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System boosts energy-harvesting efficiency

15 January 2016

A two-stage power management and storage system could dramatically improve the efficiency of triboelectric generators harvesting energy from movement.

A triboelectric generator embedded in a shoe would produce electricity as a person walked (image: Zhong Lin Wang Laboratory/Georgia Institute of Technology)

The system, developed by a team at Georgia Institute of Technology, uses a small capacitor to capture alternating current generated by 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 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.

“With a high-output triboelectric generator and this power management circuit, we can power a range of applications from human motion,” says 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.

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

Triboelectric nanogenerators use a combination of the triboelectric effect and electrostatic induction to generate small amounts of electrical power from mechanical motion (image: Zhong Lin Wang Laboratory/Georgia Institute of Technology)

“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 commercialisation of triboelectric nanogenerators because it opens up a range of new applications.”

With finger tapping as the only energy source, the power unit provides continuous direct current of 1.044 milliwatts. 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 miniaturise the circuitry to fit into an overall system

The research is described in an article published in the journal, Nature Communications.

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