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Posted by gregvan on June 24, 2007, 1:33 pm
Distillery changes seawater into drinking water:

Use an array of mirrors to focus sunlight on a tank containing
seawater.  When the seawater boils,
use the steam to drive a steam engine creating kinetic energy. After
the steam has been used by the
engine, let it condense and collect the pure water.  Remove excess
salt from the boiling tank and sell
for profit. Use the kinetic energy to drive a two water pumps. One
would pump the pure water to farms
and the other would fill the boiler tank with seawater. The condensed
steam would be safe to drink.

Diagram on http://gregvanderlaan.com

Posted by Jeff on June 26, 2007, 1:50 am
gregvan wrote:

A few calculations.

   Say water at 62F.

   The sensible heat to get one pound of water to 212F is 150 BTUs

   The latent heat of vaporization of 1 pound of water is roughly 970 BTUs.

   So 1120 BTU's/lb or rougly 9330 BTU to get 1 gallon of distilled
water, assuming 100% efficient. Not sure what the salt content will do...

   Perhaps 1000 BTUs per square foot of collector/day so you will need
lots of surface area to get any appreciable drinking water.

   If you must do this solar, perhaps just a large horizontal sheet of
plastic  weighted in the center so condensate will drip into a
collecting container. Much cheaper... And I'm not sure you'd want to
drink water that has been through a steam engine. Also,  I doubt the
steam engine efficiency would be more than 5% or so...

   Otherwise, it's good to have you thinking solar!


Posted by Anthony Matonak on June 26, 2007, 2:05 am
 Jeff wrote:

Where does all that heat go? As I understand it, you have to add that
heat to make the water boil and then you have to remove heat to make
the steam condense back to water. What if you took the heat you remove
and add it to the next batch of water? You use the incoming cold water
to cool the steam and use the steam to heat the cold water. How many
BTUs do you need then? :)

No, I don't know what multiple effect distillation means.

Posted by Jeff on June 26, 2007, 11:48 am
 Anthony Matonak wrote:

I would think you'd be able to gain much of that 150 BTUs back needed to
raise the temperature near boiling. Since the  amount of water being
preheated would be the same as the amount condensed it woule be
pointless to preheat more water, in other words the most usefull heat
you'd recover was that 150 btus and you'd still have to add 970 BTUs at
212F +.

   I did a little looking around and in the small scale most use reverse
osmosis, and in the large scale, particularly where you already have
"waste" energy, they use multistage flash distillation. Only a small
amount of incoming seawater is flashed into steam (in a partial vacuum)
and some heat is recovered as you've suggested, this is then passed into
the next stage..

   At any rate, my point is that the OPs original idea has some serious
implementation problems. The cheap lunch is expensive! Always seems to
go that way!


You use the incoming cold water

Posted by nicksanspam on June 26, 2007, 6:21 pm

If 1 ft^2 of glazing at temp Tg (F) on a perfectly insulated box transmits
200 Btu/ft^2 of sun on a 50 F day, and it has 2 Btu/h-F of outdoor airfilm
conductance, 200 = (Tg-50)2 makes Tg = 150 F, no matter what's inside, so
long as the solar power in equals the heat power out of the box, no?

If we add water to make a solar still with a black pan on the bottom and
condensation on the underside of the glazing. The vapor pressure under
the glazing Pa1 = e^(17.863-9621/(150+460)) = 8.092 "Hg, approximately. If
the water surface loses 100(Pw1-Pa1) = 200 Btu/h by evaporation (an ASHRAE
swimming pool formula), Pw1 = 10.092, so Tw1 = 9621/(17.863-ln(Pw1)) - 460
= 158.7 F, using the same approximation, and we distill about 200/1000
= 0.2 pounds of water per hour into a perimeter gutter below the glazing.

If we put another transparent water tray between the pan and the glazing,
the vapor pressure below it will be Pa2 = Pw1, and 100(Pw2-Pa2) = 200 makes
Pw2 = Pw1 + 2 = 12.092, so Tw2 = 9621/(17.863-ln(Pw2)) - 460 = 165.9 F,
and we distill a total of 0.4 pounds of water per hour (2 effects), with
condensation from the lower tray heating the upper tray. And so on.

The box won't be perfectly insulated, and the trays won't be perfectly
transparent, and some of the sun will hit the sides of the box, and some
will be reflected out of the box, and there will be some temperature and
humidity stratification, but this seems like the basic idea.


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