We have all seen the Rankine Cycle engine, most typically as the inefficient steam locomotive. The modern efficient designs use turbines to convert heat energy from two reservoirs of different temperatures to mechanical energy.
The Organic Rankine Cycle engine uses a fluid – vapor phase change other than water/steam and a wide range of compounds are available including proprietory mixtures. These mixtures allow for the conversion to mechanical energy in a wide range of applications and temperatures. Two such applications would be waste heat to energy and geothermal energy systems.
See on en.wikipedia.org