New tools for harvesting wind energy may soon look less like giant windmills and more like tiny leafless trees. A project at The 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.
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. 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” themselves would be very simple structures: think of a trunk with a few branches – no leaves required.
Early applications would include powering the sensors that monitor the structural integrity and health of civil infrastructure, such as buildings and bridges. 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.
“Buildings sway ever so slightly in the wind, bridges oscillate when we drive on them and car suspensions absorb bumps in the road,” he said. “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.”
First, through mathematical modeling, Harne determined that it is possible 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 phenomenon is called internal resonance, and it’s how certain mechanical systems dissipate internal energies. He could exploit 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. It even worked when these forces were significantly overwhelmed by extra random noise.
They tested model 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 it 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: 0.8 volts.
Then they 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 “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 – ~2 volts. https://news.osu.edu/news/2016/02/01/shaketree/
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