Cooling systems are often the largest source of water use in power plants because of the large amount of heat that must be removed to condense the steam used to drive turbine generators. Historically, this cooling was provided by water sources such as rivers and lakes, but the number of power plants using dry cooling—a cooling system that uses little to no water—has increased in recent years. Dry cooling systems have relatively high capital costs and require more energy to operate. These factors result in lower overall power plant efficiency, but dry cooling systems use about 95% less water than wet systems.
Many types of power plants generate electricity by boiling water to produce steam, which is then passed through a turbine. Plants that burn coal and biomass, nuclear plants, some natural gas plants, and even some solar facilities use this type of system. Once the steam has passed through the turbine at these plants, it must be cooled to condense back to a liquid, which is returned to the boiler or steam generator.
Most steam-generating plants in the United States use water to cool and condense steam. According to the U.S. Geological Survey, electric power generation accounts for about 40% of total water withdrawals in the United States, much of which is used for cooling.
More than 61% of the thermoelectric generating capacity in the United States uses recirculating cooling systems that reuse cooling water. These systems keep water in closed-loop piping so that the water can be used repeatedly. Plants that use once-through cooling systems account for 36% of U.S. thermoelectric generating capacity. These systems withdraw large amounts of water from nearby bodies of water to cool the condenser, and then they discharge the water back to the original source at higher temperatures.
Dry and hybrid cooling account for 3% of U.S. thermoelectric generating capacity, most of which has come online since 2000. Dry cooling systems use ambient air to cool and condense steam. These systems are classified into two types: direct and indirect systems. In direct dry cooling systems, steam is condensed using ambient air, meaning no water is consumed. In indirect dry cooling systems, steam is condensed in conventional water-cooled condensers, but the cooling water is kept in a closed system. As a result, no water is lost to evaporation, which means very little water is used.
Hybrid cooling systems are a mix between dry and wet cooling, and they can use both water and air to condense steam. These systems are typically designed to be operated as dry cooling systems during the cooler seasons and as wet cooling systems during the hotter seasons when dry systems have lower efficiency.
The 83 plants operating dry and hybrid cooling systems in the United States support about 20 gigawatts (GW) of steam-generating capacity. California has the highest number of plants with dry cooling systems (13), but Texas has the most dry cooling capacity (2.8 GW), closely followed by Virginia (2.4 GW).
The most common generating technology is the natural gas combined cycle (NGCC), representing more than 83% of the operating dry and hybrid cooling capacity. Dry cooling systems tend to be more economical for natural gas combined-cycle plants because the amount of cooling needed is much less per megawatthour than for coal or nuclear plants. More than 15% of operating generating capacity from natural gas combined-cycle plants in the United States use dry cooling technology. In the past five years (2013–2017), almost 4.3 GW of NGCC capacity using a dry cooling system came online.
Dry cooling can also be an attractive option for concentrated solar power systems. Because these systems are located in areas such as the southwestern United States, where solar resources are relatively high and water resources are relatively low, many new concentrated solar power systems have dry cooling, such as the Ivanpah and Genesis Solar plants in California and the Crescent Dunes Solar plant in Nevada.
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