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Friday, January 4, 2008

Super-efficient Solar Energy From Heat

Revisiting nano-antenna technology

January 4, 2008 Idaho National Laboratory (INL) reports that research conducted in conjunction with partners at Microcontinuum Inc. (Cambridge, MA) and Patrick Pinhero of the University of Missouri is promising a method for developing cheap solar energy technology that could be imprinted on flexible materials and still draw energy after the sun has set. The technology uses a special manufacturing process to stamp tiny square spirals, or “nanoantennas”, of conduction metal onto a sheet of plastic and the team estimates individual nanoantennas can absorb close to 80 percent of the available energy in comparison to current commercial solar panels which usually transform less that 20 percent of the usable energy that strikes them into electricity – this is even more impressive than the 30% conversion rate offered by the recently discussed development of nano flakes.

Due to their size – each interlocking spiral nanoantenna is as wide as 1/25 the diameter of a human hair - the nanoantennas absorb energy in the infrared part of the spectrum, just outside the range of what is visible to the eye. Since 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. The new approach, which garnered two 2007 Nano50 awards, was made possible by the boom in nanotechnology, but finding an efficient way to stamp out arrays of atom-scale spirals took a number of years. The INL team says that the antennas might one day be produced like foil or plastic wrap on roll-to-roll machinery and so far they have demonstrated the imprinting process with six-inch circular stamps, each holding more than 10 million antennas.

The nanoantennas could prove to be a more efficient and sustainable alternative to current commercial solar panels, which are made of processed silicon – the supply of which is lagging – and doped with exotic elements to boost efficiency. In contrast the nanoantenna circuits can be made of a number of different conducting metals, and the nanoantennas can be printed on thin, flexible materials like polyethylene. By focusing on readily available materials and rapid manufacturing the team’s aim is to make nanoantenna arrays as cheap as inexpensive carpet. The team says nanoantenna collectors might be used to charge portable battery packs, coat the roofs of homes or even be integrated into polyester fabric.

As exciting as the potential of the technology is, not all the hurdles have been passed yet. While the nanoantennas are easily manufactured, the problem of creating a way to store or transmit the electricity is yet to be solved. 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 - much too fast for electrical appliances, which operate on currents that oscillate only 60 times a second. The team is exploring ways to slow that cycling down and 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.