Electret-based Vibration Energy Harvesters Research Articles

AI-driven Discovery of Amorphous Fluorinated Polymer Electret with Improved Charge Stability for Energy Harvesting.

Electret materials are promising dielectric materials with trapped charges for various applications such as vibration energy harvesters and acoustic transducers. In the present work, we discovered ionization potential as the descriptor to quantify the charging performance for amorphous fluorinated polymer electrets. Using this descriptor, high-throughput computations, and graph neural network models, we screened 1,176,591 functional groups on the cyclic transparent optical polymers (CYTOP) and identified 3 promising electrets. The electrets were synthesized experimentally as 15-μm-thick films. The films were able to keep their both bipolar surface potentials above ±3.1kV for over 1500 hours and are estimated to have longevity of 146 years under 80°C, achieving significant improvements on charging stability among CYTOP-based polymer electrets. The excellent bipolar charging performance can greatly enhance power generation capacity of electret-based vibration energy harvesters. This work also demonstrates the use of deep learning as a new paradigm for accelerating practical materials discovery. This article is protected by copyright. All rights reserved.

Recent Progress on Mechanical Optimization of MEMS Electret-Based Electrostatic Vibration Energy Harvesters

In recent years, self-powered energy harvesting technology has been widely studied, which may provide supplementary power for wireless sensing networks with low power consumptions. The energy harvester could reduce the use of conventional power supply of batteries, and even eliminate the challenge of battery replacement for large scale networks. In this review, the fundamentals and theories of energy conversion from mechanical vibration to electrical current via variable capacitor are introduced, followed by the discussion of frequency bandwidth broadening, structural parameters, such as surface potential, initial air gap, and stopper heights, as the aspects for controlling squeeze-film air damping force and the output power. Representative examples of the design optimizations of vacuum operating pressure and perforated holes on bottom electrode are presented. Also, special electret processing and built-in corona needles for enhanced space charge stability and self-recharging capability are briefed. This review can provide the research community with a better understanding of the MEMS-compatible electrostatic vibration energy harvester, and further facilitate its development for future potential applications.

Optimization of MEMS Vibration Energy Harvester With Perforated Electrode

In this paper, electret based vibration energy harvesters are designed and fabricated with perforated electrode based on MEMS technology. Through-holes are distributed onthe fixed electrode in the device to optimize the energy harvesting process. The effect of the holes on the output power of the device are analyzed and discussed both in the finite element method (FEM) simulation and the experiments. It can be noticed that the through-holes can effectively lower the squeeze-film air damping force on the moveable proof mass at the atmosphere. Therefore, the energy loss due to the air damping could be reduced, and the output power of the device increases. Theeffects of the hole diameter and numbers on the output power of our device are also investigated in detail. By optimizing the configuration of the holes, the perforated device with the hole diameter of 400μm and depth of 100μm exhibits the highest power output at the low acceleration of 1.84 m/s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ,which proves promising application for self-powered electronics in the future.

Modelling and analysis of an out-of-plane electret-based vibration energy harvester with AC and DC circuits

An electret-based vibration energy harvester (eVEH) is a promising candidate to convert the environmental vibration into electricity efficiently. Although numerous researches exist on the structural design and applications of eVEHs, the analytical models for eVEHs to predict the outputs with different interface circuits are still lacking. In this paper, we endeavour to develop the theoretical methods to analyse the performance of eVEHs interfaced with both AC and DC circuits. Approximate analytical solutions for the models with AC and DC circuits are developed with harmonic balance method and Newton-Raphson harmonic balance method, respectively. The theoretical models and the derived approximate analytical solutions are verified by circuit simulation and experiment. The models could be utilized for estimating the steady-state performance of an eVEH without simulating the time-domain response and the power loss on diodes with different forward voltage drops and reverse current leakages in DC circuit are investigated theoretically and experimentally. The work of this paper could be utilized for theoretical analysis and optimal design of eVEHs in out-of-plane mode and set a milestone for developing more sophisticate interface circuits for eVEHs.

Modelling and Validation of Electret-Based Vibration Energy Harvesters in View of Charge Migration

One of the key problems of electret-based vibration energy harvester (eVEH) modelling is that the surface voltage itself cannot precisely reveal the charge storage characteristics of the electret fabricated by different processes. In this paper, we endeavor to interpret the charge migration mechanism of the eVEH with electrets fabricated by different processes. Based on the above analysis, a unified analytical model of the eVEH is derived accordingly. The dynamic response and voltage output predicted by this model are verified numerically by the equivalent circuit simulation and experimentally with an out-of-plane eVEH prototype. According to the analysis, the maximum power output is approximately 255 μW when the prototype works at its resonance frequency and the vibration amplitude is 1 mm. This paper elucidates the working principle of the eVEH and provides a framework for further theoretical study of eVEHs from the first principle.

A broadband electret-based vibrational energy harvester using soft magneto-sensitive elastomer with asymmetrical frequency response profile

This letter reports a broadband electret-based vibrational energy harvester (eVEH) targeting at applications in low frequency (4–10 Hz) and low excitation (0.1–0.2 g) scenarios. A soft magneto-sensitive elastomer (SMSE) cantilever with low resonance and strong softening nonlinear behaviour is utilized as the broadband oscillator. A spring suspended end-stopper attached to the electret converts low frequency excitation of the SMSE cantilever to high frequency vibrations of the eVEH for efficiency enhancement. An asymmetrical frequency response profile for the SMSE cantilever is intentionally designed to preserve its softening behaviour for broadband performance when the end-stopper is engaged. A proof-of-concept prototype is fabricated and the experiment shows that within a working frequency range of 4.0–8.2 Hz, the eVEH can harvest the average power output of 40.23 μW under a base excitation of 0.2 g.

A bipolar electret-based micro-vibration-driven energy harvester with multi-air-gap structure

Micro mechanical energy harvester as a kind of micro power source for micro electro mechanical systems (MEMS) has been paid considerable attention. Electret-based vibration energy harvester (E-MVEH) has been a research highlight due to its high sensitivity, simplicity and compatibility to the fabrication process of MEMS. In this manuscript, a novel E-MVEH with multiple-air-gap structure based on bipolar electrets is comparatively investigated. The E-MVEH with single-air-gap structure consists of an electret membrane (EM) with an electrode on one surface and a counter-electrode supported by means of micro-springs. The E-MVEH with double air gaps is made of a bipolar electret membrane put in the middle of two counter-electrodes that are supported by means of micro-springs. It is essentially equivalent to two variable capacitors in series. The E-MVEH with four air gaps consists of two double-air-gaps E-MVEH in parallel connection. When an external vibration causes a relative displacement between the electret membrane (EM) and the counter-electrode, the capacitance varies, leading to re-organization of the charges on the counter-electrode. Thus, an alternating current is generated and the vibration energy is converted into electric energy. This alternating current can be changed into direct current through a full bridge rectifier. The bipolar EM used in this study is a sandwich FEP/THV/FEP perfluoropolymer EM reported previously by our group. It can display persistent and strong electrostatic field with both polarities on the top and bottom surface, respectively. The experimental results of the output characteristic for the proposed E-MVEH show that there exists a resonant frequency in the vibration frequency range of 30−180 Hz, at which the open-circuit voltage and short-circuit current and hence the output power reach their respective maximum. The resonant frequency moves towards the low frequency with increasing vibration intensity, whereas the effect of air-gap number on the resonant frequency is rather small. On the basis of experimental results and theoretical analysis, the maximum output power is positively correlated to the vibration driving force, the surface potential and the area of the electret membrane, and the air-gap number. When the surface potential of the electret membrane is 1200±50 V, the maximum output power is only 0.19 μW for single-air-gap structure and reaches 27.02 μW for the four-air-gap structures under a vibration driving force of 0.5 N and a load resistance of 55 MΩ. For the four-air-gap structures, the maximum output power increases to 59.12 μW when the surface potential of the electret membrane increases to 2150 V. The multi-air-gap energy harvester fabricated with bipolar electret membrane can produce larger variable capacitance; therefore, the output power is significantly enhanced. This novel E-MVEH provides an up-to-date strategy for the design of self-powered sensor systems.

Electret Length Optimization of Output Power for Double-End Fixed Beam Out-of-Plane Electret-Based Vibration Energy Harvesters

Thanks to miniaturization, it is now possible to imagine self-powered systems that can harvest energy from the environment to produce electrical energy. Out-of-plane electret-based vibration energy harvesters (E-EVHs) are an effective and inexpensive energy harvester type that has attracted much attention. Increasing the capacitance of variable capacitors is an effective way to improve the output power of E-EVHs. In this paper, firstly an accurate capacitance theoretical model of a double-ended fixed beam out-of-plane E-EVHs which has 97% reliability compared with FEM (COMSOL Multiphysics) results is presented. A comparison of capacitance between the double-ended fixed beam structure and a cantilever structure of the same size indicates that the double-ended fixed beam structure has greater capacitance and capacitance variation. We apply this theoretical capacitance model to the mechanical-electrical coupling model of double-ended fixed beam out-of-plane E-EVHs and study harvesters’ output performances for different electret lengths by numerical and experimental method, respectively. There exists an optimal electret length to harvest maximum power in our simulation results. Enhanced electrostatic forces with increasing the electret length emphasizes the soft spring effect, which widens the half power bandwidth and lowers the resonance frequency. Increasing the length of the electret can reduce the resistance of the optimum load. The experimental results show trend consistent with the numerical predictions. The maximum output power can reach 404 µW (134.66 µW/cm2/g) at the electret length of 40 mm when the external acceleration and the frequency were 5 m/s2 and 74 Hz, respectively. The maximum bandwidth reaches 2.5 Hz at the electret length of 60 mm. Therefore, the electret length should be placed between 40 mm and 60 mm, while ensuring a higher output power and also get a larger bandwidth in practical applications.

Equivalent-circuit models for electret-based vibration energy harvesters

This paper presents a complete analysis to build a tool for modelling electret-based vibration energy harvesters. The calculational approach includes all possible effects of fringing fields that may have significant impact on output power. The transducer configuration consists of two sets of metal strip electrodes on a top substrate that faces electret strips deposited on a bottom movable substrate functioning as a proof mass. Charge distribution on each metal strip is expressed by series expansion using Chebyshev polynomials multiplied by a reciprocal square-root form. The Galerkin method is then applied to extract all charge induction coefficients. The approach is validated by finite element calculations. From the analytic tool, a variety of connection schemes for power extraction in slot-effect and cross-wafer configurations can be lumped to a standard equivalent circuit with inclusion of parasitic capacitance. Fast calculation of the coefficients is also obtained by a proposed closed-form solution based on leading terms of the series expansions. The achieved analytical result is an important step for further optimisation of the transducer geometry and maximising harvester performance.

Optimization for output power and band width in out-of-plane vibration energy harvesters employing electrets theoretically, numerically and experimentally

In order to improve the performances of out-of-plane electret-based vibration energy harvesters (E-VEHs) in practical environment, we investigated the dependence of output power, resonance frequency and half power bandwidth on the load resistance for E-VEHs theoretically, numerically and experimentally. A linear analytical model is presented to describe the characteristics of E-VEHs qualitatively and the frequency response function of current versus acceleration excitation is obtained by Fourier transform when the ratio of mass amplitude to air gap is less than 0.28. In particular, the coupling effect of the electrostatic force not only changes E-VEHs’ effective stiffness but also changes it’s effective damping. The analytical and numerical investigation predicted the following results: (1) an optimum value exists in the load resistance to maximize the output power; (2) enhanced electrostatic forces with decreasing the load resistance emphasize the soft spring effect, which lowers the resonance frequency; (3) a load resistance exists to maximize the half power bandwidth. A small out-of-plane E-VEHs prototype was fabricated in this paper in order to verify our predictions. The experimental results showed behaviors consistent with the numerical predictions. The output power reach a maximum 0.028 mW during frequency up-sweep and 0.0274 mW during frequency down-sweep at optimum load resistance 60 MΩ when the external acceleration were 1 ms−2 and 82.3 Hz, respectively. When increasing the load resistance to 330 MΩ, the half power bandwidth increased to 3.1 Hz during frequency up-sweep and 3.2 Hz during frequency down-sweep, increased 58.8 and 52.9% over optimum load resistance 60 MΩ, respectively. The peak output power at load resistance 330 MΩ was 0.0187 mW during frequency up-sweep and 0.0179 mW during frequency down-sweep, when the external acceleration were 1 ms−2 and 82.3 Hz, respectively. Therefore, the load resistance should be placed between 60 and 330 MΩ, while ensuring a higher output power can also get a larger bandwidth in practical applications.

Enhanced electrostatic vibrational energy harvesting using integrated opposite-charged electrets

This paper presents a sandwich-structured MEMS electret-based vibrational energy harvester (e-VEH) that has two opposite-charged electrets integrated into a single electrostatic device. Compared to the conventional two-plate configuration where the maximum charge can only be induced when the movable mass reaches its lowest position, the proposed harvester is capable of creating maximum charge induction at both the highest and the lowest extremes, leading to an enhanced output performance. As a proof of concept, an out-of-plane MEMS e-VEH device with an overall volume of about 0.24 cm3 is designed, modeled, fabricated and characterized. A holistic equivalent circuit model incorporating the mechanical dynamic model and two capacitive circuits has been established to study the charge circulations. With the fabricated prototype, the experimental analysis demonstrates the superior performance of the proposed sandwiched e-VEH: the output voltage increases by 80.9% and 18.6% at an acceleration of 5 m s−2 compared to the top electret alone and bottom electret alone configurations, respectively. The experimental results also confirm the waveform derivation with the increase of excitation, which is in good agreement with the circuit simulation results. The proposed sandwiched e-VEH topology provides an effective and convenient methodology for improving the performance of electrostatic energy harvesting devices.

Nonlinear restoring force of spring with stopper for ferroelectric dipole electret-based electrostatic vibration energy harvesters

Previously, we succeeded in developing a new electret [termed a ferroelectric dipole electret (FDE)] having an extremely high electric field using a polarized ferroelectric material. However, the pull-in, in which an oscillator sticks to the FDE under its strong electrostatic force, poses a problem for practical vibration energy harvesters. In this study, we propose use of nonlinear restoring force of a spring with a stopper in order to prevent pull-in for FDE-based vibration energy harvesters. The spring with a stopper was designed using a finite element method (FEM) analysis such that the restoring force of the spring will exceed the electrostatic force of the FDE. The proposed harvester combines the FDE and the spring successfully, and generated electricity without the pull-in. It also showed the highest figure of merit of output power and wide frequency band when compared with other available electret-based vibration energy harvesters.

Out-of-plane electret-based MEMS energy harvester with the combined nonlinear effect from electrostatic force and a mechanical elastic stopper

This paper presents the fabrication, modeling and characterization of an out-of-plane electret-based vibrational energy harvester (e-VEH) that has both positive and negative charged electret plates integrated into a single seismic mass system. Strong electrostatic spring-softening effect is induced due to the electric field provided by the double-charged electret plates. An elastic stopper is introduced for reliability concern by limiting the motion of seismic mass and meanwhile serves as a functional element to realize spring-hardening effect. The device has an overall volume of about 0.14 cm3 and is fabricated based on MEMS compatible silicon micromachining technology. When subject to weak excitations, the device exhibits an approximately linear frequency response but changes to a significantly larger broadband when strongly excited due to the combined nonlinear effect from electrostatic force and a mechanical elastic stopper. At a high excitation level of 0.48 g, the experimental results show that the device has 3 dB bandwidths of 3.7 Hz for frequency-up sweep and 2.8 Hz for frequency-down sweep, respectively, which demonstrate a large enhancement compared to the linear response (1.3 Hz). An optimal output power of 0.95 μW is also achieved with a low resonance of 95 Hz. This corresponds to a normalized power density of 37.4 μW cm−3 g−2.

Shock reliability analysis and improvement of MEMS electret-based vibration energy harvesters

Vibration energy harvesters can serve as a replacement solution to batteries for powering tire pressure monitoring systems (TPMS). Autonomous wireless TPMS powered by microelectromechanical system (MEMS) electret-based vibration energy harvester have been demonstrated. The mechanical reliability of the MEMS harvester still has to be assessed in order to bring the harvester to the requirements of the consumer market. It should survive the mechanical shocks occurring in the tire environment. A testing procedure to quantify the shock resilience of harvesters is described in this article. Our first generation of harvesters has a shock resilience of 400 g, which is far from being sufficient for the targeted application. In order to improve this aspect, the first important aspect is to understand the failure mechanism. Failure is found to occur in the form of fracture of the device’s springs. It results from impacts between the anchors of the springs when the harvester undergoes a shock. The shock resilience of the harvesters can be improved by redirecting these impacts to nonvital parts of the device. With this philosophy in mind, we design three types of shock absorbing structures and test their effect on the shock resilience of our MEMS harvesters. The solution leading to the best results consists of rigid silicon stoppers covered by a layer of Parylene. The shock resilience of the harvesters is brought above 2500 g. Results in the same range are also obtained with flexible silicon bumpers, which are simpler to manufacture.

Modeling and characterization of electret based vibration energy harvesters in slot-effect configuration

The purpose of this article is to elaborate a model and the optimization guidelines for electret based harvesters with a specific electret/electrodes configuration, namely the slot-effect configuration. Slot-effect configured harvesters have been investigated experimentally by several research groups. A model describing their dynamic behavior has also been recently proposed in the literature. However, the simplifications used in the existing model can lead to inaccuracies and a refined analysis is elaborated in the present article. The model is compared with experimental measurements on MEMS fabricated devices with a corrugated electret. The electrodes dimensioning in the MEMS device are chosen so that the harvester behaves in a quasi-linear manner over its full range of displacement. This quasi-linearity simplifies greatly the device optimization. Indeed, the behavior of the developed electrostatic harvester is shown to be very comparable to that of piezoelectric harvesters, which are very well understood and documented. The influence of several design parameters on output power performance is investigated. As long as pull-in and breakdown voltage effects can be avoided, the electret surface potential should be maximized and the air gap minimized. We also investigate theoretically the influence of three types of electret on the generated power: planar, corrugated with partial charge coverage, and corrugated with full charge coverage. With the dimensions corresponding to our MEMS devices, the output power characteristics for the three types of electret are similar. However, it is shown that this is not always true. In some conditions, corrugated electrets with full charge coverage are detrimental for the generated power.

Improved mechanical reliability of MEMS electret based vibration energy harvesters for automotive applications

Current commercial wireless tire pressure monitoring systems (TPMS) require a battery as electrical power source. The battery limits the lifetime of the TPMS. This limit can be circumvented by replacing the battery by a vibration energy harvester. Autonomous wireless TPMS powered by MEMS electret based vibration energy harvester have been demonstrated. A remaining technical challenge to attain the grade of commercial product with these autonomous TPMS is the mechanical reliability of the MEMS harvester. It should survive the harsh conditions imposed by the tire environment, particularly in terms of mechanical shocks. As shown in this article, our first generation of harvesters has a shock resilience of 400 g, which is far from being sufficient for the targeted application. In order to improve this aspect, several types of shock absorbing structures are investigated. With the best proposed solution, the shock resilience of the harvesters is brought above 2500 g.

Performance enhancement of an out-of-plane electret-based vibrational energy harvester with dual charged plates

This paper presents a novel out-of-plane electret-based vibrational power generator (EVPG) that has both negative and positive charged electret plates integrated into a single seismic mass system. Compared with the conventional single-charged two-plate configuration, the proposed device not only exhibits an enhanced output voltage magnitude but also has a wide operational bandwidth due to spring softening nonlinearity according to the experimental analysis. With the acceleration changes from 0.1g to 0.5g, the operating 3-dB bandwidth can be increased 2.6 times from 2.5 Hz to 6.5 Hz, which indicates higher energy conversion efficiency than a linear energy harvester in the practical scenario of broadband random vibrations.

Electrostatic Force-induced Broadband Effect in Electret- based Vertical Vibration Energy Harvesters using Finegrained Stainless Steel Oscillator

We propose a fine-grained stainless-steel as a promising material for a robust oscillator and investigate the dependence of frequency band width, resonance frequency, and output power on initial air gaps in electret-based vertical vibration energy harvesters. Beams of the oscillator showed a shallow side-etched depth less than 10 μm, as well as smooth edges. The oscillator succeeded in travelling over 1-mm displacement without fracture. Also, we found that broader frequency band, as well as lower resonance frequency, can be achieved with reducing the initial air gap, whereas the output power exhibited a peak value at an optimal initial air gap. The results may be attributed to the soft spring effect induced by the stronger electrostatic force. Maximum output power density and FWHM of frequency band width of our harvester are 4.7 μW/cm3 and 14 Hz at initial air gap 0.3 mm and acceleration 4.9 m/s2.

Design improvements for an electret-based MEMS vibrational electrostatic energy harvester

This paper presents several improvements to the design of an electret-based MEMS vibrational electrostatic energy harvester that have led to a two orders of magnitude increase in power compared to a previously presented device. The device in this paper has a footprint of approximately 1 cm2 and generated 175 μW. The following two improvements to the design are discussed: the electrical connection principle of the harvester and the electrode geometrical configuration. The measured performance of the device is compared with simulations. When exited by sinusoidal vibration, a device employing the two design improvements but with a higher resonance frequency and higher electret potential generated 495 μW AC power, which is the highest reported value for electret-based MEMS vibrational electrostatic energy harvesters with a similar footprint. This makes this device a promising candidate for the targeted application of wireless tire pressure monitoring systems (TPMS).

Analysis of Electret-Based MEMS Vibrational Energy Harvester With Slit-and-Slider Structure

This paper describes an analysis and a performance limit of a vibrational energy harvester with a novel slit-and-slider structure. This structure has a separable electret and microelectromechanical systems (MEMS) parts. In the MEMS parts, movable electrodes slide due to external vibration and receive electrical field that is periodically modulated by slits of fixed electrodes. The structure was fabricated based on MEMS technology and produced an ac current of 170 pA with an external vibration of amplitude of 1 m/s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at a frequency of 1166 Hz. Since the structure is separable, individual characterization of the electret and movable electrodes was performed. On the basis of their quantitative analyses, a structural model was constructed and validated. The model showed a way to optimize structural and material parameters for enhancement of output power and predicted a performance limit of 2.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> μW and 6.1% as output power and harvester effectiveness, respectively. This value of effectiveness is comparable to that of conventional non-MEMS-based large energy harvester around 1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , which indicates feasibility of MEMS-based small energy harvesters around 0.01 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> by appropriate designing.