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Thinking outside the windmill: Innovative tree-like structures turn vibrations into energy


[Image above] Credit: Sudhamshu Hebbar; Flickr CC BY 2.0

How close is the U.S. to shedding its reliance on fossil fuels?

We’re seeing green energy solutions become more economical and mainstream—think solar panels on residential rooftops and wind turbines that supply power to communities across the nation. With each green-energy development, our carbon footprint gets a little smaller.

(Check out three clean energy trends we saw in 2015 thanks to materials science.)

Wind energy is the fastest-growing source of electricity in the world, and harnessing it is one of the cleanest, most sustainable ways to generate power. According to the Department of Energy, wind generation in the U.S. tripled in the past six years and now accounts for nearly 5% of the country’s total electricity across 39 states.

Engineers are focused on developing wind turbines that are taller, stronger, and more efficient. But because wind speed varies considerably with seasons, location, and time of day or night, wind turbines’ effectiveness has its limitations.

Thinking outside the windmill, a project at Ohio State University is testing a new tool that resembles a tree-like structure for harvesting wind energy that uses vibrations from wind, traffic on a bridge, and even seismic activity to generate power.


Ryan Harne; Credit: OSU

“Buildings sway ever so slightly in the wind, bridges oscillate when we drive on them, and car suspensions absorb bumps in the road… 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,” Ryan Harne, assistant professor of mechanical and aerospace engineering at Ohio State and project leader, says in a recent OSU news release.

According to the release, the tree-like structures are “made with electromechanical materials that can convert random forces like winds or footfalls on a bridge into strong structural vibrations that are ideal for generating electricity”—technology that might prove most valuable implemented on a smaller scale in situations where other clean energy solutions like solar or giant wind turbines aren’t feasible.

Harne used mathematical modeling to determine the possibility for tree-like structures to “maintain vibrations at a consistent frequency despite large, random inputs, so that the energy can be effectively captured and stored via power circuitry,” the release explains. It’s a phenomenon called “internal resonance,” which is how certain mechanical systems are able to dissipate internal energies.

The team “exploited internal resonance to coax an electromechanical tree to vibrate with large amplitudes at a consistent low frequency, even when the tree was experiencing only high frequency forces,” the release explains. And it was successful even in environments rife with ambient noise—environments where this technology would be widely used, like bridges and buildings in high-traffic areas.

The tree-like device is constructed with two small steel beams—one that serves as the “trunk” and the other acts like a “branch.” A strip of polyvinylidene fluoride (PVDF) connects the two beams and converts the structural oscillations into electrical energy, the release explains.

When the team installed the model tree on a device that shook it back and forth at high frequencies with small, nearly imperceptible oscillations, the PVDF produced a small voltage of about 0.8 volts from the motion.

Adding more noise to the system caused the tree to display what Harne calls “saturation phenomena,” where the device channeled into a lower frequency oscillation, noticeably swaying back and forth. This upped the voltage generation to around 2 volts.

“We introduced massive amounts of noise and found that the saturation phenomenon is very robust, and the voltage output reliable. That wasn’t known before,” Harne says.

The research, published in the Journal of Sound and Vibration, is “Leveraging nonlinear saturation-based phenomena in an L-shaped vibration energy harvesting system” (DOI: 10.1016/j.jsv.2015.11.017).