Piezoelectric Power Harvesting Devices: An Overview
Ashok K. Batra1�∗, Almuatasim Alomari1, Ashwith K. Chilvery2, Alak Bandyopadhyay3, and Kunal Grover41Department of Physics, Chemistry and Mathematics (Materials Science Group), Alabama Agricultural and Mechanical University,
Normal, Alabama 35762, USA2Department of Physics and Dual Engineering, Xavier University of Louisiana, New Orleans, Louisiana 70125, USA
3Department of Electrical Engineering and Computer Science, Alabama Agricultural and Mechanical University,Normal, Alabama 35762, USA
4Henry Ford Health System, Detroit, Michigan 48202, USA
This article reviews the fundamental behavior of piezoelectric for applications in sensors and energyharvesting technologies. In fact, many devices and applications are evolving day-to-day dependingon smart materials technology such as, scanning probe microscope (SPM) and cigarette lighters.Today, vibration based energy harvesting via piezoelectric materials has become one of the mostprominent ways to provide a limited energy for self-powered wireless sensor and low power electron-ics. This review provides an insight that involves mathematical modeling of constitutive equations,lumped parameter model, mechanisms of piezoelectric energy conversion, and operating principleof a piezoelectric energy harvesting system. This article also focuses on the dielectric, piezoelectric,mechanical, and pyroelectric properties of piezoelectric and pyroelectric materials open to use fromsingle crystal such as PMN-PT through ceramics PZT and polymers such as PVDF. Recent impor-tant literature is also reviewed along with energy harvesting devices proposed for use in industrialand biomedical applications.
Keywords: Piezoelectric, Pyroelectric, Vibration Based Energy Harvesting, ConstitutiveEquations, Pyroelectric Energy Harvesting.
∗Author to whom correspondence should be addressed.
1. INTRODUCTIONEnergy harvesting remains a topic of intense interest inacademic and industrial settings since it provides a routefor the realization of autonomous and self-powered low-power electronic devices. Energy harvesting or power har-vesting or energy scavenging is defined as capturing a smallamount of energy from one or more of the surroundingenergy sources, accumulating and then, storing them forlater use. The ability to deliver sustainable electric powerto micro electromechanical systems (MEMS) or a wirelesssystem network by energy harvesting is attractive not onlybecause of the cost of batteries but it also removes the addi-tional time and cost that is necessary to replace and main-tain the batteries including installation of complex wiredsystems. This is in particular, relevant to the installation ofsensor nodes in areas that are hostile or difficult to reach.These are safety monitoring devices, structure-embeddedmicro-sensors, and medical implants. Furthermore, thereare also environmental benefits associated with limiting oreliminating the disposal of batteries.1 Thus, energy har-vesting devices provide a ‘battery-less’ solution by scav-enging energy from ambient energy sources such as light,
Piezoelectric Power Harvesting Devices: An Overview Batra et al.
heat, water and mechanical vibrations etc., and convertingit into a useable electrical power. There are also advancesin MEMS, micro-devices, nano-devices: microprocessortechnology leading to an increase in power efficiency and
Ashok K. Batra holds a Masters of Technology and Ph.D. from the Indian Institute ofTechnology, Delhi. With more than 23 years of experience in the diverse areas of solid-state physics/materials and their applications, he is presently a Professor of Physics. Hisresearch experience and interests encompass ferroelectric, pyroelectric, piezoelectric mate-rials and their applications, design, fabrication and characterization of pyroelectric, piezo-electric, photo-thermal and photovoltaic devices, nonlinear optical organic crystals, organicsemiconductors, crystal growth from solution and melt, microgravity materials research,nanocomposites and chemical sensors. Currently, Professor Batra is engaged in researchrelated to the development of ambient energy harvesting and storage devices, nanoparticles-based chemical sensors and organic photovoltaic solar cells. He has obtained variousresearch grants as the principal or co-investigator from the U. S. Army/SMDC, NSF, DHS
and NASA. The NASA grant was related to the International Microgravity Laboratory-1 experiment flown aboard theSpace Shuttle Discovery. Receiver of a NASA Group Achievement award and the Alabama A&M University School ofArts and Sciences Researcher of the Year award, he has published over 120 publications including a book, book chapters,proceedings, review articles and NASA TMs. Professor Batra is a member of SPIE, MRS, AES and AAS.
Almuatasim Alomari was born in Irbid, Jordan, 1985. He received a B.Sc. degree inphysics from the Yarmouk University, Jordan in 2007 and a M.Sc. in applied physicsfrom the Jordan University of Science and Technology, Jordan in 2011. He is currently aPh.D. student in physics/material science at Alabama A&M University, USA. His researchinterests include dielectrics, piezoelectric materials, piezoelectric composites, and smartsystems. He enjoys playing soccer, snooker, and chess.
Ashwith K. Chilvery is employed as an Assistant Professor in the Department of Physicsand Engineering at Xavier University of Louisiana. He has a Ph.D. in Applied Physicsfrom Alabama A&M University, and M.S. in Electrical Engineering from University ofSouth Alabama. His research interests are in the areas of photovoltaics, smart materialsfor energy harvesting, chemical and biological sensors and detectors. Prior to joiningXavier University of Louisiana, he was employed as Assistant Professor and Coordinatorof Physics in the Division of Natural Sciences and Mathematics at Talladega College for2.5 years. During his tenure, he served as the Co-PI/Site Coordinator for the AlabamaLouis Stokes Alliance for Minority Participation (ALSAMP) at Talladega College andplayed a vital role in enhancing the STEM recruitment and retention procedures. He is alsoaffiliated with scientific societies such as International Society for Optics and Photonics
(SPIE), Material Research Society (MRS) and Optical Society of America (OSA). Currently, Dr. Chilvery has over25 peer-reviewed publications.
Alak Bandyopadhyay is an Associate Professor of Computer Science at Alabama A&MUniversity. Dr. Bandyopadhyay has more than 25 years of research experience in algo-rithm development, numerical methods, modeling and simulation of fluid dynamics andMultiphysics problems. Dr. Bandyopadhyay has a Ph.D. in Mechanical Engineering fromUniversity of Minnesota, Minneapolis and Masters in Mechanical Engineering from IndianInstitute of Technology, Kanpur. Dr. Bandyopadhyay has vast experience in aerospacepropulsion, process modeling, laminar and turbulent incompressible and compressibleflow, and other areas of computational mechanics. Dr. Bandyopadhyay has more than30 research publications including journal and conferences.
reduced power consumption. Energy storage solutions arealso improving such as development of super-capacitorsand even structural power that will ultimately lead to suc-cessful energy harvesting products and systems.1 In this
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Batra et al. Piezoelectric Power Harvesting Devices: An Overview
Kunal Grover is a Senior Staff Physician in the Department of Internal Medicine at HenryFord Hospital in Detroit, MI. He is board certified in Internal Medicine and specializesin Hospital Medicine. Dr. Grover received his B.S. from the University of Michigan inAnn Arbor, MI and his M.D. from Wayne State University School of Medicine in Detroit,MI. He completed his residency training in Internal Medicine at Henry Ford Hospital. Hisresearch interests are in digital health, healthcare innovation, medical instrumentation, andpiezoelectric energy harvesting for implanted microsystems.
article, we provide a list of number of excellent publicationreviews and papers in the areas of piezoelectric energy har-vesting and surveys of potential devices. The objective ofthis article is to provide an overview of energy harvestingtechnologies, potential devices and associated ‘piezoelec-tric’ materials.
2. PIEZOELECTRICITY ANDPOWER HARVESTING
2.1. Fundamentals of PiezoelectricityMany materials, both natural and synthetic, exhibit piezo-electricity. Crystals which acquire a charge when com-pressed, twisted, or distorted are said to be piezoelectric.This provides a convenient transducer effect between elec-trical and mechanical oscillations. The generation of anelectric charge in certain non-conducting materials, suchas quartz crystals and ceramics, when they are subjectedto mechanical stress (such as pressure or vibration), orthe generation of vibrations in such materials when theyare subjected to an electric field. Piezoelectric materialsexposed to a fairly constant electric field tend to vibrate ata precise frequency with very little variation. The natureof the piezoelectric effect is closely related to the occur-rence of electric dipole moments in solids. Of decisiveimportance for the piezoelectric effect is the change ofpolarization ( �P ) when applying a mechanical stress. Thismight either be caused by a re-configuration of the dipole-inducing surrounding or by re-orientation of moleculardipole moments under the influence of the external stress.Piezoelectricity may then manifest in a variation of thepolarization strength, its direction or both, with the detailsdepending on(i) the orientation of �P within the crystal,(ii) crystal symmetry and(iii) the applied mechanical stress.
The change in �P appears as a variation of surface chargedensity upon the crystal faces, i.e., as a variation of theelectrical field extending between the faces caused bya change in dipole density in the bulk. For example, a1-centimeter cube of quartz with 2 kN (500 lb) of correctlyapplied force can produce a voltage of 12,500 V. Piezo-electric materials also show the opposite effect, called
converse piezoelectric effect, where the application of anelectrical field creates mechanical deformation in the crys-tal. Piezoelectric materials exhibit both a direct and areverse piezoelectric effect. Figure 1 indicates conversionof vibration/ mechanical energy into electrical energy andvice versa. The direct effect produces an electrical chargewhen a mechanical vibration or shock is applied to thematerial, while the reverse effect creates a mechanicalvibration or shock when electricity is applied. Any spa-tially separated charge will result in an electric field, andtherefore an electric potential. In a piezoelectric device,mechanical stress, instead of an externally applied voltage,causes the charge separation in the individual atoms of thematerial.2 Figure 1 indicates generation of piezoelectricityfor polar crystals, for which �P �= 0 holds without applyinga mechanical load, the piezoelectric effect manifests itselfby changing the magnitude or the direction of �P or both.For the non-polar, but piezoelectric crystals, on the otherhand, a polarization �P different from zero is only elicitedby applying a mechanical load. For them the stress can beimagined to transform the material from a non-polar crys-tal class ( �P = 0) to a polar one, having �P �= 0. Figure 2describes the mechanism of a piezoelectric effect in quartzcrystal.
2.2. Piezoelectric CoefficientsThis section reviews the physical meaning of piezoelectriccoefficients.
2.2.1. Piezoelectric Charge Coefficient (dij)The “d” coefficients, also known as the strain constant,relate the mechanical strain produced by an applied elec-tric field. It is defined as the ratio of the electric charge
Figure 1. Conversion of vibration/mechanical energy into electricalenergy and vice versa.
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Piezoelectric Power Harvesting Devices: An Overview Batra et al.
Figure 2. Mechanism of piezoelectric effect in quartz.
generated per unit area to an applied force. This chargecoefficient is usually important in the use of transducersand the piezoelectric material’s ability to perform as anactuator. The units are usually coulombs per newton.3
2.2.2. Piezoelectric Voltage Coefficient (gij)The piezoelectric voltage coefficient “g” is defined as theratio of the electric field produced to the mechanical stressapplied. High “g” coefficients are a result of producinglarge output voltages, which are necessary for sensor appli-cations. Its units are volts per newton.
2.2.3. Dielectric Constant (�ij)The relative dielectric constant is defined as the ratio ofthe permittivity of the material (�) to the permittivity offree space (�o�. This is generally measured well belowmechanical resonance. This variable is dimensionless.4
2.2.4. Coupling Coefficient (kij)The electromechanical coupling coefficient “k” is definedas the ratio of the mechanical energy stored, to the elec-trical energy applied or vice versa. Since this coeffi-cient uses the relationship of energy ratios, the units aredimensionless.4 In equation form:
dij =(�Di
�Tj
)E
=(�Sj
�Ei
)T
(1)
gij =−(�Ei
�Tj
)D
=(�Sj
�Di
)T
(2)
eij =(�Di
�Sj
)T
=−(�Tj
�Ei
)S
(3)
hij =−(�Ei
�Sj
)D
=−(�Tj
�Di
)S
(4)
where the first set of 4 terms correspond to the directpiezoelectric effect and the second set of 4 terms
correspond to the converse piezoelectric effect. Related toeach other as follows:
dij = �Tikgkj (5)
gij = dkj/�Tik (6)
eij = diqsEqj (7)
hij = giqsDqj (8)
The electromechanical coupling factor can be written interms of piezoelectric coefficients as:
K2ij =
U electricali
Umechanicalj
= e2ij
�TiksEqj
(9)
2.2.5. Efficiency of Energy ConversionThe efficiency of energy conversion, �, is described, atresonance, as follows:
� = k2/�2�1−k2��
1/Q+k2/�2�1−k2��(10)
Where, k is the coupling factor as defined in Eq. (9).
2.2.6. Piezoelectric Constitutive EquationsIn this section we will explain the basics equations whichcover electromechanical properties of piezoelectric mate-rials. When a poled piezoelectric material is mechanicallystrained it results a variation of a polarization strength,the changing in polarization appears as an electric chargeon the surface of the material. This property is called the“direct piezoelectric effect” and it is the basis operationfor sensors. Furthermore, if electrodes are attached to thesurfaces of the material, the generated electric charge canbe collected and used. This property is particularly uti-lized in piezoelectric shunt damping applications.5 Thegeneral constitutive equations commonly used to describe
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Batra et al. Piezoelectric Power Harvesting Devices: An Overview
the linear behavior of piezoelectric materials are derivedfrom basic thermodynamics principles.6�7
Sp = sEpqTq +dpkEk (11)
Di = diqTq +�TikEk (12)
where, sEpq is elastic compliance tensor at constant elec-tric field (m2/N ), �Tik is dielectric constant tensor underconstant stress, dpk is piezoelectric constant tensor (m/V ),Sp is the mechanical strain in p direction (m/m), Di is anelectric displacement in i direction (C/m2), Tq is mechani-cal stress in q direction (N/m2), and Ek is an electric fieldin k direction (V /m).8
Rewriting the above equations in the following form:
Tp = sDpqSq + gpkDk (13)
Ei = giqTq +�TikDk (14)
where, indices p�q = 1�2� �6, indexes i� k = 1�2�3refer to different directions within the material coordinatesystem. gpk is the matrix of piezoelectric constant (m2/N ),and �T
ik is impermitivity component tensor at constantstress (m/F ) which is the inverse of the permittivity matrixas well. Furthermore, the subscripts D, E, and T repre-sent measurements taken at constant electric displacement,constant electric field and constant stress, respectively.
Equations (11) and (12) express the converse piezoelec-tric effect, which describe the situation when the device isbeing used as an actuator. Conversely, Eqs. (13) and (14)express the direct piezoelectric effect, which deals with thecase when the transducer is being used as a sensor. Theconverse effect is often used to determine the piezoelectriccoefficients.
Currently, there are over 200 different piezoelectric ceram-ics that are commercially available, and amongst themPZT, BaTiO3, PMN-PT and PVDF are predominantlyused. Its properties were described in the forgoing sectionand its constitutive equations in the matrix form can bewritten as:9
⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣
S1
S2
S3
S4
S5
S6
⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦
=
⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣
s11s12s13s14s15s16
s21s22s23s24s25s26
s31s32s33s34s35s36
s41s42s43s44s45s46
s51s52s53s54s55s56
s61s62s63s64s65s66
⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦
⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣
T1
T2
T3
T4
T5
T6
⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦
+
⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣
d11d12d13
d21d22d23
d31d32d33
d41d42d43
d51d52d53
d61d62d63
⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦
⎡⎢⎢⎣E1
E2
E3
⎤⎥⎥⎦
(15)
⎡⎢⎢⎣D1
D2
D3
⎤⎥⎥⎦=
⎡⎢⎢⎣d11d12d13d14d15d16
d21d22d23d24d25d26
d31d32d33d34d35d36
⎤⎥⎥⎦
⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣
T1
T2
T3
T4
T5
T6
⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦
+
⎡⎢⎢⎣�T11�
T12�
T13
�T21�T22�
T23
�T13�T23�
T33
⎤⎥⎥⎦
⎡⎢⎢⎣E1
E2
E3
⎤⎥⎥⎦ (16)
3. MECHANICAL VIBRATIONS BASEDENERGY HARVESTING
Nowadays, vibration based energy harvesting is one ofthe most growing technologies due to decreased powerconsumption of low power electronics, such as wirelesssensor networks (WSN). In general, the simplest figureof vibration based energy harvesting can be modeled asa cantilever beam, one or more layers of piezoelectricand substrate material, and seismic mass as shown inFigure 3(a). The equivalent lumped parameters model ofFigure 3(a) can be represented as mass, spring, and dampersystem as shown in Figure 3(b).The governor equation of simple model in Figure 3 can
be written as10�11
meqx+ ceqx+keqx =−mequ (17)
The total electric power of the system can be obtained as:
P = meqTA2�2��2/�3
n�
�1−�/�n 2+ �2T ��/�n�
2(18)
where u= x�t�− y�t� is the displacement response of themass relative to the base, meq is the equivalent mass, �n =√keq/meq is the natural frequency, T = ceq/�2m�n� is the
damping ratio and ceq is the damping coefficient, and A isthe amplitude of base acceleration.
4. OPERATING PRINCIPLE OF APIEZOELECTRIC ENERGYHARVESTING SYSTEM
The process of acquiring the energy surrounding a sys-tem and converting into usable electric energy is termed aspower harvesting or scavenging. There is a dramatic rise inthe use of piezoelectric materials that provide the ability toconvert mechanical strain energy (vibrations) into electriccharge. The electric energy later can be stored or powersmall equipment with the help of power management cir-cuit. A piezoelectric generator system consists of fivemajor modules:12 mechanical energy source; mechanical
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Piezoelectric Power Harvesting Devices: An Overview Batra et al.
(b)(a)
Piezoelectric material
Substrate material
Tip mass
ceq keq
meq
RPiezoelectric
material
y(t)
x(t)
Figure 3. Simple model of vibration based energy harvesting (a) piezoelectric cantilever beam with seismic mass (b) its equivalent lumped parametermodel.
Figure 4. Schematic diagram of a power management system.
transformer; piezoelectric transducer; generator electronicsas well as intelligent energy and storage management sub-system as described with usual functions of components inFigure 4. The energy sources are translational, rotational,and acoustic energy. Mechanical transfers have two func-tions: the transformation of non-translational into trans-lational energy and matching the mechanical impedance.Power transfer circuit consists of three essential compo-nents: rectification, filtering, and optimum power transferelectronics. An intelligent energy management and stor-age sub-system is required to ensure reliable energy sup-ply. In this, controlled and reliable wake-up and sleeproutines are required in accordance with the availableenergy.
Piezoelectric materials are a set of materials that can gener-ate charge when mechanical stress is applied. Piezoelectric-ity results from the motion of dipoles naturally occurred,or artificially induced in the crystalline or molecular struc-tures of these materials. Based on their structural charac-teristics, piezoelectric materials can be divided into four
different categories: polycrystalline ceramics, single crys-tals, polymers, and composites. In single crystal materi-als, positive and negative ions are organized in a periodicfashion throughout the entire material except for the occa-sional crystalline defects. One of the most widely usedpiezoelectric single crystals is the solid solution of leadmagnesium niobate-lead titanate (PMN-PT). In contrast,ceramics, such as lead zirconate titanate (PZT) are poly-crystalline materials. Namely, they are comprised of manysingle crystal “grains” that possess the same chemical com-position. Polymers are carbon based materials composedof long polymer chains which have many repeated struc-tural units called “monomers.” These materials are muchmore flexible than ceramics and single crystals and pos-sess greater strength and flexibility. In some applications,all the above materials can be combined to form compos-ites in order to achieve certain properties that these mate-rials cannot individually provide on their own. Becauseof the strong polarizations in their crystalline structures,piezoelectric single crystals and ceramics exhibit superiorpiezoelectric properties than piezoelectric polymers. How-ever, they are considered disadvantageous for being rigid
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Batra et al. Piezoelectric Power Harvesting Devices: An Overview
and brittle. Therefore, the selection of a certain piezoelec-tric material for a specific energy harvesting application isdetermined not only by the piezoelectric properties but alsothe specific design requirements of the energy harvestingunit, such as the application frequency, the available vol-ume, and the form in which mechanical energy is fed intothe system. However, strictly from the materials perspec-tive, the important properties of piezoelectric materials forenergy harvesting applications include piezoelectric strainconstant d (induced polarization per unit stress applied, orinduced strain per unit electric field applied), piezoelec-tric voltage constant g (induced electric field per unit stressapplied), electromechanical coupling factor k (square rootof the mechanical-electrical energy conversion efficiency),mechanical quality factor Q (degree of damping; lowervalue indicates higher damping), and dielectric constant e(the ability of the material storing charge). Table I showssome typical values of these parameters for piezoelectricsingle crystals, ceramics, composites, and polymers.1 Thevalues of d, k, and e for piezoelectric single crystals andceramics are much higher than those of piezoelectric poly-mers. The g constants of the polymers are higher becauseof their much lower dielectric constants compared to thoseof the single crystals and ceramics.
Since the goal of energy harvesting is to convert asmuch input mechanical energy/vibrations into electricenergy, therefore selecting a piezoelectric material withhigh electromechanical coupling factor k, as the square ofk is the efficiency of this material converting the inputmechanical energy to the output electric energy is highlydesired. A piezoelectric ceramic with high k’s usually alsohas high d’s because under static or quasi-static condi-tions (i.e., at frequencies much lower than the resonancefrequency), k is directly related to d through elastic com-pliance and permittivity of the material.
To extract maximum amount of power, the piezoelec-tric energy harvester is preferable to operate at resonantfrequencies. However, in many cases, it is impractical tomatch the resonance frequency of the piezoelectric withthe input frequency of the host structure due to the volume
Table I. Dielectric, piezoelectric, mechanical and pyroelectric properties of some important piezoelectric materials.1
Material GaN AlN CdS ZnO BaTiO3 PZT-4 PZT-5H PMN-PT LiNbO3 PVDF
constraint of the device. This is especially common forlow frequency applications, as it usually demands a largerpiezoelectric element for energy conversion.
6. A SURVEY ON PIEZOELECTRICENERGY HARVESTING
Umeda et al. (1996, 1997), Xu et al. (1998), Goldfarband Jones (1999) measured the output power efficiency ofpiezoelectric materials.13–16 Kymissis et al. (1998) inves-tigated power generation from piezoelectric shoes; it wasdetermined that the average generated power is about1.3 mW at 0.9 Hz when the load resistance is 250 k�.17
Glynne-Jones et al. (2001) studied the electrical power ofpiezoelectric thick-film from harmonic excitation, it wasclaimed that the maximum power output was around 3 �Wunder a resonant frequency.18 Ramsay and Clark (2001)investigated the power harvesting from a 1 cm2 piezo-electric plate, it was observed that the produced poweris between microwatt to milliWatt which can be usefulin vivo bio MEMS.19
One way to improve the efficiency of energy harvestingsystems is the impedance matching between a piezoelec-tric energy harvester and electrical circuit configuration.Kasyap et al. (2002) developed a converter circuit withan impedance that could be matched to that of a piezo-electric energy harvester.20 Ottman et al. (2002) investi-gated of adding an adaptive control dc–dc converter tomaximize the output power from piezoelectric energy har-vester revealed that the output power could be enhanced byover 400% as compared without using dc–dc converter.21
Meninger et al. (2001) added an extra capacitor to providethe maximum energy transfer, which modified the trans-duction process.22 Richards et al. (2004) and Shu et al.(2006) developed an analytic formula to predict the energyconversion efficiency of piezoelectric energy harvesters incase of AC power output.23�24 Guan et al. (2007) studiedthe efficiencies of energy harvesting circuits consideringthe storage device voltages.25
Recently, piezoelectric cantilevered beams (PCB’s)under base or ambient excitation have gained increased
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Piezoelectric Power Harvesting Devices: An Overview Batra et al.
attention due to their ability to display high amounts ofstrain.26 Ambient vibrations suitable for energy harvest-ing can be found in various aspects of human experi-ence, such as buildings, bridges, trains and etc. Manyresearchers and scientists in both field of science and engi-neering have studied the linear effect of PCB’s under baseexcitation experimentally and theoretically. Sodano et al.(2003, 2004) developed a model based on Rayleigh-Ritzdiscrete formulation to predict the amount of power gen-erated from a piezoelectric cantilever beam.27�28 Similarly,Lu et al. (2004) developed a model for a cantilever typepiezoelectric generator; the output power was analyzedand its applications in MEMS were discussed.29 A threedimensional analyses of a parallel piezoelectric bimorphand triple layer piezoelectric actuators were also done inrecent years by Lim et al., (2001), Lim and He (2004),and Sodano et al., (2004).30–32 A 1-D electromechani-cally coupled piezoelectric generator model was presentedby DuToit et al. (2005).33�34 Stephen (2006) and Daqaqet al. (2007) discussed the maximum power generation andthe effect of mechanical damping using the same SDOFrelation.35�36 Ajitsaria et al. (2007) employed the SDOFrelation for predicting the voltage output analytically.37
Erturk and Inman (2008) reviewed and discussed the gen-eral solution of the base excitation problem for trans-verse vibrations of a cantilevered Euler–Bernoulli beamand predicted the output voltage and power. Further-more, multimodal energy harvesters were investigated anddesigned.38 Tadesse et al. (2009) presented a design ofmultimodal energy harvesting beam employing both elec-tromagnetic and piezoelectric transduction mechanisms,each of which was efficient for a specific mode.39 Ou et al.(2012) presented a two-degrees-of-freedom (2-DOFs) sys-tem using a two-mass cantilever beam.40 Also, a novelcompact piezoelectric energy harvester using two vibrationmodes was developed by Wu et al. (2013). The compactdesign efficiently utilizes the cantilever beam by gener-ating significant power output from both the main andsecondary beams. An experiment and simulation werecarried out using (2-DOFs) and, the results showed thatthe proposed novel method is more adaptive and func-tional in practical vibrational circumstances.41 In order toenhance the maximum output power of piezoelectric can-tilever beams as well, researchers have developed a vari-ety of techniques based on, varying shape of structurebeam using an L-shaped flexible structure,42–44 using dual-mass systems,45 changing the cross-section of a dynamicmagnifier46 and using an energy harvester with a dynamicmagnifier (EHDM).47–50
Although the piezoelectric constitutive equations arenonlinear, most of the current analyses consider only thelinearized form of these equations. Yet, the nonlinearbehavior of piezoelectric cantilever beams has been con-sidered for the actuation purpose.51–56 Recently, nonlin-ear modeling of piezoelectric energy harvesters from base
excitations has gained some attention. Triplett and Quinn(2009) included a nonlinear electromechanical coefficientterm in lumped model. It was demonstrated that the non-linearities in an electromechanical coupling could increasethe output electrical power.57 Daqaq et al. (2009) studiedenergy harvesting using a parametric excitation. In theirapproach, the dynamic response of the system was inves-tigated using a lumped-parameter model.58 Stanton et al.(2010) identified the nonlinear coefficients based on a non-linear least-squares optimization algorithm that utilizes anapproximate analytical solution obtained by the methodof harmonic balance.59 Masana and Daqaq (2011) devel-oped an electromechanical model of a clamped–clampedenergy harvester subjected to transverse excitations basedon the nonlinear Euler-Bernoulli beam theory.60 Abdelkefi(2012) performed and developed global nonlinear analy-ses for piezoelectric energy harvesters from ambient andaeroelastic vibrations.61
Piezoelectric cantilevered beam with an effect of mag-netic field has been the topic of many researchersboth experimentally and theoretically, where the resultsexhibited increased bandwidth and superior efficiencies ofelectric output power. Tang and Yang (2012) proposed amagnetic coupled piezoelectric energy harvester, in whichthe magnetic interaction is introduced by a magnetic oscil-lator. In their experiment, the bandwidth was increasedby 100% and the magnitude of output power increasedby 41%.62 Su et al. (2013) designed and developed adual-cantilever structure that consists of an outer andinner beams with magnets attached to the tips. The mag-nets generate nonlinear repulsive force between the twobeams and make the structure bistable. The new designshowed a significant improvement in the bandwidth.63
Waleed Al-Ashtari et al. studied a design of piezoelec-tric bimorph cantilevers configurations under the influenceof two permanent magnets. Theoretical and experimentalresults show that magnetically stiffened harvesters haveimportant advantages over conventional set-ups with andwithout tip mass. It was also observed that they gener-ate more power with a slight increase in the deflection ofa piezoelectric harvester and can be tuned across a widerange of excitation frequencies.64
Piezoelectric energy harvesting from aeroelastic vibra-tion has received growing interests in the last few yearsas well. The aim of an aeroelastic vibration is to convertairflow energy into electricity for aircraft sensors and wire-less electronic devices using high wind.65 Vortex-inducedvibration, flutter, buffeting, and galloping are examplesof some of these aerodynamics phenomena that can beused as a source of harvesting energy. Recently, Erturket al. (2008) investigated energy harvesting from a flow-excited morphing airfoil. In their study, the experimen-tal results showed that the maximum root mean square(RMS) level of the harvested power is about 7 �W andwas obtained when the angle of attack was set equal
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to 20 degrees with and for an electrical load resistanceequal to 98 k� at a flow velocity of 15 m/s.66 Antonand Inman (2008) attached an MFC patch to the sparwith epoxy under vacuum at locations near the roots ofthe wings. They claimed that a small battery can be fullycharged when increasing the volume of piezoelectric mate-rial. Also, they presented the possibility of installing asmall on-board camera that can take photographs duringflight.67 Erturk et al. (2008) presented an experimental andtheoretical validations for a two degree of freedom airfoilsection as a wing-based piezoaeroelastic energy harvester.In their study, they found an electrical harvested power of10.7 mW for an electrical load resistance equal to 100 k�for a freestream velocity close to the linear flutter speedwhich is 9.30 m/s.68 De Marqui et al. (2010) analyzed afrequency domain piezoaeroelastic model of a generatorwing. The level of harvested power obtained has increasedby 20 times and the linear flutter speed is increased by7.5% in the resistive-inductance circuit when compared tothe resistive one.69
Taylor et al. (2001) applied a flexible membrane withPVDF or ‘eel’ to harvest energy from ocean waves. Theresults showed that the generated voltage by the ‘eel’ ofthe order of 3 V for a water velocity of 5 m/s.70 Sim-ilar to the ‘eel’ created by Taylor et al., Pobering andSchwesinger (2004) investigated the use of a PVDF flagto harvest energy from flowing water. Their theoreticalresults showed that the harvested power can be the orderof 68 W/m3 for a river flowing at 2 m/s.71
Akaydin et al. (2010) investigated experimentally theconcept of piezoelectric energy harvesting from a turbulentflow exhibiting a thin flexible cantilever beam consistingof a PVDF layer and a mylar substrate. The measurementshowed that the maximum harvested power was about4 �W when the distance between the beam and the cylin-der is set equal to 2D.72 Akaydin et al. (2012) presenteda piezoaeroelastic energy harvester consisting of a can-tilevered aluminum shim covered by a piezoelectric mate-rial near its base, and a circular cylinder on its tip. Using afree-vibration wind-tunnel test, they reported that the max-imum harvested power was 0.1 mW for a wind speed ofabout 1.192 m/s.73 Sirohi and Mahadik (2012) investigatedtheoretically and experimentally piezoaeroelastic energyharvesting from galloping beams with a D-shaped crosssection. They reported that the maximum harvested powerwas 1.14 mW at a wind speed of 10.5 mph.74
7. PIEZOELECTRIC POWERHARVESTING DEVICES
Piezoelectric power generation devices can be divided intotwo categories:(a) self-energizing devices (self-powered solely fromstatic or dynamic movement of piezoelectric devices, and(b) energy-harvesting devices that are attached to existingstructures that vibrates during operation of the structure.
Both of these devices capture otherwise wasted mechanicalloading and convert to electrical power.
7.1. Flexible Piezoelectric Energy Harvesting fromJaw Movements
Delnavaz and Voix developed and tested a prototype ofjaw movement energy harvester and compared its perfor-mance to the analytical model predictions.75 It consists ofa flexible piezoelectric element made of flexible composite(PFC) that fits below the chin and is attached to a head-mounted device by two elastic rubber straps.75
The head-mounted devices could also have been used astactical helmets, sports helmets, or headphones. The per-son wearing the device must adjust the strap assembly to asnug fit to keep the strap under tension. Opening the mouthfurther stretches the side strap and causes: distributed forcestress in the PFC contact surface and tensile stress in PFCcross-section. Electric charge is accumulated in the PFCelectrodes are from tensile stress and the electric chargeflows through the resistive load and generates the electriccurrent in the circuit. By closing the mouth, the systemreturns to its initial position and a reverse current of samemagnitude is generated. Under optimum conditions, theycould obtain the maximum power transfer of about 7 �W.
7.2. Piezoelectric Shoe-Mounted HarvestersHarvesting mechanical energy from human motion isan attractive approach for obtaining clean and sustain-able electric energy to power wearable sensors. Therehave been several examples reported that use piezoelectricenergy harvesters mounted in shoes to harvest mechanicalenergy due to human walk or running.76
7.3. Piezo-Wind GeneratorsIn the recent past, research on piezo-wind energy genera-tors has been progressing with enhanced trend on renew-able energy sources. The wind energy can be convertedto electrical energy via integrating piezoelectric materialon leafs or converting rotary energy to linear energy tobend piezoelectric materials (PE) or PE materials can bedirectly utilized on rotary motion.Li et al., proposed a novel vertical-stalk L-type design
to harvest more energy which is composed of a poledPVDF stalk, a plastic hinge and polymer leaf.77 Oja et al.,made a tree shaped design to harvest energy by embed-ding PVDF’s on leafs and PZT’s on trunk part of thetree where the bending can be realized by strong wind.78
There have been studied rotary to linear motion windgenerators.79 Priya et al., designed a piezoelectric windmill with ten piezoelectric bimorphs in the cantilever form.It was observed that at 10 mph speed, power of 7.5 mWhad been obtained across optimum load of 6.7 kOhms.80
In the research work performed by Bryant et al., twodegrees of freedom is utilized by deflection of beam anda rotation of a flap about bearing joint which allows a
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Piezoelectric Power Harvesting Devices: An Overview Batra et al.
modal to flutter response for energy harvesting.81 Tinget al., proposed a nozzle accelerator to increase the windspeed by five times and consistently increase the dragforce in order to vibrate the piezoelectric bimorph formaximum energy harvesting.82 Robbins et al. reported onthe harvested energy by using flag-like membrane withpiezoelectric materials which is attached to an anchor rod.Wind leads flapping of the membrane and periodic stressoccur on bimorph generates voltage across electrodes.83
Recently Karthikeyan et al., describes the vertical stalk-horizontal leaf and horizontal leaf-horizontal stalk arrange-ment, Piezo tree, and the ways to harvesting then energygenerated and places it can be erected. They used PVDFfor fabricating stalk.84 Readers are referred to Ref. [85] fordetails of wind power harnessing.85
7.4. Rotary Knee-Joint HarvesterPiezoelectric harvesters have been conceptualized,designed and fabricated for energy harvesting from jointmotion of human body, including knee-joints.86
7.5. Piezoelectric Prosthetic Leg Energy HarvestersPechrach et al. reported an energy harvesting system usingsmart materials for self-power generation of upper andlower prosthetic legs.87 The smart material used for energyharvesting is piezoelectric PZT (Lead Zirconium Titanate).The bimorph structured PZT material produced a maxi-mum peak voltage of 1.8 volts and maximum peak powerof 12.5 �W. The first arm of the piezoelectric producesa downward movement, which provides an angle for themovement amplification of the second arm.
7.6. Piezoelectric PacemakerA group led by John Rogers at the University of Illinoisat Urbana–Champaign has developed a flexible, piezoelec-tric patch that harvests the mechanical energy of a beatingheart.88 The implant contains a film made of 500 nm thickribbons of lead zirconate titanate (PZT) surrounded bygold and platinum electrodes. PZT is piezoelectric, mean-ing a voltage develops across it when it is bent. The outputvoltage is used to charge a tiny battery integrated into thedevice, and the whole thing is encased in a layer of poly-imide to make it biocompatible. They found that, whenstitched at the optimal orientation onto the right ventricle,their device generated up to 0.18 �W/cm2 power. State-of-the-art pacemakers can run on as little as 0.3 �W-a poweroutput the team achieved by stacking multiple piezoelec-tric layers on top of one another.
7.7. Piezoelectric RailwaysIsrael’s Innowattech has engineered piezoelectric mate-rials for railways. An array of piezoelectric disks wasinstalled beneath the rail tracks to transform mechani-cal stresses into electrical output.89 It was observed thata railway track with traffic of 10 to 20 car trains an
hour could harvest as much as 120 kWh, which couldbe used to power infrastructural elements such as sig-naling lights or can be uploaded to the grid. In addi-tion we can determine the impact of number of wheels,weight of each wheel, the wheel’s capitation, wheelperimeter position, wheel diameter, and the speed of thetrain.
7.8. Piezoelectric Roads and HighwaysRecently, the California Energy Commission is investigat-ing the viability of deploying piezoelectric materials inCalifornia roadways for the purpose of harvesting elec-trical energy. It assessed the value of piezoelectric-basedenergy-harvesting technology to determine if the earlyresults from prototype demonstrations warrant a moredetailed demonstration in California. It was estimated thatthe cost range of the piezoelectric system to be between$0.08–$0.18/kWh. The railway application implies the useof a thinner unit for two reasons:(1) the geometry of the installation requires a thinunit, and(2) there are less inelastic forces in action in this applica-tion and fewer discs are needed per unit to harvest usefulenergy.
There are a number of cost-saving opportunities in thisinstallation. The unit is thinner, so it requires fewer piezo-electric discs, thus lowering its capital costs.90
7.9. Flexible Wearable HarvesterA flexible energy harvesters with a piezoelectric polymerPVDF in-shell structure that can generate high power fromslow motion have been proposed.91–95
8. SUMMARY AND OUTLOOKThis article presents a fundamental review of piezoelec-tric materials for applications in energy harvesting tech-nology. The Piezoelectric and Pyroelectric coefficientsof some important and common materials are describedfor ready reference. A mathematical background of con-stitutive equations, a lumped parameter model, piezo-electric theory is mentioned in this review paper. Sincethere are a very wide range of piezoelectric materi-als open to use from single crystal such as PMN-PTthrough polymers such as PVDF, dielectric, piezoelectric,mechanical, and pyroelectric properties of some impor-tant piezoelectric materials are considered. Furthermore,piezoelectric energy harvesting based on vibration andmechanical waves is still limited for only low power elec-tronics devices such as wireless sensor networks (WSN).However, a few piezoelectric commercial devices suchas an electric switch etc. are available in the market.In future, piezoelectric energy harvesting technology willplay a prominent role as more ultra-low power devicesare available along with stringent rules to protect theenvironment.
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Acknowledgments: The authors gratefully acknowl-edge support for this work through the National Sci-ence Foundation grant # EPSCoR R-II-3 (EPS-1158862).Authors thank Dr. Chance M. Glenn, Dean, Collegeof Engineering, Technology and Physical Sciences andDr. M. D. Aggarwal, Chairman, Department of Physics,Chemistry and Physics for their keen interest in this work.Authors thank Mr. Garland Sharp for fabrication of thesample holders.
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Received: 8 October 2015. Accepted: 22 October 2015.
