Solar Power
New Window Opens On Solar Energy: Cost Effective Devices Available Soon

July 11, 2008

Imagine windows that not only provide a clear view and illuminate rooms,
but also use sunlight to efficiently help power the building they are part of.

MIT engineers report a new approach to harnessing the sun's energy that could allow just that.

The work, reported in the July 11 issue of Science, involves the creation of a
novel "solar concentrator." "Light is collected over a large area [like a
window] and gathered, or concentrated, at the edges," explains Marc A. Baldo,
leader of the work and the Esther and Harold E. Edgerton Career Development
Associate Professor of Electrical Engineering.

As a result, rather than covering a roof with expensive solar cells (the
semiconductor devices that transform sunlight into electricity), the cells only
need to be around the edges of a flat glass panel. In addition, the focused
light increases the electrical power obtained from each solar cell "by a factor
of over 40," Baldo says.

Because the system is simple to manufacture, the team believes that it could be
implemented within three years--even added onto existing solar-panel systems to
increase their efficiency by 50 percent for minimal additional cost. That, in
turn, would substantially reduce the cost of solar electricity.

In addition to Baldo, the researchers involved are Michael Currie, Jon Mapel,
and Timothy Heidel, all graduate students in the Department of Electrical
Engineering and Computer Science, and Shalom Goffri, a postdoctoral associate in
MIT's Research Laboratory of Electronics.

"Professor Baldo's project utilizes innovative design to achieve superior solar
conversion without optical tracking," says Dr. Aravinda Kini, program manager in
the Office of Basic Energy Sciences in the U.S. Department of Energy's Office of
Science, a sponsor of the work. "This accomplishment demonstrates the critical
importance of innovative basic research in bringing about revolutionary advances
in solar energy utilization in a cost-effective manner."

Solar concentrators in use today "track the sun to generate high optical
intensities, often by using large mobile mirrors that are expensive to deploy
and maintain," Baldo and colleagues write in Science. Further, "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."

The MIT solar concentrator involves a mixture of two or more dyes that is
essentially painted onto a pane of glass or plastic. The dyes work together to
absorb light across a range of wavelengths, which is then re-emitted at a
different wavelength and transported across the pane to waiting solar cells at
the edges.

In the 1970s, similar solar concentrators were developed by impregnating dyes in
plastic. But the idea was abandoned because, among other things, not enough of
the collected light could reach the edges of the concentrator. Much of it was
lost en route.

The MIT engineers, experts in optical techniques developed for lasers and
organic light-emitting diodes, realized that perhaps those same advances could
be applied to solar concentrators. The result? A mixture of dyes in specific
ratios, applied only to the surface of the glass, that allows some level of
control over light absorption and emission. "We made it so the light can travel
a much longer distance," Mapel says. "We were able to substantially reduce light
transport losses, resulting in a tenfold increase in the amount of power
converted by the solar cells."

This work was also supported by the National Science Foundation. Baldo is also
affiliated with MIT's Research Laboratory of Electronics, Microsystems
Technology Laboratories, and Institute for Soldier Nanotechnologies.

Mapel, Currie and Goffri are starting 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),
voted on by all who attended the awards.


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