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Wednesday, 01/10/2007 9:55:48 AM

Wednesday, January 10, 2007 9:55:48 AM

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Holographic Solar

A novel approach to concentrating sunlight could cut solar panel costs.

By Prachi Patel-Predd

Rows of silicon solar cells alternate with rows of transparent holograms in Prism Solar's concentrators. (Courtesy of Prism Solar)

The main limitation of solar power right now is cost, because the crystalline silicon used to make most solar photovoltaic (PV) cells is very expensive. One approach to overcoming this cost factor is to concentrate light from the sun using mirrors or lenses, thereby reducing the total area of silicon needed to produce a given amount of electricity. But traditional light concentrators are bulky and unattractive -- less than ideal for use on suburban rooftops.

Now Prism Solar Technologies of Stone Ridge, NY, has developed a proof-of-concept solar module that uses holograms to concentrate light, possibly cutting the cost of solar modules by as much as 75 percent, making them competitive with electricity generated from fossil fuels.

The new technology replaces unsightly concentrators with sleek flat panels laminated with holograms. The panels, says Rick Lewandowski, the company's president and CEO, are a "more elegant solution" to traditional concentrators, and can be installed on rooftops -- or even incorporated into windows and glass doors.

The system needs 25 to 85 percent less silicon than a crystalline silicon panel of comparable wattage, Lewandowski says, because the photovoltaic material need not cover the entire surface of a solar panel. Instead, the PV material is arranged in several rows. A layer of holograms -- laser-created patterns that diffract light -- directs light into a layer of glass where it continues to reflect off the inside surface of the glass until it finds its way to one of the strips of PV silicon. Reducing the PV material needed could bring down costs from about $4 per watt to $1.50 for crystalline silicon panels, he says.

The company is expecting to pull in another $6 million from interested venture capitalists and start manufacturing its first-generation modules by the end of the year, selling them at about $2.40 per watt. Next-generation modules with more advanced technology should bring down the cost further.

In their ability to concentrate light, holograms are not as powerful as conventional concentrators. They can multiply the amount of light falling on the cells only by as much as a factor of 10, whereas lens-based systems can increase light by a factor of 100, and some even up to 1,000.

But traditional concentrators are complicated. Since the lenses or mirrors that focus light need to face the sun directly, they have to mechanically track the sun. They also heat up the solar cells, and so require a cooling system. As a result, although they redirect light with more intensity than the hologram device, "they're unwieldy...and not as practical for residential uses," says National Renewable Energy Laboratory spokesperson George Douglas.

Holograms have advantages that make up for their relatively weak concentration power. They can select certain frequencies and focus them on solar cells that work best at those frequencies, converting the maximum possible light into electricity. They also can be made to direct heat-generating frequencies away from the cells, so the system does not need to be cooled. "In this way, you are efficiently using only that part of the sunlight that really matters," says Selim Shahriar, director of the atomic and photonic technology laboratory at Northwestern University in Evanston, IL.

Also, different holograms in a concentrator module can be designed to focus light from different angles -- so they don't need moving parts to track the sun.

Prism Solar's system incorporates these advantages. Nevertheless, to be competitive with other solar technologies available today, the company might need to reduce its price below $2.40 a watt, says Christo Stojanoff, professor emeritus of engineering at the Aachen University of Technology in Germany.

CEO Lewandowski says the holographic modules will cost about $1.50 per watt in a few years, using their second-generation technology, which will have solar cells sandwiched between two glass panels containing holograms. At that price, they'll start to compete with fossil fuel-generated electricity, which now costs almost three times less than conventional solar electricity, according to San Francisco, CA-based research and consulting company Solarbuzz.

The modules' intensive use of glass could be adding to their cost, says Douglas. Still, such a novel idea for a concentrator, using holograms, could be a lucrative investment because it needs less silicon than flat-panel modules and therefore saves money. The high demand for solar cells in Germany and other European countries "has now outstripped the supply, which has [led to] a silicon shortage and a shortage of manufacturing in the photovoltaic world," he says.

Although the idea of holographic solar concentrators has been around since the early 1980s, no one has developed them commercially yet, according to Professor Stojanoff, who has investigated the technique extensively. His company, Holotec GmbH in Aachen, Germany, researches and manufactures holographic materials. Also, Northeast Photosciences, a Hollis, NH-based company, came close to manufacture, before it went defunct for reasons unrelated to the technology or to finance, he says.

So, if all goes according to plan, Prism Solar could be the first company to manufacture and sell holographic solar concentrator modules.

(To contuinue reading:)

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