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Fins 4 panels, not Aluminium? - Page 2

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Posted by gary on March 7, 2009, 11:50 pm
 
Hi,
I believe that if you build the fin/pipe connection as shown here:
http://www.builditsolar.com/Experimental/CopperAlumCollector/CopperAlumCol.=
htm

that this construction excludes water from the copper aluminum
interface, and will prevent corrosion.
The combination of paint on the aluminum and the very thin layer of
silicone both seals the area and separates the aluminum and the copper
-- a sort of belt and suspenders approach.
Since the silicone is 10 times more conductive than a very thin air
gap would be, it also improves the thermal efficiency.

I do think that a zinc chromate primer to paint any exposed metal is a
good idea.

One person sent me pictures of some fairly impressive corrosion of the
aluminum foil face sheet of polyiso insulation that was used under
copper pipes, so, I would avoid this.

If you do get galvanic corrosion between copper and aluminum the
copper the sacrificial metal in this pair, so the primary damage
(hopefully isolated to small areas) should be to the aluminum.  But, I
don't think there will be any corrosion if the collector is built
carefully.  I suppose it never hurts to take a look at your collector
every few years :)

This configuration tests to 96% of the thermal performance of a
baseline collector with copper fins soldered to copper pipes:
http://www.builditsolar.com/Experimental/PEXCollector/SmallPanelTests.htm
So, I believe that approach of wrapping the copper tube tightly with
the aluminum fin, and using a thin layer of silicone between the two
results in good thermal performance as well as good corrosion
protection.

I just took the glazing off the prototype today to do a test on the
evenness of flow rates in all the risers.  The collector has been
stagnated since late summer with only a few days of testing with water
flowing through it.  It looked exactly like it did the day it was
built -- no sign of any problems, including corrosion.

There are at least two designs (SunRay and the design built by the
Maine Solar Energy Association) that use aluminum fins on copper pipes
and have a proven track record.

Gary










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