of the battery are important factors in determining the service life of the battery, a charge and discharge controller that can control the overcharge or overdischarge of the battery pack is an essential device.
Inverter
A device that converts direct current to alternating current. Since solar cells and batteries are DC power sources,
When the load is an AC load, an inverter is essential. According to the operation mode, inverters can be divided into independent operation inverters and grid-connected inverters. Stand-alone inverters are used in stand-alone solar cell power systems to power stand-alone loads. Grid-connected inverters are used for grid-connected solar cell power generation systems. The inverter can be divided into square wave inverter and sine wave inverter according to the output waveform. The square wave inverter has a simple circuit and low cost, but has a large harmonic component. It is generally used in systems below several hundred watts and with low harmonic requirements. Sine wave inverters are expensive, but can be applied to various loads.
4. System classification
The photovoltaic power generation system is divided into independent photovoltaic power generation system, grid-connected photovoltaic power generation system and distributed photovoltaic power generation system.
1. Independent photovoltaic power generation is also called off-grid photovoltaic power generation. It is mainly composed of solar cell components, controllers, and batteries. To supply power to the AC load, an AC inverter needs to be configured. 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 with batteries that can operate independently.
2. Grid-connected photovoltaic power generation means that the direct current generated by solar modules is converted into alternating current that meets the requirements of the mains grid through the grid-connected inverter and then directly connected to the public grid.
It can be divided into grid-connected power generation systems with and without batteries. The grid-connected power generation system with battery is schedulable and can be integrated into or withdrawn from the power grid according to needs. It also has the function of backup power supply, which can provide emergency power supply when the power grid is cut off for some reason. Photovoltaic grid-connected power generation systems with batteries are often installed in residential buildings; grid-connected power generation systems without batteries do not have the functions of dispatchability and backup power, and are generally installed on larger systems. Grid-connected photovoltaic power generation has centralized large-scale grid-connected photovoltaic power stations, which are generally national-level power stations. However, this kind of power station has not developed much due to its large investment, long construction period and large area. Distributed small-scale grid-connected photovoltaics, especially photovoltaic building-integrated photovoltaic power generation, are the mainstream of grid-connected photovoltaic power generation due to the advantages of small investment, fast construction, small footprint, and strong policy support.
3. Distributed photovoltaic power generation system, also known as distributed power generation or distributed energy supply, refers to the configuration of a smaller photovoltaic power supply system at the user site or near the power site to meet the needs of specific users and support the existing distribution network economic operation, or meet the requirements of both aspects at the same time.
4. The basic equipment of the distributed photovoltaic power generation system includes photovoltaic cell components, photovoltaic square array brackets, DC combiner boxes, DC power distribution cabinets, grid-connected inverters, AC power distribution cabinets and other equipment, as well as power supply system monitoring devices 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 output electric energy from solar energy, and sends it to the DC power distribution cabinet through the DC combiner box, and the grid-connected inverter converts it into AC power supply. The building itself is loaded, and excess or insufficient electricity is regulated by connecting to the grid.
5. Advantages and disadvantages
Compared with the commonly used power generation systems, the advantages of solar photovoltaic power generation are mainly reflected in:
Solar power is called the most ideal new energy. ①No danger of depletion; ②Safe and reliable, no noise, no pollution discharge, absolutely clean (no pollution); ③It is not limited by the geographical distribution of resources, and the advantages of building roofs can be used; ④No need to consume fuel and erect transmission lines Local power generation and power supply; ⑤High energy quality; ⑥Users are easy to accept emotionally; ⑦The construction period is short, and the time it takes to obtain energy is short.
shortcoming:
①The energy distribution density of the irradiation is small, that is, it takes up a huge area; ②The energy obtained is related to the four seasons, day and night, cloudy and sunny and other meteorological conditions. The use of solar energy to generate electricity has high equipment costs, but the utilization rate of solar energy is low, so it cannot be widely used. It is mainly used in some special environments, such as satellites.
6. Application areas
1. User solar power supply: (1) Small power supply ranging from 10-100W, used in remote areas without electricity such as plateaus, islands, pastoral areas, border posts and other military and civilian life electricity, such as lighting, TV, tape recorders, etc.; (2) 3 -5KW household rooftop grid-connected power generation system; (3) Photovoltaic water pump: solves the problem of drinking and irrigating deep wells in areas without electricity.
2. Traffic fields such as navigation lights, traffic/railway signal lights, traffic warning/signal lights, Yuxiang street lights, high-altitude obstruction lights, highway/railway wireless phone booths, 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, GPS power supply for soldiers, etc.
4. Petroleum, marine and meteorological fields: cathodic protection solar power system for oil pipelines and reservoir gates, life and emergency power supply for oil drilling platforms, marine detection equipment, meteorological/hydrological observation equipment, etc.
5. Power supply for household lamps: such as garden lamps, street lamps, portable lamps, camping lamps, mountaineering lamps, fishing lamps, black light lamps, tapping lamps, energy-saving lamps, etc.
6. Photovoltaic power station: 10KW-50MW independent photovoltaic power station, wind-solar (diesel) complementary power station, various large parking plant charging stations, etc.
7. Solar buildings combine solar power generation with building materials to enable large buildings in the future to achieve self-sufficiency in electricity, which is a major development direction in the future.
8. Other fields include: (1) Matching with automobiles: solar vehicles/electric vehicles, battery charging equipment, automobile air conditioners, ventilation fans, cold drink boxes, etc.; (2) regenerative power generation systems for solar hydrogen production and fuel cells; (3) seawater Desalination equipment power supply; (4) Satellites, spacecraft, space solar power stations, etc.