1. According to the power supply method
According to the power supply mode, distributed power (photovoltaic power generation) systems can be roughly divided into three categories: independent power generation systems, grid-connected power generation systems and hybrid power generation systems.
(1) A typical independent power generation system is shown in Figure 1, using storage batteries and solar cells to form an independent power supply system to provide electrical energy to the load. When the solar battery’s output power cannot meet the load requirement, the battery will supplement it; when the output power exceeds the load requirement, the electric energy will be stored in the battery.
(2) The general grid-connected power generation system is shown in Figure 2. The solar cell control system is connected in parallel with the civil power grid. When the solar cell output power cannot meet the load requirement, the power grid will supplement it; when the output power exceeds the load requirement, the power will be delivered to the power grid.
(3) The hybrid photovoltaic power generation system is shown in Figure 3. The difference from the above two systems is the addition of a backup generator set. When the photovoltaic array power generation is insufficient or the battery storage is insufficient, the generator set can be started, which can directly supply power to the AC load, but also can charge the battery through the rectifier, so it is called a hybrid photovoltaic power generation system.
Because of the change of day and night and the seasons, as well as the influence of factors such as weather changes, photovoltaic power generation has the defect of unstable power generation, so independent power generation systems often need to use larger-capacity batteries as energy storage components to balance power supply. However, the addition of batteries to the system will bring about problems such as increased maintenance costs, increased system volume, and environmental pollution. The grid-connected power generation system can solve these problems well. As the photovoltaic power generation industry is moving rapidly from remote rural areas to urban grid-connected power generation and photovoltaic buildings, in the future, photovoltaic grid-connected power generation will be the mainstream trend of photovoltaic power generation.
The distributed photovoltaic system, that is, the household photovoltaic grid-connected system, can be combined with the building to form a rooftop photovoltaic system, and the design can reduce the construction cost and the cost of the photovoltaic power generation system. In a distributed grid-connected photovoltaic system, the electricity that is not used during the day can be sold to the local public power grid through the inverter; when electricity is needed at night, it can be purchased from the power grid. A typical distributed photovoltaic system is shown in Figure 4.
It can be seen that if the distributed photovoltaic system can be widely applied to users’ homes, it will not only make full use of the wide distribution of solar energy resources, but also achieve the goals of improving the quality of the power grid, strengthening the power grid’s peak regulation capability, and the ability to resist disasters. At present, the research on distributed photovoltaic systems is, on the one hand, the research on solar cells, which reduces the cost of each watt (W) of electricity generated by the battery to a practical stage; on the other hand, it is the research on the inverter system for grid-connected power generation, such as improving the efficiency and stability of the system, improving the control of the maximum power point of solar cells, and enhancing the peak regulation effect of the system on the grid.
2. Press system function
According to the system function, the distributed power (photovoltaic power generation) system is divided into an unschedulable photovoltaic power generation system without a battery link and a dispatchable photovoltaic power generation system with a battery pack.
(1) The undispatchable photovoltaic power generation system is shown in Figure 5. The grid-connected inverter converts the DC power generated by the photovoltaic array into AC power with the same frequency and phase as the grid voltage. When the main grid is cut off, the system automatically stops supplying power to the grid. During the day, when the AC power generated by the photovoltaic power generation system exceeds the local load requirements, the excess is fed to the grid; at other times, especially at night, when the local load is greater than the AC power generated by the photovoltaic power generation system, the grid automatically supplements the load with power.
(2) The dispatchable photovoltaic power generation system is shown in Figure 6. Compared with the undispatchable photovoltaic power generation system, the biggest difference is that the system is equipped with an energy storage link-battery pack. The capacity of the battery pack can be configured to achieve uninterrupted power supply (UPS) according to specific needs. As an active power regulator of the grid terminal, it can offset high-order spectral wave components, improve power quality, and improve the quality of grid operation.
However, due to the short life, high cost, and large size of the battery pack, the application scale of the dispatchable photovoltaic power generation system is much smaller than that of the undispatchable photovoltaic power generation system.