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Posted by Rob Dekker on September 13, 2005, 3:15 am
 


Thanks Gary, great experiment ! You are the master !

A thought came to mind :
With this PEX material, I wonder if you need the Al fins at all.

If we would create a half-circle reflector around each tube,
we can beam all light directly on the PEX.

A half-circle reflecor can be made of cut-through (along the length)
PEX tube that is twice the diameter of the collector tube, with
some reflecting mylar glued on the inside.

The would eliminate any thermal conductivity problems, by
concentrating the light/heat directly on the PEX collector tubes.
Essentially, we could build an all-PEX collector this way.
Does require more dense PEX collector tubes though (half the surface).

Rob



Posted by Gary on September 14, 2005, 1:07 am
 
Rob Dekker wrote:

I have seen collectors that used a shaped reflector around each tube
-- I think the evac tube designs use this.

It does seem like you could device a pretty simple tool to form (say)
6 inch wide trough shaped reflectors from light alum, and not having
to work out a way to thermally bond it to the PEX would be nice.

I wonder how high the surface temperatures on the PEX would get.
One thing I like about the PEX bonded to the U channel is that the U
channel protects the PEX from direct exposure to sunlight.

--


Gary

www.BuildItSolar.com
gary@BuildItSolar.com
"Build It Yourself" Solar Projects









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Posted by Rob Dekker on September 14, 2005, 8:01 pm
 
[..]

Like PEX, a thermal bond between a fin and a glass tube is hard to do.
That's where the reflectors come in.


A 6 inch trough on a 1/2" target (PEX tube) will require a sol-tracker, or at
least some sort of machanical moving of the trough, to keep it in the sun.
That makes the system more complex, and we want to keep it simple.

A reflector which is up to twice the diameter of the target tube diameter
can remain static, and still reflect all sunlight over a 180deg angle onto the
tube.

Once again, that requires tubes over half the capture surface, which is
of course more than the tubes with (Al) fins. But PEX is cheap (cheaper than Al
fins),
and mylar (to form the 2*R trough reflectors) is even cheaper, so increased
amount of material should not be problematic.


The surface temperature should be related to the amount of capture area
of each tube. So that should about the same for a 6" trough as it is for a 6" fin
(apart from efficiency differences).

For a 1/2inch tube, the 2R trough reflectors I propose would be equivalent
to a 1" fin. So lower intensity on the tubes (but we need more of them).


Yeah. Good point. Plastics typically deteriorates under UV light.
Maybe a black coating can give some protection ?


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Posted by Cosmopolite on September 14, 2005, 11:13 pm
 Rob Dekker wrote:

tube.

Al fins),

fin

    Kitec is available in " heavily UV stabilized " configuration for
outdoor use. The tubing I am using for my floors is black.




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Posted by Paul on September 15, 2005, 3:39 am
 FYI - Solargenix has Compound Parabolics that do not require tracking.
http://www.solargenix.com/hot_water_products.cfm



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