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Flexible nanoantenna arrays capture abundant solar energy

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Harvesting the sun’s energy with antennas. Researchers at Idaho National Laboratory, along with partners at Microcontinuum Inc. (Cambridge, MA) and Patrick Pinhero of the University of Missouri, are developing a novel way to collect energy from the sun with a technology that could potentially cost pennies a yard, be imprinted on flexible materials and still draw energy after the sun has set.The new approach, which garnered two 2007 Nano50 awards, uses a special manufacturing process to stamp tiny loops of conducting metal onto a sheet of plastic. Each “nanoantenna” is as wide as 1/25 the diameter of a human hair.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. The sun radiates a lot of infrared energy, some of which is soaked up by the earth and later released as radiation for hours after sunset. Nanoantennas can take in energy from both sunlight and the earth’s heat, with higher efficiency than conventional solar cells. “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. Taking antennas to the atomic level.The miniscule circuits absorb energy just like the antenna on your television or in your cell phone. All antennas work by resonance, the same self-reinforcing physical phenomenon that allows a high note to shatter glass.

Radio and television antennas must be large because of the wavelength of energy they need to pick up. In theory, making antennas that can absorb electromagnetic radiation closer to what we can see is simple: just engineer a smaller antenna. But finding an efficient way to stamp out arrays of atom-scale loops took a number of years. “It’s not that this concept is new,” Novack says, “but the boom in nanotechnology is what has really made this possible.” The INL team envisions the antennas might one day be produced like foil or plastic wrap on roll-to-roll machinery. So far, they have demonstrated the imprinting process with six-inch circular stamps, each holding more than 10 million antennas.It wasn’t immediately obvious the structures might be used for solar power. At first, the researchers considered pairing the antennas with conventional solar cells to make them more efficient. “Then we thought to start from scratch,” Novack says. “We realized we could make the antennas into their own energy harvesters.”An economical alternative. Commercial solar panels usually transform less that 20 percent of the usable energy that strikes them into electricity. Each cell is made of silicon and doped with exotic elements to boost its efficiency. “The supply of processed silicon is lagging, and they only get more expensive,” Novack says. He hopes solar nanoantennas will be a more efficient and sustainable alternative.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.Fine-tuning fine structures.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.”  A charged future. One day, Novack says, these nanoantenna collectors might charge portable battery packs, coat the roofs of homes and, perhaps, even be integrated into polyester fabric. Double-sided panels could absorb a broad spectrum of energy from the sun during the day, while the other side might be designed to take in the narrow frequency of energy produced from the earth’s radiated heat. While the nanoantennas are easily manufactured, a crucial part of the process has yet to be fully developed: creating a way to store or transmit the electricity. Although infrared rays create an alternating current in the nanoantenna, the frequency of the current switches back and forth ten thousand billion times a second. That’s much too fast for electrical appliances, which operate on currents that oscillate only 60 times a second. So the team is exploring ways to slow that cycling down, possibly by embedding energy conversion devices like tiny capacitors directly into the antenna structure as part of the nanoantenna imprinting process.”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 possibility is to create antennas with a spiral shape and place high-speed rectifiers, or special diodes, at the center to convert the electricity from AC to DC. The team has a patent pending on a variety of potential energy conversion methods. They anticipate they are only a few years away from creating the next generation of solar energy collectors. Read a news release about the research team’s;he team’s study, “Solar Nantenna Electromagnetic Collectors,” won an award for best photovoltaics paper at the American Society of Mechanical Engineers 2008 2nd International Conference on Energy Sustainability.  The paper was one of five top papers recognized at the conference.JACKSONVILLE, Fla. — 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 technology, developed at the U.S. Department of Energy’s Idaho National Laboratory, 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, the sheets could one day be manufactured as lightweight “skins” that power everything from hybrid cars to iPods with higher efficiency than traditional solar cells, say the researchers, who report their findings Aug. 13 at the American Society of Mechanical Engineers 2008 2nd International Conference on Energy Sustainability in Jacksonville, Fla. 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. (Cambridge, Mass.) and Patrick Pinhero, now at the University of Missouri. The nanoantennas are tiny gold squares 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.But 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 a cheaper, more efficient alternative to solar cells. Traditional solar cells rely on a chemical reaction that only works for up to 20 percent of the visible light they collect. Scientists have developed more complex solar cells with higher efficiency, but these models are too expensive for widespread use. Nanoantennas, on the other hand, can be tweaked to pick up specific wavelengths depending on their shape and size. This flexibility would make it 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 sheets could potentially cover building roofs or form the “skin” of consumer gadgets like cell phones and iPods, providing a continuous and inexpensive source of renewable energy.  Roberta Kwok News from: Idaho National Laboratory (INL)

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