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what coating to use on DIY flat panel collector?

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Posted by Doug on May 30, 2005, 3:48 pm
Commercially available flat panel collectors use a special selective
coating to increase their efficiency.  I'm looking for suggestions for
the best type of coating to apply to a metal panel in order to make a
very economical flat panel collector.  It's going to be used to heat
water for an outdoor shower so it's not worth spending a lot of money
on it.
I'm thinking of soldering copper pipes to a sheet of steel in a 2X6"
frame with polycarbinate covering -  that sort of thing.
Should I just use flat black spray paint?


Posted by Gary on May 30, 2005, 9:29 pm
Doug wrote:

Hi Doug,
For a single glazed collector with the absorber plate temperature
around 170F, the selective surface cuts the losses from the collector
by about 40%.  You could get roughly the same effect by using two
layers of glazing.  There used to be a film you could buy that adheres
to the collector plate, and has a selective surface, but I'm not sure
that its available anymore.  For this application, maybe neither a
selective surface or double glazing is really needed or worth the
bother.  A lot of people use black "barbecue" paint on the collector
-- clean it very carefully before painting.

This set of plans shows a way to build a flat plate collector that
does not require soldering.  Enough of these have been built in
workshops by the Maine Solar Energy Association, that I am guessing it
works OK.

You might think about getting a pre-assembled absorber plate.
Take a look at "Flat Plate Collectors" here:
I think that the SunRay ones are pretty cheap.  The Solar Energy ones
can be bought with a selective surface coating or not.
This would save you all the soldering, and might not cost much more?

You also might want to think about a small batch type solar water
heater --  there a plans for several of them here:
Scroll down to "Batch Water Heaters".
This would give you collection and storage for a few showers all in
one package.  This seems like the simplest way to me.

You could also have a look at the "Horizontal Pond Hot Water Heater"
here: http://www.builditsolar.com/Experimental/experimental.htm
(about half way down the page).  This is unproven, but might be just
the ticket for your application (which I am assuming is a summer only
outside shower).  Let me know if you decide to try this one, as there
are some "lessons learned" from the prototype.


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Posted by samuelchamb on June 2, 2005, 3:30 pm
 Doug wrote:

I just use matt black paint with red oxide paint as a primer . I have
not come across a acrilic glasing product with sufficent ware propertys.
for cheap diy pannels most seem to use 4mm house glass . the main
drawback being weight .

Posted by Sylvan Butler on June 3, 2005, 2:36 am
 On Thu, 02 Jun 2005 16:30:04 +0100, samuelchamb

Has anyone tried or seen any data on soot as an absorber coating?

It isn't a tough coating like paint, but inside a glazed enclosure it
should be durable enough.


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Posted by Duane C. Johnson on June 3, 2005, 12:58 pm
 Hi Sylvan;

 > > I just use matt black paint with red oxide paint as
 > > a primer. I have

 > Has anyone tried or seen any data on soot as an
 > absorber coating?
 > It isn't a tough coating like paint, but inside a
 > glazed enclosure it should be durable enough.

The classic standard for a "Black" absorber reference
is called "Parsons Black". I understand this is
carbon black, basically soot, mixed with Spar Varnish.
This is the definition of a NON selective coating.

An absorber surface I have experimented with is
stainless steel sheet that is heated with an oxidizing
flame using an oxyacetylene torch. Get it red hot and
the nickel in the stainless oxidizes and turns black.
This is a fairly selective surface.

See more info at:

 > sdb


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