The solar energy generation is the energy from celestial bodies outside the earth (mainly solar energy). It is the huge energy released by the fusion of hydrogen nuclei in the sun at ultra-high temperature. Most of the energy needed by human beings comes directly or indirectly from the sun. Fossil fuels such as coal, oil, and natural gas that we need for life are all formed by plants and animals buried in the ground after a long geological age after converting solar energy into chemical energy through photosynthesis and storing it in the plant body. In addition, water energy, wind energy, wave energy, and ocean current energy are all converted from solar energy.
1. Background of solar energy generation
1.1 Energy Status
With the development of the economy and the progress of society, people put forward higher and higher requirements for energy, and finding new energy has become an urgent issue facing mankind. There are four main sources of existing electric energy, namely thermal power, hydropower, nuclear power and wind power.
Thermal power needs to burn fossil fuels such as coal and oil. On the one hand, fossil fuels have limited reserves, are burning less and less, and are facing the danger of depletion; on the other hand, burning will emit carbon dioxide and sulfur oxides, which will cause the greenhouse effect and acid rain, and deteriorate the global environment.
Hydropower will inundate a large amount of land, which may cause damage to the ecological environment, and once a large reservoir collapses, the consequences will be disastrous. In addition, a country's water resources are also limited and affected by seasons.
Nuclear power is clean under normal circumstances, but in the event of a nuclear leak, the consequences are equally dire. The accident at the Chernobyl nuclear power plant in the former Soviet Union has caused 9 million people to be damaged to varying degrees; at 13:46 on March 11, 2011, a 9.0-magnitude earthquake occurred in Fukushima, Japan, triggering the Fukushima nuclear power plant accident that shocked the world, causing The 30 kilometers near the nuclear power plant will become an uninhabited area; the marine resources within a radius of 5 kilometers will be affected to varying degrees or marine organisms will mutate.
As a clean and renewable energy source, wind power has broad prospects for development. Wind energy reserves are large, and the extensive development of wind power generation is an effective way to solve the shortage of energy supply in China; wind power generation belongs to the application of clean energy and is an effective way to reduce greenhouse gas emissions.
1.2 Ideal Energy
New energy must meet two conditions at the same time: first, it has abundant reserves and will not be exhausted; second, it is safe and clean, and will not threaten human beings or damage the environment. There are two main types of new energy sources found, one is solar energy, and the other is fuel cells.
In the 21st century, solar energy will become one of the main energy sources in the world, and it is the most primitive energy source. Almost all other energy sources on the earth come directly or indirectly from solar energy. Solar energy is the energy produced by the continuous nuclear fusion reaction process of sunspots inside or on the surface. Solar energy has the advantages of abundant resources, longevity, wide distribution, safety, cleanliness, and reliable technology. Since solar energy can be converted into many other forms of energy, it has a wide range of applications. In terms of heat utilization, there are solar greenhouses, item drying and solar cookers, and solar water heaters. After years of development, solar energy generation has also been developed by leaps and bounds.
Obtaining electricity from solar energy requires photoelectric conversion through solar cells. It is completely different from other power generation principles in the past. To make solar energy generation really reach the practical level, one is to improve the efficiency of solar photoelectric conversion and reduce its cost, and the other is to realize the interconnection of solar energy generation with the power grid.
2. Classification of solar energy generation
2.1 Solar photovoltaic power generation
Solar photovoltaic power generation refers to a solar energy generation method that directly converts light energy into electrical energy without going through a thermal process. It includes photovoltaic power generation, photochemical power generation, light induction power generation and photobiological power generation. Photovoltaic power generation is a direct power generation method that uses solar-grade semiconductor electronic devices to effectively absorb solar radiation energy and convert it into electrical energy. It is the mainstream of solar energy generation today. In photochemical power generation, there are electrochemical photovoltaic cells, photoelectrolytic cells and photocatalytic cells, and photovoltaic cells are currently used in practice.
The photovoltaic power generation system is mainly composed of solar cells, storage batteries, controllers and inverters, among which solar cells are a key part of the photovoltaic power generation system, and the quality and cost of solar panels will directly determine the quality and cost of the entire system. Solar cells are mainly divided into two types: crystalline silicon cells and thin film cells. The former includes monocrystalline silicon cells and polycrystalline silicon cells. The latter mainly includes amorphous silicon solar cells, copper indium gallium selenide solar cells and cadmium telluride solar cells.
The photoelectric conversion efficiency of monocrystalline silicon solar cells is about 15%, up to 23%, and the photoelectric conversion efficiency is the highest among solar cells, but its manufacturing cost is high. The service life of monocrystalline silicon solar cells can generally reach 15 years, up to 25 years. The photoelectric conversion efficiency of polycrystalline silicon solar cells is 14% to 16%, and its production cost is lower than that of monocrystalline silicon solar cells, so it has been developed a lot, but the service life of polycrystalline silicon solar cells is shorter than that of monocrystalline silicon solar cells.
Thin-film solar cells are solar cells that use thin films such as silicon, cadmium sulfide, and gallium arsenide as substrate materials. Thin-film solar cells can be manufactured using light-weight, low-cost substrate materials (such as glass, plastic, ceramics, etc.), forming a film that can generate voltage with a thickness of less than 1 micron, which is convenient for transportation and installation. However, thin films deposited on heterogeneous substrates will produce some defects, so the large-scale mass production conversion efficiency of existing cadmium telluride and copper indium gallium selenide solar cells is only 12% to 14%, while its theoretical upper limit can reach 29% %. If the defects of cadmium telluride can be reduced during the production process, it will increase the life of the battery and improve its conversion efficiency. This requires research on the causes of defects, as well as ways to reduce defects and control quality. The solar cell interface is also critical and requires significant R&D investment.
2.2 Solar thermal power generation
The power generation method that converts solar radiation energy into electrical energy through water or other working fluids and devices is called solar thermal power generation. First convert solar energy into thermal energy, and then convert thermal energy into electrical energy. There are two conversion methods: one is to directly convert solar thermal energy into electrical energy, such as thermoelectric power generation of semiconductor or metal materials, thermal electrons and thermoelectric ions in vacuum devices Power generation, alkali metal thermoelectric conversion, and magnetic fluid power generation; another way is to use solar thermal energy to drive a generator to generate electricity through a heat engine (such as a steam turbine), which is similar to conventional thermal power generation, except that the heat energy does not come from fuel, but from solar energy. There are many types of solar thermal power generation, mainly in the following five types: tower system, trough system, disk system, solar pool and solar tower thermal airflow power generation. The first three are concentrating solar thermal power generation systems, and the latter two are non-concentrating. Some developed countries regard solar thermal power generation technology as a national research and development focus, and have manufactured dozens of various types of solar thermal power generation demonstration power stations, which have reached the practical application level of grid-connected power generation.
At present, the most promising solar thermal power generation systems in the world can be roughly divided into: trough parabolic focusing system, central receiver or solar tower focusing system and disc parabolic focusing system. The three forms that are technically and economically feasible are: 30-80MW focused parabolic trough solar thermal power generation technology (abbreviated as parabolic trough type); 30-200MW point-focused central receiving solar thermal power generation technology (abbreviated as central receiving type); 7.5 ~ 25kW point-focused parabolic disk solar thermal power generation technology (referred to as parabolic disk).
The heat transfer fluids of the concentrated solar thermal power generation system are mainly water, water vapor and molten salt, etc. These heat transfer fluids can be heated to 450 degrees Celsius in the receiver and then used for power generation. In addition, the heat storage system of this power generation method can temporarily store heat energy for several hours, so as to reserve electricity for peak times.
The parabolic trough focusing system uses a parabolic trough reflector to gather sunlight onto a tubular receiver, heats the heat transfer medium in the tube, generates steam in the heat exchanger, and drives a conventional steam turbine to generate electricity. The tower-type solar thermal power generation system uses a set of heliostats that independently track the sun to gather sunlight to a receiver on the top of a fixed tower to generate high temperature.
In addition to the above-mentioned traditional solar thermal power generation methods, research in new fields such as solar chimney power generation and solar pool power generation has also made progress.
3. Solar energy generation device
Solar energy generation is a device that uses battery components to directly convert solar energy into electrical energy. Solar cells are solid devices that use the electronic properties of semiconductor materials to achieve P-V conversion. In the vast areas without power grids, this device can conveniently realize lighting and life power for users. Some developed countries can also be connected to regional power grids. Grid-connected to achieve complementarity. At present, from the perspective of civilian use, the "photovoltaic-building (lighting) integration" technology that has matured and has begun to be industrialized abroad is mainly researched and produced in China for small-scale solar energy generation suitable for household lighting in areas without electricity. system.
The solar energy generation system mainly includes: solar cell components (arrays), controllers, batteries, inverters, users, namely lighting loads, etc. Among them, the solar battery module and the storage battery are the power supply system, the controller and the inverter are the control protection system, and the load is the system terminal.
The solar cell and the storage battery form the power unit of the system, so the performance of the storage battery directly affects the working characteristics of the system.
3.1 Battery unit
Due to technical and material reasons, the power generation of a single battery is very limited. The practical solar battery is a battery system composed of a single battery connected in series and parallel, which is called a battery module (array). A single battery is a silicon crystal diode. According to the electronic characteristics of semiconductor materials, when sunlight irradiates on the P-N junction composed of P-type and N-type homogeneous semiconductor materials with different conductivity types, under certain conditions , solar radiation is absorbed by semiconductor materials, generating non-equilibrium carriers, namely electrons and holes, in the conduction and valence bands. Same as the P-N junction barrier region, there is a strong built-in electrostatic field, so it can form a current density J, a short-circuit current Isc, and an open-circuit voltage Uoc under light. If the electrodes are drawn out on both sides of the built-in electric field and connected to the load, theoretically speaking, the loop formed by the P-N junction, the connecting circuit and the load will have a "photo-generated current" flowing, and the solar cell module will realize the power P of the load. output.
Theoretical research shows that the peak power Pk of solar cell modules is determined by the local average solar radiation intensity and the terminal power load (power demand).
3.2 Storage unit
The direct current generated by the solar cell first enters the battery for storage, and the characteristics of the battery affect the working efficiency and characteristics of the system. Battery technology is very mature, but its capacity is affected by the terminal demand and sunshine time (power generation time). Therefore, the watt-hour capacity and the ampere-hour capacity of the battery are determined by the predetermined continuous sunshine-free time.
The main function of the controller is to make the solar energy generation system always near the maximum power point of power generation to obtain the highest efficiency. The charging control usually adopts pulse width modulation technology, that is, PWM control mode, so that the whole system always operates in the area near the maximum power point Pm. Discharge control mainly refers to cutting off the switch when the battery is short of power or the system fails, such as when the battery is open or reversed. At present, Hitachi has developed a "sunflower" controller that can not only track the control point Pm, but also track the sun's moving parameters, which improves the efficiency of the fixed battery components by about 50%.
Inverters can be divided into self-excited oscillating inverters and other-excited oscillating inverters according to the excitation method. The main function is to invert the direct current of the battery into alternating current. Through the full-bridge circuit, the SPWM processor is generally used for modulation, filtering, boosting, etc., to obtain sinusoidal alternating current matching the lighting load frequency f, rated voltage UN, etc., for use by the end users of the system.
3.5 Anti-reverse charge diode
The anti-reverse charge diode of the solar photovoltaic power generation system is also called the blocking diode. In the solar cell module, its function is to prevent the solar cell array from discharging through the solar cell array when the solar array does not generate electricity or has a short-circuit fault at night. Anti-reverse charging diodes are connected in series in the solar cell square array circuit, and act as one-way conduction. Therefore, it must ensure that there is a maximum current in the loop, and it must withstand the impact of the maximum reverse voltage. Generally, a suitable rectifier diode can be selected as an anti-reverse charge diode. One board can be used without any diodes, since the controller is inherently anti-kickback. If the boards are connected in series, a bypass diode needs to be installed. If the boards are connected in parallel, an anti-recoil diode should be installed to prevent the board from being charged directly. The anti-reverse charging diode is only for protection and will not affect the power generation effect.
In a solar energy generation system, the total system efficiency ηese is composed of the PV conversion rate of the battery module, the efficiency of the controller, the efficiency of the battery, the efficiency of the inverter, and the efficiency of the load. However, compared with solar battery technology, it is much more mature than the technology and production level of other units such as controllers, inverters and lighting loads, and the conversion rate of the system is only about 17%. Therefore, improving the conversion rate of battery components and reducing the cost per unit power is the focus and difficulty of the industrialization of solar energy generation. Since the advent of solar cells, crystalline silicon has maintained its dominant position as the protagonist material. The research on the conversion rate of silicon batteries mainly revolves around increasing the energy-absorbing surface, such as double-sided batteries, reducing reflection; using gettering technology to reduce the recombination of semiconductor materials; making batteries ultra-thin; improving theories and establishing new models; photocell etc.