Good Vibrations Providing Clean Power

Swaying mechanical “trees” could soon be harvesting kinetic energy on a small scale where solar or wind may not be viable.
Published: Fri 12 Feb 2016

While it is thought that mammoth size wind turbines are the best way forward when it comes to harnessing huge wind potential [Giant Turbines To Harness Large Wind Potential],  there still seems to be a school of thought that bladeless vibrating wind structures will cut energy costs, generate less noise and not look as imposing as the 50MW monsters currently being developed. [Bladeless Wind Turbines To Cut Energy Costs by 40%] Also, these contraptions may be able to generate power from unused kinetic energy created by  various sources other than the wind.

Vibrational energy from “trees”

Ohio State University is testing whether high-tech objects that look a bit like artificial trees can generate renewable power when they are shaken by the wind—or by the sway of a tall building, traffic on a bridge or even seismic activity.

In a recent issue of the Journal of Sound and Vibration, researchers report that tree-like structures made with electromechanical materials can convert random forces—such as winds or footfalls on a bridge—into strong structural vibrations that are ideal for generating electricity.

Researchers say that the technology may prove most valuable when applied on a small scale, in situations where other renewable energy sources such as solar are not an option.

The “trees”, a very simple structure consisting of a trunk with a few branches, will feed voltages to a sensor on the underside of a bridge, or on a girder deep inside a high-rise building.

Harnessing kinetic energy

The project takes advantage of vibrational energy around us. Some sources are wind-induced structural motions, seismic activity and human activity.

Kinetic energy can be created by buildings swaying ever so slightly in the wind, bridges oscillating when cars drive over them and car suspensions that absorb road bumps.

The idea is to capture and recycle this kinetic energy which would otherwise be lost.

According to the researchers, sensors monitor the soundness of a structure by detecting the vibrations that pass through it. The aim of the project is to transform these vibrations into electricity so that structural monitoring systems can be powered by the same vibrations they are monitoring.

Today, the only way to power most structural sensors is to use batteries or plug the sensors directly into power lines, both of which are expensive and hard to manage for sensors planted in remote locations. If sensors could capture vibrational energy, they could acquire and wirelessly transmit their data in a self-sufficient way.

Generating consistent oscillations

Until now, researchers haven’t made a concerted effort to capture realistic ambient vibrations with a tree-shaped electromechanical device. This is mainly because it was assumed that random forces of nature wouldn’t be suitable for generating the consistent oscillations that bring about useful electrical energies.

Mathematical modeling made researchers brought researchers to the conclusion that tree-like structures can maintain vibrations at a consistent frequency despite large, random inputs, so that the energy can be effectively captured and stored via power circuitry. The phenomenon is called internal resonance, and it’s how certain mechanical systems dissipate internal energies.

In particular, it was determined that internal resonance can be exploited to coax an electromechanical tree to vibrate with large amplitudes at a consistent low frequency, even when the tree experiences only high frequency forces. It even worked when these forces were significantly overwhelmed by extra random noise, as natural ambient vibrations would be in many environments.

The researchers tested the mathematical model in an experiment, where they built a tree-like device out of two small steel beams—one a tree “trunk” and the other a “branch”—connected by a strip of an electromechanical material, polyvinylidene fluoride (PVDF), to convert the structural oscillations into electrical energy.

They installed the model tree on a device that shook it back and forth at high frequencies. At first, to the eye, the tree didn’t seem to move because the device oscillated with only small amplitudes at a high frequency. Regardless, the PVDF produced a small voltage from the motion: about 0.8V.

Then, the researchers added noise to the system, as if the tree were being randomly nudged slightly more one way or the other. That’s when the tree began displaying what “saturation phenomena”: it reached a tipping point where the high frequency energy was suddenly channeled into a low frequency oscillation.

At this point, the tree swayed noticeably back and forth, with the trunk and branch vibrating in sync. This low frequency motion produced more than double the voltage—around 2V.

Those are low voltages, but the experiment was a proof-of-concept: Random energies can produce vibrations that are useful for generating electricity.

The initial phase of this research was supported by the University of Michigan Summer Undergraduate Research in Engineering program and the University of Michigan Collegiate Professorship. Research is ongoing.


Further reading

Elsevier- Journal of Sound and Vibration

The Ohio State University-Turning good vibrations into energy