A wearable piezoelectric energy harvester developed by scientists from the Materials Research Institute at the Pennsylvania State University and the University of Utah produces enough power to run a personal health monitoring system.
This image shows a piezoelectric energy harvester in a wristwatch-like device. Image credit: Pennsylvania State University.
Energy harvesters are in high demand to power the millions of devices that make up the Internet of Things.
By providing continuous power to a rechargeable battery or supercapacitor, these devices can reduce the labor cost of changing out batteries when they fail and keep dead batteries out of landfills.
Certain crystals can produce an electric current when compressed or they can change shape when an electric charge is applied. This piezoelectric effect is used in ultrasound and sonar devices, as well as energy harvesting.
“Our devices we make using our optimized materials run 5 to 50 times better than anything else that’s been reported,” said Penn State Professor Susan Trolier-McKinstry.
Professor Trolier-McKinstry and colleagues used a well-known piezoelectric material — called lead zirconate titanate (PZT) — and coated it on both sides of a flexible metal foil to a thickness 4-5 times greater than in previous devices. Greater volume of the active material equates to generation of more power.
By orienting the film’s crystal structure to optimize polarization, the performance of energy harvesting was increased.
The compressive stresses that are created in the film as it is grown on the flexible metal foils also means that the PZT films can sustain high strains without cracking, making for more robust devices.
The team then designed a novel wristwatch-like device that incorporates the PZT/metal foil materials.
The device uses a freely rotating, eccentric brass rotor with a magnet embedded, and multiple PZT beams with a magnet on each beam.
When the magnet on the rotor approaches one of the beams, the magnets repel each other and deflect the beam, plucking the beam in a process that is referred to as frequency up-conversion.
The slow frequency of a rotating wrist is converted into a higher frequency oscillation.
“The design of this device is more efficient than a standard electromagnetic harvester– like those used in self-powered watches,” Professor Trolier-McKinstry said.
The team’s energy-harvesting device is described in a paper published recently in the journal Advanced Functional Materials.
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Hong Goo Yeo et al. Strongly (001) Oriented Bimorph PZT Film on Metal Foils Grown by rf-Sputtering for Wrist-Worn Piezoelectric Energy Harvesters. Advanced Functional Materials, published online July 20, 2018; doi: 10.1002/adfm.201801327
