MIT opens new 'window' on solar energy

Cost effective devices expected on market soon

Elizabeth A. Thomson, News Office
July 10, 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, to be 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.

• Fact sheet: MIT's solar concentrators

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.

Video

Click To Play. Marc Baldo discusses MIT's solar concentrator

Images

Image courtesy / Nicolle Rager Fuller, NSF

An artist's representation shows how a cost effective solar concentrator could help make existing solar panels more efficient. Enlarge image

Photo / Donna Coveney

Organic solar concentrators collect and focus different colors of sunlight. Solar cells can be attached to the edges of the plates. By collecting light over their full surface and concentrating it at their edges, these devices reduce the required area of solar cells and consequently, the cost of solar power. Stacking multiple concentrators allows the optimization of solar cells at each wavelength, increasing the overall power output. Enlarge image

Photo / Donna Coveney

Marc Baldo, associate professor of electrical engineering and computer science (left) and Shalom Goffri, postdoc in MIT's Research Laboratory of Electronics (right) hold examples of organic solar concentrators. Enlarge image

Teresa Herbert
MIT News Office
Phone: 617-258-5403
E-mail: therbert@mit.edu

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A Better Solar Collector

Thursday, July 10, 2008

A more efficient way to concentrate sunlight could reduce the cost of producing solar power.

By Kevin Bullis

Colorful concentrators: The colored plastic sheets illustrate a way to concentrate sunlight. Combinations of advanced organic dyes made into similar sheets could make solar concentrators more practical. Credit: Kevin Bullis Multimedia See a prototype solar concentrator in action.

Looking to make solar panels cheaper, MIT researchers have created sheets of glass coated with advanced organic dyes that more efficiently concentrate sunlight. The researchers, whose results appear in this week's issue of Science, say that the coated glass sheets could eventually make solar power as cheap as electricity from fossil fuels.

The researchers show that the glass sheets can reduce the amount of expensive semiconducting material needed in solar panels and provide a cheap way to extract more energy from high-energy photons, such as those at the blue end of the spectrum. "This could be the cheapest solar technology," says Marc Baldo, a professor of electrical engineering at MIT. "And I think one day, it could be competitive with coal."

The simple, flat sheets of glass have a number of advantages over previous solar concentrators, devices that gather sunlight over a large area and focus it onto a small solar cell that converts the light into electricity. Solar concentrators in use now employ mirrors or lenses to focus the light. Because the new glass sheets are lighter and flat, they can easily be incorporated into solar panels on roofs or building facades. They could also be used as windows, which, connected to solar cells, could generate electricity. What's more, mirrors and lenses require mechanical systems for tracking the sun to keep the light focused on a small solar cell. These tracking systems add cost and can break down over the decades that solar panels are made to be in service. The flat glass concentrators don't require a tracking system.

Instead of using optics, the glass sheets concentrate light using combinations of organic dyes specially designed by Baldo and his coworkers. Light is absorbed by the organic dyes coating one side of the glass sheet. The dyes then emit the light into the glass. The glass channels the light emitted by the dye to the edges of the glass, in the same way that fiber-optic cables channel light over long distances. Narrow solar cells laminated to the edges of the glass collect the light and convert it into electricity. The amount of light concentration depends on the size of the sheet--specifically, the ratio between the size of the surface of the glass and the edges. To a point, the greater the concentration, the less semiconductor material is needed, and the cheaper the solar power.

The challenge of using organic dyes as solar concentrators has been that the dyes tend to reabsorb much of the light before it can reach the edges of the glass. Baldo overcame that problem by using dyes that don't absorb the light that they emit. For example, a dye might absorb a range of colors in the light spectrum, such as ultraviolet through green, but emit light in another color, such as orange, which the dye cannot absorb.

The researchers tested how much of the light emitted by the dye makes it to the edges of 10-centimeter squares of coated glass, the largest allowed by their laboratory equipment. Based on their measurements, they project that they can make solar concentrators large enough to bring down the costs of solar power to near that of conventional electricity, given expected reductions in the cost of solar cells. "We showed much bigger concentration factors than people had shown before," Baldo says.

The researchers also tested an inexpensive way to improve the efficiency of solar cells by capturing more of the energy in sunlight. Each wavelength of light, or color, has a different amount of energy. Infrared photons have the least energy, and ultraviolet photons have the most. Different types of semiconductor materials are best for different wavelengths. It's possible to build more than one type of solar cell into a single module, but this can be more expensive than it's worth.

The dye-coated glass sheets provide a cheap way to use more than one type of solar cell in a single solar module--one solar cell tuned to work with low-energy light, and the other to work with high-energy light. Two glass sheets are stacked. The top one absorbs high-energy light and channels it to a small solar cell matched to that light. The other captures lower-energy photons and channels those to another solar cell. Based on the researchers' initial results, Baldo says, "you can almost double the efficiency of your overall system if you do this."

The researchers still need to make bigger concentrators to test their predictions. They are also working to improve the quality of the dyes, including the range of colors that they can absorb. Baldo and his colleagues have founded a company--Covalent Solar, based in Cambridge, MA--to bring the technology to market within three years. Jerry Olson, an expert in solar concentrators at the National Renewable Energy Laboratory, in Golden, CO, says that the work represents some "good steps forward." But, adds Olson, "time will tell if the projections come true."

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