Flapping Piezo-Leaf Generator for Wind Energy Harvesting

Project Member: Shu-guang Li, Hod Lipson & Francis C. Moon
To investigate the principles and feasibility of the energy harvesting from the wind as it flows around our buildings and other living ambient as an alternative to conventional rotary wind turbines.

Conceptual sketch and realization of “piezo-tree” generator, based on Dickson (Dickson, R.M., 2008, “New Concepts in Renewable Energy,” pp. 33-34, http://newrenewables.net/)

We designed a wind energy harvesting device "Piezo-Leaf Generator" by piezoelectric materials. On considering of the unpredictable wind strength and the diversely outdoor ambient, we chose the flexible piezoelectric materials, Polyvinylidene Fluoride (PVDF), as the basic component of device. The idea is to fix a piece of PVDF film on the backside of a cylinder bluff body, when the wind cross this bluff body, it will lead a vortex shedding, then the periodic pressure difference from the periodically shedding vortex could drive the Piezo-Leaf to be bending in the downstream air wake, synchronously, we collect the AC signal from the flapping Piezo-Leaf which is working on a periodic bending model, and store the electric energy to capacitor or other storage after rectifying it by a full-wave bridge.
However, because of the weak piezoelectric strain coefficient of PVDF, the preliminary Piezo-Leaf Generator’s power level was just about 100 pW, after a 2 cm diameter circular cylinder in 4.5 m/s wind, honestly, which is far away from being able to drive our light of office. Then, we tried to attach a piece of plastic film to the end of leaf along the direction of air flow, which showed about 100 times increase of power in the same condition. After that we had done a series of experiments of attachments in various shape, area, density and flexibility by a group of plastic and polymer film, then we found some totally differences in these results.
To simply treat the flapping structure as a vortex inducing pendulum-cantilever aero-elastic coupled system, the various kinds of attachments could induce the difference of mass distribution, vortex inducing force and moment. Furthermore, we had finished a series experiments about the bluff body, we found the size and shape aspects could also impact the level of power, which should be caused by the changes of wake strength and vortex shedding frequency.
Promisingly, we have been pursuing more experiments to analyze the data and optimize the structure of single Piezo-Leaf Generator in order to maximize output power. For practical application, we will build plant-like devices with hundreds or thousands of the Piezo-Leaves, like ivy, tree and forest.

Li S., Lipson H., (2009) " Vertical-Stalk Flapping-Leaf Generator For Parallel Wind Energy Harvesting", Proceedings of the ASME/AIAA 2009 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS2009.
Li, S., Yuan, J., and Lipson, H.(2011) "Ambient wind energy harvesting using cross-flow fluttering", Journal of Applied Physics, 109, 026104.[Online supplemental materials].