National Geographic : 1981 Feb 28
Pits'" 'cg \Elas~n ~o~e cR\ ptla GEO THERMAL STapping the earth's furnace G EOTHERMAL--earth heat-energyis one of our most plentiful resources. It results from the radioactive decay of rocks, which raises the earth's temperature an average of 25 degrees Celsius with each kilometer of depth. Experts estimate that 32 million quads of energy are simmering within ten kilometers of the surface of the United States. Most can never be utilized, but interest in exploitable areas is quickening. Some 2,300,000 acres of ine federal land have been leased for exploration and in development, and in 1979 drillingincreased 25 percent over 1978. rn The most common surface manifestation of geothermal Water pumped down into hot dry rock (left) circulates through fractures, picking up heat for recovery 3200 C at the surface. Radioactive granite intrusions (right)offer favorable geothermal sources. 90°oC -) energy is simply hot water. In Boise, Idaho, hot springs have heated homes since the 1890s. The earth's hot water is also used for industrial processing. Steps to produce electrical power with hot water are being taken in California's Imperial Valley, where 200 megawatts are expected to go on line in 1983. Earth heat sometimes exhibits itself as dry steam with high pressure and low water content. Such steam, driving turbines at The Geysers in California, produces electricityequal to half of San Francisco's consumption. The Department of Energy is seeking to extract heat from a third type of reservoir, hot dry rock. Such formations contain heat, but no water to bring it to the surface. Plans are to circulate water through drill holes connected by man-made fractures in the hot rock. Development and refinement of technology are necessary to make geothermal energy economically competitive with conventional sources of energy. However, experts estimate that by the year 020 geothermal could be adding 18.5 quads annually to the national energy pool.