Geothermal resources that are less than 150 – 180 degrees Celsius are considered low-temperature resources, but can often still be utilized for geothermal power production. By using a high efficiency heat exchanger, the heat from a geothermal fluid is transferred to a ‘working fluid’ that has a lower boiling point. This heating process vaporizes the working fluid and the resulting vapor is used to power a turbine using the thermodynamics of an expansion cycle. Thus, the basic concepts that apply in a binary electrical generating facility for powering the turbine are very similar to that in a steam turbine.
One commonly used working fluid is isopentane that has a boiling point of 28 degrees Celsius. By using fluids such as isopentane, geothermal resources with moderate temperatures are capable of producing large amounts electricity with most binary plants ranging from 2MW to more than 50MW.
Image courtesy of U.S. Department of Energy