A CAES system is comprised of a compressor unit with a motor, gas turbine and storage facility. Usually at night the motor compresses and stores air which is cooled by a cooler unit. Then, during peak demand hours the compressed air is supplied to the combustion chamber in the turbines to burn the fuel. Combustive gas is expanded through the turbine driving the generator to produce electrical power.
Three types of CAES exist: Diabatic CAES, Adiabatic CAES and SustainX‘s Isothermal CAES, but only diabatic plants are in operation. Diabatic CAES is the only one that uses fuel in the form of natural gas and is a mature technology with an efficiency of between 40% and 70%. Adiabatic and isothermal CAES are still under development and have no fuel input. These two CAES technologies have an efficiency at the higher end of the CAES spectrum at 70% for adiabatic and 70% to 90% for isothermal.
Diabatic CAES stores compressed air in salt caverns, depleted gas fields or aquifer areas. Gas turbines are used to heat the compressed air, when energy is needed. Air storage chambers are created by injected large amounts of water into rock salt deposits, dissolving the salt and creating a large chamber in a process that takes one and a half to two years to complete. This technology is used in the two operating commercial projects, creating a large amount of brine that needs to be disposed of. A large amount of heat energy is lost in the process.
Adiabatic CAES stores both the compressed air and heat from the compression process, but stores them separately. Heat energy is stored in a solid such as concrete or stone or in a fluid such as hot oil (up to 300 °C) or molten salt solutions (600 °C). Then heat energy is reused when energy is needed to expand the gas without the need for gas turbines.
Isothermal CAES technology attempt to maintain a operating temperature by constant heat exchange to the environment. SustainX‘s system uses electrical energy to compress air near-isothermally (i.e. at nearly a constant temperature) and then stores this high-pressure air above ground in pressure vessels and then expands the gas near-isothermically.