Solar photovoltaic power generation is one of the important components to achieve China's energy and power sustainable development strategy. Due to the strong volatility and randomness of PV output power, the instability of PV power seriously restricts the access and transmission of PV power. Energy storage technology can achieve peak-shaving, load tracking, frequency regulation and voltage regulation, power quality management, and other functions. The PV energy storage system can also store the excess power in the storage battery when the PV power plant encounters light abandonment to limit power generation, and send the power in the battery to the grid through the storage inverter when the PV power generation is lower than the limit value or when the power consumption peaks at night, the energy storage system is involved in peak and valley reduction of the grid, and the energy storage system can also use the peak and valley tariff difference to create greater economic benefits and improve the regulation ability of the system itself; as a solution for large-scale renewable energy power generation to connect to the grid As an effective support technology to solve the problem of large-scale renewable energy generation access to the grid.
The main modes of the energy storage system are energy storage system configured on the DC side of the power supply, energy storage system configured on the AC side of the power supply, and energy storage system configured on the load side.
1、Energy storage system configured on the DC side of the power supply
The energy storage system configured on the DC side of the power supply can be installed in DC systems such as photovoltaic power generation, and this design allows the battery combination photovoltaic power generation array to be mated and regulated in the DC section of the inverter, as in Figure 1. The maximum power tracking system (MPPT) in the PV grid-connected inverter is specially designed to match the PV output characteristics, and cannot meet the output characteristic curve of the storage battery at the same time. Therefore, such systems need to modify or redesign the inverter of the original system, not only to make the inverter meet the inverter requirements of the PV array, but also to add the charge and discharge controller for the battery bank, and battery energy management and other functions. Generally speaking, the system is unidirectional output, which means that the battery in the system is completely dependent on PV power charging, and the power from the grid cannot charge the battery.
The main features of this model are high system efficiency, power output from the power plant that can be dispatched from within the PV plant, the seamless connection can be achieved, good output power quality, very small output fluctuations, etc., which can greatly improve the smooth, stable and adjustable performance of PV power output, the disadvantage is that the inverter used needs to be Special design, not applicable to the existing installed most of the PV power plant to upgrade. Another disadvantage is that the battery bank in this energy storage system can only accept the power from this power generation unit to charge it, while the excess power from other nearby PV power generation units or power stations cannot charge it. In other words, this solution lacks the function of power deployment within a large power station.
2, configured in the power supply AC side of the energy storage system
Configured in the power supply AC side of the energy storage system can also be called configured in the AC side of the energy storage system, unit type AC side of the energy storage model shown in Figure 2, it uses a separate charge and discharges controller and inverter to charge the battery or inverter, this program is actually to the existing photovoltaic power generation system outside of an energy storage device, can be installed in any kind of photovoltaic power station or even wind power station or other power stations to upgrade, the formation of station energy storage system can also be built according to the needs of the grid to become a completely independent operation of the energy storage power station.
This model overcomes the problem that DC-side energy storage system cannot be dispatched with excess power, its system charging or discharging is completely controlled by the intelligent control system or by grid dispatch control, it can not only concentrate the excess power in the whole station to charge the energy storage system quickly and effectively but even dispatch the cheap low valley excess power from the grid outside the station, which makes the system operation more convenient and effective.
Another mode of AC side access energy storage system is to connect the energy storage system to the grid side, as in Figure 3. obviously, the difference between these two types of energy storage systems is only the access point, the former is to connect the energy storage part to the AC low voltage side, sharing a transformer with the original PV power station, while the latter is to form an independent energy storage system mode, directly connected to the high voltage grid.
The AC side access solution is not only applicable to grid energy storage but also widely used in relatively isolated areas such as islands to form a relatively independent mini-grid power supply system. The AC side access energy storage system can be implemented not only on new power stations, but also on already built power stations, which can be easily renovated and additionally constructed, and the circuit structure is clear, and the power generation and storage fields can be built in separate locations with less direct correlation to each other, thus also facilitating 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 a special intelligent control and scheduling system, so the cost is relatively high.
3、Load-side energy storage system
Configured in the load-side energy storage system mainly refers to the emergency power and movable electric equipment, such as rechargeable electric vehicles, electric tools, and cell phones.
This paper only discusses the technical aspects of the energy storage power plant, although the energy storage power plant has many advantages, can be implemented and applied on some special occasions, but due to the current battery efficiency, environmental protection, long life, and low prices and other key issues do not have a major breakthrough, in the current large-scale promotion of energy storage power plant may have feed-in tariffs, subsidy policies, and other issues.
