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Piezoelectric Energy Harvesting: a Systematic Review of Reviews

Jafar Ghazanfarian∗

Mechanical Engineering Department, Faculty of Engineering, University of Zanjan, P.O. Box 45195-313, Zanjan, Iran.

Mohammad Mostafa Mohammadi

Mechanical Engineering Department, Faculty of Engineering, University of Zanjan, P.O. Box 45195-313, Zanjan, Iran.

Kenji Uchino

International Center for Actuators and Transducers, The Pennsylvania State University, University Park, PA, 16802, USA

Abstract

In the last decade, an explosive attention has been paid to piezoelectric harvesters due to their flexibility in design andincreasing need to small-scale energy generation. As a result, various energy review papers have been presented bymany researchers to cover different aspects of piezoelectric-based energy harvesting, including piezo-materials, modelingapproaches, and design points for various applications. Most of such papers tried to shed light on recent progresses inrelated interdisciplinary fields, and to pave the road for future prospects of development of such technologies. However,there are some missing parts, overlaps, or even some contradictions in the review papers. In the present review ofreview articles, recommendations for future research directions suggested by the review papers have been systematicallysummed up under one umbrella. At the final section, topics for missing review papers, concluding remarks on outlooksand possible research topics, and strategy-misleading contents have been presented. The review papers have beenevaluated based on merits and subcategories and authors’ choice papers have been presented for each section based onclear classification criteria.

Highlights

• A comparative overview of reviews in the map ofpiezo-energy harvesting is presented.

• An extensive description of research lines for futureresearch is provided.

• Classification of reviews is presented based on sub-categories and merits.

• Authors’ choice papers are presented for each sec-tion.

Keywords

Energy harvesting; piezoelectric; energy conversion; re-newable energies; micro-electro-mechanical systems

Contents

1 Introduction 1

∗Corresponding author, Tel.: +98(241) 3305 4142. All authorscontributed equally to this work.

Email address: j.ghazanfarian@znu.ac.ir (JafarGhazanfarian)

2 Reviews with non-focused topics 3

3 Design and fabrication 5

3.1 Materials . . . . . . . . . . . . . . . . . . . 53.2 Structure . . . . . . . . . . . . . . . . . . . 103.3 MEMS/NEMS-based devices . . . . . . . . 143.4 Modeling approaches . . . . . . . . . . . . . 16

4 Applications 18

4.1 Vibration . . . . . . . . . . . . . . . . . . . 184.2 Biological sources . . . . . . . . . . . . . . . 204.3 Fluids . . . . . . . . . . . . . . . . . . . . . 224.4 Ambient waste energy sources . . . . . . . . 24

5 Challenges and the roadmap for future re-

search 25

1. Introduction

Due to recent developments of portable and wearableelectronics, wireless electronic systems, implantable med-ical devices, energy-autonomous systems, monitoring sys-tems, and MEMS/NEMS-based devices, the procedure ofsmall-scale generation of energy may lead to a revolutionin development of compact power technologies.

Preprint submitted to ? October 20, 2021

Figure 1 presents the output power density variationversus the actual motor power for 2000 commercial elec-tromagnetic motors. Electromagnetic motors are superiorfor the production of power levels higher than 100W. How-ever, because the efficiency is significantly dropped below100 W, the piezoelectric devices with power density in-sensitive to their size will replace battery-operated smallportable electronic equipment less than 50 W level. It isnot logical to compare the energy harvesting systems withthe MW power level. Hence, it is necessary for researchersto determine their original piezo-harvesting target, whichshould be basically the replacement of compact batteries,one of the toxic wastes in the sustainable society [1].

Figure 1: Comparison of the specific power with respect to thepower [1].

Dutoit et al. [2] provided a comparison based on thedensity of the output power, and indicated that the powerdensities of the fixed-energy density sources extensivelydrop after just 1 year of operation. So, they need mainte-nance and repair if possible. Designing an effective powernormalization scheme, strain cancelation due to multipleinput vibration components, optimizing the minimum vi-bration level required for positive energy harvesting, andthe prototype testing to eliminate the proof mass are amongthe suggestions as future works.

Advantages of the piezoelectric energy harnessing in-clude simple structure without several additional compo-nents, no need to moving parts or mechanical constraints,environment friendliness and being ecologically safe, porta-bility, coupled operation with other renewable energies,no need to an external voltage source, compatibility withMEMS, easy fabrication with microelectronic devices, rea-sonable output power density, cost effectiveness, and scal-ability. Hence, piezo-materials are an excellent candidateto replace batteries with short lifespan for powering macroto nanoscale electronic devices.

Piezo-materials can extract power directly from struc-tural vibrations or other environmental mechanical wasteenergy sources in infrastructures (bridges, buildings), biomed-ical systems, health care and medicine, and they can beused for transducers, actuators, and surface acoustic wave

device operation. Some disadvantages of the piezo-harvestersare high output impedance, producing relatively high out-put voltages at low electrical current, and rather large me-chanical impedance.

The number of review papers on piezoelectric energyharvesting has been extensively increased in the recentdecade. Due to the tremendous number of published re-view papers in this field, finding an appropriate reviewpaper became challenging. On the other hand, there arelots of overlaps, similarities, missing parts, and sometimescontradictions between different reviews. Therefore, themain motivation of the present paper is to present a sys-tematic review of the review papers on piezoelectric energyharvesting. We tried to summarize all deficits, advantages,and missing parts of the existing review papers on piezo-energy harvesting systems.

An extensive search among database sources identi-fied 91 review papers in diverse applications related to thepiezoelectric energy harvesting. As will be demonstratedlater, such papers have present different concluding re-marks for the area of usage, materials, design approaches,and mathematical models. We tried to perform a verydetailed searching procedure with several keywords andsearch engines to cover all published review papers, andto find the review papers without ”piezo” directly men-tioned in the title.

The statistics of publications during two recent decadesexcluding conference papers, extracted using the keyword”piezo AND energy harvesting” from SCOPUS are shownin Fig. 2. The results from SCOPUS included the over-all number of 4435 documents, containing 874 open accesspapers, 130 book chapters, and 36 books. The nationalnatural science foundation of China, the fundamental re-search funds for the central universities, and the nationalresearch foundation of Korea were the most frequent fund-ing sponsors. Most common subject areas were engineer-ing, material sciences, physics and astronomy, chemistry,and energy. An extrapolation shown in the figure antic-ipates publication of about 2500 articles per year duringthe coming three years.

Due to interdisciplinary nature of piezoelectric energyharvesting, prediction of behavior of the piezo-generatorsare related to different thermo-electro-mechanical sciencesas well as material engineering. We have illustrated a sys-tematic map of various aspects of piezo-energy harvest-ing in Fig. 3. Different branches of connected sciencesand applications include fabrication methods, hybrid sys-tems, performance evaluation, size, utilization methods,configurations, modeling aspects, economical points, en-ergy sources, optimization, design of an electric interface,and selection of proper materials. All sub-branches in thefigure will be discussed in subsections of the present paper.

A review article is not an omnibus of the paper col-lection. The review should be written for criticizing orpraising each paper. Evaluation of the review papers andtheir contribution to the field will be presented based on

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Year

Numberofpublications

2000 2005 2010 2015 20200

1000

2000

3000

4000

5000

Figure 2: Statistics and future estimation of publications on piezo-electric energy harvesting.

the following criteria:

1. Having a solid evaluation philosophy by the reviewer.

2. Presenting non-general future research directions inthe summary/conclusion of the paper.

3. Paying attention to the critical design aspects suchas electromechanical coupling factor or actual reso-nance frequency.

4. Many papers report the harvesting energy aroundthe resonance range. Though the typical noise vi-bration is in a much lower frequency range, the re-searchers measure the amplified resonance response(even at a frequency higher than 1 kHz).

5. If the harvested energy is lower than 1mW, whichis lower than the required electric energy to oper-ate a typical energy harvesting electric circuit witha DC/DC converter (typically around 2-3mW), it issomehow difficult to describe the system as an en-ergy harvesting device.

6. The complete energy flow or exact efficiency from theinput mechanical noise energy to the final electricalenergy in a rechargeable battery via the piezoelectrictransducer is an important part of the review fromthe applicational/industrial viewpoint.

7. Number of sub-fields covered in the review paper.

8. The review papers may provide enough theoreticalbackground of piezoelectric energy harvesting, prac-tical material selection, device design optimization,energy harvesting electric circuits to help readersprevent the ”Google syndrome” [3].

The scoring strategy is as follows: 1 point for the num-ber of conclusions reported, 1 point for the number of sub-categories covered, 2 points for paying attention to themerits, and 1 point for reporting the minimum required

energy output level. Details of scores for each parts ispresented in the tables inside brackets. The reviews withthe scores between 0-1, 1-2, 2-3, 3-4, 4-5, respectively arelabeled with E to A. It should be noted that the valueof minimum required output should be clearly addressedamong concluding remarks, conclusions, future directions,abstract, or introduction.

The outline of the paper is as follows. At the firstsection, the focus is on the reviews about the design pro-cess, structure, material considerations, size effects, andthe mathematical modeling challenges. At the second partof the article, the main theme will be evaluating applica-tions of the piezo-harvesters. The most common applica-tions include vibrational energy sources, fluid-based har-vesters, scavenging energy from ambient waste energies,and energy harnessing in biological applications. In thelast section, a summary of future challenges, research di-rections, and missing review topics will be presented.

2. Reviews with non-focused topics

The discussed papers in this section are general re-view articles without having a specific focal point. Safaeiet al. [4] presented a review of energy harvesting usingpiezoelectric materials during 2008 to 2018. This arti-cle is an update of their previous review [5], and cov-ers lead-free piezo-materials, piezoelectric single crystals,high-temperature piezoelectricity, piezoelectric nanocom-posites, piezoelectric foams, nonlinear and broadband trans-ducers, and micro-electro-mechanical transducers. Theyalso discussed several types of piezoelectric transducers,the mathematical modeling, energy conditioning circuitry,and applications such as fluids, windmill-style harvesters,flutter-style harvesters, from human body, wearable de-vices, implantable devices, animal-based systems, infras-tructure, vehicles, and multifunctional/multi-source en-ergy harvesting. Several useful illustrations have been pre-sented in the paper, which sum up different technologiesin a unified framework. However, their brief recommen-dations for future horizons in the field, including fabri-cation of piezoelectric nanofibers, piezoelectric thin films,printable piezoelectric materials, exploiting internal res-onance of structures, and development of metamaterialsand metastructures may be extended to cover other as-pects presented in Tab. 1.

Anton and Sodano [5] reviewed some general topicspublished between 2003 to 2006, discussing the efficiencyimprovement, configurations, circuitry and method of powerstorage, implantable and wearable power supplies, har-vesting from the ambient fluid flows, the micro-electro-mechanical systems, and the self-powered sensors withouta clear classification. They described that the future direc-tions are the development of a complete self-powered de-vice that includes a combination of power harvester, stor-age, and application circuitry. Also, they declared thatthe enhancement of energy generation and storage meth-ods along with decreasing the power requirements of elec-

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Figure 3: Strategic map of piezoelectric energy harvesting design aspects, modeling approaches, and applications.

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tronic devices may be a prime target. Taware and Desh-mukh [6] briefly reviewed a number of literature in the fieldof piezoelectric energy harvesting. They mentioned advan-tages and disadvantages of some of piezoelectric materials.They explained the cantilever-based piezoelectric energyharvesters, their related design points, and mathemati-cal modeling. Khaligh et al. [7] addressed the piezoelec-tric and electromagnetic generators suitable for human-powered and vibration-based devices, including resonant,rotational, and hybrid devices. A brief information hasbeen presented about the hybrid generators provided byan imbalanced rotor that needs more in-deep investiga-tions in future reviews. Sharma and Baredar [8] analyzedthe current methods to harvest energy from vibration us-ing a piezoelectric setup in low-range frequency zone byanalyzing piezoelectric material properties based on mod-eling and experimental investigations. They indicate thatthe disadvantages of the piezo-harvesters are depolariza-tion, sudden breaking of piezo layer due to high brittle-ness and poor coupling coefficient, poor adhesive prop-erties of PVDF material, and lower electromagnetic cou-pling coefficient of PZT. They discussed that the design ofhigh-efficiency energy harvesters, invention of new energyharvesting designs by exploring non-linear benefits, anddesign of portable compact-size systems with integratedfunctions are forthcoming challenges.

Mateu and Moll [9] presented an overview of severalmethods to design an energy harvesting device for mi-croelectronics depending on the type of available energy.They summarized the power consumption of the micro-electronic devices and explained the working principals ofpiezoelectric, electrostatic, magnetic induction, and elec-tromagnetic radiation-based generators. Calio et al. [10]reviewed the material properties of about 19 piezo-materials,the piezo-harvesters operating modes, resonant/non-resonantoperations, optimal shape of the beam, the frequency tun-ing, the rotational device configurations, the power densityand bandwidth, and the conditioning circuitry. They triedto present a selection guide between piezoelectric materialsbased on the power output and the operating modes. Theyconcluded that the resonant d33 cantilever beam needs tobe optimized and d15 harvester is still too complex to befabricated but has great potentials. This paper may bea good suggestion for beginners to start a research in thefield of piezoelectric energy harvesting. Batra et al. [11]reviewed mathematical modeling and constitutive equa-tions for piezo-materials, the lumped parameter model-ing, mechanisms of piezoelectric energy conversion, andoperating principles of the piezoelectric energy harvesters.Sun et al. [12] made a review on applications of piezoelec-tric harvesters. However, they put everything in a nut-shell. Such topics need more close considerations. A supershort review paper exists [13] that mainly has focused onsome points about the history of piezoelectric effect, piezo-materials, and applications like harvesting from footstepsand roads.

Although most of the aforementioned general review

papers have more or less similar titles, but their scien-tific depth and the number of reviewed items are different.Some papers like Ref. [10] have focused on design strate-gies of the piezoelectric energy harvesters. They try topresent a guide for the selection of piezoelectric materialsas harvesters. Moreover, almost all the mentioned reviewssuffer from weak classifications stemming from generalityof their topic.

The results of evaluation of generally-written reviewpapers on piezoelectric energy harvesting have been pre-sented in Table 1. The table contains different sub-categories,the range of output power, the number of reviewed articles,the merits, general conclusions, and some other extra de-scriptions. The grade for each paper has been computedbased on the number of merits, the number of subcate-gories, the number of concluding remarks, and declarationof minimum required output power.

3. Design and fabrication

3.1. Materials

The choice of suitable piezoelectric material is a crit-ical step in designing energy harvesters [14]. Thus, lotsof the review papers in the field of energy harvesters lessor more have addressed the piezoelectric materials. Dif-ferent performance metrics have been selected for com-paring piezoelectric materials on diverse applications. Inactuating and sensing applications, the piezoelectric strainand piezoelectric voltage constants are appropriate crite-ria. However, the electromechanical coupling factor, powerdensity, mechanical stiffness, mechanical strength, manu-facturability, and quality factor are the most importantfactors for energy harvesting. Also the operating temper-ature is important in material selection [15].

Li et al. [16] divided the piezoelectric materials intofour categories (ceramics, single crystals, polymers, andcomposites) based on their structure characteristics. Theydescribed the general properties of these four piezo-materialcategories, and compared some of the most important can-didate materials form these categories in terms of piezo-electric strain constant, piezoelectric voltage constant g,electromechanical coupling factor k ,mechanical qualityfactor Q , and dielectric constant e. They commentedthat piezoelectric ceramics and single crystals have muchbetter piezoelectric properties than piezoelectric polymersthat is due to the strong polarizations in their crystallinestructures. On the other hand, piezoelectric ceramics andsingle crystals are more rigid and brittle then piezoelectricpolymers. Both piezoelectric properties and mechanicalproperties are important in selection of a certain piezo-electric material for a specific piezoelectric harvesting ap-plication. Other important parameters in selecting thesuitable materials are the application frequency, the avail-able volume, and the form in which mechanical energy isfed into the system. In order to harvest maximum amountof energy, the piezoelectric energy harvester should operate

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Table 1: Overall evaluation of review papers written about non-focused topics on piezoelectric energy harvesting. ”Cons.”stands for conclusions. Numbers in brackets are scores for each item.Conclusions: 1 Efficiency/performance improvement, 2 frequency tuning, 3 safety issues, 4 costs, hybrid harvesters, 5non-linear models, 6 battery replacement, 7 miniaturization, 8 steady operation, 9 more efficient materials.Merits: 1: electromechanical coupling factor, 2: realistic resonance, 3: energy flow, 4: paying attention to the range ofoutput power.Sub-categories: 1: microscale, 2: electrostatic, 3: magnetic induction, 4: electromagnetic radiation, 5: thermal energy,6: circuit, 7: wearable device, 8: ambient fluid flow, 9: sensors, 10: material, 11: human, 12: vibration, 13: hybriddevice, 14: modelling, 15: material, 16: road and shoe, 17: fluids, 18: animals.

# Cons. Minimum re-quired output

# Refs. Merits Sub-categories

Ref. Grade Highlights

6 (0.67) µW to mW (1) 478 1, 2, 3, 4(2.00)

1, 6, 11, 14,15, 16, 17, 18(0.44)

Safaei et al. [4] A High-temperature devices,metamaterials

5 (0.56) µW (1) 90 1, 3, 4(1.5)

1, 6, 7, 8, 9,10 (0.33)

Anton and So-dani [5]

B -

5 (0.56) 375µW (1) 14 2, 4 (1.0) 11, 12, 14(0.17)

Taware andDeshmukh [6]

C -

3 (0.33) µW to mW (1) 54 1, 4 (1.0) 2, 11, 12, 13(0.22)

Khaligh et al. [7] C -

6 (0.67) - (0) 70 1, 2, 4(1.5)

14, 15 (0.11) Sharma andBaredar [8]

C Depolarization, suddenbreaking of piezo layerdue to high brittleness

4 (0.44) 100µW (1) 33 4 (0.5) 1, 2, 3, 4, 5,6 (0.33)

Mateu andMoll [9]

C A discussion on powerconsumption of microelec-tronic devices

4 (0.44) - (0) 153 1, 2, 4(1.5)

6, 11, 14, 15(0.22)

Calio et al. [10] C Optimal shapes, buckling

1 (0.11) µW to mW (1) 95 4 (0.5) 11, 12, 14,15, 16 (0.28)

Batra et al. [11] D -

3 (0.33) 1.3mW (1) 16 4 (0.5) -(0) Sun et al. [12] D Comparison with energyfrom wind, solar, geother-mal, coal, oil and gas

1 (0.11) - (0) 13 -(0) 14, 15, 16(0.17)

Sharma etal. [13]

E Historical points

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at its resonance frequency. However, in many cases suchas low frequency applications, it is impractical to matchthe resonance frequency of the piezoelectric with the inputfrequency of the host structure. They demonstrated thatfor low frequency applications in off-resonance conditionsthe piezoelectric element can be approximated as a parallelplate capacitor and for harvesting more electric energy theproduct of piezoelectric strain constant and piezoelectricvoltage constant should be high. On the other hand, fornear-resonance conditions, the optimum output power ofthe harvester is independent of piezoelectric properties ofpiezo-element but the maximum output voltage dependson piezoelectric strain constant. It is obvious that theselection of suitable piezomaterial for piezo-harvester de-pends on working condition, and it makes the selection ofpiezo-material more complex. The article has not specifiedthe minimum required power output for piezoelectric en-ergy harvesters. Also, the energy density of piezoelectricmaterials were not reported. The focus is on macroscalepiezomaterials and micro- and nono-scale materials werenot covered.

Narita and Fox [17] reviewed three categories includ-ing piezoelectric ceramics/polymers, magnetostrictive al-loys, and magnetoelectric multiferroic composites. Theirreview included describing the properties of PZT, PVDF,ZnO. They compared some of the piezoelectric materialsbased on their piezoelectric coefficients (Fig. 4). Also,they remarked some advantages and disadvantages of tra-ditional piezoelectric ceramics, piezoelectric polymers, andcomposites. They focused on characterization, fabrica-tion, modeling, simulation, durability and reliability ofpiezo-devices. Based on their analysis, the future direc-tions include the device size reduction to make them suit-able for nanotechnology, optimization, and developing ac-curate multi-scale computational methods to link atomic,domain, grain, and macroscale behaviors. Investigationof temperature-dependent properties, development of ma-terials and structures capable of withstanding prolongedcyclic loading, duration of electro-magneto-mechanical prop-erties, and fracture/fatigue studies are other recommen-dations for future research. The review does not reportedsome of the important mechanical and piezoelectric prop-erties of the piezo-materials like electromechanical cou-pling factor and quality factor, mechanical strength andmechanical stiffness, and the materials were compared basedon their piezoelectric coefficients and the output power ofthe energy harvesters.

Safaei et al. reviewed the recent progresses in thefield of piezoelectric ceramics like soft and hard PZTs,piezoelectric polymers including PVDF, piezoelectric sin-gle crystals, lead-free piezoelectrics, high temperature piezo-electrics, piezoelectric nanocomposites, and also piezoelec-tric foams. They reported the piezoelectric coefficient, andthe maximum output voltage for some of these materi-als without describing the geometry of the piezoelectricharvester. Brittleness of PZTs and existence of healthrisks in PZT ceramics due to the toxicity of lead are the

Figure 4: Piezoelectric coefficient range for some of piezoelectric ma-terials [17].

most important challenges of using PZTs, which motivatesthe development of lead-free flexible and high-performancepiezoelectric materials. They concluded that the need forenhancement of electromechanical, thermal, and biocom-patible properties has led to the introduction of new piezo-electric materials including new lead-free piezoelectrics,high-temperature piezoelectrics, piezoelectric foams, andpiezoelectric nanocomposites. The paper have eplainedlots of piezo-materials however there is not a systematiccomparison between the piezoelectric materials in termsof piezoelectric and mechanical properties. It seems thatthe main target is only reporting the recent progresses inthe field. Also the minimum required output power for thepiezoelectric harvesters was not remarked.

Zaarour et al. [18] summarized the energy harvestingtechnologies developed based on piezoelectric polymericfibers, inorganic piezoelectric fibers, and inorganic nanowire.The paper contains a review of piezoelectric fibers andnanowires with respect to the peak voltage, the peak cur-rent, the active area, and their advantages, without de-scribing the working conditions and mechanical structureof the related piezoelectric energy harvester. Maybe dueto lack of available data on properties of nano-scale piezo-electric materials, there is not any comparison betweenthe selected materials in terms of their piezoelectric andmechanical properties. The reported output powers arein the range of micro watt which is not enough for em-powering real electronic systems and circuits. They con-cluded that, standardizing the performance of the piezo-nanogenerator, developing effective packaging technology,packaging of nano-piezo-harvesters, commercializing prod-ucts for harsh environments, finding a suitable approach toenhance the electrical outputs, and augmenting the dura-bility and the output stability are some future horizons.

Yuan et al. [19] introduced the dielectric electroac-tive polymers as promising replacements for conventionalpiezoelectric materials. Electroactive polymers are lightweight,flexible, ductile, low-cost manufactured, with high strength-to-weight ratio, low mechanical impedance, and can en-dure large strains. The dielectric polymers need high volt-

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age to realize energy cycles that may lead to the break-down of the device. Piezoelectric materials are employedin energy harvesters because of their compact configura-tion and compatibility. However, these materials haveinherent limitations including aging, depolarization, andbrittleness. In comparison, electrostrictive polymers arepromising candidates to replace piezoelectric materials invibration energy harvesting cases. The challenge in designof electroactive polymer energy harvesters is to developsystems that are capable of ensuring a constant initial volt-age on the polymer at small cost.

There are some other review papers which have fo-cused on several issues in the field of piezoelectric materi-als. Piezoelectric polymers were reviewed some of paperslike Mishara’s review paper et al. [20]. High temperaturesingle crystals is the subject of Priya’s paper which madea comparative study of the main high temperature piezo-electric single crystals. Bio-piezoelectric materials weredescribed by Liu et al. [21]. Also they have reviewed mi-cro and nano fabrication techniques for micro/nano scaleenergy harvesters. useful information on micro/nano scalepiezoelectric materials may be found in Gosavi et al. [14].They defined a systematic roadmap to select the piezoelec-tric materials for micro and nanoscale energy harvesters.They pointed out that the ZnO thin film is the most widelyused structure in micro and nanoscale harvesters, and canbe economically synthesized in arbitrary sizes and shapes.A detailed comparison between traditional macro materi-als and new micro/nano piezoelectric materials in termsof dielectric, mechanical and piezoelectric properties wasperformed by Bowen et al. [22]. They mentioned somepoints about high-temperature harvesting related to theCurie temperature, light harvesting into chemical or elec-trical energy, and optimization algorithms. Their inves-tigation contains parameters like pyroelectric coefficient(harvesting from temperature fluctuations), the electro-mechanical coupling, the mechanical quality factor, theconstant-strain relative permittivity, the constant-stressrelative permittivity, the piezoelectric coefficient, and theelastic constant of piezoelectric materials. For high-strainapplications, they suggested polymeric or composite-basedsystems. Their suggested future directions are understat-ing and development of new materials and gaining strongscientific underpinning of the technology and reliable mea-surements.

Most of the review papers tried to compare the piezo-electric materials and draw a roadmap for selecting an ap-propriate material for energy harvesters. However, thechoice of material is strictly dependent on type of the en-ergy harvester, its working condition, the cost level, acces-sibility and ease of fabrication/synthesis of the piezoelec-tric material. For example, Ullah Khan and Ahmad [23]who have reviewed vibrational energy harvesters utilizingbridge oscillations, pointed out that the main selection cri-teria for piezoelectric vibrational energy harvesting are thedielectric constant, the Curie temperature, and the mod-ulus of elasticity of the material.

The piezoelectric materials with high value of elasticmodulus can be an appropriate choice for high accelerationvibrations. However, the piezoelectric materials like leadlanthanum zirconate titanate that has a high value of di-electric constant will perform very well in low-accelerationvibrational environments. Also, due to the easiness of insitu fabrication of lead zirconate titanate (PZT) with sol-gel technique, and its easy integration with the other mi-crofabrication processes, PZT has been largely utilized inmost of such applications.

As another example, we can point out the selectionof a desirable piezoelectric material for walking energyharvesting applications. Based on a review performed byMaghsoudi Nia et al. [24], this application needs an incom-bustible, chemically resistant, low-price material, whichshould be unbreakable under harsh conditions. The men-tioned criteria have made PVDF more suitable than PZTfor the most of piezoelectric harnessing from walking. Mostof the review papers have contented themselves with re-porting some electromechanical properties of piezoelectricmaterials, and provided a few information on the acces-sibility, relative cost, chemical properties, ease of fabrica-tion, and suitable working conditions of different piezo-electrics. The lack of such information indicates the needfor further research and also the necessity for making morecomprehensive and application-based reviews on piezoelec-tric materials.

The results of evaluation of the review papers on piezo-electric materials have been presented in Table 2. Thetable also contains different sub-categories, the range ofoutput power, the number of reviewed articles, the mer-its, general conclusions, and some other extra descriptions.The rank of each paper has been computed based on thenumber of merits, the number of subcategories, the num-ber of concluding remarks, and clear emphasizing on valueof minimum required output power. As indicated by table1 unless a few papers like [16] other reviews suffer fromlack of reported data on mechanical piezoelectric materi-als, their fabrication methods and other figure of merits inmaterial selection. Also, unless the paper [17] which haspointed out the needed energy for empowering the elec-tronic devices, other papers have neglected the minimumrequired energy for an energy harvester.

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Table 2: Overall evaluation of review papers written on materials in piezoelectric energy harvesting. The numbers in brackets denote non-general future lines.”Cons.” stands for conclusions.Conclusions: Efficiency/performance improvement, cost reduction, lead free materials, increasing life time, endurance, size reduction and manufacturability.Merits: 1: piezoelectric coefficients, 2: coupling factors, 3: manufacturability, 4: mechanical strength, 5: guidelines for material selection, 6: paying attentionto the minimum required output power (1 mW), 7: energy density, 8: stiffness, 9: quality factorSub-categories: 1- Micro and nano materials: 1-1 Piezoelectric micro/macro fibers, 1-2 Polymer nano fibers, 1-3 Ceramic nano-fibers, 1-4 Piezoelectricnano wires, 1-5 Micro/nano fibers/wires composites.2- macro scale materials: 2-1- Piezoelectric polymers (PVDF, Pu, P(VDF-TrFE), cellular PP), 2-2-piezoelectric ceramics (PZT, PMM-PT, PMN-PZT ...), 2-3- piezoelectric single crystals (Quartz ...), 2-4- piezoelectric foams (PDMS piezoelectric, PET/EVA/PET piezoelectret, FEP piezoelectric), 2-5- piezoelectricpowders, 2-6- piezoelectric composites (PVDF with Nanofillers, Non-Piezoelectric Polymer with BaTiO3), 2-7-bio materials.# Cons. Minimum re-

quired output#Refs.

Merits Sub-categories

Ref. Grade Highlights

6 (0.75) µW to mW (0) 120 1, 2, 3, 5, 6, 8, 9(1.75)

2-1, 2-2, 2-3,2-6 (0.33)

Li et al. [16] C 1 the current state of research on piezoelectric energy harvesting devices for low frequency (0-100Hz) applications and the methods that have been developed to improve the power outputs of thepiezoelectric energy harvesters have been reviewed. 2 The selection of the appropriate piezoelectricmaterial for a specific application and methods to optimize the design of the piezoelectric energyharvester were discussed.

6 (0.75) µW to nW (0) 478 1, 3, 6, 7 (1.15) 1, 2-2, 2-3 2-4,2-6 (0.75)

Safae et al. [4] C Reporting the recent advances in the field of piezoelectric materials. Reviewing some novel piezoelec-tric materials like piezoelectric foam and high temperature materials

9 (1) µW (0) 173 3, 4, 5 (0.75) 1-1 to 1-5(0.42)

Zaarour et al. [18] C 1 Manufacturing methods of nano fibers and wires, 2 Mentioning output voltage and currents ofnano/micro materials, 3 Comparison of nano/micro materials based on maximum voltage and currantand active area

3 (0.4) µW (0) 446 1, 3, 5 (0.75) 1-2, 1-3, 2-1,2-2, 2-3, 2-4(0.35)

Liu et al. [21] D 1 Reporting recent progresses in the field of piezoelectric materials, 2- description of fabrication tech-niques of lots of piezoelectric materials in energy harvesting applications, 3- explaining the mainfrequency bandwidth broadening techniques, 4- Classifying piezoelectric materials, fabrication tech-niques, and frequency bandwidth broadening techniques.

6 (0.75) µW to mW (0) 175 1, 3, 6 (0.75) 1-1, 2-1, 2-2,2-3 (0.33)

Narita andFox [17]

D 1 Reporting the harvested power of PZT based PEH s with different structures, 2 Reporting the recentadvances in the field op PEHs which were made of PVDF, and polymer based composite piezoelectrics.Comparing the output power of some of the piezoelectric energy harvesters.

2 (0.25) mW 50 1, 4, 7, 8 (1.15) 1, 2 (0.25) Yoan et al. [19] D 1 introducing electrostrictive and dielectric electro-active polymers, 2 performance comparison of PZT,PVDF, and DEAPs and electrostrictive polymers. Describing the industrial challenges for dielectricelectro-active polymers.

4 (0.57) µW (0) 158 1, 2, 3 (0.75) 2-1, 2-2, 2-3,2-6 (0.33)

Mishra et l. [20] D The article basically aimed at exploring the basic theory behind the piezoelectric behavior of polymericand composite systems and comparing the important types of piezoelectric polymers and composites.The article described the piezoelectric properties of lots of the piezo-polymers and polymer composites.

6 (0.75) µW (0) 216 1, 2, 9 (0.75) 2-1, 2-2, 2-3(0.25)

Bowen et al. [22] D Reviewing some resent topics like piezoelectric light harvesting, Pyroelectric based harvesting, andnano scale Pyroelectric systems

3 (0.4) µW (0) 24 2, 6, 9 (0.75) 2-1, 2-3 (0.25) Lefeuvre [25] D 1 Figure of merit for energy conversion efficiency, 2 figure of merit for piezoelectric materials, 3comparing the one, two and three stage electric power interfaces

2 (0.25) µW to mW (0) 16 1, 2, 5, 8 (1) 2 (0.1) Mukherjee andDatta [26]

D 1 Effect of load resistance on the output power of PEHs, 2 Selection criteria for piezoelectric ceramics

9

3.2. Structure

All piezoelectric energy harvesters include a mechani-cal part (or transduction part) to convert the input me-chanical energy into the electric charges in the piezoelec-tric element, and an electric part that keeps the electriccharges and converts them into a suitable form of electricoutput like direct voltage. Design of the mechanical partof a piezoelectric energy harvester usually includes the de-termination of its size, configuration, working modes, andselection of appropriate materials to enhance its perfor-mance characteristics like the output electric energy, theconversion efficiency and the working bandwidth. The sizeof the piezoelectric energy harvester may vary from microand nanoscale (lower than 0.01cm3) to macroscale (75cm3)[2].

Upon the literature, the piezoelectric energy harvesterscan be classified from different viewpoints. Form the viewpoint of operating frequency, they may be categorized intotwo main sections: the resonant type devices that operateat or near their resonance frequency, and non-resonant sys-tems that do not depend on any specific frequency. Thepiezoelectric energy harvesters may harvest energy frommotions in a unique direction or from multi-directions. Ac-cordingly, they may be single-directional or multi-directionalharvesters. Also, they have a single or several vibrationmodes (multi-modal harvesters). From the viewpoint ofgoverning dynamic models, the piezoelectric harvesters maybe linear or non-linear [27]. As indicated in Fig. 3, theirconfiguration can be classified as cantilever type, stacktype, cymbal type, circular diaphragm type, or the shelland film types.

Uchino [28] started his review by mentioning the his-torical background of the piezoelectric energy harvesting,and explaining several important misconceptions. He re-viewed the different design approaches followed by me-chanical, electrical, and MEMS engineers. He remarkedthat there are three major phases associated with piezo-electric energy harvesting: (i) mechanical-mechanical en-ergy transfer, (ii) mechanical-electrical energy transduc-tion, and (iii) electrical-electrical energy transfer to ac-cumulate the energy into a rechargeable battery. Fig. 5represents these three major phases. In order to providecomprehensive strategies on how to improve the efficiencyof the harvesting system, a step-by-step detailed energyflow analysis is essential. It was mentioned that the fiveimportant figure of merits in piezoelectrics are the piezo-electric strain constant d, the piezoelectric voltage con-stant g, the electromechanical coupling factor k, the me-chanical quality factor Qm, and the acoustic impedance Z.Also, the energy transfer rates for the piezoelectric energyharvesting systems with typical stiff cymbals and flexiblepiezoelectric transducers were evaluated for three afore-mentioned phases/steps. Moreover, a hybrid energy har-vesting device that operates under either magnetic and/ormechanical noises was introduced. It was concluded thatthe remote signal transmission, energy accumulation in

rechargeable batteries, discovering a genius idea to com-bine nano-devices in parallel, and enhancing the energydensity in medical applications have been introduced asfuture research fields. It was declared that a clear futureperspective for NEMS and MEMS piezoelectric harvestersis missing due to their low energy levels (in the order ofpW to nW). We need to discover a genius idea on howto combine thousands of nano-devices in parallel and syn-chronously in phase. Describing the performance improve-ment techniques for non-resonant and resonant energy har-vesters, are felt missing in this article.

Figure 5: Three major phases associated with piezoelectric energyharvesting [28].

Priya [29] classified the energy harvesting approachesin two categories (1) power harvesting for sensor networksusing MEMS/thin/thick film approach, and (2) power har-vesting for electronic devices using bulk approach. His re-view article covered the later category in more details. Helisted almost all the energy sources available in the sur-rounding which may be used for energy harvesting andcommented that the selection of the energy harvester ascompared to other alternatives such as battery depends ontwo main factors cost effectiveness and reliability. Also,he reported the daily average power consumption for awearable device, and of common household devices. Next,comparison of the energy density for the three types of me-chanical to electrical energy converters including electro-static, electromagnetic and piezoelectric were performed.The results were represented in Fig. 6. He concluded thatpiezoelectric converters are prominent choice for mechani-cal to electric energy conversion because the energy densityis three times higher as compared to electrostatic and elec-tromagnetics. He gave a review of piezo-harvesters appro-priate for light-weight flexible systems with easy mount-ing, large response, and low-frequency operation; calledthe low-profile piezo-transducer in on/off-resonance condi-tion. A good discussion on piezoelectric polymers, energystorage circuit, and microscale piezo-harvesting device isavailable in the article. He mentioned that the electri-cal power generated by the piezoelectric energy harvesteris inversely proportional to the damping ratio that shouldbe minimized through proper selection of the material and

10

design. He also have summarized the conditions leading toappearance of maximum efficiency in low profile piezoelec-tric energy harvesters. An interesting part of the paper isthe description of the piezoelectric material selection pro-cedure for on/off-resonance condition. However, the de-scription of performance improvement techniques for en-hancing the system frequency response are felt missing inthis article.

Figure 6: Comparison of the energy density for the three types ofmechanical to electrical energy converters [29].

Yang et al. [30] commented that from the perspec-tive of applications,the output power of the harvester andits operational frequency bandwidth are the two metricsmost useful to product development engineers. They ex-plained the materials selection procedure for piezoelectricenergy harvesters in off-resonant condition and remarkedwhy PZT’s are steel the most popular piezoelectric mate-rials for energy harvesters. They stated that linear reso-nant harvesters are not suitable for harvesting energy frombroadband or frequency-varying excitations, and in thiscondition nonlinear energy harvesters have been provento be able to exhibit a broadband performance. There-fore, researchers have explored monostable, bistable, andtristable systems and developed some frequency tuningapproaches, such as multi-cantilever structures, bistablecomposite plate designs, and passive and active stiffness-tuning technologies. On the nonlinear energy harvestersthey remarked that maintaining the nonlinear harvestersin the high-energy oscillation states, especially under weakexcitations is a difficult task. specially, with zero initialconditions, nonlinear harvesters usually follow the low-energy orbits, which results in small-amplitude voltage re-sponses. Thus maintaining the nonlinear PHE in the high-energy states is a critical problem which is possible with ac-tive and passive control. Efficiently transferring and stor-ing the generated broadband or random electric energy isanother critical problem for nonlinear PHEs. Moreover,they reviewed the different designs strategies, the opti-mization techniques, and the harvesting piezo-materials inapplications like shoes, pacemakers, tire pressure moni-

toring systems, bridge and building monitoring. They de-clared that high energy conversion efficiency, ease of imple-mentation, and miniaturization are the main advantagesof such systems. However, authors state that enhance-ment of energy efficiency of the piezo-based harvesters isstill an open challenge. They also made a systematic per-formance comparison on some of the energy harvesters.They pointed out that a considerable gap exists betweenthe achieved performance and the expected performance.Therefore, in situ testing, applying more realistic excita-tions, system-level investigations on piezo-harvesters inte-grated with the power conditioning circuits, energy storageelements, sensors, and control circuits need to be inves-tigated. This article has focused on mechanical part ofenergy harvesters and subjects like the electric interfacecircuits of the harvesters and their energy flow analysishave not been remarked.

There are some other review papers which have focusedon several issues in the field of design of piezoelectric har-vesters. Performance improvement techniques for PHEsand design optimization methods are hot topics covered byreviews [31], [16], [27] . Manual and autonomous tuningsystems for widening the operating frequency bandwidthand the future plans in this field were discussed by Ibrahimand Vahied [32]. A good review of PEH configurationssuch as cantilever beam, discs, cymbals, diaphragms, cir-cular diaphragms, shell-type, and ribbon geometries maybe found in [16]. Talib et al. [33] explained effective strate-gies and the key factors to enhance the performance ofpiezoelectric energy harvesters operating at low frequen-cies, including selection of the piezoelectric material, opti-mization of the shape, size, structure, and development ofmulti-modal, nonlinear, multi-directional, and hybrid en-ergy harvesting systems. This review paper is suitable forthe beginners who want to get acquainted with the piezo-electric materials and some designs of piezoelectric energyharvesters. They concluded that the recent developmentsare inclined towards generation of more power from low-frequency and low-amplitude ambient vibrations with re-duced required piezoelectric material. Adding a singleDOF system in the form of an extension beam or a springto the piezoelectric beam is a remarkable advise to enhancethe power output. They showed that the multi-modal en-ergy harvester exhibits a broader bandwidth when its mul-tiple resonance peaks get closer.

Brenes et al. [34] provided an overlook of existing en-ergy harvesting circuits and techniques for piezoelectricenergy scavenging to distinguish between existing similarsolutions that are different in practice. Such categoriza-tion is helpful to ponder the advantages and drawbacksof each available item. Their review is unique since theyhave classified the piezo-systems based on adaptive/non-adaptive control strategies, topologies, architectures, tech-niques form one hand, and electromechanical models fromthe other hand. The best system has been introducedwith respect to the optimized power efficiency, the designcomplexity, the strength of coupling, the multi-stage load

11

adaption, and the vibration frequency.Issues like AC-DC conversion mechanism, the passive

and active rectifications, the start-up issues, the harvester-specific interactions, the voltage conditioning, the DC-DCcharge pumps, the power regulation, and the impedancematching were discussed by Szarka et al. [35] and Dell’Annaet al. [36] . Non-linear electronic interfaces for energy har-vesting from mechanical vibrations was remarked in [37].

Tables 3 and 4 gather together the results of evalua-tion of the review papers written on design methods andthe power interface considerations, respectively. The tablealso contains different sub-categories, the range of outputpower, the number of reviewed articles, the merits, gen-eral conclusions, and some other extra descriptions. Therank of each paper has been computed based on the num-ber of merits, the number of subcategories, the numberof concluding remarks, and clear emphasizing on value ofminimum required output power.

The results of evaluation of review papers on design ofpiezoelectric energy harvesting have been presented in Ta-ble 3. The table also contains different sub-categories, therange of output power, the number of reviewed articles,the merits, general conclusions, and some other extra de-scriptions. The grade for each paper has been computedbased on the number of merits, the number of subcate-gories, the number of concluding remarks, and declarationof minimum required output power. Table 3 is designedto evaluate the review papers about design of PHEs. Themerits that are selected as the necessary considerationsin the field of design of PHEs are 1: reporting the out-put power of PHEs, 2: reporting the coupling factors andoperational modes, 3: including mathematical models, 4:attending to the motivating frequencies of PHEs, 5: At-tending to the mechanical and electrical energy conversionefficiencies. Quantitative evaluation of the papers was per-formed based on the number of merits which have been fol-lowed by the article, number of sub-categories which werecovered in the review and the number of conclusions. ac-cording to the table, unless the first papers, other papershave neglected some merits like the minimum required out-put power for the harvesters and energy flows analysis ofthem, also most of the papers have not reviewed some ofthe issues in the field as reported by paper ”*”.

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Table 3: Overall evaluation of review papers written on design of piezoelectric energy harvesters. The numbers in brackets denote the number of non-general future lines. ”Cons.”stands for conclusions.Conclusions: Efficiency/performance improvement, necessity of frequency bandwidth broadening, necessity of optimizations, increasing life time and endurance, size reduction andmanufacturability, importance of electric interface circuits, the necessity of material properties improvement.Merits: 1: reporting the output power of PHEs, 2: coupling factors and operational mode, 3: including mathematical models, 4: matching the resonance frequency of PHEs withmotivating frequencies, 5: paying attention to the energy conversion efficienciesSub-categories: Performance improvement: 1- frequency tuning approaches: 1-1-manual tuning and 1-2-autonomous tuning methods, 2- Multi-frequency systems, 3- nonlinear systems,4- frequency up conversion approach, 5- systems with free moving mass , 6- Bi directional and three directional systems, 7- Amplification techniques 8- material selection criteria 9-energy conversion efficiency 10- low profile piezoelectric harvesters 11- geometric optimization 12- Mathematical modeling of PHEs. Design improvements for 13- piezoelectric cantilevers,14- piezoelectric cymbal 15- piezoelectric stack configuration, 16- electrode optimization.... 17-performance quantification and comparison strategies...18- electronic interface circuits forPHEs 19- Hybrid energy harvesting mechanism.

# Cons. Minimum re-quired output

# Refs. Merits Sub-categories Ref. Grade Highlights

10(1) mW (1) 35 1, 2, 3, 4, 5 (2) 8, 9, 11, 13, 14,15, 17, 18, 19(0.5)

Uchino [28] A 1 describing the historical background of piezoelectric energy harvesting, 2 commenting on severalmisconceptions by the current researchers, 3 step-by-step detailed energy flow analysis in energyharvesting systems, 4- describing the key to dramatic enhancement in the efficiency, 5- importantcomments on the useful/un-useful output power level for the harvesters

6(0.85) mW (1) 75 1, 2, 3, 4, 5 (2) 9, 8, 10, 12 (0.2) Priya [29] A Describing the material selection criteria in on- and off-resonance condition, Describing the factorswhich affect the conversion efficiency of PHEs, introduction of some low profile PHEs for realizing aself powered sensor nodes

5(0.7) µW to mW (1) 338 1, 2, 3, 4, 5 (2) 3, 11, 12, 13, 14,15, 16, 17 (0.36)

Yang etal. [30]

A Analysis of different designs, nonlinear methods, optimization techniques, and materials for increasingperformance. Introducing a set of metrics for the end users of PHEs for comparison of performanceof PHEs

5(0.7) µW to mW (0) 120 1, 2, 3, 4, 5 (2) 1, 4, 8, 12, 14, 18(0.32)

Li et al. [16] B Commenting on the biggest challenges for PHEs, describing the most important limitations of piezo-electric materials

4(0.75) µW to mW (0) 446 1, 2, 3, 4, 5 (2) 2, 3, 4, 6, 12, 13,14, 15, 19 (0.52)

Liu et al. [21] B Various key aspects to improve the overall performance of a PEH device are discussed. Classificationof performance improvement approaches have been performed.

3(0.42) µW (0) 149 1, 3, 4, 5 (1.6) 1, 2, 3, 4, 5, 6(0.3)

Maamer etal. [27]

C Proposing new generic categorization, approach based on the improvement aspect of the harvester,which includes techniques for widening operating frequency, conceiving a non-resonant system andmultidirectional harvester. Evaluating the applicability of the performance improvement techniquesunder different conditions and their compatibility with MEMS technology

5(0.7) mW (0) 105 1, 3, 4 (1.2) 1, 2, 3, 4, 5, 6, 7(0.36)

Yildirim etal. [31]

C New classification of performance enhancement techniques, Comparison of lots of performance en-hancement techniques.

8(1) µW to mW (0) 66 1, 3, 4 (1.2) 1-1, 1-2 (0.1) Ibrahim andVahied. [32]

C Classifying, reviewing and comparing the different manual and autonomous tuning methods, challengeof energy consumption by self-tuning structures

4(0.6) µW to mW (0) 135 1, 2, 4, 5 (1.6) 3, 6, 8, 11 (0.25) Talib etal. [33]

C They commented that the anticipated performance of a piezoelectric harvester can be attained byachieving the trade-off between output power and bandwidth.

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3.3. MEMS/NEMS-based devicesA large number of reviews on piezo-harvesters have

been devoted to the field of MEMS/NEMS piezoelectricharvesters. Micro and nanoscale energy harvesters maybeuseful at future for easy powering or charging of mobileelectronics, even in remote areas, without the need forlarge power storage elements. MEMS-type devices includecantilever, cymbal and stack whereas NEMS type devicesare wires, rods, fibers, belts, and tubes. Generation of out-put electric current using piezoelectric energy harvestersfaces with many limitations and difficulties. Some of theselimitations are low output power, high electric impedance,crack propagation in most piezoelectric materials due tooverloading, frequency matching of the harvester with vi-brational energy sources, and fabrication/integration ofpiezoelectrics in micro/nanoscale [40].

Kim et al. [41] commented that for the elimination ofchemical batteries and complex wiring in microsystems,a fully assembled energy harvester with the size of a USquarter dollar coin should be able to generate about 100µWof continuous power from ambient vibrations. In addition,the cost of the device should be sufficiently low. The articlehave addressed two important questions that are ”how canone achieve self-powering when the power required is muchlarger than what can be achieved by MEMS-scale piezo-electric harvesters?” and ”what is the best mechanism forconverting mechanical energy into electrical energy at mm3 dimensions?”. Also, they commented that for harvestingthe power robustly, the resonance bandwidth of piezoelec-tric cantilevers should be wide enough to accommodate theuncertain variance of ambient vibrations. Thus, the reso-nance bandwidth is a significant characteristic for trappingan enough amount of energy onto the harvester and shouldbe accounted for in determining the performance of energyharvesters. MEMS technology is a cost-effective fabrica-tion technology for PHEs if it can meet the requirementsfor power density and bandwidth. Three major aspects tomake the MEMS PEHs appropriate for use in real applica-tions are the final cost of the PEH, the normalized powerdensity, and the operational frequency range (includingthe bandwidth and center frequency). They added thatpiezoelectric MEMS energy harvesters mostly have a uni-morph cantilever configuration (Fig. 7). The proof mass(M) in Fig. 7 is used to adjust the resonant frequency tothe available environmental frequency, normally below 100Hz. Recently, integrated MEMS energy harvesters havebeen developed and in comparison of MEMS PEHs someessential merits like the active area of PEH, active volume,resonant frequency, harvested power, and power densitiesin volume or area, should be considered. They reviewedchallenges of the piezo-harvesters, including the need tohigh power density and wide bandwidth of operation of thepiezoelectric systems, the non-linear resonating beams forwide bandwidth resonance, and improvements in materialsand the structural design. They concluded that the epi-taxial growth and grain texturing of the piezo-materials,the embedded medical systems, the lead-free piezoelectric

MEMS-based materials, and materials with giant piezo-electric coefficient are active research fields. They pre-sented an extensive comparison of thin-film piezo-systemsfrom various sources and concluded that the state-of-the-art of power density is still about one order smaller thanwhat is needed for practical applications.

Figure 7: Unimorph structure of piezoelectric energy harvester thathas one piezo-layer and a proof mass [41].

Toprak and Tigli [42] conducted a review on piezoelec-tric harvesters based on their size (nanoscale, microscale,mesoscale, macroscale). They also presented an interest-ing statistics that the number of publications between 2009and 2014 on piezoelectric harvesting is more than twice thesum of publications about the electromagnetic and electro-static systems. They commented that the inherent recipro-cal conversion capability is an important advantage of thepiezoelectric energy harvesters that allows them to havesimpler architectures in comparison to the electromagneticand electrostatic counterparts. It is declared that the bio-compatibility, the reconciliation with the CMOS technol-ogy, the rectification and storage losses, and enhancing theoperation bandwidth are the most challenging issues aboutsuch systems. A discussion on validity of the classical con-stitutive relations for the piezo-materials in nanoscale andpay attention to the minimum required power output ofPEHs are felt missing in the paper.

Todaro et al. [43] reviewed the current status of theMEMS-based energy harvesters using piezoelectric thinfilms, and highlighted approaches and strategies. Theycommented that such harvesters are compact and cost-effective especially for harvesting energy from environmen-tal vibrations. They believe that two main challengesof this topic to achieve high-performance devices are in-creasing the amount of generated power and the frequencybandwidth. They also introduced the theoretical princi-ples and the main figures of merit of energy conversion inpiezoelectric thin films. They compared most importantthin film piezo-materials based on the introduced figureof merit. Their recommendations for future research aredeveloping proper materials, new device architectures andstrategies involving bimorph and multimorph designs ex-

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Table 4: Overall evaluation of review papers written on power interfaces in piezoelectric energy harvesters. The numbersin brackets denote the number of non-general future lines. ”Cons.” stands for conclusions.Conclusions: Efficiency/performance improvement, necessity to considering interactions between the mechanical har-vester and the power electronics, necessity of optimizations, importance of electric interface circuitsMerits: 1: Energy flow analysis of PHEs, 2: Practical implementation of electronic interfaces, 3: including mathemat-ical models, 4: paying attention to electrical impedance matching, 5: paying attention to the energy consumption ofelectric interfaces, 6: analysis of energy conversion efficiency.Sub-categories: 1: Three Phases in Energy Harvesting Process, 2: mechanical-electrical energy transduction, 3: en-ergy flow analysis, 4: electrical-to-electrical energy transfer, 5: DC-DC converters and conversions, 6: electric impedancematching, 7: electromechanical models of PHEs, 8: requirements for power electronics, 9: AC-DC conversion with voltageconditioning, 10: DC-DC conversion with voltage conditioning, 11: Power regulation, 12: conversion efficiency of PHEs,13: rectification approaches: (13-1 resonant PEH rectifiers, 13-2- series synchronized switch harvesting on inductor (S-SSHI), 13-3- synchronized switching and discharging to a storage capacitor through an inductor (SSDCI) rectifier, 13-4-synchronous electric charge extraction (SECE), 13-5- synchronized switch harvesting on inductor magnetic rectifier (MR-SSHI), 13-6- hybrid SSHI, 13-7- adaptive synchronized switch harvesting (ASSH), 13-8- enhanced synchronized switchharvesting (ESSH), 13-9 MPPT-based PEH Rectifiers), 14: performance of rectification approaches, 15: autonomousswitch control in resonant PEH rectifiers, 16: switching techniques, 17: parallel SSHI, 18: load decoupling interfaces,19: non-adaptive MPPT control, 21: characteristics of existing adaptive control strategies, 20: the tunable OSECEtechnique, 21: two-stage load adaptation FB technique, 22: two-stage load adaptation shunt rectifier technique, 23: PSSECE technique, 24: tunable SECE technique 25: tunable USECE technique, 26: N-SECE technique, 27: FTSECEtechnique, 28: HB and FB 3-stage load adaptation technique, 29: tunable SCSECE technique, 30: four-stage topology:the SSH architecture, 31: parallel SSHI (p-SSHI) technique, 32: Series SSHI (s-SSHI), DSSH and ESSH techniques, 33:technical guidelines for the choice of an adequate circuit.# Cons. Minimum re-

quired output# Refs. Merits Sub-

categoriesRef. Grade Highlights

9(1) mW (1) 35 1, 2, 3, 4,5, 6 (2)

1, 2, 3, 4,5, 6, 7, 8,12 (0.27)

Uchino [28] A Mentioning minimum acceptable outputpower for harvesters, energy flow analysisfor cymbal type transducer, describing theelectric impedance matching technique

3(0.5) - (0) 109 1, 2, 3, 4,5, 6 (2)

12 to 34(1)

Brenes etal. [34]

B Comparison of the conditions for electric tun-ing techniques to maximize the power flowfrom an external vibration source to an elec-trical load description of necessary conditionsfor Maximum Power Point Tracking (MPPT)

8(1) µW (0) 113 2, 3, 4, 5,6 (1.66)

1, 2, 4,5, 8, 9,10, 11, 13(0.27)

Szarka etal. [35]

B Overview of power management techniquesthat aim to maximize the extracted, power ofPHEs Describing the Requirements for powerelectronics reviewing various power condition-ing techniques and comparing them in termsof complexity, efficiency, quiescent power con-sumption, startup behavior

6(0.75) -(0) 113 2, 3, 4, 5,6 (1.66)

7, 12,13 (allitems),14, 15(0.15)

Francescoet al. [36]

C 1: Almost all the rectification techniques em-ployed in PEH systems were discussed andcompared emphasizing the advantages anddisadvantages of each approach. 2: Introduc-ing the seven criteria used to evaluate the per-formance of a harvesting interface

1(0.2) - (0) 64 2, 3, 5, 6(1.33)

16, 17,18, 13-2, 13-3(0.15)

Guyomarand Lal-lart [37]

D 1: review of nonlinear electronic interfaces forenergy harvesting from mechanical vibrations,2: comparative analysis of various switchingtechniques in terms of efficiency, performanceunder several excitation conditions, complex-ity of implementation

15

ploited for bandwidth and power density improvements,progressing in synthesis and growth technologies for lead-free high quality piezoelectrics, employing new flexible ma-terials with tailored mechanical properties for larger dis-placement and lower frequencies, and taking advantage ofnon-linear effects to obtain a wider bandwidth and a higherefficiency. Specifying the minimum required output powerand attending to mechanical and electrical energy conver-sion efficiencies are felt missing in this review paper.

Dutoit et al. focused on design considerations for piezoelectric-based energy harvesters for MEMS-scale sensors. Theystated that the power consumption of tens to hundreds ofµW is predicted for sensor nodes and nowadays milli-scalecommercial node has an average power consumption of6–300 µW [2]. With the reduction of power requirementsfor sensor nodes, the application of piezoelectric energyharvesters has become viable.They stated that the poweror energy sources can be divided into two groups: sourceswith a fixed energy density (e.g. batteries) and sourceswith a fixed power density (normally ambient energy har-vesters). They suggested that the following informationbe made available in research papers to facilitate a rela-tive comparison of PEH devices: device size, the maximumtip displacement at maximum power output, the mechan-ical damping ratio, the electrical load, the device mass,and the input vibration characteristics. Also, in this pa-per a fully coupled electromechanical model was developedto analyze the response of a piezoelectric energy harvesterand the difference in optimization strategies od PEHs inon-resonant and off-resonant conditions were remarked.

Other review papers on MEMS PEHs have focused onseveral issues including ZnO nonorods and flexible sub-strates, and ZnO-base nano-devices [44], comparison ofexisting piezoelectric micro generators (including the im-pact coupled, the resonant and human-powered devices,and the cantilever-based setup) with electromagnetic andelectrostatic mechanisms [45], the description of micro andnano device fabrication techniques, performance metrics,and device characterization [14], hybrid electromagnetic-piezoelectric and triboelectric/piezoelectric MEMS-basedharvesters and their privileges [46], ZnO nanostructure-based photovoltaic, piezoelectric nano-generators, and thehybrid approach harvesting energy harvesting [47], re-porting the benefits, capacities, applications, challenges,and constraints of micro-power harvesting methods usingthermoelectric, thermophotovoltaic, piezoelectric, and mi-crobial fuel cell [40], nanostructured polymer-based piezo-electric and triboelectric materials as flexible, lightweight,easy/cheap to fabricate, being lead-free, biocompatible,and robust harvesters [48], theoretical and experimen-tal characterization methods for predicting and determin-ing the potential output of nano wire-based nanogener-ators [49], reviewing the research progress in the fieldof piezoelectric nanogenerators and describing their work-ing mechanism, modeling, and structural design [50], dis-cussing the impact of composition, orientation, and mi-crostructures on piezoelectric properties of perovskite thin

films like PbZr1-xTixO3 (PZT) in applications such aslow-voltage radio frequency MEMS switches and resonators,actuators for millimeter-scale robotics, droplet ejectors,energy harvesters for unattended sensors, and medical imag-ing transducers [51].

Table 5 presents details of evaluation of reviews on mi-cro/nanoscale energy harvesting. In summery, almost allreview articles discussed some great challenges of develop-ment of MEMS/NEMS-based piezoelectric harvesters suchas the limited bandwidth and low output power. On theother hand, there are some competitive technologies likeelectromagnetic, thermoelectric, and electrostatic energyharvesting that can be employed for scavenging the envi-ronment waste energy. Most of the comparative reviewpapers have focused on the output power and couplingcoefficient of the harvesting systems and other importantfeatures such as the lifetime, capability of working in harshenvironmental conditions, the cost level, commercial ac-cessibility, and the technology readiness level (TRL) needmore deep considerations.

3.4. Modeling approaches

Some review papers have focused on the modeling ofPHEs to clarify the physical bases behind the piezoelec-tric energy harvesting. There are a few number of reviewpapers that have totally focused on evaluation of differentmodeling approaches for piezoelectric energy harvesting.

Erturk and Inman investigated mechanical [55] andmathematical [56] aspects of the cantilevered piezoelectricenergy harvesters to avoid reuse of simple and incorrectolder models in literature. They reviewed the general so-lution of the base excitation problem for transverse andlongitudinal vibrations of a cantilevered Euler-Bernoullibeam. They proved that the classical single-degree-of-freedom (SODF) predictions may yield highly inaccurateresults, and they are just appropriate for high tip-mass-to-beam-mass ratios. Damping due to internal friction(the Kelvin-Voigt damping), damping related to the fluidmedium, the base excitation as a forcing function, and thebackward piezoelectric coupling in the beam equation areamong modeling parameters. Modelling of energy conver-sion efficiency is felt missing in the article.

Zhao et al. [57] compared different modeling approachesfor harvesting the wind energy, including the single-degree-of-freedom, the single-mode and multi-mode Euler-Bernoullidistributed-parameter models (ignored in Ref. [56]). Theyconcluded that the distributed-parameter model has a morerational representation of aerodynamic forces, while theSDOF model more precisely predicts the cut-in wind speedand the electro-aeroelastic behavior. In addition, they per-formed a parametric study on the effect of the load resis-tance, wind exposure area, mass of the bluff body, and thelength of the piezoelectric sheet on the cut-in wind speedas well as the output power level of the GPEH. Again,modelling of energy conversion efficiency is felt missing inthe article.

16

Table 5: Overall evaluation of review papers written on MEMS piezoelectric energy harvesters. The numbers in brackets denote the numberof non-general future lines. ”Cons.” stands for conclusions.Conclusions: Efficiency/performance improvement, necessity of frequency bandwidth broadening, necessity of optimizations, increasinglife time and endurance, size reduction and manufacturability, importance of electric interface circuits, the necessity of material propertiesimprovement.Merits: 1: reporting the output power or power density of MEMS/NEMS PHEs, 2: coupling factors and operational mode, 3: describingthe fabrication techniques, 4: matching the resonance frequency of PHEs with motivating frequencies, 5:paying attention to the minimumrequired output power,6: CMOS compatibility, 7: energy flow analysis,Sub-categories: 1- Micro/ nano scale materials (1-1- Grain textured and epitaxial piezoelectric films, 1-2- Lead-free piezoelectric films,1-3- Aluminum nitride piezoelectric film, 1-4- piezoelectric nano-polymers, 1-5- Polymer-Ceramic nanocomposite nano generators (NG), 1-6- Electrospun P(VDF-TrFE) nanofiber hybrid NGs, 1-7- Nylon nanowire-based piezoelectric NG, 1-8- Template-grown poly-L-lactic acid,1-9- Electrospun poly-L-lactic acid nanofibers, 1-10- ZnO-polymer nanocomposite piezoelectric NG, 1-11- ZnO nano-rods, 1-12- Nano wires,1-13- Nanowire-Composites, 1-14- PZT thin films, 1-15- piezo-polymer thin films, 1-16- piezoelectric electroactive polymers), 2- Nonlinearresonance-based energy harvesting structures, 3- energy conversion efficiency, 4- figure of merit for MEMS PHEs, 5- Material synthesis anddeposition (5-1- solution phase synthesis, 5-2- thin film deposition, 5-3- growth of polymer-based nanowires), 6- Modes of operations for MEMSPHEs, 7- design configurations for MEMS PHEs (7-1- cantilever based piezoelectric generators, 7-2- other types of piezoelectric generators),8- Microscale scale PHEs, 9- Substrate and electrode and their impact on performance, 10- MEMS device performance parameters, 11-Characterization of MEMS PHEs, 12- MEMS hybrid harvesters (12-1 Architectures of hybrid harvesters, 12-2- Mathematical models of (PZThybrid harvesters, 12-3 PZT - Tribo-electric hybrid harvester), 13- Nano-scale PHEs (13-1 working principles, 13-2- design, fabrication andimplementation of nanogenerators, 13-3- Hybrid nano-generators, 13-4- nano-rod arrays, 13-5- flexible nano generators, 13-6- ZnO nano-PHEs, 13-7- applications of nano-generators, 13-8- Flexoelectric enhancement at the nanometer scale, 13-9- Characterization of piezoelectricpotential from piezoelectric NWs, 13-10- prototypes of nano generators, 13-11- Prediction of the power output from piezoelectric NWs, 13-12-vertically aligned nanowire arrays and their fabrication, 13-13- laterally aligned nanowire arrays and their fabrication), 14- Impact coupleddevices, 15- Human powered piezoelectric generation, 16- Evolving technology of miniature power harvesters, 17- Positive prospects of micro-scale electricity harvesters, 18- Challenges and constraints of minute-scale energy harvesters, 19- CMOS compatibility, 20- biocompatibility,21- bandwidth of PHEs, 22- figure of merit for PHEs, 23- piezoelectric thin films, 24- screening effect in PHEs, 25- Energy harvesting bypiezoelectric thin films.# Cons. Minimum

requiredoutput

#Refs.

Merits Sub-categories

Ref. Grade Highlights

6(0.85) µW (1) 89 1, 2, 3, 4, 5,6, 7 (2)

1 to 4(0.25)

Kim et al. [41] A Describing figure of merits for MEMS PHEs, Mentioning the key attributes for MEMS PHEs, De-scribing minimum acceptable power density for MEMS PHEs

4(0.57) µW (0) 95 1, 3, 4 (0.6) 1-11, 13-4,13-5, 13-6(0.15)

Briscoe andDunn [44]

C 1: This review has summarized the work to date on nanostructured piezoelectric energy harvesters.2: They stated that in order to satisfy the needs of real power delivery, devices need to maximize therate of change of any strain delivered into a system in order to increase the polarization developed bythe functional layers, and improve the coupling of the device to the environment.

4(0.57) µW tomW (0)

123 1, 2, 3, 4, 5,6 (1.71)

8, 13, 18,19, 20, 21(0.25)

Toprak andTigli [42]

C 1: They commented that the size-based classification provides a reliable and effective basis to studyvarious piezoelectric energy harvesters. 2: They discussed the most prominent challenges in piezo-electric energy harvesting and the studies focusing on these challenges.

4(0.57) µW tomW (0)

145 1, 2, 3, 4, 5 ,6 (1.71)

8, 18, 22,23 (0.15)

Todaro et al. [43] C 1: The paper has reviewed the current status of MEMS energy harvesters based on piezoelectricthin films. 2: The paper has highlighted approaches/strategies to face the two main challenges tobe addressed for high performance devices, namely generated power and frequency bandwidth. 3:Comparison of lots of MEMS energy harvesters performances has been performed.

5(0.71) mW (0) 34 1, 2, 3, 4, 5(1.4)

12 (12-1 to12-3) (0.1)

Salim et al. [46] C Elaborating on the hybrid energy harvesters, reported Literature on such harvesters for recent yearswith different architectures, models, and results comparison of the present hybrid PHEs in terms ofoutput power.

6(0.85) µW tomW (0)

74 1, 2, 4, 5(1.15)

6, 7, 8, 10,25 (0.2)

Dutoit et al. [2] C Commenting on the necessary information for comparing different PHEs. Pointing on the differencebetween dominant damping components at the micro- vs. macro-scale. Developing a fully coupledelectromechanical model for analyzing the response of PHEs with cantilever configuration.

7(1) µW tomW (0)

108 1, 3, 4, 5(1.15)

1-4 to 1-10,5-3 (0.1)

Jing and Kar-Narayan [48]

C 1: Discussing the growth of nanomaterials including nanowires of polymers of polyvinylidene flu-oride and its co-polymers, Nylon-11, and poly-lactic acid for scalable piezoelectric and triboelectricnanogenerator applications. 2: discussing design and performance of polymer-ceramic nanocomposite.

6(0.85) µW tomW (0)

115 1, 2, 4, 5(1.15)

7-1, 7-2,14, 15(0.2)

Beeby [45] C Characterization and comparison of piezoelectric, electromagnetic and electrostatic MEMS generators

5(0.71) µW (0) 140 1, 3, 4, 5(1.15)

1-2, 1-15,1-16 (0.15)

Asif Khan [52] C 1: The review has covered the available material forms and applications of piezoelectric thin films.2: The electromechanical properties and performances of piezoelectric films have been comparedand their suitability for particular applications were reported. 3: Control over the growth of thepiezoelectric thin films and lead-free compositions of thin films can lead to good environmental stabilityand responses, coupled with higher piezoelectric coupling coefficients.

3(0.4) - (0) 75 1, 3, 4, 5(1.15)

1-15, 25(0.1)

Muralt et al. [51] D The article has reviewed the impact of composition, orientation, and microstructure on the piezoelec-tric properties of perovskite thin films The author described useful power levels for MEMS PHEs.

5(0.71) µW (0) 78 1, 2, 3 (0.85) 1-12, 1-13,13-7, 13-12, 13-13,18 (0.25)

Wang et al. [50] D The working mechanism, modeling, and structure design of piezoelectric nanogenerators were dis-cussed. Integration of nanogenerators for high output power sources, the structural design for in-creasing the energy harvesting efficiency in different conditions, and the development of practicableintegrated self-powered systems with improved stability and reliability are the critical issues in thefield classification of nano generators based on their desing and working modes were performed.

6(0.85) mW (0) 112 1, 4, 5 (0.85) 3, 16, 17,18 (0.15)

Selvan andAli [40]

D The capabilities and efficiencies off our micro-power harvesting methods including thermoelectric,thermo-photovoltaic ,piezoelectric, and microbial fuel cell renewable power generators are thoroughlyreviewed and reported

4(0.57) µW (0) 69 1, 2, 4 (0.85) 1-12, 13-8to 13-11,18 (0.15)

Wang [49] D 1: theoretical calculations and experimental characterization methods for predicting or determiningthe piezoelectric potential output of NWs were reviewed. 2: numerical calculation of the energyoutput from NW-based NGs. 3: Integration of a large number of ZnO NWs was demonstrated as aneffective pathway for improving the output power.

4(0.57) -(0) 80 2, 3 (0.57) 5 to 11(0.3)

Gosavi and Bal-pande [14]

D Description of some of synthesis and deposition techniques and performance parameters for MEMSPHEs

5(0.71) -(0) 100 1, 3 (0.57) 13 (13-1to 13-3)(0.05)

Kumar andKim [47]

D 1: Describing the mechanism of power generation behavior of nano-generators fabricated from ZnOnanostructures, 2: describing an innovative and important hybrid approach based on ZnO nano-structures.

17

Wei and Jing [58] presented a state-of-the-art reviewof theory, modeling, and realization of the piezoelectric,electromagnetic, and electrostatic energy harvesters. Thelinear inertia-based theory and the non-linear models havebeen described for three mentioned vibration-to-electricityconverters. They investigated some characteristics of thepiezo-harvesters such as being unaffected from external/internalelectromagnetic waves, simple structure, depolarization,brittleness of the bulk piezo-layer, the poor coupling inpiezo-film, and the poor adhesion with the electrode mate-rials. Development of new piezoelectric materials, creationof new energy harvesting configurations by exploring thenon-linear benefits, and design of efficient energy harvest-ing interface circuits are among their suggestions as futureprospects. They concluded that the non-linearity is an im-portant and effective parameter in terms of performanceenhancement. Theoretical modeling of the non-linear sys-tems with keeping reliability and stability is a challengingtask. The reviewed models have not been compared in thepaper.

Table 6 sums up the results of evaluation of the reviewpapers written about the modelling approaches. The tablealso contains different sub-categories, the range of outputpower, the number of reviewed articles, the merits, gen-eral conclusions, and some other extra descriptions. Therank of each paper has been computed based on the num-ber of merits, the number of subcategories, the number ofconcluding remarks, and clear emphasizing on Svalue ofminimum required output power.

4. Applications

4.1. Vibration

Vibration is the most common source of energy forpiezoelectric harvesters, since there is no need to convertthe input energy to the mechanical energy to produce elec-tricity in piezo-materials. Also, its abundance, accessi-bility, and ubiquity in environment, in addition to multi-ple possible transduction types have made it more attrac-tive for energy harvesting applications. The response ofpiezoelectric materials to the employed vibrations dependson their electromechanical properties like the natural fre-quency, their geometry, the electromechanical coefficients,and the damping characteristics. The design strategiesfor such types of harvesters, performance enhancementmethodologies, behavior of the energy harvesters in harshenvironment, their fatigue life, and failure mode, and theconditioning electric circuits are some of the important is-sues that should be addressed in review papers.

Kim et al. [60] summarized the key ideas behind theperformance evaluation of the piezoelectric energy har-vesters based on vibration, classifications, materials, andthe mathematical modeling of vibrational energy harvest-ing devices. They listed 17 important electro-mechanicalcharacteristics of PZT-5H, PZT-8, PVDF, and described

various configurations such as the cantilever type, the cym-bal type, the stack type, and the shell type. They ad-vised that the future opportunities for research are devel-opment of high coupling coefficient of piezoelectric mate-rials, giving the ability to sustain under harsh vibrationsand shocks, development of flexible and resilient piezoelec-tric materials, and designing efficient electronic circuitryfor energy harvesters. Siddique et al. [61] provided a litera-ture review on vibration-based micropower generation us-ing electromagnetic and piezoelectric transduction systemsand hybrid configurations. They reported some perfor-mance characteristics of the piezoelectric energy harvesterswith different materials and configurations. They claimedthat most of the recent research have been devoted to mod-ifications of the generator size, shape, and to introduce apower conditioning circuit to widen the frequency band-width of the system. Further research topics are develop-ment of the MEMS-based energy harvesters from renew-able resources and making the miniature electric devicesmore reliable. Figure 8 presents three schematic views ofmicorscale piezo-generators designed for vibration-basedenergy harvesting applications.

Sodano et al. [62], as one of the earliest reviewers ofthe field, discussed the future goals that must be achievedfor power harvesting systems to find their way towards theeveryday use, and to generate sufficient energy to powerthe necessary electronic devices. They mentioned that themajor limitations in the field of power harvesting revolvearound the fact that the power generated by the piezo-electric energy harvesters is far too small to power mostelectronic devices. Increasing the amount of energy gen-eration, developing innovative methods of accumulatingthe energy, use of rechargeable batteries, optimization ofthe power flow from a piezoelectric setup, minimizing thecircuit losses, identifying the location of power harvestingand the excitation range, proper tuning of the power har-vesting device are their predictions for future prospects ofthe vibration-based piezo harvesters.

Saadon and Sidek [63] presented a brief discussion ofvibration-basedMEMS piezoelectric energy harvesters. Theysummarized various designs of harvesters and reviewedexperimental results presented in the last 3 years beforethe date of publication of the paper. They focused onthe working modes and maximum output power of theMEMS piezoelectric energy harvesters. Harb [64] revieweda brief history of all energy harvesting methods includingthe vibration-based, the electromagnetic-based, the ther-mal or radioactive-based, pressure gradient-based, the so-lar and light-based, biological, and micro-water flow sys-tems. However, it is advised that the different types ofvibrations are the most available and the highest powerprovider sources. The review papers like the one presentedby Zhu et al. [65] are the result of an explosive utilizationof the vibration-based micro-generators in powering thewireless sensor networks. They demonstrated an overallreview of the principles and the operating strategies toincrease the operational frequency range of the vibration-

18

Table 6: Overall evaluation of review papers written on modeling of piezoelectric energy harvesters. The numbers inbrackets denote the number of non-general future lines. ”Cons.” stands for conclusions.Conclusions: Efficiency/performance improvement, necessity of frequency bandwidth broadening, necessity of opti-mizations, increasing life time and endurance, size reduction and manufacturability, necessity to improve the accuracyof PHE models.Merits: 1: giving the mathematical background of the models, 2: considering the energy losses, 3: taking into accountthe resonance and off-resonance conditions, 4: efficiency modeling, 5: mentioning the constraints and limitations of themodels, 6: mentioning the assumptions followed by the models, 7: comparison of the existing models.Sub-categories: 1- Energy conversion in PHEs with linear models, 2- Energy conversion in PHEs with nonlinear mod-els, 3- Modelling efficiency, 4- Modelling cantilever PHEs (4-1- SDOF models 4-2- distributed parameter modeling), 5-modeling the aeroelastic energy harvesting, (5-1- Flutter in airfoil sections, 5-2- vortex-induced vibrations in circularcylinders, 5-3- Galloping in prismatic structures, 5-4- VIV-/cylinder-based aeroelastic energy harvesters, 5-5- Galloping-based aeroelastic energy harvesters,5-6- Wake galloping, 5-7- SDOF models 5-8- Euler-Bernoulli distributed parametermodel).

# Cons. Minimum re-quired output

# Refs. Merits Sub-categories

Ref. Grade Highlights

6(1) mW (0) 21 1, 2, 3, 4, 5, 6, 7(2)

4-1 to 4-3(0.2)

Erturk and In-man [55, 56]

B Issues of the correct formulation for piezo-electric coupling, correct physical model-ing, use of low fidelity models, incorrectbase motion

6(1) mW (0) 48 1, 3, 4, 5, 6, 7(1.7)

5-7 to 5-8(0.2)

Zhao etal. [57]

C Comparing the performance of the mod-eling methods for GPEH, including theSDOF model, and single mode and mul-timode Euler-Bernoulli distributed param-eter models.

5(0.85) µW (0) 204 1, 2, 4 (0.85) 1, 2, 3, 4(0.8)

Wei andJing [58]

C 1 reviewing the energy conversion effi-ciency of some of the conversion mecha-nisms, 2 describing several configurationdesign for PHEs like cantilever structures,and uniform membrane structures.

6(1) mW (0) 201 3-5 (0.6) 5-1- to 5-6(0.2)

Abdelkefi [59] D Qualitative and quantitative comparisonsbetween existing flow-induced vibrationsenergy harvesters, describing some of thelimitations of existing models and recom-mending some improvement for future

19

Figure 8: (a) Geometry and position of neutral axis of piezo-composite composed of layers of carbon/epoxy, PZT ceramicand glass/epoxy [60], (b) a MEMS-based piezo-generator in 3-3mode [62], (c) schematic diagram of cross sectional view of a fab-ricated vibration-based micro power generator [61].

based micro-generators. Harne and Wang [66] reportedthe major efforts and findings about common analyticalframeworks and principal results for bi-stable electrome-chanical dynamics, and a wide variety of bi-stable energyharvesters. Based on their discussion, the remaining chal-lenges of such systems are maintaining high-energy orbits,operation under stochastic vibratory conditions, designingthe coupled bi-stable harvesters, and defining proper per-formance metrics.

In summery, different configurations of the piezoelec-tric cantilevers, their power output and the performanceenhancement strategies have been covered by the reviewpapers well. However, a systematic comparison of differ-ent configurations of piezoelectric energy harvesters, andalso their ability to sustain harsh vibrations and shocks,their fatigue life, their cost and accessibility have not beenconsidered by the reviews. Table 7 presents the resultsof evaluation of the piezo-electric energy harvesters fromvibrational sources. The table also contains different sub-categories, the range of output power, the number of re-viewed articles, the merits, general conclusions, and someother extra descriptions. The rank of each paper has beencomputed based on the number of merits, the numberof subcategories, the number of concluding remarks, andclear emphasizing on the value of minimum required out-put power.

4.2. Biological sources

Biomechanical energy harvesting provides an impor-tant alternative to electrical energy for portable electronicdevices. Hwang et al. [67] addressed the developmentsof flexible piezoelectric energy-harvesting devices by us-ing high-quality perovskite thin film and innovative flexi-ble fabrication processes. In addition, the energy harvest-ing devices with thick and rigid substrates are unsuitablefor responding to the movements of internal organs andmuscles. They commented that the electric power har-vested from the bending motion of a flexible thin film issufficient to stimulate heart muscles. Also, Easy bend-ability, higher conversion efficiency, enhanced sensing ca-pability in nanoscale, self-energy generation and the real-time diagnosis/therapy capabilities are among advantagesof such systems. Ali et al. [68] discussed the possibil-ities of utilizing the piezo-based energy conversion fromthe source of muscle relaxation and contraction, the bodymovement, the blood circulation, the lung and cardiac mo-tion in applications such as pacemakers, blood pressuresensors, cardiac sensors, pulse sensors, deep brain simula-tions, biomimetic artificial hair cells, active pressure sen-sors, and active strain sensors. The piezoelectric materialscontaining nanowires, nanorods, nanotubes, nanoparticles,thin films, the lead-based ceramics, the lead-free ceram-ics, the polymer-based materials, the textured polycrys-talline materials, and the biological piezo-materials havebeen evaluated. They proposed several challenging prob-lems such as the flexibility to fit into the shape of an organ,the proper management of power, selection of a media for

20

Table 7: Overall evaluation of review papers written on piezoelectric energy harvesting from vibration sources. ”Cons.”stands for conclusions.Conclusions: 1: Efficiency/performance improvement, 2: frequency tuning, 3: safety issues, 4: costs, 5: hybridharvesters, 6: non-linear models, 7: battery replacement, 8: miniaturization, 9: steady operation, 10: more efficientmaterials, 11: stochastic modeling.Merits: 1: electromechanical coupling factor, 2: realistic resonance, 3: energy flow, 4: range of output.Sub-categories: 1: circuits, 2: type of materials, 3: modeling, 4: noise level, 5: wearable, 6: frequency range, 7: MEMS# Cons. Minimum re-

quired output# Refs. Merits Sub-

categoriesRef. Grade Highlights

3 (0.27) µW to mW (1) 93 1-4 (2.0) 1, 2, 3, 5(0.57)

Kim et al. [60] B Comparison with electrostatic and electro-magnetic energy conversions

6 (0.55) µW to mW (1) 145 1-4 (2.0) 7 (0.14) Siddique etal. [61]

B Comparison with electromagnetic and elec-trostatic

6 (0.55) 0.17µW (1) 35 2-4 (1.5) 1, 3, 4, 5(0.57)

Sodano etal. [62]

B Insufficient output power

3 (0.27) 60µW (1) 23 2, 4 (1.0) 1, 7 (0.29) Saadon andSidek [63]

C Inadequate output power

- (0.0) 2.46mW (1) 56 3, 4 (10.) 1, 6 (0.29) Harb [64] C From thermal sources, RF sources, CMOSdevices, power management sources

7 (0.64) - (0) 50 2, 3 (1.0) 6 (0.14) Zhu et al. [65] D Focused on frequency tuning5 (0.46) - (0) 84 1, 2 (1.0) 2, 3 (0.29) Harne and

Wang [66]D Focused on bistable systems, stochastic vi-

brations

the electrical connection, enhancing the biological safety,designing the interface between the body tissue and theimplanted piezo-material, efficient encapsulation, furtherminiaturization, and conducting related experiments onsmall/large animal and human cases.

Surmenev et al. [69] described novel techniques in fabri-cation of hybrid piezoelectric polymer-based materials forbiomedical energy harvesting applications such as detec-tion of motion rate of humans, degradation of organic pol-lutants, and sterilization of bacteria. They described thedifferent methods that can be employed for the improve-ment of the piezoelectric response of polymeric materialsand scaffolds. They also reviewed biomedical devices andsensors based on hybrid piezo-composites. Similar to mostother reviews, increasing the performance is one of pro-posed future works. Others are alignment of nanofillerparticles inside the piezopolymer matrix, developing com-mon standards for consistently quantifying and evaluatingthe performance of various types of piezoelectric materials,and investigation of the structural parameters.

The internal charging of implantable medical devices(IMD) is another important biological application of piezo-electric energy harvesting. Extending the lifespan of IMDsand the size minimization have become main challenges fortheir development. For such devices, energy from the bodymovement, muscle contraction/relaxation, cardiac/lung mo-tions, and the blood circulation is used for powering medi-cal devices. Zheng et al. [70] presented an overall review ofthe piezoelectric energy devices in comparison to the tribo-electric harvesters with the source of body movement, mus-cle contraction/relaxation, cardiac/lung motions, and theblood circulation. They proposed that future opportuni-ties are fabrication of intelligent, flexible, stretchable, andfully biodegradable self-powered medical systems for mon-itoring biological signals, in vivo and in vitro treatment ofvarious diseases, optimization of the output performance,

obtaining higher sensitivity, elasticity, durability and bio-compatibility, biodegradable transient electronics, intelli-gent control of dynamic properties in vivo, improving theoperating lifetimes, and the absorption efficiency. Mhetreet al. [71] gave a brief review of micro energy harvestingtechniques and methods from the limb movement for drugdelivery purposes, dental applications, and the body heatrecovery using the piezoelectric transducers. They just an-nounced that the main challenge is to enhance the energyoutput using proper electronic circuit designs. Much moreresearch is required to harvest energy from other biolog-ical parameters such as the body temperature and respi-ration. An average amount of energy used by the body is1.07×107J per day. This amount of energy is equivalent toapproximately 800AA (2500mAh) batteries with the totalweight of about 20 kg. This considerable amounts of hu-man energy opens the road of development of energy har-vesting technologies for powering electronic devices [72].Riemer and Shapiro [72] investigated the amount of elec-tricity that can be generated from motion of various partsof the body such as heel strike, ankle, knee, hip, shoulder,elbow, arm, leg, the center of mass vertical motion, andthe body heat emission, using the piezo-harvesters andelectrical induction generators. They claimed that suchtechnologies are appropriate for the third world countries,which is to some extent doubtful referring to low perfor-mance and high cost of fabrication.

In addition to biocompatibility problems, the main chal-lenges in development of these types of energy harvestersare constructing a device that can harvest as much en-ergy as possible with minimal interference with the nat-ural function of the body. Also, the device should notincrease the amount of energy required by a person toperform his/her activities. Specially for IMDs, the life-time and efficient power output of the energy harvestersare of outmost importance. Figure 9 illustrates magni-

21

tude of harvestable energy sources from the human bodyorgans. Similar values can be predicted more or less fromorgans of animals in related applications. Xin et al. [73]

4- Walking1W, 2W

3- Typing1mW

2- Breathing100 mW

1- Upper limbs10 mW

5- Body heat emission100-525 W

6- Heel strike2-20 W

4

7- Knees36 W

8- Ankles66 W

9- Center of mass20 W

10- Hip38 W

11-Elbow2.1 W

12- Shoulder2.2W

13- Back pack50 mW

Figure 9: Available sources of energy from the human body organs.The data number 1-4 from Refs. [10], number 4-13 from [72]. Theresults are illustrated on Reza Abbasi’s ”Prince Muhammad-Beik”drawing (1620, public domain).

reviewed shoes-equipped piezoelectric energy harvesters.They described advantages and limitations of the currentand newly developing piezoelectric materials, including theflat plate type, the arch type, the cantilever type, thenanocomposite-based, the photosensitive-based, and thehybrid piezoelectric-semiconductors technologies. They an-nounced that enhancing the coupling coefficient of thepiezoelectric materials and optimizing the structure of theenergy harvester and the energy storing circuit require fur-ther investigation.

The reviewed articles about the biological applicationshave focused on highlighting new materials and structuresof biological energy harvesters and their power output.The bio-compatibility, the interference of the device withthe biological organ, reliability of the device along with itslifetime and economic issues are open topics in the field.Table 8 summarizes different highlights and descriptions

of the review articles related to the biological applications.The grade of each paper has been computed based on thenumber of merits, the number of subcategories, the num-ber of concluding remarks, and clear emphasizing on valueof minimum required output power.

4.3. Fluids

Wang et al. [74] categorized the fluid-induced vibra-tions for the purpose of energy harvesting into four cate-gories based on different vibration mechanisms: the vor-tex induced vibration, galloping, fluttering, and buffeting.They discussed the vortex-induced vibrations and buffet-ing (as forced vibration cases), galloping and flutter (aslimit-cycle vibration items) using electromagnetic, piezo-electric, electrostatic, dielectric, and triboelectric methodsalong with the corresponding numerical and experimentalendeavors. They presented a fruitful summary of the cur-rent research status on flow-induced vibration hydro/aeroenergy harvesters. It is concluded that the flow patternaround bluff bodies, the size limitations, estimation ofcosts of equipment, the maintenance costs, the lifespan,protection of equipment in the case of extreme weather,possible environmental impacts, the non-linear modeling,the intelligent regulating elements such as artificial neu-ral network, implementation of hybrid multi-purpose en-ergy harvesters, and development of new materials needto be further studied. Figure 10 presents four classes ofenergy harvesting: vortex-induced vibrations, buffeting,galloping, and fluttering, from vibration mechanisms cor-responding to fluid flows [74].

Flow-induced

vibration

Response Limit-cycle

Vortex-induced

Vibrations

BuffetingGalloping

Fluttering

Figure 10: Different classes of energy harvesting categories from flow-induced vibrations [74].

Truitt and Mahmoodi [75] reviewed effects of wind-based energy harvesting from flow-induced vibrations bybluff bodies and aeroelastic instabilities (fluttering and gal-loping). They presented an overall study of energy genera-tion density and the peak power outputs versus the band-width. After a brief review of dynamics of piezoelectricenergy harvesting, theories and principles, energy densi-ties and output powers, they concluded that the balanceof efficiency-cost-manufacturability is the future horizon ofthe topic. They suggested the use of PVDFs in fluid ex-citation applications due to their increased flexibility over

22

Table 8: Overall evaluation of review papers written on piezoelectric energy harvesting from biological applications.”Cons.” stands for conclusions.Conclusions: 1: Efficiency/performance improvement, 2: safety issues, 3: costs, 4: hybrid harvesters, 5: non-linearmodels, 6: battery replacement, 7: miniaturization, 8: steady operation, 9: efficient (flexible, stretchable, bio-compatible)materials, 10: self-powered, 11: being wearable, 12: control systems.Merits: 1: electromechanical coupling factor, 2: realistic resonance, 3: energy flow, 4: range of output.Sub-categories: 1: organ motion, 2: heel strike, 3: ankle, 4: Knee, 5: hip, 6: center of mass, 7: arms, 8: muscles, 9:cardiac/lung motion, 10: blood circulation, 11: heat emission, 12: drug delivery, 13: dental cases, 14: thin films, 15:artificial hair cell, 16: biosensors.# Cons. Minimum re-

quired output# Refs. Merits Sub-

categoriesRef. Grade Highlights

5 (0.42) 250 V, 8.7 µA(1)

71 1, 3, 4 (1.5) 1, 9, 14, 15(0.25)

Hwang etal. [67]

B Focused on thin films

9 (0.75) 11V, 283µA (1) 240 1, 4 (1.0) 1, 8, 9, 10, 16(0.31)

Ali et al. [68] B -

8 (0.67) 1µF , 20V , 50s(1)

235 1, 4 (1.0) 1, 16 (0.13) Surmenev etal. [69]

C Lead-free polymer-based, size-dependenteffects, insufficient output power of piezo-electric polymers and their copolymers

7 (0.58) 11mWcm−3 (1) 107 1, 4 (1.0) 8, 9, 10(0.19)

Zheng etal. [70]

C Comparison with triboelectric

1 (0.08) mW(1) 29 4 (0.5) 12, 13 (0.13) Mhetre etal. [71]

C -

4 (0.33) 2W (1) 38 4 (0.5) 1, 2, 3, 4, 5,6, 7, 11 (0.5)

Riemer andShapiro [72]

C Comparison with electrical induction gen-erators and electroactive polymers

4 (0.33) -(0) 53 1 (0.5) 1 (0.06) Xin et al. [73] E Shoes-equipped Geometry classification

PZTs. They concluded that the fluttering- and galloping-based methods generate a higher output power, but with anarrower frequency bandwidth in comparison to the vortexinduced methods. Also, the final vision for energy harvest-ing may be active energy harvesting in which the systemdynamics can actively change in real-time to meet chang-ing environmental dynamics. Viet et al. [76] comparedthree energy harvesting methods, including electrostatic,electromagnetic, and piezoelectric technologies to indicateprivileges of the piezoelectric harvesting in power gener-ation, transmission, structural installation, and the eco-nomic costs. Then, they reviewed different design method-ologies of harvesting energy from ocean waves. Effects oflongitudinal, bending, and shear couplings have been dis-cussed. It is concluded that due to higher energy genera-tion density, higher voltage generation capability, simplerconfiguration, and more economic benefits, the piezoelec-tric technology is superior to the other methods. Elahi etal. [77] studied the fluid-structure interaction-based, thehuman-based, and the vibration-based energy harvestingmechanisms by qualitatively and quantitatively analyz-ing the existing piezoelectric mechanisms. They reviewedthe vortex-induced vibration, fluttering, galloping, and thehuman-related structures. They commented that a signif-icant amount of research has been conducted on aeroe-lastic energy harvesters, but aerodynamic models can beimproved by taking into account steady, quasi-steady, andunsteady aerodynamics. McCarthy et al. [78] reviewedthe research done on piezoelectric energy harvesting basedon fluttering. They introduced the mathematical termsneeded to define the performance of the fluttering har-vester. They discussed effects of the Strouhal number as afunction of the Reynolds number, the wind characteristics,

and formation of the atmospheric boundary layer (ABL).They declared that the ultra-low power densities, the longreturn period of investments, and quantification and al-leviation of the fatigue damage are the most challengesfor fluttering energy harvesting. Based on their opinion,determining the fatigue life and some metrics for a piezo-electric flutter, weather and precipitation effects are activeresearch fields.

Hamlehdar et al. [79] presented a review of energy har-vesting from fluid flows. Despite the general topic of thepaper, the piezo-energy harvesting from blood as a liq-uid has been ignored. They have performed a literaturereview on energy production from vortex induced vibra-tion, the Karman vortex street, the flutter induced mo-tion, galloping, and the waves with water and air as work-ing fluids. Also, there is a short discussion on modelingchallenges. The results of the review conducted by Wonget al. [80] implies that the piezoelectric energy harvestingform the rain drop has privileges such as simple struc-ture, easier fabrication, reduced number of components,and direct conversion of vibrational energy to electricalcharge. They stated that the main challenge in this field isto design and optimize the raindrop harvester for outdooruses, being resistant against sunlight, wind, the impactforce of larger drops, being waterproof, showing appro-priate sensitivity to drops, supplying constant-rate energyover long periods of time, and optimizing the power ef-ficiency. Chua et al. [81] reviewed different types of theraindrop kinetic energy piezoelectric harvester, includingthe bridge-structure, the cantilever structure with the im-pact point near the free-end, the cantilever structure withsix impact points at varies surface locations, the cantileverstructure with impact point at the center, the PVDF mem-

23

brane or the PZT edge-anchored plate, and the collectingdiaphragm cantilevers. Also, they presented a brief sum-mary of characteristics of hybrid harvesters. It is statedthat the best parameter to compare different harvesters isthe efficiency rather than the output peak power. Thenbased on this criterion, it is found that the cantilever-typeand the bridge-type energy harvesters made of PZT arethe best choices. This is to some extent in contrast to therecommendations of Wong et al. [80].

Table 9 presents the details and highlights of reviewpapers on fluid-based piezo-energy harvesting. The gradeof each paper has been computed based on the number ofmerits, the number of subcategories, the number of con-cluding remarks, and clear emphasizing on value of mini-mum required output power.

4.4. Ambient waste energy sources

Guo and Lu [82] discussed recent advances in applica-tion of thermoelectric and piezoelectric energy harvestingtechnologies from the pavements. They found out thata pipe system cooperating with a thermoelectric gener-ator is superior in terms of cost effectiveness and elec-tricity output to piezoelectric transducers (fabricated withPZT). Based on their recommendations, the impact of thementioned energy harvesting facilities to pavement perfor-mance, life cycle assessments, optimization with respectto traffic conditions and solar radiation, and the changein vehicle fuel consumption due to additional vehicle vi-bration or resistance should be evaluated in future works.Duarte and Ferreira [83] presented a comparative study ofphotovoltaic, thermoelectric, electromagnetic, hydraulic,pneumatic, electromechanical, and piezoelectric harvest-ing technologies. Evaluation parameters are the conversionefficiency, the maximum generated power, the installationmethod, and their TRL. They declared that the essentialeconomic data of products are not yet available. Wanget al. [84] illustrated applications of the photovoltaic cells,solar collectors, geothermal, thermoelectric, electromag-netic, and piezoelectric energy extraction systems frombridges and roads in terms of energy output, benefit-costratio, and the technology readiness level. Based on theirconclusions, the grade of support of the piezoelectric har-vesters by governments is low to medium, while the so-lar and geothermal systems are strongly being supported.Pillai and Deenadayalan [85] presented a review of acous-tic energy harvesting methods and piezoelectricity as apromising technology in this category due to being sen-sitive and efficient at high frequency excitations. theydeclared that optimization of the resonator and the cou-pling of thermo-acoustic engine to the acoustic-electricityconversion transducer are open research fields. Khan andIzhar [86] reviewed the recent developments in the fieldof electromagnetic- and piezoelectric-based acoustic en-ergy harvesting. They reported sound pressure levels ofvarious ambient acoustic energy sources. A set of use-ful data about the sound pressure level (dB) and the fre-quency of various acoustic energy sources have been re-

ported. They declared that researchers were focusing onenhancing the performance of the piezoelectric membranethrough novel fabrications and optimized geometrical con-figurations. Duarte and Ferreira [87] made a compara-tive study on road pavement energy harvesting technolo-gies. They compared existing technologies based on theinstalled power (per area or volume), the conversion effi-ciency, the power density. Also, they classified the harvest-ing technologies based on their TRL (technology readinesslevels) values. It is demonstrated that the piezoelectrictechnology is at high TRL grades. But, it delivers insuf-ficient energy production rate with low economic charac-teristics.

Also, some of previously discussed papers have devoteda part of their review to piezo-based energy harvestingfrom waste energies. Performance of the electromagnetic-and piezoelectric-based vibration energy harvesters for en-ergy production from bridges has been evaluated by Khanand Ahmad [23]. They have expressed that the majorityof current harvesters are constructed based on the elec-tromagnetic effect, but the piezo-materials are commer-cially available and are easy to develop. The resonantfrequency is a critical parameter in such narrow-band low-frequency applications, which is a privilege of the electro-magnetic systems. Maghsoudi Nia et al. [24] presenteddifferent technologies of converting the kinetic energy ofthe human body during walking to electricity by locat-ing a harvesting system on the body or inserting a har-vester in the floor. In contrast to the results of Guoand Lu [82], it is recommended that the piezoelectric har-vester is a better choice for such applications, due to sim-plicity and flexibility, regardless of a lower power output.Yildirim et al. [31] reviewed amplification techniques, reso-nance tuning methods, and non-linear oscillations in appli-cations involving the ambient vibration harvesting, basedon piezoelectric, electrostatic, and electromagnetic conver-sion methods. Al-Yafeai et al. [38] reviewed methodologiesto convert the dissipated energy in the suspension dampersof a car to electricity, along with discussing the math-ematical car models and respective experimental setups.The disadvantages of the piezo-generator in comparison toother methods are poor coupling, high output impedance,charge leakage, and the low output current. However,the advantages are simple structure, no need to externalvoltage sources and mechanical constraints, compatibilitywith MEMS-based devices, high output power, and hav-ing wide frequency range. Al-Yafeai et al. [38] presented areview of design considerations for energy harvesting fromcar suspension system, including different piezo-materials,various mathematical modeling, the power dissipation, thenumber of degree-of-freedoms, the road input, the locationof the piezo-system, and the electronic circuit. Dagde-viren et al. [39] highlighted essential mechanical to electri-cal conversion processes and the key design considerationsof flexible and stretchable piezoelectric energy harvestersappropriate for soft tissues of human body, smart robotsand metrology tools. They declared that the development

24

Table 9: Overall evaluation of review papers written on piezoelectric energy harvesting from fluids. The numbers inparentheses denote number of non-general future lines. ”Cons.” stands for conclusions.Conclusions: 1: Efficiency/performance improvement, 2: frequency tuning, 3: safety issues, 4: costs, 5: hybridharvesters, 6: non-linear models, 7: battery replacement, 8: miniaturization, 9: steady operation, 10: more efficientmaterials.Merits: 1: electromechanical coupling factor, 2: realistic resonance, 3: energy flow, 4: range of outputSub-categories: 1: water waves, 2: galloping, 3: fluttering, 4: buffeting, 5: modelling, 6: wind’s vortex street, 7:instabilities, 8: raindrop, 9: mechanical design.# Cons. Minimum re-

quired output# Refs. Merits Sub-

categoriesRef. Grade Highlights

9 (0.9) >0.0289mW (1) 125 1, 2, 3, 4 (2.0) 1, 2, 3, 4(0.44)

Wang etal. [74]

A Internet of things, machine learning tools

4 (0.4) 115mW(1) 62 1 2, 3, 4 (2.0) 2, 3, 6, 7(0.44)

Truittand Mah-moodi [75]

B Active control theory

4 (0.4) 116µW/cm3 (1) 96 1, 2, 4 (1.5) 1, 9 (0.22) Viet et al. [76] B -5 (0.5) nW to mW (1) 256 1, 4 (1.0) 2, 3, 6 (0.33) Elahi et

al. [77]C Human-based sources

5 (0.5) 440µW/cm3 (1) 96 2, 4 (1.0) 3, 6, 9 (0.33) McCarthy etal. [78]

C Noise level, Atmospheric boundary layer,Fatigue life

5 (0.5) >1µW(1) 199 4 (0.5) 1, 2, 5, 6, 7(0.56)

Hamlehdar etal. [79]

C Biomimetic design

4 (0.4) -(0) 87 1, 3, 4 (1.5) 8 (0.11) Wong etal. [80]

C Size effects

4 (0.4) µW (1) 73 4 (0.5) 8 (0.11) Chua etal. [81]

C Circuit design, Hydrophilic surface

outlooks of such devices are the designs and fabricationtechniques.

Table 10 presents details and highlights of the reviewpapers on ambient and waste energy piezo-harvesting meth-ods. The grade of each paper has been computed basedon the number of merits, the number of subcategories, thenumber of concluding remarks, and clear emphasizing onvalue of minimum required output power.

5. Challenges and the roadmap for future research

Table 11 illustrates the number of published review pa-pers on each field, the year of the first and the last pub-lished review paper, and the research fund sources. It isobvious that the energy harvesting from ambient energies,the MEMS/NEMS and the fluid-based harvestinggs, andmaterial considerations, respectively have the highest rateof publication of the review papers. The numbers in brack-ets demonstrate the number of funded review papers. Al-though it is predictable that some supporters prefer to re-main anonymous, it is seen that about 46% of papers havebeen supported by a non-university organization. The lastcolumn of the table presents a list of organizations and re-spected countries that have devoted a full/partial financialsupport to the review papers on piezo-materials.

It is expected that the forthcoming review papers fo-cus on specialized topics. However, they may still containsome degree of generality. Due to the multidisciplinarynature of the field, it is vital to publish comprehensive re-views on detailed aspects of the piezoelectric harvesters.Publication of review papers with general topics is notvery welcomed anymore. The rate of publication of thereview papers on biological topics is less than expected.

Due to the rapid progress of piezoelectricity in biomedicalengineering, increasing the number of reviews in relatedfields is inevitable. We suggest the researchers to presentsome state-of-the-art articles with specific topics includ-ing progress in piezoelectric materials, new applications ofpiezoelectric energy harvesters, and new developments inMEMS and NEMS piezoelectric harvesters.

The results of comparative researches on energy har-vesters for the railway demonstrated that, even in macroscaleenergy harvesting, the piezoelectric energy harvesters arenot very successful with respect to other harvesting tech-nologies. This situation may be worst for micro and nanoscaleharvesters. We predict that the single (non-hybrid) piezo-electric energy harvesters would be the true choice onlyin some specific applications for which other harvestingsystems have inherent limitations. Thus, there is an es-sential need for making fair comparisons of all types ofenergy harvesters for specific applications. On the otherhand, we encounter the growing number of publicationson piezoelectric energy harvesters. It should be noted thatthe real world selects the energy harvesting systems withhigher performances and lower costs.

Based on the data listed in Table 11, three types ofresearch lines have been detected:

1. Pioneering topics that are still under consideration:general reviews (2005-2019), the design key points(2005-2020), the material-related studies (2009-2019),the MEMS-based devices (2006-219),

2. Pioneering topics without any recent publication ofreview papers: the modeling approaches (2008-2017),the vibration-based harvesters (2004-2015), sensorsand actuators (2007-2016),

25

Table 10: Overall evaluation of review papers written on piezoelectric energy harvesting from waste energies. ”Cons.”stands for conclusions.Conclusions: 1: Efficiency/performance improvement and optimization, 2: safety issues, 3: costs, 4: hybrid harvesters,5: non-linear models, 6: battery replacement, 7: miniaturization, 8: steady operation, 9: efficient materials, 10: controlsystems.Merits: 1: electromechanical coupling factor, 2: realistic resonance, 3: energy flow, 4: range of outputSub-categories: 1: acoustic energy, 2: modelling, 3: road pavement, 4: railway, 5: bridge.# Cons. Minimum re-

quired output# Refs. Merits Sub-

categoriesRef. Grade Highlights

4 (0.4) 241Wh/y (1) 120 3, 4 (1.0) 3, 5 (0.4) Wang etal. [84]

C Comparison with photovoltaic cell, solarcollector, geothermal, thermoelectric, elec-tromagnetic devices, Fatigue failure andlife-cycle

4 (0.4) 100mW (1) 65 3, 4 (1.0) 2, 3 (0.4) Guo andLu [82]

C Comparison with thermoelectrics

4 (0.4) 10-100W(1) 34 3, 4 (1.0) 4 (0.2) Duarte andFerreira [83]

C Comparison with electromagnetic devices,TRL level presented

3 (0.3) -(0) 80 2, 4 (1.0) 1, 2 (0.4) Pillai andDeenaday-alan [85]

D Comparison with thermo-acoustics

2 (0.2) -(0) 54 2, 4 (1.0) 1 (0.2) Khan andIzhar [86]

D Comparison with electromagnetics

2 (0.2) -(0) 97 4 (0.5) 3 (0.2) Duarte andFerreira [87]

E Comparison with solar, thermoelectric,electromagnetic devices

3. The newly developed topics: fluids (2013-2020), am-bient waste energy (2014-2020), the biological appli-cations (2011-2019).

The missing topics and the concluding future researchtopics, which need more close investigations to demon-strate their state-of-the-art are

1. Development of hybrid multi-purpose energy gener-ators to completely harness energy of any kind andwith any characteristics combining the piezo-pyro-tribo-flexo-thermo-photoelectric technologies.

2. Investigation of the mathematical models, the ana-lytical and numerical solution techniques especiallyin nanoscale geometries where the classical contin-uum mechanics principal fails or in stochastic andnon-linear situations. Some modified constitutive re-lations may need to be developed in non-continuumregimes. Also, the second law analysis and analysisof such systems from the thermodynamic viewpointare the missing topics. The ab initio first principalsimulations with atomistic nature are other challeng-ing aspects of the nanoscale piezo-harvesters. Devel-opment of opensource codes like OpenFOAM andLAMMPS to include the solvers involving the piezo-electric effect may be another future research topic.

3. Application of piezo-materials in energy saving or re-ducing energy demand of a system rather than gen-eration of energy requires a comprehensive review.An example of such energy reduction is the delay indecaying disturbances and delaying transition to tur-bulence using piezo-actuators placed on the surfaceof bluff bodies.

4. Due to the multi-physics nature of the piezoelec-tric effect, it is highly recommended to prepare re-

view papers on optimization methods or machinelearning-related topics.

5. Commercialization of the piezo-based harvesters andenhancing the technology readiness level need a se-rious attention. Perhaps, the next decade is thedecade of extensive commercialization of the piezo-harvesters.

6. Plenty of patents have been published in recent years.Even some review papers should be devoted to inves-tigation of patents presented in the field.

7. Focused reviews are needed on vibration-based piezo-harvesters in four recent years, development of piezotron-ics, and design of complete self-powered autonomoussystems.

8. The overall design of devices including all parts, in-tegrating the whole device in thin films, accumula-tion in rechargeable batteries, and taking into ac-count the energy consumption needed to store theharvested energy.

9. Optimization of device architecture and size reduc-ing configurations for portable applications, flexiblewearable compact embedding implantable devices.

10. In situ prototype testing and design of harvesterscoupled with environment and realistic applicationsto face with sunlight in outdoor applications, natu-rally occurring stochastic vibrations, the wind speedvariation, dust, noises, required flexibility to fit theshape of human organs, and waterproofness.

11. Quantification of the figure of merit for the piezo-material properties such as energy transforming orconversion efficiency and standardizing the perfor-mance of piezo-based devices.

12. Reducing the maintenance cost, enhancing the lifes-pan, ameliorating the performance, analysis of gov-

26

Table 11: Statistics of review papers published on different topics related to piezoelectric energy harvesting. The numbers in bracketsdemonstrate the number of funded review papers in each field.

Topic #reviews Period # reviews per year Non-university research fund sources

1 General 8(4) 2005-2019 0.53 National Science Foundation (USA), National Natural Sci-ence Foundation (China), Spanish Ministry of Scienceand Technology and the Regional European Develop-ment Funds (European Union), NanoBioTouch Europeanproject/Telecom Italia/Scuola Superiore SantAnna (Italy).

2 Design 15(5) 2005-2020 0.94 Texas ARP (USA), U.S. Department of Energy Wind andWater Power Technologies Office (USA), Ministry of HigherEducation (Malaysia), Natural Science and Engineering Re-search Council (Canada), National Natural Science Founda-tion (China)/EU Erasmus+ project/Bevilgning

3 Material 11(7) 2009-2019 1.00 M/s Bharat Electronics Limited (India), National NatureScience Foundation (China), Office of Basic Energy Sciences,Department of Energy (USA)/Center for Integrated SmartSensors funded by the Korea Ministry of Science (Korea),National Natural Science Foundation (China)/Shanghai Mu-nicipal Education Commission and Shanghai Education De-velopment Foundation (China), European Research Coun-cil/European Metrology Research Programme/ UK NationalMeasurement System, National Natural Science Foundation(China), China scholarship Council/China Ministry of Edu-cation/Institute of sound and vibration

4 Modeling 5(3) 2008-2017 0.5 Air Force Office of Scientific Research (USA), Air Force Of-fice of Scientific Research (USA), a NSFC project of China

5 Vibration 8(1) 2004-2015 0.67 Energy Efficiency & Resources of the Korea Institute of En-ergy Technology Evaluation/Creative Research Initiatives

6 Biology 6(5) 2011-2019 0.67 Russian Science Foundation/Alexander von Humboldt Foun-dation/European Commission, National Key R&D Projectfrom Minister of Science and Technology (China), Basic Sci-ence Research Program (Korea)/Center for Integrated SmartSensors as Global Frontier Project, R&D Center for GreenPatrol Technologies through the R&D for Global Top Envi-ronmental Technologies program funded by the Korean Min-istry of Environment, Paul Ivanier Center for Robotics andManufacturing Research/Pearlstone Center for AeronauticsResearch

7 Sensors 5(4) 2007-2016 0.5 Spanish Ministry of Education and Science, NSS-EFF/fellowship/ NSF/ Ben Franklin Technology PArt-ners/the Center for Dielectric Studies/ARO/DARPA/theMaterials Research Institute/U.S Army Research Labora-tory, Converging Research Center Program by the Ministryof Education Science and Technology (Korea), Basic ScienceResearch Program through the National Research Founda-tion of Korea

8 MEMS/NEMS 15(7) 2006-2019 1.07 National Science Foundation (China), the Basic Science Re-search Program, through the National Research Foundationof Korea, European Research Council, Ministry of Educa-tion (Malaysia), Office of Basic Energy Sciences Departmentof Energy (USA), International Research and DevelopmentProgram of the National Research Foundation of Korea

9 Fluids 8(3) 2013-2020 1.00 Ministry of Higher Education (Malaysia), National Natu-ral Science Foundation (China), Australian Research Coun-cil/FCSTPty Ltd

10 Ambient 11(4) 2014-2020 1.57 Center for Advanced Infrastructure and Transportation(USA), Portuguese Foundation of Science and Technology,European Regional Development Fund, National NaturalScience Foundation of China

27

ernment supports, the cost-benefit balance, and in-vestigations of piezo-harvesting from energy policyviewpoint.

13. Thermal design of piezo-systems including the temperature-dependent properties and high-temperature harvest-ing limitations.

14. Fabrication of new piezo-materials with the non-linearbehavior, larger displacements, lower frequencyies,wider operation bandwidth, and the frequency self-adaptation capabilities.

15. Use of meta-materials, non-toxic, biocompatible, print-able piezo-materials, nanofibers, lead-free and high-piezoelectric coefficient materials.

16. Improving the design of electrical circuitry and man-aging rectification and storage losses.

17. Modification of structural designs including fracture-fatigue studies to increases reliability, stability, anddurability of the device.

18. Design of efficient control techniques.

19. Extending the application of piezo-materials in novelfields such as internet of things.

20. Paying close attention to the use of unimorph designfor high-energy harvesting rate, obtaining realisticresonance data in order to reach compactness, inves-tigating energy output much lower than 1mW, andstep-by-step report of successive energy flow or ef-ficiency from input mechanical energy to the finalelectric energy in a rechargeable battery.

21. Focusing on applications involving elimination, re-striction, and replacement of toxic materials and en-vironmental pollution.

22. Development of designs exhibiting the highest elec-tromechanical coupling factor.

23. Considering mechanical impedance matching, elec-tromechanical transduction, electrical impedance match-ing and priority of these factors.

24. Development of other applications as energy harvest-ing devices with low energy demand.

25. Designing a grid of nano-devices (thousands) or thickfilms (10 to 30 microns) to generate minimum 1mWpower (the required electric energy to operate a typ-ical energy harvesting electric circuit with a DC/DCconverter).

26. General development directions may be remote sig-nal transmission, and energy saving in rechargeablebatteries.

However, the research on piezoelectric energy harvest-ing is not mature enough and many interdisciplinary activeresearch fields are currently available. It should be men-tioned that the progress of small-scale devices with verylow power need is tightly tied to the revolution in design ofefficient high-output power piezoelectric energy harvesters.It is recommended to lie on fundamental principles in orderto obtain unique designs for future research.

Acknowledgment

This research was supported by the Iran National Sci-ence Foundation (Grant number 98017606).

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