Energy harvester extends battery life of smart watches

Researchers from The Chinese University of Hong Kong (CUHK) have developed an embedded energy harvester that is able to sustainably power smart watches and wristbands. The device is designed to solve the problem of limited battery life in smart wearables, particularly as they take on new functions such as health monitoring, contactless payment, entertainment, living assistance and more.

Energy harvesters can be used to power up traditional watches, in which mechanical gears are equipped to increase the frequency of human motion and convert mechanical energy to electricity. But these designs have the disadvantage of incompact structure and require precision manufacture. The mechanical friction caused by the gears also leads to significant energy loss and such devices occasionally suffer mechanical failure. CUHK’s Professor Wei-Hsin Liao, Chairman of the Department of Mechanical and Automation Engineering, and his research group tried to replace mechanical gears with electromagnetic components to solve this issue.

“The low frequency of human motion results in relatively low energy conversion efficiency,” said Dr Mingjing Cai, a member of the research group. “We applied a magnetic frequency up-converter to increase the frequency of human motion, which helps the energy harvester to efficiently convert the human motion into electrical energy. With the magnetic frequency up-converter, the output power of the device can be significantly enhanced, achieving 10 times that of the published devices.”

The research team proposed a highly compact embedded generator, equipped with a motion capture unit, a magnetic frequency up-converter and a power generation unit, together with coaxial topology. Thanks to the magnetic frequency-up converter, the energy harvester is highly compact and has a very high normalised power density. Although the total volume of the harvester is 5 cm³, it can still achieve 1.74 mW power output. The power density is 10 times that of the existing devices.

Benefiting from high power output and power density, the energy harvesting system can be easily embedded in the smart watches and wristbands to provide sustainable power supply. Different from mechanical gears, the magnetic frequency up-converter uses magnetic force for transmission, avoiding the energy loss caused by mechanical friction. Also, magnetic transmission can protect the device from impact. Furthermore, the magnetic frequency up-converter has the advantages of simple structure and low cost, which will help to commercialise the device in the future.

Top image: The team has produced a miniature energy harvester while maintaining sufficient electricity output to power smart watches.

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