Re: [Paddlewise] Fresh water

From: "Patrick Maun" <patrick_at_patrickmaun.com>

> would it be at all feasible to 
> create a small solar still (ie evaporate saltwater into a collector) 
> to make a bit of fresh water. 

Absolutely.  However, there are a few problems to work out.  The solar stills
that I'm familiar with are too large to put on a kayak.  They tend to be on the 
order of a meter square.  As well, they can't be jostled, because there's not
much separating the fresh and brackish water and a typical wave will cause
the brackish to splash into the fresh.  Most of the designs must be oriented 
towards the sun to work efficiently.

One simple design, for example, is a shallow box, tilted up so it faces the sun.
The box is divided into two sections.  The "uphill" portion is large and covered
with a black surface to catch the heat.  The smaller "downhill" section is for
the captured fresh water.  The box would be, say, a little over a meter long and
a little less wide.  The fresh water section would take up the lower 10 cm or so
of the box.  The wall between the fresh and salt would be about 8-9 cm high and
the overall box about 10 cm thick.  The whole assembly is covered with glass.
A couple of tanks (bottles whatever) are used, one to contain the salt water at
the top of the box and one to collect the excess salt water near the bottom. A
third collects the fresh water from the bottom.  The brackish water is trickled 
down the upper portion and the heat evaporates some of it.  The evaporated 
water condenses on the inside of the glass and runs down the glass to collect 
in the fresh water collection area.  

That describes a version I saw working at McGill U. when I was a student there.
A fancier version is at http://www.solardome.com/SolarDome84.html.  This site
states that "approximately 8 square feet (of glass cover) will distill around 
1 gallon of water per day, over five hours of full sunlight."  Size is a drawback.

There are also designs for emergency water stills in survival books.  I think I 
first saw one in The Complete Walker, but I may be confusing it with another
book.  These kinds of stills usually involve suspending a tarp or sheet of cloth 
or plastic over a pool of brackish water and using the tarp to collect the 
condensed fresh water.  The fresh will dribble to the lowest point where it's 
collected.  

I spoke to a couple of doctors earlier this year while kayaking.  We somehow got 
onto the topic of water purification (I think a result of my evangelizing for alternative
housing and living arrangements to reduce pollution and energy consumption).  I 
pointed to a pop bottle on the deck of his kayak which was clouded on top from
the condensation of water from his cola.  "That's a still" I said.  He said he saw one
that was not much bigger for water production.  I haven't been able to track such 
a beast down, however.

A couple of the biggest problems facing third world countries is desalination and/or
purification of water (both large and small scale) and small scale refrigeration, all
with little power.  If you can solve any of these, you can be rich!

Mike





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