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Millions, perhaps billions, of people have died from drinking water polluted with disease organisms and debilitating chemicals. I have been thinking over how to make a very cheap solar still so people could pour polluted water into it and get pure water out. The functioning idea is identical to a basic solar still but the goal is to make these as cheaply as possible in a way that will yield enough drinking water for a rural village family. That requires that the solar collector be large, in order to collect enough energy, but large usually means expensive, and the people exposed to waterborne disease generally have very little money to spare.

A basic solar still diagram

A diagram of a basic solar still

The diagram above from Dummies will work but for it to be big enough for a family it would have a huge sheet of expensive and breakable glass. This solar still works by shining sunlight onto a warming pan at the bottom and evaporating the warm water and then condensing the humid moisture rising from it onto the glass at the top. This distilled water then runs down the inside of the glass into a catch trough at the bottom and then off into a storage container. The system as diagrammed is very inefficient because, for example, the condensing glass itself needs to be cool but it is also heated by the sun so it doesn’t condense very well.

A much cheaper solar still could be made of sheet plastic tubes similar to standard garbage bag material. The tubes would be hung horizontally on a line which runs along the top of the tube. These tubes could be of any size but for ease of description I will give them sizes. A basic set up would be a two meter diameter tube twenty meters long hung along an east west direction. The side facing the sun would be transparent and the opposite side would be reflective and the bottom would be black. There would be two support lines to either side at the top to give the tube some width. The ends would be closed together. During daytime the sun would heat foul water which is introduced occasionally by hand or automatically (from a source like a toilet float-valve system) to cover the black bottom with a thin layer. The water vapor would condense on the clear side, as in the diagram above, but that is inefficient, and efficiency can be improved by drawing in fresh air through V shaped (one-way) flap-slits in the sides of the tube and venting it out into a separate condenser tube. The condenser tube is placed in the sun-shadow of the reflector of the first tube and may itself have a layer of evaporating foul-water to help cooling its outside which helps with condensing. The flow of air through the system is created by a chimney made of a black plastic tube. The chimney is placed so it is in the sunshine and heats up the air inside of it which rises and this pulls the air across the heated water in the first tube and then through the condenser tube.

The cost of this system is 1. the cost of sheet plastic, 2. the welding together of the plastic sheets into the proper form, 3. Shipping it to its village location, 4. The hour to hang it up between two trees or similar local support. 5. The cost of the local polluted water. The power to operate this solar-still is from the sun and is free.

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