To generate electricity, Goffri connects the solar concentrator to solar cells. He’s making what is called a tandem solar module, a type of solar panel that uses two different kinds of cells to capture more of the energy in sunlight than a single kind could. Different wavelengths of sunlight have different amounts of energy; ultraviolet light has the most and infrared the least. Solar cells are optimized for particular colors. One designed to convert infrared light into electricity, for example, will convert most of the energy in blue light into waste heat. Likewise, red light will pass through a solar cell optimized for high-energy blue light without being absorbed. Ideally, solar cells for different wavelengths would be used in combination to collect the most sunlight, but this approach is often too expensive to be practical.
Baldo’s concentrators offer an inexpensive way to combine solar cells optimized for different wavelengths of light: different colored coatings can be paired with different types of solar cells in the same device. To make a prototype, Goffri takes a type of solar cell well suited to high-energy colors and glues it to the inside of a plastic frame; then he slides the concentrator into the frame so that its edges line up with the cells. The concentrator captures ultraviolet, blue, and green light and emits orange light that the cells convert into electricity. The lower-energy light, from the red and infrared end of the spectrum, passes through the solar concentrator to the next layer. In the prototype, the next layer is a full-size, conventional silicon solar cell that isn’t paired with a solar concentrator.
The prototype, Baldo says, can convert almost twice as much energy from sunlight into electricity as a conventional cell can, provided that the concentrator is roughly 30 centimeters square. This translates to a 30 percent decrease in the cost of solar electricity.
In the future, the cost savings can be much higher, Baldo believes. He doesn’t use a concentrator for the infrared light because, so far, no good dyes for capturing those wavelengths exist. But he is confident that such dyes can be developed. When that happens, he will be able to add a second concentrator, for little additional cost, and replace the full-size silicon solar cell with smaller, cheaper cells attached to the concentrators’ edges. If the cost of photovoltaics drops over the next several years, as expected, this setup could make solar power about as cheap as electricity from coal, he says.
There’s more work to be done in the lab, such as improving the range of colors the concentrators can absorb, which will make it possible to tailor them to specific slices of the spectrum. But Baldo says that it’s time to start moving the technology out of the lab and into the market. He and his colleagues have founded a company called Covalent Solar, which is starting to raise money. The company, based in Cambridge, MA, plans to have its first products–probably tandem solar modules–available within three years.
Kevin Bullis is Technology Review’s energy editor.
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