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re: solar options
29 jan 2003
wrote:
> np > the glazing could be a single layer of dynaglas clear corrugated
> > polycarbonate...
>www.spscorp.com... provides a decent amount of info about dynaglas...
>-- your posts appear to assume a shgc of 0.9 for dynaglas, but all i can
>find in the technical info for dynaglas on the above-mentioned web-site is:
> solar heat gain, ft.2 (btu/hr.)
> clear 218
i haven't looked at the web site, but page 33 of the nraes-33 greenhouse
engineering book says polycarbonate has a 91-94% light (par) transmittance
from 0.38 to 2.5 microns and <3% thermal transmittance at 10 microns from
a 70 f black body. duffie and beckman say the index is 1.6 vs 1.526 for
glass, which makes the fresnel loss 2((1.6-1)/(1.6+1))^2, ie 10.7%, or less,
with a layer of condensation, or more, with a layer of dust, or more, as
it ages.
>i'm assuming this is how much heat energy passes through the dynaglas
>per hour given some standard external load, but it's not clear to me
>what that standard external load is. extrapolating from your shgc of
>0.9, one would assume a standard external load of something like 242
>btu/ft^2/hr, which doesn't quite jibe with my recollection of full
>sunlight at roughly 1000 w/m^2 (317 btu/ft^2/hr).
rich komp (in "practical photovoltaics") says we might see 1 kw/m^2
("air mass 1") in the sahara at noon, but am2 is more typical, at
800 w/m^2, ie 254 btu/h-ft^2... 218/254 would be 86% transmission.
nick
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