Posted on 10/30/2009 7:11:10 AM PDT by Reaganesque
Dye-sensitized cells get a double boost from nanowires and optical fiber.
Dye-sensitized solar cells are flexible and cheap to make, but they tend to be inefficient at converting light into electricity. One way to boost the performance of any solar cell is to increase the surface area available to incoming light. So a group of researchers at Georgia Tech has made dye-sensitized solar cells with a much higher effective surface area by wrapping the cells around optical fibers. These fiber solar cells are six times more efficient than a zinc oxide solar cell with the same surface area, and if they can be built using cheap polymer fibers, they shouldn't be significantly more expensive to make.
The advantage of a fiber-optic solar-cell system over a planar one is that light bounces around inside an optical fiber as it travels along its length, providing more opportunities to interact with the solar cell on its inner surface and producing more current. "For a given real estate, the total area of the cell is higher, and increased surface area means improved light harvesting and more energy," says Max Shtein, an assistant professor of materials science and engineering at the University of Michigan who was not involved with the research.
Fiber-optic solar cells could also be used in ways that aren't possible currently. Zhong Lin Wang, professor of materials science and engineering at Georgia Tech, says fiber solar cells would take up less roof area than planar cells because long lengths of the fibers could be nestled into the walls of a house like electrical wiring.
Dye-sensitized solar cells use dye molecules to absorb light and generate electrons. The Georgia Tech group first removes the cladding from optical fibers and then grows zinc-oxide nanowires along their surface, like bristles on a pipe cleaner. Next, the fibers are treated with dye molecules, which the zinc-oxide structures absorb. The advantage of coating nanowires, rather than a smooth surface, with the dye is that the wires collectively have a very large surface area. The more dye molecules there are over a given area of such a cell, the more light it can absorb, says Wang. The dye-coated fibers are then surrounded by an electrolyte and a metal film that carries electrons off the device. The work is described online in the journal Angewandte Chemie International Edition.
"The question is, can you absorb all the light using a small amount of materials?" says Yi Cui, assistant professor of materials science at Stanford University. Building a nanostructured cell on an optical fiber provides a way to do this by increasing both the surface area covered by the dye and the effective path length of the light, he says. The longer a photon travels through a solar cell, the more opportunities it has to interact and generate an electron.
One potential stumbling block for fiber cells is getting enough light inside them in the first place. Wang's devices only collect light at their tips, so to get enough light into such a solar cell without having to track the sun, smaller fibers might be bundled together. Cui says the tips of the fibers could be made of materials that are very effective at directing light into the fiber. Another way to overcome this problem is to build fiber cells that can absorb light along their entire length, not just at the tips--which Michigan's Shtein is working on. This is tricky, because it means the cells' coatings need to be both electrically conductive and transparent, an unusual combination.
However, Shtein says that fibers that absorb light from the sides offer "an interesting architecture for light capture, because you can distribute the fibers in space in a way that helps you capture more photons more effectively than you can in a planar device." The shallower the angle at which light hits a planar cell, the more light reflects off its surface. But the light reflecting off the curved surface of a fiber at a shallow angle will hit an adjacent fiber. These cells could be designed so that it's not necessary to install them with sun-tracking systems, and they would work on cloudy days when the light is diffuse, Shtein says.
Wang says the next step is to try different materials. So far, he has built the cells on quartz optical fibers, which are relatively expensive. Next he plans to try making the cells using cheaper polymer fibers.
Now this IS cool.
Arrange cheap, but low-efficiency, solar cells around an optical fiber.
Result would be high efficiency on the surface exposed to sunlight.
Now this IS cool.
Arrange cheap, but low-efficiency, solar cells around an optical fiber.
Result would be high efficiency on the surface exposed to sunlight.
When designing optical fibers for telecommunications engineers try to avoid light "bouncing around" inside the fiber because it causes loss in amplitude of the signal beig transmitted down the fiber. Leave it to my homies at my Alma Mater to come up with a way to turn loss into gain.....again.
You are still going to need significant square footage, to get power. The 24/7, day-night mean for the CONUS is 7 Watts per square foot (averages Phoenix and Chicago, winter-summer, etc) so if you want a kilowatt you’ll need 150 square feet, more or less, even with 80% efficiency.
Note also that the comparison is between zinc oxide planar cells and zinc nanowire cells. What’s the comparison between the nanowire cells and Cu-Si cells or crystalline Si cells?
How much would a million-square-foot array of the new technology cost?
As an investor I read these periodic announcements with caution. Go back ten years and look up how many of these incredible breakthroughs actually worked, in practical terms.
And if you go back to 1957, we should have moon colonies, flying cars, robot lawn mowers (we have robot carpet vacs now), solar power, nuclear power too cheap to meter, and so forth.
“Result would be high efficiency on the surface exposed to sunlight.”
That would be the end face of the fiber. For a 1mm dia fiber, the area is small.
Fiber manufacturers have to keep in mind the use of CWDM and DWDM(multiple light wavelengths over a single strand of fiber) when constructing fiber cables. They can’t have lightwaves “bouncing around” inside fiber and have it work.
Fiber manufacturers have to keep in mind the use of CWDM and DWDM(multiple light wavelengths over a single strand of fiber) when constructing fiber cables. They can’t have lightwaves “bouncing around” inside fiber and have it work.
I think you’d need a high loss cable, or a leaky cable, so that the light hits the converter in a short distance. If you pick up your 80% of the incoming in a kilometer of coated cable you’ll go broke building a 100 KW array.
Like the guitar strings on a Hard Rock Casino sign, they are made to have strong side emissions to be visible. Um, the strings are fibers, yano.
There always is, though. There is no such thing as a perfect fiber, or connector or splice. All induce some loss into the system. This loss is measurable and determines the distance between repeaters, among other things.
With this solar power application, though, it might be desirable to scatter the light inside the fiber.....
The physics is obvious, but the practical problem is going to be making the electrical connection(s).
I’ve always heard there was about 1000 watts/square meter of potential solar power at noon on the equater and between 600-800 in the U.S.
This link tends to agree.
http://hypertextbook.com/facts/1998/ManicaPiputbundit.shtml
600 W/M2 is 60W ft2, then average that for a 24 hour cycle, and also from North to South in the CONUS, over all the seasons and account for “average” daytime cloud cover.
The nunber I used, 7 W/ft2, is from an engineering textbook.
Solar proponents like to use Phoenix AZ at noon in July for a benchmark but in Troy NY at 10AM in November the number is a bit different.
Perhaps at noon but you need the average value combined with energy storage to compare to power sources.
Unless you we only going to use the power at noon at the equator.
I think the 7w/ft2 is off by a factor of 5 for areas where large solar power facilities would actually be installed.
His response was helpful, the number was for the average CONUS.
Just thinking out loud here and I apologize as this is not my field of engineering (Civil Engineers build targets), but I would think that you would need to lay the fiber bundle in a small, highly reflective tray similar to a oversize rain gutter so that you can focus the light on to the fiber array.
But I am sure they already thought of this...
Thanks for the map. It looks like the southwest would average 30 watts/ft2.
Portland could be problematic.
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