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Effect of Humidity on Temperature

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Posted by Lee on March 6, 2007, 1:58 am

I've been experimenting with a "trickle down" solar collector.   This
is basically a sheet of black roofing iron in a box with a glass
lid.   Water runs down the sheet and is heated.

I noticed a strange phenomenon.   As condensation formed on the inside
of the glass the air temperature in the box increased by about 20%.
Also I noticed that 1 part of the glass where there was no
condensation was significantly hotter to touch from the outside than
the part with condensation.   I would have expected the opposite - as
the water evaporated I thought it would cool and also that the
condesation would block the light entering the box.

Can anyone explain this?   Maybe my observations were flawed.



Posted by Grimly Curmudgeon on March 6, 2007, 10:43 am
We were somewhere around Barstow, on the edge of the desert, when the
saying something like:

I suspect it's a mini-greenhouse effect, whereby the bounced infra-red
can't exit the box through the glass which has the condensation on it,
and the temp rises. The clear patch is allowing IR to exit and feels



Posted by Jeff on March 7, 2007, 7:56 am
 Grimly Curmudgeon wrote:

   There's not really much info on this.  Most glazings absorb IR and
it's unusual to find one that is either IR transparent or IR reflective.
No one is building collectors that are IR reflective, probably because
selective coatings are effective and IR reflective films and coatings
have so much loss.

   Still, it's an interesting idea.


Posted by Lee on March 9, 2007, 9:24 pm
 Thanks to all who replied on this topic - some very interesting
points.    Someone mentioned Harry Thomason.  It is in fact his ideas
that I am following up on.    I haven't really done any decent testing
yet but so far the trickle down collector seems to work well.    I
guess the only problem is that you need to make the inside of your box
quite water resistant to handle the constant humidity.    I'm actually
wondering whether using some other heat transfer fluid like say
cooking oil might be good but then I guess you would have to top it up
every now and then as there would be some small amount of evaporation.


Posted by Michelle P on March 6, 2007, 1:49 pm
 Lee wrote:

Water will hold more heat for a given volume than a gas (air).

Michelle P

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