The use of direct solar energy for desalting saline water has been investigated and used for some time. During World War II, considerable work went into making small solar stills for use on life rafts. This work continued after the war, with a variety of devices being made and tested.
These devices generally imitate a part of the natural hydrologic cycle in that the saline water is heated by the sun's rays so that the production of water vapor (humidification) increases. The water vapor is then condensed on a cool surface, and the condensate collected as product water. An example of this type of process is the green house solar still, in which the saline water is heated in a basin on the floor and the water vapor condenses on the sloping glass roof that covers the basin.
Variations of this type of solar still have been made in an effort to increase efficiency, but they all share the following difficulties, which restrict the use of this technique for large-scale production (we are not necessarily doing large scale though)
Large solar collection area requirements High capital cost Vulnerability to weather-related damage
Input and Output
A general rule of thumb for solar stills is that a solar collection area of about one square meter is needed to produce 4 liters of water per day (10 square foot / gallon). Thus, for a 4000-cum/d facility, a land area of 100 hectares would be needed (250 acres/mgd). This operation would take up a tremendous area and could create problems if located near a city where land was scarce and expensive.
The stills themselves are expensive to construct, and although the thermal energy may be free, additional energy is needed to pump the water to and from the facility. In addition, careful operation and maintenance is needed to prevent scale formation caused by the basins drying out and to repair glass or vapor leaks in the stills.
An application for these types of solar humidification units has been for desalting saline water on a small scale for a family or small village where solar energy is abundant but electricity is not.
Sources: The following information was obtained from a Solar Humidification website (http://www.serve.com/damien/home/solarweb/desal/solarstill.html), using information based on projects that used the solar conditions in California
Solar humidification seems to be an option depending on the materials we choose. Even though one of those devices seems very expensive, we can discuss the design of a low scale device.