Flexible Nanoantenna Arrays Capture Solar Energy

August 12, 2008

by Roberta Kwok, Idaho National Laboratory
Florida, United States [RenewableEnergyWorld.com]
Researchers have devised an inexpensive way to produce plastic sheets containing billions of nanoantennas that collect heat energy generated by the sun and other sources. The researchers say that the technology, developed at the U.S. Department of Energy's Idaho National Laboratory (INL), is the first step toward a solar energy collector that could be mass-produced on flexible materials.
While methods to convert the energy into usable electricity still need to be developed, it is envisioned that the sheets could one day be manufactured as lightweight "skins" that power products such as hybrid cars or iPods with potentially higher efficiency than traditional solar cells. The nanoantennas also have the potential to act as cooling devices that draw waste heat from buildings or electronics without using electricity.
The nanoantennas target mid-infrared rays, which the Earth continuously radiates as heat after absorbing energy from the sun during the day. In contrast, traditional solar cells can only use visible light, rendering them idle after dark. Infrared radiation is an especially rich energy source because it also is generated by industrial processes such as coal-fired plants.
"Every process in our industrial world creates waste heat," says INL physicist Steven Novack. "It's energy that we just throw away." Novack led the research team, which included INL engineer Dale Kotter, W. Dennis Slafer of MicroContinuum Inc. and Patrick Pinhero, now at the University of Missouri.
The nanoantennas are tiny gold squares or spirals set in a specially treated form of polyethylene, a material used in plastic bags. While others have successfully invented antennas that collect energy from lower-frequency regions of the electromagnetic spectrum, such as microwaves, infrared rays have proven more elusive. Part of the reason is that materials' properties change drastically at high-frequency wavelengths, Kotter says.
The researchers studied the behavior of various materials — including gold, manganese and copper — under infrared rays and used the resulting data to build computer models of nanoantennas. They found that with the right materials, shape and size, the simulated nanoantennas could harvest up to 92 percent of the energy at infrared wavelengths.
The team then created real-life prototypes to test their computer models. First, they used conventional production methods to etch a silicon wafer with the nanoantenna pattern. The silicon-based nanoantennas matched the computer simulations, absorbing more than 80 percent of the energy over the intended wavelength range. Next, they used a stamp-and-repeat process to emboss the nanoantennas on thin sheets of plastic. While the plastic prototype is still being tested, initial experiments suggest that it also captures energy at the expected infrared wavelengths.
The nanoantennas' ability to absorb infrared radiation makes them promising cooling devices. Since objects give off heat as infrared rays, the nanoantennas could collect those rays and re-emit the energy at harmless wavelengths. Such a system could cool down buildings and computers without the external power source required by air-conditioners and fans.
More technological advances are needed before the nanoantennas can funnel their energy into usable electricity. The infrared rays create alternating currents in the nanoantennas that oscillate trillions of times per second, requiring a component called a rectifier to convert the alternating current to direct current. Today's rectifiers can't handle such high frequencies.
"We need to design nanorectifiers that go with our nanoantennas," says Kotter, noting that a nanoscale rectifier would need to be about 1,000 times smaller than current commercial devices and will require new manufacturing methods. Another possibility is to develop electrical circuitry that might slow down the current to usable frequencies.
If these technical hurdles can be overcome, nanoantennas have the potential to be efficient harvesters of solar energy. Because they can be tweaked to pick up specific wavelengths depending on their shape and size, it may be possible to create double-sided nanoantenna sheets that harvest energy from different parts of the sun's spectrum, Novack says.
The team's stamp-and-repeat process could also be extended to large-scale roll-to-roll manufacturing techniques that could print the arrays at a rate of several yards per minute.
The researchers will be reporting their findings on August 13 at the American Society of Mechanical Engineers 2008 2nd International Conference on Energy Sustainability in Jacksonville, Florida.
Roberta Kwok is a Research Communications Fellow at Idaho National Laboratory.

Philippe Starck is tilting toward windmills

By Alice Rawsthorn
Friday, August 1, 2008
LONDON: There's no point in arguing with Philippe Starck, because it generally goes like this: 1. The world's most famous designer makes a well-meaning and sincere, but slightly preposterous claim. 2. You feel obliged to question the preposterous bit. 3. He comes across all hurt and boyish. 4. You feel mean.
Take Starck's claim to have "invented a concept called Democratic Design," which, he says, gives everyone high quality products at affordable prices. Sounds great, but didn't the modern movement try to do that for most of the 20th century? And how can he claim to have "won the battle" by designing "a chair that sells for less than €100," or $157, when that's still too expensive for most people? Let alone the 90 percent of the world's population who are too poor to afford the basics? What has Democratic Design done for them? "Oh please, I'm not God," pleads Starck. "I'm just a designer, and I'm doing my best."
Luckily for the underprivileged 90 percent, other designers are trying to help them. Starck is battling on another front - developing cheap, attractive, energy-saving products to "introduce everybody to ecology." The first of his Democratic Ecology products is to be launched this fall, a miniature roof-top windmill, priced between €500 and €800, which will produce up to 80 percent of a home's energy. "Imagine a Saturday afternoon, and a guy going stupidly to the supermarket to buy a useless gadget," enthused Starck. "He sees a really sexy object. 'Oh my God, it's beautiful. How much does it cost? €500? That's almost what I'd spend on a useless gadget.' He brings the windmill home, goes to his roof and 15 minutes later he sees it turning and producing energy. Wow!"
Starck's windmill is one of dozens of alternative energy sources to be coming on to the market, but there are sound reasons for taking his product seriously. One is that it's deftly designed, not least because the blades are made of transparent plastic, which will be virtually invisible up on the roof. Another is that it's designed by him, and Starck has been so successful at persuading people to buy visually seductive, but often pointless objects - plastic Louis XV chairs, gun-shaped lamps, garden gnome stools and so on - that he may well be able to do the same for something which is actually useful.
That said, it's been a long time since the design world felt that it had to take Starck seriously. He's a gentle giant, who bears a distinct resemblance to Desperate Dan, the mal barbu cowboy in the British comic "The Dandy." Now 59, he rose to fame in his native France during the 1980s, when his flair for reinventing everyday objects by casting them as something else - a lemon squeezer as a lobster, and plastic chairs as ornate Louis XV ones - was hailed as a playful, and very commercial take on then-fashionable postmodernism.
Starck has since sold hundreds of thousands of his lobster-like lemon squeezers, and nearly a million of just one of his "antique" plastic chairs, Louis Ghost. For better or worse, he has also given us the designer hotel - aided and abetted by the New York hotelier Ian Schrager - and the showstopping restrooms that now pop up in every other bar. Starck cast himself as a media star by spouting his design philosophy in "franglais" soundbites, and bragging about being able to design a chair in the time it took for an aircraft seatbelt sign to go on and off. No other designer could beat him for chutzpah and bankability, but by the mid-1990s Starck was grouching about being bored by design.
Commercially, he's still a colossus, who bags plum jobs, like the creative directorship of Richard Branson's Virgin Galactic space venture, and is shooting a reality TV show for the BBC. But at times he seems like design's equivalent of a dinosaur rocker. Starck's confession this year that he was "ashamed" that "everything I designed is unnecessary" drew derisory roars in the blogosphere. Does he still feel like that? "I regret that my job is design," he admitted. "Design stupidly produces more things, and for years I've spoken about the importance of living with fewer things. But my position is a little ambiguous."
Indeed it is. To his credit, Starck was advocating environmentalism long before it became fashionable, but he hasn't embraced it fully in his work. Nor does he seem to see the irony in rattling off a list of eco-responsible activities - organic diet, solar-powered oyster farm and so forth - ending with "the least polluting plane on the market," his private jet. Now he's hoping to redress the balance with Democratic Ecology.
The windmill is an encouraging start. Made from the same transparent plastic as his best-selling Louis Ghost chairs, Starck developed it and the other Democratic Ecology products in collaboration with Pramac, the Italian industrial group. The timing is propitious with oil prices rocketing and everyone from General Electric to the veteran oilman T. Boone Pickens investing in alternative energy. Lots of homes already sport metal wind turbines on their roofs, so why not transparent plastic ones?
Next up is a solar panel, a film that covers existing windows. Starck is also designing a prefabricated eco-house with glass walls that can be changed from clear or opaque at the push of a button. The prototype is being built for him and his family on the plot of their old home outside Paris.
An electric car is under development too, and an eco-moped. Starck has nearly finished work on a solar and hydrogen-powered boat, the first of which is to be delivered to Hotel Bauer in Venice next spring. "We're seizing every opportunity to create affordable, high-technology ecology products," he said. "It's very, very important that they're beautiful, because ecology should be a pleasure, not a punishment. One of the most beautiful boats in the world is the Venetian taxi, and our boat will be even more beautiful."