disclosed is a device of a water heat accumulator or storage medium that is composed of water and at least one solid construction material which has a structure with hollow spaces and is specifically designed to absorb the entire amount of water and autonomously retain as much thereof as possible , has a statically load - bearing structure and can thus accommodate a superstructure , and can have water - permeable and / or hygroscopic properties . all additional equipment required for operating and utilizing the heat accumulator is assigned to said water heat accumulator or storage medium . the inventive device is characterized in that the construction material is a foam product or a construction material which is composed of several identical and / or different solid construction materials that are fixedly interconnected .