Abstract: An energy storage device for temporarily storing thermal energy includes a closed storage circuit, to which heat can be supplied by a heat source and from which heat can be withdrawn by a heat consumer. A fluid container is divided into a first fluid storage chamber for colder fluid and a second fluid storage chamber for warmer fluid by a displaceable separating element. At least one pump for conveying fluid from the first fluid storage chamber into the second fluid storage chamber, and/or vice versa, is provided in the storage circuit. A power plant including the energy storage device and a method for operating the energy storage device is also provided.