What Is a Nanoantenna?
A nanoantenna, or nantenna, is an idea for a type of solar cell that, instead of harnessing visible light to create electricity, makes use of infrared radiation that is often thought of as heat and exists beyond the visible range for humans. Infrared light is emitted by the Earth itself and a wide array of industrial processes as waste energy, such as from coal-fired power plants. One version of the nanoantenna takes the shape of a microscopically small gold square or spiral of metal wire about 1/25th the diameter of a human hair that is embedded in a flexible polyethylene plastic sheet. Metals such as manganese and copper have also been studied for the nanoantenna, and, in research as of 2008, the devices have been shown to be as high as 92% efficient at converting the frequencies of infrared light that they capture into electrical energy.
Solar radiation spans a wide spectrum beyond the visible range of light. It is estimated that 44% of the light emitted by the Sun is visible with 7% in the ultraviolet range and 49% in the infrared range. When visible light impacts with the surface of the Earth or its atmosphere, it loses much of its energy in the process and most of this is later emitted back into space as longer wavelength infrared radiation. Capturing this energy using a nanoantenna array could serve two important purposes. The energy could be used to power numerous electronic devices, and it could also be drawn away from equipment like computer servers and other machinery to keep it cool and running efficiently.
One of the limitations in current nanoantenna designs, however, that may limit the production of a nanoantenna array system for some time to come, is the nature of infrared light to oscillate at high frequencies. This makes it necessary to build rectifiers into the system that would convert alternating current (AC) infrared signals to direct current (DC) power. A comparable rectifier to work with a nanoantenna would have to be scaled down by a factor of 1,000 from current models that exist on the market as of 2011 to function effectively, and this technology has not yet been developed. An alternative approach would be to create a rectifying antenna itself, which would be a combination of a nanoantenna and nano-rectifier, and which would naturally regulate infrared frequencies.
The advantages of creating nanoscopic-sized solar cell components over traditional silicon wafer solar cells may make them a revolutionary leap forward. Their efficiency at converting light is much higher than standard photovoltaic solar cells that range up to only about 15% for retail versions as of 2011. A nanoantenna solar cell could be configured to capture specific wavelengths of infrared light and could be placed on both sides of a panel to capture two different wavelengths from each side simultaneously.
Perhaps one of the most important advances over traditional solar cell technology, however, is that the functional components of a nantenna are small enough that arrays of the devices could be embedded in flexible plastic sheeting. This sheeting could then be stretched over a wide variety of irregular surfaces or electronic devices. In a research facility at the Idaho National Laboratory (INL) in the US, sheets of nanoantenna with squares about 3 inches by 3 inches wide (7.6 by 7.6 centimeters) have already been created that each contain about 260,000,000 nantenna each, and rolls of much larger sheets are possible.
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