Thursday, July 10, 2008
A more efficient way to concentrate sunlight could reduce the cost of producing solar power.
By Kevin Bullis
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."