Synopsis: Energetic Flags
The movement of a flag made of piezoelectric elements could be used to convert the wind’s mechanical energy into electrical power. In a steady wind flow, instabilities arising from the flag-wind interaction can lead to self-sustained flapping, which generates current in a coupled output circuit, as shown by a number of recent studies. However, the design of an optimal circuit remains a challenge: The circuit can induce a feedback coupling to the piezoelectric material, which, under some conditions, hinders flapping and lowers device efficiency. Yifan Xia, from École Polytechnique, France, and colleagues have now calculated that with an optimized resonant circuit, stable flapping motion, robust to velocity fluctuations, can be achieved at lower flow velocities than in previous studies. The scheme could open the way to efficient piezoelectric technologies that can harvest energy from a wide range of fluids (wind, tides, rivers, etc.)
The authors modeled a simple plane flag covered on both surfaces with piezoelectric patches and connected to a resonant circuit containing inductive and resistive elements. They found that when the flapping frequency was close to the natural frequency of the circuit, the two frequencies locked-in and the whole system resonated. By proper choice of the circuit’s frequency, the amplitude of the flapping and the energy output could be maximized. The frequency lock-in had two other advantages. First, it made flapping and energy harvest more robust: Small fluid velocity fluctuations, which could have shifted the flapping frequency, making it unstable, had little effect. Second, flapping could be induced at lower fluid velocities, at which the flag otherwise would not have flapped, extending the wind-velocity range suitable for energy harvesting.
This research is published in Physical Review Applied.