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natural system. A thorough examination of the question of unit consumption of water in power-plant cooling is not warranted here, but expert opinion ranges from Cootner and Lars (1965, p. 58) observation that water loss from a receiving stream in once-through cooling is nearly the same as in a recycle system, to an estimate by the Water Resollrces COllllcil (1968, p.4-3-2) that cooling towers have consumptive use roughly twice that of once-through systems. These differences stem from a general lack of information on evaporation from open water bodies. Although makeup water for recycling sys- tems can be measured directly with relative ease, precise measurements of evaporation from open water bodies is very difficult. Moreover, where water is in abundant sup- ply, as where once-through cooling is employed, the question of consumptive use is rather academic. Further- more, much of the consumption of water associated with once-through systems is of saline water, mainly sea- water, or from the Great Lakes where such consumption is :I sm311 consideration. Indeed, in 1970 withdrawals of saline water (MlIrray and Reeves. T972, p. 7) comprised 28 percent of the total withdrawals for thermal-electric power. This figure itself probably is disproportionately low because many power plants drawing water from estuaries or downstream of competing users are classed in Federal Power Commission reports as freshwater with- drawals, although thiS water would soon waste to the sea ifnot used in this way. Consumptive use becomes a serious consideration only where it is in competition with other socially bene- ficial water consumption. Thus, the main focus on con- sumptive use by electric-power plants and other energy industries is in the West where freshwater has high value for alternative uses. The table below shows the average evaporative requirement of modern thermal.electric plants by vari- ous classes. In each instance, most efficient design is assumed. As noted earlier, water consumed per kwhr (kilowatt hour) is governed mainly by thermal effI- ciency, although the type of cooling system employed may also affect consumption to some degree. Little future improvement can be expected in fossil-fueled plants, which already are crowding theoretical thermal- efficiency limits. Similarly, little improvement can be expected with geothermal systems, which arc con. strained by the relatively low temperature and pressure of the natural steam sources tapped. Nuclear generation, however. has a potential for significant improvements in thermal efficienc.y and water requirements. High- temperature gas-cooled reactors (HTGR) now coming into use are expected to have an overall thermal effi- ciency of 40 percent, as are breeder reactors now being planned. The other principal types of energy conversion of con- cern with respect to water consumption are conversion of oil shale to oil, coal to gas (coal gasification), and coal to oil (coal liquefaction). 01 L SHALE Oil shale may be mined either in open pit or in under. ground mines and then retorted on the surface. Below- ground retorting experiments have been tried using several methods, but until recently in situ processes were not claimed to be competitive with mining and above- ground plants. However, late in 1973 one firm announced high recoveries of oil and lower cost for a combination of underground mining and in situ retorting io which 25-30 percent of the shale is mined and retorted on the surface. The remaining shale is fractured, collapsed into the mined-out void, and retorted where it lies. Although a commercial-sized plant of any kind remains to be built, technology may change rapidly dur- ing the next decade. The operators of the two Federal Prototype Leases in Colorado probably will use above-ground processing. Pilot plants have tested many methods of mining and above-ground and in situ retorting, but none of the processes has been done on a commercial scale. The pilot mines and plants have handled a few tons of shale up to as much as 1,000 or more tons (907 metric tons) per day; the Department of Interior's Prototype Lease COflSlimplille demand of ",Qler-cuoled IlrermaJ.eleClric pial/IS He:Jt Thermal Atmosphem EV:Jporative Gallons Type' rail' efflcienl:Y dissipation Percent dissipation consumed (Btu/kwhr) (perccnl) (Btufkwhr) (Btufkwhr) per kwhr Fos~i] fueled 9,000 38 t.350 15 4.230 0.5 Nuclear to.700 32 535 5 6,741 ,8 Geotherm<J1 ... 24.000 14 1,200 5 19,440 I.B , M()~I eflid~nt design. At normal operating rales (80 pen:.:nt load factor), the water COnSlll1lption or thes.e types or plants is :Jppruximatl'ly 15 :Jrre-fljyc;]rjmw capacity for fossil-fueled, 22.acre-ffjyearjmw for nuclt'ar. and 48 acre.fljyear/mw for geothermal plants. 8