Nanoantennas envisioned as possible replacement for solar cells

By Wolfgang Gruener

Jacksonville (FL) – Imagine the possibilities of harvesting waste heat and converting it into electricity. Computer processors could be the source of power for their own cooling devices and solar cells could become dramatically more efficient by leveraging energy that is not being used today. If everything goes right, nanoantennas could even replace solar cells one day, researchers believe.

Scientists from the U.S. Department of Energy's Idaho National Laboratory believe that plastic sheets containing billions of nanoantennas that collect heat energy generated by the sun and other sources could dramatically improve the use of a type of energy we are all aware of, but have no use for so far – heat. According to research results that will be presented on August 13 at the American Society of Mechanical Engineers 2008 2nd International Conference on Energy, 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.

While described by the authors of research as solar energy, the approach does not focus on collecting sunlight, but flexible nanoantenna arrays that capture abundant solar energy mid-infrared rays radiated by the Earth as heat after absorbing energy from the sun during the day. The clear advantage of this thought is that energy can be generated at all times and not just during times when sun light is available. The scientists said that infrared radiation is an especially rich energy source because it also is generated by industrial processes such as coal-fired plants.

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.

The researchers said that they 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% of the energy at infrared wavelengths. If that in fact is true, a combination of solar cells and these nanoantennas could result in a raw energy efficiency of more than double of today’s high-end solar cell range, which is somewhere between 40% and 50% and mass-market solar cells that are just below 20%.

Starting with computer models, the team created real-life prototypes to test their computer models. 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% 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 researchers said.

You know there is always a catch and this research result is no exception. While the energy can be harvested, the scientists have found no solution yet to convert the energy into electricity. According to the scientists, “more technological advances” are necessary to turn this vision into reality. They said that 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 and today's rectifiers can't handle such high frequencies. A nanoscale rectifier suited for this application in fact would need to be about 1,000 times smaller than current commercial devices and will require new manufacturing methods, according to the scientists.

Once these hurdles are overcome, the researchers believe that nanoantennas have the potential to be a cheaper, more efficient alternative to solar cells.

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