Thermal battery

A thermal energy battery is a physical structure used for the purpose of storing and releasing thermal energy—see also thermal energy storage. Such a thermal battery (a.k.a. TBat) allows energy available at one time to be temporarily stored and then released at another time. The basic principles involved in a thermal battery occur at the atomic level of matter, with energy being added to or taken from either a solid mass or a liquid volume which causes the substance's temperature to change. Some thermal batteries also involve causing a substance to transition thermally through a phase transition which causes even more energy to be stored and released due to the delta enthalpy of fusion or delta enthalpy of vaporization. Thermal batteries are very common, and include such familiar items as a hot water bottle. Early examples of thermal batteries include stone and mud cook stoves, rocks placed in fires, and kilns. While stoves and kilns are ovens, they are also thermal storage systems that depend on heat being retained for an extended period of time. Thermal batteries generally fall into 4 categories with different forms and applications, although fundamentally all are for the storage and retrieval of thermal energy. They also differ in method and density of heat storage. Phase change materials used for thermal storage are capable of storing and releasing significant thermal capacity at the temperature that they change phase. These materials are chosen based on specific applications because there is a wide range of temperatures that may be useful in different applications and a wide range of materials that change phase at different temperatures. These materials include salts and waxes that are specifically engineered for the applications they serve. In addition to manufactured materials, water is a phase change material. The latent heat of water is 334 joules/gram. The phase change of water occurs at 0 °C (32 °F). Some applications use the thermal capacity of water or ice as cold storage; others use it as heat storage.