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.
Existing energy storage systems around the world
1. Mechanical energy storage
Mechanical energy storage mainly includes pumped hydro energy storage, compressed air energy storage and flywheel energy storage.
(1) Pumped hydro storage: the water that uses excess power as a liquid energy medium when the power grid is low is pumped from the low-lying reservoir to the high-lying reservoir, and when the power grid peaks, the water in the high-lying reservoir flows back to the lower reservoir to drive the turbine generator. Power generation, the efficiency is generally about 75%, commonly known as in 4 out 3, with daily adjustment capacity, used for peak regulation and backup.
Disadvantages: Difficulty in site selection, and its dependence on terrain; large investment cycle and high losses, including pumping and storage loss + line loss; at this stage, it is also restricted by China’s electricity price policy. Last year, more than 80% of China’s pumping and storage were exposed to the sun Sun, in August last year, the National Development and Reform Commission issued a policy on the price of pumped storage electricity. It may be better in the future, but it is definitely not the development trend of energy storage.
(2) Compressed air energy storage (CAES): Compressed air energy storage is to use the residual power when the load of the power system is at a low valley. When the power generation is insufficient, the compressed air is mixed with oil or natural gas through a heat exchanger, and then introduced into a gas turbine for power generation. There are many foreign studies and the technology is mature, and my country started a little later. It seems that Academician Lu Qiang has done more research in this area, such as cogeneration of cold and power.
Compressed air storage also has a peak shaving function, which is suitable for large-scale wind farms, because the mechanical work generated by wind energy can directly drive the compressor to rotate, reducing the intermediate conversion into electricity, thereby improving efficiency.
Disadvantages: A big disadvantage is that it is less efficient. The reason is that the temperature of the air increases when it is compressed, and the temperature decreases when the air is released and expanded. Part of the energy is lost in the form of heat during the compression of the air, which must be reheated before expansion. Natural gas is usually used as the heat source for heating air, which leads to lower energy storage efficiency. There are also conceivable shortcomings: the need for large gas storage devices, certain geological conditions and dependence on burning fossil fuels.
(3) Flywheel energy storage: It uses 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. The single technology of flywheel energy storage is basically available in China (but the gap with foreign countries is more than 10 years). The difficulty lies in developing new products with different functions according to different purposes. Therefore, the flywheel energy storage power supply is a high-tech product but original innovation. It is not enough, which makes it difficult to obtain national scientific research funding support.
Disadvantages: The energy density is not high enough and the self-discharge rate is high. If charging is stopped, the energy will be exhausted by itself within a few to dozens of hours. Only suitable for some market segments, such as high-quality uninterruptible power supplies.
2. Electrical energy storage
(1) Energy storage of supercapacitors: 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. There is nothing too complicated about supercapacitors, which is the charging of the capacitor. The rest is the problem of materials. The current research direction is whether it can achieve a small area and a larger capacitance. The development of supercapacitors is still very fast. At present, new supercapacitors based on graphene materials are very popular.
Tesla CEO Elon Musk said as early as 2011 that batteries in traditional electric vehicles are obsolete and will be replaced by new cars powered by supercapacitors in the future.
Disadvantages: Compared with batteries, its energy density leads to relatively low energy storage under the same weight, which directly leads to poor battery life and depends on the birth of new materials, such as graphene.
(2) Superconducting energy storage (SMES): a device for storing electrical energy made with the zero resistance 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.
Disadvantages: The high cost of superconducting energy storage (materials and cryogenic refrigeration systems) limits its application. Reliability and economic constraints, commercial application is still far away.
3. Electrochemical energy storage
(1) 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.
Disadvantage: If deep, fast high-power discharge, the usable capacity will drop. It is characterized by low energy density and short lifespan. Lead-acid batteries have improved their cycle life a lot this year by adding super-active carbon materials to the negative plates of lead-acid batteries.
(2) Lithium-ion battery: It is a type of battery 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 number of cycles can reach 5000 times or more, and the response is fast. It is the practical battery with the highest energy in the battery. most used. In recent years, technology has been continuously upgraded, and positive and negative materials also have a variety of applications.
The mainstream power lithium batteries on the market are divided into three categories: lithium cobalt oxide batteries, lithium batteries and lithium iron phosphate batteries. The former has high energy density, but is slightly less safe, while the latter is the opposite. Domestic electric vehicles such as BYD currently mostly use lithium iron phosphate batteries. But it seems that foreigners are playing with ternary lithium batteries and lithium iron phosphate batteries
Lithium-sulfur batteries are also very popular. They use sulfur as the positive electrode and metal lithium as the negative electrode. The theoretical specific energy density can reach 2600wh/kg, and the actual energy density can reach 450wh/kg. However, how to greatly improve the charge-discharge cycle life and use safety of the battery is also a big problem.
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 into electrical energy when needed, or can be used directly without being converted back into electrical energy. Thermal energy storage is further 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.
Disadvantages: Thermal energy storage requires a variety of high-temperature chemical thermal working fluids, and the use occasions are relatively limited.
5. Chemical energy storage
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. Germany is keen to promote this technology and has demonstration projects in operation.