Posted by nicksanspam on May 27, 2008, 8:04 pm
What happened on cloudy days?
NREL says 930 Btu/ft^2 falls on a south wall in Des Moines on an average
24.4 F December day, so 1 ft^2 of white R20 south wall with a U2 airfilm
conductance would lose about 24h(65-24.4)1ft^2/(R20.5) = 47.5 Btu/day.
We could model a dark south wall like this, viewed in a fixed font:
930/24h = 38.75 Btu/h
|--------|-->|------------www--- 65 F
which is equivalent to:
---------www-------------www--- 65 F
| 24.4+38.75/2 = 43.78 F
with a heat loss of 24h(65-43.78)1ft^2/20.5 = 24.8 Btu/day.
With an 8'x48' wall, the difference is 8x48(47.5-24.8) = 8698 Btu/day.
If a single-glazed air heater gains 0.9x930 = 837 Btu/ft^2 and loses
6h(80-24.4)1ft^2/R1 = 334 on an average December day, for a net gain
of 503, the dark wall is equivalent to 8698/503 = 17 ft^2 of air heater.
Posted by Morris Dovey on May 27, 2008, 7:25 pm
It stayed warm inside. :-)
As you'd expect, the panels aren't single glazed - and, regrettably,
there wasn't any instrumentation in place. :-(
The only winter days I was there (one of them /was/ overcast, BTW), the
heater had not been run and the indoor temperature was in the mid-70's(F).
I'm willing to believe that if the siding had been painted black, /it/
might've have been a bit warmer, but I'm not sure how much that would've
contributed to indoor comfort.
DeSoto, Iowa USA
Posted by nicksanspam on May 28, 2008, 4:22 pm
PE Norman Saunders patented "solar staircase" has mirrors for treads
and transparent risers...
But shading based on detailed summer sun angles isn't as effective as
80% vertical shadecloth over the outside of a window, since a lot of
summer sun is diffuse, eg 65% on a south wall in Austin in August.