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 ORC turbogenerator uses medium-to-high-temperature thermal oil to preheat and vaporize a suitable organic working fluid in the evaporator (4>5). The organic fluid vapor rotates the turbine (5>6), which is directly coupled to the electric generator, resulting in clean, reliable electric power.

The Rankine cycle is closed by a condenser (using cooling water) that convert saturated vapor at low pressure from the turbine generator into condensate, which is pumped back to the heat exchanger to restart the cycle.

The advantages of the ORC system are that it performs well under partial load and, because of the low working pressures and temperatures, it is not subject to the regulations of steam equipment. ORC units are suitable for biomass CHP plants with an electrical output as small as 200 kW.