In general, we divide photovoltaic systems into independent systems, grid-connected systems, and hybrid systems. According to the application form of solar photovoltaic system, the application scale and the type of load, the photovoltaic power supply system will be divided more carefully. The photovoltaic system can also be subdivided into the following six types: Small Solar Power System (SmallDC); Simple DC System (SimpleDC); Large Solar Power System (LargeDC); AC and DC Power Supply System (AC/DC); (Utility Grid Connect); hybrid power system (Hybrid); hybrid system. The following describes the working principle and characteristics of each system.
1. Small solar power system (SmallDC)
The characteristic of this system is that there is only DC load in the system and the load power is relatively small. The entire system has a simple structure and is easy to operate. Its main uses are general home-use systems, various consumer DC products, and related entertainment equipment. For example, in the western region of China, this type of photovoltaic system has been widely used and the load is a DC lamp, which is used to solve the problem of home lighting in areas where electricity is not available.
2. Simple DC system (SimpleDC)
The characteristic of this system is that the load in the system is DC load and there is no special requirement for the use time of the load. The load is mainly used during the day. Therefore, the battery is not used in the system and the controller is not needed. The system has a simple structure and is used directly. Photovoltaic modules provide power to the load, eliminating the storage and release of energy in the battery, as well as energy loss in the controller, improving energy efficiency. It is commonly used in PV pump systems, some daytime temporary equipment electricity, and some tourist facilities. The following figure shows a simple DC PV pump system. This system has been widely used in areas where there is no pure tap water for drinking in developing countries and has produced good social benefits.
3 Large Solar Power System (LargeDC)
Compared with the above two photovoltaic systems, this photovoltaic system is still suitable for DC power systems, but such solar photovoltaic systems usually have higher load power, in order to ensure reliable power supply to the load, its corresponding system The scale is also large, and it needs to be equipped with a larger array of photovoltaic modules and larger solar battery packs. The common application forms include communication, telemetry, monitoring equipment power supply, rural centralized power supply, navigation beacons, street lamps and so on. Some of China's rural PV power stations built in some non-electric areas in the western part of China are in this form. The communication base stations built by China Mobile and China Unicom in remote, non-grid areas are also powered by this type of photovoltaic system. Such as communication base station project in Shanxi Wanjiazhai.
4 AC, DC Power Supply System (AC/DC)
Different from the above three kinds of solar photovoltaic systems, this photovoltaic system can provide power for DC and AC loads at the same time. In the system structure, there are more inverters than the above three systems for converting DC power to AC power to meet the requirements. AC load requirements. Usually the load power consumption of such a system is also relatively large, so the system is also large in scale. It is used in some communication base stations with both AC and DC loads and other PV power stations with AC and DC loads.
5 Utility Grid Connect (Utility Grid Connect)
The most important feature of this solar photovoltaic system is that the direct current generated by the photovoltaic array is directly connected to the mains network after the grid-connected inverter is converted into the alternating current that meets the requirements of the mains grid, and the power generated by the PV array in the grid-connected system is not only provided for exchange. Outside the load, excess power is fed back to the grid. On cloudy or rainy days, the photovoltaic array is powered by the grid when no power is generated by the PV array or the generated power cannot meet the load demand. Because electric energy is directly input into the power grid and the battery is dispensed with, the process of storing and releasing the battery is omitted, and the power generated by the PV array can be fully utilized, thus reducing the energy loss and reducing the cost of the system. However, a dedicated grid-connected inverter is required in the system to ensure that the output power satisfies the grid power requirements for voltage, frequency, and other indicators. Because of the efficiency of the inverter, there will still be some energy loss. Such systems are often able to use mains and solar PV arrays in parallel as a source of local AC loads. Reduced the load shedding rate of the entire system. And the grid-connected PV system can play a role in peaking the utility grid. However, the grid-connected photovoltaic power supply system as a decentralized power generation system will have some adverse effects on the traditional centralized power supply system, such as harmonic pollution and islanding.
6 Hybrid Power System (Hybrid)
In addition to the array of solar photovoltaic modules used in this type of solar photovoltaic system, an oil generator is also used as a backup power source. The purpose of using a hybrid power supply system is to comprehensively use the advantages of various power generation technologies to avoid their respective shortcomings. For example, the advantages of the above-mentioned several kinds of independent photovoltaic systems are the low maintenance. The disadvantage is that the output of energy depends on the weather and is not stable. Hybrid power supply systems that use diesel generators and photovoltaic arrays in combination can provide weather-independent energy compared to stand-alone systems with a single source of energy. Its advantages are:
1. The use of hybrid power systems can also achieve better utilization of renewable energy. Because stand-alone systems using renewable energy are usually designed for worst-case scenarios because renewable energy sources are variable and unstable, the system must be designed with the minimum period of energy production. Since the system is designed according to the worst case, the capacity of the system is excessive at other times. The excess energy generated during the highest peak of solar radiation cannot be used and wasted. The performance of the entire independent system is thus reduced. If the worst-month situation is very different from other months, it may result in wasted energy equal to or even exceeding the design load requirement.
2. With high system availability. In a stand-alone system, because of the change and instability of the renewable energy, the system will be in a situation where the power supply can not meet the load demand, that is, there is a load shortage situation, and using the hybrid system will greatly reduce the load shortage rate.
3. Less maintenance and less used fuel compared to diesel generators alone.
4. Higher fuel efficiency. Under low load conditions, the fuel efficiency of the diesel engine is very low, which can result in wasted fuel. In the hybrid system, comprehensive control can be performed so that the diesel engine operates near the rated power, thereby improving fuel efficiency.
5. Load matching better flexibility. After the hybrid system is used, the hybrid system can be applied to a wider range of load systems because the diesel generator can provide greater power in real time, for example, large AC loads, impact loads, and the like can be used. It can also better match the load and the system's power generation. As long as the backup energy is turned on at the peak of the load, it can be done simply. Sometimes, the size of the load determines the need to use a hybrid system. Large loads require large currents and high voltages. If you just use solar energy, the cost will be high.
Hybrid systems also have their own shortcomings:
1. The control is more complicated. Because of the use of multiple energy sources, the system needs to monitor the working conditions of each energy source, deal with the mutual influence between various sub-energy systems, and coordinate the operation of the entire system. As a result, its control system is more complex than the stand-alone system. Process the chip for system management.
2. The initial project is larger. The design, installation and construction of the hybrid system are all larger than those of the independent project.
3. Need more maintenance than stand-alone systems. The use of the oil machine requires a lot of maintenance work, such as replacing the oil filter, fuel filter, spark plug, etc., and also need to add fuel to the fuel tank.
4. Pollution and noise. Photovoltaic systems are noiseless, emission-free clean energy sources, but because diesel engines are used in hybrid systems, noise and pollution are unavoidable.
Many communication power sources and civil aviation navigation equipment power supplies in remote and unpowered areas are used in hybrid systems because they require high power supplies in order to achieve the best price/performance ratio. Many rural PV power plants built in Xinjiang and Yunnan in China use an optical/Chai hybrid system.
7 On-grid Hybrid Power System (Hybrid)
With the development of the solar photovoltaic industry, there has emerged a grid-connected hybrid power supply system that can make comprehensive use of solar photovoltaic module arrays, mains electricity and auxiliary oil engines. This kind of system is usually an integrated integration of controller and inverter. It uses a computer chip to fully control the operation of the entire system, comprehensively uses all kinds of energy to achieve the best working conditions, and can also use the battery to further improve the system's load and power supply protection rate. For example, AES's SMD inverter system. The system can provide qualified power for local loads and can work as an online UPS (Uninterruptible Power Supply). Power can also be supplied to the grid or obtained from the grid.
The way the system works is usually to operate the mains and solar power in parallel. For local loads, if the PV module generates enough power to use the load, it will directly use the power generated by the PV module to supply the load. If the PV module generates more power than the demand of the immediate load, it can return the excess power to the grid; if the PV module generates insufficient power, the utility will be automatically activated, and the demand for the local load will be supplied by the utility, and when local When the power consumption of the load is less than 60% of the rated commercial capacity of the SMD inverter, the utility will automatically charge the battery to ensure that the battery is in a floating state for a long period of time; if the utility power fails, the mains failure or the mains If the quality is not qualified, the system will automatically disconnect the mains and turn it into an independent work mode. The battery and the inverter provide the AC power required by the load.
Once the utility power returns to normal, that is, the voltage and frequency are restored within the above-mentioned normal state, the system will disconnect the battery, change to grid-connected mode and be powered by the mains. Some grid-connected hybrid power systems can also integrate system monitoring, control, and data acquisition functions into the control chip. The core components of this system are controllers and inverters.
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