Solar power without solar cells: A hidden magnetic effect of light could make it possible

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April 13, 2011
Solar power without solar cells: A hidden magnetic effect of light could make it possible

ANN ARBOR, Mich.—A dramatic and surprising magnetic effect of light discovered by University of Michigan researchers could lead to solar power without traditional semiconductor-based solar cells.

The researchers found a way to make an "optical battery," said Stephen Rand, a professor in the departments of Electrical Engineering and Computer Science, Physics and Applied Physics.

In the process, they overturned a century-old tenet of physics.

"You could stare at the equations of motion all day and you will not see this possibility. We've all been taught that this doesn't happen," said Rand, an author of a paper on the work published in the Journal of Applied Physics. "It's a very odd interaction. That's why it's been overlooked for more than 100 years."

Light has electric and magnetic components. Until now, scientists thought the effects of the magnetic field were so weak that they could be ignored. What Rand and his colleagues found is that at the right intensity, when light is traveling through a material that does not conduct electricity, the light field can generate magnetic effects that are 100 million times stronger than previously expected. Under these circumstances, the magnetic effects develop strength equivalent to a strong electric effect.

"This could lead to a new kind of solar cell without semiconductors and without absorption to produce charge separation," Rand said. "In solar cells, the light goes into a material, gets absorbed and creates heat. Here, we expect to have a very low heat load. Instead of the light being absorbed, energy is stored in the magnetic moment. Intense magnetization can be induced by intense light and then it is ultimately capable of providing a capacitive power source."

What makes this possible is a previously undetected brand of "optical rectification," says William Fisher, a doctoral student in applied physics. In traditional optical rectification, light's electric field causes a charge separation, or a pulling apart of the positive and negative charges in a material. This sets up a voltage, similar to that in a battery. This electric effect had previously been detected only in crystalline materials that possessed a certain symmetry.

Rand and Fisher found that under the right circumstances and in other types of materials, the light's magnetic field can also create optical rectification.

"It turns out that the magnetic field starts curving the electrons into a C-shape and they move forward a little each time," Fisher said. "That C-shape of charge motion generates both an electric dipole and a magnetic dipole. If we can set up many of these in a row in a long fiber, we can make a huge voltage and by extracting that voltage, we can use it as a power source."

The light must be shone through a material that does not conduct electricity, such as glass. And it must be focused to an intensity of 10 million watts per square centimeter. Sunlight isn't this intense on its own, but new materials are being sought that would work at lower intensities, Fisher said.

"In our most recent paper, we show that incoherent light like sunlight is theoretically almost as effective in producing charge separation as laser light is," Fisher said.

This new technique could make solar power cheaper, the researchers say. They predict that with improved materials they could achieve 10 percent efficiency in converting solar power to useable energy. That's equivalent to today's commercial-grade solar cells.

"To manufacture modern solar cells, you have to do extensive semiconductor processing," Fisher said. "All we would need are lenses to focus the light and a fiber to guide it. Glass works for both. It's already made in bulk, and it doesn't require as much processing. Transparent ceramics might be even better."

In experiments this summer, the researchers will work on harnessing this power with laser light, and then with sunlight.

The paper is titled "Optically-induced charge separation and terahertz emission in unbiased dielectrics." The university is pursuing patent protection for the intellectual property.

U-M Sustainability fosters a more sustainable world through collaborations across campus and beyond aimed at educating students, generating new knowledge, and minimizing our environmental footprint. Learn more at http://sustainability.umich.edu/.

http://ns.umich.edu/htdocs/releases/story.php?id=8368

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Solar power: breakthrough could herald big drop in costs

Solar power is generated by photovoltaic cells, but two scientists are exploring different materials that could foster voltage from light's magnetic effects.
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By Mark Clayton, Staff writer / April 15, 2011

Scientists at the University of Michigan have discovered a new effect from an old property of light, which they say could lead to an "optical battery" that converts sunlight to electricity at a fraction of the cost of today's photovoltaic cells.
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Light has electric and magnetic qualities. Scientists had long thought, however, that the effects of light's magnetic field were so weak as to be irrelevant.

No so, says Stephen Rand, a professor of physics. Along with doctoral student William Fisher, he persisted in probing the long-ignored weak magnetic field that light produces when traveling through a nonconductive material, such as glass.

The breakthrough – unveiled Friday in a scientific paper in the Journal of Applied Physics – shows that if light is intense enough, it can, when traveling through nonconductive material, generate voltage from magnetic effects 100 million times stronger than earlier expected. Such magnetic effects produce a strong electric field that can be harnessed for electric power production, Dr. Rand and Mr. Fisher say.

"This could lead to a new kind of solar cell without semiconductors and without absorption to produce charge separation," Rand said in a statement. "In solar cells, the light goes into a material, gets absorbed and creates heat. Here, we expect to have a very low heat load. Instead of the light being absorbed, energy is stored in the magnetic moment."

He continues, "Intense magnetization can be induced by intense light and then it is ultimately capable of providing a capacitive power source."

Of course with every scientific breakthrough, there's the challenge of how to make it practical. In this case, the problem is that the intensity of the light must be about 10 million watts per square centimeter. Ordinary sunlight is much less than even one watt per square centimeter.

But that doesn't deter Fisher, who says that new materials (transparent ceramics, perhaps), when combined with focused sunlight, could work at lesser intensities.

"We show that sunlight is theoretically almost as effective in producing charge separation as laser light is," says Fisher in a phone interview. "It turns out we can in principle develop a voltage along the direction of the beam of light."

He adds, "Enough sunlight, focused into an optical fiber, could generate electricity – that's is a simple way to think about it."

In experiments planned for this summer, the two scientists plan to harness this power using laser light and – after that – sunlight. Fisher says that with improved materials (various kinds of glass, for example), sunlight could produce electricity at perhaps 10 percent efficiency – roughly equal to the rate at which commercial solar cells today convert sunlight to electricity.

"The breakthrough is really on the cost side," Fisher says. "All we need are lenses to focus the light and a fiber to guide it. Glass is made in bulk, and it doesn't require much processing, either."

But the breakthrough is unlikely to be implemented in solar power production for several years, perhaps even a decade, Fisher cautions. Yet he does not foresee any hurdles that can't be overcome.

"It's doable," he says.

http://www.csmonitor.com/Environment/20 ... p-in-costs