Structure-borne elastic wave energy harvesting enhanced by metamaterial concepts
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Existing research on vibration-based energy harvesting has been mainly focused on the harvesting of vibrational energy available at a fixed position in space. Such an approach is convenient for designing and employing linear and nonlinear vibration-based energy harvesters, such as base-excited cantilevers with piezoelectric laminates undergoing persistent excitation that yield modal vibrations. This theoretical and experimental research is centered on the harvesting of structure-borne propagating elastic waves in one-dimensional and two-dimensional settings. Specifically, it is aimed to enhance the harvested elastic wave energy by exploiting concepts from metamaterials and phononic crystals. First, the focus is placed on a one-dimensional beam configuration for piezoelectric energy harvesting from bending waves through optimal resistive-reactive electrical loading, spatially localized obstacle for harvesting local reflections, and a multifunctional energy-harvesting electromechanical non-reflective boundary condition in a semi-infinite setting. Next, two-dimensional efficient wave energy harvesting concepts are explored by means of novel wave mirror and lens concepts in elastic plates. Mirror concepts are studied thoroughly with a focus on their scattering characteristics. In this context, structurally embedded mirrors and bandgap-based mirrors are presented. As an alternative approach for plane wave focusing, elastic lenses are designed by creating a gradient distribution of the refractive index of the phononic crystals (PC) and locally resonant (LR) unit cells. To this end, a Gradient-Index Phononic Crystal Lens (GRIN-PCL), a 3D printed GRIN-PCL, and an omnidirectional Luneburg lens are fabricated and experimentally validated. In addition, wave focusing is explored with GRIN lenses composed of LR subwavelength unit cells towards enabling enhanced low frequency energy harvesting. Overall, this work provides electroelastic models and metamaterial-based approaches to efficient elastic wave energy harvesting. Beyond enhanced energy harvesting, ramifications of this work range from MEMS implementation to 3D printed platforms for structural integration in sensing applications and nondestructive testing.