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8 Geothermal Energy
Pages 397-421

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From page 397...
... Furthermore, geothermal deposits of the types most useful at present—natural steam and hot water reservoirs—are rather localized, mainly in the western states and often far from potential users. Still, even the small currently useffil part of the resource can be important in a world of rising fuel prices and declining supplies.
From page 398...
... GEOTHERMAL RESOURCE TYPES The geothermal resource is divided, for the purposes of this report, into six categories. HOT WATER RESERVOIRS In many places in the United States, especially in the western states, are underground reservoirs of geothermally heated water, some of which are tapped for space heating and the like.
From page 399...
... NORMAL GEOTHERMAL GRADIENT AND HOT-DRY-ROCK RESOURCES The various types of steam and hot water reservoirs are relatively easy to tap, because they supply their own working fluids. However, most of the potentially exploitable geothermal heat is stored in dry rock.
From page 400...
... For the purposes of this study, however, the USGS estimates have been retabulated using somewhat more conservative assumptions. The USGS, for example, includes in its resource base estimates of all of the geothermal heat above 15°C to a depth of 10 km, and assumes that within these limits all heat at temperatures above 90°C could be used for space heating and similar applications and that temperatures above 150°C could be exploited for electricity generation.
From page 402...
... depth except where noted. bHeat contents above 50°C to depths of 6-7 km, in onshore reservoirs only, not including the heat content of dissolved natural gas.
From page 403...
... Elsewhere, natural steam fields are rare; the USGS has identified another steam field in Yellowstone National Park
From page 404...
... However, The Geysers field apparently extends far beyond the area so far developed,5 and Reed and Campbell6 estimate a maximum potential generating capacity of up to 5000 MWe. There are good physical, geological, and historical reasons to believe that natural steam fields of commercially exploitable size are rare.
From page 405...
... Energy Content Based on this study's retabulation of estimates in USGS Circular 726,7 the identified resource is estimated to be 30,905 quads in reservoirs at temperatures between 80°C and 180°C, and 2442 quads in the single listed reservoir with a temperature higher than 180°. On the basis simply of the relative land area involved, it is estimated that the accessible geopressured resource base consists of 67,991 quads of heat at temperatures between 80°C and 180°C and 5372 quads at temperatures above 180 C, or a total of 73,363 quads of heat above 80°C (Table 8-1)
From page 406...
... HOT DRY ROCK Particularly where the crust is thin or has been disturbed by volcanism or faulting, higher than normal geothermal gradients are often encountered. This offers the possibility of reaching usefully high temperatures with shallower, less expensive holes than would be needed where the geothermal gradient is normal (about 30 C/km of depth)
From page 407...
... Most such areas are in sparsely populated parts of the western United States, and it is assumed that, if environmental considerations permit, half this area ultimately may be accessible to geothermal energy development. This yields an estimated resource base of 52,500 quads in rock with initial temperatures between 80°C and 180 C and 111,000 quads in rock hotter than 180 C, for a total of 163,500 quads (Table 8-1)
From page 408...
... So-called normal-gradient geothermal energy represents most of the nation's geothermal resource base. Exploitation of normal-gradient resources presents in essence the same technical problems as that of hot dry rock.
From page 409...
... MOLTEN MAGMA The extreme case of hot dry rock is magma, or molten lava, which may be found at temperatures higher than 650 C, in pools at the surface or in reservoirs below volcanoes. Aside from a few in national parks, the existence of such bodies and their depths are generally speculative, and practical means of extracting heat from them have yet to be demonstrated.
From page 410...
... Even the highest-grade geothermal resources, natural steam fields, allow a generating efficiency of only 20 percent, compared to typical efficiencies of about 35 percent for conventional thermal power plants. This means that to be a competitive source of heat for generating electricity, geothermal heat must be at most about half the cost of competing sources, Btu for Btu.
From page 412...
... , but estimates for future units are considerably higher P4 For an 80 percent load factor this gives a delivered cost for power of 9.2 mills/kWh, including the steam cost and the 0.5 mill/kWh paid the field operator to dispose of excess condensate, but not including distribution or customer service costs or general company overhead. Greider-5 estimated the capital cost of a typical generating plant using natural steam at $210/kWe installed, with which were associated capital investments of 5148/kWh for field development and 515/kWh for local transmission (20 miles)
From page 413...
... For a given generating capacity, large increases are required in the number of both production and injection wells and in either the number or the size of most surface facilities. Thus, for 50-MWe power plants using geothermal water of low salinity at 149°C, Swink and Schultz list estimated total system costs of $1190 52885/kWe of installed generating capacity, for a wide variety of possible power cycles, and corresponding generating costs of 26.7-64.7 mills/kWh.
From page 414...
... Generating costs, for example, have been estimated as low as 21 mills/lcWh for certain reservoir characteristics and selected technologies. HOT DRY ROCK Since the technology ffir extracting heat from hot dry rock has not yet been fully developed, cost estimates are necessarily speculative.
From page 415...
... In a systematic examination of the effects of geothermal gradients, well depths, flow rates, and rock temperatures, they present the possibility of generating costs less than 10 mills/kWh where gradients are very high and up to about 30 mills where they are normal, in both cases with relatively high flow rates through the system. While their cost estimates filr hot dry rock are within the range of similar estimates for natural hot water systems where reinfection is required, they of course assume the success of a technology that has not vet been demonstrated NORMAL-GRADIENT SYSTEMS The possibilities and costs of extracting and using geothermal heat in areas where the geothermal gradient is normal (approximately 30°C/km of depth)
From page 416...
... Federal and state courts and executive agencies differ on the question of whether geothermal resources are to be considered minerals, subject to equivalent taxation with oil and gas, or water resources, which are not given special tax treatment but are subject, especially in the West, to complicated state regulations designed to allocate water rights. Finally, looking ahead to wide commercialization of this resource, there is an institutional mismatch between those who explore for and produce geothermal heat and those who purchase the heat for power generation.
From page 417...
... In summary, the ultimate resource potential of geothermal energy is very difficult to estimate because of the lack of development and assessment of the most abundant resources, and because of the absence of a demonstrated technology for extracting the energy from hot dry rock or geopressured bones. In consequence of this situation the estimates set forth in Table 8-4 indicate the relatively modest total contribution that could be expected from geothermal sources within the time period of this study.
From page 419...
... One potentially large source of rather low temperature geothermal energy is the geopressured brines of the Gulf Coast. These brines may also hold very large amounts of dissolved natural gas.
From page 420...
... B Greider, "Status of Economics and Financing of Geothermal Energy Power Production," in Ptoceed logs Second United Nations Symposium on the Development and Use of Geothermal Resources available from Supenatendent of Documents (Washington, D.C.: U S
From page 421...
... L Weeden, "Geopressured Geothermal Energy—Will it Work?


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