| nicksanspam@ece.villanova.edu 2006-12-26, 9:25 am |
| buck <nowayjose@covad.net> wrote:
>If I want to collect as much solar heat through a window as possible, and
>don't care about insulating values will I be better off with a single pane
>or a double pane glass?
Single, or none :-)
>... would a double pane save more of the heat loss at night and therefor
>it would be a wash???
Double is likely better, for a window on a living space that's 70 F for
24 hours per day. Where I live near Phila, 1000 Btu/ft^2 falls on a south
wall on a 30 F average January day. If each pane has 90% transmission and
adds R1, one would transmit 900 Btu/day and lose 24h(70-30)1ft^2/R1, for
a net gain of -60 Btu/day. Two would have a 330 Btu/day gain, 3 would have
409, 4 would have 416, and more than 4 would have less net gain...
10 FOR P=1 TO 10'number of panes
20 SUN=1000*.9^P'sun passing through window (Btu/ft^2)
30 LOSS=24*(70-30)/P'24-hour heat loss through window (Btu/ft^2)
40 PRINT P,SUN-LOSS
50 NEXT
# panes net gain (Btu/day)
1 -60
2 330
3 408.9999
4 416.0999
5 398.4899
6 371.4409
7 341.1539
8 310.4671
9 280.7537
10 252.6783
If S Btu/ft^2 falls on the wall on an average T (F) temp day, the net
gain = S0.9^P-24h(70-T)/P; dgain/dp = 0 makes 24(70-T) = -P^2ln0.9x0.9^PS,
eg P = e^(P+21)/19 for Phila. Plugging in P = 2 on the right makes P = 3.4
on the left, then 3.6, 3.64, and 3.65...
A single pane window (or a single layer of polycarbonate plastic) on a low-
thermal mass sunspace that gets cold at night might gain 900 Btu/day and lose
6h(70-30)1ft^2/R1 = 240, for a net gain of 660. Two panes might gain 810 and
lose 120, for a net gain of 690... not much more, but condensation where
moist house air touches the indoor glazing surface would be less likely.
Nick
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