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re: hair brained solar cistern???
9 jan 2003
news wrote:
>you can build a 100% solar home, bt it will cost!
maybe not. a 2k ft^2 2-story house with r40 6.5" urethane sip walls and
130 ft^2 of u0.38 south windows with 46% solar transmission (shgc = 0.46)
and a thermal conductance of 214 btu/h-f needs 24h(65-27.4)214 = 193k btu
on an average december day in worchester, ma. if 300 kwh/mo of electrical
energy use contributes 34k of that and 0.46x130x860 = 51k comes in south
windows, we only need 107.6k more.
a square foot of r1 vertical south air heater or sunspace glazing with 90%
solar transmission and 80 f air on the inside would gain 0.9x860 = 774 btu
of sun and lose about 6h(80-27.4)1ft^2/r1 = 316, for a net gain of 458. we
might heat the house on an average day with 107.6k/458 = 235 ft^2 of extra
south glazing, eg a 32'x8' tall single layer of polycarbonate glazing on
the outside of an open stud wall with sips on the inside, or over exposed
posts and beams at the south edge of a second floor cantilevered 4' to the
south of the first floor, forming a 4'x32' arcade with a transparent wall
beneath, for a medieval look. uk planners might like this, from a distance.
we need about 18h/24hx193k = 145k btu of overnight heat. with a 10 f daily
temp swing, 7k btu/f of inherent house thermal mass and furnishings with
a short (2 hour) time constant could store 70k btu. we might store the rest
in 150 10'x4" pvc water pipes among rafters in 600 ft^2 of basement ceiling.
we need 5(193k-34k) = 795k btu for 5 cloudy 27 f days, at (65-27)214 = 8132
btu/h. with 2250 btu/h-f of thermal conductance to room air, the pipes could
warm the house with 65+8132/2250 = 69 f water. a 4x8x8' tall epdm-rubber-
lined r40 sip boxful cooling from 69+795k/(256+64) = 118 to 69 f would lose
24h(118-65)256ft^2/r40 = 8,140 btu/day to house air. we might add 50k btu/day
for showers, with a 42 gallon galvanized pressurized tank inside and two
unpressurized plastic drums to warm house air as greywater heat exchangers.
a square foot of 45 degree south roof would get 0.707(860+480) = 948 btu on
an average december day. trickling 130 f water between a dark metal roof and
2 layers of polycarbonate glazing, we might gain 0.8x848 = 758 btu and lose
about 6h(130-30)1ft^2/r2 = 300, for another net gain of 458 and 58140/458
= 127 ft^2 of roof collector, eg a 12'x12' patch...
nick
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