The technology of storing energy by compressing air and keeping it in a suitable reservoir. Surplus electrical energy is used to compress the air, which is stored in an underground cavern or in a special container. Old mines are often used as suitable spaces. When needed, the air is released on an air turbine to produce electricity. The air heats up strongly during compression. This heat can be dissipated during compression and used for another purposes. This, however, reduces the amount of energy stored. The other option is to store also the heat in thermal storage and drive the turbine with hot air, which will increase the efficiency of electricity generation.