Photovoltaic power generation systems are divided into two types: off-grid photovoltaic power generation systems and grid-connected photovoltaic power generation systems. Among them, the grid-connected photovoltaic power generation system is further divided into countercurrent type, non-countercurrent type, switching type, direct, alternating current, mixed type and regional type.
1. Composition and function of off-grid power generation system
Off-grid power generation systems (also known as independent power generation systems) can be divided into three types: supplying power to DC loads, supplying power to AC loads, and with or without batteries. A complete off-grid power generation system is mainly composed of power generation components, controllers, and battery packs. If the load is an AC load, an inverter needs to be configured. The functions of the components of the off-grid power generation system are as follows.
(1) Power generation components
Power generation components include photovoltaic modules and wind turbines.
The controllers include photovoltaic controllers and wind-solar hybrid controllers, whose main task is to regulate and control the generated electrical energy. On the one hand, the adjusted energy is sent to the DC load or the AC load; on the other hand, the excess energy is sent to the battery pack for storage. When the generated electricity cannot meet the load needs, the controller sends the battery power to the load. After the battery is fully charged, the controller should control the battery not to be overcharged. When the electric energy stored in the battery is discharged, the controller shall control the battery not to be over-discharged to protect the battery. When the performance of the controller is not good, it has a great impact on the service life of the battery, and ultimately affects the reliability of the system.
(3) battery pack
The main task of the battery pack is to store energy, so as to ensure the load power consumption at night or in cloudy and rainy days.
The inverter is the core component of the off-grid power generation system, which is responsible for converting DC power into AC power for use by AC loads. Due to the difficulty of maintenance in relatively backward and remote areas, in order to improve the overall performance of the photovoltaic wind power generation system and ensure the long-term stable operation of the power station, high requirements are placed on the reliability of the inverter. In addition, due to the high cost of new energy power generation, the efficient operation of the inverter is also very important.
2. Grid-connected photovoltaic power generation system
According to reports, before 2010 in China, most solar cells were used in independent photovoltaic power generation systems. From 2011 to 2020, the mainstream of the photovoltaic power generation market will shift from independent power generation systems to grid-connected power generation systems. In the next 15 years, China will invest more than 200 billion yuan to make full use of the advantages of long sunshine hours in the western region and coastal areas to build solar heating systems, solar thermal application demonstration projects and large-scale solar photovoltaic power stations, and vigorously develop the solar energy industry. It is estimated that by 2015, China’s photovoltaic production will reach 1GWp, ranking among the world’s advanced levels, and will reach 35GWp in 2020. Therefore, the construction of grid-connected solar power generation systems is expected to rise in the market.
The grid-connected photovoltaic power generation system is a system composed of a grid-connected control inverter connected to the local power grid. It can also redistribute the power of the photovoltaic power generation system through the power grid for peak shaving power. Figure 1 shows the physical space structure of grid-connected power generation.
(1) Direct and AC grid-connected power generation system This system is to directly supply the DC power generated by the photovoltaic power generation system to the electrical equipment, as shown in Figure 3. The system is sometimes used in conjunction with the power system, the main purpose is to improve the reliability of the power supply.
(2) Hybrid grid-connected power generation system When the power provided by solar photovoltaic power generation is insufficient (such as continuous rainy weather, short sunshine time in winter, etc.) and other energy sources need to be used to supplement, wind power generation, fuel cell power generation, etc. Other power generation systems are used together with photovoltaic power generation systems, and such systems are called hybrid grid-connected power generation systems.
(3) The grid-connected power generation system of solar photovoltaic and fuel cells is to comprehensively utilize energy, improve the comprehensive utilization rate of energy, save electricity costs, and reduce environmental pollution. Battery grid-connected system.
(4) Wind and solar complementary grid-connected power generation system When the power provided by photovoltaic power generation is insufficient, wind power can be used; when wind power is insufficient, photovoltaic power can be used, such a system is called wind and solar complementary grid-connected The power generation system is shown in Figure 3. The wind and solar complementary grid-connected power generation system utilizes both solar energy and wind energy to generate electricity, so the utilization of meteorological resources is more sufficient. It can realize day and night power generation. Under suitable meteorological conditions, the wind-solar complementary grid-connected power generation system can improve the continuity and stability of the power supply of the system. Since the wind is usually strong when there is no sunlight at night, the complementarity is good, and the solar panel configuration of the system can be reduced, thereby greatly reducing the cost of the system. The initial investment and power generation cost of the system per unit capacity are lower than independent photovoltaic systems. When the system has surplus power generation, it can supply power to the grid system (selling electricity); when the power generated by the system is insufficient, it can supply power from the grid system (buying electricity).
(5) Counter-current solar grid-connected power generation system When the power generated by the solar photovoltaic system is sufficient, the remaining power can be sold to the power system, that is, to supply power to the power grid system; when the power provided by the solar photovoltaic system is insufficient, it can be purchased from the power grid. Electricity, that is, the use of external power system to supply power. Such a system is called a countercurrent grid-connected power generation system, as shown in Figure 4.
(6) Non-countercurrent grid-connected power generation system The solar photovoltaic power generation system will not supply power to the power system (sell electricity) even if the power generation is sufficient, but when the power supply of the solar photovoltaic power generation system is insufficient, it can buy electricity from the power system. Such a system is called a grid-connected power generation system without counterflow.
(7) Switching grid-connected power generation system The system can be divided into the following two types.
①Switching grid-connected power generation system When it is cloudy, rainy, lack of sunlight, night or insufficient battery capacity, the switcher can automatically supply power to the load directly from the power grid to the power system side. When designing, if a large-capacity battery is used, the investment cost is large; if a small-capacity battery is used, the cost can be significantly reduced.
② Self-running switching grid-connected system When the power system is suddenly cut off due to various reasons, the photovoltaic power generation system can automatically separate the power system from the photovoltaic power generation system through the protection device.