Solar still

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Solar still built into a pit in the ground

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 on top and drips down to the side, 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 to put together solar still was presented on the TV show Survivorman, in which the host 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, Les Stroud created a solar still by using urine for the source of water, arranged in the sand beside a cup.

[edit] 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.
Solar cooker -- A similar device used to cook food.