Energy harvesting devices 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, energy harvesters 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. Using a well-known piezoelectric material called PZT, researchers created a wearable energy harvesting device about the size of a wrist-watch that produces enough power to run a personal health monitoring system.
The PZT was coated on both sides of a flexible metal foil to a thickness four or five 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 — known as the figure of merit — 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 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.
The researchers are working on adding a magnetic component to the current mechanical harvester to scavenge energy over a larger portion of the day when there is no physical activity.