Compressed air energy storage, or CAES for short, is a technology that uses compressed air to store energy. At present, compressed air energy storage technology is the second largest technology considered suitable for GW-level large-scale electric energy storage after pumped hydro energy storage. Its working principle is that during periods of low electricity consumption, electric energy is used to compress air to high pressure and stored in caves or pressure vessels, so that the electric energy is converted into the internal energy of the air and stored; during peak periods of electricity consumption, high-pressure air is discharged from the air storage chamber. It is released and burned in the combustion chamber to use fuel combustion to heat up and drive the turbine to generate electricity.

　　A complete compressed air system consists of five key equipment: compressor, cooler, pressure vessel, regenerator, turbine and generator. The functions of each component are as follows:

　　Compressor: compresses air and converts electrical energy into air internal energy. The air pressure can reach 70-100 bar and the temperature can reach 1000 ° C;

　　Cooler: heat exchange equipment used for cooling before storage in pressure vessels to prevent the pressure of air from reducing in pressure vessels or caves;

　　Pressure vessel: stores the cooled air. If cave storage is used, it needs to meet the geological conditions with high pressure resistance and good sealing;

　　Regenerator: heat exchange equipment or combustion chamber, which increases the air temperature to about 1000°C, allowing the turbine to operate stably for a long time to improve turbine efficiency;

　　Turbine: The air is depressurized by the turbine and the internal energy is converted into kinetic energy;

　　Generator: Mostly synchronous generators, which convert kinetic energy into electrical energy.

　　There are many problems in the compressed air system at present, the most important of which is that it is too restricted by geographical conditions like pumped hydropower storage. To build a compressed air system, special geographical conditions are required to serve as a large air storage chamber, such as highly airtight rocks. Caves, salt caves, abandoned mines, etc. This limitation is one of the important factors affecting the promotion of this technology. In addition, the system efficiency of traditional air compression systems is only 40%-55%, which is lower than the 80% efficiency of pumped hydro storage.

　　From its principle, it can be known that a large part of the energy stored in compressed air is converted into heat energy during the compressed air process and is not effectively utilized. This is an important reason for the low efficiency of this technology. In order to improve the efficiency of the compressed air system, the heat generated during the compression process can be stored in a heat storage device. This part of the heat can be used to preheat the compressed air during the power generation process to achieve the purpose of recovering heat. This improved technology is called Adiabatic compressed air energy storage system (AA-CAES). At present, there is no actual demonstration project of this system put into operation. The biggest challenge facing this system is how to ensure the constant heat storage of the heat storage device and how to achieve a more economical and reasonable system design.

　　Compressed air energy storage technical parameters

　　Compressed air energy storage technology advantages and disadvantages

　　As the demand for energy storage continues to grow, compressed air energy storage technology is becoming increasingly popular as the only large-scale energy storage technology with energy storage levels comparable to pumped hydro energy storage. Its advantages are as follows:

　　• Fast startup time (<15 minutes)

　　• High energy density and power density

　　• Black start capability

　　• Low daily operating costs

　　• Small underground storage space on the Earth’s surface

　　• Long equipment life and low wear and tear

　　• Compressed air has low self-discharge rate

　　• For adiabatic compressed air, its system efficiency is high (70-75%), and it does not require the use of traditional fossil energy to heat compressed air, making it truly carbon neutral.

　　Compressed air energy storage technology has frequency modulation (secondary and tertiary frequency modulation), voltage regulation, peak load regulation, load balancing, static reserve, and black start capabilities. The future application space is very broad, and the technology has good regional relevance and is widely used in my country. The Three Norths region has huge development potential and can also be used for offshore wind power energy storage (North Sea Salt Cave).

　　However, compressed air energy storage is also restricted by various factors, such as:

　　• High investment cost and long investment return (investment return > 25 years)

　　• To build the system, certain geological conditions must be met (pressure-sealed caverns), and the salt cavern has a high

　　• For adiabatic systems, the regenerator self-discharge rate is high

　　• For non-adiabatic systems the efficiency is relatively low (<55%)

　　There is also little experience with the technology, with only two (older) non-adiabatic compressed air energy storage projects currently operating; there are currently no demonstration projects for adiabatic systems in operation. Considering geographical factors, cave excavation will also have a certain impact on the environment, and the number of suitable locations for caves is limited. Generally, caves are used for natural gas or oil storage, which is more economical. Finally, the competition in decentralized storage systems is becoming increasingly fierce. With small-scale energy storage capacity, the competitiveness of this technology is temporarily insufficient to compete with other technologies.

　　Compressed air energy storage case

　　Huntorf is a power station put into commercial operation in Germany in 1978. It is still in operation and is the world's largest compressed air energy storage power station. The compressor power of the unit is 60MW, the energy release output power is 290MW, and the maximum rated output time is 2 hours. The system stores compressed air in an abandoned mine cave 600m underground. The total volume of the mine cave reaches 3.1×105m3, and the pressure of the compressed air can reach up to 10MPa. The unit can continuously inflate for 8 hours and generate electricity for 2 hours. The power station was connected to the grid more than 5,000 times from 1979 to 1991, with an average startup reliability of 97.6%. The power station adopts a natural gas supplementary combustion solution, and the actual operating efficiency is about 42%. The actual efficiency after deducting supplementary combustion is 19%.

　　The McIntosh compressed air energy storage power station in Alabama, USA, was put into commercial operation in 1991 and was the second commercial compressed air energy storage power station put into operation in the world. The compressor unit power of this system is 50MW and the power generation power is 110MW. The gas storage cave is 450m underground, with a total volume of 5.6×105m3 and a compressed air storage pressure of 7.5MPa. It can achieve continuous air compression for 41 hours and power generation for 26 hours. It takes about 9 minutes for the unit to reach full load from start-up. The power station is remotely controlled automatically by the Alabama Power Company's energy control center. Similar to Huntorf, natural gas supplementary combustion is still used, and the actual operating efficiency is about 54%, and the actual efficiency after deducting supplementary combustion is 20%.

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