Distributed power sources are intermittent and fluctuating power sources, and their power generation has obvious randomness and uncertainty, and it is easy to have an impact on the operation of the power grid and even cause power accidents. In order to make full use of renewable energy and ensure the reliability of its power supply, it is very important to equip energy storage equipment reasonably and effectively. How to choose a suitable energy storage method, the main consideration is what role the energy storage equipment needs to play in the distributed power generation system. After meeting the technical and functional requirements, and then considering the economic performance requirements, the following mainly discusses the choice of energy storage methods in the grid-connected distributed power generation system:
When distributed power sources are connected to the grid, to smooth system disturbances, maintain dynamic balance between power generation and electricity consumption, and maintain system voltage and frequency stability, the energy storage equipment in the system is required to have millisecond-level response speeds and corresponding capacity power compensation capabilities.
Lead-acid batteries have mature technology, high reliability, and low material prices. At present, they have been widely used in distributed power generation systems. However, lead-acid batteries have limited charging time and limited power, and the charging mode is generally limited to constant current charging. Take the wind power grid-connected operation system as an example. Due to the large volume of lead-acid batteries and frequent charging and discharging, frequent maintenance is required, which increases the cost of the system.
The energy storage density of sodium-sulfur battery is as high as 140kWh/m3, long cycle life, high charging and discharging efficiency (>90%), no memory effect, maintenance-free for long-term use, nearly 2/3 of the installed capacity can be used to smooth the load, and the technology is relatively mature. The all-vanadium redox flow battery can discharge 100% deeply, and its energy storage life is very long, and the battery capacity can be easily increased by increasing the amount of electrolyte. With the increasing maturity of flow battery technology, all vanadium flow batteries can greatly improve the efficiency and stability of new energy power generation.
The pumped storage unit has a capacity of 2000MW. Although its efficiency is not very high compared to some batteries, it is reliable and stable in operation, large in scale, and long in life. Its biggest disadvantage is that the fixed cost for distributed power generation systems is too high.
The energy density of the flywheel energy storage system is large, up to 108J/m3, the charging efficiency is high, the number of charging and discharging is unlimited, the occupied space is relatively small, the operating temperature is -40~+50℃, and the maintenance is simple. Some countries have integrated flywheel energy storage technology into wind power generation systems. Tests have shown that the introduction of flywheel energy storage equipment can greatly improve the power output performance of distributed wind power generation systems and increase economic benefits.
The biggest advantage of compressed air energy storage technology used in distributed power generation systems is the low investment cost and the good economic benefits of power generation. Combining compressed air energy storage equipment with gas turbines, the capacity can reach hundreds of megawatts, the efficiency is about 60%, and the service life is long. It can be used for cold start and black start. At present, the 8~12MW miniature compressed air energy storage system has been a hotspot in the research of distributed grid-connected energy storage technology, and its application prospects are very broad.
The power density of supercapacitors is very high, equivalent to 5~10 times that of ordinary batteries. It is sensitive, has a response time of less than 1s, is completely discharged, has no memory effect, and is simple to maintain. The operating temperature is -40~+70℃, and the maintenance is simple. Especially for small independent distributed photovoltaic power generation systems and fuel cell power generation systems, it is a very ideal energy storage device. Moreover, the technical characteristics of the super capacitor and the battery energy storage device can complement each other. The high power density of the super capacitor can smooth the system power fluctuations well, and it only needs to store energy equivalent to the peak load of the system. The battery can effectively store the energy at base load. The combination of the two can make the energy storage equipment have good load adaptability, effectively reduce the volume of the device, and improve the reliability and stability of the distributed power supply.
The superconducting magnetic energy storage system (SMES) has large-capacity power compensation characteristics, but the superconducting magnetic coil technology with a capacity higher than 100MWh has certain difficulties. Compared with the response time of compressed air and pumped storage energy storage systems in minutes, although SMES has a response speed of milliseconds and its power transmission efficiency advantage is very obvious, the system investment and operating costs are higher. The superconducting magnetic energy storage system has the characteristics of fast response speed and sensitive power transmission, so that it can quickly and effectively absorb or emit power when the power supply fails or the intermittent power supply is unstable in the distributed electric power generation system, which greatly improves the power supply stability and safety of the distributed power generation system.