An organic Rankine cycle (ORC), is fundamentally the same as a steam Rankine cycle; however, it uses a lower boiling point organic fluid to better match its operation to lower temperature heat sources. ORC systems can achieve better efficiencies than steam turbines for smaller systems (less than a few MW_e).

The Rankine cycle, also called the Rankine vapor cycle, is a thermodynamic cycle that converts heat into mechanical energy. The Rankine cycle is name after William Johnson Macquorn Rankine, a 19th century Scottish engineer and physicist known for his research in the thermodynamic properties of steam.

The Rankine cycle operates in the following steps: 1-2-3 Isobaric Heat Transfer. High pressure liquid enters the boiler from the feed pump (1) and is heated to the saturation temperature (2). Further addition of energy causes evaporation of the liquid until it is fully converted to saturated steam (3).