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).

A Rankine cycle system requires four main components, a boiler, a turbine, a condenser, and a water pump. The most common and relevant example of the Rankine cycle is the steam turbine system, which uses water in its liquid and gaseous phases as the working fluid.

* The efficiency of the ORC system is always optimized: on average at 50% load, electrical efficiency is still 90% of the nominal electrical efficiency.

The Rankine cycle is a heat engine with a vapour power cycle. The common working fluid is water. The cycle consists of four processes as shown in Figures 1 (a) and 1 (b): 1 to 2: Isentropic expansion (Steam turbine)1 An isentropic process, in which the entropy of working fluid remains constant.