Status of Geothermal Research and Development in the World Situation mondiale de la Recherche et du Développement géothermiques

The heat flow observed on the earth's surface (on averageof 59 mW/m2) mainly derives from the heat generated by the decay of radioactive elements (U238, U235, Th232, K40) in the crust. The distribution of heat flow values is closely tied to the phenomena described in theplate tectonicstheory: most of the surface geothermal anomalies and, consequently, the industrially exploitable geothermal areas, are located in correspondence to spreading ridges (geothermal fields of lceland, Kenya, Ethiopia, etc. ) and subduction zones (Indonesia Japon, Indian and Chinese Himalayas, Chile, etc. ). However, geothermal fields can also be found in intraplate zones in areas with a normal heat flow value (e. g. , Paris basin) or slightly higher (e. g. Hungarian basin). Generally these fields produce a low-enthalpy fluid that can be used for non-electric exploitation. The best-known geothermal systems, and the only ones exploited so far, belong to the 'hydrothermal convective' type. These occur wherever a fluid circulation, mainly of meteoric origin, is able to develop in sufficiently permeable rocks near a heat source (such as a magmatic intrusion) or at depths at which it can be heated by the normal geothermal gradian. This type of system is usually divided intowater-dominatedsystems, where the continuous fluid phase in the reservoir is liquid andvapour dominatedsystems where steam is the continuous phase. Other geothermal systems have still to be utilized and probably will be exploited as progress is made in technology. These are: a) geopressured systems, found in some subsident sedimentary basins containing high-temperature connate waters of near lithostatic pressure; b) the so-called 'hot dry rocks', which can be used ta create artificial systems by artificial fracturing of the high-temperature rocks and induced circulation of closed-circuit fluids. Geothermal energy is exploited: a) To generate electricity. The total installed geothermoelectric capacity throughout the world (1979) is 2,063 MWe. Nowadays the most advanced countries where geothermal energy is concerned are the USA (908 MWe), Italy (421 MWe), New Zealand (202 MWe) and Japon (171 MWe). Research is under way in various countries, directed at begining production or increasing present levels. By 1985 the installed geothermoelectric capacity should be around 6,500 MWe. b) For non-electric uses. There are various applications of geothermal energy, the main ones being in space-heating and agriculture (especially g