Solar still

Solar still built into a pit in the ground

Gathering water through the use of a pit (or 'solar still') is another much employed survival technique

A solar still is a very simple way for distilling water, powered by the heat of the sun, especially when distillation equipment is unavailable. A few basic types of solar stills are cone shaped, boxlike, and pit. For cone solar stills, impure water is inserted into the container, where it is evaporated by the sun through clear plastic. The pure water vapor condenses at the high point and drips down off the low point, where it is collected and removed. The most sophisticated of these are the box-shaped types. The least sophisticated are the pit types.

Solar stills are used in cases where piped or well water is impractical, such as in remote homes or during power outages. In Florida and other hurricane target areas that frequently lose power for a few days, solar distillation can provide an alternate source of clean water.

Solar stills are occasionally used on a longer term basis in developing world settings. However, they produce a relatively small amount of clean water, and even smaller amounts where the source water is saline or brackish. If the amount of water is inadequate, a compromise method is to mix the distilled water with the brackish or saline water purified with other methods - this gives a more adequate quantity, while still lowering the salinity, and improving the taste. A larger scale version of the concept of the solar still is the Water Pyramid, which uses an inflatable dome as the condensing surface and can be applied in tropical, rural areas.

Knowing how to put together a solar still is often billed as a useful survival skill and could provide an important means of potable water in the event of a wilderness emergency. Nevertheless, under typical conditions makeshift solar stills rarely produce enough water for survival, and the sweat expended in building one can easily exceed its daily output. Solar stills can extract water from moisture in the ground but to increase the amount of moisture available to a solar still, water (fresh or saline) can be added inside or along the edges of the still. Where no water sources are readily available, urine or shredded vegetation can be used inside the pit. To prevent losing moisture by taking apart the still to retrieve collected water a length of plastic tubing can be used to sip water as it accumulates.

A simpler way to put together a solar still was presented in the Kalahari Desert episode from Season 2 of the TV show Survivorman, in which Les Stroud simply wrapped a plastic bag around a leafy branch on a live tree. The transpiration from the tree leaves provided the water source. No cup was used; water dripped straight into the bag. In the same episode, he created a solar still by using urine for the source of water, arranged in the sand beside a cup.

In an episode of the Discovery Channel show Man vs Wild, host Bear Grylls shows how to build a solar still to distill urine and sea water.

References

Jackson, R. D.//van Bavel, C. H. M. (1965) Solar Distillation of Water from Soil and Plant Materials: A Simple Desert Survival Technique. Science 149(3690)1377-1378

See also

• Solar water disinfection - uses the sun's Infrared energy and heat to disinfect water, but does not distill (evaporate and condense) the water.
• Watermaker - A filter to obtain clean water from seawater; used in large vessels
• Solar cooker -- A similar device used to cook food.

External links

• Making a solar still
• Example of a PV-powered solar still
• Watercone – a commercial implementation
• SEA Panel — manufacturer of personal solar stills