Thermal energy storage system

A thermal energy storage system for absorption of heat from a heat source (1) having a temperature above a predetermined temperature and release of heat to a heat sink (3) having a temperature below the predetermined temperature. The system comprises a heat storage mass (14) comprising a condensed state material which changes between the solid and liquid phases at about the predetermined temperature and a compartment for containing the heat storage mass. The compartment (13) includes means providing an area for transfer of heat between a fluid and the material. The material may be an aliphatic diamine corresponding to the general formulawhere n is an integer between 4 and 14 inclusive, bis(hexamethylene)triamine, 3-aminobutylcyclohexylamine or p-phenylenediamine. Mixtures of these amines with each other may be used, particularly including certain eutectic mixtures having melting points near room temperature. Also useful are mixtures containing at least one of said amines and water or another agent for lowering the melting point of the amine.