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In any energy harvester, one of the key targets is to maximize the power output. Though the harvested power using the proposed model is significantly higher than the existing models of same kind, additionally, two approaches are suggested to further maximize the power output using the unit AEMM model,

1. Multi-frequency/multi-modal harvesting 2. Geometric optimization

Multi-frequency Harvesting

In current state of the art, low frequency energy harvesting at multiple frequencies is very challenging using the unit cell design. Figure 7.4 confirms that using proposed AEMM model, four local resonance (energy trapping) mode can be introduced within 1 KHz range. It has already been reported that, placing a piezoelectric material perpendicular to the loading direction in between the core resonator and the cavity wall is the appropriate orientation for harvesting energy from mode Q (Ref. Figure 7.7). It is hypothesized that, with proper placement of the piezoelectric material significant energy can be scavenged from the other modes (P, R and S) as well. To harvest energy at those modes, tentative orientation of the piezoelectric material is described in Table 7.1.

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Table 7.1: Tentative orientation of the smart material to scavenge energy at different models of vibration

Mode Vibration Pattern Tentative Piezoelectric Material Orientation

P

Core resonator is vibrating along thickness direction of

the unit cell.

In between resonator and free surface of the matrix material, keeping piezoelectric material and AEMM thickness axis aligned.

R

Core resonator is rotating about the width axis of the

structure.

In between resonator and cavity wall, keeping piezoelectric material and AEMM thickness

axis parallel.

S

Core resonator is rotating about the thickness axis of

the structure.

In between resonator and cavity wall, keeping piezoelectric material thickness axis perpendicular to the AEMM thickness axis.

In early studies, PZT 5H is employed as energy conversion medium in harvesting energy from mode Q. It has be observed without PZT placement in the structure, mode Q is found at ~ 415 Hz, however the mode shifts to ~ 430 Hz with the PZT addition. It has also been found that, PZT orientation and placement significantly manipulates the vibration modes in the AEMM. Other vibration modes in the AEMM (P, R and S) are extinct after placing the PZT inside. We anticipate that, since the mass and stiffness of the piezoelectric material is considerable compared to the constituents of the unit cell, hence it plays significant role in vibration patterns of the constituents. By selecting the appropriate piezoelectric material, its shape and placement significant energy can be scavenged from the P, R and S modes without affecting the vibration modes due to the addition of the PZT material.

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Figure 7.9: PZT rotation about the thickness axis of the unit cell (a) 300 (b) 600. Displacement plot at 500 Hz with PZT (c) 300 (d) 600 PZT orientation.

To understand the PZT effect in vibration modes in little more detail, a numerical study is performed at various orientations of the PZT in the unit cell. Keeping all other parameters constant, the piezoelectric material is rotated about the thickness axis of the cell with an interval of 300. Figure 7.9 (a-b) shows two sample orientations (300 and 600) of the PZT. Figure 7.9 (c-d) confirms the mode manipulation feature of the AEMM through PZT orientation. Where with 00 PZT orientation only Q mode is exist, additional local resonance modes can be introduced with the rotated PZT orientation. Interestingly it has been observed that, in the proposed model, there are two local resonance modes always exists;

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1) along the loading axis; and 2) along the PZT thickness axis. The loading direction is recalled in Figure 7.9a with double sided arrow. Since in mode Q, both loading and PZT thickness axis coincides, hence only one mode is exist with 00 PZT orientation. Note that, the second mode doesn’t exist only if the PZT thickness axis is perpendicular to the loading axis. Inspired from the outcome, additional studies (such as, using multiple PZT, using variable PZT, optimize orientation etc.) are envisioned to further introduce new local resonance modes and optimize the power output from a unit cell AEMM.

Geometric Optimization

Recalling the effective mass equation in chapter-3, effective mass (hence, local resonance) of the system is strongly depends on mechanical properties and geometric configuration of the cell constituents. It is hypothesized that, power output and local resonance frequency of the system can be altered significantly through the variation of cell geometry and material selection.

7.9 Chapter Summary

This chapter demonstrates the energy harvesting capabilities of an acousto-elastic metamaterial (AEMM) in the ground of simultaneous wave filtering and energy harvesting. In addition to wave filtering and energy harvesting the proposed model is also capable of obtaining several other objectives which are not available in previously introduced models. It is also confirmed that multi-frequency energy harvesting is possible using the proposed AEMM model, which is challenging to obtain using traditional phononic crystal based energy harvesters. It is shown that by setting a piezoelectric wafer inside the soft matrix of AEMM, significant electric potential can be recovered and the amount of power that can

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be harvested from a unit cell with unit excitation, is significantly higher (~36μW against 10KΩ) compared to the existing low frequency harvesters of same kind. A multi-cell model for broadband energy scavenging is proposed which is a heuristic design to demonstrate the concept, but any application specific structure can be manufactured at sub wave length scale by applying the similar physics. It is shown that the systematic selection of the core mass, placement of piezoelectric wafer and coupling local, structural and matrix resonance in a multi-cell system could result a broadband energy scavenging device. It has been found that PZT geometry and orientation plays important role in manipulating vibration modes and additional local resonance modes can be introduced through altering PZT orientation. Since power output is one of the key features of the energy scavengers, hence various optimization approaches are proposed to optimize the power output from the proposed AEMM model. We believe that the proposed harvester possess enough potential, flexibility and novelty to be employed in many engineering applications.

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