Research Papers

Fractal-Inspired Multifrequency Structures for Piezoelectric Harvesting of Ambient Kinetic Energy

[+] Author and Article Information
D. Castagnetti

Department of Engineering Sciences and Methods,  University of Modena and Reggio Emilia, 42122 - Reggio Emilia (RE), Italydavide.castagnetti@unimore.it

J. Mech. Des 133(11), 111005 (Nov 11, 2011) (8 pages) doi:10.1115/1.4004984 History: Received October 14, 2010; Revised August 22, 2011; Published November 11, 2011; Online November 11, 2011

Energy harvesting devices capable of converting freely-available ambient energy into electrical energy have received significant attention recently. Ambient kinetic energy is particularly attractive for conversion since it is almost ubiquitous and easily accessible. Piezoelectric energy harvesting devices are promising due to their simple configuration and high conversion efficiency. This paper studies multifrequency structures for piezoelectric energy harvesting of ambient kinetic energy, inspired by fractal geometry. Identifying such structures that are simple and efficient is challenging. We propose four fractal-inspired structures and we examine them at both micro and macroscales. We calculate their frequency response up to 100 Hz with computational modeling, and we also examine the effect of the fractal geometry iteration level. We use a cantilever plate example as a reference to validate computational results against analytical ones. A quantitative criterion to assess the harvesting efficiency of the proposed structures is introduced using the bending strain associated with each mode shape. Results show that a large number of eigenfrequencies is obtained, evenly distributed below 100 Hz, particularly in the macroscale. In addition, the iteration level of the fractal geometry affects the number and distribution of eigenfrequencies in the range of interest. Comparison with a conventional batch of cantilevers of the same size as the proposed structures shows noticeable improvement in electric charge generation.

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 1

A single (a) and batch (b) of cantilevers converter with tip mass

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Figure 2

The proposed structures inspired by fractal geometry criteria: iteration level F1 in the left-hand column and F2 in the right-hand column

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Figure 3

A batch of cantilevers of the same size and providing the same eigenfrequencies as structure 1 in the macroscale

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Figure 4

First mode shape for each of the structures in Fig 1 (iteration level F1)

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Figure 5

Bar charts showing the global average value, Εg (black bars), and optimized average value, Εopt (white bars) of the bending strain, for all the proposed structures at both iteration levels

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Figure 6

Partition of the surface to calculate the optimized average strain, Εopt , (opposite hatch) and paths of nodes for calculating the average strain (bold lines)

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Figure 7

Bar chart of the electric charge provided by structure 1 in the macroscale, fractal iteration level F1 (black bars), and by the batch of cantilevers (white bars)




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