Imagine your iPod or Blackberry has lost its charge. Now imagine taking it out of your pocket and laying it on your desk in you cubicle for a few minutes. After soaking up the light for a few minutes, the phone powers back to life with enough juice to make a call. No, it wouldn't be by magic, rather this is the scenario envisioned by researchers at the U.S. Department of Energy's Idaho National Laboratory who have developed a unique kind of flexible solar cell.
The new cells consist of massive arrays of nanoantennas, which can collect energy from light and other sources. The INL team discovered a way to mass produce these arrays on flexible sheets of plastic. The only crucial problem remaining unresolved is developing additional components to harvest the collected energy by transforming it to electricity.
At the American Society of Mechanical Engineers 2008 2nd International Conference on Energy Sustainability, engineers from the INL presented their findings and revealed their visions of the tech which they hope will one day cover cars and consumer electronics in replenishing skins. The skins could also act as cooling devices by drawing away waste heat, according to the researchers.
The nanoantennas absorb a targeted wavelength range of mid-infrared rays. The Earth continuously emits these rays thanks to the solar energy that it absorbs during the day. This would allow for continuous solar panel operation, in theory. Traditional panels can only absorb visible light and thus are idle at night.
The INL physicist who led the team, Steven Novack, describes, "Every process in our industrial world creates waste heat. It's energy that we just throw away."
Mr. Novack worked with INL engineer Dale Kotter, W. Dennis Slafer of MicroContinuum, Inc. and Patrick Pinhero, now at the University of Missouri to design the nanoantennas -- tiny gold squares or spirals set on polyethylene, a plastic commonly found in plastic bags. The effort marks perhaps the first successful effort to capture infrared rays with nanoantennas. Past efforts have been able to harvest other lower-frequency wavelengths but have fallen short with high-frequency wavelengths like IR. This is due in part to the fact that materials' properties change at high frequencies.
Gold was selected after testing it, manganese and copper's reactions to IR rays. After careful computer design, an antenna which could collect 92 percent of the energy from infrared rays was achieved in theoretical simulations.
Next, the researchers moved to making a prototype, etching silicon wafers with the antenna pattern. A just slightly less efficient prototype was produced that harvest 80 percent of the energy. Finally a stamp-and-repeat method was used to emboss thousands of the antennas on thin sheets of plastic. The plastic skin produced is currently undergoing efficiency testing, but is expected to perform similarly to the first prototype.
As heat typically is emitted as IR rays from many objects, the antennas could cool objects by collecting these rays and reemitting them at a harmless wavelength. This could be used on a large scale, or on a smaller scale for computer component cooling.
A major obstacle remains in that though the device already produces alternating current, it alternates at a rate of trillions of times per second, far to fast for modern rectifiers to convert to DC current. Further, the current smallest rectifier would need to be shrunk to a thousandth of its size to fit next to the nanoantenna. This would require new manufacturing techniques. An alternative might be to develop nanodevices to slow down the alternating current to more manageable levels.
The light at the end of the tunnel, so to speak, should these problems be overcome, is the production of much cheaper and more efficient solar cells. Current cells only have an efficiency of around 20 percent, due to the inherent inefficiency of the chemical reactions used to harvest visible light. More exotic cell materials have promised higher efficiencies, but they remain too expensive and difficult to utilize.
Nanoantennas on the other hand can harvest rays much more efficiently. Further, they can be formed in multiple layers, with each layer tuned to a different part of the spectrum based on the antenna design. Mr. Novack imagines manufacturers to eventually be producing "several yards per minute" of the material.
The program is a part of the U.S. Department of Energy's ongoing alternative energy investments