Abstract: This invention concerns the release of stored heat energy to do useful work. In particular it concerns a method for the release of stored heat energy to do useful work, and in further aspects it concerns other methods, apparatus and systems. In a first aspect, the invention is a method for releasing stored heat energy to do useful work. The method comprising: Storing heat energy in a heat storage medium. Arranging a surface of the heat storage medium and a surface of a heat collecting element of a heat consuming engine or process, such that a surface of the heat collecting element is in proximity to, in contact with or embedded in a surface of the heat storage medium. Then, transferring stored heat energy from the surface of the heat storage medium to the surface of the heat collecting element.