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Collector absorber

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Posted by Kiyu on January 7, 2005, 8:31 pm
 
I'm fooling around with solar collectors for hot water again and was trying to
make a small test standard to compare other absorbing arrangements with. Today
while I was picking up the materials I would need (I had planned to split
copper pipes and mash them flat to make an economical copper absorber) when I
found rolls of copper flashing 10" wide by 20' long. This solved my problem of
having an affordable copper absorber but the material is only .005 thick once
the backing is removed. I noticed on Sun-Ray's site their copper absorbers are
.008, but even that sounds thin to me
Would anyone here know how much the reduced thickness affects heat transfer
and how much should I space my 1/2" copper tubes running across the backplane?
Their spacing is something like 3 1/2" I believe so I should probably use
less, a guesstimate of 3". I don't really know where to look to find such
information or if there is a standard for pipe spacing/absorber thickness.

Thanks
Kiyu

Posted by Gary on January 7, 2005, 10:33 pm
 
Kiyu wrote:

Solar Thermal Engineering, Lunde provides a way to estimate fin
efficiency -- if I did the math right, here are some sample fin
efficiencies:

fin thickness      spacing      efficiency
0.005 inch         6 inches      84%
0.01 inch          6 inches      92%
0.015 inch         6 inches      94%

fin thickness      spacing      efficiency
0.005 inch         3 inches      96%
0.01 inch          3 inches      98%
0.015 inch         3 inches      98.5%

This is all for copper fins.

Fin efficiency is defined as the ratio of the heat that a real fin
(like yours) would transfer to the fluid to that of a perfect fin.
The way I read it, if you use the 3 inch spacing the 0.005 should be
quite thermally efficient.
Three inch seems pretty tight to me compared to collectors I have seen?
I wonder if you will run into any difficulties bonding it to the
copper tubing because of the thin gage?  Please let us know.
The answers above assume good thermal bonding between the tube and the
fin.

If I might ask, how much does the copper flashing cost, and what kind
of places carry it?

Gary





Posted by Kiyu on January 8, 2005, 3:06 am
 

Thanks Gary. That is exactly the sort of info I was looking for.
I'm not sure how the bonding will go.....lots of flux and lots of
experimenting to get it to bond with the minimum amount of heat/solder. I am
concerned that the flashing is so thin and I'll have problems. Because of the
differences in thickness I think I'll try heating the tubing first and
allowing the flowing solder to heat the flashing. It will be interesting.
I found the stuff at Lowes and was $9.95 for the roll. They have some that is
a bit narrower and costs less but I figured I'd get the larger roll to allow
for a few screwups.

My first attempt with using copper tubing imbedded in concrete (for an
absorber) was pretty sad. It worked but I had to kick start it to get it to go
- poor design. Finding the flashing at such a good price would seem to make
fooling with any other absorber a waste of time but it is fun trying to make
it work - I have a few ideas to improve it. (My Xmas present from the wife was
all the cash I needed to fill the backyard with solar claptrap.<G>)

My next door neighbor is interested in your hot air collector, after I build
one first of course<G>, and so the next project will be to make a free
standing single unit & connect it to the back of the house.

Thanks again
Kiyu
(Tim W)


Posted by John Canivan on January 16, 2005, 7:20 am
 : quoted-printable

Whats wrong with aluminum?
John    www.jc-solarhomes.com
  I'm fooling around with solar collectors for hot water again and was =
trying to
  make a small test standard to compare other absorbing arrangements =
with. Today
  while I was picking up the materials I would need (I had planned to =
split
  copper pipes and mash them flat to make an economical copper absorber) =
when I
  found rolls of copper flashing 10" wide by 20' long. This solved my =
problem of
  having an affordable copper absorber but the material is only .005 =
thick once
  the backing is removed. I noticed on Sun-Ray's site their copper =
absorbers are
  .008, but even that sounds thin to me
  Would anyone here know how much the reduced thickness affects heat =
transfer
  and how much should I space my 1/2" copper tubes running across the =
backplane?
  Their spacing is something like 3 1/2" I believe so I should probably =
use
  less, a guesstimate of 3". I don't really know where to look to find =
such
  information or if there is a standard for pipe spacing/absorber =
thickness.

  Thanks
  Kiyu
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<DIV><FONT face="Arial Rounded MT Bold">Whats wrong with aluminum?</FONT></DIV>
<DIV><FONT face="Arial Rounded MT Bold">John&nbsp;&nbsp;&nbsp; <A
href="http://www.jc-solarhomes.com">www.jc-solarhomes.com</A>  </FONT></DIV>
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  <DIV>"Kiyu" &lt;<A
in message
  <A
  =
ews.verizon.net</A>...</DIV>I'm
  fooling around with solar collectors for hot water again and was =
trying
  to<BR>make a small test standard to compare other absorbing =
arrangements with.
  Today<BR>while I was picking up the materials I would need (I had =
planned to
  split<BR>copper pipes and mash them flat to make an economical copper
  absorber) when I<BR>found rolls of copper flashing 10" wide by 20' =
long. This
  solved my problem of<BR>having an affordable copper absorber but the =
material
  is only .005 thick once<BR>the backing is removed. I noticed on =
Sun-Ray's site
  their copper absorbers are<BR>.008, but even that sounds thin to =
me<BR>Would
  anyone here know how much the reduced thickness affects heat =
transfer<BR>and
  how much should I space my 1/2" copper tubes running across the
  backplane?<BR>Their spacing is something like 3 1/2" I believe so I =
should
  probably use<BR>less, a guesstimate of 3". I don't really know where =
to look
  to find such<BR>information or if there is a standard for pipe
  spacing/absorber =
thickness.<BR><BR>Thanks<BR>Kiyu</BLOCKQUOTE></BODY></HTML>


------=
Posted by Gary on January 17, 2005, 11:20 pm
 John Canivan wrote:

Seems like Al would work fine if you double the thickness to account
for the lower thermal conductivity.
But, I wonder what the long term corrosion experience has been with Al
fins and copper tubes?
You would also need a good way to bond the Al to the Cu?
Gary




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