The Rankine thermodynamic cycle is a concept wherby a set of processes involving a working fluid in a closed loop is such that thermal power is converted into mechanical power, and thereafter possibly into electricity. Traditionally, the working fluid is water (thus steam, when vaporized). In the Organic Rankine Cycle (ORC), the working fluid is an organic substance. Organic means that the molecules of the fluid contain a carbon atom. The implementation of the ORC concept is an engine or power plant. An examplary process flow diagram is shown in the figure below.
The use of an organic fluid in place of steam is in general advantegeous if the thermal energy source is at low/medium temperature (~100 to 600 oC), and/or the power capacity is small (few kW to few MW). In these cases the proper selection of the fluid allows to obtain comparatively higher efficiency and solves several technological problems related to the desing of the expander. The vapor-liquid critical point of the fluid and its molecular complexity are the main parameters affecting all the fluid properties influencing the design of the system.
The temperature-entropy thermodynamic diagram of the fluid is most often used to visualize the ORC processes (see figure above). The line separating the liquid, saturated, and superheated thermodynamic states of the fluid is unique for each fluid.