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Posted by Brian Graham on November 6, 2006, 8:42 pm
 
If you're asking how effective serpentine copper tubes under glass will pick up
heat from the sun, let me give you a quick example.

I had a small copper tube on the surface of my wood stove. It picked up next to
no heat at all. I changed it to a steel box covering one whole surface, and I
heat my 40 gallons of water with that alone.

So, I therefore threw out my plans to build solar panels on the cheap with
serpentine copper under glass and bought two "real" 4x8 solar panels. They keep
my 40 gallons near boiling all summer.
--
Brian


If I'm going to make my own panel for a radiant slab, which is a better way
to go?  Run the tubing in a serpentine fashion, or like a manifold system?
I plan to use copper tubing under glass.


thanks!
--
Steve Barker






Posted by SJC on November 6, 2006, 9:11 pm
 


  Steve,

If you decide not to build your own, here is a company that I think has good
prices.

http://www.sunraysolar.com/collectors.php

I have no affilliation with this company, I just came across them on the web.

Posted by Steve Barker LT on November 7, 2006, 7:40 am
 THANKS for all the replies.  I guess I'll just experiment.  Buying a
commercially available unit is out of the question.  I could build one the
size of a football field for what they want for a 4x8.

--
Steve Barker




Posted by Jeff on November 7, 2006, 12:07 pm
 Steve Barker LT wrote:

   I was hoping Mike in NZ would weigh in...

   Several things, look through:

http://builditsolar.com

   Search the archives, I've pasted in below some posts from Mike in NZ
that will get you started:

Hi Jeff

1/2" soft drawn copper tubing is very difficult to straighten sufficiently
so when clamped between the aluminium, does not make a very good contact, I
have used straight lengths of hard drawn copper to better effect, brazing to
1'' or 25mm headers.

For 22g aluminium 140mm tube spacing would be better, use following - 0.5mm
thickness = 125mm spacing, 0.75mm = 140mm, 1.0mm = 150mm

Rather than press a groove into sheet, I have used mini corrugated aluminium
sheet which is an almost exact profile for 1/2'' or 15mm copper tube. Cut up
another sheet into strips and clamp behind each tube every 4'' with a rivet
each side of the copper tube. Use a smear of the white heatsink paste used
for electronics work between the copper and aluminium.

To prevent corrosion (electrolysis) between dissimilar metals, I sprayed the
copper tubes after brazing to headers with a thin coating of self etching
primer, comes in aerosol cans. Allow to harden fully for 24 hours or leave
it in the sun for a day before clamping up, or it will scratch. Once the
whole thing is constructed, spray paint the sun facing side only with the
same flat black etch primer. Two thin coats, no top coat is required.

Twin wall polycarbonate glazing is very effective, however it has a very
high coefficient of expansion, so for a panel 8' or 2.4metre length allow
for a vertical movement of 1/2'' approx. Have a wide flange surrounding the
panel edge at least 30mm wide covered by a 30 by 50mm angle flashing to
clamp the glazing in place. Use screws along the sides not through the
polycarbonate. With this set-up no rubber seals are required as they can
self stick to the polycarbonate and restrict its movement, which over time
will cause it to crack. Run the polycarbonate vertically and seal the top
edge with self adhesive aluminium tape before clamping into place.

Place a small drain hole (10mm) in the lowest bottom corner of the panel to
allow ventilation and drain off any internal condensation.


Cheers
Mike



Hi Jeff

Yes I have built several installations and sorted out most of the potential
problems.

Back clamping strip, I used a jigsaw and cut several strips the length of
the riser pipes from the mini - corrugated sheet, so they fit neatly around
the pipes when clamped by pop rivets, use split type pop rivets, they spread
out much more than normal compression pop rivets and don't tend to pull
through the rear strip as much. Mini corrugate is 0.5mm thick.

For larger surface area panels or multiple panels daisy chained together,
1'' internal header pipe works better than the next size smaller 3/4'' as
there is better flow distribution between them. I use approx 1 sq foot of
panel area per gallon of water to be heated ( 50 litres per sq Metre), this
is for solar radiation levels here in Wellington New Zealand, may be
different at your location.

For natural convection systems pass the cool water in the bottom and exit at
diagonally opposite top.
For active pumped system the exit water is best at top same side as input,
this is due to the Bernoulli effect which tends to suck the water down the
riser tubes due to the flow past them in the headers. If the exit water is
taken from the same side as the input there is better flow distribution in
the risers, especially when 2 or 3 panels are coupled together.


You can also use copper foil, available in rolls 0.25mm thick by 300mm
width, easily cut with hand scissors, as the heat surface. push a piece of
dowel across it to make a depression to partially wrap the copper riser,
solder to the risers. Bit more messy to work with, but takes about same time
to build as setting 200 rivets per panel.

For the fin to glazing distance use 20mm, any more results in excessive
turbulence in the air captured within the cavity causing higher losses.
You can glue some small silicon rubber standoffs to the top of the fins,
this will prevent the poly glazing touching the fins and melting, probably
not a problem with twinwall poly as its more rigid, the single wall 1mm poly
gets quite flexible as it heats up.
If you are using polycarbonate sheet for the glazing, it will expand approx
15mm in length over a 2.4 metre sheet, so be careful not to clamp this too
tightly to your case or it will buckle as it expands.
If the pipes are supported by the headers, then near the centre just roll up
a bit of glassfibre and lay across the foil to give some support there. I
usually have a small air gap 5mm between the fins and the foil faced
insulation. Waste of time painting the rear of the fins, the heat is
reflected back from the foil into the fin.
For insulation I use 25mm polyurethane sheet on the bottom with layer of
25mm foil faced glassfibre batt covering that and folded up on all sides,
this prevents the poly getting excessively hot and seals any condensing
products from settling on the underside of your glazing.
For very cold locations then thicker polyurethane could be used with an
extra piece along each side, combined with twinwall would make the panel
more immune to freezing in the winter.



*********************************************************


I'll post up some details of my collectors when I get em done. I've had
some trouble getting it all together. Being a city slicker it's hard to
find some materials that are available either in Rural areas or in
countries outside the US. Here, in Atlanta, Ga, I see only mass
merchandise materials.

   For glazing use SunTuf drom Home Depot. Use only polycarbonate with a
UV guard which SunTuf has. ~ $ square foot. Do *not* use styrofoam
insulation, it won't hold up to the heat. Use Polyisocyanurate (lexan),
which at HomeDepot and Lowes and is called "R Max", it will  have an
aluminized coating. I can get it only in 1/2" so my boxes have a 3/4"
cross members with one sheet attached to  the bottom of that and another
on the top with the air space between them.

   You can model your collector with this:

<URL: http://sel.me.wisc.edu/codepro/new_codepro.html  />

   Gary at builditsolar.com found that, but he has so much on his site
it can be hard to find it through all  the other great projects.

   Unless you buy preassembled absorber plates, not a bad idea at all,
it is hard to find selective coatings. One source that sells small
quantities (1 gallon) is Solkote. They'll make sure you know what you
are doing before they'll sell it to you (nice guys). It's about $5 UPS
delivered.

<URL: http://www.solec.org/  />

My boxes are 2' x 10.5'  by ~4 1/2" and run about $00 each. About half
that in copper!

   If cost is an issue and you can mount the collectors close to  the
walls of a south facing room, consider an air collector. Much cheaper to
build. There's plans in the archives for these

Posted by Jeff on November 7, 2006, 12:12 pm
 <snip>

The polycarbonate is lexan, not the polyiso.

Jeff


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