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re: solar heated greenhouse saga, update dec 4, 2002 (with question)
5 dec 2002
lynn coffelt wrote:
>what manner of insulation would be necessary to keep, say, 50 gal drum of
>150f water for a couple of months with a target of maybe 100f remaining??
c = 8x50 = 400 btu/f. in, say, 55 f air, 100 = 55+(150-55)exp(-24x60/rc), so
rc = -24x60/ln((100-55)/(150-55)) = 1927 hours, and r = rc/c = 4.8 h-f/btu.
the drum has about 25 ft^2 of surface, so you need an r value of 25r = r120,
eg a styrofoam box with 2' thick walls...
>practical? probably not, but with the last tomato picked thanksgiving
>(started producing in june) and rough calculations made, i think the average
>cost was only about $50 each. practicality hasn't been achieved this year,
>but there's always next year.
pe howdy reichmuth's ecotope concentrating greenhouse at pragtree farm near
seattle works fine. you might go take a look. bill sturm built a 12k ft^2
bubblewall tomato greenhouse in calgary. with no heat storage, he saved 82%
on propane on -20 f nights with foam, compared to -20 f nights with the
walls unfoamed.
december is the worst-case month for greenhouse heating in seattle, with
420 btu/ft^2 on a south wall and 250 on a level surface on an average 40.5 f
day. you might put up a 30'x48' plastic film greenhouse with foam a foot
thick and a nighttime thermal conductance of 2969 ft^2/r24 = 124 btu/h-f.
you could make a 2' high bench along each ew wall with a 2' diameter poly
duct full of water inside. with an r10 foamboard top and an r10 north wall
and a 2' tall layer of r1 polycarbonate with 90% solar transmission along
the south wall, each linear foot of duct inside the south bench would gain
2ft^2x420x0.9^3 = 612 btu/day and lose about 6h(t-40.5)2ft^2/r2 (for the
south wall during the day) plus 18h(t-40.5)2ft^2/r24 (for the south wall
at night) plus 24h(t-65)4ft^2/r10 (for the rest of the walls.) this makes
the water temp 90 f, after a long string of average days, if the south duct
contributes no heat at night on an average day.
the greenhouse would need 5x24(55-40.5)124 = 216k btu to stay 55 f inside
for 5 cloudy days in a row. the 9651 pounds of water inside the south duct
could act as a ballast foundation and supply 216k btu by cooling from 90
to 90-216k/9651 = 67.6 f.
the north bench could take care of overnight heat storage on an average day.
with a 1000 cfm fan moving air past the duct most of the time, the day time
constant would be about 9651/1000 = 9.65 hours. at night, the resistance
from water to outdoors would be 1/1000+1/124 = 0.00906, for a 9651x0.00906
= 87.5 hour night time constant. if the greenhouse is 70 f for 6 hours on
an average day and the south duct water has temp tn at dawn and reaches
a max temp td at dusk, td = 70+(tn-70)e^(-6/9.65) = 32.4+0.537tn. the duct
would cool to tn = 40.5 + (td-40.5)e^(-18/87.5) = 7.53+0.814td by dawn.
substitution makes td = 64.7 and tn = 60.2 f. at dawn, 2176 btu/h would
flow out of the water, making the greenhouse air temp 40.5+2176/124 = 58 f.
the north duct might work well as a dehumidifier if the inner glazing temp
is greater than 64.7. we might ensure that with some shadecloth near the
inner film to act as a parasitic heater.
nick
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