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So I was thinking that as the world gets more congested (and polluted), clean drinking water will be a major issue, especially in third world countries. The very old fashion way of getting clean water out of sea water is evaporation and condensing the clean water vapor into some collection unit. Looking (way) back at my high school days, I remember seeing water boil at room temperature when it was under a high vacuum. It came to me to put these together (not an original idea I have found from doing patent searches but none that I have found have my little twist). The basic concept is this:

A large vessel (air tight) contains a smaller vessel of sea water. The air is drawn out of the larger vessel via a vacuum pump causing the boiling point of the sea water to lower to room temperature. This moisture laden vapor created is again sucked out via the pump, through cooling coils (submerged in the sea to keep cool) and pumped to a clean water collection tank.

My little twist is this... I'd use a multi-speed bicycle to drive the vacuum pump. No electricity needed. When the peddling gets too hard... shift to a different gear. Once the optimum vacuum is reached, the peddling stops while the water boils. The vapor created will lower the vacuum pressure so peddling will have to continue on and off to pump out the moisture and maintain the optimum vacuum pressure.

I plan on building a prototype after the new year to test the feasibility of the unit. I hope to find out the following:

-Does it work? Can a bicycle powered vacuum pump generate enough vacuum to lower the boiling point of sea water down to room temperature? (need some sea water too... I live in Michigan - guess I'll have to make some-ironic, isn't it)
-Does the amount of work needed (sweat power) offset the amount of water gained? A lot of work = a little bit of water.
-Can the process be scaled up? 55 gallon drum possibly?
-Can a unit such as this work to draw water from the air, such as in a tropical forest? If yes, could one harness wind power to drive the pump? But how would it be cooled/condensed...?

What about the people who don't live by the ocean or in a rain forest? Hey, I didn't say I'd solve everybody's clean water issue..

Also, please visit my website http://www.lifeback.org and send anybody there wich you think could use that service (prosthetic accessories)
Great idea, Chris. But the numbers do not gel. Whatever the mechanism used, one still needs 550 kilocalories of heat to evaporate 1 kg of water. Also one has to pump down to about 1/25 th of an atmosphere to boil water at 30 degC.
Better use a passive solar still which gives about 5 litres distilled water/day/sq.mtr without any effort at less than USD 25/sqmtr.
The water boils under vacuum, but the rate of boiling is still controlled by the latent heat of vaporization supplied.
As a denizen I can tell you that the problem in the third world is the unavailability of safe drinking water rather than fresh water.
Wouldn't the 550 kilocalories of heat that is needed to evaporate 1 kg of water be supplied from the ambient temperature? In other words the temperature in the room in which the evaporation was being done would decrease to supply the energy needed to evaporate the water. Evaporation of water is an entropy-driven reaction. Then you could recompress the evaporated water outside so that the heat would be released outside. That way the device could serve as an air conditioner as well as a desalinizing machine. It sounds to me like the idea could work. :-?

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