Title:
Advances in Energy Focusing, Localization and Harvesting

Contents:
Metamaterial-based energy harvesting (MBEH) has been recently proposed as a breakthrough technology to drastically enhance energy conversion efficiency. The metamaterial refers to an artificial structure engineered to possess exotic properties such as a phononic bandgap. The phononic band gap is defined as a certain range of frequencies within which mechanical waves cannot be transmitted through phononic crystals (PnC). The phononic bandgap can be used to focus and/or localize mechanical waves on an energy conversion medium for highly dense energy harvesting.

This invited talk mainly discusses about how the phononic bandgap contributes to the enhancement of energy harvesting performance. Two innovative design concepts for the MBEH using the phononic bandgap will be presented as: 1) a quarter-wave stack (QWS) parabolic mirror for energy focusing and 2) a defect-mode PnC for energy localization. For energy harvesting, piezoelectric materials are attached onto a focal region of the parabolic mirror and defect spots of the supercell.

The first half of this talk will present the QWS parabolic mirror to avoid wave cancellation at the focal region. When the phase between the incident and reflected waves are different, destructive interference can occur, thereby causing diminution of energy focusing. The backbone idea of the QWS unitcell configuration is thus to make all the reflected and incident waves in-phase at the focal region for maximizing the reflectivity while minimizing the wave cancellation.

In the last half of this talk, we introduce the defect-mode PnC which confines the energy from incident waves into the defects. The supercell is configured by removing unitcells from the perfect PnC. A narrow passband within the phononic bandgap exhibits a resonance nature of the cavities, which trap the energy. Topology optimization is performed to find the optimal location of the defect for maximizing the trapped energy.