Characteristics and classification of photovoltaic power generation systems

This chapter introduces the characteristics and classification of photovoltaic power generation systems

The photovoltaic power generation system has no greenhouse gas emissions during the power generation process, so it is called “green” energy. It has the following main advantages:
① No need for fossil fuels, solar energy is spread all over the world, and it is inexhaustible;
②No moving or rotating parts, no pollution, no noise, and the life span can be more than 30 years;
③The construction period is short, even for large-scale photovoltaic power plants, from design to construction and installation can be completed in 3 to 6 months;
④) The operation and maintenance cost is very low, only about 1/10 of the conventional power station;
⑤ It can be adjusted to local conditions, set up nearby, and the application is very convenient;
⑥Modular structure, adjustable scale, as large as GW-level centralized photovoltaic power station, as small as distributed household rooftop photovoltaic system, and even solar electronic products.

The disadvantages of photovoltaic power generation systems are fluctuations and discontinuities, just like wind power and other renewable energy sources. There are three technical ways to deal with this kind of intermittent energy utilization:
①System configuration energy storage device
② Grid-connected power generation, “store” the generated energy on the grid for storage and use adjustment;
③Multi-energy complementary power generation, and construction of microgrid

The classification of photovoltaic power generation systems (PV systems) is usually divided into two categories: independent and grid-connected according to the relationship with the public grid. Among them, the independent type is divided into dedicated load and general load according to the load type, or divided into hybrid type, non-energy storage battery type, and battery combined type according to the system composition. Grid-connected systems are divided into two types: reversible flow and irreversible flow.

The so-called stand-alone photovoltaic power generation system refers to a closed system that has no relationship with the power system. It is usually used for the power supply of portable equipment, to supply power to areas or equipment far away from the existing power grid, and for any power supply occasions that do not want to contact the power grid.
(1) Photovoltaic power generation system with dedicated load
The photovoltaic power generation system with dedicated load may be constructed and designed only according to the requirements of its load. Therefore, the output power is DC or AC at any frequency, which is more suitable. This kind of system is technically feasible using frequency conversion speed regulation. For example, in the case of a motor load in a photovoltaic water pump system, starting from a variable frequency can suppress the inrush current and at the same time make the frequency converter miniaturized.
(2) Photovoltaic power generation system with general load
The photovoltaic power generation system with general load is a power supply system that targets unspecified loads within a certain range. As a load, it is usually an electrical product, which is more convenient for industrial operation. If it is a DC load, the inverter can be omitted. Of course, the actual situation may be both AC and DC loads. Generally, a battery energy storage device is required to store the electricity generated by the solar panel during the day in the battery for use at night or in rainy days. If the load is only for agricultural machinery, it is not necessary to install a photovoltaic power generation system for battery load. It can also be divided into an on-site load system and a distributed load system. The former is used as a power source for households or some equipment in remote areas, and it is a system that generates and uses electricity on site. The latter requires small-scale distribution lines to be able to supply power to loads far away from the location of the photovoltaic power station.
The grid-connected type is divided into two types: reversible flow system and irreversible flow system (see figure). In the PV system, if surplus power is generated, the reversible flow system adopts the system of purchasing the surplus power by the power company. Nowadays, almost all residential PV systems use reversible flow systems.

Grid-connected system
Grid-connected system

 Figure Grid-connected photovoltaic system

Irreversible flow system, the power demand in the area is usually greater than the output power of the PV system, so it is used without generating reverse flow power. In this type of photovoltaic system, it is impossible to confirm that its surplus power does not flow into the grid in the reverse direction. Therefore, even if a small current is generated, the system should have the function of automatically reducing the output power of the PV system or suspending the operation of the PⅤ system, that is, installing an anti-backflow device. .