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While the solar still is an excellent method of providing small quantities of fresh water at low cost, the amount of collector area needed rules it out as a supply for large cities. In the United States a single home may use 16,000 gallons of water a month; thus the Symi still could provide for only a few families accustomed to using water as we do. This is because the solar still as used at Symi, while cheap to operate, is very inefficient. It is a "single-effect" still, and makes use of only a tiny fraction of the energy in solar heat. The heat of vaporization of the water is largely wasted.
In an effort to adapt the solar still for more efficient production of fresh water, the United States Government has underwritten the cost of a new "multiple-effect" still installed at Puerto Penasco, Mexico, on the Gulf of California. Designed by Carl Hodges, a young scientist at the University of Arizona, the 10,000-square-foot pilot plant produces up to 5,000 gallons a day for trial use by residents of the small Mexican fishing town. Prior to the solar still, their only supply of fresh water was trucked in at high cost.
To provide one million gallons of fresh water a day, enough for a town of about 50,000 people, a solar still of the Symi type would have to have an area of about 6,000,000 square feet. This would be a plant measuring about 2400 feet on a side, with concrete basins, and acres of plastic covers. For a million people, a solar plant would have to be about 2 miles on a side.
It would be wrong to dismiss the solar still as impractical for these reasons, however. We saw in earlier chapters that the theoretical energy required for converting salt water to fresh was only 2.6 kwh per 1000 gallons. A single yard of area exposed to direct sunlight receives a kilowatt of continuous energy. Averaged over an 8-hour day of sunshine, perhaps 1/2 kilowatt is available on a continuous basis. In other words, one square yard represents a potential of 4 kilowatt-hours per day, enough to convert 1430 gallons of water instead of the 11/2 gallons at Symi. And the sunshine is free. In theory there is room for a thousand-fold improvement in efficiency of the solar still. The Puerto Penasco multiple-effect design, three times as efficient as the Symi still, is a modest step in this direction.
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