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Solar Concentrator Increases Collection with Less Loss
UNIVERSITY PARK, Pa. -- Converting sunlight into electricity is not economically attractive because of the high cost of solar cells, but a recent, purely optical approach to improving luminescent solar concentrators (LSCs) may ease the problem, according to researchers at Argonne National Laboratories and Penn State.
Using concentrated sunlight reduces the cost of solar power by requiring fewer solar cells to generate a given amount of electricity. LSCs concentrate light by absorbing and re-emiting it at lower frequency within the confines of a transparent slab of material. They can not only collect direct sunlight, but on cloudy days, can collect diffuse light as well. The material then guides the light to the slab's edges, where photovoltaic cells convert the energy to electricity.
"Currently, solar concentrators use expensive tracking systems that need to follow the sun," said Chris Giebink, assistant professor of electrical engineering, Penn State, formerly of Argonne National Laboratory. "If they are a few tenths of a degree off from perfection, the power output of the system drops drastically. If they could maintain high concentration without tracking the sun, they could create electricity more cheaply."
LSCs can do this, potentially concentrating light to the equivalent of more than 100 suns but, in practice, their output has been limited. While LSCs work well when small, their performance deteriorates with increasing size because much of the energy is reabsorbed before reaching the photovoltaics.
Typically, a little bit of light is reabsorbed each time it bounces around in the slaband, because this happens hundreds of times, it adds up to a big problem. The researchers, who included Giebink and Gary Widerrecht and Michael Wasielewski, Argonne-Northwestern Solar Energy Research Center and Northwestern University, note in the current issue of Nature Photonics that "we demonstrate near-lossless propagation for several different chromophores, which ultimately enables a more than twofold increase in concentration ratio over that of the corresponding conventional LSC."
The key to decreasing absorption is microcavity effects that occur when light travels through a small structure with a size comparable to the light's wavelength. These LSCs are made of two thin films on a piece of glass. The first thin film is a luminescent layer that contains a fluourescent dye capable of absorbing and re-emitting sunlight. This sits on a low refractive index layer that looks like air from the light's point of view. This combination makes the microcavity and changing the luminescent layer's thickness across the surface changes the microcavity's resonance. This means that light emitted from one location in the concentrator does not fit back into the luminescent film anywhere else, preventing it from being reabsorbed.
"We were looking for some way to admit the light, but keep it from being absorbed," said Giebink. "One of the things we could change was the shape and thickness of the luminescent layer."
The researchers tried an ordered stair step approach to the surface of the dye layer. They looked at the light output from this new configuration by placing a photovoltaic cell at one edge of the collector and found a 15 percent improvement compared to conventional LSCs.
"Experimentally we are working with devices the size of microscope slides, but we modeled the output for larger, more practical sizes," said Giebink. "Extending out results with the model predicts intensification to 25 suns for a windowpane sized collector. This is about two and a half times higher than a conventional LSC."
The researchers do not believe that the stair step approach is the optimal design for these LSCs. A more complicated surface variation is probably even better, but designing that will take more modeling. Other approaches may also include varying the shape of the glass substrate, which would produce a similar effect and potentially be simpler to make.
"We need to find the optimum way to structure this new type of LSC so that it is more efficient but also very inexpensive to make," said Giebink.
The U.S. Department of Energy supported this work. Argonne National Laboratory has filed for a patent on this application.
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Contact: Dr. Giebink may bereached at 814-865-2229 or at ncg2@psu.edu.
MSNBC also covered this solar technology story
Looks like this penn state tech has made it to the big time...
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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.