Development of a Broadband Energy Harvesting Technique Utilizing Acoustic Metamaterials

Abstract A novel deterministic method to harvest energy within a broadband frequency (0 ∼ 25 kHz) from a mass-in-mass metamaterial is presented. Traditional metamaterials are composed of multiple materials (named as resonators and matrix) with different mechanical properties (e.g., stiffness, density). Typically, the mass-in-mass metamaterial consists of a core material, surrounded by an outer shell. The core material is usually made up of a material with higher density and stiffness, while the outer shell is composed of a less dense and lower stiff material. The subwavelength structures within the shell serve to redirect the acoustic waves, resulting in negative density. In this work, the stiffness of matrix materials is altered systematically to allow diversified property mismatch between the constituent components to introduce local resonance in the unit cell. During the material property manipulations, the properties of the resonator are kept unchanged. By manipulating the subwavelength structures in the outer shell, it is hypothesized to optimize the local resonance with higher density of states. A finite element based commercial solver COMSOL Multiphysics is used to develop the dispersion curve, and an in-house MATLAB code is developed to estimate the density of states at a series of eigen-frequencies. To illustrate the broad-band resonances and the energy harvesting at these frequencies, the dispersion relation, and corresponding DOS are used as a predictive tool. While local resonance leverages wave energy passing through the acoustic metamaterials trapped within the relatively soft matrix as dynamic strain energy, a strategic and deterministic methodology is investigated to obtain a broadband local resonance frequency. A correlation is established between the local resonance and the density of states. Using the proposed geometric configurations, the matrix properties are manipulated and a repeated higher density of states within a broadband frequency range is achieved. By predicting a higher density of states, the possibility of energy harvesting capability within this broadband frequency range is enhanced significantly. Hence, the frequency band is utilized to harvest the trapped energy by embedding a smart material inside the matrix which is capable of electromechanical transduction (e.g., lead zirconate titanate). A series of parametric frequency domain studies are carried out to estimate the power densities for a unit external displacement excitation. This concept has been proved numerically by harvesting energy at a broadband frequency with a power density of ∼ 10μW/in2. The harvested power can directly be utilized to power wireless network sensors, wearable devices, smart buildings, internet of things, and so on.