A spiral-shaped harvester with an improved harvesting element and an adaptive storage circuit

A piezoelectric energy harvester consists of a spiral-shaped piezoelectric bimorph to transfer mechanical energy into electric energy, an electrochemical battery to store the scavenged electric energy, and a rectifier together with a step-down dc-dc converter to connect the two components as an integrated system. A spiral-shaped harvesting structure is studied in this paper because it is very useful in the microminiaturization of advanced sensing technology. The aim of employing a step-down dc-dc converter in the storage circuit is to match the optimal output voltage of the piezoelectric bimorph with the battery voltage for efficient charging. In order to raise the output power density of a harvesting element, moreover, we apply a synchronized switch harvesting on inductor (SSHI) in parallel with the piezoelectric bimorph to artificially extend the closed-circuit interval of the rectifier. Numerical results show that the introduction of a dc-dc converter in the storage circuit or a SSHI in the harvesting structure can raise the charging efficiency several times higher than a harvester without a dc-dc converter or an SSHI.     

Analysis of a piezoelectric bimorph plate with a central-attached mass as an energy harvester

This article analyzes the performance of a piezoelectric energy harvester in the flexural mode for scavenging ambient vibration energy. The energy harvester consists of a piezoelectric bimorph plate with a central-attached mass. The linear piezoelectricity theory is applied to evaluate the performance dependence upon the physical and geometrical parameters of the model bimorph plate. The analytical solution for the flexural motion of the piezoelectric bimorph plate energy harvester shows that the output power density increases initially, reaches a maximum, then decreases monotonically with the increasing load impedance, which is normalized by a parameter that is a simple combination of the physical and geometrical parameters of the scavenging structure, the bimorph plate, and the frequency of the ambient vibration, underscoring the importance for the load circuit to have the impedance desirable by the scavenging structure. The numerical results illustrate the considerably enhanced performances by adjusting the physical and geometrical parameters of the scavenging structure.     

Vibration energy harvester with sustainable power based on a single-crystal piezoelectric cantilever array

We designed and fabricated a bimorph cantilever array for sustainable power with an integrated Cu proof mass to obtain additional power and current. We fabricated a cantilever system using single-crystal piezoelectric material and compared the calculations for single and arrayed cantilevers to those obtained experimentally. The vibration energy harvester had resonant frequencies of 60.4 and 63.2 Hz for short and open circuits, respectively. The damping ratio and quality factor of the cantilever device were 0.012 and 41.66, respectively. The resonant frequency at maximum average power was 60.8 Hz. The current and highest average power of the harvester array were found to be 0.728 mA and 1.61 mW, respectively. The sustainable maximum power was obtained after slightly shifting the short-circuit frequency. In order to improve the current and power using an array of cantilevers, we also performed energy conversion experiments.     

Low-frequency meandering piezoelectric vibration energy harvester

The design, fabrication, and characterization of a novel low-frequency meandering piezoelectric vibration energy harvester is presented. The energy harvester is designed for sensor node applications where the node targets a width-to-length aspect ratio close to 1:1 while simultaneously achieving a low resonant frequency. The measured power output and normalized power density are 118 μW and 5.02 μW/mm(3)/g(2), respectively, when excited by an acceleration magnitude of 0.2 g at 49.7 Hz. The energy harvester consists of a laser-machined meandering PZT bimorph. Two methods, strain-matched electrode (SME) and strain-matched polarization (SMP), are utilized to mitigate the voltage cancellation caused by having both positive and negative strains in the piezoelectric layer during operation at the meander's first resonant frequency. We have performed finite element analysis and experimentally demonstrated a prototype harvester with a footprint of 27 x 23 mm and a height of 6.5 mm including the tip mass. The device achieves a low resonant frequency while maintaining a form factor suitable for sensor node applications. The meandering design enables energy harvesters to harvest energy from vibration sources with frequencies less than 100 Hz within a compact footprint.     

Micromachining of a bimorph Pb(Zr,Ti)O3 (PZT) cantilever using a micro-electromechanical systems (MEMS) process for energy harvesting application

We designed and fabricated a bimorph Pb(Zr,Ti)O3 (PZT) cantilever with an integrated Si proof mass to obtain a low resonant frequency for an energy harvesting application. The cantilevers were fabricated on the micro-electromechanical systems (MEMS) scale. A mode of piezoelectric conversions were d31 and d33 mode in cantilever vibration Therefore, we designed and fabricated a single cantilever with d31 unimorph, d31 bimorph, d33 unimorph, and d33 bimorph modes. Finally, we fabricated a device with beam dimensions of about 5,400 microm x 480 microm x 14 microm (< +/- 5%), and an integrated Si proof mass with dimensions of about 1,481 microm x 988 microm x 450 microm (< +/- 5%). In order to measure the d31 and d33 modes, we fabricated top and bottom electrodes. The distance between the top electrodes was 50 microm and the resonant frequency was 89.4 Hz. The average powers of the d31 unimorph, d31 bimorph, d33 unimorph, and d33 bimorph modes were 3.90, 9.60, 21.42, and 22.47 nW at 0.8 g (g = 9.8 m/s2) and optimal resistance, respectively.     

Nonlinear interface between the piezoelectric harvesting structure and the modulating circuit of an energy harvester with a real storage battery

This paper studies the performance of an energy harvester with a piezoelectric bimorph (PB) and a real electrochemical battery (ECB), both are connected as an integrated system through a rectified dc-dc converter (DDC). A vibrating PB can scavenge energy from the operating environment by the electromechanical coupling. A DDC can effectively match the optimal output voltage of the harvesting structure to the battery voltage. To raise the output power density of PB, a synchronized switch harvesting inductor (SSHI) is used in parallel with the harvesting structure to reverse the voltage through charge transfer between the output electrodes at the transition moments from closed-to open-circuit. Voltage reversal results in earlier arrival of rectifier conduction because the output voltage phases of any two adjacent closed-circuit states are just opposite each other. In principle, a PB is with a smaller, flexural stiffness under closed-circuit condition than under open-circuit condition. Thus, the PB subjected to longer closed-circuit condition will be easier to be accelerated. A larger flexural velocity makes the PB to deflect with larger amplitude, which implies that more mechanical energy will be converted into an electric one. Nonlinear interface between the vibrating PB and the modulating circuit is analyzed in detail, and the effects of SSHI and DDC on the charging efficiency of the storage battery are researched numerically. It was found that the introduction of a DDC in the modulating circuit and an SSHI in the harvesting structure can raise the charging efficiency by several times.     

基于谐振器的多模态压电换能结构研究

传统压电悬臂梁俘能器一般通过其基频与环境中某一激振频率的匹配来收集这个频率上的振动能,为俘获环境中多个激振频率上的能量,提出一种在压电悬臂梁上附加谐振器的多模态换能结构,由该结构组成的俘能器的前两阶共振频率可以被设计在给定的激振频率上,从而有效地收集分布在这两个频率上的能量.实验表明附加谐振器有效地提高了俘能效率,其中一阶模态的俘能效率相对附加等量质量块时提高了71％,二阶模态的俘能效率与原压电悬臂梁的一阶模态相当.     

Electromechanical modeling and analytical investigation of nonlinearities in energy harvesting piezoelectric beams

Piezoelectric materials are extensively applied for vibrational energy harvesting especially in micro-scale devices where other energy conversion mechanisms such as electromagnetic and electrostatic methods encounter fabrication limitations. A cantilevered piezoelectric bimorph beam with an attached proof (tip) mass for the sake of resonance frequency reduction is the most common structure in vibrational harvesters. According to the amplitude and frequency of applied excitations and physical parameters of the harvester, the system may be pushed into a nonlinear regime which arises from material or geometric nonlinearities. In this study nonlinear dynamics of a piezoelectric bimorph harvester implementing constitutive relations of nonlinear piezoelectricity together with nonlinear curvature and shortening effect relations, is investigated. To achieve this goal first of all a comprehensive fully-coupled electromechanical nonlinear model is presented through a variational approach. The governing nonlinear partial differential equations of the proposed model are order reduced and solved by means of the perturbation method of multiple scales. Results are presented for a PZT/Silicon/PZT laminated beam as a case study. Findings indicate that material nonlinearities of the PZT layer has the dominant effect leading to softening behavior of the frequency response. At the primary resonance, different frequency responses of the extracted power can be distinguished according to the excitation amplitude, which is due to harmonic generation as a result of piezoelectric nonlinearity. The extracted power is analytically computed and validated with a good agreement by a numerical solution.     

Time and frequency domain analysis of piezoelectric energy harvesters by monolithic finite element modeling

The successful design of piezoelectric energy harvesting devices relies upon the identification of optimal geometrical and material configurations to maximize the power output for a specific band of excitation frequencies. Extendable predictive models and associated approximate solution methods are essential for analysis of a wide variety of future advanced energy harvesting devices involving more complex geometries and material distributions. Based on a holistic continuum mechanics modeling approach to the multi‐physics energy harvesting problem, this article proposes a monolithic numerical solution scheme using a mixed‐hybrid 3‐dimensional finite element formulation of the coupled governing equations for analysis in time and frequency domain. The weak form of the electromechanical/circuit system uses velocities and potential rate within the piezoelectric structure, free boundary charge on the electrodes, and potential at the level of the generic electric circuit as global degrees of freedom. The approximation of stress and dielectric displacement follows the work by Pian, Sze, and Pan. Results obtained with the proposed model are compared with analytical results for the reduced‐order model of a cantilevered bimorph harvester with tip mass reported in the literature. The flexibility of the method is demonstrated by studying the influence of partial electrode coverage on the generated power output.     

Phase-matched air ultrasonic transducers using corrugated PVDF filmwith half wavelength depth

A new type of large area PVDF film air transducer is proposed. The transducer features a high power output and a sharp beam angle. Conventionally known curved length mode resonators with two clamps at both ends have a resonance frequency determined by the curvature. In the present work, PVDF was formed into alternating concave and convex multiple curved sections, eliminating clamps, i.e., a periodic corrugation structure using a single PVDF film. Each convex and concave section has a common resonance frequency. A common excitation voltage induces vibration for each section, and the vibration direction is normal to the film surface. The vibration phase of convex section is shifted 180 degrees from the concave section. These waves add constructively to form a strong acoustic beam when corrugation height is approximately one-half of the wavelength. The corrugation height controls propagation path difference, canceling excitation phase difference. The design principle based on a uniform vibration mode is presented. Experimental investigations using 8.8×2.5-, 10×5-, and 20×20-cm² transducers are presented. Side lobes unique to this corrugation structure have been observed. A theoretical analysis of the side lobes is also presented. According to the theory, choosing the corrugation height appropriately will reduce side lobes to -15 dB with regard to the main lobe, and the observed side lobe height agreed with the theoretical result     

宽频压电振动能量采集器的力-电输出特性分析

压电振动能量采集器是一种新型的力(加速度)-电耦合转换输出器件,为了提高单自由度悬臂梁压电振动能量采集器的输出功率和工作频带,通过在单自由度悬臂梁压电振动能量采集器模型基础上增加一个弹性放大器的方法,构造形成了具有两自由度的宽频压电振动能量采集器。利用ANSYS有限元软件建立了宽频压电能量采集器的有限元力-电耦合模型,数值分析了模型中各参数(如质量比、阻尼比以及负载电阻等)对系统力特性(速度、加速度等)和电输出特性(电压、电流、输出功率等)的影响。研究结果表明：大的质量比和小的阻尼比能够提高压电悬臂梁能量采集器的输出功率并拓展其工作频带;短路谐振状态下的匹配电阻能够使能量采集器产生较大的输出电流,而开路谐振状态的匹配电阻能够使能量采集器产生较大的输出电压,优化后的短路谐振和开路谐振最大输出功率分别达到4386.5 mW/g2和4263.4 mW/g2。频带宽度达到10 Hz,且是SDOF系统的5倍。     

Z型梁结构压电式能量采集性能分析

振动能量采集能够将外部环境中的振动能转化为电能,具有绿色可持续、节能环保、设计灵活等优势,在工业、生物、医学、军事等领域具有广阔的应用前景.为使振动型能量采集器适应更为复杂多变的工作环境,提高其采集功率和工作频带,提出一种多梁结构-Z型梁结构压电式能量采集器.理论分析了该采集器的固有振动特性,并通过有限元分析了结构尺寸...     

Simulation Guided Coaxial Electrospinning of Polyvinylidene Fluoride Hollow Fibers with Tailored Piezoelectric Performance

Electrospun polyvinylidene fluoride (PVDF) piezoelectric fibers have high potential applicability in mechanical energy harvesting and self-powered sensing owing to their high electromechanical coupling capabilities. Strategies for tailoring fiber morphology have been the primary focus for realizing enhanced piezoelectric output. However, the relationship between piezoelectric performance and fiber structure remains unclear. This study fabricates PVDF hollow fibers through coaxial electrospinning, whose wall thickness can be tuned by changing the internal solution concentration. Simulation analysis demonstrates an increased effective deformation of the hollow fiber as enlarging inner diameter, resulting in enhanced piezoelectric output, which is in excellent agreement with the experimental results. This study is the first to unravel the influence mechanism of morphology regulation of a PVDF hollow fiber on its piezoelectric performance from both simulation and experimental aspects. The optimal PVDF hollow fiber piezoelectric energy harvester (PEH) delivers a piezoelectric output voltage of 32.6 V, ≈3 times that of the solid PVDF fiber PEH. Furthermore, the electrical output of hollow fiber PEH can be stably stored in secondary energy storage systems to power microelectronics. This study highlights an efficient approach for reconciling the simulation and tailoring the fiber PEH morphology for enhanced performances for future self-powered systems.     

Bimorph piezoelectric vibration energy harvester with flexible 3D meshed-core structure for low frequency vibration

This paper proposes a bimorph piezoelectric vibration energy harvester (PVEH) with a flexible 3D meshed-core elastic layer for improving the output power while lowering the resonance frequency. Owing to the high void ratio of the 3D meshed-core structure, the bending stiffness of the cantilever can be lowered. Thus, the deflection of the harvester and the strain in the piezoelectric layer increase. According to vibration tests, the resonance frequency is 15.8% lower and the output power is 68% higher than in the conventional solid-core PVEH. Compared to the solid-core PVEH, the proposed meshed-core PVEH (10 mm × 20 mm × 280 μm) has 1.3 times larger tip deflection and the maximum output power is 24.6 μW under resonance condition at 18.7 Hz and 0.2G acceleration. Hence it can be used as a power supply for low-power-consumption sensor nodes in wireless sensor networks.     

SH-waves at a corrugated interface between a dry sandy half-space and an anisotropic elastic half-space

Summary A problem of reflection and transmission of a plane SH-wave incident at a corrugated interface between a dry sandy half-space and an anisotropic elastic half-space is investigated. Rayleigh's method of approximation is applied to derive the reflection and transmission coefficients for the first and second order approximation of the corrugation. The expressions for reflection and transmission coefficients for the first order approximation of the corrugation are obtained in closed form for a special type of interface, and the energy partition relation is derived. It is found that these coefficients are proportional to the amplitude of corrugation and are functions of elastic properties of the half-spaces and also of the angle of incidence. Numerical examples illustrating the effects of the sandiness, the anisotropy, the corrugation of the interface, the frequency, and the angle of the incidence on the coefficients are presented.     

Analysis of the left-handed corrugated circular waveguide

Infinite circular corrugated waveguide is analysed to investigate its ability to support modes with backward wave behaviour. Such waveguides provide an alternative structure, easier to manufacture than those already reported based on rectangular symmetry with corrugated walls or filled with frequency selective surfaces. The corrugations if sufficiently deep provide a guiding structure with the required series capacitance and shunt inductance to allow left-handed propagation within some frequency bands. These backward waves are analysed using the surface impedance model of propagation in corrugated waveguides to predict their properties. Interpreting the physical meaning of the analysis, the authors discuss how backward waves are related to resonances in corrugated structures. The relationship between power flows in the guide and the behaviour of the group velocity for such guides is shown. A full wave simulator is also applied to validate these results and the case of a dielectric filled waveguide is considered showing the improved ability to support left-handed modes. The authors present the results of a parametric study of how left-handed propagation depends on the corrugation depth. Potential applications of backward waves in corrugated circular waveguides are proposed.     

An experimental study into the evaporation heat transfer and flow characteristics of R-134a refrigerant flowing through corrugated tubes

The effects of corrugation pitch on the evaporation of R-134a flowing inside horizontal corrugated tubes are investigated. The test section is a tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water flowing in the annulus. Smooth and corrugated tubes having similar inside diameters are used as the inner tube. Three corrugated tubes having a corrugation depth fixed at 1.5 mm and corrugation pitches of 5.08, 6.35 and 8.46 mm are examined. The data obtained from smooth tube are plotted and compared with the flow pattern map established by Censi et al. (2003) and Zurcher et al. (2002). The effects of average vapour quality, equivalent Reynolds number and corrugation pitch are discussed. The maximum ratios of heat transfer enhancement factor to pressure drop penalty factor (Nu_c/Nu_s)/((ΔP/L)_c/(ΔP/L)_s) is approximately 1.2.     

楞型对瓦楞结构材料瓦楞方向准静态力学性能的影响

目的研究不同楞型瓦楞结构材料在准静态条件下对瓦楞方向相关力学性能的影响。方法通过有限元模拟的方法,在准静态压缩条件下,得到不同楞型的瓦楞结构材料在瓦楞方向上的变形模式、应力-应变曲线等,通过能量效率法对其峰应力、密实化应变、平均抗压强度和单位体积吸收能量等进行对比分析。结果在同一壁厚条件下,A,C,B,E这4种楞型的峰应力、平均抗压强度、单位体积吸收能量依次增大;对于任一楞型来说,峰应力、平均抗压强度、单位体积吸收能量随壁厚的增大而增大,且与其呈线性关系;随着壁厚的增大,A,C,B,E这4种楞型的峰应力、平均抗压强度、单位体积吸收能量的增长幅度依次增大。结论楞型对瓦楞结构材料瓦楞方向的力学性能有显著影响,在其他条件相同的情况下,A,C,B,E这4种楞型的力学性能依次增强。     

Optimal design of cubic nonlinear energy harvester device for random vibrations

Linear energy harvesters have a narrow frequency bandwidth and hence operate efficiently only when the excitation frequency is very close to the fundamental frequency of the harvester. Consequently, small variations of the excitation frequency around the harvester’s fundamental frequency drop the energy output making the harvesting process inefficient. To extend the harvester’s bandwidth, some recent solutions call for using electromechanical devices with stiffness-type nonlinearities. This work deals with the optimisation of the performance of a single degree-of-freedom electromagnetic energy harvester whose mechanical behaviour has a Duffing-type nonlinearity, as for suspended masses, to reduce the size of energy harvesting devices without affecting their power output. The vibration input is assumed as a broadband Gaussian white noise base acceleration. It is analytically shown that the optimum load resistance of the device is different to that which is dictated by the principle of impedance matching.     

Corrugation Reinforced Composites: A Method for Filling Holes in Material‐Property Space

Material‐property space is filled with holes representing desirable combinations of properties, such as high strength and high necking strain. One way to fill those holes is to use architectured materials. In this work, Finite Element Modeling (FEM) simulations are performed to evaluate composites with a corrugated reinforcement architecture across a range of volume fractions and corrugation heights for a model copper‐steel system. The corrugated reinforcement geometry shows large improvements in necking strain, which increases with corrugation height, without sacrificing strength, and fills a desirable region in material‐property space. Additionally, it is found that the necking strain of a matrix material can be increased by adding a less ductile reinforcing material provided it has a highly corrugated geometry. The improvement in necking strain seen in these composites is attributed to a boost in work hardening that results from an evolving reinforcement alignment as the corrugation unbends.