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Better Solar Still

Coming soon: the still better still.
  [vote for,

Inflatable solar stills are often carried as survival equipment on liferafts.

A typical such still looks like a mini circular liferaft with a conical lid. The bottom of the "raft" is dark, and the lid is transparent. You put salt water in the bottom, and tether the thing to your liferaft. Sunlight heats the salt water, which then condenses as fresh water inside the lid, and runs down to be collected around the edge. See link for picture.

One problem with such stills is that, in choppy water, the salt water in the bottom can splash up and get into the fresh water being collected in the rim.

Another problem is that the condensation has to happen inside the transparent lid, which will be quite warm (at least as warm as the surrounding air).

I suggest a better version.

First, imagine an inflatable ring. Now attach two transparent, inflatable cones to it - above and below. So now you have a sort of spindle-shaped double cone, with the inflatable ring running around the equator. The cones can be opened up using a seal like on zip-loc bags.

Now add a string, running across the centre of this device (ie, diametrically across the ring), and from this string hang your saltwater container. The saltwater container (which can also be inflatable, for packability) is almost as wide as the inside of the ring (you want a lot of surface area), and is deep enough to stop the saltwater splashing out if the sea is a little choppy.

We'll also add some ballast (anything will do) in the end the bottom cone.

The whole thing floats in the sea (tethered to your liferaft), with one of the cones sticking down into the sea, the other one sticking up in the air, and the saltwater container held by the string in the middle.

It works much like a conventional solar still, with water evaporating from the saltwater container, then condensing and being collected in the bottom. However, it's better because:

1) The saltwater reservoir is not in contact with the sea, and will therefore get hotter than it would in a conventional solar still.

2) As well as condensing on the inside of the upper cone (which will be at air temperature), the water can also condense on the inside of the bottom cone, which sits below the waterline and will therefore, generally, be cooler. The combination of (1) and (2) should greatly improve the thermal efficiency of the whole thing.

3) Because the saltwater container hangs from the string in the middle of the device, it should be less liable to being tipped and splashed into the fresh water; the fresh water, on the other hand, collects in the bottom where it is less likely to be splashed into the saltwater container.

MaxwellBuchanan, Jun 01 2011

(?) Typical solar still http://www.seamarkn.../info_SM030-SS.html
Thermally inefficient, and also a rip-off at £122! [MaxwellBuchanan, Jun 01 2011]


       "First, imagine an inflatable ring"   

       Marked for tagline (+)
normzone, Jun 01 2011

       Would it not be easier just to be rescued ?
8th of 7, Jun 01 2011

       It might but, with limited payload capacity on a helicopter, would you be able to bring your twelve discs and complete works of Shakespeare?
MaxwellBuchanan, Jun 01 2011

       With appropriate use of MP3 files and the Compleat PDF's of Williame Shaikspere, we do not anticipate a problem. But the leather upholstered wing armchair, the Aubusson carpet, the polished oak bookcases, the Sheraton escritoire, the Ming vases, the collection of Etruscan ceramics in their glazed display cabinet, the life size bust of Palmerston in Carrera marble, the elephant's-foot umbrella stand, the Rembrandts, the Adam fireplace, the cellar of vintage Port, Madeira and Sherry, four footmen, six maids, the Assistant Second Bookbinder-In-Ordinary, the Coal Urchin, the Deputy Warden of the Fire Irons ... yes, you're right, it all adds up.   

       Good thing you're not suggesting moving one of the main Libraries. That would be quite difficult.
8th of 7, Jun 01 2011

       Why would I need and elephant's foot umbrella stand? Elephant's foot umbrellas are so hard to find these days.
MaxwellBuchanan, Jun 01 2011

       Solar stills are old hat. Reverse osmosis hand pumps are where it's at.
mitxela, Jun 02 2011

       But I have been told that, if you hand-pump a R/O osmosis pump, you risk losing more water in sweat than you get back from the pump.
MaxwellBuchanan, Jun 02 2011

       //The cones can be opened up using a seal// but what if there aren't any in that part of the ocean?
marklar, Jun 02 2011

       You'd manage it one way or an otter.
8th of 7, Jun 02 2011

       A friend and I were once called upon (during an adavnced wilderness course) to improvise a solar still, and we ended up building something very much like this out of a few sheets of poly, some broken tent poles, a bootlace, and a one-gallon ziplock bag. The bottom cone was dug out of sand and lined with poly, and some extra tent pole segments were arranged in the upper cone to keep rigid(ish). It worked well but very, very slowly, and once the ziplock bag reservoir slopped when the wind gusted, meaning we had to clean it out and start over (we were using microbe- contaminated bogwater and, er, urine). Still, I can see a well-engineered and professionally- manufactured version of this being very effective.
Alterother, Jun 02 2011

       Don't forget some 1/4 inch plastic tubing...you're gonna need that.
Ling, Jun 02 2011

       you'll need a better system for keeping it from slopping, like a way to take out the fresh water as soon as it's freshened.
Voice, Jun 02 2011

       The simplest way is not to have the saltwater sloshing around. Put a sponge in the saltwater reservoir, so that it can be saturated with seawater but not free to slop around.
MaxwellBuchanan, Jun 02 2011


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