Water is a necessity for life. Clean water is needed for health.
1. The oceans have more water than we can use, but it contains dissolved materials, mainly salt, ten times more than we can live with, 100 times more than hard (well) water. The challenge is to find an inexpensive way of extracting the dissolved materials from ocean water so it can be used for irrigation and drinking water.
2. The sun provides constant power for evaporation. The evaporative energy has not been used except on smaller scales for solar stills. It has not been used to supply more than drinking water or for agriculture, possibly greenhouse use.
3. The sun's power can also be turned into electrical energy that can be used to disassociate the dissolved electrons from the ocean water. No one has yet found a means to do that successfully although it appears to have the most potential for an inexpensive solution.
4. The dreamed solution is to embed photoelectrical cells within a very long double walled clear poly tube. The inner walls of the double wall tube are porous or permeable and will transfer, by electrolysis, the positive ions to one side of the tube and the negative ions to the other through the porous film. The electrical source is a low voltage current created by solar cells embedded in the poly tubing. The poly needs to withstand direct sunlight (with the solar cells embedded) without breaking down, for at least 3-5 years. The permeable film, bonded to the outer film at top and bottom, allows the ions of dissolved minerals in the ocean water to pass through to one side or the other. The length and long periods of low current are factors in providing large quantities of irrigation and drinking water. The tubing must be capable of 'floating' in the ocean within a meter of the surface to ensure maximum sunlight is usable, and not on the surface so that smaller wave action is not causing breaking or undue deterioration of it.
Opening and discharge areas require additional development. The positive ions are collected for further refinement. See this also http://news.bbc.co.uk/2/hi/technology/7313203.stm
Markets for this technology are found in many tropical or subtropical nations with sheltered or semi-sheltered coastal waters.