Making Motions Multitask

Imagine you’re walking down the street and talking on your cell phone. You’ve been talking a while, but you’re not worried about the battery dying because the motion of your walking is creating enough electricity to charge your phone.

This isn’t possible with current energy-harvesting technology, which is designed for consistent motions, such as walking at a steady pace on a treadmill. But a team of engineers at Duke has come up with a theory for improving those devices so that they can more efficiently capture energy from everyday motions.

“A traditional linear harvester would only be able to take advantage of a very limited frequency,” says Benjamin Owens ’10, a graduate student at the Pratt School of Engineering. That may work fine in a lab setting, where variables like speed and direction can be controlled. But in real-world applications, linear devices would capture only a fraction of the energy expended in walking down a crowded street.

The Duke engineers reworked the principles to account for a greater range of frequencies. Their model uses magnets to change the orientation of piezoelectric material, which generates electricity when bent, essentially allowing it to “tune” to a greater range of motions.

“Being able to capture more of the bandwidth would make it more likely that these types of devices would have practical uses in the real world,” says Brian Mann, associate professor of mechanical engineering and senior researcher on the study. Potential applications might range from a cellphone to a pacemaker or cardiac defibrillator. Mann is investigating whether such nonlinear devices could power sensors on buoys by extracting electricity from the motions of waves or could be applied to even larger devices.