Novel vibration-based nonlinear energy harvesters : design, analysis, and applications

Abstract

Kinetic energy is ubiquitous in nature, daily life, and industrial surroundings, from human motion to vehicle vibration. Electricity generated from ambient kinetic energy resources can provide clean and sustainable power supplies compared to conventional batteries. Due to the huge potentials of exploiting renewable and clean energy in powering portable and wireless electronic devices, the field of kinetic/vibration-based energy harvesting has been increasingly attractive to researchers. Although large numbers of valuable reports, there are still problems that needed to be settled. Firstly, most of the piezoelectric harvesters are facing the challenge of capturing low-frequency power. Secondly, most of the ocean wave converters are linear ones, and exploiting nonlinear benefits to enhance the performance is far from development. Thirdly, how to explicitly characterize and design the critical parameters of the harvesting systems is a major challenge. Inspired by these, in the present thesis, mainly three major research topics have been focused as follows, in terms of structural design, nonlinearity design, methodology utilization, and the potential applications of the bio-inspired X-shaped structured harvesting systems. Firstly, in the low-frequency range, the conventional piezoelectric-based cantilevered harvesters could not effectively capture ambient power, since the natural frequencies of the beam harvesters are generally in a higher range, which cannot effectively match the external sources, even by using the existing nonlinear approaches. To settle this problem, several novel ultralow frequency X-structured tunable broadband piezoelectric harvesters (Chapters 2 and 3) have been proposed. The proposed devices can be regarded as advantageous versions of the cantilever-based configurations for the ultralow frequency range, which holds potentials in low-frequency energy harvesting. Secondly, exploring beneficial nonlinearities including nonlinear stiffness and damping concerning ocean wave energy harvesting is still very limited and yet to be further investigated. The existing three-spring mechanism has many limitations including high resonant frequency at different equilibria and the smaller vibration stroke it can bear with. In the present study, a specially designed bistable X-structured system is proposed as a novel point absorber. This work (Chapter 4), for the first time, shows that both bistable and quasi-zero stiffness could benefit ocean wave energy harvesting due to low resonant frequency and large vibration stoke in all working ranges created by the proposed bistable X-structured harvester. Finally, a straightforward parametric design method for characterizing the critical structural parameters of nonlinear energy harvesting systems is desired. To this aim, a novel Volterra series-based frequency domain method is established in this study for a typical X-structured nonlinear energy harvesting system (Chapter 5). A hybrid linear and nonlinear characteristic output spectrum (lnCOS) method, is thus proposed to achieve concise parametric characteristic expression for the nonlinear output frequency response (OFRF) and power generation function (PGF) of the system. This work for the first time provides explicit expressions of the vibration responses/power generations concerning the structural parameters of the X-structured energy harvesting system, which cannot be achieved using traditional approaches such as the Harmonic balance method or numerical simulation. In summary, by exploiting beneficial nonlinearities of the bio-inspired X-structured harvesters, the research work above aims to advance the field of nonlinear energy harvesting, in terms of designing piezoelectric and electromagnetic types of harvesters, characterizing critical system parameters, and providing innovative technologies for energy harvesting field and the development of smart city and ocean.