Abstract: In an expansion device for use in a working medium circuit, for performing a process sequence corresponding to that of a Clausius Rankine cycle or an Organic Rankine cycle, a working unit comprising two interleaved scroll elements is provided of which one is stationary whereas the other is supported for orbiting relative to the stationary scroll element so as to form separate expansion regions between the scrolls moving, upon orbiting of the movable scroll, from the center of the interleaved scroll elements circularly outwardly with increasing volumes in which a working medium admitted at the center can expand for driving the movable scroll element.