The potential for hot water reservoirs in the 80°C–180°C range is based on a retabulation of USGS Circular 726,2 omitting reservoirs in national parks. This defines a resource base of 784 quadrillion Btu (quads) (Table 8–1). This yields a calculated total heat content of 3982 quads for U.S. hot water reservoirs at temperatures between 80°C and 180°C (Table 8–1). The potentially producible energy above 180°C is calculated to be 505 quads of heat, and the total resource base in the temperature range from 180°C to 850°C is calculated at 8035 quads (Table 8–1). If half this heat can be recovered, 1991 quads of useful heat is potentially producible from U.S. hot water reservoirs with temperatures between 80° and 180°C (Table 8–2), and 4018 quads from hot water reservoirs warmer than 180°C.
At present no electricity is produced in the United States from hot water geothermal sources, and direct use of geothermal hot water in this country amounts to only about 15 MWt.3 However, both types of use are expected to increase steadily in the United States throughout the next few decades, as they already have in several other countries.
The relatively cooler reservoirs have a major disadvantage in that larger volumes of water must be withdrawn and reinjected to produce a given amount of heat, at a risk of subsidence and aquifer disruption. Given reasonable care, however, nonelectrical uses of such deposits should do little environmental damage because of their small scales.
With hotter, more saline reservoirs (such as those in the Imperial Valley of California) the volumes of fluid that must be extracted and reinjected to produce a given unit of heat are considerably smaller, but accidental spills present hazards of soil salination (already a natural problem in the Imperial Valley) and water pollution. Also, at least some reservoirs produce brine high in the air pollutant hydrogen sulfide. The land-use conflict between geothermal development and agriculture is also serious, as is the possibility of crop damage by geothermal effluents.
Except for variable amounts of noncondensible gases such as carbon dioxide and hydrogen sulfide, natural geothermal steam is usually quite pure and can be piped directly from a well to a turbine generator system. This is the case at The Geysers. Elsewhere, natural steam fields are rare; the USGS has identified another steam field in Yellowstone National Park