Abstract: A method of charging and discharging a heat accumulator is disclosed, when a heat exchange medium is passed through a heat accumulator medium. The heat exchange medium can either be heated or cooled external to the heat accumulator medium. If it is heated, it gives up its heat to the accumulator medium, and in doing so changes from the gaseous to the liquid state. If it is cooled, it absorbs heat from the accumulator medium, and in doing so changes from the liquid to the gaseous state. In both cases, the opposite change of state is brought about by the heating or cooling external to the heat accumulator medium.