In an energy conversion method and a thermal power plant for converting heat into mechanical or electric energy using a working medium, a vapor state in the working medium is generated at a first pressure in a steam generator. The vaporized working medium is expanded to a lower second pressure in a steam expanding device. An energy obtained by the expansion process is discharged. The expansion of the steam state is carried out using a saturation line of the working medium. The working medium is thereby separated into a non-condensed portion and a condensed portion in a separating device. The non-condensed portion is then compressed into a compressed non-condensed portion in a compressor. The compressed non-condensed portion is cooled and condensed into a compressed condensed portion. The compressed condensed portion and the initially condensed portion are then heated, and both portions are returned to the steam generator together.