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Design variations for solar space heater

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Posted by Johnnyo on July 14, 2008, 5:41 pm
I'm in the planing and design phase for an 8ftx16ft collector for
supplemental winter space heating for our home in rural PA.   Firm
requirements include vertical wall mounting, fan circulation and
opposite corner air inlet and outlet (cold in at the bottom and hot
out at the top).  Glazing will be Sun-Lite HP and back of panel
insulation will be rigid polyiso panels.

Reading posts from the SMEs on this forum as well as many other
sources suggests that there are a lot of potential variations in
absorber materials (window screen, aluminum sheet, filter fiber,
felt), baffle/air channel configuration, absorber placement etc...

So the big question is, does anyone know of documented comparative
testing along these lines?  I've looked but have not found much, so
I'm considering building a 4ftx8ft scale test bed where I can easily
swap out or change the "innards" in support of an iterative design

(a newbie member here)

Posted by Morris Dovey on July 14, 2008, 5:56 pm
Johnnyo wrote:

Interesting. Why have you made fan circulation and opposite corner
intake/discharge firm requirements?

Yup, you do have your work cut out for you...

Suggest you build your test bed for full height at whatever width seems
convenient - so as to get a more accurate picture of flow and output

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by Johnnyo on July 14, 2008, 6:27 pm
the configuration of the rooms and windows on the south side of my
house drove the intake/discharge locations.   i chose fan circulation
since i already have several suitable DC fans and a PV panel to drive

the final collector would be 8ft tall by 16ft wide, so i thought a
test bed with the same aspect ratio would be ideal.  if i go with the
full height, by maybe 4ft wide, wouldn't i miss important horizontal
flow effects that i would need to address in the full size version?
i've been assuming (possibly incorrectly) that with the corner intake/
discharge location, i will end up using baffles to create a horizontal
serpentine air flow ( a 3 channel S flow, or maybe 5 channels?)

Posted by Morris Dovey on July 14, 2008, 8:48 pm
 Johnnyo wrote:

Hmm - ok. This doesn't compute, but I'll assume that you know what
you're doing here.

It's nice to have this stuff, but fans are only needed if you hose up
the design so badly that natural convection won't work.

Please, no! No serpentine baffles. You want air to flow into, through,
and out of the collector the collector as quickly and freely as you can
manage. To achieve that, you need to minimize horizontal flows and
optimize the daylights [pun not really intended] out of the vertical flows.

I know it probably seems counter-intuitive, but you want the collector
to run as cool as you can manage.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by nicksanspam on July 15, 2008, 12:27 pm

The glazing might be 2 $4 4'x16' sheets of Dynaglas corrugated
polycarbonate greenhouse roofing from Griffin in Morgantown, PA
installed as "solar siding."

A 70 F room on a 30 F day and a C cfm fan in full sun (250 Btu/h-ft^2)
and fully-mixed solar-warmed air at temperature T (F) near the glazing
would look something like this, viewed in a fixed font:

0.9x250x8x16 = 28.8K Btu/h       T
         ---                     |         1/C
|-------|-->|--------------------*---------www--- 70
         ---                     |
    R1/(8x16) = 1/128            |
30 ------www---------------------

which is equivalent to this:

         1/128      |        1/C
   -------www----------------www--- 70
  | 30+28.8K/128 = 255 F   I --->

I = (255-70)/(1/128+1/C) = 23.7KC/(128+C) Btu/h with collection efficiency
E = 100I/28.8K = 82C/(128+C)% and average heater air temp T = 70+I/C.

C = 100 cfm makes I = 10.4K Btu/h and E = 36% and T = 174 F.
C = 500 cfm makes I = 18.9K Btu/h and E = 66% and T = 108 F.

What is your fan cfm?

With no fans, just holes at the top and bottom to allow thermosyphoning:

                    T      I --->
         1/128      |        ---
   -------www---------------|-->|-- 70
  |                          ---                    
  | 255 F  

According to an empirical chimney formula, I = 16.6Asqrt(H)dT^1.5 Btu/h,
with 2 A ft^2 vents and an H' vertical separation. With a 16'x4" slot at
the top and bottom and H = 8', I = 250(T-70)^1.5, and T = 255-I/128, so
T = 70+((255-T)/1.96)^(2/3). Plugging in T = 100 F on the right makes
T = 88.4 on the left. Repeating makes T = 89.3, then 89.2, with
I = 21.1K Btu/h and E = 73%.

These collectors can be more efficient with a "transpired absorber,"
some sort of mesh that allows 70 F air to flow up between the mesh
and the glazing and back from south to north through the solar-warmed
mesh into the house. This keeps cooler air near the glazing and reduces
reradiation loss through the glazing. The house wall behind the mesh
should be dark, eg dark green or black.

Gary Reysa has done some of that. I like his air heater design:


I'd use a single layer of black fiberglass window screen for the mesh.

Why fuss around with a smaller version, especially if you insist on fans?


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