Abstract: An energy storage device for a power plant includes a heat exchanger arranged in a floating manner in a lower basin that is fillable with water via a first supply line. A second supply line supplies water from the lower basin. A third supply line is in fluid communication with the heat exchanger. A heat pump provides coolant to the heat exchanger via the third supply line such that energy is extracted via the heat exchanger while freezing of the water in the lower basin or in the form of sensible heat from the water in the lower basin, wherein the energy is passed on to a consumer for heat dissipation or for cold dissipation.