Ultrathin Acoustic Energy Harvesting Device By Acoustic Metamaterial

Energy harvesting attracts wide attentions with increasing emphasis on energy in recent years,which is studied to gather ambient energy and convert it into electric energy,such as wind,solar energy,vibrations,and so on.Among these harvestable energy in environment,acoustic energy,which is ubiquitous independent of season and climate,is an abundant,sustainable,and clean energy source.While acoustic energy is commonly wasted via noise insulation or noise absorption,acoustic energy harvesting enables full utility of sound and converts it to electricity,with great application potentials in diverse scenarios,such as controlling noise and producing power for micro-devices.However,acoustic waves innately have long wavelength and low power density,posing a fundamental challenge on study of acoustic energy harvesting.Traditional acoustic energy harvesting devices,based on Helmholtz resonators and quarter-wavelength resonators,usually exhibit bulky sizes especially in response to low-frequency acoustic wave,which hinder their practical applications.The emergence of acoustic metamaterials with unconventional acoustical properties enables many extraordinary ways of sound wave manipulation and help downscale acoustic energy harvesting devices to subwavelength with innovative structures.Furthermore,as the two-dimensional brunch of acoustic metamaterial,acoustic metasurfaces with subwavelength thickness have developed rapidly in the past years for their extraordinary abilities in diverse areas,including sound wave manipulation,noise reduction and so on,which offer abundant possibilities for design of innovative acoustic functional devices.Several researches on acoustic energy harvesting devices by acoustic metamaterials are proposed,exhibiting advantages over traditional acoustic energy harvesters in sub-wavelength dimension and innovative mechanism,yet still limited in some aspects such as thickness,rigidity efficiency and bandwidth.The usually low and broad frequency of airborne acoustic wave in diverse applications call for further miniaturization of acoustic energy harvesting devices.This makes it pivotal to explore the mechanism for realizing an ultrathin and high-efficiency acoustic energy harvester.This research proposes a design of ultrathin acoustic energy harvesting device based on acoustic metasurface,realizing high-efficiency absorption and energy conversion of low-frequency acoustic wave,which provides new idea for downscaling of acoustic energy harvesters.This article is divided as the following sections:In Chapter ?,theoretical and experimental works on acoustic metamaterials and acoustic energy harvesting are systematically reviewed,which serve for the background of the research.The current progress and potentials of acoustic energy harvesting are introduced,as well as the limitations of acoustic energy harvesting devices in some aspects,which are in need of further exploration and improvement.In Chapter ?,the related theories of acoustic energy harvesting are introduced and calculated in detail,for better understanding of mechanism of acoustic energy harvesting,including wave equation,lumped parameter model and energy harvesting systems.Piezoelectric and Electromagnetic energy harvesting systems are theoretical analyzed via mechano-electrical analog circuits,and related parameters are derived,founding a theoretical model for the research.In Chapter ?,we design an ultrathin metasurface-based acoustic energy harvester.In the research,we propose the mechanism that utilizes acoustic metasurface to reduce effective wavelength so that strong local energy is produced within deep-subwavelength dimension and highefficiently energy conversion are realized by piezoelectric method.Based on coiling up space,we designed the labyrinthine structures capable of confining low-frequency energy within narrow folded channels at resonance.Theoretical model is analyzed to derive effective acoustical parameter and absorption coefficient,which are also verified by numerical simulations.A piezoelectric plate is judiciously designed resonating at the same resonance frequency of acoustical structure and placed where acoustic energy is strongly localized.Our designed acoustic energy harvester is downscaled to as thin as 1/149 wavelength in thickness and 1/11 wavelength in width while keeping flat shape and mechanical rigidity.The simulation result shows it produces voltage of 0.75 V under 2Pa acoustic incidence at 442 Hz.Thanks to the subwavelength width of the designed unit,our proposed matasurface-based energy harvester is not sensitive to the variation of incident direction,which is also verified by numerical simulations that the design produces voltage of 0.4V-0.6V when incident by plane waves with large angles.In Chapter ?,a broadband and ultrathin acoustic energy harvesting device is studied based on the ultrathin unit designed in the last chapter,we propose the method of coupling hybrid structures to enhance working bandwidth,which is crucial to improve the application potential of acoustic energy harvesters.The designed broadband acoustic energy harvesting device still exhibits deep-subwavelength thickness.In the research,we design a hybrid supercell comprising 4 unit cells resonating at different frequencies by specifically adjusting the parameter of each unit cell.Theoretical model is established based on lumped parameter theory.In order to obtain the best broadband performance of the device,we utilize genetic algorithm optimization to simplify the proceeding of parameters optimization.Acoustic pressure distributions are analyzed by numerical simulation.Four piezoelectric plates are connected in parallel with each being designed specifically to work at the resonance frequency of each unit,so that the connected piezoelectric system of the device obtains multiple vibration modes.The simulation results demonstrate that our design produces output power of 5.4?W-11?W at 428-460 Hz,realizing the expansion of bandwidth compared to single unit cell.Finally,the main conclusions of this research and prospect for future work are stated in Chapter ?.