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article imageMicrowave harvesters could pluck electricity out of thin air

By Robert Myles     Nov 16, 2013 in Technology
Durham - Researchers recently perfected a prototype device to produce electricity. It scavenges microwaves in the atmosphere that would otherwise be dispersed. Its function could be compared to harvesting energy out of thin air.
Looking remarkably similar to an array of solar panels, the device, constructed from inexpensive materials, is tuned to collect microwave signals, used by many household devices and gadgets, which would otherwise disappear into the ether, converting them into usable electricity.
The power-harvesting device has been perfected by researchers at the Pratt School of Engineering at Duke University, Durham, North Carolina. Their application has implications for how the likes of cellphones or e-readers are recharged. Ultimately, rolled out on a larger scale, it might be applied to reducing household energy bills and, as a consequence, reducing carbon emissions.
In its prototype form, the device wirelessly converts stray microwave signals to direct current voltage sufficient to recharge a cellphone battery. Looking very similar to solar panels, which work by converting light energy into electricity, the energy harvester, say the Pratt School researchers, could be adapted so that it tunes in and collects a variety of signals, be they satellite, Wi-Fi or even sound waves, converting them into usable power.
The researchers highlight the use of metamaterials as the key to the functioning of their energy gatherer. Metamaterials don’t occur naturally. Instead, they’re artificial materials engineered so that their geometrical configuration, patterns on a microscopic scale, can affect light-waves, sound-waves and even microwave radiation. It’s by developing these properties that the North Carolina researchers have produced a workable “charger” whose output compares favorably with phone chargers in everyday use.
Unlike conventional phone chargers, however, which continue to rack up electricity bills even when plugged in and not connected to a phone, the new device won’t fill power companies’ coffers at precisely zero benefit to the consumer.
Allen Hawkes, an undergraduate engineering student, conceived the microwave harvesting device, working with graduate student Alexander Katko and lead investigator Steven Cummer, professor of electrical and computer engineering.
Their prototype consists of series of five fiberglass and copper energy conductors wired together on a circuit board configured in such a way that it converts unseen electro-magnetic radiation such as microwaves into a usable 7.3 volts of electricity. By comparison, a typical Universal Serial Bus (USB) charger used to charge small electronic devices such as e-readers, provides about 5 volts of power.
Through careful design the researchers have achieved energy efficiency of 37 percent, comparable to now-familiar solar cells.
Commenting on the research, Alexander Katko said, “It's possible to use this design for a lot of different frequencies and types of energy, including vibration and sound energy harvesting."
He added, "Until now, a lot of work with metamaterials has been theoretical. We are showing that with a little work, these materials can be useful for consumer applications."
In its most everyday form, the application could be used to develop cell phones which incorporate metamaterial panels, continually charging on the move and picking up free electromagnetic radiation from the atmosphere.
More ambitiously, say the researchers, metamaterials might be used to coat the ceiling of a room and scavenge power from domestic Wi-Fi signals that would otherwise be lost. And it may not be a case of a lot of work to do away with a single phone charger. According to Professor Cummer, the design of the electromagnetic harvester is scalable.
As Cummer put it, "Our work demonstrates a simple and inexpensive approach to electromagnetic power harvesting. The beauty of the design is that the basic building blocks are self-contained and additive. One can simply assemble more blocks to increase the scavenged power."
That drawer full of chargers, some old, some new and some of unknown provenance but kept “just in case” could soon be history.
Full details of the Pratt School of Engineering research are published under the title “A microwave metamaterial with integrated power harvesting functionality” in the journal Applied Physics Letters.
Their research was supported by a Multidisciplinary University Research Initiative from the US Army Research Office.
More about metamaterials, electronic harvesters, microwave scavenging, Green energy, micro electricity generation
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