Abstract: Installation for producing motive power, operating on a hybrid cycle of gas and vapor such as steam. The installation has a heat accumulator for storing a heat-exchange fluid which can also act as a fluid fuel for a heat source adapted to heat the fluid of the heat accumulator. Heat can be drawn from the accumulator in the form of a flow of hot fluid, for use as required by a motive power producing machine which may be a vapor turbine. Fluid can also be drawn from the accumulator, to act as a fuel for the heat source, supplementary fluid being supplied to the accumulator to make up for the amount drawn off, whereby the accumulator contents are continuously renewed, to reduce thermal decomposition of the fluid fuel. The heat source can be a gas turbine for producing a basic power output, the turbine exhaust gases being used to heat the heat-exchange fluid by way of a heatexchanger connected to the heat accumulator.