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[DPRG] Solar still

Subject: [DPRG] Solar still
From: Randy M. Dumse rmd at newmicros.com
Date: Mon Dec 23 16:23:01 CST 2002

Ed,

You'll get about 100 watts per square meter on peak sunny days.
With conversion inefficiencies, you'd be doing good if that
turned into 70 watts of heating. So with a big dish, only a few
hundred watts. To get the solar heating you're looking for,
you'd need something like an acre of collectors.

Think of it this way. Sun light is strong enough in the summer
to heat your pool to 80 degrees. As the sun is lower in the sky
in the winter, the light is less normal to the surface of the
earth, so it contains less energy per square on the ground. What
you're looking for is 1) to collect enough energy to make up
that difference, 2) and collect enough more energy to make up
for the heat loss to the cold stuff around the pool (that was 80
degrees or better, just like the pool, in the summer). So if the
sun is 46 degrees lower in the sky (difference from Tropic of
Cancer for summer to Tropic of Capricorn for winter) so you are
only getting abou 72% of the incoming solar energy on your pool
that you would in summer. So if you had a reflector that could
put the heat from an area 40% larger than than your pool, you'd
have the same amount of energy coming in.

But! that's only the first half of the problem. The other half
is everything around the pool is cooler, so you have to manage
the heat loss (the energy flowing out from the poll by transfer
to the colder bodies). I don't remember the formulas, except
perhaps it goes by the second or third power or such. Meaning, a
little difference in temperature... not much escape. A big
difference in temperature (such as your pool and the surrounding
ground in winter)... A HUGE AMOUNT of escape. You can poke
around here
http://scienceworld.wolfram.com/physics/topics/Heat.html if you
want to look for a better description of the math of heat
transfer.

Randy

> What I was
> hoping to find were
> formulas to help calculate how much heat I would get
> if I were to make a 6
> foot, or larger still.


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