Abstract: A transportable power system, suitable, for example, for powering a vehicle or a transportable machine tool, comprises a storage container for receiving and storing, in liquid form, a working fluid which is nevertheless gaseous at standard temperature and pressure and of non-toxic and/or non-inflammable nature, and self-contained heat supply means together with pump means by which the liquid working fluid is pumped through heat exchange means to convert it to its gaseous form and thence through the heat supply means, which is conveniently a thermal storage unit, wherein the temperature of the gas is raised preparatory to being expanded through means for converting the heat energy of the working fluid into mechanical work. The heat supply means should have a thermal capability sufficient to cause the temperature of the gas to rise to at least 200.degree. C above its critical temperature.