Chris Giebink, Penn State
An LSC is illuminated by a laser beam (central spot) resulting in luminescence that is emitted from the edges and projected onto a white business card. The faintly visible concentric rings and different colors of light on the business card result from microcavity effects.
By John Roach
Admit it. You fried an ant under a magnifying glass. It's OK. We did it too. Now scientists are reporting a breakthrough in a similar technology that could bring down the cost of solar power.
About 50 percent of the cost of solar power is due to the materials and manufacturing of solar cells, essentially pieces of silicon that convert sunlight into electricity. By concentrating the sunlight, you can get the same amount of power with fewer cells.
One way to do this is with a magnifying glass, like we do when we fry ants. But this is a bit tricky when we want to concentrate sunlight all day long because we have to make sure the glass is directly aligned with the sun.
"In order to do that, you have to track the sun … and that drives up the cost of your concentrating system," Chris Giebink, an assistant professor of electrical engineering at Pennsylvania State University, told me today.
Luminescent solar concentrators
The approach he and his colleagues are improving upon is a decades old technology called luminescent solar concentrators. These contraptions concentrate light by absorbing it with special dyes that re-emit about 75 percent of the light within the confines of a transparent slab of material.
The trapping effect is similar to the way optical fibers use light to transmit data. "It is trapped so it is guided towards the edges and that's where you stick your solar cells," Giebink explained.
The bigger you make the LSC, the more concentrated the light that's fed to the solar cells on the edges. In theory, these things can concentrate the light to the power of 100 suns — all without tracking the sun since they work at any angle and even concentrate diffuse light on cloudy days.
"On paper, it sounds really good," Giebink said. "In practice, the reason you don't see these things is because they don't work very well."
The biggest problem is that much of the sunlight that is absorbed by the dye and reemitted into the glass either bounces off the glass and gets reabsorbed by the dye and lost or reemitted in a direction where it is no longer trapped, which has about a 25 percent chance of occurring.
"Since we are bouncing through this thing hundreds of times, that adds up to a big problem. It has prevented these things from getting anywhere close to their theoretical potential," Giebink said.
Preventing re-absorption
He and his colleagues have now found a way to prevent the light from being reabsorbed by the dye en route to the edge of the glass.
To do this, they made an LSC with two very thin films stacked on a layer of glass.
The first film — about 100 nanometers thick — is a luminescent layer containing the dye that absorbs and reemits sunlight. This layer sits on top of a low refractive index layer, "which essentially means from the standpoint of light it looks a lot like air," Giebink explained.
This combination creates what is called a microcavity. The researchers found if they changed the thickness of the luminescent layer, the microcavity would change in a way that prevents the light reemitted by the dye from being reabsorbed when it bounces off the bottom of the glass.
"We've changed the thickness of one of the films such that light essentially can't fit in that thin film anymore and as a result it is reflected back with very high efficiency, close to 100 percent," Giebink said.
Their experimental results suggest this approach allows them to get to about 25 suns for a window pane sized collector, which is 2.5 times greater than a conventional LCS.
Going forward, the researchers need to optimize the design so that it is both cheap to manufacture and has the desired effect. After all, it won't bring down the cost of solar power if the concentrator cost as much as the solar cells it's meant to replace.
"We've shown the general idea works, but how exactly to build one of these things is not entirely clear," Giebink said.
Complementary approach
The breakthough is compatible with another approach to this problem reported by researchers at the Massachusetts Institute of Technology in 2008 that focused on creating dyes that are less susceptible to reabsorbing the light they reemit.
"We took any dye that you want and decreased the probability of re-absorption a lot just by how we structure the concentrator itself," Giebink explained. "We ought to be able to combine the two approaches. That's the direction we are going now."
If it all works out, the researchers estimate it could reduce the cost of solar power systems by about a factor of two, he added, which could help make solar energy more price competitive with coal and oil, easing the transition away from fossil fuel energy.
More on solar power technologies:
The researchers, who included Giebink and Gary Widerrecht and Michael Wasielewski with Argonne-Northwestern Solar Energy Research Center and Northwestern University, published their findings in current issue of Nature Photonics.
John Roach is a contributing writer for msnbc.com. To learn more about him, check out his website. For more of our Future of Technology series, watch the featured video below.
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