Yuantai Hu

Broadband piezoelectric energy harvesting devices using multiple bimorphs with different operating frequencies

This paper presents a novel approach for designing broadband piezoelectric harvesters by integrating multiple piezoelectric bimorphs (PBs) with different aspect ratios into a system. The effect of 2 connecting patterns among PBs, in series and in parallel, on improving energy harvesting performance is discussed. It is found for multifrequency spectra ambient vibrations: 1) the operating frequency band (OFB) of a harvesting structure can be widened by connecting multiple PBs with different aspect ratios in series; 2) the OFB of a harvesting structure can be shifted to the dominant frequency domain of the ambient vibrations by increasing or decreasing the number of PBs in parallel. Numerical results show that the OFB of the piezoelectric energy harvesting devices can be tailored by the connection patterns (i.e., in series and in parallel) among PBs.

A piezoelectric power harvester with adjustable frequency through axial preloads

We study the technique to adjust the performance of a piezoelectric bimorph vibrating in the flexural mode through axial preloads, which is useful for a power harvester to effectively scavenge energy from ambient mechanical vibrations/noise with varying-frequency spectra. The external circuit connected to the bimorph is simplified as an impedance in the analysis. Analytical solutions are derived. The analyses show that resonance happens when the natural frequency of the bimorph is adjusted adjacent to the external driving frequency by preloading, and the output power density can be raised many more times in that case. The mechanism for an axial preload to improve the bimorph performance at varying-frequency vibrations is examined in detail.

Transmitting electric energy through a metal wall by acoustic waves using piezoelectric transducers

The feasibility of transmitting electric energy through a metal wall by propagating acoustic waves using piezoelectric transducers is examined by studying the efficiency of power transmission and its dependence upon the relevant system parameters for a simplified system consisting of an elastic plate sandwiched by two piezoelectric layers. One of these layers models the driving transducer for generating acoustic wave, and the other layer models the receiving transducer for converting the acoustic energy into electric energy to power a load circuit. The output voltage, the output power, and the efficiency of this system are expressed as explicit functions of the system parameters. A numerical example is included to illustrate the dependence of the system performance upon the physical and geometrical parameters.

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

Effects of electrodes with varying thickness on energy trapping in thickness-shear quartz resonators

We study the possibility of using electrodes of varying thickness for strong energy trapping in quartz thickness-shear resonators as an alternative for contoured resonators. A theoretical analysis is performed. Results show that nonuniform electrodes can produce strong trapping of thickness-shear modes

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.

Adjusting the resonant frequency of a PVDF bimorph power harvester through a corrugation-shaped harvesting structure

We propose a corrugated polyvinylidene fluoride (PVDF) bimorph power harvester with the harvesting structure fixed at the two edges in the corrugation direction and free at the other edges. The resonant frequency of a corrugated PVDF bimorph is readily adjusted through changing either its geometrical configuration or the span length, which can keep the harvester operating at the optimal state in environments with different ambient vibrations. The governing equations of a PVDF bimorph with a corrugation shape, are derived from the transfer-matrix technique. Statistical results show that the adaptability of a harvester to the operating environment can be improved greatly by designing the harvesting structure with adjustable resonant frequency.

A piezoelectric energy harvester based on flow-induced flexural vibration of a circular cylinder

This article proposes a new piezoelectric structure for energy harvesting from flow-induced vibrations. It consists of a properly poled and electroded flexible ceramic cylinder. When it is in a flow perpendicular to its axis, the flow exerts a transverse force on the cylinder due to asymmetric vortex shedding, which drives the cylinder into flexural vibrations with an electrical output. A one-dimensional model is derived for the motion of the cylinder, which allows an analytical solution from which the basic behaviors of the energy harvester are calculated and examined. For a cylinder of 40 cm in length and 1 cm in diameter in flowing air with a speed of 5 m/s, the output power is of the order of 10 − 3 W . It becomes significantly higher if the flow speed is increased.

Vibration of a thickness-twist mode piezoelectric resonator with asymmetric, nonuniform electrodes

We studied the effect of electrodes with varying thicknesses on thickness-twist modes in a piezoelectric plate resonator of crystals of 6 mm symmetry. The focus is on the effects of asymmetric electrodes, which do not seem to have been examined before. A theoretical analysis is performed using the theory of linear piezoelectricity. A trigonometric series solution is obtained that is relatively rare from the equations of piezoelectricity. Numerical calculations are made based on the series solution. Results show that asymmetric, nonuniform electrodes have a strong effect on vibration mode shapes. This effect offers the possibility of using nonuniform electrodes in design to achieve various goals. The quantitative results in this paper are useful for the purpose.

Theoretical modeling of a thickness-shear mode circular cylinder piezoelectric transformer

We propose a piezoelectric transformer operating with thickness-shear modes of a circular cylinder arid perform a theoretical analysis on the transformer. An exact solution from the three-dimensional equations of piezoelectricity is obtained. The output voltage, input admittance, and efficiency of the transformer are determined. The basic behaviors of the transformer are shown by numerical results