There seems to be two bits of water involved, as you probably realize. One bit, several gallons, is sitting in a saggy plastic sheet, whose bulge forms a lens. You ask a good question, could one float a skin of oil on that to keep it from evaporating?
The bit that gets purified is in a separate container held (manually?) at the focal point of the water lens, and gets hot. But as the sun moves so does the focal point, so a very patient person has to be shoving the heating container along every few minutes. This doesn't seem very practical to me.
The bit that gets purified is in a separate container held (manually?) at the focal point of the water lens, and gets hot. But as the sun moves so does the focal point, so a very patient person has to be shoving the heating container along every few minutes. This doesn't seem very practical to me.