September/October 2008

Intensifying the Sun

A new way to concentrate sunlight could make solar power competitive with fossil fuels.

By Kevin Bullis

Marc Baldo poses with a collection of glass sheets coated with light-emitting organic dyes. The dyes absorb light and reëmit it into the glass, which channels it to the edges of the sheets. Baldo uses the devices to concentrate sunlight, making solar power cheaper. Credit: Porter Gifford

Multimedia   See how the solar concentrator works.   A demonstration of how the solar concentrators are made.

In his darkened lab at MIT, Marc Baldo shines an ultraviolet lamp on a 10-­centimeter square of glass. He has coated the surfaces of the glass with dyes that glow faintly orange under the light. Yet the uncoated edges of the glass are shining more brightly--four neat, thin strips of luminescent orange.

The sheet of glass is a new kind of solar concentrator, a device that gathers diffuse light and focuses it onto a relatively small solar cell. Solar cells, multilayered electronic devices made of highly refined silicon, are expensive to manufacture, and the bigger they are, the more they cost. Solar concentrators can lower the overall cost of solar power by making it possible to use much smaller cells. But the concentrators are typically made of curved mirrors or lenses, which are bulky and require costly mechanical systems that help them track the sun.

Unlike the mirrors and lenses in conventional solar concentrators, Baldo's glass sheets act as waveguides, channeling light in the same way that fiber-optic cables transmit optical signals over long distances. The dyes coating the surfaces of the glass absorb sunlight; different dyes can be used to absorb different wavelengths of light. Then the dyes reëmit the light into the glass, which channels it to the edges. Solar-cell strips attached to the edges absorb the light and generate electricity. The larger the surface of the glass compared with the thickness of the edges, the more the light is concentrated and, to a point, the less the power costs.

Baldo, an associate professor of electrical engineering, published his findings recently in Science. On their basis, he projects that his solar concentrators could be made big enough for the electricity they help generate to compete with electricity from fossil fuels. Indeed, says Baldo, panels equipped with the concentrators "could be the cheapest solar technology."

Secret Ingredient
The process for making Baldo's solar concentrators begins down the hall in another lab. A postdoctoral researcher, Shalom Goffri, takes several bottles filled with colorful dye powders from a cabinet and measures the powders into small vials. Some of the dyes were developed for use in car paints; others have been used in organic light-emitting diodes. Both types of dyes can last for years in the sun, a quality essential for solar concentrators. Once he has measured out the powders, Goffri adds a solvent to each to make a liquid ink.

The next steps take place inside a sealed box, so that Goffri doesn't inhale the solvents used to make the dye. He reaches into the box, using thick black gloves mounted in its glass front, and carefully mixes together different inks. Determining the right combination of inks solved a fundamental problem that researchers have encountered with this type of solar concentrator. If the glass sheet is coated with a dye that absorbs sunlight in, say, the green-to-blue range of the solar spectrum and emits light of the same wavelength, the emitted light will be quickly reabsorbed by the dye, and little of it will ever reach the edge of the glass. The problem has limited the size of these solar concentrators, since the further the light needs to travel to the edges, the less of the light will make it.

By using certain combinations of dyes interspersed with other light-absorbing molecules, Baldo makes coatings that absorb one color but emit another. The emitted light is not quickly reabsorbed by the coatings, so more of it reaches the edges of the glass sheet.

The coatings that Goffri is making absorb ultraviolet through green light and emit orange light. Once Goffri has prepared the final mixture, he pours a small amount on a 10-centimeter-wide glass square--the largest that can fit inside a device that spins the glass at 2,000 revolutions per minute to spread the ink evenly. Within a minute or two, the solvent has evaporated and the process is finished. The solar concentrator, with its coating of orange dye, is complete.