Desalinating water by evaporation requires 540 calories per gram, a huge energy cost. There are other methods (reverse osmosis followed by "polishing", or the Zarhin process if it can ever be made to work, and few other) that are much less energy intensive. Solar is not such a bad partial solution for arid and warm areas with a lot of sunshine.
The main problem is that solar flux is "just" 1 KW per square meter, and most Si ased cells convert only 10% of that to electricity. I have a meeting on the 16th on just that problem with people from Alcoa, they are interested in solar energy and are interested in a patent I have that allows very low cost concentrators to reduce the amount of Si required (the man cost in a solar cell is the silicon).
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