Inspired by the damping mechanisms which sustain trees under wind and seismic loads, researchers at The Ohio State University and the University of Michigan are investigating the potential for the development of energy harvesting systems that efficiently convert the same motion- and wind-based excitations into electric power. In a paper in the Journal of Sound and Vibration, they report demonstrating 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.
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, 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.
Trees leverage a variety of damping mechanisms for self-preservation when subjected to wind and seismic loads. Some features such as collision effects (e.g., branches rubbing) and aerodynamic phenomena (e.g., leaves flapping) contribute to dampen the swaying vibrations. The remaining energies are dissipated via structural/material factors, about which several interesting hypotheses have been proposed.
… The themes of these studies suggest that nonlinearity and multimodality play critical roles in the dynamical behaviors of trees for the purposes of structural damping. Indeed, the occurrence of an internal resonance suggests that some trees exploit particularly unique energy transfer characteristics. For different purposes altogether, energy harvesting structures are designed to efficiently absorb and electrically dissipate the vibrations to which they are subjected. While to date there have been numerous energy harvesting investigations focused on nonlinearity or multimodality as individual features, there are few that have considered exploiting both phenomena concurrently to improve energy conversion.
…In spite of the active explorations on the potential for nonlinearity and multimodality to enhance energy harvesting system performance, there are several important questions which remain to be answered, particularly regarding how these two features are best integrated and exploited via internal resonance. … This research seeks to address these important questions by investigating a prototypical, monostable energy harvesting platform which exploits the 1:2 internal resonance.—Harne et al.
The idea of using tree-like devices to capture wind or vibration energies may seem straightforward, because real trees clearly dissipate energy when they sway. Although other research groups have tested the effectiveness of similar tree structures, until now, they haven’t made a concerted effort to capture realistic ambient vibrations with a tree-shaped electromechanical device—mainly because it was assumed that random forces of nature wouldn’t be very suitable for generating the consistent oscillations that yield useful electrical energies.
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.
In particular, he determined that 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, as natural ambient vibrations would be in many environments.
Harne and his colleagues 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 0.8 volts.
Then they added noise to the system, as if the tree were being randomly nudged slightly more one way or the other. The tree began displaying what Harne called “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 2 volts.
Those are low voltages, but the experiment was a proof-of-concept: Random energies can produce vibrations that are useful for generating electricity.
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 tree-like structures feeding voltages to a sensor on the underside of a bridge, or on a girder deep inside a high-rise building.
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 is a truly self-sufficient way.
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.
R.L. Harne, A. Sun, K.W. Wang (2016) “Leveraging nonlinear saturation-based phenomena in an L-shaped vibration energy harvesting system,” Journal of Sound and Vibration, Volume 363, Pages 517-531, doi: 10.1016/j.jsv.2015.11.017