The binary cycle is the most common application of geothermal power plants because it is designed for low temperature geothermal resources (lower than 150 ºC) which constitute the majority of those available worldwide. In a binary cycle plant, hot water is pumped to surface via a production well and then passed through a heat exchanger. A second “working fluid” then passes through the heat exchanger to harvest that heat. The working fluid is chosen for its chemical properties and more specifically because it tends to change state and become a vapour at a lower temperature than water.
The now heated working fluid then becomes steam and is passed through an electrical turbine which spins to generate electricity. Once the fluid has passed through the turbine it then returns to a liquid form to be re heated within the heat exchanger to complete the cycle once again. Additionally, the geothermal water used to heat the working fluid has been reinjected into the geothermal reservoir to be re heated inside the Earth and produced once gain.
Flash Steam Turbines
A third scenario exists where a geothermal resource exists at a temperature where both water and steam are produced up the production well. In these flash steam turbines the liquid portion of the water is separated and reinjected to the reservoir and the steam portion is used in the power producing turbine before it is reinjected.
Dry Steam Turbine
Dry steam turbines are used in applications where the geothermal resource is a very high temperature and exists only as steam. Unlike the binary cycle, dry steam applications do not require a heat exchanger or a working fluid and the hot steam is sent directly through the power producing turbine.