Energize Weekly, September 5, 2018
Power generation is one of the biggest consumers of water—used for cooling—in the U.S., but a small, though increasing number of plants are using dry cooling technology, according to the federal Energy Information Administration (EIA).
Dry-cooled and hybrid plants, which use both water and air cooling, account for only 3 percent of current steam-powered generating capacity, but they are set to make up a larger portion of generation capacity between 2018 and 2022.
In part, this is because these systems are more compatible with natural gas combined-cycle plants, which are now being built, the EIA said. Combined-cycle plants account for 83 percent of the operating dry and hybrid cooling systems.
“Dry cooling systems tend to be more economical for natural gas combined-cycle plants because the amount of cooling needed is much less per megawatt-hour than for coal or nuclear plants,” the EIA said.
More than 15 percent of natural gas combined-cycle plants in the U.S. use dry cooling technology. Between 2013 and 2017, nearly 4.3 gigawatts (GW) of natural gas-fired plants with dry cooling systems came online.
The 83 plants operating dry and hybrid cooling systems account for about 20 GW of steam-generating capacity. California has the highest number of plants with dry cooling systems—13. Texas has the most dry cooling capacity—2.8 GW. Virginia is a close second with 2.4 GW.
Dry cooling systems, which use ambient air to cool hot steam coming out of generating turbines, have comparatively high capital costs and require more energy to operate, which reduces overall plant efficiency. These systems, however, use 95 percent less water than wet systems.
Hybrid cooling systems are a mix that are able to use 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 EIA said.
Plants that boil water to power steam turbines have to cool that steam, condense it back to liquid and return it to the boiler or steam generator. Water has been the prime coolant used, coming from sources such as lakes and rivers. The U.S. Geological Survey estimates that electricity generation accounts for 40 percent of all water withdrawals in the country.
There are two types of dry cooling systems. With direct dry cooling systems, steam is condensed using ambient air, and no water is consumed. In indirect systems, dry 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.
“Dry cooling can also be an attractive option for concentrated solar power systems,” the EIA said. “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.”
