Waste heat disposal from U.S. geothermal power plants—An update

Abstract

Dissipation of the heat rejected from geothermal power plants is a major concern because the inherently low efficiencies result in heat rejection rates that are three to four times greater per kW of installed capacity than is typical of fossil- or nuclear-fueled stations. The most cost-effective methods of waste heat dissipation involve the evaporation of water, yet most of the important hydrothermal resources of the U.S. are located in areas where cooling tower makeup water for power plants is in short supply. Flashed-steam power cycles can use condensate derived from the geofluid for tower makeup unless reinjection is necessary, as is already required at some sites. Condensate is not available from binary cycles because the geofluid is reinjected. Geothermal station makeup water requirements have been estimated at 50–100 m 3/yr per kW of electrical capacity. Some of the more interesting and significant methods that are currently being studied in the U.S. for reducing waste heat dissipation system costs and water consumption are (1) allowing plant power output to vary with ambient conditions, (2) use of ammonia to transport waste heat from the turbine condenser to air-cooled coils, (3) development of a plastic-membrane type wet/dry tower, (4) marketing of steam turbines that can tolerate a wider range of back pressures, (5) use of circulating water storage to delay heat dissipation until more favourable ambient conditions exist, (6) development of tubes with enhanced heat transfer surfaces to reduce condenser capital costs, and (7) use of evaporative condensers to reduce costs in binary cycles. Many of these projects involve large-scale tests that are now installed and producing some preliminary data. Definitive results from some of the tests may not be available until mid-1982 or later.