Abstract: An energy accumulator for storing large quantities of heat in the form of superheated water includes an excavation which is divided into upper and lower compartments by a horizontal diaphragm. The lower compartment is filled with superheated water from a heat source such as a nuclear or other steam generating facility. The water in the lower compartment is maintained in its superheated condition by a heavy mass of water in the upper compartment which applies its weight and pressure through the diaphragm to the superheated water in the lower compartment. Means are provided for drawing the superheated water from the lower compartment to a heat exchanger in which the heat is transmitted to a fluid heat carrying medium which can be pumped to a location where the heat is to be used, such as for urban heating. The circulatory system associated with the superheated water is closed and the mass of water employed in the system is constant.