RADIATION SNATCHED from leaky microwave ovens to power gadgets
Sudden explosion in meals-for-one predicted (by no one)
By Bill Ray, 24th September 2013
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.
Harnessing microwaves ... how the tech works
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. ®
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