Wednesday, September 30, 2009

Solar Electricity Power Towers Face Water Use Constraints

A solar power tower, left, is a solar thermal plant that uses different technology than the photovoltaic solar panels that produce electricity directly from the sun. In such plants, mirrors heat a liquid (liquid sodium) to heat water to create steam that drives an electricity-generating turbine. As in a fossil fuel power plant, that steam must be condensed back to water and cooled for reuse.

The conventional method is called wet cooling. Hot water flows through a cooling tower where the excess heat evaporates along with some of the water, which must be replenished constantly. An alternative, dry cooling, uses fans and heat exchangers, much like a car’s radiator. Far less water is consumed, but dry cooling adds costs and reduces efficiency — and profits.

In California alone, plans are under way for 35 large-scale solar projects that could generate 12,000 megawatts of electricity, equal to the output of about 10 nuclear power plants.

When Solar Millennium announced plans to build two large solar farms in Nevada, the project experienced difficulties when they revealed that its method of cooling the power plants would consume 1.3 billion gallons of water a year, about 20 percent of this desert valley’s available water. It has created conflicts between some people who hope to make money selling water rights to the company and others concerned about the project’s impact on the community and the environment, particularly the effects that industrialization will have on wildlife, their desert solitude and, most of all, their water. Some neighborhood residents are also worried about having water from their wells. Solar power tower projects can demand a huge amount of water and certain areas of Nevada are being inundated with requests from renewable energy developers that far exceed the amount of available water.

Water disputes forced Solar Millennium to abandon wet cooling for a proposed solar power plant in Ridgecrest, California after the water district refused to supply the 815 million gallons of water a year the project would need. The company subsequently proposed to dry cool two other massive Southern California solar farms it wants to build in the Mojave Desert.

BrightSource Energy’s dry-cooled Ivanpah project in Southern California would consume an estimated 25 million gallons a year, mainly to wash mirrors. But a wet-cooled solar trough power plant barely half Ivanpah’s size proposed by the Spanish developer Abengoa Solar would draw 705 million gallons of water in an area of the Mojave Desert that receives little rainfall.

NextEra Energy Resources, a subsidiary of the utility giant FPL Group, plans to build a wet-cooled plant in a dry area east of Bakersfield, California. NextEra wants to tap freshwater wells to supply the 521 million gallons of cooling water the plant, the Beacon Solar Energy Project, would consume in a year, despite a state policy against the use of drinking-quality water for power plant cooling.

Tessera Solar is planning a large project in the California desert that would use only 12 million gallons annually, mostly to wash mirrors. But because it would draw upon a severely depleted aquifer, Tessera may have to buy rights to 10 times that amount of water and then retire the pumping rights to the water it does not use. For a second big solar farm, Tessera has agreed to fund improvements to a local irrigation district in exchange for access to reclaimed water.

Opponents complain that allowing projects to use fresh water would remove any incentives that developers have to use technologies that minimize water use. (NYT, 9/29/09)

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