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most efficient collector for solar water heating

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Posted by codechump on December 23, 2008, 2:19 pm
I wonder if anyone can help? I'm looking to build a home solar water
heater. Can you tell me what is more efficient? A flat collector
(assuming it's insulated from convection) or a parabolic trough
covering the same area (with it's collector insulated also)?


Posted by Robert Scott on December 23, 2008, 2:08 pm
codechump@gmail.com wrote:

It depends on the kind of weather you have.  A parabolic trough is good for
nothing if the sun is diffuse, as on a very hazy, partly cloudy day.  But a flat
panel can still pick up some useful heat on such days.  However if you have
bright direct sun to work with, then the parabolic trough may be better only
because it has less hot surface area which can leak heat to ambient, and can
therefore generate higher temperatures.

Are you thinking about efficiency in terms of BTUs per square foot?  That may
not be the best measure of efficiency, unless you truly are space-constrained
and money is no object.  Perhaps a better measure is BTUs per dollar.  If the
choice is between a 30 square foot paraboloic trough versus a 60 square foot
flat plate collector (which costs less than the 30 square foot paraboloic
trough, especially if you count tracking hardware), then I think the flat plate
will win the BTUs per dollar contest.

Robert Scott
Ypsilanti, Michigan

Posted by azuredu on December 23, 2008, 5:12 pm
 On Dec 23, 3:08 pm, n...@dont-mail-me.com (Robert Scott) wrote:


The diffuse radiation is greatly exagerated by solar panel vendors.

According to my luxmeter measurements, diffuse radiation rarely
exceeds 150W/m^2. On the other hand, a typical flat panel collector
loses about 4W/m^2.K. If you do it yourself, the loss will probably be
higher. Counting optical efficiency, you hit zero output as long as
the water temperature is more than 30C above the environment. So it's
only of some use during the summer.

Even then, the contribution of diffuse radiation is probably at most
0.5kWh/m^2 for a favorite day for that. What can you do with that toy
energy? And should you compare this with the cost of the panel?

Posted by codechump on December 23, 2008, 8:00 pm
 On Dec 23, 9:08am, n...@dont-mail-me.com (Robert Scott) wrote:

Thanks for your suggestions. To provide more information, I'm in Mid-
Florida. The sun here is either on or off for short periods during a
passing storm, year round. I did a small experiment and built the
solar cooker at this link. In December I can bring the water in the
black jar to a boil!

Does this sway the suggestion? I'm thinking that pumping water through
the collector pipe of a parabolic trough will get it pretty hot in the
Summer time!

Posted by azuredu on December 23, 2008, 9:27 pm
 On Dec 23, 9:00 pm, codech...@gmail.com wrote:

I am field testing my new home made parabolic trough. We are at winter
solstice, and I have got a 570W/m^2 thermal output for a sun at a mere
22.8 degrees from the horizon. When dry burning without water in the
tube, it heats up to 200C in just a few seconds! Had to put it off
quickly for things were getting melted.

Sorry but the details are still not yet ready. Maybe in 2 to 3 weeks.
Just a picture for the time being. It will cost much less than a flat


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