solar desalination

Article on desalination costing 50 cents a cubic meter.

I have been thinking about vacuum distillation for desalination ever
since I heard a piece on NPR about arsenic in wells in pakistan.

The way I see it, you get cool water in, and run that through some heat exchange pipes to warm it up as steam condenses on it and drips off to be collected -- pumped out when it collects -- then the still-relatively-cool inbound water does another heat exchange dance with exiting brine, then goes into the evaporation chamber, where there is a vacuum causing the water to boil off leaving the salts, and the inbound flows are adjusted to tune it for maximum
throughput and/or reduced energy use.

A tower is used to create the vacuum, since atmospheric pressure will only push water up so far, at the top of the water column there is vacuum and water will readily evaporate.

I have drawn small designs suitable for camping use in my notebooks several times. I had been under the impression that vacuum distillation is part of how having a municipal water tower helps purify water, but the wikipedia entry on water towers says that the purifying aspect of water towers is merely due to the suppression of backflow from leaking pipes when there is positive pressure in the mains.

The idea is to recycle the heat of condensation is later evaporation, so the only energy input is to offset heat of solution (is that the right term? chemistry class was over two decades ago), and a vacuum is used to lower the required temperatures, so that a low-intensity heat source, such as painting the pipes black and letting the sun do its thing, iw sufficient to drive the desallination process.

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