Harvesting Energy out of Thin Air

Researchers capture electromagnetic signals to power small electronics

21 October 2011
Photo: Gary Meek

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It’s hard to walk around a city and not come across a person talking on a cellphone or a driver using a GPS navigator. In a world that increasingly relies on wireless devices, electromagnetic energy is in the air practically everywhere. A team of researchers at Georgia Tech has found a way to harness that energy and use it to power sensors, microprocessors, and communications chips.


Electromagnetic energy is transmitted from radio and cellphone towers, television transmitters, and satellite communication systems. “It’s all around us, but no one has been able to tap into it,” says Manos Tentzeris, an IEEE Fellow and professor of electrical and computer engineering at Georgia Tech.


Tentzeris and his team of researchers, including IEEE members Rushi Vyas and Vasileios Lakafosis, have created a self-powered sensor that relies on an ultrawideband antenna to capture energy from the 100 megahertz and greater frequencies generated by all those systems. The scavenged energy is converted from AC to DC, and then stored in a capacitor that gradually fills until the sensor is fully charged. The team has used the antenna to power a number of sensors, including sensors that detect heat and humidity, as well as biosensors that monitor physiological changes in humans. And Tentzeris says that as the technology advances, it has the potential to charge other devices.


“The antenna could easily be made smaller and implanted in a device to charge, for example, a small thin-film lithium-ion battery that may be able to power cellphones and tablet computers,” he says. 


The team’s work was presented in July at the IEEE Antennas and Propagation Symposium link to in Spokane, Wash.

PUTTING IT ALL TOGETHER

To build the antenna and sensors, Tentzeris and his team use a standard inkjet printer to deposit droplets of an emulsion made with conductive nanospheres onto either paper or a flexible polymide film. The droplets form RF components, including ultrawideband antennas and transmission lines.


The team has been printing antennas and RF components with inkjets since 2004. They have worked to perfect their ink recipe and have experimented with printing on different materials to achieve greater circuit speed.


“Our ultrawideband antenna can scavenge energy from a wider range of frequencies than ever before,” Tentzeris says. “We can now print antennas that can scavenge energy at up to 60 gigahertz if we print on a polymide film.” 


POWERING UP 

The energy-scavenging antenna, which the team is testing in its lab at Georgia Tech, has yielded hundreds of microwatts of power. That is enough energy to be a primary source of power for networks of small wireless sensors and microprocessors. And they are now testing the antenna with a solar powered device to see if it can generate enough electricity to provide a primary source of energy for low-power sensors, and circuits, or even a backup source of power for solar-powered devices.


“The scavenging antenna, when used in parallel with a solar power source, might be able to supply electricity at night when the sun is not present,” Tentzeris says. “All that’s needed is a miniature alternator.” 


The team has tested the antenna in Atlanta and Tokyo, big cities that buzz with electromagnetic energy. But how would the antenna fare in areas with smaller populations? Tentzeris says it could still scavenge some energy as long as there are TV towers nearby.


“The antenna could work well within the vicinity of TV broadcast stations,” he says. “With limited signal strength in the air in non-urban areas, it would still have the ability to charge a device, albeit slowly.”


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