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re: solar collector glazing
20 apr 2001
stanley pesis   wrote:

>...i understand that the primary, if not only, function of the glazing
>is to prevent heat loss through convection during the time the sun shines
>on the collector. 

it also reduces reradiation loss. the hot collector radiates heat back
at the glazing, which can block outgoing ir while passing most of the
incoming shortwave solar radiation. this is "the greenhouse effect."
glass and polycarb have it, with a 2% longwave ir transmittance, vs
polyethylene film, which transmits 80% of longwave ir. 

>therefore it doesn't really matter about double or triple glazing and
>in fact such glazing lowers the efficiency of the collector.

that depends on the amount of sun and the outdoor and collector temps.
for instance, with 250 btu/h-ft^2 and a 30 f outdoor temp and a collector
temp t, 2 layers of r1 glazing with 90% solar transmission will be more
energy-efficient than 1 if 0.81(250)-(t-30)/r2 > 0.9(250)-(t-30)/r1, ie
t > 75 f. but 1 layer is more cost-effective... 

>nick, i believe, has suggested the use of a polycarbonate sheet...
>is that stuff available in 5'x11' sheets or must one buy much larger
>amounts of material and cut it down to proper size?

you can buy flat clear replex polycarb in $300 49"x50'x0.020" thick rolls
from 877-rimol-gh, or maybe find someone to sell you a piece of a roll... 

>one posting indicated the sheet was capable of being cut with a scissors;
>how resistant to puncture is polycarbonate.


>i'd be trying to mount the sheet within an aluminum frame.

you might use holes and screws. it will be looser on a warm day...

>i guess one alternate possibility might be to stretch the polycarbonate
>sheet across the face of the collector and wrap it around the edges to
>the back of the collector and use duct tape to keep it in place.

the duct tape sounds rather temporary, and this 0.020" polycarbonate
is a little too stiff to bend that way.

btw, the glazing will be cooler than the collector. a single layer might
average the collector and outdoor temps, so a 400 f collector might make
the glazing 250 f on a 100 f day.

you might use an overhang for shading (reflective underneath :-) or vent
the box at the top with a $12 leslie-locke afv-1b automatic foundation vent
that opens at 150 f to lower the stagnation temp. an 8"x16" (0.9ft^2) vent
at the top and an 8"x16" hole at the bottom of a 4'x8' collector would make
cdt = 0.9x250x4x8 = 16.6(0.9)sqrt(8)dt^1.5, so dt = 31 f. 


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