The off-grid photovoltaic power generation system is mainly composed of solar cell components, photovoltaic controllers, energy storage batteries, photovoltaic inverters, AC and DC distribution boxes, and photovoltaic supports. Off-grid photovoltaic power generation systems can be divided into the following forms according to the characteristics of the electrical load.
1. Battery-free DC photovoltaic power generation system
The battery-less DC photovoltaic power generation system is shown in Figure 1. The characteristic of the system is that the electrical load is a DC load, and there is no requirement for the load usage time, and the load is mainly used during the day. The solar battery is directly connected to the electric load, and when there is sunlight, it generates electricity for the load to work, and when there is no sunlight, it stops working. The system does not need to use a photovoltaic controller, and there is no electricity storage device. The advantage of this system is that it reduces the loss of electrical energy through the photovoltaic controller and the storage and release of the battery, and improves the utilization efficiency of solar energy. The most typical application of this system is solar photovoltaic water pumps. The application of solar photovoltaic pumps can not only directly pump water for irrigation when the sun is full, but also use photovoltaic pumps to pump water into a reservoir for storage, and convert solar energy into potential energy for use at night and on cloudy and rainy days.
2. DC photovoltaic power generation system with battery
The DC photovoltaic power generation system with storage battery is shown in Figure 2. The system consists of solar cells, photovoltaic controllers, storage batteries, DC loads, etc. When there is sunlight, the solar cell converts light energy into electrical energy for use by the load, and at the same time stores electrical energy in the storage battery. At night or in rainy days, the battery supplies power to the load. This system is widely used, ranging from solar lawn lights and garden lights to large mobile communication base stations and microwave relay stations far away from the grid, and power supply in remote rural areas. When the system capacity and load power are large, a square array of solar cells and battery packs are required.
3. AC and AC, DC hybrid photovoltaic power generation system
The AC and AC, DC hybrid photovoltaic power generation system is shown in Figure 3. Compared with the DC photovoltaic power generation system, the AC photovoltaic power generation system has an additional photovoltaic inverter to convert DC power into AC power to provide electrical energy for the AC load. When there is sunlight, the photovoltaic cell converts light energy into DC power to charge the energy storage battery, and at the same time converts the DC power into AC power through the photovoltaic inverter to provide power to AC users or loads. At night or in rainy days, the DC power stored in the energy storage battery is converted into AC power through a photovoltaic inverter to supply power to the load. The AC and DC hybrid system can supply power for both DC loads and AC loads.
4. Mains complementary photovoltaic power generation system
The utility power complementary photovoltaic power generation system is shown in Figure 4. The so-called commercial complementary photovoltaic power generation system is based on solar photovoltaic power generation in the independent photovoltaic power generation system, supplemented by ordinary 220V AC power. In this way, the capacity of the battery components and storage batteries in the photovoltaic power generation system can be designed to be smaller. Basically, when there is sunshine on the day, the electricity generated by solar energy is used on that day, and the mains energy is used to supplement it in rainy days. In most areas of our country, there are basically more than two-thirds of the fine weather throughout the year, so the system has more than two-thirds of the year to use solar power, and the remaining time to use city power to supplement energy. This form not only reduces the one-time investment of solar photovoltaic power generation system, but also has significant energy saving and emission reduction effects. It is a good transitional method for solar photovoltaic power generation in the promotion and popularization process. In principle, this form is similar to the grid-connected photovoltaic power generation system without countercurrent that will be introduced below, but it cannot be equated to grid-connected applications.
Application example: In a certain urban street lamp renovation, if all ordinary street lamps are replaced with solar street lamps, the one-time investment is very large and cannot be realized. However, if the ordinary street lights are modified, the original mains power supply lines and light poles are kept in place, energy-saving light source lamps are replaced, and the form of mains complementary photovoltaic power generation is adopted. Considering the number of consecutive cloudy and rainy days), it constitutes a complementary solar street lamp with municipal electricity, and the investment is reduced by more than half, and the energy-saving effect is significant.
5. Photovoltaic power generation system capable of automatic switching
The photovoltaic power generation system that can automatically switch is shown in Figure 5. The so-called automatic switching means that the off-grid system has the function of automatic two-way switching with the public grid. First, when the photovoltaic power generation system is insufficient due to cloudy, rainy days, and its own failure, the switcher can automatically switch to the public grid power supply side, and the grid will supply power to the load; second, when the grid suddenly loses power for some reason , The photovoltaic system can automatically switch to separate the grid from the photovoltaic system and become an independent photovoltaic power generation system. Some photovoltaic power generation systems with switching devices can also cut off the power supply to the general load and switch on the power supply to the emergency load when needed.
6. Wind-solar hybrid and wind-solar diesel hybrid power generation system
Wind-solar hybrid and wind-solar diesel hybrid power generation systems are shown in Figure 6. The so-called wind-solar complementation refers to the integration of the wind power generation system into the photovoltaic power generation system, so that solar energy and wind energy complement each other according to their respective meteorological characteristics. Generally speaking, as long as the weather is fine during the day, the photovoltaic power generation system can operate normally, and when there is no sunlight at night, the wind power is usually relatively high. The wind power generation system just makes up for the shortcomings of the photovoltaic power generation system. The wind-solar hybrid power generation system uses both solar and wind energy to generate electricity, making full use of meteorological resources, enabling day and night power generation, and improving the continuity and stability of the system’s power supply. However, it is not suitable for use in areas with poor wind resources.
In addition, where more important or high requirements for power supply stability, diesel generators, photovoltaic power generation systems, and wind power generators must be used to form a wind-solar and diesel-powered complementary power generation system. Diesel generators are generally in standby state or low-power operation standby state. When wind and solar power generation is insufficient and battery energy storage is insufficient, diesel generators are supplemented for power supply.