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"Designer" for solar reflectors

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Posted by brian white on November 18, 2009, 4:49 am
 
The idea here is that if you line up a reflector with the path of the
sun across the sky, the best shape for getting maximum concentration of
light over one hour, 2 hours, 3, etc is NOT going to be parabolic.  In
fact, nobody knows what shape it will be.
Check 5 minutes into this video from Bill Gross for an idea. (It will
not be THAT complicated)

So I thought some way to guesstimate it would be useful.
The idea is pretty simple. In my case, I want to design a reflector that
sends all the light to the target for 2 hours. So I use a laser to
simulate the sun at the start of the 2 hours and another one to simulate
the sun at the end of the 2 hours. As long as the light from both lasers
bounce off the reflector and both hit the target, the reflector is
correctly shaped. Basically, you set up the shape of the reflector one
strip at a time until you get the whole thing done.
I think very little work has been done on this (because the math is too
complicated) so if a few people make these little "designers" and play
around with them, I think very useful shapes and concepts will show up.
There is no "best" shape but perhaps close approximations will show up
fairly quickly.
The designer idea is at the link below and I will add to it over the
next few days  Brian >
http://www.instructables.com/id/Method-for-designing-reflectors-for-unattended-sol/

Posted by Josepi on November 18, 2009, 12:32 pm
 
Good presentation!

However, there are lots of lies in that presentation.

- After all the bullshit about the poor parabolic reflector design they end
up with a parabolic reflector, anyway.

- The other main lie is after $50,000 of research the end result is a
parabolic reflector with twelve tracking "pettles" for $ each, with
tracking motors and microprocessors. Sign me up! I will even give them $2
for 100% profit. Not likely!

- Then the whole video ends up with a PV promotion for the worst inefficient
design layout I have ever seen. Too much blank space in a compact module.

Entertaining though.


http://www.instructables.com/id/Method-for-designing-reflectors-for-unattended-sol/  



Posted by Robert Scott on November 18, 2009, 1:44 pm
 The supposed "benefit" of this unique multi-petal reflector scheme was that it
avoids the problems of off-angle cosine loss as experienced by static flat PV
panels.  However the off-angle cosine loss can eliminated by breaking the PV
panel up into smaller PV panels, about the size of one of the inventor's "petal"
reflectors, and motorizing each little PV panel, just like he motorized the
petal reflectors.  It is still questionable whether the gain in power by
tracking is worth it.  If a tracking collector costs twice what a static
collector of the same size costs, then I think I will just buy twice as many
static collectors instead.  The overall off-angle cosine loss is not so great
that it justifies a 2:1 price difference.  The real advantage in concentrating
collectors occurs when ultra-high temperatures are generated, which allows more
efficient thermal engines.  But then you are comparing two different
technologies: PV and thermal engines.  And that comparison is likely to depend
on the scale.

In the invention cited there is no great breakthrough in reflector shape design.
When a reflector is broken up into individual little pieces, then it does not
matter much what exact shape the pieces are.  The fact that each piece is
individually motorized makes them all reflect light to the same place.  And in
fact, as Josepi pointed out, they are still using parabolas.




Robert Scott
Ypsilanti, Michigan

Posted by dow on November 19, 2009, 4:18 pm
 
If you think about a little bit of the surface of a reflector, it must
act like a plane mirror. As the sun moves across the sky, the
drection  in which sunlight is reflected by this little mirror *must*
change. So it basically impossible to design a reflector that will
reflect all the sunlight that strikes it to a stationary target over
an extended period of time.

What is possible, however, is to design a reflector such that *part*
of its surface will reflect sunlight to the target at some specific
time, then another part will do so at a later time, and so on. There's
a shape, which I know you have seen, called a "compound parabola" that
can be used to do this. Basically, this consists of two or more
parabolas side by side and fused together. The parabolas share a
common focal point, but are aimed in slightly different directions. At
some time, one of the parabolas exactly reflects sunlight to the
focus. A bit later, the sun has moved, and another of the parabolas
reflects sunlight to the focus. If the objective is to send sunlight
to an extended object such as a cooking pot, the compound parabola
will do this fairly well continuously for a considerable time. Light
does not have to be reflected exactly to the focal point in order to
hit the pot, so one parabola reflects sunlight to the pot for a few
minutes, then another parabola does it for the next few minutes, and
so on. However, at any given time, only part of the total area of the
reflector is reflecting light to the pot.

There is  no reason, apart from simplicity, why the parabolas should
be side by side. They could be cut into many pieces and put together
in any pattern. However, the basic fact that only part of the total
area is reflecting light to the pot at any given time will always be
true.

                             dow



Posted by dold on November 19, 2009, 5:39 pm
 

The video shows something resembling an old Victrola audio horn that
focuses varying light angles to a focal point, some with multiple bounces.

--
Clarence A Dold - Hidden Valley Lake, CA, USA  GPS: 38.8,-122.5

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