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 slab and, 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 fluorescent 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 window
pane 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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The above story is reprinted from materials provided by Penn State.Note: Materials may be edited for content and length. For further information, please contact the source cited above.
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