THE DALLES, Ore., April 24 (UPI) -- Add another requirement to the clean-energy checklist: low water usage.

Two Virginia Tech researchers released a study this week examining the water-use requirements for 11 different energy sources, ranking them in terms of efficiency.

One of the most important aspects of the study was to raise awareness of the role water plays in energy production, said Rachelle Hill, a recent Virginia Tech graduate who co-authored the study.

"We need to do more research and really study what goes into energy production, not just what comes out," Hill told United Press International.

And a lot of water goes in. According to the study, U.S. thermoelectric power plants consume 136 billion gallons of water per day, averaging out to 25 gallons for every kilowatt-hour produced. Water is used in almost every aspect of energy, from the cooling and cleaning of machinery to the production of steam to turn turbines. And as energy demand rises in coming decades, so will the amount of water required to produce it.