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September 17th, 2008

Double-duty windows harvest solar energy

Published on September 17th, 2008 | By: pwray@ceramics.org
Diagram illustrates how a cost effective solar concentrator might make existing solar panels more efficient. MIT says its dye-based concentrator would function without the use of tracking or cooling systems and, thus, reduce overall solar costs, compared to other concentrator technologies. Dye molecules coated on glass absorb sunlight and re-emit it at a different wavelengths. Light is trapped and transported within the glass until it is captured by solar cells at the edge. Some light passes through the concentrator and can be absorbed by lower voltage solar cells underneath. Alternatively, the partially transmissive concentrator can function as a window. Note: graphic is not to scale. (Credit: NSF)

(Credit: NSF)

What if your house’s windows offered double-duty performance by not only letting in sunlight but also harvesting its energy to efficiently and cost-effectively power your home? That’s the concept behind the organic solar concentrator, a new kind of solar powering device, recently developed by MIT researchers and reported on in Science. “Light is collected over a large area [like a window] and gathered, or concentrated, at the edges,” explains Marc Baldo, project leader and MIT associate professor of electrical engineering and computer science. As a result, rather than covering a roof with expensive solar cells, the cells only need to be placed around the window’s edges. An added benefit is that the focused light increases the electrical power attained from each solar cell “by a factor of over 40,” according to Baldo. MIT’s solar concentrator entails a mixture of two or more dyes coated onto a flat panel of glass or plastic. The dyes work together to absorb light across a range of wavelengths and, then, re-emit it at a different wavelength and transport nearly all of it to the pane’s edges. Solar cells placed around the panel’s edges then pick up this concentrated light energy. Similar solar concentrators were developed in the 1970s by impregnating dyes in plastic. The idea was abandoned, however, because, among other things, too much light was lost en route and never reached the edges. MIT has overcome this obstacle by mixing the dyes in specific ratios and applying the mixture only to the surface of the glass, allowing some level of control over light absorption and emission. Says Jon Mapel, an MIT research team member:

“We made it so the light can travel a much longer distance. We were able to substantially reduce light transport losses, resulting in a tenfold increase in the amount of power converted by the solar cells.”

MIT’s solar concentrator improves on concentrators in use today. Baldo notes,

“[Existing concentrators] track the sun to generate high optical intensities, often by using large mobile mirrors that are expensive to deploy and maintain. Solar cells at the focal point of the mirrors must be cooled, and the entire assembly wastes space around the perimeter to avoid shadowing neighboring concentrators.”

Baldo says his team still needs to improve the concentrator’s durability and efficiency (estimated at 6.8 percent) before it can be taken to market. He estimates these changes and more can be accomplished within three years. He even has plans to adapt MIT’s concentrator so it can be added to existing solar-panel systems to increase their efficiency by 50 percent at minimal cost. He says this, in turn, will substantially reduce the cost of solar electricity. In addition to Jon Mapel, Baldo’s research team also includes Michael Currie and Timothy Heidel, also graduate students in MIT’s Department of Electrical Engineering and Computer Science. The team’s fifth member is Shalom Goffri, a postdoctoral associate in MIT’s Research Laboratory of Electronics Mapel, Currie and Goffri have started a company, Covalent Solar to develop and commercialize the new technology. Earlier this year Covalent Solar won two prizes in the MIT $100K Entrepreneurship Competition.  The company placed first in the Energy category ($20,000) and won the Audience Judging Award ($10,000). See video above for a demonstration of this light-concentrating technique.

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