Electronics engineer Yoshihiro Kawahara at the University of Tokyo says he was inspired by the notion of “space solar power”, – in which colossal solar panel farms placed in orbit will one day beam energy down to Earth in the form of microwaves that are converted to electricity.
A microwave oven uses a device called a magnetron to generate electromagnetic waves with a wavelength of 12.5 centimetres and a frequency of 2.4 gigahertz – enough for vibrating water molecules to heat food. Although a waveguide delivers the microwaves into the food chamber some still escape through the gap around the oven door and through the metal-meshed window. So, for consumer safety reasons, the US Food And Drug Administration stipulates that leakage from a microwave oven cannot exceed a power density of 5 milliwatts per square centimetre at approximately 5 cm from the oven surface.
With a team from Georgia Institute of technology in Atlanta, Kawahara began studying the energy leakage from a range of ovens to see what useful power levels might be harvestable to replace button cell batteries in kitchen gadgets.
Kawahara’s leakage tests on a range of popular ovens, including those manufactured by Sharp, Panasonic, Whirlpool and National. The average leakage is generally lower than the legal limit at around 0.5 milliwatts per square centimetre, he told a conference on ubiquitous computing in Zurich, Switzerland on 11 September. That made around 1 milliwatt of power available in front of the oven.
To harness that energy, they then designed a power harvester the size of a US quarter, or UK 10 pence piece, that combined with a 1-cm-long microwave antenna to generate an electric current that could charge a circuit. “The energy accumulated over a two-minute run of the microwave oven was enough to operate some low-power kitchen tools for a few minutes,” says Kawahara. So by leaving gadgets close to the microwave, they would be gradually charged up enough to operate. He says the harvester is small enough to be embedded in most kitchen gadgets.
Michael Rodrigues, a researcher in energy harvesting technology at University College London, says the microwave scavenging technique has promise in a growing area: it could fuel development of energy-neutral sensor networks that make homes smarter without boosting their carbon footprint, he says.