Why is Cooling Necessary?



Thermoelectric power plants boil water to create steam, which then spins turbines to generate electricity. The heat used to boil water can come from burning of a fuel, from nuclear reactions, or directly from the sun or geothermal heat sources underground. Once steam has passed through a turbine, it must be cooled back into water before it can be reused to produce more electricity. Colder water cools the steam more effectively and allows more efficient electricity generation.


Types of Cooling

Even though all thermoelectric plants use water to generate steam for electricity generation, not all plant cooling systems use water. There are three main methods of cooling:

Once-through systems take water from nearby sources (e.g., rivers, lakes, aquifers, or the ocean), circulate it through pipes to absorb heat from the steam in systems called condensers, and discharge the now warmer water to the local source. Once-through systems were initially the most popular because of their simplicity, low cost, and the possibility of siting power plants in places with abundant supplies of cooling water. This type of system is currently widespread in the eastern U.S. Very few new power plants use once-through cooling, however, because of the disruptions such systems cause to local ecosystems from the significant water withdrawals involved and because of the increased difficulty in siting power plants near available water sources.
Wet-recirculating or closed-loop systems reuse cooling water in a second cycle rather than immediately discharging it back to the original water source. Most commonly, wet-recirculating systems use cooling towers to expose water to ambient air. Some of the water evaporates; the rest is then sent back to the condenser in the power plant. Because wet-recirculating systems only withdraw water to replace any water that is lost through evaporation in the cooling tower, these systems have much lower water withdrawals than once-through systems, but tend to have appreciably higher water consumption. In the western U.S., wet-recirculating systems are predominant.
Dry-cooling systems use air instead of water to cool the steam exiting a turbine. Dry-cooled systems use no water and can decrease total power plant water consumption by more than 90 percent. The tradeoffs to these water savings are higher costs and lower efficiencies. In power plants, lower efficiencies mean more fuel is needed per unit of electricity, which can in turn lead to higher air pollution and environmental impacts from mining, processing, and transporting the fuel. In 2000, most U.S. dry-cooling installations were in smaller power plants, most commonly in natural gas combined-cycle power plants.
About 43 percent of thermoelectric generators in the United States use once-through cooling, 56 percent recirculating, and 1 percent dry-cooling (2008 data). In 2008, some 30 percent of electricity generation involved once-through cooling, 45 percent recirculating cooling, and 2 percent dry-cooling. (In some cases, those same power plants also produced electricity using non-steam systems, such as combustion turbines.)


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