Glossary of terms
Geothermal power plants are generally of two types: the dual flash and the binary. In the dual flash system superheated water (above 212 degrees Fahrenheit) is released from pressure near the surface, where it "flashes" into steam in a separator. The water that does not turn to steam is sent to a second separator where the pressure is further reduced and another portion of it flashes to steam. The steam from the separators is directed to the turbines. The steam turns the turbines, which in turn rotate an electric field inside the generator, creating electricity. The steam is cooled and condensed into water and injected into the geothermal reservoir.
In the binary system or heat exchange system the geothermal water is pumped through a heat exchanger and injected back into the geothermal reservoir. The heat exchanger contains a second fluid that has a low boiling point, and the heat from the geothermal water is transferred to this second fluid, making it boil and vaporize. It is this vapor which pushes against the turbine blades. The vapor is condensed and recycled through the heat exchanger. This method enables the use of lower temperature geothermal water to produce electric power.
Nuclear power plants are generally of two types; those with a pressurized-water reactor and those with a boiling-water reactor. Both types of plants use steam to drive turbines which in turn generate electricity. In a pressurized-water reactor, water is heated by a nuclear reaction, but is kept under pressure to avoid turning to steam. The heat from this pressurized water is transferred to another system of water pipes where steam is allowed to form (and is then routed to the turbines). In a boiling-water reactor, the heat from the nuclear sources is allowed to boil the water it comes in contact with, turning it to steam. This steam is routed to the turbines, and then condensed, to be routed back to the boiler again.
Water withdrawal requirements at fossil-fuel and nuclear power generation facilities depend primarily on whether or not the cooling water is recirculated. "Once through cooling" requires the largest amounts of water withdrawal because it is not recirculated within the facility. The water is withdrawn from a source, circulated through the heat exchangers, and then returned at a higher temperature. This technology was common in older facilities, but is outdated for new facilities because of increasingly restrictive thermal requirements, such as tighter thermal controls on return water.
The more water-efficient alternatives to once-through cooling include cooling ponds and cooling towers. A cooling pond is a shallow reservoir with a large surface area to remove heat from circulation water. The rate of heat loss may be enhanced through the use of spray nozzles. Cooling ponds are used where land is relatively inexpensive, cooling water is limited, or thermal loading is restricted. Cooling ponds are advantageous over once-through cooling because the circulation water in the system can be reused, thus reducing the overall water-withdrawal requirement.
A cooling tower is a structure designed to remove heat from water. The heated circulating water is sprayed into the tower, exposing the water droplets to the cooling process of evaporation. Cooling is due to radiation from the sides of the tower, contact of the water with the cooler air, and evaporation of the water. Cooling towers are commonly used where land and water are expensive, or local regulations prohibit the release of thermal waters.
If it is not possible to get withdrawal amounts from a power plant directly or from the USDOE, a coefficient to estimate the gallons of water used per kilowatt hour of electricity generated may be calculated. This coefficient can be multiplied by the amount of electricity generated over a specified time period for plants of similar heating processes and cooling methods.
The net annual production of electricity, in gigawatt-hours, will also be reported. Some power plants may report power production in kilowatt- or megawatt-hours that will have to be converted to gigawatt-hours. Some industrial facilities also generate thermoelectric power. These facilities are called `cogeneration' facilities. The amount of water used by the industry to generate electricity should be reported as a power generation water use, if the data provided from the industrial facility is sufficient to differentiate water used in power generation and the amount of water used for the industrial processes of the plant.
Some of the electricity generated at a thermoelectric power plant is used to run the power plant itself. It is important to ask a plant operator if the estimates the gross or net amount of power produced. The 1995 EUOWITUS effort is requesting the net amount of power produce.
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Denver Research Institute, 1981, Water and energy in Colorado's future: the impacts of energy development on water use in 1985 and 2000: Boulder, Colo., Westview Press, 303 p.
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Office of Management and Budget, 1987, Standard industrial classification manual, 1987: Washington, D.C., U.S. Government Printing Office, 705 p.
U.S. Department of Energy, 1994, Electric Power Annual, 1993: Washington, D.C., U.S. Department of Energy, Energy Information Administration Publication DOE/EIA-0348(93), 200 p.
U.S. Department of Energy, 1994, Electric Power Monthly, December 1994: Washington D.C., U.S. Department of Energy, Energy Information Administration Publication DOE/EIA-0226 (94/12), 212 p.
U.S. Department of Energy, 1994, Inventory of Power Plants in the United States, 1993: Washington, D.C., U.S. Department of Energy, Energy Information Administration Publication DOE/EIA-0095 (93), 400 p.