From: dpe@inel.gov (Don Palmrose)
Subject: Re: Request: info on desalination and solar energy
Organization: INEL EG&G Idaho
Date: Wed, 18 Nov 92 02:43:39 GMT

In article <1992Nov17.183658.29104@impmh.uucp>, dsg@impmh.uucp (Dave Gordon) writes:
>
 
> Yep, it *is* feasible; remember that you can convert sunlight to heat
> (which is all you want for desalination by distillation) with near 100%
> efficiency.  Solar radiation is about 1kW/m^2 maximum, say 1/2 that
> on average during the day (we're talking about *hot* countries here),
> so it doesn't take an enormous area to collect significant amounts of
> energy.  And you can just use more seawater as the heat sink on the
> condenser side of the still.
> 
> Israel already has solar desalination plants, but then I don't think
> oil is "dirt-cheap" there.  Maybe the oil-producing countries don't
> use solar, but then maybe they should (if the Israelis would sell them
> the technology ;-).
> -- 
> 
> ===============================================================================
> Dave | dsg@imp.co.uk | +44 753 516599 | "C^i tio ne estas .subskribo"
> ===============================================================================
> 
>

The kind of dsalination plants I am familiar with are evaporative where you
lower the pressure in a fairly large chamber, spray heated seawater at about
180 degrees F, and let the water flash to steam.  This is repeated in stages
for maximum efficiency of extracting the fresh water from the brine but also
to efficienctly use of the heat source.  Namely, the incoming sea water is
initially heated by condensing the fresh water that flashed to steam in the
evaporator chamber.  The residue from the first chamber is sucked into the
next because it is kept at a lower pressure than the first, so more fresh
water flashes in the second stage.  The resulting high density brine is pumped
back into the ocean after going through acouple of more heat exchangers to
give up their heat to more incoming sea water.

It is plain to me that solar collectors would make *excellent* heat sources 
for heating the sea water to the proper temperature.  But the system still 
needs pumps to move the sea water/brine/fresh water around.  Also, air 
ejectors are needed to lower the evaporative chambers pressure to below 
atmospheric.  Where is the power for the pumps coming from?  Do the Israelis 
have a system that is totally independent of an outside electrical source?  
Are the solar energy collectors also generating electricity for the 
desalination plant's auxiliaries loads?

The reason I ask is that I see a great potential for such a solar based
desalination plant to be very effective in water poor areas like the Carribean
Islands and several South Pacific Islands.  The big handicap for these people
is electricity is still needed to power the pumps.  Since they are also 
electricity poor, having a solar heated desalination plant does not do them
much good if they still have to build an oil-fired electrical power plant to
provide the electricity to the pumps.

Thanks.

Don Palmrose





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