A binary cycle uses a secondary organic fluid with a lower boiling point than water.
In a binary geothermal plant, hot underground water passes through a heat exchanger rather than directly driving the turbine. The heat vaporizes a second working fluid, commonly an organic compound, whose vapour expands through a turbine and produces electricity. The geothermal water can then be reinjected underground.
This arrangement allows electricity generation from geothermal resources that are hot but not hot enough to provide abundant natural steam. It also keeps the geothermal fluid and the turbine loop largely separate, which can reduce issues caused by dissolved minerals and gases.
Dry-steam plants send natural steam directly to a turbine, while flash-steam plants reduce the pressure of hot water so part of it rapidly vaporizes. Binary-cycle technology is therefore especially useful for moderate-temperature resources and closed-loop operation.