As additional solar photo voltaic and wind generation displaces fossil fuel generation, each megawatt-hour generated could save as much as 600 gallons of water that would otherwise be lost to fossil plant cooling. Because wind energy generation uses a negligible amount of water, the 20% Wind Scenario would avoid the consumption of 4 trillion gallons of water through 2030, a cumulative reduction of 8%, with annual reductions through 2030. The annual savings in 2030 alone would be approximately 450 billion gallons. This savings would reduce the expected annual water consumption for electricity generation in 2030 by 17%. (Department of Energy, http://www1.eere.energy.gov/windandhydro/pdfs/20-2030_jwalker.pdf)
In contrast to the low requirement for water for wind turbine generated electricity, fossil-fuel-fired thermoelectric power plants consume more than 500 billion L of fresh water per day in the United States alone (Virginia Water Resources Research Center, in Blacksburg, Va.). Nuclear power uses almost triple the amount of water as fossil fuels. Biodiesal, not so green from the perspective of water use, requires an astronomical 40 times as much water as coal for irrigation and and for turning the legumes into fuel. See the short article:
How Much Water Does It Take to Make Electricity
John Farrell compared the water consumption of fossil fuel power plants to various solar technologies, noting that wet-cooled concentrating solar thermal power (think big mirrors) uses more water per megawatt-hour (MWh) than any other technology. The following chart illustrates the amount of water used to produce power from various technologies.
Minimizing the use of water in energy production is very important, because, unlike for energy sources, there is no replacement for water, and worldwide water supplies are in short supply. According to United Nations global statistics, 2.6 billion people do not have adequate water for basic sanitation, and 1.1 billion people do not have clean drinking water. We don't usually think of our Land of 10,000 Lakes as short on water, but corn production for ethanol draws down water tables, and the size of ethanol refineries in Minnesota has had to be limited because the refineries use so much water that they draw down water tables.