Geothermal energy is the energy contained as heat in the Earth's interior. The paper describes the internal structure of the Earth together with the heat transfer mechanisms inside the mantle and crust. It also shows the location of geothermal fields on specific areas of the Earth. The Earth's heat flow and geothermal gradient are defined, as well as the types of geothermal fields, the geologic environment of geothermal energy, and the methods for geothermal exploration Geothermal energy, as natural steam and hot water, has been exploited for decades to generate electricity, both in space heating and industrial processes. The geothermal electrical installed capacity in the world is 7173.5 MWe (December 1996), and the electrical energy generated is 38 billion kW h/year, representing 0.4% of the world total electrical energy which was 13,267 billion kWh in 1995. The thermal capacity in non-electrical uses (greenhouses, aquaculture, district heating, industrial processes) is 8664 MW1 (end of 1994). Electricity is produced with an efficiency of 10-17%, and the geothermal kWh is generally cost-competitive with conventional sources of energy, in the range 3-12 US cents/kWh. The geothermal electrical capacity installed in the world is probably comparable with that from biomass. but almost twice that from solar or wind sources summed together. In developing countries, where total installed electrical power is still low, geothermal energy can play a significant role: in El Salvador 15% of electricity comes from geothermal steam, 15% in Nicaragua. 21% in the Philippines, and 6% in Kenya and Costa Rica. Financial investments in geothermal electrical and non-electrical uses worldwide are summarised. Present technology makes it possible to control the environmental impact of geothermal exploitation. The future use of the geothermal energy from advanced technologies such as the exploitation of geopressured reservoirs, hot dry rock systems and magma bodies is briefly discussed. While the viability of hot dry rock technology has been proved., research and development are still necessary for the other two sources. Finally, a brief discussion follows on training of specialists, geothermal literature, on-line information, and geothermal associations. © 1997 Published by Elsevier Science Ltd
