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Posted by schooner on April 27, 2007, 11:39 am
 
Thanks.  Just curious, how are you testing/comparing your panels to
determine the improved performance?



Posted by Morris Dovey on April 27, 2007, 12:32 pm
 
schooner wrote:

| Thanks.  Just curious, how are you testing/comparing your panels to
| determine the improved performance?

Generally, by the most unscientific method: side-by-side comparisons.
If a given change doesn't produce a truly obvious performance
improvement, then it doesn't go into production panels.

Occasionally I build a panel incorporating a number of marginal
improvements to see if they add up to a significant improvement in
performance - but so far that hasn't happened. Real improvements have
seemed to stand on their own.

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/Absorber.html



Posted by Jeff on April 26, 2007, 6:03 pm
 Morris Dovey wrote:

   That brings up two questions.

1) Glazing. Of the glazings tested by Gary in an earlier thread, only
polyethelene was IR transparent. I know that ordinary window glass also
blocks IR. If the IR can't get through the glazing it doesn't amatter if
the absorber can absorb it.

2) Losses due to IR reradiation. Tests on selective surfaces have shown
greater efficiency for water collectors, particularly at higher . Such
surfaces are whiter in the IR range.

   Jeff


Posted by Morris Dovey on April 26, 2007, 6:50 pm
 Jeff wrote:
| Morris Dovey wrote:
|| I've put up a web page (link below) with a graph that may provide a
|| bit of food for thought for those designing their own solar heating
|| panels.
|
|    That brings up two questions.
|
| 1) Glazing. Of the glazings tested by Gary in an earlier thread,
| only polyethelene was IR transparent. I know that ordinary window
| glass also blocks IR. If the IR can't get through the glazing it
| doesn't amatter if the absorber can absorb it.
|
| 2) Losses due to IR reradiation. Tests on selective surfaces have
| shown greater efficiency for water collectors, particularly at
| higher . Such surfaces are whiter in the IR range.

Good questions indeed. <g>

Gary's measurements would seem to indicate that major improvements can
be made in solar glazing materials. I haven't worked with selective
absorber surfaces (mine are all shiny/glossy), and my approach to
re-radiation has been geometric more than anything else. I haven't
built a water collector for more than three decades, so will take your
word for their greater efficiency.

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/Absorber.html



Posted by DavidMDelaney on April 27, 2007, 12:51 am
 
I don't know how the "absorber" in the Desoto Solar Panel is designed,
but it has been well known for decades how to design at least one kind
of cover that transmits about 90% of the energy of sunlight. If you
combine such a cover with a black absorber inside the collector that
has a high absorbtance across the same band of wavelengths, which is
easy to do, you can get more than 80% percent of the energy of
sunlight turned into low temperature energy (low compared to the sun)
inside the collector. Then you have to figure out how to keep it
there.

The starting point is a cover made of glass with a very low content of
iron oxide, (Fe2O3), so called "water white" glass -- if you can find
it.  It's called water white because the edge of it looks white,
whereas the edge of glass with a larger amount of Fe2O3 looks green.

Low iron glass has a very good, sharp sided flat topped bandpass
filter for sunlight.  You can see its bandpass characteristics in
FIgure 5.7.1 on p. 231 of Duffie and BEckman, SOlar Engineering of
Thermal Processes, 2nd. ed. The short wavelength cutoff is at about
0,25 micron and the long wavelength cutoff is at about 2.8 micron. The
visible spectrum, on the other hand, is from 0.38 micron to 0.78
micron. See page 7 of D & B. You can read the fraction of the total
energy of sunlight in the glass bandpass from Table 1.3.1a on page 8
of the same book. It's about 98%.   The flat top of the low iron glass
bandpass filter is pretty constant at 90% transmissivity throughout
this range.

On the other hand, 2.8 microns is a much shorter wavelength than the
range of wavelengths that contains nearly all of the energy of the
radiation of bodies at the sorts of temperatures than can be produced
inside solar thermal collectors by the absorbtion of sunlight. In
other words, glass, whether white or green, is completely opaque to
the thermal radiation from the interior of the solar collector. The
glass may reradiate thermal energy to the environment, but not because
it's transmitting it -- the glass is getting hot itself.

By the way, green edged glass is much inferior to white edged glass,
having a deep notch in the middle of the same bandpass. See Figure
5.7.1.

David



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