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Posted by on August 2, 2006, 9:23 am
>>I am planning on building a solar heater on my shop (south side), the
>>size would be 12'x40'...
R19 walls and an R60 roof and 102 ft^2 of R2 windows and 0.5 ACH of air
leaks and a 34x54/R60 = 31 Btu/h-F roof conductance and 102/2 = 51 for
windows with 136' of non-collector wall with (12.5x136-102)/R19 = 84 and
0.5x12.5x34x54/60 = 191 for 191 cfm of air leaks totals 357 Btu/h-F.
>... With a 70 F shop air temp, R1 Dynaglas... with 90% solar transmission
>might collect 0.9x902 = 812 Btu/ft^2 and lose 6h(70-26)1ft^2/R1 = 264
>for a net gain of 548, ie 12x40x548 = 263K Btu/day.
If the shop is used 6h/day and it doesn't store much heat, it needs about
6h(70-26)357 = 94.2K Btu/day, which leaves 168.8K for the house, collected
at 168.8K/6h = 28.1K Btu/h. A 2'-4' deep heater with 70 F air in the space
between black fiberglass window screen with 67% open area with the top of
the screen attached to the south edge of the heater ceiling and the bottom
attached near the shop wall and 150 F air in the space under the heater
ceiling might heat 150-28.1K/1600 = 132 F water with 320' of fin-tube pipe
or 2 used 800 Btu/h-F auto radiators. Hot collector air might thermosyphon
up, travel through the radiators and then back down a duct into the bottom
of the collector, with a fan and a shop temp thermostat and an occupancy
sensor to keep the shop 70 F.
>How can we predict the performance of an air heater with a mesh absorber
>with some warm shop air flowing from south to north through the mesh and
>hotter air thermosyphoning from the space above the mesh through some auto
>radiators and back down a duct into the lower part of the heater, to
>the north of the mesh? The shop air flow tends to keep air moving into
>the mesh, like a breathing wall, but if we reduce the flow of shop air,
>at some point, hot air will want to naturally circulate south from the top
>of the space to the north of the mesh through the mesh, downwards next to
>the cool glazing, and back north through the mesh near the bottom.
So at the top of the mesh we have 70 F air to keep the shop warm flowing
north through the mesh fighting 150 F air that wants to naturally circulate
south through the mesh, because its hot air column to the north of the mesh
is lighter than the 70 F column of air next to the glazing. If the 70 F air
weighs 0.075 lb/ft^3 and 140 F air weighs 0.075(460+70)/(460+150) = 0.065
lb/ft^3 and the heater is 12.5' high, the 150 air bouyancy force would be
12.5(0.075-0.065) = 0.0125 psf, or 0.0024 "Hg, within the range of a fan.
The shop needs 15.7K Btu/h, ie 15.7K/(150-70) = 200 cfm min, ie 0.4 cfm/ft^2
of mesh. If cfm = 27900Asqrt(dP) with A in ft^2 and dP in psf, each square
foot of mesh needs an A = 0.4/(27900sqrt(0.0125)) = 0.00013 ft^2 hole, ie
a 0.018 in^2 hole, ie a 0.15" hole. We might jab 1/4" holes with flaps using
a pencil through dark Typar mesh on a 1' grid behind the window screen and
use a smaller fan to move 10 cfm of 70 F air through the mesh when the flaps
are closed and the 200+ cfm fan isn't running.
Nick
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