Broadband Vibrational Energy Harvesting:Research Of Methods And Structural Design

The operation of the IoT systems relies on a large number of distributed independent power supplies,which brings great challenges to the development of the Internet of Things technology.Vibrational energy,as one of the most widespread renewable energy sources,exists in a variety of forms in natural environments.Vibrational energy harvesting technology converts ambient vibrational energy into electrical energy which is able to be output for storage,providing a rich source of distributed energy for the IoT systems.Vibrational energy harvesters are usually designed on the basis of a resonance principle,which require the structural resonant frequency must be closely matched with the ambient excitation frequency.In order to adapt to the low-frequency and random characteristics of vibration excitations in natural environment,this thesis summarizes and studies the existing broadband vibrational energy harvesting methods and divides them into two categories.According to the two methods,two novel energy harvesters are designed to achieve broadband vibrational energy harvesting.In addition,this thesis also proposes another new approach for broadband vibrational energy harvesting,namely,multifrequency synergetic poly-stable vibration method,which absorbs the advantages of the existing two methods and remedies the congenital defects of them.The main works and research contents of this thesis include the following four parts:(1)According to the difference of working principles,this thesis divides the existing broadband vibrational energy harvesting methods into two categories: linear multifrequency resonant method and nonlinear multistable method.In this thesis,the physical modeling,broadband principle analyses,and parameter researches of both methods are carried out,and the congenital defects of them are analyzed.The existing problems of the broadband vibrational energy harvesters designed based on above two methods are pointed out by illustrations.For linear multi-frequency resonant method,there are several common problems about the existing energy harvesters,such as the mutual independence between different energy harvesting units,the low structural utilization rate,and the specificity of energy harvesting directions.The core problem of nonlinear multistable method is that the introduction of potential energy barriers hinders the collection of lowintensity vibrational energy.(2)On the basis of linear multi-frequency resonant method,a multimodal vibrational energy harvester using a double-branched beam is designed to achieve multidirectional broadband vibrational energy harvesting.Utilizing the finite element simulation analysis,the four modes of vibration as well as the corresponding four natural frequencies of the double-branched beam in low frequency range are studied,and the vibrational directions as well as structural deformations of different vibration modes are researched.In the experiments,simulation results are verified by measuring the dynamic strain of the double-branched beam under swept excitations in horizontal and vertical directions.By comparing with a traditional straight cantilever beam,the improvements of generation performance of this double-branched beam are studied in terms of open-circuit voltage,working bandwidth,output power density,and generation efficiency.This doublebranched beam structure is proved to have the ability of multidirectional vibrational energy harvesting.(3)On the basis of nonlinear multistable method,a bistable vibrational energy harvester using a seesaw-type cantilever beam is designed to achieve bistable motion under lowintensity excitations.By analyzing and calculating the energy transfer process of the seesaw-type structure in nonlinear motion,the mechanism of overcoming the potential barrier during bistable motions is theoretically studied and demonstrated.In the experiments,the seesaw-type structure is compared with a traditional bistable cantilever beam under the same low-intensity excitation in terms of vibrational displacement,opencircuit voltage,and output power density.The wider working bandwidth and the characteristic of easily overcoming potential barriers are verified by comparison experiments.(4)In addition to the two existing methods,this thesis proposes a new approach for broadband vibrational energy harvesting,namely,multi-frequency synergetic poly-stable vibrational method.This approach assembles a plurality of cantilever beams with different natural frequencies into an array.The beams in the array repel each other through their tip magnets,and thus achieve a new multistable physical phenomenon,namely,synergetic poly-stable motion.In this thesis,the magnetic interactions between different beams are analysed by utilizing magnetic dipole model,and the potential energy function of the synergetic poly-stable system is then derived.The analysis of potential energy function and stable equilibrium states proves the existence of synergetic poly-stable states,reveals the condition of realizing the synergetic poly-stable motion,and demonstrates the novel feature that the maximum number of stable equilibrium states has an exponential dependence on the number of beams in the array.The correctness of theoretical analysis and the broadband characteristics brought by this multi-frequency synergetic poly-stable vibrational method are further verified by comparing the synergetic poly-stable beam array with a corresponding linear beam array in terms of open-circuit voltage,output power density,and power generation.