Random vibrations turn tiny trees into power plants

Electrical energy was converted from structural oscillations of a small "trunk and branch" device

Step aside windmills, there's a new way to harvest kinetic energy in the works. A research team at the Ohio State University has created electromechanical devices that look like tiny leafless trees and can generate electricity when they are moved by seismic activity, the slight swaying movements of a tall building, or the vibrations from traffic on a bridge.

Before we start envisioning great fields of wriggling tree-like power generators, it should be stated that this idea is for situations where small amounts of power are required.

In a study published last month, the researchers described their experiments with the new vibrational energy-harvesting platform. "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. "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."

The initial goal is to power low-voltage sensors that maintain the structural integrity of civil structures such as bridges or the girders deep inside high rise buildings. Currently this job is undertaken by battery or grid-powered sensors, methods that are expensive and hard to maintain in remote locations. Sensors that could capture vibrational energy could do their job in a completely self-sufficient way.

In the past, researchers have assumed that random movements generated in nature couldn't possibly be the most suitable option for creating the consistent oscillations needed to make usable electricity. As such, artificial, non-random vibrations have been used in experiments. By contrast, the Ohio team has explored ways to capture energy generated in a more natural, random way.

Through mathematical modeling, Harne worked out that it is possible for tree-like structures to maintain vibrations at a consistent frequency despite large, random inputs, thanks to internal resonance, a phenomena that allows certain mechanical systems to dissipate internal energy. The energy could then be captured and stored via power circuitry.

Harne and his colleagues tested the model by building a device made from two steel beams forming an L shape (similar to a trunk and branch) supported by a clamp and fixed to a structure that shook back and forth at high frequencies. The beams were connected by a strip of polyvinylidene fluoride (PVDF) to convert the structural oscillations to electrical energy.

When the device responded to high frequencies, it oscillated with only small amplitudes, barely visible to the naked eye. Nevertheless, it produced about 0.8 volts. But when the researchers added random noise to the system, the tree began displaying what Harne calls "the saturation phenomena." It reached the tipping point where high frequency energy was suddenly channeled into a low frequency oscillation. At this point, the tree swayed back and forth, with the trunk and branch vibrating in sync. This low frequency motion produced around 2 volts of electrical energy, more than double the voltage and enough to achieve proof of concept.

"We introduced massive amounts of noise, and found that the saturation phenomenon is very robust, and the voltage output reliable," Harne said. "That wasn't known before." He hopes to continue to develop the idea.

The study has been published in the Journal of Sound and Vibration.

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