Two-Stage Power Management System Boosts Energy-Harvesting Efficiency

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A two-stage power management and storage system could dramatically improve the efficiency of triboelectric generators that harvest energy from irregular human motion such as walking, running or finger tapping.

The system uses a small capacitor to capture alternating current generated by the biomechanical activity. When the first capacitor fills, a power management circuit then feeds the electricity into a battery or larger capacitor. This second storage device supplies DC current at voltages appropriate for powering wearable and mobile devices such as watches, heart monitors, calculators, thermometers – and even wireless remote entry devices for vehicles.

By matching the impedance of the storage device to that of the triboelectric generators, the new system can boost energy efficiency from just one percent to as much as 60 percent. The research was reported December 11 in the journal Nature Communications.

“With a high-output triboelectric generator and this power management circuit, we can power a range of applications from human motion,” said Simiao Niu, a graduate research assistant in the School of Materials Science and Engineering at the Georgia Institute of Technology. “The first stage of our system is matched to the triboelectric nanogenerator, and the second stage is matched to the application that it will be powering.”

Triboelectric nanogenerators use a combination of the triboelectric effect and electrostatic induction to generate small amounts of electrical power from mechanical motions such as rotation, sliding or vibration. The triboelectric effect takes advantage of the fact that certain materials become electrically charged after they come into moving contact with a surface made from a different material. However, the output is alternating current, which can power applications such as LED lighting – but is not ideal for mobile devices.

Ordinary alternating current can be converted to direct current by using a transformer – but such a device requires consistency in the number of cycles per second. Because biomechanical energy sources such as walking or finger tapping produce fluctuating amplitude and variable frequencies, a standard transformer can’t be used. In addition, the output from a triboelectric generator tends to have high voltage and low current – while applications for it require just the opposite: low voltage and higher current.

To address the problem, Niu and collaborators under the supervision of Professor Zhong Lin Wang at Georgia Tech developed their power management system, which converts the fluctuating power amplitudes and variable frequencies to a continuous direct current.

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