An energy conversion device for generating electricity includes a heat exchanger adapted to receive heat from a heat source. A closed magnetohydrodynamic (MHD) circuit includes a section for extracting heat from a heat exchanger so as to raise the temperature of the flowable electrically conductive material within the MHD circuit. The heat exchanger includes a heat-conductive support adapted to be heated, and a heat-extracting section of the MHD circuit extracts heat from the support. The support may be a metal plate having a sinuous pipe arrangement in heat-conductive contact against each face, one of these pipe arrangements being part of a circuit for heating the plate and the other pipe arrangement being part of the MHD circuit. The heat source may be combustion of a fuel, solar, geothermal, chemical reaction, or waste heat in the form of a hot gas. The heat conductive support may be a metal tube heated, usually by a hot gas, in which case the heat-extracting section of the MHD circuit includes a helical pipe surrounding the tube. The heat-providing means may include a lens arrangement for concentrating solar radiation on one face of the heat exchanger plate.