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0-14
15-29
30-44
45-59
60+
FIGURE 13. Geothermal Gradient Contour Map of the United States.
Geothermal energy derives from the immense thermal reservoir of the earth's interior. Heat from molten rock (magma) beneath the earth's crust or from natural radioactive decay transfers to rock and water closer to the surface. In certain regions of the earth, the hot waters are close enough to the surface to be commercially exploited in heating applications, or, in the case of high-grade steam reserves, in electrical power generation.
One question that commonly arises regarding geothermal energy is whether or not it is a renewable resource. The answer hinges on how the resource is developed. Certainly the heat within the earth, like the sun, is limitless compared to human activity. However, the waters that are tapped in geothermal development are finite. Hydrothermal (hot water) aquifers will be diminished whenever water is withdrawn faster than it is recharged. Overexploitation at some facilities in California, for example, resulted in a lower than expected output. If water is reinjected into the field after extracting heat (as is done in some locations), then the resource may be said to be truly renewable. Otherwise, it is simply mined, much as a petroleum reserve.
Areas with significant geothermal resource occur where the earth's crust is relatively thin, such as along the boundaries of tectonic plates. Geysers, hot springs, volcanoes, and seismic activity, all of which are noticeably absent in Texas, mark such regions. In the U.S., the best geothermal resources occur along the Pacific rim (California to Alaska) and in Hawaii (see Figure 13). California has the largest geothermal electric facilities in the nation, with about 1100 MWe, most concentrated at the Geysers steam field in the northern part of that state.
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The increase in temperature with depth below the ground is highest in areas with volcanic and seismic activity. This map indicates Hot Dry Rock potential.
A significant portion of the energy consumed in the United States requires relatively low temperatures. Energy needed for space and water heating, fish farming and greenhouse heating, enhanced oil recovery, and desalinization can take advantage of low temperature hydrothermal resources if such resources are present where the energy is consumed.
Texas does not possess any easily accessible field with the high temperatures required for electric power generation. It does, however, possess some low-temperature hydrothermal reserves that have seen limited use. As shown in Figure 14, these resources occur mainly in two bands, one that cuts a swath through the central part of the state, and a second that borders the Rio Grande in the Trans-Pecos. Temperatures in the Central Texas hydrothermal aquifers range from about 90° to 160°F at depths from 500 to 5,000 feet. Historically the waters have seen some application in spas and therapeutic baths. Where waters are potable, a number of smaller communities have tapped them for their municipal supply, without making use of the heat. A recent project in Marlin, however, employed geohermal well water to heat a local hospital. In the Trans-Pecos, thermal waters have likewise supplied resort baths, with scant need for more extensive development owing to the region's remoteness.
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 Potential Hydrothermal or Geopressure Source
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In addition to the state's low-temperature hydrothermal resource, large zones of hot, highly pressurized fluids occur in deep strata under the Gulf Coast. This so-called "geopressured-geothermal" resource was studied extensively in the 1970's and 1980's and a test well was operated by the Department of Energy at Pleasant Bayou near Houston. Typically, geopressured zones are at depths on the order of 15,000 feet and the fluid itself is a hot (about 300 deg. F), high-pressure brine with methane dissolved in it. Interest in the resource is probably driven as much by the potential methane recovery as by its geothermal character. To date, development has not proven economical. Hot brine, however, may someday be used in enhanced oil recovery schemes. Since the resource is not renewable, it must be mined to be used.
A final, long-term geothermal energy prospect is the extraction of heat from zones of "hot dry rock" (HDR). In the envisioned HDR facility, high-pressure water injected underground at one point is collected at a distance well after it has been heated by passing through fractured, hot rock. The scheme is presently in its infancy. One study suggested that Texas has moderately good resource in the eastern part of the state (see Figure 13).
Texas does not have the sort of readily accessible, high-temperature hydrothermal resource that can be used to generate electricity. The resource in the central part of the state can, however, have an impact in low-temperature applications such as space heating or aquaculture. Several municipalities that presently introduce warm aquifer water in drinking supplies could capture beneficial heat with the addition of a heat exchanger. The geopressured-geothermal resource will become more attractive only in the context of higher energy prices. Hot dry rock's potential value is presently unknown.
Known
Known
Known
