January 04, 2008

80% efficient solar power from nanoantennas

From nanowerk, a new approach for solar power, which garnered two 2007 Nano50 awards, uses a special manufacturing process to stamp tiny square spirals of conducting metal onto a sheet of plastic. Each interlocking spiral "nanoantenna" is as wide as 1/25 the diameter of a human hair.

Update: Al fin has an excellent article that discusses some more details and provides more analysis of this technology

Because of their size, the nanoantennas absorb energy in the infrared part of the spectrum, just outside the range of what is visible to the eye. "I think these antennas really have the potential to replace traditional solar panels," says physicist Steven Novack, who spoke about the technology in November at the National Nano Engineering Conference in Boston. So far, they have demonstrated the imprinting process with six-inch circular stamps, each holding more than 10 million antennas.

The team estimates individual nanoantennas can absorb close to 80 percent of the available energy. The circuits themselves can be made of a number of different conducting metals, and the nanoantennas can be printed on thin, flexible materials like polyethylene, a plastic that's commonly used in bags and plastic wrap. In fact, the team first printed antennas on plastic bags used to deliver the Wall Street Journal, because they had just the right thickness.

By focusing on readily available materials and rapid manufacturing from inception, Novack says, the aim is to make nanoantenna arrays as cheap as inexpensive carpet.

The real trick to making the solar nanoantenna panels is to be able to predict their properties and perfect their design before printing them in the factory. While it is relatively easy to work out the physics of one resonating antenna, complex interactions start to happen when multiple antennas are combined. When hit with the right frequency of infrared light, the antennas also produce high-energy electromagnetic fields that can have unexpected effects on the materials.
So the researchers are developing a computer model of resonance in the tiny structures, looking for ways to fine-tune the efficiency of an entire array by changing factors like materials and antenna shape. "The ability to model these antennas is what's going to make us successful, because we can't see these things," Novack says. "They're hard to manipulate, and small tweaks are going to make big differences."

"At this point, these antennas are good at capturing energy, but they're not very good at converting it," says INL engineer Dale Kotter, "but we have very promising exploratory research under way." Kotter and Novack are also exploring ways to transform the high-frequency alternating current (AC) to direct current (DC) that can be stored in batteries. One potential candidate is high-speed rectifiers, special diodes that would sit at the center of each spiral antenna and convert the electricity from AC to DC.

They anticipate they are only a few years away from creating the next generation of solar energy collectors.

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