Re: [Vo]:Stationary Fresnel Array (Hybrid)

[Stephen A. Lawrence]

Stephen A. Lawrence wrote:

Jones Beene wrote:

It is perhaps possible to provide a *stationary*
Fresnel array which has "virtual" tracking.

Imagine a fixed, stationary array of Fresnel lens and
underlying photocells, somewhat as in this image:

http://www.sinosolargroup.com/en/images/toushe33.JPG

... and, with such a panel sited on a south-facing
roof, exactly the same way as a normal fixed
solar-panel would be sited - except that this one is
requiring 500 time less area of actual photocells than
the normal array.

This site lists the advantage of 500:1 concentration:

http://www.emcore.com/solar_photovoltaics/terrestrial_concentrator_photovoltaic_arrays

Needless to say, since Nanosolar gives only perhaps a
5-to-1 cost advantage with their printed cell, and it
is a far less-efficient cell, the comparative
advantages of any kind of concentrator array, in cost,
would be *huge* - except- for the one issue.
That issue being the need, added complexity and
aesthetics (for home use) which 2-axis tracking
demands.

Here is how to overcome most of that added (tracking
cost) and other issues, while still keep the solar
array fixed and stationary.
It is not a unique idea, as it has been suggested for
other uses, but it may be unique when it is combined
with a Fresnel concentrator, especially the kind of
Fresnel which itself is already combined with an
angled cone secondary. These are called
"self-focusing" but that is a misnomer. They are also
called "non-imaging"

http://en.wikipedia.org/wiki/Non-imaging_optics

I'm not sure about this article.  Among other things it says:

Imaging optics can concentrate sunlight to, at most, the same flux found at the surface of the sun.

This is false as written; a lens of f/0.5 produces an image of the sun with flux equal to the flux at the surface of the sun, and a larger lens -- or shorter focal length -- produces a flux /larger/ than that at the surface of the sun. There's something related and true which they may be trying to say, but the statement on the page, as written, is not correct. This leads me to wonder how firmly grounded the rest of that particular article is.

Want to produce a spot that's brighter than the surface of the sun? No prob; you can get the materials from Edmund's catalog. Maybe you can't build a classical glass lens with f/<0.5 but there's no prob making a Fresnel lens to do the job.

If you can find a big Fresnel lens of f/<0.5 you're all set. (It's possible the refractive indices of available plastics don't allow such short f/number lenses; I haven't tried to work out the geometry of the lens surface which would be required.) If you can't, you can make one; take three or four ordinary 12" square solar furnace-style Fresnel lenses (available from Edmund's, or at least they used to be) outside on a sunny day, stack them up in a sandwich (which cuts the focal length by a factor equal to the number of lenses in the sandwich), focus the sun, and voila, you've blown a hole in the concrete sidewalk. (Or at any rate you can blow a small chip out of it; I've done it, using a single 12" square lens, never mind the stack.) But don't look at the spot unless you're equipped with something appropriate, like welder's glasses; it is very bright indeed.

As I recall the basic "solar furnace" Edmund lens was not much over f/1 to start with, so four of the them stacked should be neatly under f/0.5.

To find the focal length of a stack of lenses, express their focal lengths in diopters and just add them up. A 1 foot diameter circular lens with 18" focal length is an F/2 lens.

Er, rather, it's an 1/1.5 lens.

18" = 0.46 meters = 2.2 diopters. Four of them, stacked, have a focal length of about 8.7 diopters, or 0.11 meters, or 4.5 inches. For a 12" diameter lens, that's f/0.38, and it should do the job.

A page I happen to have on image brightness:

http://physicsinsights.org/simple_camera_brightness_1.html

And here's a not-quite-airtight proof that you can't build a telescope which will make things look brighter when you look through it (sorry, you can never see things with your eyes the way they look in the Hubble photos, even if they let you go up on the shuttle and look in the eyepiece [if it had an eyepiece :-) ] ) :

http://physicsinsights.org/simple_optics_brightness_1.html

This type of optics has some added loss, but it seems
to provide a great deal more latitude than a normal
Fresnel. It will self-focus (in the patent claims at
least) within plus or minus 30 degrees of direct focus
(or better).
With an initial south facing placement, the use of
non-imaging optics will allow you to dispense with one
axis of tracking, just as with the parabolic trough;
but still requires the one axis for early morning and
late evening.
OK - the further enhancement is to implement this type
of limited self-focusing along with a "mirrored-slat
louver array", which lays above the Fresnel array.
The mirrored slats are long, thin polished metal, and
are all operated in unison to track the sun on one
axis from a single stepper motor, but there are huge
advantages to being both thin and tracking on *one
axis* only. Since the slats can be thin polished
aluminum, but otherwise similar to louvered shades,
they would only add a modest increase in thickness to
the fixed array, even it they are protected with
another layer of glass.
They should be enclosed so that they are protected
from weather and storms; and that external layer of
glass also protects the plastic Fresnel layer - which
can then be the thinest and cheapest variety that
Michael Foster can make ;-) It should be noted that
many window manufacturers now provide (for shade)
steerable louvers placed between two panes of glass
during manufacture without too much added cost.

As mentioned: glass, Fresnels lens, and mirrors have a
cost structure that is hundreds of times less (per
unit of surface area) than silicon or even printed
photocells; consequently, even with what may seem at
first to be a more complex hybrid arrangement - the
"net cost" per watt of output should be much less than
anything out there - and less than the dollar-per-watt
claim of Nanosolar...
...or so it would seem.

Jones