Energy storage device and method for its operation

The device (2-5) has a transducer (9) which compresses air from output pressure to a storage pressure using energy and releases the air from reservoir pressure to outlet pressure. A liquid piston converter with a liquid piston compresses and expands the air. A pump pumps a piston liquid using the energy and a piston fluid drivable fluid motor drives the shaft. The heat transfer fluid heat is discharged from the piston converter, and heat transfer fluid heat is supplied in the converter, so that the heat transfer fluid from the piston liquid is fluidly separated. Independent claims are included for the following: (1) energy storage system; and (2) method for operating energy storage device.