252 



UNITED STATES MINERAL RESOURCES 



to the ground surface. Under present technology, 

 rocks with too few pores, or with pores that are not 

 connected, do not constitute an economic geothermal 

 reservoir, however hot the rocks may be. 



Water in a geothermal system also serves as the 

 medium by which heat is transferred from a deep 

 igneous source to a shallow geothermal reservoir at 

 depths shallow enough to be tapped by drill holes. 

 Geothermal reservoirs are located in the upflowing 

 parts of major water convection systems. Cool rain- 

 water percolates underground from areas that may 

 consist of tens to thousands of square kilometers. 

 At depths of 2-6 km, the water is heated by contact 

 with hot rocks (in turn probably in contact with 

 molten rock). The water expands upon heating and 

 then moves buoyantly upward in a column of rela- 

 tively restricted cross-sectional area (1-50 km=). 

 The driving force of these large circulation systems 



is gravity, eifective because of the density difference 

 between cold downward-moving recharge water and 

 hot, upward-moving geothermal water. 



EXPLOITATION 



The primary use of geothermal resource to date 

 is for the generation of electricity (fig. 27). Under 

 existing technology, geothermal steam (after sepa- 

 ration of any associated water) is expanded into a 

 low-pressure (5-7 bar) turbine which drives a con- 

 ventional electrical generator. Geothermal heat is 

 also used directly (table 49) in the heating and air- 

 conditioning of buildings, in the heating of hot- 

 houses and soil for agricultural purposes, and in 

 product processing. In addition, warm waters from 

 springs and wells are widely used for bathing, rec- 

 reational, and therapeutic purposes, particularly in 

 central Europe and in Japan (Komagata and others. 



400 

 300 - 

 200 - 

 100 - 





 300 



200 

 100 - 



200 



100 



I 



I 



NEW ZEALAND 



j 



I 



.^J 



Figure 27. — Growth of geothermal generating capacity by countries 1900-72. Sources — Italy: [Italy] ENEL, [1970]. U.S.A.: 

 Bruce (1971). New Zealand: McKenzie and Smith (1968) ; Smith (1970). Japan: Hayashida and Ezima (in press) ; Mori 

 (in press). Mexico: Alonso-Espinosa and others (1968). U.S.S.R.: Tikhonov and Dvorov (1970); Facca (in press). Ice- 

 land: Ragnars and others (1970). 



