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BW Businessworld

New Rays

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Starting from sometime in the past century, when human beings became aware that solar energy is non-polluting and abundant, companies and research and development institutions have been spending considerable money and energy to make it cheap as well. Engineers have tried a slew of approaches to harvest solar energy, but with incremental improvements. Now there is a solar energy technology, nantenna, that promises to deliver a magnitude increase in performance — if it goes from a prototype to a commercial version.

Developed by a team of scientists led by Patrick Pinhero from the University of Missouri, it has a new approach towards using solar energy. Solar cells, no matter what they are made of, use the property of some materials to produce electricity — called the photoelectric effect — when exposed to sunlight. Another kind of solar energy technology, solar thermal, uses the Sun's energy to heat water and then produce electricity through a turbine. The Missouri invention uses a series of tiny antennas that can absorb Sun's radiation and generate electricity. There is, however, a crucial difference here compared to photovoltaic cells: the antenna can use 90 per cent of the radiation, while photovoltaic cells can use around 20 per cent.

It is a revolutionary technology, even though it is early days for it, and the scientists think it will take five years before it can become a commercial device. Yet, there are many uses for it in the meantime. It could produce electricity from waste heat in factories. It could detect contraband materials in airports. You could use them in the skin of electronic devices for continual charging. The nantenna has its uses in security applications as well, and some of these could make their way to the world before a large-scale energy harvesting device does. But it can become a game-changer if it ever makes it to the power-generation market. And there are no serious theoretical bottlenecks obvious now.

The idea of a nantenna — a short form for nano-antenna — originated in the 1970s. Tiny antennas of the right material can absorb radiation of specific frequencies proportional to their size. By designing antennas of different sizes, one can design systems that can absorb radiation of different frequencies. The absorbed radiation produces an alternating current that is converted to direct current using a rectifier. However, there are enormous technological challenges to designing tiny antennas that can absorb a visible and infrared radiation from the Sun. Pinhero and his team are supposed to have surmounted several such challenges and developed a prototype. They were assisted by engineers from the University of Colorado, who developed a diode to act as a rectifier, and private firm MicroContinuum in Massachusetts, which is developing high-volume manufacturing process.

The biggest advantage of this technology is it can use radiation that lies outside the scope of photovoltaics. A substantial amount of radiation reaches the Earth's surface between wavelengths of 800 and 900 nanometeres, even when it is cloudy, but it is very difficult to design solar cells that can use this radiation. You can design panels with antennas that absorb Sun's radiation on one side and the Earth's radiated heat on the other. As the Earth radiates heat at night, the device could produce some electricity even at night. Pinhero's team has fabricated the device on a flexible substrate, making it ideal for installations on rooftops and other places. So it pays to keep a watch on the development of nantennas. They could power the world one day.

(This story was published in Businessworld Issue Dated 30-05-2011)