The water requirements of power plants are a "hidden cost" that should be more fully considered in discussions about clean energy policies, a new report warns.

Roughly 62 percent of nuclear plants use closed-loop cooling systems that withdraw 700 to 1,100 gallons of water per megawatt-hour from nearby water bodies, with most of the withdrawal being lost to evaporation, according to the report released by Environmental Working Group and the Civil Society Institute, an environmental think tank.

Closed-loop systems pull water from rivers and streams and circulate the resource through a plant.

Similarly, coal plants use closed-loop systems for 61 percent of their operations. With coal, the systems require 500 or 600 gallons of water per megawatt-hour and lose most of it to evaporation, the groups said.

Reduced water supplies in drought-prone areas also pose a risk for biomass plants -- which rely heavily on closed-loop cooling -- and natural gas production because of the water needs for hydraulic fracturing operations, they said.

"The government and energy industries are literally flying blind as they plan for continued reliance on coal, natural gas, nuclear power and industrial biomass to meet our energy needs," said Grant Smith, an analyst at the Civil Society Institute.

Power plants and fracking are big users

The report broadly looks at multiple external costs of power production from fossil fuels, biomass and renewables, including their life cycle greenhouse gas emissions, air pollution impacts, subsidy levels and land-use effects.

The goal is to challenge the notion that a clean energy standard -- such as one backed by President Obama -- should be measured through a single emission number associated with a given fuel, said Pam Solo, president of the Civil Society Institute, in a foreword to the analysis.

In an interview, Smith said it is important for policymakers to understand that many costs of power are not specifically on an electric bill. He said that dry cooling equipment in theory could replace many water-intensive systems on power plants but would be cost-prohibitive for many utilities.

The report emphasizes water because information about its impact on multiple fuels used for electricity generally is not compiled in one place, he said.

The main alternative cooling systems at power plants, "open loop" systems, withdraw more water than closed-loop mechanisms but return water back to a waterway. They pose a risk, as well, Smith said, since they require large amounts of water to be consistently cool enough to lower a temperature at a plant effectively.

Streams get too hot to cool

The water link with power plants came to the forefront this summer after Connecticut's nuclear power plant shut down one of its units for the first time in its history because Long Island Sound's water was too warm for cooling purposes.

The nuclear industry maintains that its operating ability remained stable this summer and largely mirrored previous years, despite the situation in Connecticut. In a recent interview, one industry representative said warm water temperatures are a "rare" problem for the nation's 104 nuclear power plants.

Krista Lopykinski, an Exelon spokeswoman, also told the Associated Press that there was nothing unusual about this past summer's operations at six Illinois plants. The coal industry further maintains that the generation costs of the fuel are much lower than the costs for wind and solar, despite any consideration of future water strain.

A peer-reviewed study in Nature Climate Change this summer reported that climate change could reduce the capacity of nuclear and coal plants by 4.4 to 16 percent by reducing the ability of their cooling systems to function (ClimateWire, June 4).