From: gary@ke4zv.uucp (Gary Coffman)
Subject: Re: Request: info on desalination and solar energy
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group
Date: Wed, 18 Nov 1992 18:23:52 GMT

In article <1992Nov18.024339.8885@inel.gov> dpe@inel.gov (Don Palmrose) writes:
>
>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?

Don, the system you describe sounds like a shipboard system. It puts a
premium on the amount of *space* it occupies, and on the amount of
*fuel* it consumes. The type of desalination plant we're talking about
is much simpler. It consists of a transparent cover over shallow black
trays. Salt water is slowly fed into the trays by gravity flow. The water
evaporates and condenses on the inside of the transparent covering. It
then drains down to channels at the lower edges of the cover and flows
by gravity out of the system. To make lots of desalinized water, this
system has to be *big*, but if space isn't at a premium, it practically
runs itself, and the fuel is free. Some pumping is required to keep the
salt water reservoir filled, and to carry away the desalinized water,
but this can be a batch process from tanks that can be done by solar
steam engines, or by hand power. 

Such systems produce about 4 liters of fresh water per square meter per 
day. To supply my county's daily consumption, 4 million liters, would
require a collector surface of about 1 million square meters, or a 
square 1 km on a side. That's about $12,000,000 worth of land at local
real estate prices. Our current water treatment plant cost more than
that and it starts with fresh lake water.

Note also that these plants can be made to *float* and the pumping
can be done by wave action, thus taking up no valuable land area
or requiring any special energy source for the pumps. A storm could
play hell with the plant though.

Gary
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