Of all the energy-harvesting technologies presently in development, piezoelectric devices offer some of the most intriguing possibilities. They work by converting mechanical stress, which can take the form of movement-caused vibrations, into an electrical charge. This means that things such as shoes, roads, keyboards - or anything else that moves or is subjected to movement - could be outfitted with piezoelectrics, which would produce power. Unfortunately, the range of vibrations that any one device can harness is presently quite limited. Research being conducted at North Carolina's Duke University, however, could change that.

Piezoelectric harvesters typically operate in a linear fashion, in that they can only be tuned to a particular frequency of vibration. While that may work well in the lab, where the same vibration can be produced over and over again, the real world tends to be a lot more random. Duke engineer Brian Mann decided that in order to generate a practical amount of electricity, the devices would need to be able to take advantage of a wider range of frequencies - essentially, they would need to be nonlinear.

His research team proceeded to rework the mathematical principles used in the design of linear piezolelectric devices. They then built a physical laboratory model, in which magnets were used to bend a cantilevered arm made of piezolelectric material. By changing the orientation of the magnets, they were able to simulate the random nature of vibrations encountered in everyday life. However, by applying their new principles to the material, they were able to tune the arm so that it could harvest a wider range of those vibrations than would otherwise be possible.

"This nonlinear approach offers significant improvements in electricity production, sometimes on the order of one magnitude," said Mann. "More importantly, being able to capture more of the bandwidth would make it more likely that these types of devices would have practical uses in real world. These nonlinear systems are self-sustaining, so they are ideal for any electrical device that needs batteries or is in a location difficult to access."

A paper on the research was recently published in the journal Physica D: Nonlinear Phenomena.