In the poem, Rime of the ancient Mariner, the famous line “Water , water everywhere and not a drop to drink” tells the misfortune of a sailor whose ship find calm waters, not being able to move for days. Unfortunately a simple device like a Solar Still could have save the sailor’s misery and that of many other seamen that got stuck on the horse latitude in the days of sailing ships. Solar still is one of the devices that could have been invented as early as the time glass was invented.
Solar Still is a simple device that evaporates and condenses water to purify it. Sea water or brackish water can be collected and used as a feed. When exposed to sunlight (Direct not diffuse sunlight), the water evaporates using salt behind. The evaporated water vapour than condenses over a cooler film. It coalesces with other vapour particles and forms drops. These drops can be made to slide down the film and can be collected in a vessel.
The simplest form of solar still can be made using a clear plastic sheet, a bin liner and a pot. A small cavity in the ground can be dug up that will act as a vessel for salt water. The bin liner can be placed on the pit surface. It will fulfil two purposes:
1) The black surface will absorb the incoming solar radiation
2) It will water proof the cavity, preventing salted water getting absorbed into the ground
The diagram explains its functionality.
It has been debated that in a case where a person is stranded in a desert, whether the buried pit solar still would bring any benefit to survival. There will be energy lost in digging up to pit and with it body will also lose water. The water that will be recovered by a still in a desert can be lower than what is lost in digging up the pit. Therefore one has to weight the options. Once however the pit is dug, it can be used for multiple days therefore in the long run it definitely pays back the water invested.
There are many different kinds of solar stills. From the buried pit, water cone, transpiration bag to wick still each having their own advantages.
The use of portable /floatable solar still has featured in movies such as “Life of Pi” and Robert Redford’s “All is Lost”. These portable solar stills were originally made for the pilots that got stranded on the ocean. On a sunny day they can produce up to 2.4 litres of fresh water. On a cloudy day they can produce as much as 1.4 litres of potable water.
Companies like Aquamate still produce these floatable sea worthy solar still and they can be purchased for price of around $150.