Compressed air energy storage
![A pressurized air tank used to start a diesel generator set in Paris Metro](/Images/godic/202501/05/Bouteille_air_démarrage4322.JPG")
![A compressed air locomotive by H. K. Porter, Inc., in use at the Homestake Mine between 1928 and 1961.](/Images/godic/202501/05/Compressed_Air_Loco4322.jpg")
![Schematic views of a near isothermal compressor and expander. Left view with piston fully retracted right view with piston fully inserted.](/Images/godic/202501/05/Crowley's_isothermal_compressor_&_expander4322.jpg")
Compressed air energy storage (CAES) is a way to store energy generated at one time for use at another time using compressed air. At utility scale, energy generated during periods of low energy demand (off-peak) can be released to meet higher demand (peak load) periods. Small scale systems have long been used in such applications as propulsion of mine locomotives. Large scale applications must conserve the heat energy associated with compressing air; dissipating heat lowers the energy efficiency of the storage system.