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re: glazing revisited
11 mar 1999
doug otis   wrote:

>the glass is to shield against convection cooling, air sealing
>away the heat mechanically.

glass and polycarbonate also block longwave ir reradiation. glass
passes most of the solar wavelengths shorter than 3 microns and blocks
most of the heat that tries to escape back out from the collector, eg
the heat from an 80 f surface with a black body radiation spectrum 
with a power peak at 10 microns. 

>using a nitrogen gas sealed under the glass would increase
>the insulation properties.

air is 80% nitrogen. perhaps you are thinking of argon. 

>also you should not care about the ir spectrum with respect to the
>glass.  this frequency does not provide much of the energy used
>to heat the panels.  there is far more energy in higher
>frequency light with energy proportional to frequency.

the ir caring has more to do with keeping the heat in than collecting it. 

>the coating on the metal should block ir to work the best at
>trapping the ir light and preventing it radiating away.

it would be nice to have a coating with poor longwave ir emission,
vs "blocking," which any opaque material does, no? we'd like a
low-emissivity selective surface, but they seem expensive.   

>you could look to see which shield exposes uv
>sensitive paper fastest for your selection.

glass mostly blocks uv, which seems irrelevant here. 

>you could do a comparative measure of two small panels for heat rise.  

that sounds like fun. a collector race. i raced some bayer urethane
film against some greenhouse polyethylene film a few months ago,
side by side over a 2'x4'x3" thick plywood box painted black on the
inside, with a baffle between them and 1.5" of styrofoam over the back.
the urethane won, with a stagnation temp of 214 f in full sun, vs 207
for the poly, inside my 70 f poly film sunspace.

nick




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