In a world full of motion and solar radiation, energy can be harvested from many natural systems; how about a field of grass?
Jared Hobeck, Postdoctoral Research Fellow in Professor Dan Inman’s Adaptive Intelligent & Multifunctional Structures (AIMS) Lab, is harnessing the movement of synthetic grass blades in wind to generate energy to power small electronics and in-situ sensors.
His grass blades, which are little more than thin vertical sheets of metal, use the direct piezoelectric effect to convert the swaying motion of grass into electric pulses. Jared explains the origin of this research, which was featured on a recent episode of Nature Knows Best:
“SAIC (Science Applications International Corporation) released a very open-ended design request; they wanted something to harvest energy in low-velocity, highly turbulent fluid flow. I submitted four ideas, three of which followed more traditional turbine/propeller-type designs, one of which I considered out-of-left-field [the grass design]. They surprisingly asked me to pursue that [unconventional] one.”
Jared’s initial concept for the piezoelectric grass stemmed from observations of natural systems:
“For inspiration, I started watching videos of things moving in wind and water; seaweed, sea grass, leaves, different types of turbines, and vortex-induced vibration generators. I realized that [energy harvesting] devices like turbines are a one-shot deal – if some mechanical parts get tangled in debris or break, the machine won’t produce power. However, with grass, there is a built-in redundancy – if a single piece fails, the rest can remain in motion. It’s a simple, outside-the-box solution.”
To characterize the power production capabilities of his grass design, Jared placed a small array of the grass in a wind tunnel. Initially, the system was a bit finicky – it required specific wind speeds and turbulence patterns to produce energy. However, after experimental testing in Michigan’s 2’ x 2’ wind tunnel, Jared’s team had a breakthrough: they realized that above a critical wind speed, the “field” of grass would achieve a resonance condition where large synchronized wave-like motion was caused by the array’s own self-sustaining turbulence:
“This property was completely unexpected and exactly the selling point we needed to make our piezoelectric grass design viable. Before, all the energy production was localized to a specific part of the array; this new discovery meant that the entire array could achieve and maintain significant levels of energy production. Interestingly, this same phenomenon [of grass resonance] has been documented in agricultural journals. Farmers noticed that their crops were being destroyed when they achieved a certain resonant motion in the wind that exceeded their stress limit.”
Moving forward, Jared considers room for further experimentation with the geometry and configuration of the grass blades to optimize their power production. Initial exploratory experiments suggest that a simple orientation of the piezoelectric grass in rows and columns could be the most efficient.
Jared envisions widespread possible applications for this technology. Due to its inherent redundancy and minimal required maintenance, these grass-like arrays could be placed in remote locations, both on land and underwater, to power scientific instruments or sensors. They could be scaled to fit inside ventilation systems and other regions of airflow to power cell phones and other small electronics.
To learn more, check out Episode 105 of Nature Knows Best on Hulu, Amazon or Roku.