Thermoelectric power plants with once-through cooling systems generated 30 percent of U.S. electricity. These power plants use steam-driven turbines to generate their energy.
They do this by drawing in water from rivers and lakes, circulating it through pipes to absorb the steam’s heat, and discharging the heated water back into the river or lake. Federal regulations to protect the environment govern a power plant’s water use and the maximum temperature of the used cooling water that is discharged back into rivers or lakes.
However, according to a new study – “Factors that reduce once-through cooling capacity and thus power output include not just environmental regulations, but warming surface waters, and drought.”
The study, conducted by Candise L. Henry, a 2018 Ph.D. graduate of Duke’s Nicholas School, and Lincoln F. Pratson, Gendell Family Professor of Energy and the Environment at the Nicholas School of the Environment, Duke University, was published in the peer-reviewed journal, Environmental Science & Technology on March 8.
The study found that the future generating capacity of these “once-through cooling systems” will be undercut by droughts and rising water temperatures linked to climate change, and further impacted by federal regulations that limit water use. The solution, say the researchers is not to scrap regulations but to “scrap the old cooling systems.”
“If we want to have reliable electricity and, at the same time, protect the lakes and rivers that provide cooling water to the plants, we need to retrofit the plants with recirculating cooling systems,” said Pratson.
Why recirculating cooling systems are better
The researchers analyzed seven years of operational and meteorological data for 52 eastern US power plants with once-through cooling systems that spanned the years 2007 to 2014 when severe droughts affected much of the Southeast.
The team was able to track local water temperatures and flow rates and how any changes affected each plant’s maximum power output. They then ran a computer model using seven different water-availability and temperature scenarios to determine what percentage of change in generating capacity was due to rising water temperatures and what percentage was due to decreased water supply, as would be seen in drought-like conditions.
The researchers were then able to determine warming and water flow trends for the coming century, estimating the likely future impacts of each factor.
“Past studies bundled the impacts of drought, water temperatures, and environmental regulation together,” Pratson notes. “By pulling them apart, we gain a much clearer picture of what the big threats will be and what we can do to mitigate them.”
The results and conclusions
The study found that should surface waters warm 3 degrees Centigrade and river flows drop 20 percent – both of which are probable by the end of the century – drought-related impacts will account for about 20 percent of all capacity reductions at power plants with once-through cooling systems.
“These reductions include capacity curtailments or shutdowns that could occur when local surface water levels drop below a plant’s intake structure,” says the study.
“It’s surprising that the impacts of drought will be so much larger than those of warmer temperatures, which we estimate will account for little more than 2 percent of reductions,” said Henry. “But it’s also surprising that drought impacts will be so much smaller than regulatory impacts.”
”Fortunately, nearly all of these impacts could be mitigated by switching to recirculating cooling systems,” Henry said.
In a recirculating cooling system, water from the cooling tower absorbs the steam’s heat. The heat is then routed back to the tower where the heat is released through evaporation. In this type of system, heated water is not discharged into surface waters and the plant only needs to replace a small portion of water lost to evaporation.
Of the 1,655 operable cooling systems in the United States, 875 (53 percent) reuse water either through a cooling tower or a cooling pond, while 719 (43 percent) do not reuse water, according to the U.S. Energy Information Administration.
This system makes nuclear power plants less vulnerable to drought impacts and environmental regulations.
“Right now, it’s fairly common for plants to be granted provisional exemptions from rules governing the maximum allowable temperature of discharged water, but if regulations become more stringent under future administrations, we could see more curtailments or shutdowns of once-through power plants,” said Henry, who is now a postdoctoral researcher at the Carnegie Institution for Science at Stanford University.
