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re: big metal buildings!
8 feb 2001
nick pine wrote:
>chuck wrote:
>>i have a friend who is in the process of building 2 - 60x80 buildings...
>maybe solar heat, with some sort of sunspace on the south side...
thinking about the 60x80... nrel says the average january temperature
is 30 f where i live near phila, and 1000 btu/ft^2 of sun per day falls
on a south wall and 729 btu/ft^2 of beam sun falls on a solar attic...
a 60x80x12' tall building with rv exterior surfaces has about 12960ft^2/rv
btu/h-f of conductance. keeping it warm for 5 cloudy days with g gallons of
(say) 180 f water cooling to 90 f makes 5x24(70-30)12960/rv = 8g(180-90), so
g = 86.4k/rv, eg 5760 gallons with r15 walls and ceiling and floor, eg r7.2
atlas energy-shield polyisocyanurate 1" double-foil foamboard on the walls
and ceiling, with 1.5" styrofoam under a radiant concrete floor.
we might store 6144 gallons of hot water in a 4'x8'x24' plywood box lined
with a single $240 20x40' piece of folded epdm rubber roofing material.
on an average day, the shop needs 24h(70f-30f)12960ft^2/r15 = 829k btu
for 24 hours or 207k over 6 hours. r1 sunspace glazing (a single layer of
vertical dynaglas corrugated polycarbonate) with 90% transmission might
collect 900 btu/ft^2 over 6 hours and lose 6h(100f-30f)1ft^2/r1 = 420 for
a net gain of 480 btu/ft^2, so 207k/480 = 432 ft^2 of sunspace glazing (eg
12'x36') could keep the shop warm for 6 hours. (this could store some heat
in the floor and furnishings, and we might have a night setback and internal
heat gain and air infiltration, but let's ignore all that for now.)
a solar attic might gather the other 829k-207k = 620k btu/day. a square
foot of solar aperture under 90% transmissive roof glazing with a 90%
reflector and 90% target glazing can gather 0.9x0.9x0.9x729 = 531 btu,
so we need about 620k/531 = 1167 ft^2 of solar roof aperture, eg 80x16',
with a ridgeline 16' above the walls, roughly a 6:12 pitch, and dynaglas
(about $1.50/ft^2, with a 20 year expected lifetime) over the south half
and a parabolic shape under the north half, lined with nielsen's reflective
mylar film (10 cents/ft^2 in 4' wide rolls.)
at dawn, the sun would focus on a line 16^2/(4x30') = 2.1' from the north
wall, and the focus line would move north during the day, so we might use
a 4'x80' horizontal epdm rubber collecting trough under 4' of polycarbonate
glazing along the north attic floor. with 4:1 solar concentration and an
average of only 3 hours of beam sun, the heat loss to the warm attic would
be small, even at 180 f, and the heat loss from the storage tank would
contribute to the average daily heat gain of the building.
this system could also provide lots of hot water and daylight for the shop,
with a few skylights in the ceiling.
nick
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