school energy&environment

High temperature geothermal systems

In high temperature geothermal systems, the underground waters are very hot, usually, over 140 ° C. Temperatures can be even higher, such as, for example, in Larderello (Toscana) (260° C), Cerro Prieto (Messico) (388° C) or S. Vito (Campi Flegrei, Campania) (400 °C): the latter area has registered the highest temperature ever observed in a geothermal system. In these systems the heat flux is 3-4 times higher than normal and can generally be found in correspondence with cooling magma intrusions, between 3 and 15 km of depth. Geothermal systems can present both an ascent of “dry” and overheated vapour, in the absence of liquid water (this constitutes the so called “vapour-dominated systems”), and an ascent of a mix of liquid water and vapour (“water-dominated systems”). Vapour, drawn through wells and systems of pipes, is used to give power to a turbine system, which in turn produces electricity. Vapour-dominated systems are the most productive, because in water-dominated systems the liquid phase has to be separated and eliminated and this entails an expenditure of energy. Vapour-dominated systems are quite rare and there are only four, spread around the globe: Larderello and M. Amiata (Italy), The Geysers (California), Matsukawa (Japan) e Kawah Kamojang (Indonesia), while the most important water-dominated ones are in Wairakei (New Zealand) and Cerro Prieto (Mexico). The production of electricity from geothermal fields is an Italian initiative: it started in Larderello in 1904, followed only many years after by the Wairakei plants (New Zealand) in 1958 and The Geysers (California) in 1960: Italy was a precursor in the exploitation of geothermal energy and to this day our country is one of the major world producers. Currently the main producers of geothermal electricity are, in order of productivity, the USA, followed by the Philippines, Indonesia, Mexico, Italy, New Zealand, Iceland and Japan. The discovery of new geothermal fields is an exceptional event, however technological research allows a continuous increase in productivity of the existing fields.