Binary Cycle Power Plant

A binary cycle power plant generates electricity from geothermal resources too cool for a conventional steam plant, typically below 180 degrees Celsius, by running two separate fluid loops: the hot geothermal fluid passes through a heat exchanger and transfers its heat to a second working fluid, usually an organic compound or an ammonia water mixture with a much lower boiling point, and it is that second fluid, vaporized by the transferred heat, that drives the turbine and generator. The earliest known binary cycle plant operated on the Italian island of Ischia from 1940 to 1943, producing 250 kilowatts using ethyl chloride as its working fluid, and the first commercial scale binary plant, an 11 megawatt facility in the East Mesa field of Southern California, began operating in 1979. As of December 2014 roughly 203 binary cycle plants were operating in 15 countries, accounting for 35 percent of the world geothermal power plants by number, though because they draw on lower temperature, lower efficiency resources they generated only about 1,250 megawatts of the world total geothermal output. The technology ability to use comparatively cool resources has made it central to expanding geothermal power beyond the small number of locations hot enough for a conventional flash steam plant.