NOVELTY - A secondary fluid (L2) is heated in a heat exchanger (V1) connected to a displacement devices (G,H) with higher and lower volumetric capacities and an energy converter (W). Outlet fluid (L2) from device (G) is connected to the inlet of a cold reservoir (J) after being cooled in a heat exchanger (V3) by a fluid (L3) and to the heat exchanger (V1) via the displacement device (H) and heat exchanger (V2).    USE - For energy converter which is a kind of energy pump that transforms energy in the form of relatively small temperature differences between the energy source and cooling medium to energy in the form of mechanical work such as waste heat from power plants, process industry, etc.    ADVANTAGE - Thermal expansion of fluids is utilized in a way that a small thermal expansion under high pressure is changed to a large movement advantageously. Low temperature sources are utilized to produce or regain energy that is classified as waste energy.    DETAILED DESCRIPTION - The heat exchanger (V1) extracts heat from a source (C) with a primary fluid (L1) and transfers the heat to the secondary fluid (L2). The outlet of secondary fluid from the displacement device (G) is cooled in a heat exchanger (V2) by the secondary fluid coming out of the displacement device (H) before being cooled again in the heat exchanger (V3). The displacement devices (G,H) consist of a double acting hydraulic cylinder with essentially same pressure on both sides of the piston arranged with a relationship between the piston rod side's cross-sectional area and area of the piston that is calculated so that ratio of the volumetric capacity between the displacement devices adjust to the coefficient of thermal expansion and compressibility factor of the secondary fluid. The energy converter is designed to convert mechanical energy to other forms of energy preferably electrical power. An INDEPENDENT CLAIM is also included for the method of transforming energy from heat energy to other forms of energy.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic arrangement of the heat exchangers and displacement devices.    Source (C)    Displacement devices (G,H)    Cold reservoir (J)    Fluids (L1-L3)    Heat exchangers (V1-V3)    Energy converter (W)