[Prev][Next][Index][Thread]

Re: Today's high, in the solar attic



>Dan Settles <dan_settles@cellbio.duke.edu> wrote:
  
>I was wondering if you are approaching the upper temperature limit for the
>polycarbonate...

Three more thoughts: 

My polycarbonate plastic roof is very thin, a single layer 0.020" thick, and
the plastic itself has a fairly high thermal conductivity, so it will
probably have a uniform temperature that is halfway between the attic air
temperature and the outdoor temperature, like a paper cup used to boil water
over a candle.

Even if the attic were perfectly insulated, the attic air temperature has
a natural limit, based on radiation losses. If the peak solar power into
a square foot of glazing were 300 Btu/hour, and there were no thermal storage
in the attic, and the outdoor temperature were 30 F, and all of the solar
power left through the glazing by radiation (it doesn't, in this case--
Table 4.2 of _Greenhouse Engineering_ (3rd revision, August, 1994, published
by the Northeast Regional Agricultural Engineering Service at Cornell,
152 Riley-Robb Hall/Cooperative Extension/Ithaca, NY 14853-5701) lists the
solar and infrared transmissivities of polycarbonate as 0.85 and 0.01,
_better_ than a single layer of window glass, listed at 0.85 and 0.02--
meltdowns are less likely with polyethylene film :-), which has solar and
infrared transmissivities of 0.92 and 0.81) we have a maximum attic air
temperature of roughly 122 F, from the following formula:

300 = 0.174 x 10^-8 ((Ta+460)^4 - (30+460)^4).

If the R-value of the inside air film under the plastic were 2/3, and the
plastic itself and outside air film were perfectly conductive, and the plastic
passed no infrared radiation at all, the maximum attic air temperature would
be closer to 478 F, using Ohm's law for heatflow:

300 = (Ta-30)/R0.67.

Double glazing would be more dangerous.

Nick


Follow-Ups: References: