256 



UNITED STATES MINERAL RESOURCES 



productivity are unlikely to be abundant. White 

 (1965) estimated resources recoverable as elec- 

 tricity to 3 km to be 2 X 10" calories (=2.32x10" 

 kwhr). This energy is less than one ten-millionth of 

 the total amount of heat above 15°C in the outer 

 10 km of the earth (White, 1965, p. 2) . 



Utilization of a greater proportion of the heat 

 stored in the outer 10 km of the earth depends on 

 achieving one or more of the following items : 



1. Technological advances that would allow elec- 



trical generation from low-temperature reser- 

 voirs. 



2. Breakthroughs in drilling technology that would 



permit low-cost drilling of holes to depths 

 greater than 3 km. 



3. Development of techniques of artificial stimula- 



tion that would increase the productivity of 

 geothermal reservoirs. 



4. Expansion of the use of low-grade geothermal 



resources for such purposes as space heating, 

 product processing, agriculture, and desalina- 

 tion. 

 Several of these breakthroughs may occur in the 

 reasonably near future; if they do, the recoverable 

 resource estimates of White (1965) will have to be 

 revised upward to reflect the major changes in 

 basic assumptions. Four possible breakthroughs de- 

 serve specific mention: 



1. Much attention is currently being paid to the 



possible generation of electricity from low- 

 temperature geothermal waters, using a sys- 

 tem whereby the geothermal heat is used in a 

 heat exchanger to boil a secondary fluid such as 

 isobutane or freon. This low-boiling fluid (as a 

 gas) drives a turbine, is condensed, and then 

 returns to the heat exchanger (Jonsson and 

 others, 1969). A generating unit based on the 

 heat-exchange principle and using intake wa- 

 ter at 81°C is reported to be in pilot operation 

 at Paratunka, Kamchatka, U.S.S.R. (Facca, 

 in press). U.S. industry interest in this gen- 

 erating mode is high (Anderson, 1973), al- 

 though no pilot or prototype plant has yet 

 been built. 



2. Successful demonstration of the technical feasi- 



bihty of geothermal self-desalination (U.S. 

 Bur. Reclamation, 1972) could greatly en- 

 hance the economic position of geothermal 

 resources. Particularly in water-short parts 

 of the world, geothermal energy may be the 

 preferable energy source for desalination, 

 either of the geothermal brine itself or of other 

 saline waters near the geothermal develop- 

 ment. 



3. Research at the LASL (Los Alamos Scientific 



Laboratory) has recently been focused on the 

 development of a nuclear drill that would bore 

 holes in rock by progressive melting rather 

 than by chipping, abrading, or spalling 

 (Smith, 1971). If development of this "nu- 

 clear subterrene" is successful and its use is 

 relatively inexpensive, extraction of geother- 

 mal energy from depths as much as 10 km 

 may become feasible. 



4. One possible application of the nuclear subter- 



rerf^ proposed by LASL is to drill to depths 

 gr.. ir than 5 km in regions where tempera- 

 tures may be abnormally high but where per- 

 meability is low. LASL proposes to hydrofac 

 the hot rock to increase permeability and 

 expects that extraction of heat by water cir- 

 culated through the crack will result in 

 thermal-stress cracking and a continuously 

 enlarging crack system (Aamodt and Smith, 

 1973; Harlow and Pracht, 1972). Geothermal 

 reservoir stimulation by various methods (in- 

 cluding nuclear devices) was recently the sub- 

 ject of a symposium of the American Nuclear 

 Society (Kruger and Otte, 1973). 

 Another confusing aspect of geothermal resource 

 estimation involves the units of energy in which 

 the estimates are expressed by various authors. 

 Geothermal energy is heat, and the resource and 

 reserve estimates therefore should be expressed in 

 calories, joules, or Btu's. But historically the major 

 use of geothermal energy has been to generate elec- 

 tricity, and resource estimates commonly have been 

 expressed in units of electrical energy (kilowatt- 

 hours or megawatt-years) or in terms of installed 

 electrical capacity (kilowatts or megawatts). In 

 converting from calories to kilowatt-hours, how- 

 ever, one cannot blindly use the energy conversion 

 factors given in standard tables (for example. 

 Handbook of Chemistry and Physics) . These con- 

 version factors, although mathematically and physi- 

 cally accurate, do not take into account the ther- 

 modynamic inefficiencies in converting heat to 

 electricity via a turbine and generator. For exam- 

 ple, in units 3 and 4 at The Geysers, Calif, (a vapor- 

 dominated geothermal system), only 14.3 percent 

 of the energy delivered to the turbine is actually 

 converted to electricity (Bruce, 1971). Almost all 

 the remaining 85.7 percent is discharged as heat 

 to the atmosphere, with only a small fraction of 

 heat being returned to the reservoir in condensate 

 from the cooling towers. 



The best fossil-fuel generating plants in the 

 United States have a thermal efficiency of about 



