GEOTHERMAL-ENERGY FROM THE EARTH - ITS POTENTIAL IMPACT AS AN ENVIRONMENTALLY SUSTAINABLE RESOURCE

Geothermal energy technology is reviewed in terms of its current impac t and future potential as an energy source. In general, the geothermal energy resource base is large and well distributed globally. Geotherm al systems have a number of positive social characteristics (they are simple, safe, and adaptable systems with modular 1-50 MW [thermal (t) or electric (e)] plants capable of providing continuous baseload, load following, or peaking capacity) and benign environmental attributes ( negligible emissions of CO2, SOx, NOx, and particulates, and modest la nd and water use). Because these features are compatible with sustaina ble growth of global energy supplies in both developed and developing countries, geothermal energy is an attractive option to replace fossil and fissile fuels. In 1997, about 7,000 MWe of base-load generating c apacity and over 15,000 MWt of heating capacity from high-grade geothe rmal resources are in commercial use worldwide. A key question is whet her these levels can grow to a point where geothermal energy is more u niversally available and thus have a significant impact on global ener gy supplies in the twenty-first century. Such an achievement will requ ire the economic development of low-grade resources. The current statu s of commercial and emerging technologies for electric power productio n and direct heat use is reviewed for the major geothermal resources i ncluding hydrothermal, geopressured, hot dry rock, and magma. Typicall y, high-temperature resources (>150 degrees C) provide base-load gener ating capacity while lower-temperature resources provide energy for ge othermally assisted heat pumps and for direct use in domestic, agricul tural, and aquacultural heating applications. Critical development iss ues relating to resource quality and distribution, drilling costs, and reservoir productivity are discussed in the context of their economic impact on production costs. Advanced drilling and improved heat minin g methods are suggested as approaches to increase the worldwide use of geothermal energy by reducing field development costs. With these imp rovements, lower-grade resources can compete in growing global energy markets that are currently controlled by abundant and low-cost fossil fuels.