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Journal of Fluids and Structures
journal homepage: www.elsevier.com/locate/jfs
Tree-inspired piezoelectric energy harvesting
William B. Hobbs a, David L. Hu a,b,Ã
School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA School of Biology, Georgia Institute of Technology, Atlanta, GA 30332, USA
a r t i c l e i n f o
Article history: Received 23 July 2010 Accepted 6 August 2011 Keywords: Vortex induced vibrations Piezoelectric energy harvesting Tandem cylinders
We design and test micro-watt energy-harvesters inspired by tree trunks swaying in the wind. A uniform ﬂow vibrates a linear array of four cylinders afﬁxed to piezoelectric energy transducers. Particular attention is paid to measuring the energy generated as a function of cylinder spacing, ﬂow speed, and relative position of the cylinder within the array. Peak power is generated using cylinder center-to-center spacings of 3.3 diameters and ﬂow speeds in which the vortex shedding frequency is 1.6 times the natural frequency of the cylinders. Using these ﬂow speeds and spacings, the power generated by downstream cylinders can exceed that of leading cylinders by more than an order of magnitude. We visualize the ﬂow in this system by studying the behavior of a dynamically matched ﬂowing soap ﬁlm with imbedded styrofoam disks. Our qualitative visualizations suggest that peak energy harvesting occurs under conditions in which vortices have fully detached from the leading cylinder. & 2011 Elsevier Ltd. All rights reserved.
1. Introduction Energy harvesting is the process by which ambient energy is captured from external sources (thermal, wind, solar and hydrodynamic). One way to harvest energy is through the use of piezoelectricity, the ability of certain materials such as bone (Fukada and Yasuda, 1957), wood (Fukada, 1955) and ceramics to generate electric ﬁelds in response to mechanical strain. In this study, we investigate the feasibility of using biologically inspired kinetic sculptures to harvest energy by swaying in the wind. We examine primarily the use of ceramic piezoelectric transducers, but the principles we ﬁnd may be applied to transducers composed of other materials. While primarily used for sensing pressure, ceramic piezoelectric transducers have been recently implemented in several designs for ﬂuid ﬂow energy harvesting. They are envisioned for use in generating micro- and milli-watts for powering remote sensor networks and small-scale electronic devices. Some piezoelectrics replace electrical generators in conventional cam-driven rotating turbine designs (Priya et al., 2005). Other designs have been implemented to operate entirely differently from rotating designs. One example is the piezoelectric eel, an underwater sheet of piezoelectric polymer that oscillates in the wake of a bluff body (Taylor et al., 2001). The ‘‘oscillating blade’’ generator resembles a stalk of corn, in which a piezoelectric transducer connects a steel leaf spring to leaf-like ears (Schmidt, 1992). Vortex-induced vibration is one of the primary mechanisms by which moving ﬂuids cause objects to undergo oscillation, and is often capitalized upon in piezoelectric energy harvesting. The subject has a long history of experimental, theoretical and computational work, summarized in reviews by Bearman (1984), Williamson and Govardhan (2004), and
Ã Corresponding author at: School of Mechanical Engineering, Georgia Institute of Technology, 801 Ferst Drive, MRDC 1308, Atlanta, GA 30332-0405, USA. Tel.: þ 1 404 894 0573; fax: þ 1 404 894 8496. E-mail address: firstname.lastname@example.org (D.L. Hu).
0889-9746/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jﬂuidstructs.2011.08.005
Please cite this article as: Hobbs, W.B., Hu, D.L., Tree-inspired piezoelectric energy harvesting. Journal of Fluids and...