14 Organic Rankine Cycle (Binary) Geothermal Power Plants

We extend our discussion of heat pumps now to heat engines. A discussion of heat engines employing the Rankine Cycle is appropriate here because of the strong similarities to the mechanical vapor compression cycle. Further, there is a continuing interest and trend to employ power cycles in low-temperature geothermal applications similar to those previously discussed for absorption cooling (i.e., <100 °C). Even at geothermal resource temperatures up to ∼175 °C, closed-loop Rankine cycles are typically employed in a binary-type configuration. Such cycles are referred to as organic Rankine cycles (ORCs) because a secondary fluid is employed as the working fluid. The ‘organic’ working fluid is typically a hydrocarbon with a boiling point lower than water.