By 2050, wind and solar energy are likely to play greater roles in generating electricity in the US than might otherwise be the case, if water scarcity joins curbs on carbon-dioxide emissions as a factor in selecting technologies to feed the grid, according to a new study.
The study marks the first time analysts have tried to account for water availability as well as emission-reductions in trying to identify the most economical mix of technologies to meet mid-century energy demand, especially in regions where global warming is expected to bring still-drier conditions, according to Mort Webster, an associate professor of engineering systems at the Massachusetts Institute of Technology in Cambridge, Mass., and the study's lead author. The study was published Sunday in the journal Nature Climate Change.
“If you're reducing greenhouse gases, you're probably doing it, at least in part, because water is a problem. Water's a bigger problem than temperature,” Dr. Webster says.
Yet up to now, studies that have tried to glean the most economically viable mix of technologies for generating electricity have focused on the mix needed to meet some sort of cap on CO2 emissions.
“But that never considered the water,” he says, even as other studies looked at the water consumption that coal, nuclear, or gas-fired plants require for cooling and for steam to spin the turbines running the generators.
Researchers call this confluence of water and energy production in a world facing an ever-warming climate the water-energy nexus.
The issue made headlines in August 2008, when the Tennessee Valley Authority had to temporarily shut down three reactors at its Browns Ferry nuclear power plant in Athens, Ala., after a drought reduced water levels in the Tennessee River and a heat wave boosted the water temperatures. In principle, the plant still could have used the water for cooling its reactors, but the temperature of the effluent pumped back into the river would have exceeded limits set to protect aquatic life. Two years later, low water levels forced the utility to throttle back the reactors there to 50 percent capacity.
Similar concerns over water temperature and availability have affected nuclear plants from Kansas and the Connecticut coast to Europe.
Nuclear plants are not the only types of generating facilities affected.
In Texas, the state's power plants should be able to tap existing surface-water supplies through 2030, according to a study published in January that looked at the impact of weather variability on the state's electric utilities and their future access to water.
But population growth and the need for more power plants are expected to force utilities to slake their thirst from other sources. These range from aquifers containing drinkable or brackish groundwater to some limited additional consumption of water currently being used to irrigate "low value" crops, the report suggests.
All of these are likely to be more expensive than currently available surface water supplies, according to the report prepared by energy and water specialists at Argonne National Laboratory in Argonne, Ill.; Sandia National Laboratory in Albuquerque, N.M.; and the University of Texas at Austin.
Given Texas' history of drought and the the prospect that droughts will become more frequent and severe there as the climate warms, Webster and two MIT colleagues focused on the state as a testbed for estimating the most economical mix of technologies for generating electricity by mid-century, when water and CO2 emissions both are factors. In addition, the Lone Star state is a grid unto itself, independent from the rest of the nation's grid, so it makes for a tidy study area.
Webster's team performed modeling studies that tried to address the mix of generating and cooling technologies needed to meet projected demand in the most economical manner.
With no CO2-emission limits and no limits on water use, coal-fired plants using water for cooling would represent more than half of the state's generating capacity in 2050, the team estimates, followed by two approaches to using natural gas. One of those would also require cooling water. Nuclear energy is nowhere to be found in the mix.
If the power-generation sector is required to reduce CO2 emissions by 75 percent by 2050 but plenty of water is available, coal's contribution drops precipitously. Additional nuclear plants and gas-fired power plants fill the gap. The contribution from renewables doesn't show up in either of these first two scenarios.
Add a 50 percent cut in allowable water withdrawals, however, and the mix changes yet again, with wind energy appearing for the first time amid gas-fired plants and fewer nuclear plants – all using more water-miserly technologies for cooling.
“Many technologies that do not seem cost-effective as part of a low-carbon system, such as wind and solar, may be justified when considering the full set of environmental challenges including water use,” the researchers conclude.
And while the study focuses on Texas, its general conclusion is likely to hold for other arid parts of the country with growing populations, Webster says.
The study carries with it large uncertainties, the team acknowledges. Among them, the price of fuels, capital costs of new technologies, and the efficiency with which they generate electricity.
As a first stab at exploring the potential impact of restrictions on water use and CO2 emissions in utility planning, the study also doesn't account for the roles political opposition to nuclear energy or increasing unease with fracking and its byproducts could play in guiding the future course of utility planning. This year alone, a mix of political opposition and competition from cheaper natural gas has prompted US utilities to reverse their decisions to extend the licenses of four nuclear plants and instead close them.
Still, the study represents a useful first-look at the question of generating electricity in a looming dual age of CO2 emissions restrictions and declining water resources in key regions of the country.
“They're on the right track,” says Christopher Harto, an environmental and energy analyst at the Argonne National Laboratory. “How they designed their study and the assumptions they made, you can always have questions about those things. But there's a rate at which nature provides water. When you start bumping up against that limit, you start running into some significant conflicts.”