After 300 years of simple heatflow physics, most people still doubt
that a house can be close to 100% solar heated outside of the Southwest,
inexpensively. We might move 8' D-cubes into regional Infestations of Doubt,
with 2" double-foil foamboard walls and ceiling and polycarbonate over
the south wall and an EPDM-rubber-lined heat storage tank under the floor,
which also serves as a ballast foundation. We might deploy these devices
near churches and schools and green building conferences and Renewable
We might warm a small low-mass house with sunspace air for 6 hours on
an average January day in Phila with 250 Btu/h-ft^2 of direct sun and a 34 F
average daytime temp and simultaneously store 18 hours of overnight heat
in hot water, using a low-power PV pump in a slow draindown system...
If we collect 200 Btu/h of 170 F overnight heat in B ft^2 of Rich Komp's
homemade Big Fins (a 1/2" copper pipe pounded into a groove in a strip of
brown-painted aluminum coil stock) behind A ft^2 of R1 glazing with 90%
solar transmission (eg $1/ft^2 Dynaglas) and we also collect 50 Btu/h of
T (F) air and 200 = 225B-(170-T)1.5B = 1.5TB-30B for the Fins,
T = (200+30B)/(1.5B).
If A=B and 225B%0+(T-34)B, B=1.6 ft^2 and T3 F, with a 250/(1.6x250)
= 0.625 collection efficiency. We could move 50 Btu/h in a 50/(103-70)
= 1.5 cfm airstream.
If the Fins cost more than the Dynaglas, which is likely, with more labor
and rising aluminum coil stock (60 cents/ft^2) and copper pipe prices,
we could make B smaller and A larger, with a lower cost and a higher temp
and a lower collection efficiency, but easier hot air thermosyphoning.
For instance, A = 2 ft^2 makes 450 = 250+(T-43)2, so T = 143 F, and
200 = 225B-(170-143)B makes B = 1 ft^2. Scaling this up to A = 4'x8',
we could move 16x50/(143-70) = 11 cfm with an 8' height difference
through upper and lower sunspace vents, each having an area
Av = 11cfm/(16.6sqrt(8'(143F-70F)) = 0.0273 ft^2, ie 4 in^2.
Scaling up more, 2 $12 8"x16" automatic foundation vents might handle
16.6x8x16/144sqrt(8)(143-70)^1.5/(16x50) = 32 4'x8' glazing panels,
collecting 25.6K Btu/h of 143 F air and 102.4K Btu/h of 170 F water
(or more, with an average Fin temp less than 170 F) with about $1K of
Dynaglas and $1K of Fins, and this can be dry in full sun as a draindown
system with no damage. What's the equivalent evacuated tube system cost?
Leslie Locke's AF1B foundation vent has aluminum louvers and a bimetallic
coil spring which opens the louvers when the air temp near the spring rises
to about 60 F, but that soft threshold temp can be changed by turning
the spring mounting screw, and the spring can be removed and reversed
to make the louvers close vs open as the air temp rises, so it can work
as a crude room air temp thermostat in a thermosyphoning sunspace system.
For more accurate room temp control, we might add a $20 thermostat and
a $50 2-watt Honeywell 6161B1000 damper motor.