Rotational Piezoelectric Energy Harvesting: a Comprehensive Review on Excitation Elements, Designs, and Performances

Review Rotational Piezoelectric Energy Harvesting: A Comprehensive Review on Excitation Elements, Designs, and Performances

Haider Jaafar Chilabi 1,2,* , Hanim Salleh 3, Waleed Al-Ashtari 4, E. E. Supeni 1,* , Luqman Chuah Abdullah 1 , Azizan B. As’arry 1, Khairil Anas Md Rezali 1 and Mohammad Khairul Azwan 5

1 Department of Mechanical and Manufacturing, Faculty of Engineering, Universiti Putra Malaysia, Serdang 43400, Malaysia; [email protected] (L.C.A.); [email protected] (A.B.A.); [email protected] (K.A.M.R.) 2 Midland Reﬁneries Company (MRC), Ministry of Oil, Baghdad 10022, Iraq 3 Institute of Sustainable Energy, Universiti Tenaga Nasional, Jalan Ikram-Uniten, Kajang 43000, Malaysia; [email protected] 4 Mechanical Engineering Department, College of Engineering University of Baghdad, Baghdad 10022, Iraq; [email protected] 5 Department of Mechanical Engineering, College of Engineering, Universiti Tenaga Nasional, Jalan Ikram-15 Uniten, Kajang 43000, Malaysia; [email protected] * Correspondence: [email protected] (H.J.C.); [email protected] (E.E.S.)

Abstract: Rotational Piezoelectric Energy Harvesting (RPZTEH) is widely used due to mechanical rotational input power availability in industrial and natural environments. This paper reviews the recent studies and research in RPZTEH based on its excitation elements and design and their influence on performance. It presents different groups for comparison according to their mechanical Citation: Chilabi, H.J.; Salleh, H.; inputs and applications, such as fluid (air or water) movement, human motion, rotational vehicle tires, Al-Ashtari, W.; Supeni, E.E.; and other rotational operational principal including gears. The work emphasises the discussion of Abdullah, L.C.; As’arry, A.B.; Rezali, K.A.M.; Azwan, M.K. Rotational different types of excitations elements, such as mass weight, magnetic force, gravity force, centrifugal Piezoelectric Energy Harvesting: A force, gears teeth, and impact force, to show their effect on enhancing output power. It revealed that a Comprehensive Review on Excitation small compact design with the use of magnetic, gravity, and centrifugal forces as excitation elements Elements, Designs, and Performances. and a fixed piezoelectric to avoid a slip ring had a good influence on output power optimisation. One Energies 2021, 14, 3098. https:// of the interesting designs that future works should focus on is using gear for frequency up-conversion doi.org/10.3390/en14113098 to enhance output power density and keep the design simple and compact.

Academic Editor: Keywords: piezoelectric; rotational energy harvesting; energy harvester excitation elements; rota- Abdessattar Abdelkeﬁ tional energy harvester design; factors affecting energy harvester performance

Received: 3 April 2021 Accepted: 10 May 2021 Published: 26 May 2021 1. Introduction

Publisher’s Note: MDPI stays neutral Energy harvesting is defined as the process of converting mechanical energy such with regard to jurisdictional claims in as distortion energy, vibration, or other kinetic energy into electrical energy [1]. Fast published maps and institutional affil- development in wireless sensor networks (WSN), and storage power with better efficiency iations. solutions will finally increase using of self-power devices in health care, monitoring of the environment, and automotive applications [2]. However, limitation of power source and batteries, such as volume, weight, and short lifetime, which is much less than the WSN life, changing the batteries frequently, and devices in hard reach area [3]. All these limitations make the powering of microdevices and WSNs using energy harvester considering a Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. feasible approach in our environment due to its small size, low power consumption, and This article is an open access article special working environment [4,5]. distributed under the terms and Piezoelectric energy harvesting is one of the innovative approaches that has been conditions of the Creative Commons evolved and implemented for harvesting power for microdevices using different types of Attribution (CC BY) license (https:// mechanical power sources [6]. Piezoelectricity is the electric charge accumulated in certain creativecommons.org/licenses/by/ solid materials (Piezoelectric) such as crystals, ceramics, and Polyvinylidene Fluoride 4.0/). PVDF in response to applied mechanical stress [7]. Depending on the application, and

Energies 2021, 14, 3098. https://doi.org/10.3390/en14113098 https://www.mdpi.com/journal/energies Energies 2021, 14, 3098 2 of 29

the availableness of the mechanical power sources, various types of energy harvesting are obtainable most of the time. Mechanical energies, such as vibration, fluid-flow, human motion, etc., are energies of changeable frequencies and amplitude, which could be named random energies that available everywhere [8]. Rotational mechanical energy is one of the significant power sources for piezoelectric energy harvesting. Various types of rotational power sources have been used in the past few years for piezoelectric energy harvesting, such as rotational machines [9–18], vehicle tire [19–23], human motion [24–28], and even wind [29–36] and fluids [31,32,37–41]. They have been applied in various applications, such as tire pressure monitoring system TPMS [42–44], WSNs in any rotary machine [11,45,46], wearable device and medical implants [47–51], and others. The Rotational Piezoelectric Energy Harvesting (RPZTEH) operation principle is based on the plucking of piezoelectric for excitation, which results in piezoelectric vibration, bending, or pressing, and thus voltage is generated. This plucking could be done using different excitation elements. Researchers have used various excitation elements in RPZTEH such as magnetic [52–54], mass weight [44,49], gravitational force [55–57] centrifugal force [58,59], and gears teeth force [60,61], or a compilation of these elements [42,62,63]. They have utilised these elements for frequency up-conversion of rotational frequency, which is considered lower in general than the piezoelectric resonant frequency [9,62,64], or widen the broadband range [20,65,66]. Most of the previously published review papers mentioned rotational piezoelectric energy harvesting partly and focused mainly on frequency up-conversion or bandwidth widening [2,67–72]. However, frequency up-conversion methods continue to be enhanced. Hence, rotational power sources, especially wind, human motion, some rotary machines, and some vehicle tires, are generally considered as low frequencies, compared to piezoelectric frequency. In some cases, the wiring for output power transfer is still an issue because the piezoelectric is rotating with the system and thus a slip ring or any other wireless transfer is needed, and the device has become even more complicated [4,43,45,72,73] The rapid repetitive bending and impact on the piezoelectric with the direct excitation contact can also be considered an issue because it will reduce its lifetime; however, this can be avoided in different ways and suitable excitation elements [37]. Besides choosing suitable excitation elements, the compact, simple, and right-size design for the suggested application is quite important [43,72]. Using gear (and gear teeth) as a rotational mechanical input for piezoelectric energy harvesting excitation has been considered a good and new method. Only a few works have been accomplished using the gear as input in rotational piezoelectric energy harvesting. This paper aims to present a comprehensive review of RPZTEH from the mechanical input method concept and applications. To the authors’ knowledge, there is no such review specifically for rotational energy harvesters. This paper’s originality is mainly on comparing various designs and excitation elements and their influence on performance. It also focuses on past work performance and key design comparison perspectives on the various mechanical inputs and applications, such as fluids (air, water) movement, rotational vehicle tires, human motion, and other rotational operational principals. Various excitation elements such as mass (the mass weight work as a force), magnetic, centrifugal force, gravity, gears, or others were discussed. Each mechanical input study is examined in- depth, including working principles and main findings. Each of the above four mechanical inputs has been subdivided into comparison of different designs and excitation elements and the influence of excitation elements and design on performance (challenges and issues). This review summarises several important aspects of the previous energy harvesting studies including methods, materials type, polarity, rotational speed, output power, power density, and wiring. The comparison tables and graphs of performances are also included in this paper. The advantage or disadvantage of each mechanical input study was identified, and future research recommendations were discussed. Energies 2021, 14, x FOR PEER REVIEW 3 of 30

are also included in this paper. The advantage or disadvantage of each mechanical input Energies 2021, 14, 3098 3 of 29 study was identified, and future research recommendations were discussed.

2. Methodology 2. MethodologyIn the present review, the comparisons of excitation elements, design and their influence Inon theperformance present review, are divided the comparisons into four types of excitation according elements, to mechanical design and inputs: their fluids influ- (air,ence water) on performance movement, are human divided motion, into four rotational types according vehicle tires, to mechanical and other inputs: rotational fluids oper- (air, ationalwater) movement,principals. For human each motion, one, mechanical rotational inputs, vehicle excitation tires, and type, other design, rotational methodology, operational andprincipals. wiring have For each been one, characterised mechanical and inputs, thoroughly excitation reviewed. type, design,The type methodology, of excitation andhas beenwiring subdivided have been according characterised to excitation and thoroughly elements reviewed., such as The mass, type gear, of excitation magnetic, has spring, been centrifugal,subdivided gravity according and to impact excitation forces. elements, Each group such may as mass, contain gear, one magnetic, or more of spring, the excita- cen- tiontrifugal, elements gravity to andconclude impact which forces. group Each has group the maximum may contain influence one or more on rotational of the excitation piezoe- lectricelements energy to conclude harvesting which enhancement group has.the The maximum design comparison influence onalso rotational discusses piezoelectric the piezoe- lectricenergy rotation, harvesting which enhancement. may affect the The type design of wiring comparison for output also discusses power transfer. the piezoelectric The fixed piezoelectricrotation, which cantilever may affect uses the direct type ofwiring, wiring while for output a rotating power one transfer. requires The either fixed a piezoelec- slip ring ortric Arduino cantilever and uses Bluetoo directth wiring, for output while power a rotating transfer. one requiresFor each eitherof the afollowing slip ring orsection. Arduino At theand end Bluetooth of each for section, output a powersummary transfer. table Forwas eachmade of by the the following author with section. full Atdetail the and end calculatedof each section, power a density summary for tablerelated was previous made by works, the author as in Tables with full 1–4. detail A plot and has calculated also been madepower by density the authors for related for all previous the designs, works, due as to in rpm Tables and 1–4. output A plot power has also density, been to made give by a clearthe authors idea about for allthe the best designs, excitation due elements to rpm and and output which one power has density, the highest to give power a clear density, idea andabout as thein Figures best excitation 5, 9, 13, elementsand 18, respectively. and which one Figure has the1 show highest the powermethodology density, schemati and as inc diagram.Figures 5, 9, 13, and 18, respectively. Figure1 show the methodology schematic diagram.

FigureFigure 1. MethodologyMethodology schematic schematic diagram. diagram. 3. Fluids Movement Applications 3. Fluids Movement Applications In this section, a comparison has been made on the studies that have used fluid In this section, a comparison has been made on the studies that have used fluid in- including wind (or air), and water (or rain) as a mechanical input source for RPZTEH. Each cluding wind (or air), and water (or rain) as a mechanical input source for RPZTEH. Each of these two groups is divided according to excitation elements. Their design, methodology, of these two groups is divided according to excitation elements. Their design, methodol- and power density have been reported for each study. ogy, and power density have been reported for each study. 3.1. Comparison of Different Designs and Excitation Elements Firstly, the studies that use air (wind) as a mechanical input source for RPZTEH have been reported. Using air only as an excitation element, Stamatellou & Kalfas [29] utilised air swirled to create a turbulent flow field and harvest energy, as shown in Figure2a. The piezoelectric mounting mode allows for varying the orientation and position of piezoelec-

tric to optimise output power. A power density of 0.031 µW/mm3 was harvested in a EnergiesEnergies 2021 2021, 14, 14, x, xFOR FOR PEER PEER REVIEW REVIEW 4 4of of 30 30

3.1.3.1. Comparison Comparison of of Different Different Designs Designs and and Excitation Excitation Elements Elements Firstly,Firstly, the the studies studies that that use use air air (wind) (wind) as as a am mechanicalechanical input input source source for for RPZTEH RPZTEH have have Energies 2021, 14, 3098 beenbeen reported. reported. Using Using air air only only as as an an excitation excitation element, element, Stamatellou Stamatellou & & Kalfas Kalfas [ 29[29] ]utilised utilised4 of 29 airair swirled swirled to to create create a aturbulent turbulent flow flow field field and and harvest harvest energy, energy, as as shown shown in in Figure Figure 2a 2a. The. The piezoelectricpiezoelectric mounting mounting mode mode allows allows for for varying varying the the orientation orientation and and position position of of piezoelec- piezoelec- tric to optimise output power. A power density of 0.031 µW/mm3 3was harvested in a col- triccollection to optimise time output of 20 spower. cycle duration A power from density 37 mm of 0.031 PZT µW/mm at a 300 rpm, was withoutharvested using in a acol- slip lection time of 20 s cycle duration from 37 mm PZT at a 300 rpm, without using a slip ring lectionring or time any of output 20 s cycle power duration transfer from since 37 themm PZT PZT does at a not300 rotaterpm, without with the using system. a slip ring oror any any output output power power transf transferer since since the the PZT PZT does does not not rotate rotate with with the the system. system.

(a()a ) (b(b) ) (c()c )

FigureFigureFigure 2. 2. (a 2.()a Simultaneous)( aSimultaneous) Simultaneous measurement measurement measurement of of the ofthe thepressure pressure pressure and and and piezoelectric piezoelectric piezoelectric film film ﬁlm output output output voltage voltage voltage [ 29[29] [;29 ]; ]; (b(b) (View)b View) View of of rotating of rotating rotating energy energy energy harvesting harvesting harvesting system system system [ 55[55 []55; ](;c]; ()c (View)c )View View of of ofenhanced enhanced enhanced energy energy energy harvest harvest harvestererer [74 [74 []74. ].].

Similarly,Similarly,Similarly, Febbo Febbo Febbo et et al. et al. al.[55 [55 []55 performed] performed] performed a anovel anovel novel design design design alternative alternative alternative to to a to asimple asimple simple PZT PZT PZT beam beam beam thatthatthat fixed fixed fixed the the the beam beam beam to to a to ahub, ahub, hub, by by byutilising utilising utilising a abeam, abeam, beam, spring, spring, spring, gravity, gravity, gravity, centrifugal centrifugal centrifugal forces, forces, forces, and and and twotwotwo masses masses masses as as excitation as excitation excitation elements, elements, elements, as as shown as shown shown in in Figure in Figure Figure 2b 2b2. b.The. The The power power power density density density ranges ranges ranges from from from 0.0790.0790.079–0.32–0.32–0.32 μW/mm μW/mmµW/mm3.3 .The The3. The power power power transfer transfer transfer using using using an an acquisition an acquisition acquisition system system system (Arduino) (Arduino) (Arduino) to to avoid to avoid avoid slipslipslip rings. rings. rings. Future Future Future work work work may may may use use use two two two PZT PZT PZT since since since they they they have have have two two two beams beams beams to to get toget getmore more more power, power, power, andandand a adurability adurability durability test test test will will will be be beuseful useful useful to to extend to extend extend the the the harvester harvester harvester lifetime. lifetime. lifetime. TheTheThe same same same authors authors authors enhanced enhanced enhanced their their their model model model [ 74[74] []by74 by ]adding byadding adding a asingle single a single side side spring side spring spring stop, stop, stop,in in additionadditionin addition to to the the to two two the-flexible- two-flexibleflexible beam, beam, beam, and and the andthe spring spring the spring that that joined thatjoined joined the the two two the dense twodense dense masses, masses, masses, as as shownshownas shown in in Figure Figure in Figure 2c 2c. .This This2c. design Thisdesign design generates generates generates more more morepower power power than than the thanthe previous previous the previous one, one, and one,and the andthe 3 rectifiedrectifiedthe rectified output output output power power powerdensi density densityty is is 0.31 0.31– is2.59– 0.31–2.592.59 μW/mm μW/mmµW/mm3 3at at 5 05–0150–at150 50–150rpm. rpm. The The rpm. novelty novelty The novelty of of this this of devicedevicethis deviceis is that that the isthe that contact contact the contactforce force was was force maximum maximum was maximum at at the the lowest atlowest the lowestrotation rotation rotation speed. speed. Thus, speed. Thus, it it Thus,pro- pro- it posedposedproposed as as an an alternative asalternative an alternative solution solution solution for for generated generated for generated power power power at at the the atlow low the excitation excitation low excitation frequency. frequency. frequency. However,However,However, Y. Y. Yang Y. Yang Yang et et al. et al. al.[ 30[30 []30 ]have have] have used used used im impact impact-inducedpact-induced-induced resonant resonant resonant to to have to have have piezoelectric piezoelectric piezoelectric beambeambeam effective effective effective excitation excitation excitation vibration vibration mode. mode. mode. The The The harvester-basedharvester harvester-based-based knowledge knowledge knowledge designed designed designed is isthat; is that;that;an an impactan impact impact could could could excite excite excite resonance resonance resonance beneath beneath beneath any any any operating operating operating conditions. conditions. conditions. A A configurationA configuration configuration has hashasbeen been been made made made ofof of twelvetwelve twelve PZT PZT beams beams and and and a a aseven sevenseven ball, ball,ball, as as shown shown in in in Figure Figure Figure 3 a.3a. The The The harvester harvester harvester 3 µ 3 powerpowerpower density density density at at a at aspeed aspeed speed of of 200 of 200 200 rpm rpm rpm and and and across across across 20 20 20,000,000,000 ohms ohms ohms was was was 1.3 1.3 1.3 μW/mm μW/mmW/mm.3 This. This. This power power power cancancan be be bestored stored stored in in a in acapacitor acapacitor capacitor and and and used used used in in WSN; in WSN; WSN; however, however, however, this this this WSN WSN WSN must must must rotate rotate rotate with with with the the the systemsystemsystem or or add or add add a aslip aslip slip ring ring ring or or other or other other output output output transfer transfer transfer device. device. device.

(a()a ) (b(b) ) (c()c )

FigureFigureFigure 3. 3. ( 3. a()a ()Piezoelectrica Piezoelectric) Piezoelectric windmill windmill windmill diagram diagram diagram shows shows shows piezoelectric piezoelectric piezoelectric bimorph bimorph bimorphss sarrangement arrangement arrangement [ 30[30 []30; ](; b];(b) ()bSchematic )Schematic Schematic illustration illustration illustration andandand working working working mechanism mechanism mechanism of of the of the theinvestigated investigated investigated Magnetically Magnetically Magnetically Coupled Coupled Coupled Piezoelectric Piezoelectric Piezoelectric Wind Wind Wind Energy Energy Energy Harvester Harvester Harvester (MPWEH). (MPWEH). (MPWEH). The The The symmetrical opposite magnetic arrangements [75]; (c) Prototypes of nonlinear wind harvester: configuration of tangential symmetricalsymmetrical opposite opposite magnetic magnetic arrangements arrangements [75 [75]; ];(c ()c Prototypes) Prototypes of of nonlinear nonlinear wind wind harvester: harvester: conf conﬁgurationiguration of of tangential tangential designdesign and and configuration configuration of of radial radial design design [ 52[52]. ]. design and conﬁguration of radial design [52].

Furthermore, Bai & Havr [66] studied an air-ﬂow energy harvester by combining two piezoelectric beams with a composition of free-standing bi-morph thick ﬁlm, and a windmill with a free-spinning fan; all have been fabricated and assembled. The maximum power density is 0.058 µW/mm3 at a fan rotation speed of 750.6 rpm and 87 Hz magnetic Energies 2021, 14, 3098 5 of 29

force frequency. Further optimisation can be done by designing the harvester to be more compact and miniaturised, which is considered essential for the conventional wind turbine. Rezaei-Hosseinabadi et al. [65] also designed a wide-band piezoelectric energy harvester and optimised it to get the maximum energy using mass and magnetic as excitation forces. The prototype consists of PZT that vibrates due to the interactions between perme- ant magnetics in the small turbine and the magnetic attached to the piezoelectric beam. It was found that the optimum output power density was 0.59 µW/mm3, and no need for a slip ring for output power transfer; typical wiring was used since the PZT does not rotate. Alternatively, Zhao et al. [75] presented an innovative method for RPZTEH with wind and use mechanisms of force amplification and magnetic coupling as excitation only. By arranging the magnetic in a symmetrical opposite way, they maximised the effective force and significantly reduced the resistance torque, as in Figure3b. The experimental results show that it can work continuously for more than 12,000 s with wind speed from 3–10 m/s, and the maximum power density was 3.3 µW/mm3. This device could describe as a wide speed range of wind energy harvester and better robustness. Moreover, using magnetic force for excitation, Çelik et al. [54] present a configuration consisting of a PZT layer of 70 × 32 × 1.5 mm dimensions and a harvester of 30 cm length from propeller to the tail end with 16 cm propeller diameter. The system’s maximum power density in dynamical regime simulation is 4.76 µW/mm3 at 600 rpm. This study confirmed that a piezoelectric harvester with a magnet at its tip could harvest chaotic and regular dynamics on different rotation speeds. Additionally, a built-in wind generator has been designed to supply sensor nodes with power in hard reach or remote locations from low wind speed by Karami et al. [52] using magnetic only as excitation. Two configurations have been made: a tangential and a radial one, as in Figure3c. The measuring maximum output power density is 15.5 µW/mm3 in 200 rpm, 16 Hz, and a magnetic gap of 25 mm. This work presents a novel nonlinear PZT wind turbine, whose wind speed startup is low and harvests enough power for the operation of a common WSN. Alternatively, an energy harvester mathematical modelling using mass, gravity, and centrifugal forces for excitation has been presented by Nezami et al. [76]. The harvester can convert the slow mechanical rotation into PZT vibration by using two magnets and a small desk for large applications, such as wind turbine blades. In the range of 35–45 rpm, the output power becomes significant, where the disk motion is chaotic by switching between bouncing and passing. The power density is 2.83 µW/mm3, and a slip ring was used to transfer it since the PZT rotates with the system. The studies that use water as a mechanical input source for RPZTEH, have been reported below. Another approach for energy harvesting from raindrops besides the direct impact; is an indirect one using a PZT turbine or watermill and magnetic force for excitation. Since raindrop’s maximum speed is reached far before hitting the ground, raindrops’ kinetic power is related to the square of the raindrops’ speed. So, there is no difference in the harvested power from rainfall on higher elevation or ground level. Moreover, raindrops can be collected using a water tank at a higher platform. The rainwater with gravitational energy could be released into the PZT turbine arm [39,40]. This will rotate the turbine, and thus the PZT will be bend and harvest energy. This way gives a higher force on PZT than a single raindrop at each time, and thus it will harvest more power. The estimated output average energy from this harvester is 10 W/cm3 to 100 W/cm3 [40]. Furthermore, a piezoelectric energy harvester and hydro-electromagnetic have been designed to give power to the smart type of water meter system using water flow and magnetic force for excitation by Cho et al. [37]. A stainless-steel turn-buckle water wheel of 90 mm diameter and two magnets was proposed in this work, as shown in Figure4a. Output power density maximum root mean square for the piezoelectric harvester was 0.6222 µW/mm3 at 10 kΩ. The power harvested by piezoelectric will be used for the warning system of water leakage as a water leakage detector. Further work on the threshold voltage detector as a key issue can be investigated as future work. Energies 2021, 14, x FOR PEER REVIEW 6 of 30

of 90 mm diameter and two magnets was proposed in this work, as shown in Figure 4a. Output power density maximum root mean square for the piezoelectric harvester was 0.6222 µW/mm3 at 10 kΩ. The power harvested by piezoelectric will be used for the warn- Energies 2021, 14, 3098 6 of 29 ing system of water leakage as a water leakage detector. Further work on the threshold voltage detector as a key issue can be investigated as future work.

(a) (b) Figure 4.Figure (a) Pipe 4. (a-)flow Pipe-flow energy energy harvester harvester system system using using magnets magnets and withand nowith contact no contact [37]; (b) A [37 general]; (b) A general viewview forfor a a vortex-induced vortex-induced piezoelectric piezoelectric energy harvesterenergy harvester [32]. [32]. Conversely, in the An et al. study, a novel vortex-induced PZT energy converter Conversely,(VIPEC) is proposed in the An to harvest et al. study, the ocean’s a novel kinetic vortex energy-induced in the underwater PZT energy environ- converter (VIPEC)ment is proposed [32]. The harvester to harvest consists the ocean of a storage’s kinetic circuit, energy PZT patches, in the cylinder, underwater and a pivoted environment [32]. Theplate harvester attached consists to its tail, of as a shownstorage in circuit, Figure4 b.PZT The patches, maximum cylinder, output power and a density pivoted plate µ 3 attachedreaches to its 0.035tail, asW/m shownwith in a Figure resistance 4b. of The 10 k Ωmaximum. The VIPEC output simple structurepower density makes it reaches quite suitable for underwater mooring cables or other Underwater Mooring Platforms 3 0.035 µW/m(UMPs). with a resistance of 10 kΩ. The VIPEC simple structure makes it quite suitable for underwaterSimilarly, mooring Cellini et al. cables [77] assessed or other energy Underwater harvesting feasibilityMooring from Platforms the mechanical (UMPs). Similarly,buckling Cellini of Ionic et Polymer-Metal al. [77] assessed Composites energy (IPMC) harvesting induced feasibility by a steady from flow of the fluid. mechanical bucklingThe harvester of Ionic consists Polymer of- aMetal slider-crank Composites mechanism, (IPMC) two (IPMC)induced fixed by at a bothsteady ends, flow and of fluid. a paddlewheel. Experimental results demonstrate that IPMC output power range from The harvester1 pW–1 consists nW as the of flow a slider speed-crank varies mechanism, from 0.23–0.54 two m/s, (IPMC) and the fixed shunting at both resistance ends, and a paddlewheel.different Experimental from 1–1000 Ω. results The design demonstrate is unique; however, that IPMC the output output power power needs range to be from 1 pW–1 nWenhanced as the and flow make speed the designvaries more from compact. 0.23–0.54 m/s, and the shunting resistance different from 1–1000 Ω. The design is unique; however, the output power needs to be enhanced 3.2. The Influence of Excitation Elements and Design on Performance (Challenges and Issues) and make the design more compact. As shown in Table1, a comparison has been made between variable factors, divided into input, output, and comments. In general frequency (rpm), Piezoelectric size, dimen- 3.2. The sion,Influence and material of Excitation type, resistance, Elements andand whetherDesign theon Performance Piezoelectric rotate(Challenges or not, usingand Issues) a Asslip shown ring orin not,Table all 1, these a comparison elements affect has piezoelectric been made energy betwe harvestingen variable output factors, power, divided as shown in Table1. Output power and power density are also included in this table. into input,Although, output, excitation and comments. elements affect In thegeneral design, frequency however, as (rpm), shown in Piezoelectric Table1, for the size, same dimension, andexcitation material elements, type, resistance, the output powerand whether density is the different Piezoelectric depending rotate on the or design. not, using The a slip ring or materials not, all used these in elements the RPZTEH affect were mainlypiezoe PZT;lectric however energy some harvesting researchers output used PVDF power, as shown inand Table PZT composite. 1. Output power and power density are also included in this table. Alt- hough, excitation elements affect the design, however, as shown in Table 1, for the same excitation elements, the output power density is different depending on the design. The materials used in the RPZTEH were mainly PZT; however some researchers used PVDF and PZT composite.

Energies 2021, 14, 3098 7 of 29

Table 1. Summary comparison for rotational piezoelectric energy harvesting studies with detailed information.

Input Output Comments Optimal Power Mechanical Number Ref. Excitation Volume Polarisation Material Type Frequency PZT Rotate Piezo Dimension (mm) rpm Resistance Power (µW) Density Power Wiring Elements (mm ) Mode Use (Hz) or Not 3 (Ω) (µW/mm3) Source 1 [29] air swirler 96.2 37 × 13 × 0.2 / PVDF 300 6 47 3 0.03 Air Not Normal wiring storage rotates 2 [55] M + Sp + Gr 328 50.8 × 25.4 × 0.254 d31 PZT 5A 150 13.1 10 k 105 0.32 Air Yes with the system 3 [74] Sp + M + Gr 326.5 50.8 × 25.4 × 0.254 d31 QP16N 150 2.5 5 × 105 845 2.588 Air Yes Bluetooth 4 [30] imp 470 (47 × 20 × 0.5) / PZT 200 3.33 20 k 613 1.3 Air Yes supercapacitor 5 [66] M + Mg 4.26 16 × 3.5 × 0.076 / PZT-bimorph 751 12.5 200 k 0.25 0.06 Air Not ﬁxed piezo the application Q220-A4– 6 [65] M + Mg 115 31.8 × 7.12 × 0.508 d31 3696 61.6 3300 / 0.59 Air Not within the 303YB rotation area 7 [75] Mg 400 40 × 10 × 1 / (PZT-5H) 546 9.1 3 × 105 1320 3.3 Air Not / 8 [54] Mg 3360 70 × 32 × 1.5 / PZT layer 600 10 8000 k 16000 4.76 Air Not Normal wiring 9 [52] Mg 323 (50 × 12.7 × 0.127) × 4 d31 PZT-5A 200 16 247 k 5000 15.5 Air Not Normal wiring 10 [76] M + Gr + Mg 776 40.2 × 25.4 × 0.76 d31 (PPA-2011) 30 34 25 k 2200 2.835 Air Yes slip ring 11 [37] Mg 315 45 × 35 × 0.2 d31 PZT-ceramic 120 / 10 k 196 0.6222 Wat Not / vortex- 12 [32] / / d31 PZT / 16.49 1 × 105 / 0.035 1E + 09 Wat Not / induced ionic polymer Water ﬂow + 13 [77] 166.4 52 × 16 × 0.2 / metal 372.4 1 50 1 × 109 Wat Not / slide-crank composites M for mass, Mg for magnetic, Gr for gravity, Sp for spring, Wat for water, air for air, imp for impact force, and /for no information from the authors. Energies 2021, 14, 3098 8 of 29 Energies 2021, 14, x FOR PEER REVIEW 9 of 30

In Figure5, all the above studies have been plotted with rpm and output power density to have a clear idea about the best excitation elements with the highest power In Figure 5, all the above studies have been plotted with rpm and output power den- density.sity to have As a shown clear idea in theabout ﬁgure, the best the excitation studies elements that use with magnetic the highest force power as an density. excitation elementAs shown get in an the excellent figure, the output studies power that use density magnetic [52 ,force54]. Bothas an ofexcitation them use element only get magnets an forexcellent excitation output and power able toden harvestsity [52, excellent54]. Both of output them use power. only However,magnets for Karami excitation et al.and [52 ] haveable anto addedharvest advantage: excellent output their outputpower. powerHowever, density Karami is higher et al. and[52] hashave a an lower added rotational ad- speed.vantage: Additionally, their output usingpower airdensity only is as higher an excitation and has a gives lower the rotational lowest outputspeed. Addition- power. Yet, theally, feasibility using air ofonly energy as an harvesting excitation gives from the a swirling lowest output ﬂow using power. a piezoelectricYet, the feasibility beam of has beenenergy examined. harvesting from a swirling flow using a piezoelectric beam has been examined.

FigureFigure 5. 5.Power Power density density comparison comparison forfor differentdifferent studiesstudies on rotational piezoelectric piezoelectric energy energy harve harvestingsting for for fluid ﬂuid input input mechanical power source with different excitation elements and as follows: M for Mass, Mg for magnetic, Gr for gravity, mechanical power source with different excitation elements and as follows: M for Mass, Mg for magnetic, Gr for gravity, Sp Sp for spring, and air swirler for air swirler. for spring, and air swirler for air swirler. Regarding the design aspect, Zhao et al. [75] and Karami et al. [52] present a design Regarding the design aspect, Zhao et al. [75] and Karami et al. [52] present a design with an innovative method and a novel PZT cantilever and harvest good output power. with an innovative method and a novel PZT cantilever and harvest good output power. Both authors used magnetic only for excitation, and their prototypes are small and com- Both authors used magnetic only for excitation, and their prototypes are small and compact. pact. Karami et al. [52] have one more advantage: the PZT in his design does not rotate Karami et al. [52] have one more advantage: the PZT in his design does not rotate with with the system and thus no-slip ring or any other device is needed for output power the system and thus no-slip ring or any other device is needed for output power transfer. transfer. Then again, Y. Yang et al. [30] and Nezami et al. [76] are both able to harvest Thenenough again, power; Y. Yang however, et al. [ 30both] and prototypes Nezami etare al. bulky [76] areand both larger able. In to general, harvest Karami enough et power; al. however,[52] have boththe highest prototypes output are power bulky density and larger. with only In general, one excitation Karami element et al. [ 52to] be have less the highestcomplicated, output and power it comes density with with a small only onecompact excitation design. element One of to the be interesting less complicated, designs and itthat comes future with works a small should compact focus design.on is applying One of magnetic the interesting and gravity designs forces that as future excitation works shouldelements focus but on with is applyinga small compact magnetic design and to gravity achieve forces maximum as excitation output elementspower density. but with a small compact design to achieve maximum output power density. 4. Human Motion Applications 4. Human Motion Applications In most human motion applications, the harvester needs a frequency up-conversion so thatIn most its frequency human motion will be applications,near or at the thepiezoelectric harvester resonant needs a frequencyfrequency up-conversionto get maxi- somum that output its frequency power. will This be section near or has at made the piezoelectric a comparison resonant for different frequency designs to getand maximum excita- outputtion elements power. on This the section studies has that made used ahuman comparison motion for as differenta rotational designs input andmechanical excitation elements on the studies that used human motion as a rotational input mechanical power

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source.power It has source. been It subdivided has been su intobdivided groups into according groups to according excitation to elements, excitation such elements as (mass,, such magnetic,as (mass, and magnetic, gravitational and gravitational Forces), (mass Forces), and magnetic(mass and force), magnetic (magnetic force), (magnetic force only), force (mass),only), and (mass), (magnetic and (magnetic and gravity). and gravity). The design The anddesign excitation and excitation elements elements inﬂuence influence on outputon output power power density de havensity been have reported been reported as well. as well. 4.1. Comparison of Different Designs and Excitation Elements 4.1. Comparison of Different Designs and Excitation Elements Balancing between excitation elements and design is quite essential to produce a Balancing between excitation elements and design is quite essential to produce a sim- simple device with sufﬁcient output power. Sometimes, using the same excitation element ple device with sufficient output power. Sometimes, using the same excitation element can produce different output power due to their implementation in the design. can produce different output power due to their implementation in the design. One of the challenges of human motion energy harvesting is random excitation and One of the challenges of human motion energy harvesting is random excitation and low frequency. Pillatsch et al. [64] used a successful frequency up-conversion strategy: low frequency. Pillatsch et al. [64] used a successful frequency up-conversion strategy: introducing an inertial device containing PZT beam buckling principle using the mag- introducing an inertial device containing PZT beam buckling principle using the magnetic netic coupling of two permanent magnets and a rotating mass in half disk shape with a coupling of two permanent magnets and a rotating mass in half disk shape with a gravi- gravitational force as excitation elements, as shown in Figure6a. This device can work fromtational 0.5 to 4force Hz, andas excitation its maximum elements, power as density shown wasin Figure 0.023 6aµW/. This mm device3 at 2 can Hz wor (120k rpm). from 0.5 3 Theto device 4 Hz, is and quite its simple, maximum has fewerpower component density was numbers, 0.023 μ isW wear-reducing,/ mm at 2 Hz (120 and avoidsrpm). The physicaldevice contact. is quite simple, has fewer component numbers, is wear-reducing, and avoids phys- ical contact.

(a) (b) (c)

FigureFigure 6. (a )6. Harvester (a) Harvester prototype prototype isometric isometric view v andiew sectionand section view view [64]. [ (64b)]. Diagram (b) Diagram structure structure of the of wristthe wrist driven driven rotational rotational energyenergy harvester harvester using using magnetically magnetically plucked plucked PZT PZT [78]; [ (78c)] The; (c)The energy energy harvester harvester diagram diagram [72]. [72].

HalimHalim et al. et [ 78al.] [ fabricated78] fabricated a wrist-worn a wrist-worn rotational rotational energy energy harvester harvester using using plucked plucked piezoelectricpiezoelectric for for wearable wearable applications applications that that use use mass mass and and magnetic magnetic forceforce as excitation and andwithout without gravity. gravity. It It is is made made of of an an eccentric eccentric rotor rotor consisting consisting of of multiple multiple magnets magnets and and pie- piezoelectriczoelectric clampedclamped toto thethe hub centre,centre, and a a magnet magnet at at each each beam beam tip, tip, as as in in Fi Figuregure 6b6b.. This Thisdevice device can can harvest harvest a a more power densitydensity 0.34610.3461µ μW/mmW/mm33 atat 1.251.25 HzHz only. only. Its Its design design is is compactable,compactable, light, light, and and small, small, and and the the conﬁguration configuration allows allows the the arrangement arrangement of multiple of multiple magnetsmagnets in terms in terms of number, of number, size, size, and and gap gap to tune to tune the system.the system. Furthermore,Furthermore, using using the the same same excitation excitation elements, elements Fan, Fan et et al. al. [ 24[24]] proposed proposed aa multi-multipur- purposepose harvester containing containing a a ferromagnetic ferromagnetic ball ball with with 14 14 mm mm diameter diameter and and four four PZT PZT beams. beams.The Theprototype prototype has hasbeen been fabricated fabricated and and tested tested,, and and it was it was found found that that,, for for the the rotational rota- tionalmoti motion,on, it can it can harvest harvest a power a power density density of of 1.91 1.91 μW/mmµW/mm3 at3 atthe the rotating rotating frequency frequency of of 1 Hz 1 Hz(60 (60 rpm), rpm), which which is ismore more than than the the previous previous device device and and with with less less Hz. Hz. This This method method is prom- is promisingising for for low low-power-power devices devices implementing implementing self self-sustained-sustained power power.. Moreover,Moreover, a novel a novel hybrid hybrid multimodal multimodal device device was was presented presented by by Larkin Larkinet et al.al. [[7272]] us- usinging mass mass and and magneticmagnetic forceforce as excitation excitation also. also. AA configuration conﬁguration has has been been made made of an of un- an unbalancedbalanced rotary rotary-disk,-disk, three three PZT PZT-beams,-beams, and and two two permanent permanent magnet magnetss that that act act as asa proof a proofmass mass at ateach each PZT PZT end, end, as asin inFigure Figure 6c6.c. This This device device can can harvest harvest more more po powerwer density density (4.18 3 (4.18μW/mmµW/mm3) and) and has has been been suggested suggested for for several several applications applications,, such asas wornworn onon anan ankle ankle or or bicycle.bicycle. However, However, this this device device needs needs to to be be scaled scaled down down and and use use a slip a slip ring, ring, and and it also it also worksworks in a in much a much higher higher frequency frequency range range 20.2 20.2 Hz (1212Hz (1212 rpm). rpm).

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OnOn the the other other hand, hand, using using magnetic magnetic force force only only as as the the excitation, excitation, Sriyuttakra Sriyuttakra et et al. al. [79 [79]] introducedintroduced a practicala practical optimisation optimisation test test to theto the RPZTEH RPZTEH for low-frequencyfor low-frequency applications applications like wearablelike wearable devices. devices. When When the rotor the is rotor driven, is driven, a pushing a pushing force is applied force is from applied thetwo from magnets’ the two magneticmagnets’ forcemagnetic attached forceat attached the PZT at farthe endPZTin far the end stator in the and stator on and the rotor’son the rotor rotational’s rota- 3 disk.tional The disk. harvested The harvested power power density density ranges ranges from from 0.06–2.259 0.06–2.259µW/mm μW/mm,3 at, at10–100 10–100 rpm rpm (0.167–1.667(0.167–1.667 Hz) Hz)respectively. respectively. The The harvested harvested power power is is quite quite good, good, has has a a wider wider frequency frequency range,range, and and the the harvester harvester size size is is small small and and compact. compact. Moreover,Moreover, a a novelnovel wearablewearable energy harvester harvester that that converts converts the the low low frequency frequency of hu- of humanman limb limb into into high high frequency frequency has has been been proposed proposed in inthis this study study by byLi, Li,Keli, Keli, et al. et [ al.25] [.25 Mi-]. Micromachinedcromachined nickel nickel cantilever cantilever and and NdFeB NdFeB magnet magnet were were glued glued and and fixed fixed on a on polydime- a poly- dimethylsiloxanethylsiloxane (PDMS) (PDMS) film, film,as shown as shown in Figure in Figure 7a. The7a. generated The generated power power density density at 300 rpm at 3 300(5 Hz) rpm is (5 2.91 Hz) μW/mm is 2.91 µ3.W/mm The packaging. The packaging technique technique of flexible of devices flexible needs devices to be needs improved, to be improved,and the device and the needs device to be needs weaved to be into weaved the structures into the structures of tissue. of tissue.

(a) (b)

FigureFigure 7. 7.(a ()a Photos) Photos show show wearable wearable applications applications where where two two ﬂexible flexible harvesters harvesters are are ﬁxed fixed on on a a human human elbow elbow and and knee knee with with CloseClose views views [25 [25];] (;b ()b Piezoelectric) Piezoelectric knee-joint knee-joint energy energy harvester harvester with with frequency frequency up-conversion up-conversion by by magnetic magnetic plucking plucking [48 [48]. ].

Additionally,Additionally, KuangKuang etet al [ [4848]] produce produce a aKnee Knee-Joint-Joint Energy Energy Harvester Harvester (KEH). (KEH). A pro- A prototypetotype has has been been made made of of eight eight PZT PZT mount mounteded on on the the inner hub, anan outerouter ringring ofof 88 88 mm mm innerinner diameter, diameter, and and 64 64 slots slots are are equally equally positioned positioned all all around around the the outer outer ring ring inner inner edge. edge. EachEach slit slit allows allows oneone primaryprimary magneticmagnetic (pm) to be fixed ﬁxed and and a a secondary secondary magnetic magnetic (sm) (sm) is isfixed ﬁxed in in the the tip tip of of the the piezoelect piezoelectricric beam, beam, as asin Figure in Figure 7b7. b.The The output output power power increased increased with withthe the gap gap decreasing decreasing between between pm pm & & sm sm and and increasing increasing rotational speed. speed. The The maximum maximum powerpower density density is is 3.94 3.94µ W/mmμW/mm33at at 3232 PMs,PMs, and and uses uses eight eight piezoelectrics piezoelectrics and and a a 1.5 1.5 mm mm gap gap betweenbetween the the pm pm and and sm sm at at a a rotating rotating speed speed of of 0.9 0.9 Hz Hz (54 (54 rpm). rpm). This This study study makes makes fair fair use use ofof magnetic magnetic force force without without any any signs signs of of performance performance decreasing. decreasing. However,However, in in this this study, study, Mohamad Mohamad Hanif Hanif et et al. al. [49 [49] was] was utilised utilised only only the the mass mass concept concept forfor excitation. excitation. TheyThey havehave designeddesigned aa prototype consistsconsists of PZT cantilever, cantilever, a a rotor rotor with with a apole pole on on it itfor for PZT PZT plucking, plucking, and and aluminium aluminium proof proof mass, mass, as in asFigure in Figure 8a. The8a. power The power density 3 densityis 6.5 μW/mm is 6.5 µW/mm3 at a frequencyat a frequency of 18 Hz of (1080 18 Hz rpm) (1080 which rpm) is which much is powerful much powerful than the than pre- thevious previous technique. technique. This device This device’s’s advantage advantage is to generate is to generate enough enough electricity electricity to power to power elec- electronictronic devices, devices, only only by byusing using the thelow low-frequency-frequency vibration vibration of human of human motion, motion, without without rely- relyinging on onexternal external power. power. Pillatsch et al. [47] discovered a method of wirelessly actuating a rotor inside of a formerly presented rotational PZT harvester by using magnetic and gravity forces as excitation. A prototype has been made of the internal rotor, PZT beam, and magnetic, as shown in Figure8b. The maximum output power density is 13.6 µW/mm3, at 25 Hz. This harvester can charge the capacitor, even if there is no movement from the person, so, it could be a valuable addition to the already existing motion energy harvester, get red the primary concern of failure at rest.

(a) (b)

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On the other hand, using magnetic force only as the excitation, Sriyuttakra et al. [79] introduced a practical optimisation test to the RPZTEH for low-frequency applications like wearable devices. When the rotor is driven, a pushing force is applied from the two magnets’ magnetic force attached at the PZT far end in the stator and on the rotor’s rotational disk. The harvested power density ranges from 0.06–2.259 μW/mm3, at 10–100 rpm (0.167–1.667 Hz) respectively. The harvested power is quite good, has a wider frequency range, and the harvester size is small and compact. Moreover, a novel wearable energy harvester that converts the low frequency of human limb into high frequency has been proposed in this study by Li, Keli, et al. [25]. Mi- cromachined nickel cantilever and NdFeB magnet were glued and fixed on a polydime- thylsiloxane (PDMS) film, as shown in Figure 7a. The generated power density at 300 rpm (5 Hz) is 2.91 μW/mm3. The packaging technique of flexible devices needs to be improved, and the device needs to be weaved into the structures of tissue.

(a) (b) Figure 7. (a) Photos show wearable applications where two flexible harvesters are fixed on a human elbow and knee with Close views [25]; (b) Piezoelectric knee-joint energy harvester with frequency up-conversion by magnetic plucking [48].

Additionally, Kuang et al [48] produce a Knee-Joint Energy Harvester (KEH). A prototype has been made of eight PZT mounted on the inner hub, an outer ring of 88 mm inner diameter, and 64 slots are equally positioned all around the outer ring inner edge. Each slit allows one primary magnetic (pm) to be fixed and a secondary magnetic (sm) is fixed in the tip of the piezoelectric beam, as in Figure 7b. The output power increased with the gap decreasing between pm & sm and increasing rotational speed. The maximum power density is 3.94 μW/mm3 at 32 PMs, and uses eight piezoelectrics and a 1.5 mm gap between the pm and sm at a rotating speed of 0.9 Hz (54 rpm). This study makes fair use of magnetic force without any signs of performance decreasing. However, in this study, Mohamad Hanif et al. [49] was utilised only the mass concept for excitation. They have designed a prototype consists of PZT cantilever, a rotor with a pole on it for PZT plucking, and aluminium proof mass, as in Figure 8a. The power density is 6.5 μW/mm3 at a frequency of 18 Hz (1080 rpm) which is much powerful than the previous technique. This device’s advantage is to generate enough electricity to power elec- Energies 2021tronic, 14, 3098 devices, only by using the low-frequency vibration of human motion, without rely- 11 of 29 ing on external power.

(a) (b)

Figure 8. (a) Exploded view of the rotational motion energy harvester device [49]; (b) The harvester rotor design with the motion of magnetic and internal rotor [47].

4.2. The Influence of Excitation Elements and Design on Performance (Challenges and Issues) Table2 summarises different variables, divided into input, output, and comments. Excitation elements greatly affect the shape of the design and make it suitable for certain applications and help harvest sufficient output power. However, as shown in the Table, for the same excitation elements, the output power density is changing, and this is due to design change and its effect on output power. Figure9 compares all these study designs according to rpm and the output power density to have a clear idea about the best excitation elements based on power density. As indicated in the figure, almost all the studies used magnets as an essential element for excitation. However, Pillatsch et al. [47] can harvest the highest output power with added gravity force to the magnetic force as an excitation element at 25 Hz. In contrast, Pillatsch et al. [64] used the same excitation elements (magnetic and gravity forces) and added mass to them but failed to get as high as Pillatsch et al. [47]; however, its frequency was much lower which is 2 Hz only. These revealed that using the same excitation elements with the right better design gives a better result; however, achieving good power at very low frequency is also important especially in human motion applications where the frequency is low and irregular. Regarding the design of the prototype, Mohamad Hanif et al. [49] and Pillatsch et al. [47] both have the best simple design with the best output power. Mohamad Hanif et al. [49] used a straightforward design with only mass for excitation and harvest a good output power in less frequency compared to Pillatsch et al. [47] which used magnetic force with mass for excitation. However, Pillatsch et al. [47] have discovered a method using the coupling reluctance of a magnetic to an outer rotor with at less one permanent magnetic and gravitational force. This method can charge the capacitor, even if there is no movement from the user. On the other hand, Fan et al. [24] and Larkin et al. [72] can harvest sufficient output power but with a more complicated bulky design. In general, Pillatsch et al. [47] present the best simple, small, and compact design with higher output power. It could also be a valuable addition to the existing motion energy harvester and get red the primary concern of failure at rest. One of the interesting designs that future works should focused on is applying gravity and magnetic forces as excitation elements but with compact and small design to achieve maximum output power density. Energies 2021, 14, 3098 12 of 29

Table 2. Summary Comparison for rotational piezoelectric energy harvesting studies with detailed information.

INPUT Output Comments Optimal Power Mechanical Excitation Volume Piezo Dimension Polarisation Material Type Frequency PZT Rotate © Ref. rpm Resistance Power (µW) Density Power Wiring Elements (mm ) (mm) Mode Use (Hz) or Not 3 (Ω) (µW/mm3) Source 1 [64] M + Mg + Gr 1850 / / PZT 120 4 150 k 43 0.0232 HM Yes / piezo-electric 2 [78] M + Mg 18.06 12 × 3.5 × 0.43 / 75 1.25 95 k 6.25 0.3461 HM Not Normal wiring M1100 ceramic 3 [24] M + Mg 73.1 42 × 7.1 × 0.245 / PZT-5H 60 1 80 k 140 1.92 HM Yes Normal wiring 4 [72] M + Mg 1012 33 × 14 × 0.73 d31 PZT5A 1212 20.2 75 4230 4.18 HM Yes slip ring 5 [79] Mg 103.8 6.4 × 31.8 × 0.51 d31 PZT-5A4E 100 1.667 / 234.5 2.259 HM Not Normal wiring 6 [25] Mg 1.2 5 × 4 × 0.06 d31 PZT 300 5 40 k 3.5 2.92 HM Yes / PZT bimorph 7 [48] Mg 1471 38.1 × 12.7 × 0.38 / 54 0.9 15 k 5800 3.94 HM Yes / 5H 8 [49] M 200 40 × 10 × 0.5 / PZT5H) 1080 18 12 k 1300 6.5 HM Yes Normal wiring 9 [47] Mg + Gr 7.22 19.5 × 1 × 0.37 PZT bimorph 1500 25 151 k 100 13.9 HM Yes Normal wiring M for mass, Mg for magnetic, Gr for gravity, HM for human motion, and/for no information from the authors. Energies 2021, 14, x FOR PEER REVIEW 13 of 30

Energies 2021, 14, 3098 lower which is 2 Hz only. These revealed that using the same excitation elements with 13the of 29 right better design gives a better result; however, achieving good power at very low frequency is also important especially in human motion applications where the frequency is low and irregular.

FigureFigure 9. Rotational9. Rotational piezoelectric piezoelectric energy energy harvesting harvesting forfor humanhuman motion mechanical mechanical input input with with excitation excitation elements elements and and as as follow: M for Mass, Mg for magnetic, and Gr for gravity. follow: M for Mass, Mg for magnetic, and Gr for gravity.

5. RotationalRegarding Vehicle the de Tiressign of Applications the prototype, Mohamad Hanif et al. [49] and Pillatsch et al. [47] both have the best simple design with the best output power. Mohamad Hanif et al. [49]In used this a part, straightforward a comparison design has beenwith madeonly mass on the for studies excitation that and use harvest rotational a good vehicle out- tire asput input power mechanical in less frequency for piezoelectric compared energy to Pillatsch harvesting. et al. It [47 has] which been subdividedused magnetic into force groups accordingwith mass to for the excitation. excitation However, elements Pillatsch such as mass, et al. magnetic,[47] have discovered gravity, and a centrifugalmethod using force orthe a combinationcoupling reluctance of them. ofThe a magnetic design, to methodology, an outer rotor and with power at less output one permanent density have mag- been reportednetic and for gravitational each study asforce. well. This method can charge the capacitor, even if there is no movement from the user. 5.1. ComparisonOn the other of Different hand, Fan Designs et al. [ and24] and Excitation Larkin Elementset al. [72] can harvest sufficient output powerXie, but Kwuimy with a etmore al. [complicated19] presented bulky a rotational design. In energy general, harvester Pillatsch using et al. a[47 PZT] present bistable buckledthe best beam,simple, with small, a rotatingand compact disk design in low with rotational higher speed, output and power. magnetic It could force also onlybe a as excitationvaluable addition as in Figure to the 10 a.existing the Dual motion Attraction energy Magnetsharvester withand get Single red Repulsionthe primary (DASR) concern can enhanceof failure the at power rest. One and of obtain the interesting a wide broadband designs that frequency future ofworks 6–14 should Hz (560–840 focused rpm) on withis anapplying output powergravity densityand magnetic of 0.99–1.54 forces µasW/mm excitation3. The elements device but uses with a contactless compact and method small for harvestingdesign to achieve energy; maximum however, sizeoutput needs power to bedensity. reduced, and it needs to be more compatible. Furthermore, Fu & Yeatman [20] also used magnetic coupling as excitation for fre- quency5. Rotational up-conversion Vehicle inTires low-frequency Applications rotational energy harvesting. A prototype consists of a piezoelectricIn this part, cantilever a comparison harvester has been with mad a magnetice on the onstudies its tip that and use arotating rotational magnetic vehicle on thetire rotating as input host, mechanical as shown for in piezoelectric Figure 10b. Atenergy a rotational harvesting. speed It ofhas 15–35 been Hz subdivided (900–2100 into rpm ), thegroups power according density was to 5.031 the excitationµW/mm 3. elements This design such is simple as mass, in structure, magnetic, compact, gravity, and and using non-contact plucking, and can be used in various rotational energy harvester methods.

Energies 2021, 14, x FOR PEER REVIEW 14 of 30

centrifugal force or a combination of them. The design, methodology, and power output density have been reported for each study as well.

5.1. Comparison of Different Designs and Excitation Elements Xie, Kwuimy et al. [19] presented a rotational energy harvester using a PZT bistable buckled beam, with a rotating disk in low rotational speed, and magnetic force only as excitation as in Figure 10a. the Dual Attraction Magnets with Single Repulsion (DASR) can enhance the power and obtain a wide broadband frequency of 6–14 Hz (560–840 rpm) Energies 2021, 14, 3098 with an output power density of 0.99–1.54 µW/mm3. The device uses a contactless14 method of 29 for harvesting energy; however, size needs to be reduced, and it needs to be more compatible.

(a) (b)

FigureFigure 10. 10.(a) ( Energya) Energy harvester harvester design design [19]; [19 (b]); Energy(b) Energy harvester harvester diagram: diagra Outlinem: Outline of magnetic of magnetic coupling coupling and parametersand parameters of structureof structure [20]. [20]. Above all, Manla et al. [80] have carried out the experiment using magnetic force and centrifugalFurthermore, force Fu for & excitation. Yeatman [ The20] also harvester used magnetic includes, coupling a tube with as excitation a piezoelectric for fre- thunderquency ﬁx up on-conversion each end and in alow magnetic-frequency adhesive rotational to each energy thunder. harvesting. At a rotation A prototype speed from con- 3–5.5sists rps of (180–330a piezoelectric rpm), ca thentilever output harvester power density with a ismagnetic from 0.0031–0.054 on its tip andµW/mm a rotating3. This mag- designnetic is on a novelthe rotating approach, host, low as shown in mass, in volume, Figure 10b. and At also a rotational compact. However,speed of 1 utilising5–35 Hz the(900– magnets’2100 rpm), stiffness the power by changing density its was size 5.031 or axial µW/mm gap3 will. This enable design the is harvestersimple in tostructure, be used com- in otherpact, applications and using wherenon-contact different plucking, input force and can is applied. be used in various rotational energy har- vesterFurthermore, methods. X. Wu et al. [4] also studied a combination of centrifugal and magnetic forces andAbove implemented all, Manla themet al. as[80 excitation] have carried elements out the in aexperiment novel design using of see-saw magnetic structure. force and Thiscentrifugal design consists force for of excitation. two magnetic The supportharvester seesaw includes, structure a tube and with PVDF a piezoelectr ﬁx belowic thunder them. Atfix 16 on Hz each (965 end rpm and and a 115.4 magnetic km/h adhesive for an actual to each car), thunder. the power At adensity rotation is speed 1.2 µW/mm from 3–35.5 usingrps a(18 slip0–330 ring rpm), to transfer the output the power. power This density unique is from design 0.003 could1–0.054 avoid µW/mm the effect3. This of huge design centrifugalis a novelforce, approach, due tolow its in balance mass, volume, distinctive. and also In future compact. work, However, different uti piezoelectriclising the mag- materialsnets’ stiffness maybe by test changing to overcome its size the or temperature axial gap will issue enable in high-speed. the harvester to be used in other applicationsAlong with, where Zhang different et al. 2016input [ 42force], presented is applied. an energy harvester using magnets, Energies 2021, 14, x FOR PEER REVIEW mass, andFurthermore, gravity force X. Wu for et excitation. al. [4] also studied The prototype a combination consisted of centrifugal of; aluminium and magnetic beam,15 of 30 PZTforces adhesive and implemented on it, two magnets, them as and excitation an adjusting elements pole in to a change novel design the distance of see between-saw struc- them,ture. as This in Figure design 11 consistsa. The experimentof two magnetic shows support that stochastic seesaw structure resonance and will PVDF enhance fix below the performancethem. At 16 and Hz that (965 the rpm capture and power 115.4 km/his 12-times for an better actual than car), using the the power noise density excitation is 1.2 andonly 50µW/mm-times and 50-times 3better using athan better slip ringusing than to using gravity.transfer gravity. the The power. Thepower power This density unique density is design is0.52 0.52 μW/mm couldµW/mm avoid3 3atat the 6 6 Hz, Hz,effect and sinceand ofthe sincehuge harvester the centrifugal harvester is attached force, is attached due to to the to its the wheelbalance wheel centre distinctive. centre,, a cablecable In future transmission transmission work, candifferent can be usedbe piezoe- used to to powerpowerlectric the the TPMS. materials TPMS. maybe test to overcome the temperature issue in high-speed. Along with, Zhang et al. 2016 [42], presented an energy harvester using magnets, mass, and gravity force for excitation. The prototype consisted of; aluminium beam, PZT adhesive on it, two magnets, and an adjusting pole to change the distance between them, as in Figure 11a. The experiment shows that stochastic resonance will enhance the performance and that the capture power is 12-times better than using the noise excitation only

(a) (b)

FigureFigure 11. 11.(a) Energy(a) Energy harvester harvester fixed ﬁxed to tothe the wheel wheel centre centre [42 [42]];; ( (bb)) Rotating Rotating tire energyenergy harvesterharvester model,model, with with real real one one photo photo [63 [63].].

Similarly, mass, magnetic, but with centrifugal force, has been used as the design excitation elements by Shange et al. 2018 [63]. A prototype consists of two magnetic masses with the same polarities and a cantilever beam with piezoelectric attached to it, which is applied on a rotating energy harvester tire, as in Figure 11b. In a driving test, the rotating frequency bandwidth could be broadened from 15–25 km/h to 10–40 km/h, and the generated power density is 11.26 μW/mm3. The advantage of this design is that it broadens the bandwidth of rotating frequency and concurrently stabilised high energy path oscillations. Along with, Gu & Livermore 2010 [59] have presented a self-tuning energy harvester using mass and centrifugal forces as excitation. The prototype was made of a radial cantilever piezoelectric at a distance from the centre with a mass tip at its end. The output power density range 1.5–5.3 μW/mm3, using a slip ring for output power transfer. This design showed significantly enhanced performance compared with the untuned harvester. As well as Gu & Livermore 2012 [58] also produced a self-tuning energy harvester’s rotational applications using centrifugal force only for excitation. The harvester turns into a vertical plane, and it consists of two beams: a piezoelectric cantilever and a thin driving cantilever with mass put at his end. At 15.2 Hz, the power density is 1.53 μW/mm3, although the PVDF beam output power is less than PZT ceramic, its durability is higher, besides, using a thicker PVDF beam can increase the output power. Furthermore, Zhu et al. [44] have made a prototype of a piezoelectric beam with one end fix and a mass on the other end, and with using mass, vibration, and centrifugal force for excitation. A road test has been done to check the energy harvester output power from the wheel vibration excitation. At 952.2 rpm, the output power density is 6.122 µW/mm3. This device is feasible to supply power for TPMS; however, a slip ring was needed to transfer output power. Hence, Guan & Lia [43] have used mass, spring, and gravity forces for excitation. The prototype is made of; piezoelectric mounted on a beam and fixed near the edge of a rotating frame with mass at its end, as in Figure 12. Using a slip ring for power transfer, the power density is 2.168 µW/mm3 at 13 Hz (810 rpm). This application could be applied in TPMS, but the housing and mounting must consider as a future issue. Besides, it will not harvest energy, at a very slow speed or at rest; however, this can be solved by using a super capacitor or rechargeable battery.

Energies 2021, 14, 3098 15 of 29

Similarly, mass, magnetic, but with centrifugal force, has been used as the design excitation elements by Shange et al. 2018 [63]. A prototype consists of two magnetic masses with the same polarities and a cantilever beam with piezoelectric attached to it, which is applied on a rotating energy harvester tire, as in Figure 11b. In a driving test, the rotating frequency bandwidth could be broadened from 15–25 km/h to 10–40 km/h, and the generated power density is 11.26 µW/mm3. The advantage of this design is that it broadens the bandwidth of rotating frequency and concurrently stabilised high energy path oscillations. Along with, Gu & Livermore 2010 [59] have presented a self-tuning energy harvester using mass and centrifugal forces as excitation. The prototype was made of a radial cantilever piezoelectric at a distance from the centre with a mass tip at its end. The output power density range 1.5–5.3 µW/mm3, using a slip ring for output power transfer. This design showed significantly enhanced performance compared with the untuned harvester. As well as Gu & Livermore 2012 [58] also produced a self-tuning energy harvester’s rotational applications using centrifugal force only for excitation. The harvester turns into a vertical plane, and it consists of two beams: a piezoelectric cantilever and a thin driving cantilever with mass put at his end. At 15.2 Hz, the power density is 1.53 µW/mm3, although the PVDF beam output power is less than PZT ceramic, its durability is higher, besides, using a thicker PVDF beam can increase the output power. Furthermore, Zhu et al. [44] have made a prototype of a piezoelectric beam with one end fix and a mass on the other end, and with using mass, vibration, and centrifugal force for excitation. A road test has been done to check the energy harvester output power from the wheel vibration excitation. At 952.2 rpm, the output power density is 6.122 µW/mm3. This device is feasible to supply power for TPMS; however, a slip ring was needed to transfer output power. Hence, Guan & Lia [43] have used mass, spring, and gravity forces for excitation. The prototype is made of; piezoelectric mounted on a beam and fixed near the edge of a rotating frame with mass at its end, as in Figure 12. Using a slip ring for power transfer, the power density is 2.168 µW/mm3 at 13 Hz (810 rpm). This application could be applied in TPMS, but the housing and mounting must consider as a future issue. Besides, it will Energies 2021, 14, x FOR PEER REVIEW 16 of 30 not harvest energy, at a very slow speed or at rest; however, this can be solved by using a super capacitor or rechargeable battery.

FigureFigure 12. 12.The The prototype prototype of the of piezoelectricthe piezoelectric harvester harvester structure structure ﬁxes on fixes the frame on the [43 frame]. [43].

Moreover, Y. Wang, et al. [81] proposed a design that changes its resonance frequency with different rotational speeds. Mass and spring force have been used for excitation but with centrifugal force instead of gravity force. The design is made of a trapezoidal cantilever with a rubber plate and a tensional spring to join the trapezoidal base’s. When the wheel rotates at 200–700 rpm the output power density is 1.06–2.12 μW/mm3, and a slip ring was used. This design’s novelty is its ability to change its resonance frequency with different rotational speeds. Alternatively, Rui et al. 2019 [82] presented a piezoelectric composite fibre materials energy harvester installed in spoke for the TPMS. The gravity influence of the free end mass generates a periodic excitation during the rotation, and the structure of the safety limiting is designed. The experimental results show that the power density was 1.67 μW/mm3. This harvester in the spokes has good application prospects and higher practicability than other harvesters installed on the tire. However, some improvements can be made to enhance output power, such as using bimorph instead of uni-morph and install more harvester, one at each spoke. Similarly, Rui et al. 2020 [83] using mass, centrifugal, and gravity forces as excitation and proposed a PZT energy harvester to provide power for low-consumption appliances in intelligent car wheels. The harvester is implanted in the spoke and has the self-tuning ability due to centrifugal force in the rotational motion. An experimental prototype has been built with macro fibre composite, and the generated output power density ranges from 1.437–14.804 μW/mm3 at a 40–120 kph on a real road test. The harvester passive self- tuning creates broadband to match vehicle speed. Furthermore, applying a High-Efficiency Compressive-Mode Piezoelectric Energy Harvester (HC-PEH) in a rotational system was studied theoretically and experimentally by Wang et al. [84]. The HC-PEH is comprised of a proof mass, two elastic beams, and a centre of a flex-compressive which consists of one PZT and two bow-shape curved in metal plates. The experimental results show that the maximum output power density was 26.993 μW/mm3 at 21.67 Hz, with an external resistor of 40 kΩ. Notably, the harvester performance in a rotational system is better than the translation one; thus, it is quite adaptable for rotational system applications.

5.2. The Influence of Excitation Elements and Design on Performance (Challenges and Issues): As shown in Table 3, a comparison has been made between variable factors, divided into input, output, and comments. In general frequency (rpm), PZT size, dimension, and material type, resistance, and whether the PZT rotate or not, using a slip ring or not, all

Energies 2021, 14, 3098 16 of 29

Moreover, Y. Wang, et al. [81] proposed a design that changes its resonance frequency with different rotational speeds. Mass and spring force have been used for excitation but with centrifugal force instead of gravity force. The design is made of a trapezoidal cantilever with a rubber plate and a tensional spring to join the trapezoidal base’s. When the wheel rotates at 200–700 rpm the output power density is 1.06–2.12 µW/mm3, and a slip ring was used. This design’s novelty is its ability to change its resonance frequency with different rotational speeds. Alternatively, Rui et al. 2019 [82] presented a piezoelectric composite fibre materials energy harvester installed in spoke for the TPMS. The gravity influence of the free end mass generates a periodic excitation during the rotation, and the structure of the safety limiting is designed. The experimental results show that the power density was 1.67 µW/mm3. This harvester in the spokes has good application prospects and higher practicability than other harvesters installed on the tire. However, some improvements can be made to enhance output power, such as using bimorph instead of uni-morph and install more harvester, one at each spoke. Similarly, Rui et al. 2020 [83] using mass, centrifugal, and gravity forces as excitation and proposed a PZT energy harvester to provide power for low-consumption appliances in intelligent car wheels. The harvester is implanted in the spoke and has the self-tuning ability due to centrifugal force in the rotational motion. An experimental prototype has been built with macro fibre composite, and the generated output power density ranges from 1.437–14.804 µW/mm3 at a 40–120 kph on a real road test. The harvester passive self-tuning creates broadband to match vehicle speed. Furthermore, applying a High-Efficiency Compressive-Mode Piezoelectric Energy Harvester (HC-PEH) in a rotational system was studied theoretically and experimentally by Wang et al. [84]. The HC-PEH is comprised of a proof mass, two elastic beams, and a centre of a flex-compressive which consists of one PZT and two bow-shape curved in metal plates. The experimental results show that the maximum output power density was 26.993 µW/mm3 at 21.67 Hz, with an external resistor of 40 kΩ. Notably, the harvester performance in a rotational system is better than the translation one; thus, it is quite adaptable for rotational system applications.

5.2. The Influence of Excitation Elements and Design on Performance (Challenges and Issues) As shown in Table3, a comparison has been made between variable factors, divided into input, output, and comments. In general frequency (rpm), PZT size, dimension, and material type, resistance, and whether the PZT rotate or not, using a slip ring or not, all these elements that affect on piezoelectric energy harvesting output power has been reported in Table1, also, both output power and power density have been reported as well in this table. Although, excitation elements greatly affect shaping the design and make it suitable for certain applications and help harvest sufficient output power. However, as clearly shown in Table3, for the same excitation elements the output power density is changing, and this is due to design change and its effect on output power. Figure 13 compares all these designs according to rpm and the output power density to have a clear idea about the best excitation elements. As shown in the Figure, almost 78% of the reported studies have used centrifugal force for excitation. However, this was not enough to harvest maximum output power. As shown, the prototype that adds mass as excitation alone/or with gravity force gets a very good result [44,84]. On the other hand, the applications that add a magnet to centrifugal force for excitation get the lowest results [4,80]. However, Manla et al. [80] presented a harvester that contains a tube with a piezoelectric thunder fix on each end. This design is a novel approach, having low values of mass, minimum volume, and compact. X. Wu et al. [4] also presented a see-saw structure design, this unique harvester design could avoid the effect of huge centrifugal force due to its balance distinctive. Energies 2021, 14, 3098 17 of 29

Table 3. Summary Comparison for rotational piezoelectric energy harvesting studies with detailed information.

Input Output Comments Optimal power Mechanical Excitation Volume Piezo Dimension Polarisation Material Frequency PZT Rotate Number Ref. rpm Resistance Power (µW) Density Power Wiring Elements (mm ) (mm) Mode Type Use (Hz) or No 3 (Ω) (µW/mm3) Source 1 [19] Mg 42.2 22 × 8 × 0.24 / PZT-5H 840 14 110 k 65.4 1.55 VT No Direct wiring 2 [20] Mg 3.98 26.5 × 1.5 × 0.1 / / 2100 300 / 20 5.03 VT No Direct wiring 3 [80] Mg + Cn 64.5 25.4 × 12.7 × 0.2 / PZT 5A 330 5.5 / 3.5 0.05 VT Yes / 4 [4] Mg + Cn 30 25 × 12 × 0.1 D31- PVDF 965 16 600 k 36 1.2 VT Yes Slip ring 5 [42] M + Mg + Gr 22.9 22.9 × 10 × 0.1 / PZT-ceramic 552 9.2 252 k 12 0.52 VT / / 6 [63] M + Mg + Cn 21.3 22.9 × 9.3 × 0.1 / / 365 6.08 150 k 240 11.3 VT Yes / 7 [58] Cn 80 25 × 6.4 × 0.5 d31 PZT 972 16.2 / 123 1.54 VT Yes slip ring 8 [59] M + Cn 130 41.3 × 6.3 × 0.5 / PZT 792 13.2 / 700 5.38 VT Yes slip ring 9 [44] Cn + M 24.5 70 × 5 × 0.07 d33 PZT-5H 952 15.9 1000 k 500 20.4 VT Yes slip ring 10 [43] M + Sp + Gr 380 28.5 × 50 × 0.267 / PSI-5A4E 810 13.5 / 825 2.17 VT Yes slip ring 11 [81] M + Sp + Cn 66 22 × 6 × 0.5 D31 PVDF 700 11.7 3300 k 140 2.12 VT Yes slip ring 12 [82] M + Gr + Cn 600 150 × 20 × 0.2 / PVDF 504 / 2 × 105 1003 1.6717 VT Yes slip ring 13 [83] M + Gr + Cn 424.2 101 × 20 × 0.21 / PZT (MFC) 1008 16.79 2 × 105 6280 14.804 VT Yes Sd card 14 [84] M + Gr + Cn 612 51 × 20 × 0.6 d31 PZT-5H 1300 21.67 40 k 16,520 26.993 VT Yes slip ring M for mass, Mg for magnetic, Cn for centrifugal, Gr for gravity, Sp for spring, VT for vehicle tires, and/for no information from the authors. Energies 2021, 14, x FOR PEER REVIEW 18 of 30

[4,80]. However, Manla et al. [80] presented a harvester that contains a tube with a piezo- Energies 2021, 14, 3098 electric thunder fix on each end. This design is a novel approach, having low values18 of of 29 mass, minimum volume, and compact. X. Wu et al. [4] also presented a see-saw structure design, this unique harvester design could avoid the effect of huge centrifugal force due to its balance distinctive.

FigureFigure 13. 13.Rotational Rotational piezoelectric piezoelectric energy energy harvesting harvesting forfor vehiclevehicle tires mechanical mechanical input input with with excitation excitation elements elements and and as as follow:follow: M forM for mass, mass, Mg Mg for for magnetic, magnetic, Cn Cn for for centrifugal, centrifugal, GrGr forfor gravity, and Sp for for spring. spring.

NowNow in in termsterms of design, design, few few studies studies present present a very a very simple, simple, small, small, and compact and compact design design[44,59 [44,8,593]., 83Zhu]. et Zhu al. et[44 al.] design [44] design their model their after model several after tests several to wheel tests vibration to wheel charac- vibration characteristicsteristics and ene andrgy energy harvester harvester test at testthe road. at the Gu road. & Livermore Gu & Livermore [59] have [59 enhanced] have enhanced their theirself- self-tuningtuning energy energy harvester harvester by adding by adding mass massforce beside force beside centrifugal centrifugal force for force excitation. for excitation.This This design design showed showed significantly significantly enhanced enhanced performance performance comparing comparing with with the the untuned untuned harvester.harvester. Rui Rui etet al.al. [[8383]] presentpresent a harvester with with the the ability ability of of passive passive self self-tuning-tuning ability ability byby centrifugal centrifugal forceforce toto createcreate a broadband matching matching vehicle vehicle speed. speed. There There are are some some other other designsdesigns that that areare notnot veryvery compactcompact [ 1919,,4433,,8811]].. For For example, example, Fu Fu & &Yeatman Yeatman [20 [20] prototyp] prototypee needsneeds to to reduce reduce thethe sizesize and make it it more more compact; compact; however, however, the the device device uses uses a contactless a contactless wayway to to harvest harvest energy. energy. Additionally, Guan & Lia [43] application could be applied in TPMS, but it must be Additionally, Guan & Lia [43] application could be applied in TPMS, but it must considered; the housing and mounting as a future issue. Besides, it will not harvest en- be considered; the housing and mounting as a future issue. Besides, it will not harvest ergy, when the tire moves very slow or at rest; however, this can be solved by using a energy, when the tire moves very slow or at rest; however, this can be solved by using a supercapacitor or rechargeable battery. The design produced by Y. Wang, et al. [81] is also supercapacitor or rechargeable battery. The design produced by Y. Wang, et al. [81] is also a little bit complicated with many parts and excitation elements. For the output power a little bit complicated with many parts and excitation elements. For the output power transfer, a slip ring was used; however, the novelty of this design is that its ability to transfer, a slip ring was used; however, the novelty of this design is that its ability to change change its resonance frequency with different rotational speeds. In general, Zhu et al. [44] itsand resonance Rui et al. frequency [83] has the with simpl differentest compact rotational design speeds.with a significantly In general, enhanced Zhu et al. perfor- [44] and Ruimance. et al. One [83] of has the the interesting simplest designs compact that design future with works a significantly should focus enhanced on is applying performance. grav- Oneity and of the centrifugal interesting forces designs as excitation that future elements works but should with focusa compact on is and applying small design gravity fit and centrifugaleasily in vehicle forces tire as excitationto achieve elementsmaximum but output with power a compact density. and small design fit easily in vehicle tire to achieve maximum output power density.

6. Other Rotational Operational Principal Energy Harvester in General This section compares the studies of various authors, who have suggested that their designs can make use of any rotational machine as an input mechanical power source for rotational piezoelectric energy harvester, has been done. It has been divided into two sections: any rotational machine in general and a rotational machine using gears. Each Energies 2021, 14, x FOR PEER REVIEW 19 of 30

6. Other Rotational Operational Principal Energy Harvester in General This section compares the studies of various authors, who have suggested that their designs can make use of any rotational machine as an input mechanical power source for rotational piezoelectric energy harvester, has been done. It has been divided into two sections: any rotational machine in general and a rotational machine using gears. Each sub- section is divided into different groups according to excitation elements for each study, their design, methodology, and output power density have been reported. Energies 2021, 14, 3098 19 of 29

6.1. Comparison of Different Designs and Excitation Elements

Firstly, in this subsectionsection, the is divided studies into that different have groups been accordingdesigned to to excitation work without elements forspecific each study, rotational mechanicaltheir input design, (any methodology, application) and outputhave been power reported density have in different been reported. sections according to their excitation elements. 6.1. Comparison of Different Designs and Excitation Elements Grzybek and Micek et al. [85] present an investigation on piezoelectric energy har- Firstly, in this section, the studies that have been designed to work without specific vesting using Macrorotational Fiber Composite mechanical input (MFC) (any whic application)h was directly have been glued reported on inthe different rotating sections shaft without addingaccording any mechanical to their excitation structure, elements. using shaft stress and rpm as excitation. At 56.17 kΩ, and 20 HzGrzybek the output and Micek power et al. density [85] present is 2.08 an investigation μW/mm3. This on piezoelectric harvester energy can be harvest- used to supply powering for using WSN Macro used Fiber in Composite measuring (MFC) shaft which parameters, was directly e.g., glued stress. on the This rotating sen- shaft without adding any mechanical structure, using shaft stress and rpm as excitation. At sor can replace the conventional sensor that required conduits and a slip3 ring for output 56.17 kΩ, and 20 Hz the output power density is 2.08 µW/mm . This harvester can be used transfer. to supply power for WSN used in measuring shaft parameters, e.g., stress. This sensor can Conversely, Ramírezreplace theet al. conventional [73] designed sensor a that device required of two conduits cantilever and a slipbeams ring forwith output a mass transfer. at the free end and connectConversely,ed with Ram springírez et as al. [excitation73] designed elements, a device of as two shown cantilever in beamsFigure with 14a. a mass This work’s primaryat influence the free end is and to provide connected a withfinite spring element as excitation of one dimension elements, as shownof modelling in Figure 14a. This work’s primary influence is to provide a finite element of one dimension of modelling a three-dimension a rotating three-dimension energy rotating harvester. energy The harvester. maximum The maximum output power output power density density is is 0.0229 μW/mm3 at 60.0229 Hz (300µW/mm rpm),3 at an 6 Hz Arduino (300 rpm), is an used Arduino to transfer is used to the transfer output the voltage output voltage via via Bluetooth to avoid Bluetoothslip ring tomechanisms avoid slip ring complexity. mechanisms complexity. Future work Future may work include may include develop- developing ing a rotational energya rotational harvester energy more harvester complex more to complex investigate to investigate the generation the generation of power. of power.

(a) (b)

FigureFigure 14. (14.a) piezoelectric (a) piezoelectric beam using beam a tip using mass [ 73a tip]; (b )mass The harvester [73]; (b design,) The harvester with its excitation design, force with using its magnetics excitation [12]. force using magnetics [12]. W. Wu [12] has made a prototype of two piezoelectric beams connected in parallel with a magnet of 2.5 m diameter and 2 mm in height fixed on the piezoelectric beam free W. Wu [12] hasend made and on a theprototype rotating host,of two respectively piezoelectric to provide beams magnetic connected force, as in in parallel Figure 14 b. It with a magnet of 2.5was m found diameter that at and 546 rpm,2 mm the in maximum height outputfixed on power the density piezoelectric is 22.82 µ W/mmbeam 3free. Besides, end and on the rotatingthe magnetic host, respectively force will widen to theprovide PZT broadband magnetic energy force, harvester, as in Figure and optimising 14b. It the was found that at 546system rpm, was the done maximum by using output different power configurations density of is magnetic 22.82 μW/mm force. 3. Besides, Furthermore, Ma et al. [86] designed a hybrid energy harvester of triboelectric- the magnetic force will widen the PZT broadband energy harvester, and optimising the electromagnetic-piezoelectric induced by a multi-function magnet. In the piezoelectric system was done bygenerator using different (PEG) unit, configurations a PZT is put in the of topmagnetic shell centre force. beside a permanent magnet that Furthermore, Mais fixed et al. to [ the86] end designed of the PZT. a hybrid The attractive energy and harvester repulsive of forces triboelectric between the-elec- passing tromagnetic-piezoelectricand permanent induced magnets by a multi bend- thefunction PZT and magnet. therefore In energy the piezoelectric was harvested, gener- as shown in 3 ator (PEG) unit, a PZTFigure is 15 puta. The in PEGthe outputtop shell power centre density beside is 0.4213 a µpermanentW/mm under magnet 50 MΩ .that This is hybrid harvester shows good durability and stability and demonstrates a new energy harvester fixed to the end of thekind PZT. that can The effectively attractive harvest and rotational repulsive mechanical forces between energy. the passing and permanent magnets bend the PZT and therefore energy was harvested, as shown in Figure

Energies 2021, 14, x FOR PEER REVIEW 20 of 30

15a. The PEG output power density is 0.4213 μW/mm3 under 50 MΩ. This hybrid har- Energies 2021, 14, 3098 20 of 29 vester shows good durability and stability and demonstrates a new energy harvester kind 15a. The PEG output power density is 0.4213 μW/mm3 under 50 MΩ. This hybrid har- that can effectivelyvester harvest shows goodrotational durability mechanical and stability energy. and demonstrates a new energy harvester kind that can effectively harvest rotational mechanical energy.

(a) (b) (c) (a) (b) (c) Figure 15. (a) Operation principle of PZT generator [86]; (b) the diagram of the simulated model FigureFigure 15. 15.(a) Operation(a) Operation[45 principle]; (c) principlea schematic of PZT generator of design PZT showing [86generator]; (b) the the diagram two[86 ]magnetic; (b of) the simulatedanddiagram all other model of parts. the [45 simulated[9]]; (.c ) a schematic model design showing[45]; (c the) a twoschematic magnetic design and all othershowing parts. the [9]. two magnetic and all other parts. [9]. To avoid slip ring mechanism complexity for data transmission, an Arduino board To avoid slip ring mechanism complexity for data transmission, an Arduino board To avoid sliphas ring been mechanismimplemented ascomplexity a data gaining for system data transmission,to transfer voltage an through Arduino Bluetooth board by M. Lihas et beenal. [45 implemented]. The prototype as a dataconsists gaining of a systemmagnetic to transfercircuit that voltage fixed through to the beam Bluetooth free by has been implementedend,M. and Li as the et a al.cantilever data [45]. gaining The is prototypeput insystem the magnetic consists to tran of circuitsfer a magnetic voltageair gap circuit as through in Figure that ﬁxed 15b.Bluetooth toAt thea rotation beam by free M. Li et al. [45]. speedTheend, prototypeof and588 rpm the cantilever (9.8 consists Hz), the is putof powe a in magnetic ther density magnetic 2.73 circuit circuit μW/mm that air3 gap. Futurefixed as in towork Figure the may beam15b. include At free a rotation an end, and the cantileverinvestigationspeed is of put 588 on in rpm the the (9.8 harvester magnetic Hz), the radial power circuit location density air gap influence 2.73 asµ W/mmin onFigure the3. Future output 15b. work At power a mayrotation’s perfor- include an speed of 588 rpmmance (9.8investigation Hz),and developing the onpowe the a harvesterr MEMSdensity-scale radial 2.73 generator. location μW/mm inﬂuence 3. Future on the work output may power’s include performance an andFu & developing Yeatman [9] a MEMS-scale presented a generator. harvester for low-power sensing applications. A pro- investigation on the harvester radial location influence on the output power’s perfor- totype ofFu mass & Yeatman and magnets [9] presented as excitation a harvester elements for for low-power PZT were sensing used to applications. change kinetic A pro- mance and developingenergytotype into a MEMS of electric mass- andscaleenergy, magnets generator. as in asFigure excitation 15c. The elements output for power PZT density were used at 660 to change rpm is kinetic5.2 Fu & YeatmanμW/mm energy[9] 3presented. This into work electric presentsa harvester energy, a theoretical as for in Figure low model- power15c. and The insensing- outputdepth bistable powerapplications. densityfrequency atA up 660pro--con- rpm is 3 totype of mass andverting 5.2magnets µharvesterW/mm as .analysing, Thisexcitation work giving presents elements basic a guidance, theoretical for PZT and modelwere understanding andused in-depth to change the bistable design kinetic of frequency low - up-converting harvester analysing, giving basic guidance, and understanding the design energy into electricfrequency energy, broadband as in Figure energy 15c.harvester. The output power density at 660 rpm is 5.2 ofRamírez low-frequency et al. [87 broadband] presented energy an experimental harvester. validation with numerical analysis to μW/mm3. This work presents a theoretical model and in-depth bistable frequency up-con- the novelRam piezoelectricírez et al. [rotational87] presented motion an experimental device made validation with E-shape with multi numerical-beams. analysis Centrif- to the verting harvesterugal analysing,novel force piezoelectricand givingmass have rotationalbasic been guidance, used motion for excitation device and understanding made elements, with E-shape and the the multi-beams. harvester design comprisesof Centrifugallow- frequency broadbandof twoforce energyE-shape and mass multipleharvester. have beams been used connected for excitation by a rigid elements, beam, and and as the shown harvester in Figure comprises 16a. of two E-shape multiple beams connected by a rigid beam, and as shown in Figure 16a. Ramírez et Theal. [maximum87] presented output an power experimental density of 0.61 validation μW/mm3 withachieved numerical when running analysis in a lowto - µ 3 frequencyThe maximum from 0.5 outputto 3 Hz power (30–180 density rpm). of Finally, 0.61 W/mm the proposedachieved device when provides running a good in a low- the novel piezoelectricfrequency rotational from 0.5mo totion 3 Hz device (30–180 made rpm). with Finally, E-shape the proposed multi- devicebeams. provides Centrif- a good energy harvesting solution at a very low rotational frequency such as 30 kW wind tur- ugal force and massenergy have harvestingbeen used solution for excitation at a very low elements, rotational and frequency the harvester such as 30 kWcomprises wind turbines. bines. of two E-shape multiple beams connected by a rigid beam, and as shown in Figure 16a. The maximum output power density of 0.61 μW/mm3 achieved when running in a low- frequency from 0.5 to 3 Hz (30–180 rpm). Finally, the proposed device provides a good energy harvesting solution at a very low rotational frequency such as 30 kW wind turbines.

(a) (b)

FigureFigure 16. ( 16.a) Sche(a) Schematicmatic illustration illustration of rotational of rotational piezoelectric piezoelectric energy energy harvester harvester using using multi multi-beam-beam [87]; [87];( (bb)) Rotational Rotational motion motion design design using using magnetic magnetic and and two two piezoelectric piezoelectric beams beams [ 10[10]]. .

(a) (b) Figure 16. (a) Schematic illustration of rotational piezoelectric energy harvester using multi-beam [87]; (b) Rotational motion design using magnetic and two piezoelectric beams [10].

Energies 2021, 14, 3098 21 of 29

Furthermore, Zou et al. [10] presented a magnetically coupled design of an energy harvester using two inverted piezoelectrics and their free ends points to the rotating shaft, as in Figure 16b. The excitation elements are a combination of centrifugal and magnetic forces all alone with mass and gravity force. The obtained output power density at 420 & 550 rpm (7 & 9.17 Hz) is 14.1 & 13.3825 µW/mm3. The results have shown that this harvester is suitable for low frequency rotational, and vibration energy can be harvested in multiple frequency bands. F Khameneifar et al. [11] present a harvester comprises a piezoelectric beam with tip mass fixed on a rotating host, and the mass and gravity are the elements of piezoelectric excitation. At 132 rad/s and 40 kΩ, the maximum output power density can be reached to 9.16 µW/mm3 at 1260.5 rpm (21 Hz). They have found that with the proper size and parameters, this design can harvest enough power to operate wireless sensors in rotating mechanisms like tires and turbines; however, an experimental test will be useful to utilise these results. Similarly, using the same excitation elements, Hsu et al. [56] present a design consists of a piezoelectric beam with a tip mass fixed to a clamp which can be driven to rotate along with a machine tool spindle. The maximum output power density at optimum resistance of 400 kΩ, and 720 rpm is 18.51 µW/mm3. Furthermore, this method flexibility in the various geometrical model building allows the procedure to be used in a more complex geometries model beam harvester, however, it is difficult to be done with the classical beam theory. Moreover, Farbod et al. [57] have also used mass and gravity force as excitation elements to study vibration energy harvester. A design has been made of a cantilever beam with a tip mass on its end and piezoelectric ceramic mounted along the beam fixed on a rotating shaft. The output power density is 25.43 µW/mm3 at 21.96 Hz, and it is transferred using a slip ring. Furthermore, more power can be generated by using more than one harvester; however, further experimental studies need to be made for system power generation evaluation. Alternatively, a gear was used as a frequency up-conversion method; more frequency can be obtained and higher frequency to match the piezoelectric resonant frequency. More- over, no need for more slip ring or any other extra device for output power transfer, normal wiring can be used since the PZT is stationary and does not need to rotate within the system. P. Janphuang et al. [88] have tested the gear teeth impact on piezoelectric energy harvesting. A prototype consists of 16 teeth gear, with 25 rpm motor rotating speed, and gear that supply impacts force on the bimorph MEMS piezoelectric transducer. The maximum output power density is found to be 0.3 µW/mm3 with 25 rpm and 100 µm depth tip. The output power reaches a good level to be used in applied applications, and it can be simply risen by increasing, beams number on the gear, the rotational speed, or gear teeth number. Furthermore, Wei & Duan [61] have presented a piezoelectric rotating energy electrical harvester using gear with polygon shape along with multiple cantilever piezoelectric. The generator has accomplished a 7.14 µW/mm3 open-circuit output power density at 300 rpm. The produced power could be used as AC and Dc power with a delivered power of about 80% (4.5 µW/mm3) for AC and 40% (2.55 µW/mm3) for DC. This generator’s broadband feature makes it ideal for scale-scale fluid-driven power-generation systems operating with low-frequent broadband excitations. Moreover, Park et al. [60] have demonstrated a new design based on shape optimisation, with a gear of 36 teeth as a frequency up, and different shapes of cantilevers are used to compare with the normal one. Without area constraint but with a wider tip, it was found that the output average power density is 0.874 µW/mm3. Moreover, it concludes that the design with a wider free tip improves the output power level. Future work by making the design more practical can be done by reducing the friction by enhancing the tip excitation and selecting a material with higher durability and efficiency. Similarly, Pattanaphong Janphuang et al. [89] carried out a compact energy harvester configuration to convert low-frequency mechanical oscillation into usable energy by using Energies 2021, 14, 3098 22 of 29

Energies 2021, 14, x FOR PEER REVIEW 22 of 30 an Atomic Force Microscope (AFM) like the piezoelectric MEMS cantilever coupled to a rotational gear. It was found that at a rotating speed of 19 rps (1140 rpm), the output average 3 power density is 3.42 µW/mmSimilarly, .Pattanaphong Wear-less Janphuang coating and et al. materials[89] carried out have a compact been energy investigated harvester to reduce the problemconfiguration of wearing to and convert enhance low-frequency this harvester’s mechanical oscillation longevity into usable and efficiency.energy by using Additionally, Aan gullwing-structural Atomic Force Microscope rotational(AFM) like the piezoelectric piezoelectric MEMS energy cantilever harvester coupled using to a rotational gear. It was found that at a rotating speed of 19 rps (1140 rpm), the output av- the mechanism of gear-inducederage power density oscillation is 3.42 μW/mm is proposed3. Wear-less by coating Yang and et materials al. [90]. have The been prototype investi- consists of two nonlineargated to blockedreduce the bridgeproblem of which wearing uses and enhance a flexible this polymerharvester’s longevity substrate and andeffi- a piezoelectric unit naturallyciency. buckled. The harvester can generate an output power density 3 Additionally, A gullwing-structural rotational piezoelectric energy harvester using of 6.54 µW/mm atthe 7.8 mechanism Hz. These of gear results-induced show oscillation that thisis proposed approach by Yang promises et al. [90]. self-poweredThe prototype sensors, specifically atconsists low-frequency of two nonlinear and blocked changeable bridge which ambient, uses a like flexible tire polymer pressure substratemonitoring. and a Pattanaphong Janphuangpiezoelectric unit et al. naturally [91] demonstrated buckled. The harvester a configuration can generate an realised output power mechanism density of 6.54 μW/mm3 at 7.8 Hz. These results show that this approach promises self-powered of frequency up-conversionsensors, specifically using piezoelectric at low-frequency and and changeable a rotating ambient, gear. like The tire frequency pressure monitor- up can be achieved using theing. Atomic Force Microscope (AFM), such as piezoelectric beam plucked by rotating gear teeth. TheyPattanaphong have found Janphuang that et the al. output[91] demonstrated power density a configuration at a rotational realised mecha- speed of 19 rps (1140 rpm)nism is 10 of µfrW/mmequency up3.-conversion Furthermore, using piezoelectric the performance and a rotating can gear. be The enhanced frequency by up can be achieved using the Atomic Force Microscope (AFM), such as piezoelectric beam using more durableplucked harvesting by rotating material gear teeth. or applyingThey have found a frictionless that the output coating power density while at keeping a rota- the system compacttiona andl extendingspeed of 19 rps its (1140 lifetime. rpm) is 10 μW/mm3. Furthermore, the performance can be Furthermore, Chilabienhanced et by al. using [92] more fabricated durable aharvesting frequency material up-conversion or applying a frictionless energy harvester. coating while keeping the system compact and extending its lifetime. The prototype consists ofFurthermore, the planetary Chilabi gear, et al. interchangeable [92] fabricated a frequency planet up cover,-conver andsion PZT energy installs har- on the fix ring gear, asvester. shown The prototype in Figure consists 17. At of athe resistance planetary gear, of 50 interchangeable kΩ, 1500 rpm, planet and cover, 6.25 and Hz bending frequency ofPZT the installs PZT on the the output fix ring gear, power as shown density in Figure was 17. 9.59 At a mW/cmresistance of3. 50 This kΩ, 1500 harvester rpm, and 6.25 Hz bending frequency of the PZT the output power density was 9.59 mW/cm3. novelty of providingThis the harvester desired novelty output of providing power the from desired a fixed output rotational power from speeda fixed rotational could be a good start for futurespeed enhancement. could be a good start Additionally, for future enhancement. the way Additionally, of system the rotation way of system made ro- an impact on PZT by planettation made gear an teeth impact smother, on PZT by resulting planet gear inteeth PZT smother, longer resulting lifetime. in PZT longer lifetime.

Figure 17. The harvester prototype with a section view in the A-A vertical line, and all part names [92]. Figure 17. The harvester prototype with a section view in the A-A vertical line, and all part names [92]. 6.2. The Influence of Excitation Elements and Design on Performance (Challenges and Issues) 6.2. The Inﬂuence of ExcitationAs shown Elements in Table 4 and, a comparison Design on has Performance been made between (Challenges variable factors, and Issues) divided into input, output, and comments. In general, frequency (rpm), PZT size, dimension, and As shown in Table4, a comparison has been made between variable factors, divided into input, output, and comments. In general, frequency (rpm), PZT size, dimension, and material type, resistance, and whether the PZT rotate or no using a slip ring or not, all these elements that effect piezoelectric energy harvesting output power has been reported in the Table, also, both output power and power density have been reported as well in this table. Although, excitation elements greatly affect shaping the design and make it suitable for certain applications and help harvest sufﬁcient output power. However, as clearly shown in the Table, for the same excitation elements the output power density is changing, and this is due to design change and its effect on output power. Energies 2021, 14, 3098 23 of 29

Table 4. Summary Comparison for rotational piezoelectric energy harvesting studies with detailed information.

Input Output Comments Optimal Power Mechanical Volume Piezo Dimension Polarisation Material Frequency PZT Rotate Number Ref. Excitation Eements rpm Resistance Power (µW) Density Power Wiring (mm ) (mm) Mode Type Use (Hz) or Not 3 (Ω) (µW/mm3) Source 1 [85] Shaft stress 357 85 × 14 × 0.3 d31 PZT 1200 20 56,170 744 2.084 RM Yes slip ring 2 [73] M + Sp + Gr 239 20.57 × 0.254 × 45.8 d31 QP16N 360 12.7 / 5.5 0.02 RM Yes Arduino 3 [12] Mg + Sp force 53.9 35 × 2.2 × 0.7 / PZT 546 34.1 400 k 1230 22.8 RM Not Normal wiring 4 [86] Mg 15.12 21.6 × 3.5 × 0.2 / (PZT) 45 0.75 5 × 105 6.37 0.4213 RM Yes slip ring 5 [45] M + Mg 57.6 12 × 6 × 0.8 NA PZT 588 9.8 3.3 M 157 2.73 RM Yes slip ring 6 [9] Mg + bistable 10.1 33.5 × 2 × 0.15 d31 PZT 660 11 52.3 5.2 RM Not Normal wiring wireless data 7 [87] M + Cn 327.7 50.8 × 25.4 × 0.254 d31 PZT(MFC) 180 0.5–3 1000 200 0.6102 RM Yes acquisition 8 [10] M + Mg + Gr + Cn 40 20 × 10 × 0.2 d31 (PZT) 550 9.17 50 k 535 13.4 RM Yes / 9 [11] M + Gr 840 50.8 × 31.8 × 0.26 d31 PZT-ceramic 1261 21 / 7700 9.17 RM Yes / 10 [56] M + Gr 86.4 24 × 18 × 0.2 / (PZT-5A) 720 12 400 k 1600 18.5 RM Yes slip ring 11 [57] M + Gr 89 50.8 × 38.1 × 0.13 d31 PVDF& PZT 1318 22 400 k 6400 25.4 RM Yes slip ring 12 [88] G 4.2 / / / 25 0.4 2.7 1.26 0.3 RMG Not Normal wiring 13 [61] G 138.66 33 × 22 × 0.191 / PZT 300 6 / 6000 2.55 RMG Not Normal wiring 14 [60] G + M 4.26 152 × 0.028 / PZT-5A 15 500 1000 3.72 0.87 RMG Not Normal wiring 15 [89] G 3.5 3.5 / / 1140 19 4700 12 3.43 RMG Not Normal wiring PZT- 16 [90] Gr 61.2 10 × 18 × 0.17 × 2 / 468 7.8 30 k 400 6.54 RMG Not / ceramics 17 [92] PGr 157.8 46.4 × 6.8 × 0.25 d31 PZT 5H 1500 6.25 50,000 1566 9.59 RMG Not Normal wiring 18 [91] G + M + Mg 1.2 5 × 3 × 0.08 d31 PSI5A4E, 1140 19 180 k 12 10 RMG No Normal wiring M for mass, Mg for magnetic, Cn for centrifugal, Gr for gravity, Sp for spring, G for gear, Pgr for planetary gear, RMfor rotationa motion, RMGfor rotational machine (gear), and/for no information from the authors. Energies 2021, 14, x FOR PEER REVIEW 24 of 30

33 × 22 Normal 13 [61] G 138.66 / PZT 300 6 / 6000 2.55 RMG Not × 0.191 wiring 152 × PZT- Normal 14 [60] G + M 4.26 / 15 500 1000 3.72 0.87 RMG Not 0.028 5A wiring Normal 15 [89] G 3.5 3.5 / / 1140 19 4700 12 3.43 RMG Not wiring 10 × 18 PZT-ce- 16 [90] Gr 61.2 × 0.17 × / 468 7.8 30 k 400 6.54 RMG Not / ramics 2 46.4 × Normal 17 [92] PGr 157.8 6.8 × d31 PZT 5H 1500 6.25 50,000 1566 9.59 RMG Not wiring 0.25 G + M + 5 × 3 × PSI5A4 Normal 18 [91] 1.2 d31 1140 19 180 k 12 10 RMG No Energies 2021, 14, 3098 Mg 0.08 E, wiring24 of 29 M for mass, Mg for magnetic, Cn for centrifugal, Gr for gravity, Sp for spring, G for gear, Pgr for planetary gear, RMfor rotationa motion, RMGfor rotational machine (gear), and/for no information from the authors. In Figure 18, a plot has been made for all the designs, due to rpm and output power In Figure 18, a plot has been made for all the designs, due to rpm and output power density, to give a clear idea about the best excitation elements and which one has the highest density, to give a clear idea about the best excitation elements and which one has the high- powerest powe density.r density. As shown As shown in the in Figure, the Figure, a very a goodvery outputgood output power power result result has been has made been by Hsumade et al.by [ 56Hsu] and et al. Farbod [56] and et al. Farbod [57] using et al. mass[57] using and gravity mass and and gravity Wu [12 and] using Wu magnetic [12] using and springmagnetic force and and spring also getforce good and results. also get A good compilation results. A of compilation mass, spring of force,mass, andspring gravity force, has beenand usedgravity by has Ram beenírez used et al. by [73 Ramírez] and get et the al. lowest[73] and results. get the However, lowest results. for Ram However,írez et al. for [73 ] theRamírez primary et al. inﬂuence [73] the isprimary to provide influence a ﬁnite is to element provide ofa finite one dimensionelement of one accomplished dimension of modellingaccomplished three-dimension of modelling rotatingthree-dimension energy harvester.rotating energy One ofharvester. the interesting One of designsthe inter- that futureesting worksdesigns should that future focus works on includes should focus developing on includes a rotational developing energy a rotational harvester energy more complexharvester to more investigate complex power to investigate generation. power generation.

FigureFigure 18. 18.Rotational Rotational piezoelectric piezoelectric energy energy harvesting harvesting forfor rotatingrotating machine including including gear gear with with excitat excitationion elements elements and and as as follow:follow: M M for for Mass, Mass, Mg Mg for for magnetic, magnetic, Cn Cn for for centrifugal,centrifugal, Gr Gr for for gravity, gravity, Sp Sp for for spring, spring, G Gfor for gear, gear, PGr PGr for for planetary planetary gear, gear, and bistable for bistable force. and bistable for bistable force.

Regarding the design aspect, Wu [12], Fu & Yeatman [9], Farbod et al. [57], and Pattanaphong Janphuang [91] present; the simple, smaller size, and compact design. On

the other hand, Ramírez et al. [87] and Hsu et al. [56] present a more complicated prototype. However, Ramírez et al. [87] present the novel piezoelectric rotational motion device made with E-shape multi-beams and provides a good solution for energy harvesting at low rotational frequency. Hsu et al. [56] present a method with ﬂexibility in the various geometrical model building that allows the procedure to be used in a more complex geometries model beam harvester. This method is challenging to be done with the classical beam theory. In general Wu [12] and Farbod et al. [57] present the best prototype for the prospective of; simple excitation elements, design, and maximum power output. Similarly, Pattanaphong Janphuang [91] and Chilabi et al. [92] present a good design that its output power reaches a good level to be used in applied applications, and it can be simply risen by increasing, beams number on the gear, the rotational speed, or gear teeth number. One of the interesting designs that future works should focused on applying gravity spring, and magnetic forces as excitation elements and using gears, but with compact small design considering less friction with gears to achieve maximum output power density. Energies 2021, 14, 3098 25 of 29

7. Conclusions This paper reviews the recent studies and research in rotational piezoelectric energy harvesting based on its excitation elements and design and its influence on performance. Depending on the application, and the availableness of the mechanical power sources, various types of energy harvester are obtainable most of the time. The materials used in the RPZTEH were mainly PZT, however some researchers used PVDF and PZT composite. Mechanical energies such as vibration, fluid-flow, human motion, etc., are energies of changeable frequencies and amplitude, which could be named as random energies that are available everywhere. Human motion and some other rotational mechanical power source have a low rotational frequency, and thus, it will not reach the piezoelectric resonance frequency. Consequently, researchers have started to find ways of bandwidth widening techniques and frequency up-conversion methods. The comparison of excitation elements and design and their influence on performance for various mechanical Inputs are divided into four types according to mechanical inputs: fluids (air, water) movement, human motion, rotational vehicle tires, and other rotational operational principal. The fluid power source has been divided into two sections air (wind) and water (or rain). There are some excellent works for air or wind and where various types of excitation elements have been utilised; however, no plane has been done for very high wind speed. Furthermore, more experiments can be done on more conventional available places such as the air inside the air conditioning tunnel or air conditioning air exhaust. For water flow or rain as the input power source, only a few studies have been conducted; however, the power density is small. Based on this review it was found that with a small compact design, applying magnetic and gravitational forces as excitation elements can achieve higher output power density. Different excitation elements have been used for frequency up-conversion and output power enhancement for human motion as rotational input mechanical power sources. However, this power source’s main concern is its low frequency and need for a light and compact harvester. Many studies have used different novel designs with various excitation elements to wideband the bandwidth and thus harvest more power in different speed ranges. Furthermore, more compact designs need to be fabricated using PVDF materials considering their flexibility with human body parts movements. More real-life experiments need to be done to see its feasibility in human daily use. Based on this review it was found that applying gravity and magnetic forces as excitation elements with a small compact designs can achieve higher output power density. For using vehicle tires as mechanical input power source, different designs have been used; some of them need to have a field test on a real vehicle to see its feasibility others need to resize it so that it fits in the tire. However, our main concern is to focus on fabricating a prototype that harvests enough power in the wideband frequency range, especially at low vehicle speed. based on this review it was found that applying gravity and centrifugal forces as excitation elements with a small compact design that fit easily in vehicle tire can achieve higher output power density. There are not many studies that use the gear as an excitation element for frequency up-conversion. However, its output power reaches a good level to be used in many applications. Besides, it can be increased by simply used more gear teeth or increasing the piezoelectric number. More studies can be done to reduce that contact between the gear teeth and piezoelctric or make it contactless (use magnetic) to increase piezoelectric lifetime and output power and make the gear harvester more compact. Based on this review it was found that with a small compact design and using gear; applying gravity spring, and magnetic forces as excitation elements can achieve higher output power density, taking into account less friction with the gear teeth. In general, many essential factors influence the rotational piezoelectric harvester output power, which made a wide range of output power from nano to milliWatts. Design with a fixed piezoelectric cantilever is more favourable for future work. In this case, normal wiring can be used for the output power transfer, without a slip ring or any extra Energies 2021, 14, 3098 26 of 29

component that makes the design less compact. Generally, for most rotational piezoelectric energy harvesting applications, it can be said that the magnetic force had a good influence on optimising output power considering the pole direction and the number of magnets. The studies revealed that the design of a small compact prototype with a fixed piezoelectric to avoid slip ring and using magnetic, gravity, and centrifugal forces had a good influence on output power optimisation. Future work should also focus on using gear for frequency up-conversion to enhance output power density and keep the design simple and compact.

Author Contributions: Conceptualisation, H.J.C.; writing—original draft preparation, H.J.C.; writing —review and editing, H.S.; visualisation, H.J.C.; supervision, E.E.S.; project administration, H.S.; funding acquisition, E.E.S.; review and editing, A.B.A.; review and editing, K.A.M.R.; review and editing, W.A.-A.; review and editing, L.C.A.; review and editing, M.K.A. All authors have read and agreed to the published version of the manuscript. Funding: The founder for this research is UPM Faculty of Engineering, grant number 9607500. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: A special thanks for Ahmed B. Atrah for help and guide through the work. Many thanks are also extended to Moaath Al-Rifaay and Nuradeen M. Nasidi for help and guidance. Conﬂicts of Interest: There are no conﬂict of interest to disclose.

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