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Boffins improve on tech that extracts DC power from ambient Wi-Fi

Series of 8 spin-torque oscillators improves efficiency, but parallel design better for wireless transmission

A research team from the National University of Singapore and Japan’s Tohoku University say they have improved on the use of spin-torque oscillators (STO) to harvest and convert Wi-Fi signals into energy through a series configuration that lit up a battery-less LED.

"There is a lot of ambient Wi-Fi wasted, the question is can we harvest the wasted energy and convert to something useful?" Professor Yang Hyunsoo from the National University of Singapore's Department of Electrical and Computer Engineering explained to The Register.

He believes the resultant green energy could eventually eliminate the need for batteries.

While devices that convert AC electromagnetic waves into DC electricity are not new, previous tech is immature and in need of refinement.

Lining up multiple STOs can increase the power and change the frequency of the technology, but it will likely increase the device size to make most desirable applications impractical. Meanwhile, an STO on its own is limited in the range of frequencies it can respond to.

To improve efficiency and usability of the technology, the boffins came up with an array of eight STOs connected in a series that converted 2.4 GHz Wi-Fi electromagnetic radio waves into an electrical current that was transmitted to a capacitor and lit a 1.6V LED. Five seconds of charging the capacitor powered the LED for one minute.

The team also looked at a parallel design of the STOs and concluded it was better for wireless transmission, while the series layout was more efficient at harvesting energy.

Team researcher Dr Raghav Sharma said in a canned statement:

Aside from coming up with an STO array for wireless transmission and energy harvesting, our work also demonstrated control over the synchronising state of coupled STOs using injection locking from an external radio-frequency source. These results are important for prospective applications of synchronised STOs, such as fast-speed neuromorphic computing.

fig from nature comms wifi harvesting paper

Schematic of the circuit used for the energy harvesting. The wireless signal from the horn antenna or microstrip patch-antenna with a power of 0 dBm is irradiated on the array of eight electrically connected STOs in series at zero magnetic field and dc bias. The rectified signal is stored in the capacitor, which is then supplied to the dc–dc boost converter for supplying the voltage to power the red LED

Yang spoke to The Register about its applications:

There’s a quite wide range of applications, for example, remote sensors. You think of all these AI and IoT devices where there is no way to change the battery, so if you can harvest the energy, there’s no reason to change the battery.

In the future it can be used for RFID, charging wearable, we have had inquiries for medical devices. That way they don’t have to change old medical device batteries through surgery.

Surgeons and biomedical engineers shouldn’t yet get their hopes up. Yang added:

Our work is proof of concept, whether to commercialize is a different story, but we show it is possible. We’re getting lots of inquiries from different people, but as a scientist I don’t want to overcommit.

The paper, "Electrically connected spin-torque oscillators array for 2.4 GHz WiFi band transmission and energy harvesting", was published this week in Nature Communications. ®

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