Organic Rankine Cycle (ORC) systems are used for generating electricity from low to medium temperature heat sources in the range of 175 °F to 1,000 °F. The organic Rankine cycle is a modification of the traditional steam Rankine cycle (SRC) that is the basis for most of the central station power generation in the U.S. Instead of steam, ORCs use organic compounds that have better generation efficiencies at lower heat-source temperatures as the working fluid. In a Rankine cycle (either SRC or ORC), a liquid working fluid is pumped to elevated pressure before entering a heat recovery boiler. The pressurized fluid is vaporized using energy captured from a waste heat stream, and then expanded to lower temperature and pressure in a turbine, generating mechanical power that can drive an electric generator. The low-pressure working fluid is then exhausted to a condenser where heat is removed by condensing the vapor back into a liquid. The condensate from the condenser is then returned to the pump and the cycle is repeated.

The figures below show a six-stage thermodynamic cycle in which, first, a working fluid is pumped from a lower pressure to a higher pressure (3 to 4). The high-pressure liquid goes through the recuperator to pick up heat from the expanded vapor exiting the expander (4 to 5). Then the high-pressure fluid enters a boiler where it is heated at a constant pressure until it becomes a dry saturated vapor( 5 to 6). Next, this vapor expands through a turbine where mechanical work is converted into electrical energy (6 to 1). The high temperature vapor goes through the recuperator to be cooled by the liquid exiting the condenser (1 to 2). The wet vapor, then, enters a condenser where it is condensed back into a saturated liquid and the cycle starts again (2 to 3). For most applications, the cycle includes a regeneration section to improve cycle efficiency. The following figures show a simplified schematic of an ORC cycle and the same ORC cycle shown on a temperature/entropy diagram.

ORCs have long been used to generate power in geothermal power plants and a variety of packaged ORC systems are now available to generate emissions-free electricity from waste heat recovered from boilers, ovens, kilns and other types of furnaces, from gas turbine and engine exhaust, or from byproduct heat released from exothermic reactions (e.g., fertilizer production) or incinerators.