The composition and working principle of photovoltaic power generation system

Photovoltaic power generation system is a renewable energy device that uses photovoltaic panels to convert solar energy into electrical energy and store it in the battery of the system or directly for load use. The basic solar photovoltaic power generation system includes photovoltaic panels, converters, controllers, batteries or other energy storage and auxiliary power generation equipment, as shown in Figure 1. Photovoltaic panels are composed of solar cell components in series and parallel according to the needs of the system. In the daytime, it receives sunlight and directly converts solar energy into electrical energy. One part supplies DC or AC load to work, and the other part charges the battery pack through anti-reverse charge diodes. At night or rainy days, the photovoltaic panels cannot work, and the battery pack supplies power to the DC or AC load to work. The photovoltaic panel is the core component of the photovoltaic system. The converter usually converts the unstable direct current output by the photovoltaic panel into alternating current, which is used by the alternating load or connected to the grid to charge the energy storage device. Energy storage devices generally use batteries, especially lead-acid batteries, to store the remaining power generated by the photovoltaic panels. In the case of insufficient power supply from the photovoltaic panels, the batteries provide auxiliary power to the load. As the control core of the whole system, the controller is responsible for detecting the operating parameters of the system, controlling the charge and discharge of the battery, and controlling the power output of solar energy and battery according to the demand of load or power grid, so that the system can automatically and stably operate and work in the best state. If there is an automatic tracking device, it is also centrally controlled by the same controller to track changes in the sun’s position.

The composition and working principle of photovoltaic power generation system
Figure 1 – Structural composition of solar photovoltaic power generation system

The composition of the independent photovoltaic power generation tracking system is shown in the left part of point A in Figure 1. It mainly includes the condenser, the tracking of the maximum power point and the control of the battery charging and discharging. Concentrators are usually used to improve the conversion rate of photovoltaic cells. Commonly used concentrators include parabolic troughs, CPC concentrators, fluorescent concentrators, Fresnel lenses, holographic concentrators, and central receiving concentrators. They can all improve the conversion rate of photovoltaic cells to varying degrees.

According to the relationship between the photovoltaic system and the grid, it can be divided into independent photovoltaic power generation systems and grid-connected photovoltaic power generation systems. Independent photovoltaic power generation system means that it is not connected to the grid and uses 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, it is supplemented by the storage battery, and when the output power exceeds the load requirement, the electric energy is stored in the storage battery. The grid-connected photovoltaic power generation system connects the solar cell control system and the civil power grid in parallel. When the solar cell’s output power cannot meet the load requirement, the grid will supplement it; and when the output power exceeds the load requirement, the power will be delivered to the grid. In the grid-connected photovoltaic power generation system, there are also AC grid-connected devices and electric energy metering devices.

The composition of an independent photovoltaic power generation system is related to the load. Both DC and AC loads include photovoltaic cells, battery packs, and control circuits. If the load of an independent system is a DC load without an inverter circuit, it can be directly connected to the battery, and the output voltage of the battery is boosted (decreased) and then provided to the load. This type of system has a simple structure and low cost. Due to the different load DC voltages (such as 12, 24V, etc.), it is difficult to achieve system standardization and compatibility, especially for domestic electricity, the load is mainly AC, and the DC system is also difficult to achieve grid-connected operation. Therefore, AC photovoltaic inverter power supply is gradually replacing DC photovoltaic power supply. The main difference between an AC photovoltaic inverter power system and a DC photovoltaic power system is that an inverter is added between the load and the battery, and the inverter assumes the function of converting DC voltage into AC voltage. The photovoltaic array is installed outdoors to receive solar energy, and the battery is charged through the charge controller. The inverter circuit converts the direct current into the three-phase or single-phase alternating current required by the load.