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)
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?)
The glazing might be 2 $64 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
R1/(8x16) = 1/128 |
which is equivalent to this:
1/128 | 1/C
| 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 | ---
| 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?
Thanks Nick and Morris! You are certainly schooling me here.
So i'm finally getting it - the fans are really of no value....better
to save the PV panel for some other project.
Nick - Thanks for the tip on the local Dynaglass source - i was really
wanting to use twinwall polycarb but have not been able to find it
locally, and the shipping/crating charges are steep for a small order
off the internet. Dynaglass seems a great solution.
It looks like the temps are getting rather high so i'm getting a bit
worried about code issues if i stay with a plywood back (thinking of
using existing exterior wall sheathing) and wood sides for the box.
(Thanks to Gary in his articles for pointing this out). Does anyone
know if lining the collector interior with foil faced polyiso would
satisfy the codes? Would think to use aluminum flashing for the
intake & exhaust.
Thanks again guys!
On Jul 15, 8:27 am, email@example.com wrote:
The least I can do is intercede on behalf of the poor fan - it really
is easy to justify using one. Do give it a chance, it will improve the
bottom line. Unless the PV panel is free, I'd go with AC.
So how to choose a fan? If other things are equal, when you increase
air flow, the collector runs cooler. That is the direction to go, up
to that certain personal tradeoff point. By that I mean that for air
collectors in general, a cfm is usually reached at which the fan is
too noisy or too expensive.
Anyway, please let us know how your tests come out, and tell us what
fans you try. My range (your mileage may vary), for a 30-35 square
foot collector would be: try fans ranging from 70 cfm (nice and quiet)
to 150 cfm (great thermal efficiency, but noisy.) The middle of that
range is a nice tradeoff. 105 to 120 cfm will usually do the job.
Hey, now that i'm on a roll, I'll advocate for a Very High Surface
Area Absorber. Just my 2 cents here, but I would choose a black
polyester felt, in a fiber density that catches say 80 percent of the
light on the first pass. Some seem to prefer screen, or a similar open
material, but I would go for a nice thick felt, or two layers of
Open area is not your friend in an air cooled absorber. An easy way of
evaluating a candidate absorber fabric is to check out the amount of
light passing through a sample. Carefully look through it toward the
sun. You should see a few glints of light; if too much light is
getting through, add a layer.
If you let lots of light through an absorber that's too open, it will
hit the planar back wall of the collector, which, is less efficient at
transferring heat to the air than the absorber. A good strategy is to
intercept as much light as possible as it makes its first pass through
the absorber. What gets through should then be directed back to the
absorber by a reflective back wall, rather than being absorbed by a
black back wall. The absorber is more efficiently cooled by the air
flow, so the collector runs cooler.
Let's say you decide to minimize openness, and to intercept most of
the light using a felt absorber, and that you also use a reflective
back wall. Then, please follow through and use Nick's suggested air
flow pattern - where air moves from the front, through the absorber,
to the rear. This is a "massively parallel" flow pattern. A serpentine
air flow, by contrast, will cause the downstream end of the absorber
to run hot. The ideal is for all areas of the absorber to run at the
same temperature. Any hot spots on the face of an absorber are energy
losers - these areas diaproportionately radiate IR energy out through
And do use a forward-leaning absorber orientation (leans forward at
the top). Feed room temp air into the collector near the bottom, in
front of the absorber. The presence of cooler inlet air next to the
glazing reduces conduction losses, as mentioned by (Nick or Morris)
above. The outlet is from the area behind the absorber. The collector
has a cooler "face," and a hotter "core."
If you send me an email, I'll send back a .pdf of plans for a low cost
collector that uses these principles more or less. If you request it
I'll also attach an ASHRAE efficiency curve that shows a theoretical
72% at intercept. The test was run on Western Michigan University's
test stand. It was the highest efficiency of any air system they had
ever tested. Their testing program no longer exists as I understand.
- Bill Kreamer
-- End Of Hot Air Rant (sorry, couldn't help myself) --
By all means, give fans a try - I did and it may be an instructive
experience for you, as well.
I have a personal preference for devices with the least possible number
of moving parts. It's possible to design a panel that achieves a very
high efficiency without a fan, associated control system, and power
source - so I did (and so can anyone else).
At http://www.iedu.com/DeSoto/SC_Madison.html there're photos of a pair
of the panels being installed. You might find the venting interesting.
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