How it Works: Water for Power Plant Cooling
In the United States, 90 percent of electricity comes from conventional thermoelectric power plants—coal, nuclear, natural gas, and oil—that require cooling. The remaining ten percent is produced by hydroelectric and other renewable energy facilities. Some renewable energy technologies are thermoelectric as well, including certain types of concentrating solar, geothermal, and biomass power plants.
Contents:
1. Why is Cooling Necessary?
2. Types of Cooling
3. Electricity Generation Technologies that Use Cooling
4. Other Key Issues
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 [1]
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.[2] 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, 92 percent of all U.S. dry-cooling installations were in smaller power plants and most commonly in natural gas combined-cycle power plants.[3]
According to estimates from the U.S. Department of Energy's National Energy Technology Laboratory, 43 percent of current thermoelectric power plants use once-through cooling, 56 percent use wet-recirculating, and 1 percent use dry-cooling.[4]
Table 1: Water withdrawn and consumed for power plant cooling, in gallons of water required per megawatt-hour of electricity produced[5]
(Click to enlarge)
Electricity Generation Technologies that Use Cooling
Conventional Electricity
- Coal
- Nuclear
- Natural Gas
- Oil
Renewable Electricity
- Concentrating solar (solar thermal electric)[6]
- Geothermal
- Biomass
Other Key Issues
- Siting: The geographic location of power plants has a huge impact on cooling technology options, water availability, type of water used for cooling, and environmental impacts. Solar and geothermal power plants, for example, must be sited in areas with high solar radiation and geothermal energy, respectively—locations that may be arid and far from conventional water resources. In these situations, dry cooling may be an option, or alternative water sources may be available, but such choices can affect power plant performance and local environments.
- Water type: Although many power plants use freshwater for cooling, waste water and salt water are other possibilities with advantages and disadvantages. Salt water is an obvious and abundant option for coastal power plants, for example, but such plants face similar challenges as inland plants with regard to damaging the local aquatic ecosystems through excessive withdrawals or thermal pollution.
[4] DOE. 2009. Water Requirements for Existing and Emerging Thermoelectric Plant Technologies.
[5] U.S. Government Accountability Office (GAO). 2009. Energy-Water Nexus: Improvements to Federal Water Use Data Would Increase Understanding of Trends in Power Plant Water Use.
