Johnnyo wrote:
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.
The glazing might be 2 $64 4'x16' sheets of Dynaglas corrugated
polycarbonate greenhouse roofing from Griffin in Morgantown, PA
installed as "solar siding."
i chose fan circulation since i already have several suitable DC fans and
a PV panel to drive them.
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:
T
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%.
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...
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.
So the big question is, does anyone know of documented comparative
testing along these lines?
Gary Reysa has done some of that. I like his air heater design:
http://www.motherearthnews.com/multi...id=74688&seq=1
I'd use a single layer of black fiberglass window screen for the mesh.
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 process.
Why fuss around with a smaller version, especially if you insist on fans?
Nick