Photovoltaic power generation is a technology that uses the photovoltaic effect at the semiconductor interface to convert light energy directly into electricity. The key component of this technology is the solar cell. Solar cells are connected in series and then encapsulated and protected to form large-area solar cell modules, which are combined with power controllers and other components to form a photovoltaic power generation device.
1、Photovoltaic effect
If light shines on the solar cell and light is absorbed in the interface layer, photons with sufficient energy can excite electrons form covalent bonds in P-type silicon and N-type silicon, resulting in electron-hole pairs. The electrons and holes near the interface layer are separated from each other by the electric field of the space charge before they are compounded. The electrons move toward the positively charged N region and the holes move toward the negatively charged P region. The charge separation through the interface layer will produce an outward testable voltage between the P and N regions. At this point, 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 0.6 V. The more electron-hole pairs generated at the interface layer by light, the higher the current. The more light energy absorbed by the interfacial layer, the larger the interfacial layer, i.e., the larger the cell area, and the larger the current formed in the solar cell.
2、Principle
Sunlight shines on the semiconductor p-n junction, forming new hole-electron pairs. Under the action of the electric field of the p-n junction, holes flow from the n-zone to the p-zone, and electrons flow from the p-zone to the n-zone, forming a current when the circuit is connected. This is the working principle of the photoelectric effect solar cells.
There are two ways of solar power generation, one is the light-thermal-electricity conversion method, and the other is the light-electricity direct conversion method.
(1) The photo-thermal-electric conversion method generates electricity by using the thermal energy generated by solar radiation, generally by converting the absorbed thermal energy from solar collectors into steam from the work mass, and then driving a turbine to generate electricity. The former process is the light-to-heat conversion process; the latter process is the heat-to-electricity conversion process, the same as ordinary thermal power generation. The disadvantage of solar thermal power generation is the very low efficiency and high cost, it is estimated that its investment is at least 5 to 10 times more expensive than ordinary thermal power plants.
(2) The direct conversion of light - electricity way is the use of the photoelectric effect, the direct conversion of solar radiation energy into electricity, light - electricity conversion of the basic device is the solar cell. A solar cell is a kind of photovoltaic effect due to the direct conversion of solar energy into electrical devices, is a semiconductor photodiode, when the sunlight shines on the photodiode, the photodiode will turn the sun's light energy into electrical energy, producing current. When many cells are connected in series or in parallel, they 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 life, as long as the sun exists, solar cells can be a long-term investment and use; compared with thermal power generation, nuclear power generation, solar cells do not cause environmental pollution.
3、System composition
Photovoltaic power generation system is composed of solar cell arrays, battery banks, charge and discharge controllers, inverters, AC distribution cabinets, solar tracking control systems, and other equipment. The role of some of the equipment is
Battery array: In the case of light (whether from the sun or other luminous bodies), the battery absorbs light energy, and the accumulation of dissimilar charges at both ends of the battery, that is, the "photovoltaic voltage", which is the "photovoltaic effect". Under the effect of the photovoltaic effect, solar cells produce electric potential at both ends, converting light energy into electricity, which is a device for energy conversion. Solar cells are generally silicon cells, divided into monocrystalline silicon solar cells, polycrystalline silicon solar cells, and amorphous silicon solar cells three.
Battery: The role of the battery is to store the solar cell array by the light emitted when the electrical energy is and can be powered to the load at any time. The basic requirements of the solar cell power generation battery pack used are a. low self-discharge rate; b. long service life; c. strong deep discharge capacity; d. high charging efficiency; e. less maintenance or maintenance-free; f. wide operating temperature range; g. low price.
Controller: It is the equipment that can automatically prevent the battery from over-charging and over-discharging. As the cycle charge and discharge times and depth of discharge of the battery is important factor to determine the service life of the battery, so the charge and discharge controller that can control the battery overcharge or over-discharge is essential equipment.
Inverter: It is the equipment that converts DC power into AC power. As solar cells and batteries are DC power sources, and the load is AC load, the inverter is essential. According to the operation mode, inverters can be divided into stand-alone operation inverters and grid-connected inverters. Stand-alone inverters are used for solar cell power systems that operate independently to supply power to independent loads. Grid-connected inverters are used for grid-connected solar cell power generation systems. Inverters can be divided into square wave inverters and sine wave inverters according to the output waveform. Square wave inverters have simple circuitry and low cost, but have large harmonic components, and are generally used for systems with less than a few hundred watts and low harmonic requirements. Sine wave inverter has a high cost, but it can be applied to various loads.
4、System classification
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. Mainly composed of solar modules, controllers, batteries, if you want to power the AC load, you also need to configure the AC inverter. Independent photovoltaic power plants include village power supply systems in remote areas, solar household power systems, communication signal power, cathodic protection, solar street lights, and other kinds of photovoltaic power generation systems with batteries that can operate independently.
(2) Grid-connected photovoltaic power generation is the direct current generated by the solar modules through the grid-connected inverter into AC power that meets the requirements of the utility grid and is then directly connected to the public grid.
It can be divided into a grid-connected power generation systems with batteries and without batteries. Grid-connected power generation systems with batteries are dispatchable and can be connected to or disconnected from the grid as needed, and also have the function of a backup power supply for emergency power supply in case of grid outages. Grid-connected power generation systems with batteries are often installed in residential buildings; grid-connected power generation systems without batteries do not have dispatchability and standby power functions and are generally installed in larger systems. Grid-connected photovoltaic power generation has centralized large-scale grid-connected photovoltaic power plants are generally national power stations, the main feature is to transmit the generated energy directly to the grid, by the grid unified deployment of power supply to users. But this kind of power station investment, long construction period, covers a large area, has not been too much development. And decentralized small grid-connected photovoltaic, especially photovoltaic building-integrated photovoltaic power generation, due to the advantages of small investment, fast construction, small footprint, and strong policy support, is the mainstream of grid-connected photovoltaic power generation.
Distributed photovoltaic power generation system, also known as decentralized power generation or distributed energy supply, refers to the configuration of smaller photovoltaic power supply systems at or near the user site 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 modules, photovoltaic square brackets, DC sink boxes, DC distribution cabinets, grid-connected inverters, AC distribution cabinets, and other equipment, in addition to power supply system monitoring devices and environmental monitoring devices. Its operation mode is that under the condition of solar radiation, the solar cell array of photovoltaic power generation system will convert the solar energy output, through the DC sink box centralized into the DC distribution cabinet, by the grid-connected inverter reverse into AC power supply to the building's own load, excess or deficient power through the connection to the grid to regulate.
5、The advantages of photovoltaic power generation
Compared with the commonly used power generation systems, the advantages of solar photovoltaic power generation are mainly reflected in
Solar power generation is known as the most ideal new energy source. ①No danger of depletion; ②Safe and reliable, no noise, no pollution emissions outside, absolutely clean (no pollution); ③Not limited by the geographical distribution of resources, can take advantage of the building roof; ④No need to consume fuel and set up transmission lines can be on-site power generation and supply; ⑤High quality of energy; ⑥Easy to accept from the user emotionally; ⑦Short construction period, short time spent to obtain energy.
6、Application areas
I. Users of solar power: (1) small power supply ranging from 10-100W, used in remote areas without electricity, such as plateaus, islands, pastoral areas, border guard posts, and other military and civilian living electricity, such as lighting, TV, recorders, etc.; (2) 3-5KW family rooftop grid-connected power generation system; (3) photovoltaic water pump: to solve the deep well drinking and irrigation in areas without electricity.
Second, the field of transportation such as beacon lights, traffic/railroad signal lights, traffic warning/sign lights, Yu Xiang street lights, high altitude obstacle lights, highway/railroad wireless telephone booths, unattended road shift power supply, etc.
Third, communication/communication field: solar unattended microwave relay station, fiber optic cable maintenance station, broadcasting/communication / paging power supply system; rural carrier phone photovoltaic system, small communication machine, GPS power supply for soldiers, etc.
Fourth, oil, marine, meteorological field: oil pipelines and reservoir gates cathodic protection solar power system, oil drilling platform life and emergency power, marine detection equipment, meteorological/hydrological observation equipment, etc.
V. Household lighting power supply: such as garden lights, street lights, portable lights, camping lights, hiking lights, fishing lights, black lights, rubber cutting lights, energy-saving lamps, etc.
Sixth, photovoltaic power station: 10KW-50MW independent photovoltaic power station, scenery (diesel) complementary power station, a variety of large parking plant charging stations, etc.
Seven, solar architecture will be combined with solar power generation and building materials, making the future of large buildings to achieve power self-sufficiency, is a major future development direction.
Eight, other areas include: (1) and automotive supporting: solar cars / electric vehicles, battery charging equipment, automotive air conditioning, ventilation fans, cold drinks box, etc.; (2) solar hydrogen plus fuel cells regenerative power generation system; (3) desalination equipment power supply; (4) satellite, spacecraft, space solar power station, etc.
