CHICAGO—Currently, the world has the capacity to manufacture over 40 Gigawatts of solar panels each year, the vast majority of them silicon-based. And it's easy to see why: our expertise with processing the material has led to a staggering drop in costs, making photovoltaics (PVs) much more cost-competitive than just about anyone had predicted.
But that manufacturing innovation hasn't been matched on the basic research side; it's been over a decade since the last time anyone set a new efficiency record for silicon cells. And, even as manufacturing costs have dropped, the cost of support equipment and installation has remained stubbornly high and is an ever-increasing slice of the total price of PV systems.
That's got people thinking that it might be time that we get more power out of each installation. At the meeting of the American Association for the Advancement of Science, two researchers spelled out how they were finding ways to take an expensive material and make it cheap enough to be deployed on the same scale as silicon.
The material in question is gallium arsenide, which can be fashioned into solar cells with efficiencies twice those of silicon. The high cost of the material, however, has limited its use to applications like satellites. But two research groups have come up with ways to get much more out of GaAs.
Gallium goes thin
Both teams have figured out how to make extremely thin layers of GaAs. Harry Atwater's group at Caltech has developed a process that allows them to peel hundreds of thin layers off a large aggregate of the material, much like individual graphene sheets can be peeled off a block of graphite. The end result is an extremely thin film of GaAs (he passed some samples around to the audience).
John Rodgers, who works at the University of Illinois at Urbana-Champaign, grows thin layers of GaAs separated by a thin sacrificial layer. When the sacrificial layer is etched away, you're left with a collection of thin GaAs chips; the silicon wafer they were grown on can then be recycled, cutting down on the costs significantly. A plastic stamp can then pick up the chips and "print" them onto just about any surface, including one pre-patterned with wiring.
In the rare cases where GaAs chips are used here on Earth, they're typically used in what's called a concentrated solar system, where lenses pump as many photons into the chips as they can manage without melting. But these tracking and focusing systems add significantly to the cost of these systems. Both groups are thinking of doing some focusing, but going about it in different ways.
Rodgers, who can print large arrays of tiny GaAs chips, is managing costs by keeping things simple: his team's process simply involves dropping a plastic sphere that acts as a lens on top of the chip. There are some ideas about how to manufacture more specialized spheres that focus the light more efficiently, but, for now, simplicity is the selling point.
Trapping the photons
Caltech's Atwater is making the focusing device a more central part of his system for reasons that focused on the physics of what happens inside the chips. When a photon is absorbed, it creates a free electron and a positively charged "hole." There are three things that can happen to this pair. One is that they end up at electrodes, producing a useful current. One is that they recombine uselessly, releasing the energy as heat. The third is that they recombine by releasing another photon.
For Atwater, the key to an efficient photovoltaic material is minimizing the wasteful recombination of electrons and holes. And that means getting them to re-emit a photon—in his view, a good photovoltaic material is also a good LED. In these materials, photons get absorbed and re-emitted a hundred times before being productively harvested, and the inside of the material is a sea of photons.
The danger here is that some of the photons escape back out of the material. To limit that, the GaAs can be put on a reflective backing that sends the stray photons right back into the chip. The front has to let sunlight in, but then keep photons from escaping. To do that, he's testing a system that looks a bit like two U's with their bottoms fused (technically, it's back-to-back parabolic lenses connected by a narrow aperture). This takes photons from a broad area and funnels them into the PV chip. The other end of the U acts like a reflective cap, making it very hard for a photon to escape from the chip without being reflected back into it.
Grab all the wavelengths
At this point, we're pumping lots of photons into the GaAs device and keeping them there until they're absorbed. The next way to up the efficiency is to have multiple independent photovoltaic devices, each tuned to different wavelengths. Traditionally, this has been done with layered devices, where each layer takes out a specific chunk of the spectrum.
And that's what Rodgers' team is already doing, by placing a standard triple-junction cell (which absorbs three different chunks of the spectrum) on top of a fourth cell that grabs yet another chunk. Atwater hopes to use the optic devices his team is working on to split the light into colors and direct them to independent photovoltaic devices.
Both of these faculty members have started companies to try to commercialize their work, and the devices they're making are already above the 40-percent efficiency mark—double that of silicon cells. Rodgers' company already has a 5MW capacity plant, and he said that scaling up production to an 80MW capacity plant would let them produce devices that are cost-competitive with coal.
Lots of great sounding technologies never make it past the demonstration phase, or they get caught up in market forces beyond their developers' control. But in this case, market forces are clearly working in the devices' favor. As installation costs become an ever larger fraction of the total cost of a solar installation, it's clear that getting more out of each installation is likely to be critical.
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