8
Geothermal Energy

The United States’ vast resource of geothermal heat—thought to come from the radioactive decay of elements in the earth’s crust—is certain to become increasingly useful as time goes on and fuel prices go up. At present it provides very little useful energy, and the technical and economic barriers it faces make it unlikely to become one of the nation’s main energy sources before the end of this century, if ever. 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 useful part of the resource can be important in a world of rising fuel prices and declining supplies. If current research and development are successful in solving some rather intractable technical problems, and if the economics of energy become more favorable, the contribution of the now inaccessible parts of the resource could be a significant factor in reducing oil imports.

Geothermal energy now contributes less than 1/40 of a quad in the United States, all from steam and hot water fields. At The Geysers, near San Francisco, the only geothermal electric power plant in the United States and the largest in the world, with a 565-megawatt (electric) (MWe) capacity in mid-1979 and expanding, exploits one of this country’s rare commercial-size steam reservoirs. In addition, about 15 megawatts (thermal) (MWt) of thermal energy from hot water reservoirs are used directly in various places for space heating, low-temperature industrial process heat, and similar uses.

Where it is usable, geothermal energy can be a very economical source of heat for direct use. However, most of the more accessible, higher-grade



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 8 Geothermal Energy The United States’ vast resource of geothermal heat—thought to come from the radioactive decay of elements in the earth’s crust—is certain to become increasingly useful as time goes on and fuel prices go up. At present it provides very little useful energy, and the technical and economic barriers it faces make it unlikely to become one of the nation’s main energy sources before the end of this century, if ever. 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 useful part of the resource can be important in a world of rising fuel prices and declining supplies. If current research and development are successful in solving some rather intractable technical problems, and if the economics of energy become more favorable, the contribution of the now inaccessible parts of the resource could be a significant factor in reducing oil imports. Geothermal energy now contributes less than 1/40 of a quad in the United States, all from steam and hot water fields. At The Geysers, near San Francisco, the only geothermal electric power plant in the United States and the largest in the world, with a 565-megawatt (electric) (MWe) capacity in mid-1979 and expanding, exploits one of this country’s rare commercial-size steam reservoirs. In addition, about 15 megawatts (thermal) (MWt) of thermal energy from hot water reservoirs are used directly in various places for space heating, low-temperature industrial process heat, and similar uses. Where it is usable, geothermal energy can be a very economical source of heat for direct use. However, most of the more accessible, higher-grade

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems hot water reservoirs are in parts of the western states remote from industry and population. Because steam can be pipelined efficiently for only a few miles, there is little market for this heat unless it can be converted to electricity, which could be transmitted hundreds of miles with relatively small losses. Power generation from the highest-grade geothermal resources is technically feasible now; rises in the prices of other fuels and advances in geothermal technology could render it economically feasible as well. In addition to the technical and economic problems of locating and exploiting the different kinds of geothermal resources, development faces a number of institutional constraints. Federal and state leasing policies, for example, often conflict. Leasing itself is slow and costly to bidders. The tax status of geothermal development is unclear; the resources are treated as minerals in some states and as water resources in others. The form of future utility contracts with geothermal steam producers is uncertain. All of these problems will retard development and increase costs unless they are corrected. Their influences are discussed more fully later in this chapter, under the heading “Future Development of the Geothermal Resource.” It is apparent that federal and state governments will largely determine the speed with which the present small geothermal industry can be expanded. By sponsoring research and development in exploration and production techniques, federal funds can provide the technical means. By streamlining leasing procedures, putting the geothermal resource on an equal tax footing with oil and gas, and providing financial assistance, especially in high-risk ventures, federal legislation could greatly speed the day when geothermal energy assumes a competitive place in the U.S. energy market. The timing and extent of these measures will dictate when that day arrives. 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. Some electricity is generated abroad by such deposits, but because of their higher salinity the hotter geothermal brines, more suitable for this purpose, are very corrosive to generating

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems equipment. This is a very large part of the geothermal resource, though, and the Department of Energy devotes a large proportion of its geothermal funds to investigating economical ways of using it. NATURAL STEAM RESERVOIRS Under rare geological conditions the pressures in hydrothermal reservoirs are so low that the water has boiled to steam, as at The Geysers power development. While this form of reservoir is the rarest, it is the easiest and most economical to tap, largely because there is little corrosion problem and little need to deal with the large amounts of brine that must be drawn from and reinjected into a hot water reservoir. It is a very inexpensive source of electricity, but because of its rarity it is not likely ever to contribute more than a few gigawatts of generating capacity to this country. GEOPRESSURED RESERVOIRS In some deep sedimentary basins in the United States, notably along the Gulf Coast, are deposits of brine, highly pressurized by the weight of the overlying land. These deposits are mostly at temperatures below 180°C and are assumed to be nearly or completely saturated with natural gas. The temperatures are generally too low and the deposits too deep for economical electricity generation, but the credit for producing gas as a by-product, if it is exploitable along with the heat, offers some potential for space heating and similar direct uses, if the costs of alternatives rise rapidly. 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. Even the normal geothermal gradient (about 30°C/km of depth) provides throughout the world temperatures usable for electric power generation (180°C) at the accessible drilling depth of 5.5 km and temperatures useful for direct heating (80°C) at a depth of 2.2 km. In many places the geothermal gradient is higher than this. Where it is higher than 40°C/km, the resource is called “hot dry rock.” Though these two types are by far the largest in heat content, they cannot yet be exploited. To do so, it will be necessary not only to supply the working fluid by injecting water, but also to find or create in some way a permeable network of channels through which the water can flow to be

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems heated. A number of projects around the world are investigating ways to exploit the resource, but none has reported results that can be applied commercially. MOLTEN MAGMA The final and most speculative part of the resource is the heat in molten rocks, or magma. Very few accessible magma bodies are known, and there is no technology for recovering heat from them, but their high temperatures and great heat content make them potentially very important. ESTIMATED HEAT CONTENTS AND PRODUCIBILITY Table 8–1 summarizes the total estimated heat contents of each of these six geothermal resource types in the United States, divided by temperature range. These estimates are based on U.S. Geological Survey (USGS) Circular 726,1 the most authoritative and up-to-date source of data on the geothermal resource of the United States. 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. This study considers 6 km a generally more reasonable maximum depth for geothermal drilling, 80°C the minimum commercially useful temperature for low-temperature heating applications, and 180°C the minimum temperature for electricity generation. Note that the currently useful part of the resource, hot water and steam reservoirs, is a very small share of the total. Table 8–2 lists estimates of the amounts of energy actually producible from geothermal reservoirs of the six types. The uncertainty of the estimates in these tables is obviously very great Geothermal energy is a largely unexplored resource; from 1970 to 1974 about 130 geothermal wells were drilled, while at least 140,000 oil and gas wells were completed. In fact, the size of the resource can be debated endlessly. Obviously, though, the sum is so large that production is limited not by the size of the accessible resource but by technology, economic considerations, and legal, social, and environmental issues.

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems TABLE 8–1 Estimated Thermal Energy Contents of Accessible U.S. Geothermal Reservoirs of Several Types (quads)a Reservoir Type Reservoirs at Temperatures from 80°C to 180°C Reservoirs at Temperatures Above 180°C Total Identified and Undiscovered Identified Undiscovered Total Identified Undiscovered Total Hot water 784 3,198 3,982 505 7,530 8,035 12,017 Natural steam 0 0 0 106 73 179 179 Geopressuredb 30,905 37,086 67,991 2,442 2,930 5,372 73,363 Normal gradientc 947,000 — 947,000 306,000 — 306,000 1,253,000 Hot dry rockd 52,500 — 52,500 111,000 — 111,000 163,500 Molten magmas 0 0 0 0 3,500 3,500 3,500 TOTAL 1,031,189 40,284 1,071,473 420,053 14,033 434,086 1,505,559 aHeat contents above 80°C to 6-km (19,685 ft) depth except where noted. National Parks are not included. bDoes not include heat content of reservoir rock or dissolved natural gas, or energy recoverable mechanically from high-pressure fluid. Includes entire heat content of fluid above 50°C, in onshore reservoirs only, to depths of about 6–7 km. cAssumed geothermal gradient, 30°C/km. dAssumed geothermal gradient, 40°C/km.

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems TABLE 8–2 Estimated Total Potentially Producible Thermal Energy from Accessible U.S. Geothermal Reservoirs of Several Types (quads)a Reservoir Type Heat Contents 80°–180°C Above 180°C Total Hot water 1,991 4,018 6,009 Natural steam 0 45 45 Geopressuredb 2,244 177 2,421 Normal gradientc 9,470 3,060 12,530 Hot dry rockd 525 1,110 1,635 Molten magmase 0 35 35 TOTAL 14,230 8,445 22,675 aHeat contents above 80°C to 6-km (19,685 ft) 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, Economical production technology not yet demonstrated. cAssumed geothermal gradient, 30°C/km. Heat-extraction technology not yet demonstrated. dAssumed geothermal gradient, 40°C/km. Heat-extraction technology not yet demonstrated. ePractical heat-extraction technology not yet developed. TECHNICAL AND ENVIRONMENTAL CONSIDERATIONS HOT WATER RESERVOIRS In this type of underground reservoir the water circulates convectively throughout the reservoir at a nearly uniform temperature, ranging from only slightly above atmospheric temperatures to 350°C or higher. Hydrostatic pressure is generally high enough to keep it from boiling even when greatly superheated, and the water generally remains in the reservoir long enough to become saturated with minerals. Hotter reservoirs usually are more saline than cooler ones, due to the increased solubility of most minerals at high temperatures. Geothermal waters with total dissolved solids contents ranging from less than 0.1 percent to more than 30 percent are known in the United States. The cooler, less saline waters are used directly in many places for such purposes as space heating. The hotter, more saline waters are used in a few places abroad for generating electricity, but this is not economical in the United States.

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems Energy Content 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. Environmental Considerations 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. NATURAL STEAM RESERVOIRS 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

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems and suggests the existence of another in Mount Lassen National Park, but because of their locations, these cannot be developed. Energy Content From data in USGS Circular 726,4 the accessible U.S. resource base of natural steam is estimated at 179 quads of heat above 80°C (Table 8–1). If one fourth is assumed potentially producible, this represent 45 quads of useful energy (Table 8–2). The technologies for producing and using natural steam commercially are well developed. However, there is no reliable method for locating and evaluating new steam reservoirs. Except in Yellowstone Park and in extensions of The Geysers field, no major steam discoveries have been confirmed in recent years. Unless new fields are discovered, all U.S. expansion of natural steam use will be at The Geysers. There, in 1975, the net installed generating capacity was 502 MWe, and plans at the time were to increase capacity to 1238 MWe by 1981, and ultimately to about 2000 MWe. Long delays in licensing and certification have interfered with this schedule, and the rate at which even the proven reserves at The Geysers will be developed is still in question. 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. It is evident, however, that The Geysers field is not the only such reservoir. We therefore assume here that one or more discoveries will occur, adding a total productive capacity of the order of two thirds that of The Geysers, which appears to be unique in size if not in kind. Environmental Considerations Steam reservoirs have one great environmental (and economic) advantage; the only water that must be withdrawn is the actual steam that goes through the turbines. Potential pollutants are, therefore, largely restricted to relatively volatile gases, although some dust and a few rocks come up. There is no need to deal with gargantuan volumes of dirty water and tons of silica scale. (However, The Geysers condensate does contain many environmentally harmful chemicals and must be reinjected.) The relatively small quantities of water involved and the already low pressures in these reservoirs render subsidence hazards small or nonexistent.

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems GEOPRESSURED RESERVOIRS Geopressured brine reservoirs are relatively common in deep sedimentary basins. In particular, large areas along the Gulf coasts of Texas and Louisiana are underlain by thick beds of water-filled sandstones and shales. The weight of the overlying sediments produces pressures up to several thousand pounds per square inch above normal hydrostatic pressure. The water does not circulate deeply, so that it reaches only moderately elevated temperatures, but its volume is so great that the total heat content is thousands of quads. The brine is believed in general to be saturated with natural gas, which could conceivably be an important supplement to the nation’s fuel supply. There is no direct evidence that heat or natural gas or both can be extracted economically from geopressured hot water reservoirs, but large-scale field experiments to investigate this have begun in Texas and Louisiana. 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). This is only heat in the reservoir fluid and does not account for a small amount of mechanical energy potentially recoverable by flowing the naturally pressurized water through a turbine, or for the much larger energy content of dissolved natural gas. If the 3.3 percent recovery factor derived by Papadopulos and his colleagues8 is representative for this resource, the potentially producible heat from accessible geopressured reservoirs in the United States can be estimated at 2421 quads (Table 8–2). However, relatively rapid decreases in reservoir pressure were observed when geothermal water was tapped by exploratory oil and gas wells in Gulf Coast geopressured areas, leading some reservoir engineers to believe that it may be impossible to maintain the high per-well production rates necessary to economical exploitation. Accordingly, the fraction of the heat that may actually be producible remains uncertain. There are strong differences of opinion about the technical and economic feasibility of producing commercial power from geopressured

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems reservoirs. There is, however, general agreement on several major issues. First, the resource base (Table 8–1) is large enough to deserve a thorough investigation. On the other hand, the relatively low temperatures and the high cost of drilling the necessary deep holes make it unlikely in the near future to become an economical source of heat for generating electricity. If, however, the geopressured brine is nearly or completely saturated with natural gas and if this can be economically separated, dried, and pipelined, then the credit for natural gas recovery may make this a very economical source of relatively low-grade heat. It is anticipated that by about 1981 enough information will have been collected to permit an intelligent decision on the feasibility of constructing electrical generating plants using geopressured reservoirs. As much as 25,000 MWe of generating capacity might eventually be installed along the Gulf Coast. It is, of course, also possible that the results of research now beginning will discourage this. Environmental Considerations There is little information on the chemical compositions of these brines beyond their salinities (1.5–9.0 percent), but their geological origin as coastal sediments and the presence of large amounts of methane strongly suggest large concentrations of hydrogen sulfide, a harmful air pollutant. A more certain and ultimately even more serious environmental problem is the hazard of subsidence, since the pressure in these reservoirs is due simply to the weight of the overlying land. It seems clear that subsidence problems will limit exploitation. To be sure, full reinjection of the brine might prevent subsidence, but the pumping required would use up all the captured mechanical energy and more. 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). For purposes of discussion, a gradient of 40°C/km is the dividing line between the normal gradient and hot-dry-rock resource types. The main technical barrier to exploiting this part of the geothermal resource is the lack of a method for extracting heat from deeply buried dry rock. The approach most widely investigated is to use water as a working fluid. However, much of the hot-dry-rock resource is embodied in impermeable rocks, which lack channels through which the water can flow

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems to be heated. In such a case it is necessary to create artificially a large permeable region through which the water can circulate before being withdrawn and used. Bodvarsson and Reistad9 have discussed the possibility of recovering heat by forced circulation of water through fractured fault zones or open contacts between lava beds or between dikes and the surrounding country rock. Others have investigated the creation of such regions by using conventional and nuclear explosives and by injecting water to flash explosively to steam.10,11 None has reported commercializable results. Once a large enough permeable region has been found or created, water must be injected until a steam pressure or water table high enough for extraction is built up. No one knows whether usable per-well flow rates can be achieved in this manner or how much water might have to be invested. Energy Content While information on the subject is fragmentary, it is conservatively estimated that 5 percent of the land area of the United States is underlain by rock in which the geothermal gradient is 40°C/km or more. This suggests a resource base of at least 327,000 quads of heat at temperatures above 80°C and depths less than 6 km, of which about 105,000 quads would be in rock at temperatures between 80° and 180°C and about 222,000 quads in rock hotter than 180°C. 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). There is no technical basis for assuming that any energy will be recoverable from this part of the geothermal resource. An assumed recovery factor of only 1 percent, however, would indicate 1635 quads of producible heat at depths less than 6 km, of which 525 quads would come from rock at temperatures between 80°C and 180°C and 1110 quads from rock hotter than 180°C (Table 8–2). Environmental Considerations The chief environmental problem with this scheme is that very large volumes of water may have to be invested to bring the reservoir up to producible condition. The amount required will be considerably less if the reservoir is to generate steam rather than hot water. If the rock is naturally

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems TABLE 8–3 Representative Heat, Plant, and Generating Costs for Geothermal Energy Systems of Several Types (constant 1975 U.S. dollars) Reservoir Type Temperature (°C) System Costs (dollars per kilowatt of installed capacity) Heat Cost (dollars per million Btu) Generating Cost (mills per kilowatt-hour) Field Developmenta Generating Plant Other Total Hot water (1) 90–125 130–500 none — — 0.40–2.50 (Nonelectric) Hot water (2) 150–270 160–600 400–700 15b 600–1300 0.40–1.50 15–45 Natural steam 240 150 255–280 15b 420–445 0.35 11–14 Geopressured (1) 80–200 390–450 none — — 0.75–2.00c (Nonelectric) Geopressured (2) 180–200 390–450 440–550 130–260d 970–1150 — 20–35e Normal gradient 80–200 300–700 300–600 15b 600–1300 — 30–60 Hot dry rock 180–300 230–600 300–600 15b 600–1200 — 20–40 aIncluding reinjection system. bLocal transmission (20 miles). cWithout credit for methane production. dNatural-gas separation and pipeline to market (50 miles). eWith credit for methane produced at $2 per thousand cubic feet.

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems was about $0.32 per million Btu. (This price, however, may not represent the present cost of finding, developing, and producing steam from a new field elsewhere; the 1976 cost of steam at The Geysers was 11.35 mills/kWh or $0.53 per million Btu, and the 1977 cost was then estimated at about 14.5 mills/kWh, or approximately $0.67 per million Btu.) The capital cost for Generating Unit no. 14, now being built at The Geysers, is $149 per kilowatt (electric) (kWe) (not including the wells and steam-collection system), but estimates for future units are considerably higher.14 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. Greider15 estimated the capital cost of a typical generating plant using natural steam at $210/kWe installed, with which were associated capital investments of $148/kWh for field development and $15/kWh for local transmission (20 miles), representing a total investment of $373/kWe in the complete system. (For geothermal developments in which heat is produced and used at the same site, it is usually the capital investment in the complete system that is stated as a “plant cost.” This is equivalent to including in the capital cost of a coal-fired power plant its pro rata share of the investment in the coal mine and the transportation system furnishing coal to the plant.) Greider16 estimated that electricity from such a plant should sell for about 10–13 mills/kWh at the plant bus-bar (before transmission). However, to allow for the addition of the hydrogen sulfide abatement or recovery system now required, Greider suggests that the investment in the generating plant should be increased to the range of about $255–$280/kWe (1975 dollars), which would raise the price of power somewhat HOT WATER RESERVOIRS The cost of heat from hot water geothermal systems varies widely with the depth and temperature of the reservoir, the production rates and load factors of the wells, the chemical characteristics of the fluid, and the distance from producer to consumer. For a variety of typical nonelectric uses, Towse17 gives a range of $0.372–$5.217 per million Btu, including the costs of reinjection wells and of distribution systems where needed. Omitting one obviously uneconomic case (heating a single dwelling) reduces the top of this range to $2.24 per million Btu. In general, the lowest costs are for large-scale, on-site, industrial users with steady loads; the highest are for residential heating, where individual loads are small and intermittent and distribution lines are long. By excluding other relatively small-scale space-heating applications and two cases for which wells were fairly deep and brine concentrations very high, the cost range in

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems Towse’s list is further reduced to $0.372–$ 1.184 per million Btu, with an average of $0.86. When geothermal heat from hot water wells is to be used for generating electricity, this broad range of costs is extended further by the fact that, at lower temperatures, more water is needed to deliver a given amount of heat, while the efficiency with which the heat can be used decreases rapidly. 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 Schultz18 list estimated total system costs of $1190–$2885/kWe of installed generating capacity, for a wide variety of possible power cycles, and corresponding generating costs of 26.7–64.7 mills/kWh. For water from the same reservoir, but with a lower production rate per well, Bloomster19 estimates a generating cost of 26 mills at the plant. For 150°C–200°C hot water sources in which the water flashes directly to steam for the turbine, Milora and Tester20 estimate a total system cost of $650–$1826/kWe and a generating cost of 16.1–43.0 mills/kWh. Substituting a binary cycle plant, in which the hot water is used to heat a second fluid that in turn drives the turbine, would reduce total system cost to $632–$1773/kWe and generating costs to 15.7–41.8 mills/kWh. For a 200°C low-salinity reservoir and an isobutane binary cycle, Bloomster predicts an electric power cost of 16.4 mills/kWh at the plant. Greider,21 for a typical binary system using water at 204°C, estimates the price of electricity at 16–20 mills/kWh. Apparently, for generating electricity, the higher temperature hot water systems should be competitive with energy sources of other types because both heat and power plant costs are comparatively low. (The average generating cost for conventional power plants is now about 25 mills/kWh.) However, costs increase rapidly as reservoir temperature or production rate per well decreases and as depth or salinity increases. GEOPRESSURED RESERVOIRS With the possible exception of one or two on the Gulf Coast, geopressured reservoirs are in the class of lower temperature hot water reservoirs considered above. However, in general they are at relatively great depths (3–5 km or more), and well costs are correspondingly high. So, therefore, are heat costs. For a large on-site user, Towse22 estimates a heat cost of $0.77 per million Btu for geopressured water at 82°C from wells about 2.6 km deep, including the cost of reinjection. For a similar user of higher-grade heat, he estimates a heat cost of $1.16–$1.18 per million Btu for water at 121°C from wells 4 km deep, again including reinjection costs.

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems The Gulf Coast geopressured water is generally believed to be nearly or completely saturated with dissolved natural gas. Besides contributing substantially to the fossil fuel supply of the United States, this gas, credited against the cost of producing the geothermal water, could reduce the cost of heat enough to make geopressured water an economical source of low-grade heat for direct use. Power generating costs would not be so competitive. A Dow Chemical study reported by Weeden23 estimates a heat cost of $2.00 per million Btu for geopressured water from a 4.5-km-deep reservoir at 163°C, including reinjection costs. Credit for recovering gas reduces this to $0.63 per million Btu. The study included cost estimates for a 25-MWe steam power plant with methane separators, hydraulic turbines to recover mechanical energy, and two disposal wells for each production well. The estimated total capital investment came to $2420/kWe of capacity. With credit for gas production, estimated generating costs came to 46 mills/kWh, almost twice today’s average generating cost for conventional facilities. The report concluded that electricity generation would be economical only with higher temperatures, more efficient conversion techniques, or higher natural gas prices. Other authorities have produced cost estimates that vary widely according to the water temperature, necessary drilling depth, and conversion technology. Generating costs, for example, have been estimated as low as 21 mills/kWh for certain reservoir characteristics and selected technologies. HOT DRY ROCK Since the technology for extracting heat from hot dry rock has not yet been fully developed, cost estimates are necessarily speculative. Most methods proposed for heat production from this source involve drilling an injection hole and a recovery hole to depths comparable to those required to reach geopressured reservoirs, so that in similar formations drilling costs might be similar for the two cases. At least initially, however, hot-dry-rock systems will probably be developed in igneous or metamorphic rock, in which drilling costs should be much higher than in the soft Gulf Coast sediments. No salable by-product such as methane would be expected from such a system, but at least in part this and higher per-foot drilling costs should be countered by the higher temperatures reached at a given depth. Bodvarsson and Reistad24 suggest that recovering heat by circulating water through fractured fault zones or open contacts between lava beds or

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems between dikes and the surrounding country rock may be at least marginally economic for producing low-temperature heat where the geothermal gradient is only normal (giving, for example, a rock temperature of 135°C at a depth of 3.5 km). The economic potential is significantly better where the gradient is higher. For a hydraulically fractured system in granite at a depth of 3.9 km and a temperature of 250°C, Milora and Tester25 estimate a total capital cost of $625/kWe (1975 dollars) for a 100-MWe binary cycle plant, and a generating cost of 15.6 mills/kWh. 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 for hot dry rock are within the range of similar estimates for natural hot water systems where reinjection is required, they of course assume the success of a technology that has not yet 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) have already been treated indirectly, in the discussion of hot dry rock. Costs would be increased, of course, by the need to explore and drill to greater depths and by a small decrease in production rates per well, because of the pressure drop involved in producing fluid through a longer string of casing. With the flexibility in location provided by normal-gradient systems and with the cost decreases expected from improved drilling and energy conversion technology, these deep natural energy supplies may become economic in the future, if a proven technology becomes available. MOLTEN MAGMA No estimates for the costs of heat and electric power from molten rock will be possible until at least a primitive heat-extraction technology has been developed. It can only be hoped that the high intensity of the heat in molten magma will compensate for the probably high cost of the sophisticated systems that will be required to recover it.

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems FUTURE DEVELOPMENT OF THE GEOTHERMAL RESOURCE Besides the technical and economic obstacles already described, geothermal energy faces a number of financial and institutional barriers. The extent to which these difficulties will be solved in the future, and the costs of doing so, are extremely uncertain. At present it is possible only to identify the major problems and the general development trends that could be expected to follow from their solutions. INSTITUTIONAL PROBLEMS In addition to the technical and economic problems of locating and exploiting the different kinds of geothermal resources, development of a geothermal industry faces a number of institutional constraints. Federal leasing policy is an especially important one. One feature of federal law that tends to retard development is the 20,000-acre maximum lease size for any company in any one state. The leasing process itself is long and costly enough to bar small firms and to substantially increase the costs of even the large successful bidders. Furthermore, the time lag between lease application and competitive sales is now about 3–4 years. Leasing is further complicated by the fact that resource areas are often broken up among federal, state, and private ownership. This means that a given project might be subject to regulation by authorities from federal to local levels, with disparate or conflicting standards. Another delaying influence is the tax treatment of geothermal development. In general such development is not eligible for many of the tax benefits that oil and gas development receive, even though they all require similar drilling operations. 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. Most of the former are oil companies, who have shown a preference for fast-write-off, high-risk opportunities. The latter are utilities, whose long planning horizons bias them in favor of low-risk, regulated, fixed rates of return. This may produce conflicts between producers and users in negotiating contracts for steam. All of these institutional problems must be dealt with along with the

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems technical and economic ones to which each geothermal resource type is subject. The extent to which federal and state leasing policies, taxations, and regulation can be rationalized will largely determine the effectiveness of current and future research, development, and demonstration in exploration and production technologies. It will also affect the costs of production and therefore the competitiveness of the price at which geothermal energy can be sold. FUTURE TRENDS IN GEOTHERMAL ENERGY PRODUCTION The Supply and Delivery Panel26 constructed the three potential production scenarios shown in Table 8–4. The numbers are of course extremely speculative, but they serve to illustrate the range of possibilities under three levels of effort aimed at reducing technical, economic, and institutional constraints. In the business-as-usual scenario all present constraints remain in effect. The national-commitment scenario assumes that geothermal energy is given a high priority and that most institutional problems are corrected, that financial incentives such as loan guarantees are offered to the industry for high-risk projects, that a strong and consistent demonstration plant program is enacted, and that federal funding greatly improves the technologies for exploration. The enhanced-supply scenario falls between these two extremes. 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 brines. 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. The maximum potential realizable by 2010 with a national commitment is much lower than that of nuclear fission, and substantially lower even than what is at least theoretically achievable with a national commitment to all areas of solar development. CONCLUSIONS ON GEOTHERMAL ENERGY Sources of geothermal energy are not indefinitely sustainable in the same sense as solar energy. However, their total energy is so large that their potential as an energy source will depend mainly on their economic producibility, not on resource considerations. At present, the only usable geothermal resources are deposits of hot water or natural steam. In the long-term future, it may be possible to

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems TABLE 8–4 Estimated Installed Geothermal Energy Capacity for Generating Electricity and for Nonelectrical Uses (megawatts and quads) Year Installed Generating Capacity (megawatts) Equivalent (quads)a Heat Required (quads) Hot Water Natural Steam Geopressured Normal Gradient Hot Dry Rock Molten Magma Total Electricalb Nonelectrical Total Business as usual 1990 720 1,800 200 10 200 0 2,930 0.09 0.35 0.02 0.37 2010 7,800 4,500 2,100 710 3,700 60 18,870 0.56 2.26 0.18 2.44 Enhanced Supply 1990 1,000 2,500 310 10 260 10 4,090 0.12 0.49 0.05 0.54 2010 15,400 5,000 3,100 1,510 6,800 360 32,170 0.96 3.85 0.32 4.17 National Commitment 1990 2,600 4,000 600 60 1,000 10 8,270 0.25 0.99 0.06 1.05 2010 25,600 6,500 4,200 2,600 21,000 1,000 60,900 1.85 7.28 0.96 8.24 aQuads per year assuming continuous operation of full generating capacity. bQuads per year assuming 20 percent overall conversion efficiency and 80 percent load factor.

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems extract heat from the natural thermal gradient in the earth’s crust and from unusually hot rock formations lying close to the earth’s crust. As there is no demonstrated technology for using these resources, the cost and the amount of energy that might be producible can be only grossly estimated. The use of dry rock depends on developing a fracture system large enough to be economical as a source of heat. The possibilities of achieving this, and the environmental effects of doing so, are speculative. The only widespread potential geothermal resource, the natural thermal gradient, is the most speculative in practical exploitability. As an indefinitely sustainable source, it also suffers the inherent disadvantage that the normal heat flux from the inside of the earth is only about 1/1000 of the solar energy flux falling on the same area. 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. If the heat and gas can be exploited simultaneously, this might be an attractive resource. Too little is known about it today: Considerable effort is justified in assessing its potential. Considered in all of its potential, the geothermal resource represents extremely large amounts of energy. However, for a variety of technical, economic, geographical, and institutional reasons geothermal energy will probably not be a major contributor to the national energy system until well into the twenty-first century, if ever. It may, however, become an important source of inexpensive heat for localized use at relatively small scales. While it warrants serious exploration and continued development, it cannot be considered among the most important of the long-term energy alternatives. NOTES    1. D.W.White and D.L.Williams, eds., Assessment of Geothermal Resources of the United States—1975, U.S. Geological Survey Circular 726 (Washington, D.C.: U.S. Government Printing Office, 1975).    2. Ibid.    3. J.H.Howard, ed., Present Status and Future Prospects for Nonelectrical Uses of Geothermal Resources (Berkeley, Calif.: Lawrence Livermore Laboratory (UCRL-51926), 1975).    4. White and Williams, eds., op. cit.    5. T.C.Urban, W.H.Diment, J.H.Sass, and I.M.Jamieson, “Heat Flow at The Geysers, California, U.S.A.,” in Proceedings, Second United Nations Symposium on the Development and Use of Geothermal Resources, available from Superintendent of Documents (Washington, D.C.: U.S. Government Printing Office, 1975).    6. M.J.Reed and G.E.Campbell, “Environmental Impact of Development in The Geysers Geothermal Field, U.S.A.,” in Proceedings, Second United Nations Symposium on

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems    the Development and Use of Geothermal Resources, available from Superintendent of Documents (Washington, D.C.: U.S. Government Printing Office, May 1975).    7. White and Williams, eds., op. cit.    8. S.S.Papadopulos, R.H.Wallace, Jr., J.B.Wesselman, and R.E.Taylor, “Assessment of Onshore Geopressured Geothermal Resources in the Northern Gulf of Mexico Basin,” in D.E.White and D.L.Williams, eds., Assessment of Geothermal Resources of the United States—1975, U.S. Geological Survey Circular 726 (Washington, D.C.: U.S. Government Printing Office, 1975).    9. G.Bodvarsson and G.M.Reistad, “Econometric Analysis of Forced Geoheat Recovery for Low-Temperature Uses in the Pacific Northwest,” in Proceedings, Second United Nations Symposium on the Development and Use of Geothermal Resources, available from Superintendent of Documents (Washington, D.C.: U.S. Government Printing Office, May 1975).    10. Y.D.Diadkin and Y.M.Pariisky, “Theoretical and Experimental Grounds for Utilization of Dry Hot Rock Geothermal Resources in the Mining Industry,” in Proceedings. Second United Nations Symposium on the Development and Use of Geothermal Resources, available from Superintendent of Documents (Washington, D.C.: U.S. Government Printing Office, May 1975).    11. M.C.Smith, R.L.Aamodt, R.M.Potter, and D.W.Brown, “Man-Made Geothermal Reservoirs,” in Proceedings, Second United Nations Symposium on the Development and Use of Geothermal Resources, available from Superintendent of Documents (Washington, D.C.: U.S. Government Printing Office, May 1975).    12. R.L.Smith and H.R.Shaw, “Igneous-Related Geothermal Systems,” in D.E.White and D.L.Williams, eds., Assessment of Geothermal Resources of the United States—1975, U.S. Geological Survey Circular 726 (Washington, D.C.: U.S. Government Printing Office, 1975).    13. F.J.Dan, D.E.Hersam, S.K.Kho, and L.R.Krumland, “Development of a Typical Generating Unit at The Geysers Geothermal Project—A Case Study,” in Proceedings, Second United Nations Symposium on the Development and Use of Geothermal Resources, available from Superintendent of Documents (Washington, D.C.: U.S. Government Printing Office, May 1975).    14. Dan, Hersam, Kho, and Krumland, op. cit.    15. B.Greider, “Status of Economics and Financing of Geothermal Energy Power Production,” in Proceedings, Second United Nations Symposium on the Development and Use of Geothermal Resources, available from Superintendent of Documents (Washington, D.C.: U.S. Government Printing Office, May 1975).    16. B.Greider, personal communication, January 1977.    17. D.F.Towse, “Economic Considerations” in J.H.Howard, ed., Present Status and Future Prospects for Nonelectrical Uses of Geothermal Resources (Berkeley, Calif.: Lawrence Livermore Laboratory (UCRL-51926), 1975).    18. D.G.Swink and R.J.Schultz, Conceptual Study for Total Utilization of an Intermediate Temperature Geothermal Resource, (Idaho Falls, Idaho: Aerojet Nuclear Co. (ANCR- 1260), April 1976).    19. C.H.Bloomster, “An Economic Model for Geothermal Cost Analysis,” in Proceedings, Second United Nations Symposium on the Development and Use of Geothermal Resources, available from Superintendent of Documents (Washington, D.C.: U.S. Government Printing Office, May 1975).    20. S.L.Milora and J.W.Tester, Geothermal Energy as a Source of Electric Power (Cambridge, Mass.: MIT Press, 1976).    21. Greider, “Status of Economics and Financing of Geothermal Energy Power Production,” op. cit.

OCR for page 397
Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems       22. Towse, op. cit.    23. S.L.Weeden, “Geopressured Geothermal Energy—Will It Work?” Ocean Industry, June 1976, pp. 119–126.    24. Bodvarsson and Reistad, op. cit.    25. Milora and Tester, op. cit.    26. National Research Council, U.S. Energy Supply Prospects to 2010, Committee on Nuclear and Alternative Energy Systems. Supply and Delivery Panel (Washington, D.C.: National Academy of Sciences, 1979).