Method for the conversion of thermal energy of a low-temperature heat source with temperatures of maximum 200°C in a triangular process, hereinafter referred to as "Extended Trilateral Cycle" (E-TLC), into volume change work and subsequently technically utilisable energy, which passes through the following states or steps: • a first process state with a liquid working mixture under a first pressure at a first temperature • an increase in the pressure and an increase in the temperature of the working mixture to a second pressure and a second temperature without vaporisation of the working mixture, the second pressure and the second temperature being higher than the first pressure and the first temperature • a second process state with the liquid working mixture at the second pressure and heated to the second temperature • a flash evaporation of the working mixture initiated by external pressure reduction with partial evaporation of the working mixture with simultaneous performance of volume change work by the volume of the newly generated or volume increase of the already existing vapour • a third process state with the working mixture again under the first pressure, partially vaporised and cooled to the first temperature • condensation of the vaporised portion of the working mixture at the first temperature with removal of entropy to form a liquid, thereby restoring the first process state characterised in that • the working mixture consists of a working medium and a heat medium which is not chemically identical to the working medium, the heat medium having a higher vaporisation temperature than the working medium at the same pressure • during flash evaporation, only the working fluid evaporates and the heat medium remains in a liquid state at all times • the working fluid vaporises completely from the working mixture and only the heat medium remains liquid • the flash evaporation of the working fluid is polytropic • the additional thermal energy required for the polytropic flash evaporation of the working fluid is transferred from the heat medium to the evaporating working fluid and the heat medium cools as a result. • a first process state with a liquid working mixture under a first pressure at a first temperature • an increase in the pressure and an increase in the temperature of the working mixture to a second pressure and a second temperature without vaporisation of the working mixture, the second pressure and the second temperature being higher than the first pressure and the first temperature • a second process state with the liquid working mixture at the second pressure and heated to the second temperature • a flash evaporation of the working mixture initiated by external pressure reduction with partial evaporation of the working mixture with simultaneous performance of volume change work by the volume of the newly generated or volume increase of the already existing vapour • a third process state with the working mixture again under the first pressure, partially vaporised and cooled to the first temperature • condensation of the vaporised portion of the working mixture at the first temperature with removal of entropy to form a liquid, thereby restoring the first process state characterised in that • the working mixture consists of a working medium and a heat medium which is not chemically identical to the working medium, the heat medium having a higher vaporisation temperature than the working medium at the same pressure • during flash evaporation, only the working fluid evaporates and the heat medium remains in a liquid state at all times • the working fluid vaporises completely from the working mixture and only the heat medium remains liquid • the flash evaporation of the working fluid is polytropic • the additional thermal energy required for the polytropic flash evaporation of the working fluid is transferred from the heat medium to the evaporating working fluid and the heat medium cools as a result.