As the U.S. Warms, Power Plants Face New Water Limits

By Andrew Freedman

The power sector is responsible for a large share — about 40 percent — of greenhouse gas emissions in the U.S., particularly thermoelectric-generating stations, such as coal-fired power plants. And so it is not without a hint of irony that a recent study concluded that the effects of global warming, particularly drought and heat waves, will increasingly limit the generating capacity of these power plants — thereby making them both contributors to and victims of global warming.

The New Madrid Power Plant in Missouri.
Credit: AEC, Inc.

The study, conducted by a team of U.S. and European researchers and published in Nature Climate Change in June, found that increasing water temperatures and reduced river flows during the summer months constitute the achilles heal of the power sector, making power plants vulnerable to significant disruptions in service at the very times when they are needed most. The study's findings were based on computer modeling of how climate change will affect summer river flows and water temperatures at 61 U.S. power plants and 35 plants in Europe. Previous studies had only examined changes in river flows, overlooking the cooling water issue.

Thermoelectric facilities need large amounts of water for cooling purposes, taking in relatively cool water from rivers, lakes, or the ocean, pumping that water through plant components to bring temperatures down, and discharging warmer water in return. Such plants use “once-through cooling” systems. Other power plants take in less water and circulate it, pumping it through the plant and into large cooling towers, where it eventually evaporates into the atmosphere. 

Withdrawals by thermoelectric plants account for about half of total water use in the U.S., according to a 2005 U.S. Geological Survey study, the most recent year for which such comprehensive data is available.

Regardless of which type of cooling system is in use, cooling water is the lifeblood that sustains thermoelectric power plant operations, and reductions in the availability of such water can cause a complete shutdown of a major facility.

During the scorchingly hot and dry summer of 2012, low water levels and hot temperatures forced several power plants in the Midwest and the East  to curtail their output or cease operating altogether for a time. The drought that reached its peak during late summer has been the worst to strike the U.S. since the 1950s, and comparable in some ways to the Dust Bowl-era droughts of the 1930s. Climate projections show that drought conditions as well as heat waves are expected to become more intense and frequent in coming decades as the climate continues to warm. 

Credit: U.S. Department of Energy.

In the U.S., power plant operators must comply with laws such as the Clean Water Act, as well as state regulations that prohibit nuclear and other plants from operating once water temperatures go above a certain threshold, in part because it could compromise the safe operation of the facility, and also because discharging very warm water can kill fish and other marine life. Conflicts between the need for electrical power on the one hand, and environmental concerns on the other, tend to arise during periods of hot, dry weather, when river levels drop and water temperatures spike. Electricity demand is often heightened during such periods, due to air conditioner use, making any disruptions in service especially costly for utilities.

This past summer there were several instances when power plants had to shut down after running into water temperature thresholds. For example, the Millstone nuclear plant in Waterford, Conn. had to partially shut down in mid-August because the waters of Long Island Sound had become too warm to cool the plant, a development that plant operators had never encountered before.

In July, the New York Times reported that the Braidwood Generating Station, a nuclear plant about 60 miles southwest of Chicago, had to be granted a special waiver to continue operating after the water it was taking in for cooling purposes hit 102°F, two degrees above the legal operating limit for the plant.

And according to an article in the Washington Post, the Illinois Environmental Protection Agency also granted special exceptions to four coal-fired power plants, along with three other nuclear plants this summer, in order to allow them to discharge water that exceeded water temperature limits.

The study comes at a time when the power sector is facing increasing electricity demands as well as pressure to reduce their greenhouse gas emissions. Given the long lifespan and huge costs of building power plants and electrical infrastructure, the authors recommend that warming-related cuts in power generation be factored into the planning process, including the likelihood that cooling water will be a more limited commodity in the future.

The U.S. Drought Monitor map for Sept. 18, 2012, showing the persistent drought conditions across much of the Central and Western U.S.
Credit: NOAA/USDA.

“It is important for the electricity sector to have realistic projections of both water availability and water temperature to be able to anticipate and adapt to changes in cooling-water availability,” the study states. 

According to the study, thermoelectric power plants in the U.S. could see a summer average decrease in generating capacity of between 4.4 to 16 percent during the 2031-2060 period. The reason for the large range of that estimate is that the amount of the decrease depends heavily on the exact cooling type of a particular power plant as well as its location, and how much greenhouse gases are emitted between now and the 2030s.

In addition to the general decrease in summer electrical generating capacity, the study found that the probabilities of extreme reductions in power production will increase as well. For example, at one power plant the researchers studied, a 1,200 megawatt station in New Madrid, MO., low river flows and increased water temperatures could cut usable capacity down to as low as 400 megawatts on a regular basis by the 2040s, with such severe disruptions taking place every seven-to-10 years, on average.

The study projects that the largest water temperature increases will take place in the southern part of the Mississippi River Basin and in the East, although other studies have projected greater warming in Western states compared to the Southeast. 

In a bit of a silver lining, the study found that natural-gas fired power plants are less vulnerable to cooling water-related disruptions, since they tend to have smaller water demands than coal-fired plants. Natural gas has been gaining ground against coal in recent years, mainly due to a decline in natural gas prices relative to coal.

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