As any kid who’s set leaves ablaze with a magnifying glass knows, concentrated sunlight packs a lot more punch than its unmanipulated counterpart. That’s why more and more solar systems, including photovoltaic variants, feature a concentrating capability.
MIT’s Marc Baldo works on concentrator technologies. Early on, though, he decided against pursuing standard approaches: sun-tracking mirrors that beam light at photovoltaic surfaces from several directions, for example, or bulky concentrating lenses that fit over, say, a rooftop solar panel.
Why? Baldo, an associate professor of electrical engineering, says it’s partly the challenges posed by the very high temperatures some conventional systems generate. He also wants technologies that work consistently in locales not known for lots of sunshine: Cambridge, Mass., for instance. “You’re unlikely to see conventional concentrators in places like this,” he says, “because if it’s cloudy, they won’t work.” His group’s systems are not only a lot less intrusive than conventional ones but work when the clouds roll in.
Baldo has built on a concept tried in the ’70s. It involves coating a light-transmitting material — glass, or a plastic called polycarbonate — with a dye. The dye absorbs many of the incoming photons, the massless particles that make up light waves. It then re-emits photons at a lower frequency: in other words, it fluoresces. Those photons, in effect redirected (the technical term is “guided”) by the glass or polycarbonate, go to the material’s edges. At that point, they enter what’s basically a standard photovoltaic system, but one that fits, window-frame-like, around the dye-coated material’s sides.
In early experiments, the Baldo group’s approach didn’t work well. A lot of the photons, like freed-up fish that dart back into a trawler’s nets, were reabsorbed by the dye.”If a concentrator absorbs its own light,”notes Baldo, “that’s catastrophic, because the photons never get to the edge of the plate.”
The group, though, found a way to minimize such absorption: use two dyes. The tactic creates a kind of daisy chain that greatly boosts photon flows in the glass or plastic. “A photon comes into one of the dyes, which then transfers the energy to the other dye,” Baldo explains. “The second dye emits a photon that can’t be readily reabsorbed, so that photon gets to the edge.”
The concentrator promises to feed the photovoltaic device bordering the glass or plastic plate 40 times more photons than that system would get if it were a standard PV technology. Right now, though, the Baldo group’s system — partly because of limits on the wavelengths of light you can collect in the first place — doesn’t yield as much electrical output as a standard photovoltaic array with a comparable sunlight-absorbing area. It’s about seven percent efficient, while typical commercial systems are in the 14 to 16 percent range.
As the group works to boost efficiencies, the researchers are thinking about ways to apply the technology. One possibility is in skylights, where the glass or polycarbonate could double as a window. “You can tune the dyes so the window remains clear,” notes Baldo. “At the same time, you’d be bleeding off some of the energy in sunlight and sending it to the solar cells.”
The work has gotten enough attention that Baldo often receives unsolicited calls and emails, including some from ’70s-era concentrator researchers. “Many of them tell me they would have kept working on this but the money went away,” says Baldo. “They’re still passionate about seeing it succeed, though.”