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Increase in collector temps due to snow relection? - Page 4

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Posted by SolarFlare on December 14, 2005, 1:39 am
 
If the lat. was not so close to 45 deg it wouldn't be
so easy to forget. It would be more obvious. I, too,
have had this problem.

message

You're right nick, on

(41+23.5)).


Posted by Duane C. Johnson on December 11, 2005, 1:14 am
 
i Solar;

SolarFlare wrote:
 > "daestrom" wrote:

 > > > Just wondering if anyone has noticed any
 > > > increase in their air collector

 > > > temps due to sun reflection from snow.
 > > > Does this have any impact on the collector or
 > > > is the reflected sun of no advantage?

 > > It's a very real thing. In some areas that get
 > > a lot of snow, you can get more energy if you
 > > mount the collecter vertical on a south wall than
 > > if you mount it at latitude minus 23 degrees.
 > > The loss in direct radiation because non
 > > perpendicular angle can be made up for in the
 > > gain in ground reflected radiation.

 > > The only 'hitch' is you need a nice open area to
 > > the south from the collector that is covered with
 > > snow.

 > > http://rredc.nrel.gov/solar/pubs/bluebook/appendix.html

 > > Under 'Global solar radiation', the radiation
 > > reflected from the ground is discussed. They suggest
 > > that an albedo of 0.2 is typical for green
 > > vegetation. I think snow can be as high as 0.9.

That's what I recall also.

 > > daestrom

 > This is a thermal group. I doubt there would be much
 > "thermal" radiation off snow. PV?...definitely.

Obviously there would be negligible thermal gain due
to radiation from cold snow. Especially if the snow
is colder than the thermal receiver.

But that isn't the point.

The Sun's radiation that is not converted to heat in
the snow is reflected, 90% or so. This radiation is
turned to heat in a thermal collector. The total
passive concentration over snow is often significantly
higher than 1X.

Of course this works for both thermal an PV systems.

Often my dual axis trackers look like they are pointing
in the wrong direction. Then I realize they are
integrating the light from a reflection. Their smarter
than one would expect sometimes %^)

Duane

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Posted by SolarFlare on December 11, 2005, 6:12 am
 Interesting point but some holes inthat argument.

What is the energy they track by and is it the same
energy you want the most of?

How do you know your solar trackers are not pointing in
the wrong direction?

We are talking thermal radiation, not light radiation,
They are different wavelengths, absorbed, radiated,
bent and passed through different mediums completely
different.

If your solar trackers are working from thermal sensor
the way some passive units do they may be much less
efficient as they attempt to run as hot as they can
instead of as PV efficient as they can.




http://rredc.nrel.gov/solar/pubs/bluebook/appendix.html

suggest

significantly

pointing

smarter


Posted by Iain McClatchie on December 12, 2005, 5:50 am
 This is a good point:

And this shows why you are confused:

It's all photons, which I think you know.  The sun has a
spectrum it emits.  Some energy is radio, more is IR,
still more is visible (the peak is in there somewhere,
maybe yellow IIRC), then less UV.

For thermal collection, we don't care what the wavelength
is, just so long as the absorber can collect it.
Interestingly, the best panels are made with "spectrally
selective" materials, like black chrome, which are actually
quite shiny to deep IR and do not absorb it well at all.
They absorb the higher frequency stuff, like visual and UV,
where the bulk of the power is.  They are designed
specifically not to absorb deep IR because any material
emits exactly as well as it absorbs, and for a surface at
~180 F, deep IR is going to be the peak of the emitted
spectrum.

Google on "black body emission spectrum" should get
you what you need.

I don't know what the reflection spectrum of clean snow
is, but if it reflects 90% of the total energy in the sun's
spectrum, that's going to be a big effect.

You are right about the sun reflecting off glass too, by
the way.  The SRCC measures this when they rate
solar panels, and it takes a noticeable bite out of the
system efficiency.  Gobi 410s (which I'm doing my
calculations with) reflect 9% at 60 degrees strike angle.
I suspect they had to do some engineering to get it
that low.

PV is somewhat more fussy about the wavelengths
because the diode junctions require specific wavelengths,
or smaller, to cause the electron energy jump which gets
the electron up over the energy barrier.  Wavelengths
longer than the correct one make no electricity and just
heat.  Wavelengths shorter than the correct one (and thus
higher energy) all give the same energy, and the shorter
they are the more heat is made.  Very clever panels
incorporate multiple layers of junctions to catch different
portions of the spectrum, and are more expensive, and
are not used on houses.


Posted by Me on December 11, 2005, 7:00 pm
 

So speaks one who obviously never did much skiing in the springtime.


Me        one who did.......

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