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re: solar thermal calc'ing for angles
10 aug 1998
raul almquist wrote:
> in my locality we experience the sun at an angle of 66 degree in the
>summer and 22 degrees in the winter.
i guess you live at about 44.5 north latitude.
> now what i would like to know is where one can get, or else figure
>out... the angle the sun is from late fall to late spring, to use in
>building roof over hangs for optimized solar gain when needed during the
>cold months and to limit or eliminate solar gain during the hot and humid
>months.
you can find this shading info at http://rredc.nrel.gov, in their
"solar radiation data manual for buildings."
you can calculate it (approximately) based on your 66 and 22 (the
difference should be 47, ie 90-lat+/-23.5) degree max solar elevations
at noon on 6/21 and 12/21, by finding the ns length of an overhang that
will admit all of the noon sun on 12/21 and none of the noon sun on 6/21.
the overhang would be spaced enough above the window so that the lower
edge of its shadow just touches the upper edge of the window on 12/21...
. .
. . w
so if the window and overhang look like this .______
(in a non-proportional font like courier), . |
tan(22) = s/w, 22 . . | s
so s = w tan(22), and . . . . . .|
. .
tan(66) = (h+s)/w, .
so w = (h+s)/tan(66) .
= (h+w tan(22))/tan(66) . . h
= (h+0.404w)/2.25 .
= 0.445h+0.180w, so 66 ..
0.82w = 0.445h, and . . .
w = 0.54h
for instance, if h = 4', w = 2.17' and s = w tan(22) = 0.88',
if i did the trig right.
laren corie points out that you can do a lot better in the summertime
with movable overhangs. he extends the ceiling joists out to the south
and stacks boards flat on top in the summer, and stacks them up closer
in to the glazing in wintertime. he says varying the board width makes
this interesting to look at.
nick
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