Geothermal energy technology and current status: an overview

Geothermal energy is the energy contained as heat in the Earth's interior. This overview describes the internal structure of the Earth together with the heat transfer mechanisms inside 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 of exploration for geothermal resources including drilling and resource assessment. Geothermal energy, as natural steam and hot water, has been exploited for decades to generate electricity, and both in space heating and industrial processes. The geothermal electrical installed capacity in the world is 7974 MWe (year 2000), and the electrical energy generated is 49.3 billion kWh/year, representing 0.3 % of the world total electrical energy which was 15,342 billion kWh in 2000. In developing countries, where total installed electrical power is still low, geothermal energy can play a significant role: in the Philippines 21% of electricity comes from geothermal steam, 20% in El Salvador, 17% in Nicaragua, 10% in Costa Rica and 8% in Kenya. Electricity is produced with an efficiency of 10-17%. The geothermal kWh is generally cost-competitive with conventional sources of energy, in the range 2-10 UScents/kWh, and the geothermal electrical capacity installed in the world (1998) was 1/5 of that from biomass, but comparable with that from wind sources. The thermal capacity in non-electrical uses (greenhouses, aquaculture, district heating, industrial processes) is 15,14 MWt (year 2000). Financial investments in geothermal electrical and non-electrical uses world-wide in the period 1973-1992 were estimated at about US$22,000 million. Present technology makes it possible to control the environmental impact of geothermal exploitation, and an effective and easily implemented policy to encourage geothermal energy development, and the abatement of carbon dioxide emissions would take advantage from the imposition of a carbon tax. The future use of geothermal energy from advanced technologies such as the exploitation of hot dry rock/hot wet rock systems, magma bodies and geopressured reservoirs, is briefly discussed. While the viability of hot dry rock technology has been proven, research and development are still necessary for the other two sources. A brief discussion on training of specialists, geothermal literature, on-line information, and geothermal associations concludes the review.