USA / Japan: Lekkende magnetrons

Bron: www.theregister.co.uk/2013/09/24/boffins_harvest_microwave_energy_outside_the_door/ .
24 sept. 2013

RADIATION SNATCHED from leaky microwave ovens to power gadgets

Sudden explosion in meals-for-one predicted (by no one)
By Bill Ray

A collaboration between universities in Tokyo and Atlanta has spawned a device for harvesting power leaked from domestic microwave ovens – turning wasted waves into free energy.

Microwaves pump out energy in the 2.4GHz band: the industrial, scientific and medical (ISM) radio space popularised by Wi-Fi and Bluetooth. The casing of the oven contains almost all the energy used to heat the ''meal for one'', but some escapes to interfere with nearby wireless networks and that's the energy scooped up by the boffins and their new circuit.

The electronics in their microwave ''rectenna'' consists of an antenna, a diode and a capacitor, we're told. The incoming waves induce a tiny voltage across the antenna's terminals; the other components rectify and step up the voltage to a mighty 1.8V, enough for most gadgets.

Domestic microwave ovens are governed by regulations that restrict their leakage to five milliwatts per square centimetre, at a distance of five centimetres, as New Scientist explains. The researchers discovered leakage was well below that across the brands they tested, but were able to harvest a good proportion of that power to run kitchen appliances including a thermometer and countdown timer.

Equally interesting, to this correspondent at least, was the range of frequencies in which the tested ovens operated. According to the published study PDF, readable up to page five an oven from Sharp will blank out Wi-Fi in channel 7, while one from Whirlpool will only affect wireless networks in the top few slots (10 & 11), which could matter to some.

But the point of the project was to see if useful energy could be extracted from the stray radiation, and the conclusion was that it could. A low-power device could run happily on the harvested power, and a wireless sensor could be charged to run for hours from a two-minute burst of microwaves, all from energy that would otherwise be wasted.

It's not the first attempt to harvest energy from radio, as Yoshihiro Kawahara and his fellow boffins note. We've reported on energy harvested from TV signals and RFID transmissions, but those have side effects or use deliberately generated signals, while the leaks from a microwave oven are literally wasted without intervention.

How useful it is to harvest them is debatable: the energy is only there when the microwave is in use, and while most homes might indeed have a microwave oven, it may not be in use all that often, but practicability isn't what this kind of research is about. The question is ''how''; ''why'' comes later.

en:
www.newscientist.com/article/dn24242-leaky-microwaves-can-power-your-kitchen-gadgets.html#.UkMdm4apXJo .

Leaky microwaves can power your kitchen gadgets
20 sept. 2013

If you are fed up with replacing the batteries in your kitchen gadgets why not use the energy that leaks from your microwave oven to power them instead? Engineers in Japan have managed to scavenge enough energy to run low-power devices such as oven thermometers, cooking timers and digital scales this way.

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.


Interessant omdat erin staat hoeveel straling gelekt wordt, maar het beste is natuurlijk om helemaal geen magnetron te gebruiken.



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