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Two sheet flat panels

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Posted by Graham Parkinson on October 13, 2004, 12:10 am
Saw a cheap solar collector today:

Junked galvanized steel truck cooling panels from a reefer rig.

These are about 1" thick and about 2' by 8' feet, crimped and soldered
around the edges with 1/2" copper cooling pipes.   These would make cheap
collectors and made me wonder why the focus is on bonding fins to pipes for
collectors rather than building twin sheet panels (possibly with baffles
inside to promote uniform heat transfer)

- Graham

Posted by Gary on October 13, 2004, 3:06 am
Graham Parkinson wrote:

I'm not sure I understand how these are built.
Is the top plate "crimped and soldered" to the bottom plate along
all four edges?
Are the half inch copper pipes just along the top and bottom edge
to deliver fluid to one end and pick it up from the other end, with
the fluid flowing between the sheets?  Or is there a pattern of the
half inch pipes between the top and bottom sheets?  Or?


I've noticed that the commercial water type collectors run about $0
to $0 per sqft, while the assembled absorber panels are about $ per
sqft.  This suggests that the glazing, cabinet, and final assembly
labor account for a lot of the cost.  I'm thinking about making my
roof collector with preassembled absorber panels, but using a single
sheet of PolyCarbonate for glazing with a wood frame that encloses
several of the absorber panels.  You can get 4ft by 12 ft absorber
panels, and could enclose (say) 6 of them in one frame with glazing
support ribs every 4 ft.  This makes a panel that costs about $0ish a
sqft, and I don't see why it would not have a long and happy life?



Posted by Graham Parkinson on October 13, 2004, 6:06 am

These cooling plates are crimped on 4 edges and probably have internal
baffles forcing flow
around inside the plates.  They have standard 1/2" Cu pipe fittings brazed
into notches in the crimp.

Your design sounds like it would work well.  The big issue with
polycarbonate is allowing for thermal expansion and contraction.  Some kind
of grooved or overlapping bezel is the best way to attach the poly to the
wood (ie screw down some strips of thin Al to hold the poly against a gasket
and the frame)  Poly cracks easily and is very unforgiving of stress
concentrations caused by drilling holes or sawing so plan on putting
fasteners in the rim not the plastic.  You might want to use preserved wood
for the frame too.

- Graham

Posted by nicksanspam on October 13, 2004, 10:10 am

Because pipes and fins use less material and weigh less and can take more
pressure and won't corrode and store less water and lose less heat overnight?


Posted by Gary on October 13, 2004, 2:31 pm
 nicksanspam@ece.villanova.edu wrote:

On the other hand, if you are using a collector that is not
pressurized, then the collectors made from two panels of sheet metal
use cheaper material, and avoid the very labor intensive job of
thermally joining the riser pipes to the fins.  If the panels drain
when when not collecting, then losing heat overnight is not an issue?

The article in Homepower issue 59, "Low Cost Solar House Heating"
describes a system that drips water between two sheets of corrugated
aluminum roofing that "cost $ each". The water circulation system
consists of 1 inch plastic pipe headers with numerous push in small
diameter drip irrigation tubes that feed the area between the metal
sheets.  The panels are glazed with glass.  Heated water is picked up
at the bottom of the panels with a gutter system, and is delivered to
a sump at around 140F when sunny.  Water from the heated sump is
pumped through tubing embedded in the house floor.  His system of 10
panels is reported to have cost $000, and is said to heat a 4000 sqft
house in New Mexico.  At the time of the article, the system was 7
years old and "still working well".
Providing the same collection area in commercial fin and tube panels
would be around $000 just for the panels.



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