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finish for rear of solar collector.

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Posted by R L driver on June 8, 2006, 12:51 am
 
Hi Guys, Well I've made the box, insulated it with 2" foil covered
urethane foam, made the collector from pipes and clip fins . Obviously
the front of the collector should be painted matt black to encourage
heat absorption. But my brain has frozen ..... what colour should the
rear of the collector be?
I guess we dont want to radiate any heat out again so is the answer
"shiny to reflect the heat back in". The clip fins are aluminium. Thanks
in advance for you advice
Steve the grease

Posted by Gary on June 8, 2006, 1:33 am
 
R L driver wrote:

Hi Steve,
2 inches of urethane is enough that it probably makes almost no difference what
you paint it -- the losses out the back are going to be very small compared to
the glazed side losses.  That said, I suppose something with low IR emissivity
would be good (to make it an inefficient radiator of heat) -- I think that
limits you to things like alum foil -- like the alum foil that may already be on
the insulation?
A lot of commercial collectors just use one inch of the urethane on the back, so
you should be in very good shape.

Gary


--


Gary

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









Posted by Jeff on June 8, 2006, 3:54 am
 Gary wrote:

  I think he meant the back of the fin, and I think the answer is about
what you said. Don't paint it, leave it shiny.

  I'm getting closer to building mine. What do you do about the back of
the fin, how do you mount that? Also, how close to the glazing can the
front of the fin be?


   From that program you posted it looks like 2" is about where you
start  flattening off in efficiency.

   Jeff


Posted by Gary on June 8, 2006, 2:59 pm
 Hi Jeff,

...


My actual building experience on full size water collectors is zero.
I am working on the 240 sqft (6 bays) one for my house now, and just facing
these same issues of detail design.  I plan to buy prebuilt absorber sheets.  My
initial thought on mounting them is to basically support them only at the
manifolds, and with supports that allow some thermal movement.  This will
probably allow some contact between the back of the fins and the insulation
behind the fins in a few spots -- which is probably OK??

I'd be glad to hear any advice anyone has??



That sounds good.  I'll also check the Rabl book and see what they say -- I'll
post it if different.

Gary



--


Gary

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









Posted by Jeff on June 8, 2006, 4:23 pm
 Gary wrote:
 > Hi Jeff,
 >
 > ...
 >
 >>
 >>  I'm getting closer to building mine. What do you do about the back of
 >> the fin, how do you mount that? Also, how close to the glazing can the
 >> front of the fin be?
 >
 >
 > My actual building experience on full size water collectors is zero.
 > I am working on the 240 sqft (6 bays) one for my house now, and just
facing
 > these same issues of detail design.  I plan to buy prebuilt absorber
 > sheets.  My
 > initial thought on mounting them is to basically support them only at the
 > manifolds, and with supports that allow some thermal movement.  This will
 > probably allow some contact between the back of the fins and the
insulation
 > behind the fins in a few spots -- which is probably OK??

I think some contact is OK. A lot of contact with the aluminized sheet
on the polyiso is probably bad.

   My thinking is that you probably want to lay them on small squares of
insulation (like standoffs), with the foil side down.

Alternatively, flip the insulation so the foil side is on the back and
embed the the fins. With single glazing I'm thinking stagnation temps
around 180 F on a hot day windless day. Well within the polyiso range. I
do like the benifits of that radiant barrier though as the they work
best with high temps, and that would negate that.

   Alternatively, just support the manifolds and let everything else
fly. I'm unsold on the big fin for efficiency and am thinking fins out
of that 12" or so flashing. I think the big fin is probably best when
you want a lot of unglazed collector and a low temp rise.

I'm thinking a half inch or so between the glazing and the fin.

   Now, where's Mike? Mike's got it going on!

   Also, does Nick have any quibbles with that program, I'm sure he must...

   Jeff
 >
 > I'd be glad to hear any advice anyone has??
 >
 >
 >>
 >>> A lot of commercial collectors just use one inch of the urethane on
 >>> the back, so
 >>> you should be in very good shape.
 >>
 >>
 >>
 >>   From that program you posted it looks like 2" is about where you
 >> start  flattening off in efficiency.
 >
 >
 > That sounds good.  I'll also check the Rabl book and see what they say
 > -- I'll
 > post it if different.
 >
 > Gary
 >
 >
 >


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