Ⅰ. Types of energy storage systems
Existing energy storage systems are mainly divided into five categories: mechanical energy storage, electrical energy storage, electrochemical energy storage, thermal energy storage and chemical energy storage.
At present, the world’s highest proportion is pumped hydro storage, with a total installed capacity of 127GW, accounting for 99% of the total energy storage capacity, followed by compressed air energy storage, with a total installed capacity of 440MW, and sodium-sulfur batteries ranking third with a total capacity of 316MW.
Ⅱ. Comparison of various energy storage systems
1. Mechanical energy storage
Mechanical energy storage mainly includes pumped water storage, compressed air energy storage and flywheel energy storage.
Pumped-storage: when the power grid is low, the water used as a liquid energy medium is pumped from the low-lying reservoir to the high-lying reservoir. When the power grid peaks, the water in the high-lying reservoir flows back to the lower reservoir to drive the turbine to generate electricity. The efficiency is generally as follows: About 75%, commonly known as in 4 out 3, the pumped water energy storage system has daily adjustment capability for peak regulation and backup.
Compressed air energy storage (CAES): Compressed air energy storage is the use of the remaining electricity when the load of the power system is low, and the air compressor is driven by the motor to press the air into the airtight large-capacity underground cave as the air storage chamber.
When the power generation of the system is insufficient, the compressed air is mixed with oil or natural gas through the heat exchanger, and then introduced into the gas turbine to generate power.
The compressed air energy storage system also has a peak shaving function, which is suitable for large-scale wind farms, because the mechanical work generated by wind energy storage can directly drive the compressor to rotate, reducing the intermediate conversion into electricity, thereby improving efficiency.
Flywheel energy storage: the use of a high-speed rotating flywheel to store energy in the form of kinetic energy. When energy is needed, the flywheel slows down to release the stored energy.
2. Electrical energy storage system
Supercapacitor energy storage: The supercapacity is obtained with an electric double layer structure composed of an activated carbon porous electrode and an electrolyte. Unlike batteries, which utilize chemical reactions, the charging and discharging process of supercapacitors is always a physical process. Short charging time, long service life, good temperature characteristics, energy saving and green environmental protection.
Superconducting energy storage (SMES): A device that stores electrical energy using the zero-resistance property of superconductors. The superconducting energy storage system roughly includes four major parts: superconducting coil, low temperature system, power regulation system and monitoring system.
The development of superconducting material technology is the top priority of superconducting energy storage technology. Superconducting materials can be roughly divided into low temperature superconducting materials, high temperature superconducting materials and room temperature superconducting materials.
3. Electrochemical energy storage system
Lead-acid battery: It is a battery whose electrodes are mainly made of lead and its oxides, and the electrolyte is sulfuric acid solution. At present, it is widely used in the world, the cycle life can reach about 1000 times, the efficiency can reach 80%-90%, and the cost performance is high.
Lithium-ion battery system: It is a type of battery energy storage system that uses lithium metal or lithium alloy as the negative electrode material and uses a non-aqueous electrolyte solution.
Mainly used in portable mobile devices, its efficiency can reach more than 95%, the discharge time can reach several hours, the cycle number can reach 5000 times or more, and the response is fast. It is the practical battery with the highest energy in the battery.
Sodium-sulfur battery: It is a secondary battery with metal sodium as the negative electrode, sulfur as the positive electrode, and ceramic tube as the electrolyte diaphragm. At present, more than 200 power stations with such energy storage systems have been built in Japan, Germany, France, the United States and other places, which are mainly used for load leveling, peak shifting and improving power quality.
Flow battery: a high-performance battery that uses positive and negative electrolytes to separate and circulate separately. The power and energy of the battery are not related, the energy stored depends on the size of the storage tank.
As a result, energy can be stored for hours to days, and the capacity can reach the MW level. This hybrid energy storage system has multiple systems, such as iron-chromium system, zinc-bromine system, sodium-bromine polysulfide system and all-vanadium system, among which vanadium batteries are the most popular.
4. Thermal energy storage
Thermal energy storage: In a thermal energy storage system, thermal energy is stored in the medium of an insulated container and converted back to electrical energy when needed, or can be directly used without being converted back to electrical energy. Thermal energy storage is divided into sensible heat energy storage and latent heat energy storage. Thermal energy storage can store a lot of heat, so it can be used to generate electricity from renewable energy sources.
5. Chemical energy storage system
Chemical energy storage: use hydrogen or synthetic natural gas as the carrier of secondary energy, use excess electricity to produce hydrogen, either directly use hydrogen as the energy carrier, or react with carbon dioxide to become synthetic natural gas (methane), hydrogen or synthetic In addition to being used for power generation, natural gas can also be used in other ways such as transportation.