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Efficiency of (flat) thermal collectors?

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Posted by Paul Ciszek on January 3, 2009, 5:18 am
What is a typical efficiency for non-concentrating solar thermal
collectors?  i.e., what fraction of the sunlight hitting them is
typically translated into heat inside the house?

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Posted by Morris Dovey on January 3, 2009, 5:46 am
Paul Ciszek wrote:

For air-heating collectors the number should be somewhere between 60%
and 80% - but I suspect that it may be closer to 40-50% for a "typical"

Water heating panels should be somewhat lower due to the need to
incorporate a powered control system and pump.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by separatus on January 5, 2009, 7:52 pm
SRCC rates a SunEarth panel at 68%-71% at high noon on average.
However, remember this is the highest you will ever get, and only for
moments per day. Flat collectors have a bell curve, making almost
nothing at 8:30AM for example. In fact, for a flat plate some 80% of
the daily heat collection is between 10:00AM and 2:00PM. That is one
of the reasons evacuated tubes make more heat... an evacuated tube
essentially goes to it's rated maximum efficiency and stays there all

Michael Eckhard
TrendSetter Solar Products, Inc.

Posted by Morris Dovey on January 6, 2009, 3:42 pm
 separatus wrote:

For that particular panel - which, for anyone who didn't pull up the
SunEarth specs, appears to be a (year-round) rooftop DHW panel. Other
designs for other applications (such as space heating) can and do
improve on that range.

An over-generalization. It's worth pointing out that not all "flat
collectors" are/have "flat plates", and that the 10am - 2pm interval is
dependent on a number of unstipulated factors.

Hmm - interesting assertion.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by separatus on January 6, 2009, 5:36 pm
Yes, that particular panel which is considered a very good panel. I am
not aware of any flat-plate collector at this time with significantly
higher output. If you are aware of a 4x10 collector with a rated
output 10% higher than the EC40 collector from SunEarth, I would love
to see the make and model here.

You are right I did not run an FChart for this application. However, I
am curious as to whether you actually intend to dispute that flat
plate collectors have a heating curve that follows a bell curve?

Not really. With the same absorber area and the same maximum
conversion efficiency, an evacuated tube will trounce any flat plate
collector on the market. Just because the SRCC rating sheets don't
account for it, their performance projections (annualized) show it

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