The last two decades have witnessed several advances in microfabrication technologies and electronics, leading to the development of small, low-power devices for wireless sensing, data transmission, actuation, and medical implants. Unfortunately, the actual implementation of such devices in their respective environment has been hindered by the lack of scalable energy sources that are necessary to power and maintain them. Batteries, which remain the most commonly used power sources, have not kept pace with the demands of these devices, especially in terms of energy density. In light of this challenge, the concept of vibratory energy harvesting has flourished in recent years as a possible alternative to provide a continuous power supply. While linear vibratory energy harvesters have received the majority of the literature’s attention, a significant body of the current research activity is focused on the concept of purposeful inclusion of nonlinearities for broadband transduction. When compared to their linear resonant counterparts, nonlinear energy harvesters have a wider steady-state frequency bandwidth, leading to a common belief that they can be utilized to improve performance in ambient environments. Through a review of the open literature, this paper highlights the role of nonlinearities in the transduction of energy harvesters under different types of excitations and investigates the conditions, in terms of excitation nature and potential shape, under which such nonlinearities can be beneficial for energy harvesting. [DOI: 10.1115/1.4026278]

On the Role of Nonlinearities in Vibratory Energy Harvesting: A Critical Review and Discussion

The dramatic reduction in power consumption of current integrated circuits has evoked great research interests in harvesting ambient energy, such as vibrations, as a potential power supply for electronic devices to avoid battery replacement. Currently, most vibration-based energy harvesters are designed as linear resonators to achieve optimal performance by matching their resonance frequencies with the ambient excitation frequencies a priori. However, a slight shift of the excitation frequency will cause a dramatic reduction in performance. Unfortunately, in the vast majority of practical cases, the ambient vibrations are frequency-varying or totally random with energy distributed over a wide frequency spectrum. Hence, developing techniques to increase the bandwidth of vibration-based energy harvesters has become the next important problem in energy harvesting. This article reviews the advances made in the past few years on this issue. The broadband vibration-based energy harvesting solutions, covering re...

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Toward Broadband Vibration-based Energy Harvesting

This paper reviews possible strategies to increase the operational frequency range of vibration-based micro-generators. Most vibration-based micro-generators are spring-mass-damper systems which generate maximum power when the resonant frequency of the generator matches the frequency of the ambient vibration. Any difference between these two frequencies can result in a significant decrease in generated power. This is a fundamental limitation of resonant vibration generators which restricts their capability in real applications. Possible solutions include the periodic tuning of the resonant frequency of the generator so that it matches the frequency of the ambient vibration at all times or widening the bandwidth of the generator. Periodic tuning can be achieved using mechanical or electrical methods. Bandwidth widening can be achieved using a generator array, a mechanical stopper, non-linear (e.g. magnetic) springs or bi-stable structures. Tuning methods can be classified into intermittent tuning (power is consumed periodically to tune the device) and continuous tuning (the tuning mechanism is continuously powered). This paper presents a comprehensive review of the principles and operating strategies for increasing the operating frequency range of vibration-based micro-generators presented in the literature to date. The advantages and disadvantages of each strategy are evaluated and conclusions are drawn regarding the relevant merits of each approach.

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Strategies for increasing the operating frequency range of vibration energy harvesters: a review

The last decade has witnessed significant advances in energy harvesting technologies as a possible alternative to provide a continuous power supply for small, low-power devices in applications, such as wireless sensing, data transmission, actuation, and medical implants. Piezoelectric energy harvesting (PEH) has been a salient topic in the literature and has attracted widespread attention from researchers due to its advantages of simple architecture, high power density, and good scalability. This paper presents a comprehensive review on the state-of-the-art of piezoelectric energy harvesting. Various key aspects to improve the overall performance of a PEH device are discussed, including basic fundamentals and configurations, materials and fabrication, performance enhancement mechanisms, applications, and future outlooks.

A comprehensive review on piezoelectric energy harvesting technology: Materials, mechanisms, and applications

In an effort to eliminate the replacement of the batteries of electronic devices that are difficult or impractical to service once deployed, harvesting energy from mechanical vibrations or impacts using piezoelectric materials has been researched over the last several decades. However, a majority of these applications have very low input frequencies. This presents a challenge for the researchers to optimize the energy output of piezoelectric energy harvesters, due to the relatively high elastic moduli of piezoelectric materials used to date. This paper reviews the current state of research on piezoelectric energy harvesting devices for low frequency (0–100 Hz) applications and the methods that have been developed to improve the power outputs of the piezoelectric energy harvesters. Various key aspects that contribute to the overall performance of a piezoelectric energy harvester are discussed, including geometries of the piezoelectric element, types of piezoelectric material used, techniques employed to match the resonance frequency of the piezoelectric element to input frequency of the host structure, and electronic circuits specifically designed for energy harvesters.

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Energy harvesting from low frequency applications using piezoelectric materials

We analyze temperature-induced frequency shift in a thickness mode bulk acoustic wave resonator with a layer of another material for temperature compensation. The perturbation integral by Tiersten is used to calculate frequency shifts in the resonator under a temperature change. It is shown that, with a proper design of the compensation layer, temperature sensitivity of the resonator can be reduced or made zero.

Analysis of Temperature Compensation in a Plate Thickness Mode Bulk Acoustic Wave Resonator

A two-dimensional model was established to study the dynamic characteristics of a quartz crystal resonator with the upper surface covered by an array of hemispherical material units. A frequency-dependent equivalent mass ratio was proposed to simulate the effect of the covered units on frequency shift of the resonator system. It was found that the equivalent mass ratio alternately becomes positive or negative with change of shear modulus and radius of each material unit, which indicates that the equivalent mass ratio is strongly related to the vibration mode of the covered loadings. The further numerical results show the cyclical feature in the relationship of frequency shift and shear modulus/radius as expected. The solutions are useful in the analysis of frequency stability of quartz resonators and acoustic wave sensors.

Frequency shift of a crystal quartz resonator in thickness-shear modes induced by an array of hemispherical material units

We propose a system to transmit and store electric energy by using transmitting element, a chargeable battery, and a
rectifier together with a dc-dc converter to connect the two components as an integrated system. The transmitting
element is modeled by two piezoelectric transducers. One is as the driving transducer for generating acoustic wave; the
other is as the receiving transducer for converting the acoustic energy into electric energy. A dc-dc converter employed
in the storage circuit is to match the optimal output voltage of the receiving transducer with the battery voltage for
efficient charging. A synchronized switch harvesting on inductor (SSHI) in parallel with the receiving transducer is
introduced to artificially extend the closed circuit interval of the rectifier. This analysis extends a previous one by
considering that influence of wall thickness which always exists in the application. The characteristics of the
energy-transmitting element are studied. Performance of the energy-transmitting element is optimized by synthetic
adjusting parameters of the element, and carefully choosing input frequency of electric source.

Wireless energy transmission through a sealed wall using the acoustic-electric interaction of piezoelectric ceramics

An improved harvester available in scavenging energy from the operating environment with either weaker or stronger vibration levels is studied. To ensure the optimal harvester performance, a Cuk dc-dc converter is employed into the modulating circuit. This paper reports how this harvester scavenges maximal energy from varying-level vibrations and store energy into an electrochemical battery. Dependence of the duty cycle upon the external vibration level is calculated, and it is found that: 1) for weaker vibrations, the charging current into the battery is smaller than the allowable current, and thus all the optimal output power of the harvesting structure can be absorbed by the battery. In this case, the duty cycle should be fixed at 1.86%; 2) for stronger external forcing, the allowable charging current of the battery is smaller than the optimal harvested current. This indicates that just a portion of the scavenged energy can be accepted by the battery. Thus, the duty cycle should be decreased gradually with the increase of the vibration level. Finally the energy transmission process and the roles of each electronic element are analyzed. It is shown that a Cuk converter can greatly raise the efficiency of such a harvester, particularly when subjected to a weaker ambient vibration.

An improved piezoelectric harvester available in scavenging-energy from the operating environment with either weaker or stronger vibration levels

A theoretical model of piezoelectric phononic crystal with tuned band gaps has been proposed. The piezoelectric phononic crystal consists of a beam and periodic array of piezoelectric patches. Each pair of piezoelectric patches is connected in parallel by resonant shunting circuits independently. The effect of structure and material parameters on Bragg and locally resonant band gap has been investigated. The numerical results show Bragg band gap is tuned 34Hz by the axial force. Synchronization can be maintained spontaneously since it is not necessary to adjust parameter of every unit as the way that moving band gap by circuit parameter needs to change every shunting circuit synchronously. This property can enhance the applicability of phononic crystals.

Huan Xue

Papers

Analysis of Temperature Compensation in a Plate Thickness Mode Bulk Acoustic Wave Resonator

Frequency shift of a crystal quartz resonator in thickness-shear modes induced by an array of hemispherical material units

Wireless energy transmission through a sealed wall using the acoustic-electric interaction of piezoelectric ceramics

An improved piezoelectric harvester available in scavenging-energy from the operating environment with either weaker or stronger vibration levels

Band gap frequencies of piezoelectric phononic crystals tuned by axial force