Solar photovoltaic power generation is one of the important components of China's energy and electricity sustainable development strategy. Due to the strong fluctuation and randomness of photovoltaic power output, the instability of photovoltaic power severely restricts the access and transmission of photovoltaic power. Energy storage technology can realize peak load cutting, load tracking, frequency and voltage regulation, power quality control and other functions. Photovoltaic energy storage system can also be encountered in photovoltaic power station to abandon limit when generating excess electricity into light energy storage batteries, photovoltaic power generation is lower than the peak limiting values or night through storage inside the battery into electricity grid inverter, peak energy storage systems in power grid peel and energy storage system still can use the peak valley price difference to create greater economic benefits, improve the regulating ability of system itself; As an effective support technology to solve large-scale renewable energy generation access to the power grid.
The main modes of the energy storage system include 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, etc.
1. Energy storage system configured on the DC side of power supply
The energy storage system configured on the DC side of the power supply can mainly be installed in the DC system such as photovoltaic power generation. In this design, the battery combination photovoltaic power generation array can be connected and regulated in the DC section of the inverter, as shown in Figure 1. The photovoltaic power generation system and the battery energy storage system in the system share an inverter, but due to the large difference between the charging and discharging characteristics of the battery and the output characteristics of the photovoltaic power array, the maximum power tracking system (MPPT) in the photovoltaic grid-connected inverter in the original system is specially designed to match the photovoltaic output characteristics. The output characteristic curve of the storage battery cannot be simultaneously satisfied. Therefore, such systems need to transform or redesign the original system inverter, not only need to make the inverter can meet the requirements of photovoltaic array inverter, but also need to increase the battery charge and discharge controller, and battery energy management and other functions. Generally speaking, the system is one-way output, that is to say, the battery in the system is completely dependent on photovoltaic power generation to charge, the power grid is not able to charge the battery.
Figure 1: Energy storage system configured on the DC side of the power supply
Electricity from photovoltaic array in the system automatic dc inverter in front and battery balance, the main characteristic of this model is system of high efficiency and power output can be made of photovoltaic power station internal operation, can achieve a seamless connection, the output power quality is good, the output fluctuation is very small, etc., can greatly improve the photovoltaic output smooth, stability and control performance, The disadvantage is that the inverter used needs special design and is not suitable for upgrading the existing photovoltaic power stations that have been installed. Another disadvantage is that the batteries in the system can only receive power from the same unit to charge them, not the excess power from other nearby PHOTOVOLTAIC units or power stations. In other words, it lacks the internal power distribution function of a large power station.
2. Energy storage system configured on the AC side of the power supply
Configuration is also called the power ac side of the energy storage system configuration 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 discharge controller and inverter to battery charging and inverter, this scheme is actually to the existing photovoltaic power generation system plugins an energy storage device, It can be upgraded and installed in any kind of photovoltaic power station or even wind power station or other power station to form the in-station energy storage system. It can also be built into a completely independent energy storage power station according to the needs of the power grid.
This model overcomes the dc energy storage system can't be extra electricity unified scheduling problem, the system charge or discharge completely controlled by the intelligent control system or controlled by the power grid scheduling, it not only can focus within the total surplus electricity to rapid and efficient energy storage system of charging, even cheap low excess power grid scheduling stand outside, Make the system run more convenient and effective.
Figure 2: Side energy storage system configured at ac low voltage
Another mode of the energy storage system connected to the AC side is to connect the energy storage system to the power grid, 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 power station mode and directly connects the energy storage system to the high-voltage power grid.
Ac side access scheme is not only suitable for power grid energy storage, but also widely used in relatively isolated areas such as islands, forming relatively independent micro-grid power supply system. The ac side access energy storage system can not only be implemented in new power stations, but also can be easily transformed and additional construction for existing power stations. Moreover, the circuit structure is clear, and the power field and energy storage field can be constructed separately, with little direct correlation with each other, so it is also convenient for operation control and maintenance. The disadvantage is that since power generation and energy storage are independent from each other, the coordination and control between them requires a special intelligent control and scheduling system, so the cost is relatively high.
Figure 3: Energy storage system on high voltage side of AC power supply
3. Equipped with load side energy storage system
The load side energy storage system mainly refers to emergency power supply and mobile electric equipment, such as rechargeable electric vehicles, power tools and mobile phones.
This article simply discuss the technology of energy storage power station, although energy storage power station has many advantages, implementation and application in some special occasions but due to the battery of high efficiency, environmental protection, long life and low price key problems such as no big breakthrough, in the current large-scale promotion of energy storage power station and possibly feed-in tariffs, subsidies, and other issues.
