Harvesting Energy from WiFi Signals

The technology uses small smart devices known as spin-torque oscillators (STOs) and is capable of powering small electronics.

A chip embedded with roughly 50 spin-torque oscillators. Image courtesy of NUS.

This research was a joint project between NUS and Tohoku University (TU) in Japan, and its results were published in Nature Communications on 18 May 2021.

 

Harvesting 2.4GHz Radio Frequencies

STOs are a class of oscillators capable of generating microwaves, and they are increasingly utilized in wireless applications. However, their low output power and broad linewidth are commonly referred to as one of the reasons they’re not as widely adopted for commercial purposes.

In order to overcome these limitations, researchers from NUS and TU created an array of eight STOs connected in series. Thanks to the disposition of its STOs, the array successfully managed to harvest 2.4 GHz electromagnetic radio waves emitted by WiFi devices, and convert them into a direct voltage signal to light up a 1.6-volt LED.

Initial experiments showed that, after five seconds of charge, the STOs could light up the LED for one minute. As part of the new research, the scientists also compared the novel, parallel STO array disposition with traditional on-chip STO systems.

The examination revealed mixed results, as the parallel configuration proved to be better for wireless transmission due to improved time-domain stability, spectral noise behavior, and control over impedance mismatch.

On the other hand, series connections proved to be more advantageous for energy harvesting thanks to the additive effect of the diode-voltage generated by STOs.

 

A Step Toward More Energy-efficient Applications?

According to Professor Yang Hyunsoo from the NUS Department of Electrical and Computer Engineering, the new research aims to exploit the potential behind unutilized WiFi signals.

"We are surrounded by WiFi signals, but when we are not using them to access the Internet, they are inactive, and this is a huge waste,” explained Hyunsoo, who spearheaded the project.

Hyunsoo believes the new findings also hold promise to lead toward greener applications.

“Our latest result is a step towards turning readily-available 2.4GHz radio waves into a green source of energy, hence reducing the need for batteries to power electronics that we use regularly,” the Professor said.

This way, small electric gadgets, and sensors could be powered wirelessly by using radiofrequency waves as part of other Internet of Things (IoT) applications. 

“With the advent of smart homes and cities, our work could give rise to energy-efficient applications in communication, computing, and neuromorphic systems," Hyunsoo added.

 

An International Collaboration

The innovative research was carried out by the research team of Professor Guo Yong Xin from the NUS Department of Electrical and Computer Engineering together with Professor Shunsuke Fukami from TU and his team.

Professor Yang Hyunsoo (left) with Dr Raghav Sharma (right), showcasing the new chip. Image courtesy of NUS

"Aside from coming up with an STO array for wireless transmission and energy harvesting, our work also demonstrated control over the synchronizing state of coupled STOs using injection locking from an external radio-frequency source,” explained Dr. Raghav Sharma, the first author of the paper.

Moving forward, the researchers are looking to increase the number of STOs in the array they had designed, to enhance the system’s energy harvesting ability.

The teams confirmed they will also test their energy harvesters for wirelessly charging other electronic devices and sensors. In addition, the researchers will now seek the input of industry partners to explore the development of on-chip STOs for self-sustained smart systems.

This will potentially open up further possibilities for the design and manufacturing of wireless charging and wireless signal detection systems.

For more information about the new findings, you can read the full paper here.