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Two-stage triboelectric nanogenerator may power our sensors… and phones

January 12, 2016 - 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.


January 12, 2016
By Anthony Capkun

The system from Georgia Institute of Technology researchers 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, etc.

PHOTO Shoe: A triboelectric generator embedded in a shoe would produce electricity as a person walked. Courtesy Zhong Lin Wang Laboratory.

By matching the impedance of the storage device to that of the triboelectric generators, the new system can boost energy efficiency from just 1% to as much as 60%.

“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 & Engineering at Georgia. “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.”

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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 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 AC, which is not ideal for mobile devices.

Ordinary AC can be converted to DC via a transformer, but that would require consistency in the number of cycles per second, say the researchers. Because biomechanical energy sources such as walking or finger tapping produce fluctuating amplitude and variable frequencies, a standard transformer cannot 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.

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

The system can work with any triboelectric generator that produces a minimum of 100µW. The system requires some power to operate, but compensates by increasing the overall output as much as 330X to reach mW levels.

“It doesn’t matter what kind of mechanical motion or what frequency of mechanical motion you have as long as the energy input is high,” said Niu. “This is a critical step in the commercialization of triboelectric nanogenerators because it opens up a range of new applications.”

PHOTO Fold: 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. Courtesy Zhong Lin Wang Laboratory.

With finger tapping as the only energy source, the power unit provides continuous DC of 1.044 mW. The unit can work continuously with the motion, allowing devices to be operated even as the device charges the battery or capacitor.

Beyond portable electronics, Niu believes the system could be useful in powering networks of sensors, allowing long-term operation without the need for replacing batteries.

“In a sensor network, you would have so many devices that you could not replace all of the batteries,” he said. “This technology would allow you to power the sensors by harvesting energy from the environment and then directly providing energy for each component of the network.”

With the energy management circuitry demonstrated in this proof-of-concept, the next step will be to miniaturize the circuitry to fit into an overall system, said Zhong Ling Wang.

— With files from John Toon.