Solar photovoltaic power generation is one of the important components to realize the sustainable development strategy of energy and electric power in our country.

Due to the strong volatility and randomness of photovoltaic output power, the instability of photovoltaic power seriously restricts the access and transmission of photovoltaic power. Energy storage technology can realize functions such as peak shaving and valley filling, load tracking, frequency regulation and voltage regulation, and power quality management. The photovoltaic energy storage system can also store excess electric energy in the energy storage battery when the photovoltaic power station encounters light abandonment and limit power generation. The energy storage system participates in the peak-shaving and valley-filling of the power grid, and the energy storage system can also use the peak-valley price difference to create greater economic benefits and improve the system’s own adjustment capabilities; as a solution to large-scale renewable energy generation connected to the grid An effective support technology.

The main modes of the energy storage system are the energy storage system configured on the DC side of the power supply, the energy storage system configured on the AC side of the power supply, and the energy storage system configured on the load side.

Photovoltaic energy storage

1. Energy storage system configured on the DC side of thepower supply

The energy storage system configured on the DC side of the power supply can be mainly installed in a DC system such as photovoltaic power generation. This design allows the battery to be combined with a photovoltaic power generation array to be matched and regulated in the DC section of the inverter, as shown in Figure 1.

The photovoltaic power generation system and battery energy storage system in this system share an inverter, but due to the large difference between the charge and discharge characteristics of the battery and the output characteristics of the photovoltaic power generation array, the maximum power of the photovoltaic grid-connected inverter in the original system The tracking system (MPPT) is specially designed to match the output characteristics of photovoltaics, and cannot meet the output characteristic curve of the energy storage battery at the same time.

Therefore, this type of system needs to modify or redesign and manufacture the inverter of the original system, not only to make the inverter meet the inverter requirements of the photovoltaic array, but also to add a charge and discharge controller for the battery pack, and battery energy management and other functions. Generally speaking, the system is one-way output, which means that the battery in the system is completely charged by photovoltaic power generation, and the power of the grid cannot charge the battery.

The power generated by the photovoltaic power generation array of the system is automatically DC balanced with the battery at the front end of the inverter. The main feature of this mode is that the system has high efficiency. Good quality, very small output fluctuation, etc., can greatly improve the smoothness, stability and controllability of photovoltaic power generation output. The disadvantage is that the inverter used needs a special design, which is not suitable for most of the existing photovoltaic power plants that have been installed. Do upgrades. Another disadvantage is that the battery pack in the energy storage system can only be charged by the power of the power generation unit, but cannot be charged by the excess power of other adjacent photovoltaic power generation units or power stations. That is to say, this solution lacks the function of power deployment within the large power station.

2. Energy storage system configured on the AC side of the power supply

The energy storage system configured on the AC side of the power supply can also be called an energy storage system configured on the AC side. The energy storage mode of the unit type AC side is shown in Figure 2, which uses a separate charge and discharge controller and inverter To charge or invert the battery, this solution is actually to add an energy storage device to the existing photovoltaic power generation system, which can be upgraded and installed in any current photovoltaic power station or even wind power station or other power stations to form energy storage in the station The system can also be built into a completely independent energy storage power station according to the needs of the grid.

This mode overcomes the problem that the DC-side energy storage system cannot perform unified dispatch of excess power. Its charging or discharging of the system is completely controlled by the intelligent control system or the grid dispatching control. It can not only concentrate the excess power in the whole station to the energy storage The fast and effective charging of the system can even dispatch the cheap low-peak surplus power of the power grid outside the station, making the system operation more convenient and effective.

Another mode of the energy storage system connected to the AC side is to connect the energy storage system to the grid side, as shown in Figure 3. Obviously, the difference between these two energy storage systems is only the access point. The former connects the energy storage part to the AC low-voltage side and shares a transformer with the original photovoltaic power station, while the latter forms an independent energy storage system. Power station mode, directly connected to the high-voltage power grid.

The AC side access scheme is not only suitable for grid energy storage, but also widely used in relatively isolated areas such as islands to form a relatively independent micro-grid power supply system. The energy storage system connected to the AC side can not only be implemented in new power stations, but also can be easily modified and additionally constructed for existing power stations. The circuit structure is clear, and the power generation field and energy storage field can be constructed separately. There are few direct correlations, so it is also convenient for operation control and maintenance. The disadvantage is that since power generation and energy storage are independent of each other, the coordination and control between them requires an additional set of special intelligent control and dispatching system, so the cost is relatively high.

3. Configure the energy storage system on the load side

The energy storage system configured on the load side mainly refers to emergency power sources and mobile electric equipment, such as rechargeable electric vehicles, electric tools and mobile phones.

This article only discusses the technical issues of energy storage power stations. Although energy storage power stations have many advantages and can be implemented and applied in some special occasions, there are no major breakthroughs in key issues such as high efficiency, environmental protection, long life and low price of batteries. In the current large-scale promotion of energy storage power stations, there may be issues such as on-grid electricity prices and subsidy policies.