Abstract: Disclosed is a thermal storage tank for the long term storage of a thermal storage medium whose energy is available on demand for satisfying cooling or heating loads. The tank may be earth excavated and include side walls of a lightweight, high strength concrete that retains its thermal insulating characteristics even when exposed to groundwater. A unique inverted frustrum shape for the tank offers numerous advantages. In association with a cold storage tank, a novel submerged, single point ice introduction system provides a much improved formation of the ice mass in the tank. Also provided for the cold storage tank is a novel floating top that provides a ballast effect to keep the ice mass submerged.