Picking up good vibrations: artificial trees harvest energy

February 05, 2016 // By Peter Clarke
Researchers from the University of Ohio have shown that tree-like structures made with electromechanical materials are suitable for converting winds or structural vibrations into electricity.

The "trees" would be relatively simple structures – a trunk with a few branches and no leaves – and they may not be scaled up to sit among conventional forests or compete with windmills or solar farms. More likely they would be used at the small scale to power sensors that monitor the structural integrity of buildings, bridges and other civil engineering structures.
 

"Buildings sway ever so slightly in the wind, bridges oscillate when we drive on them and car suspensions absorb bumps in the road," said project leader Ryan Harne, assistant professor of mechanical and aerospace engineering at Ohio State, and director of the Laboratory of Sound and Vibration Research, in a statement. "In fact, there's a massive amount of kinetic energy associated with those motions that is otherwise lost. We want to recover and recycle some of that energy."

Harne envisions tiny "trees" feeding voltages to a sensor on the underside of a bridge, or on a girder deep inside a high-rise building. In this way structural monitoring systems could be powered by the vibrations they are monitoring. 

Previously it was assumed that mechanical structures would not be good at collecting energy from a wide spectrum of vibration. Harne showed by mathematical modeling that this was not the case and that it should be possible to exploit internal resonance to get an electromechanical tree to vibrate at large amplitudes at a consistent low frequency even when the tree was experiencing random inputs or only high-frequency forces.

To test the math Harne and colleagues built a tree-like structure out of small steel beams connected by an electromechanical material polyvinylidene fluoride (PVDF) that would convert the movement into electrical energy. From a noisy high-frequency input it was possible to put the structure into low frequency oscillation producing an energy source at 2V. 

"In addition, we introduced massive amounts of noise, and found that the saturation phenomenon is very robust,