The Rankine cycle is a thermodynamic cycle widely used by power plants to convert water into steam, then expanding that steam through a turbine to produce mechanical work. The organic Rankine cycle (ORC) is a refinement of this technology, using an organic working fluid in place of water.

The low speed ORC process has an electricity output of 400–1100 kW and electricity to heat ratio of 0.18–0.22. The ORC-concept has two circuits, one for thermal oil and another for organic fluid.

The working principle of the organic Rankine cycle is the same as that of the Rankine cycle: the working fluid is pumped to a boiler where it is evaporated, passed through an expansion device (turbine, screw, scroll, or other expander), and then through a condenser heat exchanger where it is finally re-condensed.

Organic Rankine Cycles differ from traditional steam Rankine Cycles in their use of an organic liquid as opposed to steam as the working fluid. The term ORC is also applied generally to any Rankine cycle with a low-grade heat source.