Posted by Vortex on February 19, 2006, 5:23 pm
A few months ago I was given 4 solar heating panels by the father of a
friend. He built them in the late 70's to a CAT (Centre for Alternative
Technology) design, basically:
* 1m^2 collector made of sheet copper to which about 10m of 15mm pipe is
* Collector backed with aluminium foil
* 3/4" plywood box, insulated with Fibre Glass
* A glass window on the front
The panels were used for domestic hot water for 25 years (obviously backed
up by conventional heating). I want to refurbish them with a view to
getting minimum another 10 years use.
Q. I'm reluctant to use a glass window for safety reasons. I am thinking
a good alternative would be twinwall polycarbonate, which claims light
transmission efficiency of 82%. Is twinwall polycarbonate the most
appropriate material for this application?
Posted by Anthony Matonak on February 19, 2006, 6:05 pm
I'm no expert in these things but I'd say your best bet is to
put glass on them. Tempered glass should be used, otherwise they
will be way too prone to break if anything were to fall on them.
The above link shows that twinwall polycarbonate is only good
up to 275F. It's quite possible that those panels could experience
temperatures in excess of this if the water was not circulating.
Posted by vortex2 on February 20, 2006, 8:24 am
I forgot to mention I need "football-proof" panels!
Yes, tempered glass is safe, but I suspect not so cheap when broken.
Posted by Anthony Matonak on February 20, 2006, 9:37 am
Well, you won't lose much trying twinwall, corrugated fiberglass
or the like. If it doesn't work out then you could always go with
regular window glass and cover that with steel mesh or iron bars
to keep the footballs away.
Posted by Gary on February 19, 2006, 8:31 pm
I am going to respectfully disagree with Anthony a bit on this one.
I think you would be OK with the twinwall.
If you have the panels tilted at an angle equal to your latitude (best year
round collection), then the panels should probably be protected with shade cloth
or something like that during the summer anyway. This is really true for glazed
panels as well -- the antifreeze (and maybe other parts) will deteriorate if
exposed to high stagnation temps. If you look (for example) on the Heliodyne
collector site (the Cadillac of collectors), they say to protect the panels in
If you mount the panels vertically, they will not overheat. I have vertical
mounted, polycarbonate glazed panels that are at stagnation all summer, and they
don't get above about 180F.
You could also use single wall polycarbonate. Home Depot and the like carry a
corrugated polycarbonate (SunTuf) that is $ per sqft, and is guaranteed for
"life". You may need to add a light glazing support at the panel midspan, as
the PC is not as stiff as glass, and will deflect -- especially when at
operating temperature. One advantage of the single wall over the twinwall from
an overheating point of view is that the losses will be a bit greater, and
therefore the internal temps will be lower. In addition, the single wall PC
sees ambient on one face, and the inside collector temperature on the other face
-- so its temperature is somewhere between the two -- not as high as the
stagnation temperature of the collector.
I think that you will find that getting custom sizes of tempered glass is pretty
expensive, and by the look of the panels they may not be worth spending that
much. At $ per sqft for the PC, you are not out much if things (for any
reason) go south on the project.
Here is the PC panel I use:
Its now two years old with no sign whatever of deterioration.
"Build It Yourself" Solar Projects
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