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Photovoltaic cells have a characteristic known as band
gap that determines how much electricty incoming
photons can produce. A photon with an energy lower
than the band gap will not produce any electricity (it will
pass through or produce only heat). A photon with an
energy larger than the
band gap will produce electric
energy equal to the band gap (and heat out of it's excess
energy). The PV cell, therfore, only generates optimum
output from frequencies exactly equal to its band gap.
Some of the latest high end cells have multiple different
band gaps to produce the highest possible energy output.
What I am considering is a single axis concentrator, that
in addition to the lens or mirror,
also contains a dispersive prism. The incoming light is
concentrated, spread into a rainbow and encounters
rows of PV cells, each optimized for the section of
spectrum it receives. (Obviously, the result would be
somewhat piecewise, depending on the band gaps
produceable and the resoultion of PV production). This
should result in a fairly high total efficiency and be
somewhat cheaper to manufacture than multiple band
gap cells.
IKECE
IKECE
Much good stuff in this poorly titled idea about the problems of heat in space. [bungston, Jul 30 2010]
VHESC
http://www.darpa.mi..._Cells_(VHESC).aspx
Very High Efficiency Solar Cell [pashute, Jul 22 2011]
SolarDon's link
http://www.sol-solution.net
Welcome SolarDon. This is where links go in HalfBakery. Unusual, but that's the way it works here. [pashute, Aug 19 2011]
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In single band gap, are out of gap frequencies harmful or just wasted? If harmful this could be addressed with a filter. |
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In fact, go both ways: concentrate, disperse, grab the stripes that match your PV materials, then re-combine the leftovers into white-ish light (you might have to selectively dump some more strips to get a good balance) and use the result for illumination. |
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[bungston] Out of gap frequencies either produce heat and no energy (under) or heat and energy (over). As long as the concentration isn't too high, these are not generally harmful, but also part of the reason that high concentration cells need active cooling. |
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Intriguing - and a relatively inexpensive proof-of-concept for the home baker [+] |
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I have a friend who's doing a PhD on pv cells. When she explained about band gaps and all that, I came up with almost exactly the same idea as this. |
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Basically, it would cost too much. The multiple band-gap cells are more expensive than normal, but much cheaper than *lots* of normal ones, and a huge prism. |
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I thought about that, but you aren't looking at a huge prism, you're looking at a small, if long, prism, which shouldn't be that expensive. |
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Under the optimum situation, you'd have a single row of junctions (cells) of any given bandgap that intercepts all of the frequencies up to the next band gap and row of junctions. In practice, you'd probably need more than one junction in each frequency band, but still not that many. The entire thing could be produced on a single wafer, and be no wider than a standard module unit. As a result you wouldn't be looking at more than 2 or so times as many modules as a normal concentrator. |
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You'd need a dedicated semi-conductor construction line, but any specialized system requires that. You'd also be able to get an ideal response from a wider range than even the multijunction cells. |
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I wonder if it is possible to change the frequency of light with little or no energy loss. Besides going the speed of light or near. Maybe not, or spectroscopy would be unreliable for astronomers. |
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Erm, you cannot put a prism in a beam of light and spead it out neatly into a spectrum unless the beam is very narrow and pretty much parallel. The classic prism pictures all feature the sunlight coming in from a slot cut in a window blind (for instance). The bands of coloured light are roughly the width of the original slot. |
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Plain old sunlight raging unobstructedly down on a prism will not split out into a pretty spectrum. It may have a red edge and a blue edge, but the main body of the prism's output is gonna be all mixed up as white. |
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// a single axis concentrator, that ... also acts as a dispersive prism // |
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Um, how? As we have discussed elsewhere, you cannot make a narrow beam of light out of a wide beam of light very easily. You'd need a lot of truck in the way of mirrors and lenses above the prism, or you'd just be using a narrow slot out of the available light. |
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Somewhere, sometime, this might be useful, but I'd just put up some more PV cells. |
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A single axis (trough) style concentrator will focus
the light into a single line. The addition of a
single
lens would turn this into roughly parallel light. |
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Put the lens in the focal line of the trough, direct
it
from there into a prism, then onto the solar cells.
It's optically viable. |
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And yes, I'm aware that every optical step results
in some losses, but I'm not convinced it would be
more than the gains from the cells. Oh, and
you're right, I was probably to cute with trying to
combine them. Original post rephrased to
indicate separate mirror and prism. |
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//Typos fixed// There's still a 'therfore' in there. [+] |
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I think it sounds like an excellent idea, but I don't know enough about photovoltaics to tell for sure. |
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/ Somewhere, sometime, this might be useful, / |
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This concept might be good for solar panels in space. On earth, waste heat can leave thru air convection or be conducted away to the earth. It is harder to get rid of heat in space as only radiance is left (see link, where I learned about this). I can imagine waste frequencies heating the apparatus to the point of damage. Eliminating these in advance would help reduce heating of the array. |
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I could imagine a space based collector: a parabolic mirror. The fearsome power of the collected sun would pass thru the prism, which will not be damaged as the energy largely passes thru. Unneeded frequencies go off into space. The useful one goes on to the panel. |
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If this is in orbit and one of those unneeded frequencies winds up hitting a spot on earth, it would make a very neat looking star for a short period should anyone be looking up at the right time. Depending on angles and orbits, the star could change colors. |
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THIS IS ALREADY BEING DONE!
Check out the Rainbow Concentrator by Sol Solution www.Sol-Solution.net. There is also a patent 7206142 for the idea. |
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I think you may be able to invalidate their patent. I
wonder if that makes for any way of getting a share
in it, but you definitely deserve it. |
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The patent predates my post by a couple of years,
so it's all theirs. The two approaches aren't quite
identical, but I'd have to do a serious patent
search to determine if there is a closer match
(their prior art sections suggests there might be).
I figure this doesn't fall under widely known to
exist though, so I'll leave it up. |
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Besides, I got someone to join just so he could
point out that they're already doing it, which is
kind of cool. |
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Will this do the trick?
I think it should. |
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Have your concentrating mirror have one curve, top to bottom. But being divided in to a series of separate curved channels side to side. Get the focal lengths such that each section produces a short vertical line of light at the focus of each section, and a point behind that. Rearrange the sections such that these vertical lines of light are very close together. Then place a suable defraction grating at this point. The defraction grating will split the light into its colours. |
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Better results might be achieved if the defraction grating is laid out with narrower gaps between the central slits, becoming progressively wider as the slits are further from the centre. All of the slits should be the same width as the lines of light. |
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LEDs work as photovoltaics if you shine light on
them*. Therefore, perhaps you could take an old
laptop screen, stick a multi-prismatic Fresnellish
lens on the front to direct red, glue and green light
to the relevant pixels, and use it as a solar panel. |
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Equally, of course, perhaps you couldn't. |
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*but probably not very efficiently |
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