Energy storage technology is mainly divided into electricity storage and heat storage. At present, the energy storage methods are mainly divided into three types: mechanical energy storage, electromagnetic energy storage, and electrochemical energy storage.
Energy storage technology is mainly divided into physical storage (such as pumped storage, compressed air energy storage flywheel energy storage, etc.), chemical storage (such as lead-acid batteries, redox flow battery, sodium sulfur battery and lithium ion battery) and electromagnetic energy (such as superconducting magnetic energy storage, super capacitor energy storage three categories, etc.). According to the characteristics of energy storage technology, flywheel, superconducting magnetic energy storage and supercapacitor energy storage for the need to provide short larger pulse power applications, such as response to voltage sag and instantaneous power, improve the quality of the user's power, suppressing low frequency oscillation of power system and improve the system stability and pumping applications; applications of energy storage, compressed air energy storage and electrochemical battery storage for large system peaking and emergency power supply, renewable energy into large scale and large capacity etc..

At present, the most mature way of storing energy is pumped storage. It needs to build up two reservoirs in the upper and lower reaches. In the low load period, pumped storage equipment is in the working state of the motor, and the downstream reservoir's water is pumped to the upstream reservoir to store. At the peak load, the equipment is in the generator's working state, and the water is stored in the upstream reservoir. The energy conversion efficiency is about 70% to 75%. However, due to the factors such as high site selection, long construction period and slow response to dynamic regulation, the large-scale popularization and application of pumped storage technology is restricted to a certain extent. At present, the total installed capacity of the global pumped storage power station is 90 million KW, accounting for about 3% of the global power installed capacity.
Compressed air energy storage is another way of storing energy in large-scale industrial applications. By using this energy storage mode, surplus electric energy can be used to drive air compressors in the low load period of power network, and air high pressure is sealed in the caves, abandoned mines and expired oil and gas wells. The compressed air is released at the peak load of power grid to promote the power generation of the steam turbine. Because of its high efficiency, long life and fast response, and energy conversion efficiency is about 75%, compressed air energy storage is one of the potential energy storage technologies.
1 mechanical energy storage
Mechanical energy storage includes: pumped storage energy, compressed air energy storage, and flywheel energy storage.
1. Pumped storage energy
Pumped storage is to pump water from the lower reservoir to the upper reservoir in the low load period of electric power, and to convert electricity into gravitational potential energy, and to release the water power in the upper pool during the peak load period. The release time of pumped storage can range from several hours to several days, and the comprehensive efficiency is between 70% and 85%. It is mainly used for peak shaving, valley filling, frequency modulation, phase modulation, emergency standby and so on. The construction of pumped storage power station is restricted by terrain, and the transmission loss is larger when the power station is far away from the electric area.
2. Compressed air energy storage
The compressed air in the power grid load trough used for compressed air, the air pressure seal in the abandoned mines, the settlement of the undersea gas tank, caves, oil and gas expired or new gas wells, the release of compressed air in peak load power driven turbine. Compressed air is mainly used for power peak adjustment and system reserve. The construction of compressed air energy storage power station is restricted by terrain, and has special requirements for geological structure.
3. Flywheel energy storage
The flywheel energy storage uses the motor to drive the flywheel to rotate at a high speed. The electric energy is converted into mechanical energy to be stored. When needed, the flywheel drives the generator to generate electricity. The flywheel system operates in a high vacuum environment. Its characteristics are no friction loss, small wind resistance, long life, no impact on the environment, and almost no maintenance. It is suitable for power grid FM and power quality assurance. The disadvantage of flywheel energy storage is that the energy density is low. The cost of ensuring the security of the system is very high, and it can not show its advantages in small occasions. It is mainly used to supplement the battery system at present.

2 electromagnetic energy storage
Electromagnetic energy storage includes superconducting energy storage, capacitive energy storage, and supercapacitor energy storage.
1. Superconducting energy storage
Superconducting magnetic energy storage system (SMES) using coil made of superconductor magnetic energy storage, power transmission without energy conversion form, with fast response (MS), high conversion efficiency (more than 96%), specific capacity (1-10 Wh/kg) / power ratio (104-105kW/kg) and other advantages, can achieve real-time the capacity of energy exchange and power compensation in power system. SMES can fully meet the requirements of voltage support, power compensation, frequency regulation, stability and power transmission capacity of the transmission and distribution network.
2, supercapacitor energy storage
Super capacitor according to the electrochemical double layer theory developed, can provide a strong power pulse, charging at the electrode surface ideal polarization state, the charge will attract around the ions in the electrolyte solution, which is attached to the electrode surface, the double layer formed, composed of electric double layer capacitor. Electric load smooth and short time, high power quality can be used for occasions of peak power in the power system, such as high power DC motor start-up support, state voltage restorer, improve the level of power during the voltage sag and transient interference.

3 electrochemical energy
The electrochemical energy storage includes lead-acid battery, lithium ion battery, liquid current battery, sodium sulphur battery and so on. The liquid - flow battery has the potential to store energy in a large scale, but the most widely used battery is the lead-acid battery.
4 prospects for development
Energy storage is widely used in power systems, covering all aspects of power generation, transmission, distribution and end users. The energy storage technology of the power grid system includes pumped storage, compressed air, flywheel, chemical battery, supercapacitor and so on. In addition to the mature pumped storage energy, other energy storage technologies are still in the early stage of industrialization or research and development. However, governments have already realized the importance of the energy storage industry and are making great efforts to develop energy storage technology.
In order to create a clean and sustainable future, the Chinese government is shifting the policy center to clean energy technology. By the end of 2013, China power installed capacity of 1250 GW, which contains 91.4 gigawatts of wind power (accounted for 7.3%). In addition to thermal power and hydroelectric power, wind power is also the third largest source of power in China. Chinese photovoltaic installed capacity of 18.1 GW, accounting for 1.5% of the country, surpassing the United States as the world's largest PV market.

The demand for energy storage in China is expanding as a result of the rapid growth of power generating capacity. In 2013, the total installed capacity of China pumped 21.5 gigawatts of energy storage, other storage technology, the installed capacity of 65 mw. Last year, the power peaking load energy demand is 95 GW in 2014 is expected to increase to 110 GW, which reflects the tremendous prospects for development. In addition, the integration of wind power and photovoltaic power generation will create a huge demand for energy storage, 2014 respectively in 5.6 and 3.8 gigawatts gw.
There are various kinds of energy storage technology in power grid system. Taking China as an example, lithium battery, lead-acid battery and fluid technology are mainly used. In 2013, the three methods accounted for 60%, 20% and 14% respectively.
The importance of energy storage technology to the development of electric vehicles is more intuitionistic. Batteries are involved in electric vehicles charging, cruising mileage, and safety problems. For example, the security accidents caused by the battery have weakened the consumer's confidence and affected the development of electric vehicles. For China, the development of electric vehicles, in addition to the replacement of oil, can also solve the city's car exhaust and noise pollution.