All thermal power plants need water. In one of the great ironies of physics, solar thermal power plants, which work best in areas of nearly year-round sunshine, may run through about as much water as a conventional fossil-fueled steam plant that is typically sited alongside a river or on the coast.

One widely used solar-thermal configuration runs synthetic oil through a tube along the focal line of parabolic trough mirrors that track the sun. The sun heats the oil to approximately 735 degrees F. It then passes through a heat exchanger where it generates steam to drive a conventional turbine.

Larry Stoddard, an associate vice president and solar energy engineer with the Energy Division of Black & Veatch, said that the solar thermal plants now in operation typically use closed-loop wet cooling systems requiring 700 to 1,000 gallons per megawatt hour (MWh). A 100-MW plant running at a 30% capacity factor might consume5 180 million to 260 million gallons per year.

Most of the U.S. solar thermal capacity is located in the Mohave Desert in California. The largest development has 354 MW of net generating capacity.

The water, Dr. Stoddard said, comes from wells that typically were previously developed along with purchased water rights, but the “the California Energy Commission is making use of wet cooling more difficult.”

The alternative is for future installations to use air-cooled (also known as dry cooling) systems, which consume about one-tenth the quantity of wet systems, he said, but that still comes to about 50,000 to 70,000 gallons per day for a 100-MW plant.

In an article last year,6 Bill Kemp, Black & Veatch vice president, and Rich Rudden, senior vice president, pointed out that air-cooled systems operate most efficiently in the colder northern climes. Dry-cooled generating units suffer decreasing operating efficiency as temperatures rise, driving down fuel efficiency and output capacity. In general, the efficiency and capacity penalty compared to a wet cooling system is 2% to 4% annually in a moderate climate – rising to as much as 10% on a hot day. The impact on a plant sited in the desert might be even more severe. Dry systems also cost about 12% more than wet cooling systems.

Solar power stations have another, although relatively small, claim on water – the need to periodically wash the parabolic mirrors. The mirrors focus the sunlight, so if there is any scatter from the dust, efficiency suffers, Dr. Stoddard said. “You need real clean mirrors.”

— Samuel Glasser, Black & Veatch

5 Water “use” includes all withdrawals from water sources, even if almost all of that water is later returned to the source. Water “consumption” is water that is not returned to the source, but rather is lost to evaporation, percolation, plants and animals, etc.

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