A binary cycle is a method for generating electrical power from geothermal resources and employs two separate fluid cycles, hence binary cycle. The primary cycle extracts the geothermal energy from the reservoir, and secondary cycle converts the heat into work to drive the generator and generate electricity. Binary cycles permit electricity generation even from low temperature geothermal resources (<180°C) that would otherwise produce insufficient quantities of steam to make flash power plants economically viable. However, due to the lower temperatures binary cycles have low overall efficiencies of about 10-13%. Introduction In contrast to conventional geothermal power generation methods like dry-steam or flash, which use a single open cycle, a binary cycle has two separate cycles operating in tandem, hence binary cycle. The primary cycle extracts heat from the geothermal reservoir and provides this to the secondary cycle, which converts heat into work (see Heat Engine) to drive a generator and produce electricity. Thermodynamically, binary cycle power plants are similar to coal-fired or nuclear power plants in that they employ Rankine Power Cycles, the main difference being the heat source and the choice of cycle working fluid. Primary cycle The geothermal reservoir's hot in-situ fluid (or geofluid) is produced to the surface via a wellbore, if necessary assisted by a pump. On the surface, the hot geofluid transfers some of its heat to the secondary cycle, via a heat exchanger, thus cooling in the process. The cold geofluid is then reinjected into the geothermal reservoir via a separate wellbore, where it is reheated. The primary cycle is considered an "open" cycle. Secondary cycle Cold high-pressure working fluid is heated and vapourised in a heat exchanger by the hot geofluid. The hot high-pressure vapour is expanded in a turbine before being cooled and condensed in a condenser. To close the loop, the cold low-pressure liquid is repressurised via a feed pump. The secondary cycle is a closed cycle. The two main secondary cycle configurations are Organic Rankine cycles (ORC) or Kalina cycles, the main difference being the choice of working fluid; an organic fluid (commonly a hydrocarbon or refrigerant) or a water-ammonia mixture respectively. History The earliest example of a binary cycle geothermal power plant is thought to have been located on Ischia, Italy, between 1940-1943. The plant is thought to have used Ethyl Chloride as the working fluid at an effective capacity of 250 kW. However, owing to the Second World