ORC: How it works In a typical ORC design, a thermal energy source feeds an evaporator to drive an expander or “reverse compressor”, which in turn generates the electricity. The provider of the heat can be natural, such as in the case of geothermal energy, or waste heat from any number of industrial waste heat sources.

This process was developed in 1859 by Scottish engineer William J.M. Rankine. This is a thermodynamic cycle which converts heat into mechanical energy—which usually gets transformed into electricity by electrical generation.

Organic Rankine cycle (ORC) machines are used to convert low-temperature heat resources into power. Examples include systems making use of waste heat in a cogeneration system, low temperature geothermal electricity generation, solar ponds, and potentially OTEC.

A real Rankine cycle or non-ideal cycle used in actual power plants do not undergo isentropic compression and expansion by pump and turbine respectively. These processes are irreversible compared to the ideal cycle, and there is an increase in entropy as shown in the T-s diagram below.