Large-scale vibration energy harvesting.pdf

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Article

Journal of Intelligent Material Systemsand Structures24(11) 14051430

The Author(s) 2013Reprints and permissions:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/1045389X13486707

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Large-scale vibration energy harvesting

Lei Zuo and Xiudong Tang

AbstractNowadays, harvesting energy from vibration is one of the most promising technologies. However, the majority of cur-rent researches obtain 10 mW to 100 mW power, which has only limited applications in self-powered wireless sensorsand low-power electronics. In fact, the vibrations in some situations can be very large, for example, the vibrations of tallbuildings, long bridges, vehicle systems, railroads, ocean waves, and even human motions. With the global concern onenergy and environmental issues, energy harvesting from large-scale vibrations is more attractive and becomes aresearch frontier. This article is to provide a timely and comprehensive review of the state-of-the-art on the large-scalevibration energy harvesting, ranging from 1 W to 100 kW or more. Subtopics include energy assessment from largevibrations, piezoelectric materials and electromagnetic transducers, motion transmission and magnification mechanisms,power electronics, and vibration control. The relevant applications discussed in this article include vibration energy har-vesting from human motion, vehicles, transportations, and civil structures. The unique challenges and future researchdirections of large-scale vibration energy harvesting are also discussed.

KeywordsEnergy harvesting, control, piezoelectric, electromagnetic, vibration

Introduction

With the global energy crisis and environment con-cerns, many technologies in energy harvesting, such assolar, wind, geothermal, and hydraulic power plants orfarms, have been developed. Since vibration existseverywhere, such as the vibration of floor and wall,machines, pumps, vehicle chassis, railway train or tracks,and human motions, etc. it becomes a good alternativeenergy source and receives more and more attention inrecent years. The research has resulted in a wealth of theliterature and some promising applications, such as low-power electronics and self-powered wireless sensors(Chalasani and Conrad, 2008). Hundreds of articles havebeen published in the past 5 years (Figure 1), includingover 10 review articles (Collins, 2006; Galhardi et al.,2008; Paradiso and Starner, 2005; Park et al., 2008;Saadon and Sidek, 2011; Sodano and Inman, 2004). Sofar, all the review articles and the majorities of theresearch on vibration energy harvesting focus on thesmall scale ( \ 100 mW). In real world, the vibrations canbe very large, for example, the vibrations of buildings orbridges, vehicle systems, ocean waves, even humanmotions. Harvesting large amount of vibration energypromises more meaningful applications. Though relativefew, there are still some interesting initiatives in theresearch laboratories and industry on harvesting vibra-tion energy on the order of Watts or even more. The

purpose of this article is to review the state-of-the-art andchallenges of energy harvesting from vibrations, with the

emphasis on the large-scale vibration energy harvesting.A typical vibration energy harvesting system consists

of a mechanical system with external excitation, atransducer that converts the vibration energy into elec-tric energy, mechanisms for motion transmission andmagnification, power electronics and energy storageelements, and energy management and control strate-gies, as shown in Figure 2. This article is organizedaccording to the components of such a typical vibra-tion energy harvesting system. First, the assessment of large-scale vibration harvesting potential from differentsources is conducted in section Power assessment of

large-scale vibration. Then, different transducers aresummarized in section Transducers, where two of themost popular transducers, piezoelectric materials andelectromagnetic transducers, are reviewed and com-pared. In section Motion and magnification mechan-isms, several mechanisms for motion transmission and

Department of Mechanical Engineering, State University of New York atStony Brook, Stony Brook, NY, USA

Corresponding author:Lei Zuo, Department of Mechanical Engineering, State University of NewYork at Stony Brook, Stony Brook, NY 11794, USA.Email: [email protected]


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magnification, which plays important roles in large-scale vibration energy harvesting, are summarized.Power electronic circuits, energy management, and con-trol strategies are reviewed in sections Energy harvest-ing circuits and power management and Vibrationcontrol. The existing challenges and future researchdirections are presented in section Challenges andfuture directions.

Power assessment of large-scale vibration

The vibrations could be large in many situations. If theenergy in large-scale vibration can be successfully har-vested, it can serve as either an on-site generator or apower source to feed the grid. In this section, the factsof available energy in different vibration systems willbe analyzed and summarized. The feasibility of harvest-ing significant amount of vibration energy out of thosesystems is also discussed.

Harvestable power in regenerative vehiclesuspensionsFigure 3 shows the energy flow of a 2.5L 2005 Camry,where 1/5 of the fuel energy is converted into mechani-cal energy, and less than half of mechanical energy istransfer to the driving wheel (Bandivadekar et al.,2008). Typically, only 10%16% of the available fuel

energy is used to drive the vehicle, which is to overcomethe resistance from road friction and air drag (USDepartment of Energy, 2011). There are three opportu-nities to improve the fuel efficiency: recovery of wasteheat, regenerative braking, and regenerative shockabsorbers. More information on energy harvesting

from the heat waste of the vehicles can be found incomprehensive review articles by Bell (2008) and Yangand Francis (2009). Through the vast investment in thepast two decades, regenerative brakes have been suc-cessfully commercialized in many hybrid vehicles andgreatly increased the fuel efficiency by recoveringenergy during braking. Energy harvesting from vehiclesuspensions is still in the research and development(R&D) stage.

When the vehicle travels on the road, the roadroughness, accelerations, decelerations, and unevennesswill excite the undesired vibration. Traditionally, oil

shock absorbers are used in parallel with the suspen-sion springs to ensure the ride comfort, road handling,and safety, by dissipating the undesired vibrationenergy into waste heat. On the other hand, active sus-pension has been demonstrated for full-scale vehicleswith very impressive performance, for example, byBose Corporation (Rani, 2005). However, the powerconsumption, cost-effectiveness, and reliability are of serious concern, and active suspension is barely used inautomotive industry. Regenerative shock absorbershave been proposed to convert the kinetic energy of theundesired vibration into useful electricity and to reducethe vibration. Although a few researchers have alreadylooked into the potential of energy harvesting in thevehicle suspensions (Goldner et al., 2001; Kawamoto etal., 2007; Martins et al., 2006; Zhang et al., 2007), thenumbers vary in large range, from negligible 46 W(Zhang et al., 2007) to unreasonably high 7500 W(Goldner et al., 2001) in a passenger car.

Zuo and Zhang (2011) assessed the energy potentialof vehicle suspension systems through an integratedmathematical modeling of roadvehicleharvester sys-tem, which was also verified by road tests. In thismodel, the excitation from road irregularity is modeledas a stationary random process, where the displacementpower spectral densities (PSDs) of different roads aresuggested by the international standard organization(ISO 2631-1:1997, 1997), as shown in Figure 4. Theconcept of system H 2 norm (Zhou, 1996) is used to

Figure 1. Increasing research interest on vibration energyharvesting indicated by the numbers of relevant articles in thedatabases engineering village (EI) and web of science (ScienceCitation Index (SCI)).

Figure 2. Typical components of a vibration energy harvesting system.

1406 Journal of Intelligent Material Systems and Structures 24(11)


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obtain mean value of the power dissipated by the con-ventional shock absorbers. The conclusion is that for a

middle-sized passenger car (3500 lb) with four shockabsorbers, average powers of 100, 400, and 1600 W areavailable for harvesting while driving 60 mile/h onClass B (good), Class C (average), and Class D (poor)roads, respectively. And the energy potential for trucks,railcars, and off-road vehicles is on the order of 110kW. The theoretical modeling is validated by road testusing a super compact vehicle, as shown in Figure 5.

Assuming 75% energy harvesting efficiency, theregenerative shock absorbers of a middle-sized passen-ger car can recover 300 W electricity on the average(Class C) road at 60 mile/h. Such 300 W of electricity

must not be underestimated. As noted in a generalmotors (GMs) article (Yang, 2009), the typical electric-ity usage of a vehicle is about 250350 W with alloptional systems turned off, which is currently gener-ated by the alternator driven by the engine crank shaft.The alternator has a typical power capacity of 500600W and an efficiency of 55% (Bradfield, 2008).Considering the efficiency of engines and alternators,300 W of electrical power means about 1800 W of petroleum fuel power.

The average energy usage of fossil cars is 80 kWhper 100 km (62.5 miles), while the one of electric orhybrid is less than 20 kWh per 100 km (MacKay, 2009),which means the energy consumption rates of fossil cars

and electric cars are 76.8 and 19.2 kW at 60 mile/h,respectively. Therefore, the 300 W of electricity har-

vested (equivalent 1800 W fuel power) means 2.4%9%increase of fuel efficiency for the vehicles (calculated as1800 W/76.8 kW1800 W/19.2 kW). This estimation isconsistent with the literature in the thermoelectric wasteheat recovery, for example, 390 W means 4% fuel effi-ciency for a BMW (Fairbanks, 2011).

Harvestable power from civil structuresAnother promising alternative energy technology is toharvest vibration energy from civil structures. Civilstructures, such as tall buildings, communication

towers, and long-span bridges, are very susceptible tothe dynamic loadings of wind, earthquake, traffic, andhuman motions, and thus, large vibration exists inthese civil structures. Large vibration amplitudes candamage the structures or the secondary components orcause discomfort to its human occupants (Kareem etal., 1999). For example, the Tacoma Narrow Bridgecollapsed from wind-induced vibration 4 months afteropening in 1940 (Scott, 2001), as shown in Figure 6. In1972, wind-induced vibration also caused more than 65panels of window glass, weighing 500 pounds each, tofall and crash on the sidewalks hundreds of feet belowthe Hancock Tower in Boston (Schwartz, 2001). Thosetwo examples show that the wind-induced vibration

Figure 3. Vehicle energy flows of a 2.5L 2005 Camry (Bandivadekar et al., 2008).

Figure 4. Integrated roadvehicleharvester modeling.

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could be huge. Figure 7 compares the sizes of some tallbuildings with the 100 kW1.5 MW wind turbines,which gives a sense of wind forces and power acting onthe tall buildings.

Modern buildings and bridges are constructed withsteel or concrete, which have very low inherent damp-ing. Typical damping ratios are z = 0.5%1%. And

the vibration amplitude at the resonant frequency canbe 50100 times larger than the static deformation(quality factor fix this Q = 1/2 z = 50100). Hence, thesupplemental damping becomes the most importantstrategy to control the vibration. This includes viscousfluid dampers, viscoelastic dampers, metallic yield dissi-paters, friction dampers, and tuned mass dampers(TMDs). The vibration energy can be converted intoelectricity, by replacing the viscous dissipative elementswith energy transducers. Among all these energy dissi-paters, the TMD and its variants (tuned liquid damper,and so on) become increasingly popular and compose

the greatest percentage of the supplemental dampingsystems currently in use (Housner et al., 1997; Kareemet al., 1999; Soong and Spencer, 2002). The TMD,which is composed of a mass up to several hundred orthousand tons connected to the structure with springand viscous dissipative devices (VDDs), has beeninstalled in many buildings around the word, for exam-ple, Citigroup Tower in New York (410 ton TMD,1978 installation), John Hancock Tower in Boston (600ton TMD, 1977), Trump World Tower in New York(600 ton, 2001), Chicago Spire (1300 ton, in construc-tion), Taipei 101 Tower (730 ton, 2004), Crystal Towerin Japan (540 ton, 1990), Chifley Tower in Sydney (400ton, 1993), BronxWhitestone Bridge in New York (94

(a)

0

0.1

0.2

0.3

0.4

0 20 10040 60 80 0

100

200300

400

500

600

700

Vehicle speed (mph)

A v e r a g e p o w e r p o t e n a

l ( W

)

R M S s u s p e n s i o n v e

l o c i t y

( m / s )

Middle size car 3550lb,Four shock absorbers

400W

Stony Brook campus road

Sensor

(c)

(b)

0 10 20 30 40 50 600

50100150200250300350400450500

Time (Sec)

I n s t a n t p o w e r a t o n e a b s o r b e r

( W )

Super compact car 2400lb, 25mphOne of the four absorbers

Figure 5. (a) Predicted suspension velocity and harvesting power potential for a middle-sized passenger car on average road (ClassC), (b) recorded power potential from one shock absorber of a super compact vehicle at 25 mile/h on Stony Brook campus roadand (c) Experimental setup (Zuo and Zhang, 2011).RMS: root mean square.

Figure 6. Wind-induced torsional vibration of Tacoma NarrowBridge in 1940 (Scott, 2001).



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ton, 1988), and many others (Gamble et al., 2009; Sunet al., 1995). These TMDs can reduce the vibrationresponse of the structures by 40%60% via energy dis-sipation. They are typically either suspended with pen-dulums (including multistage pendulums or invertedpendulums) or supported with bearings (mechanical,hydraulic, or rubber) with mechanical guides on thetop of the roof or the top floor. Figure 8 shows suchtwo typical implementations.

Instead of dissipating the vibration energy, Tang andZuo (2010, 2011b) proposed regenerative TMD to har-vest the vibration energy, where simultaneous energy

harvesting and vibration control have been demon-strated on a three-storey building prototype. Ni et al.(2011) estimated the power potential that is availablefor harvesting in typical buildings, by considering thewind dynamics and buildingTMD dynamics. It isshown that more than 85 kW of power is available forharvesting from 76-storey building with TMD in highwind events of 13.5 m/s at a standard height of 10 m. Inthe state of arts, oil-based VDDs are used in the TMDs.Such a large power rate already creates a lot of chal-lenges, and forced liquid cooling or the heat-resistantdesign has to be used. Figure 9 shows the energy

Figure 7. Size comparison of some civil structures with wind turbines (Ni et al., 2011).

Figure 8. Typical implementations of TMDs in structures: (a) 730 ton of metal ball (18 # diameter) suspended in Taipei 101, (b) 410ton of concrete block (30 # 3 30# 3 9# ) on the top of Citigroup tower, and (c) simplified model of the classic TMD.TMD: tuned mass damper.

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dissipated by one of the eight oil dampers (VDDs) in

the TMD on Taipei 101 in a wind-induced vibration(Haskett et al., 2004). We can see that the estimationagrees well with the recorded data.

It should be noted that such dual functional gen-erators are not limited by TMDs or distributed dam-pers linked in a truss. Other examples includeintegration of electromechanical dampers in core andoutrigger structural systems commonly employed fortall buildings at the interbuilding connections and atconnections of the building core to outer tube/fram-ing systems.

Energy harvesting potential from railway tracksWhen the train moves on the track, the track willdeflect vertically, as illustrated in Figure 10(a). Railcarscan weigh from 30 ton (empty) to 140 ton (loaded)each. A typical freight train exerts a load around20,00030,000 pounds on the track surface and induces

1/8 $ 3/8 $ track deflection (Bowness et al., 2007;Farritor et al., 2006; Igwemezie, 2007; Phillips, 2011).The frequency at which the track deflects depends onthe distance between the two bogies of the train.Usually, the freight trains have different cart lengths,so the distance between bogies is not uniform.

The frequency is around 0.61.8 Hz calculatedby assuming that the train is moving at a speed of 2575 mile/h.

The average power available on the railway track sitedue to the moving train can be estimated using the fol-lowing equation

Pavg = NFD

T =

FDDT 1

where N is the number of wheels passing through, D isthe track deflection under wheel load, F is the normalforce exerted by the wheel on the track, T is the totaltime taken by the train to pass by, and DT is the aver-age time for each wheel to pass by.

Assuming that a four-wheel railcar has 100 tonweight and 80 feet length, the four wheels will pass overin 1.36 s ( DT = 0 :34 ) at a train speed of 40 mile/h. Theaverage power potential will be about 2 kW under 1/4 $

track deflection according to equation (1). If 5% of thesupport force is provided to a harvester at the trackside,the harvestable energy from up and down track motionwill be 200 W. This amount of power is sufficient formost of the trackside electric facilities. Typically, thelight-emitting diode (LED) signal lights require a power

of 810 W, grade-crossing gate requires a power of 150200 W, and axle counter requires a power of 100 150 W (Penamalli, 2011). It should be noted that this is just a ballpark estimation. The actual power availabledepends on the speed of the train, the weight of thetrain, the type of railway track, ballast, road founda-tion, and so on.

Figure 10. Railroad track vibration: vertical track deflection (Bowness et al., 2007; Igwemezie, 2007).

Figure 9. Power dissipated by one of the eight VDDs in theTMD of Taipei 101 in wind-induced vibration (Haskett et al.,2004).VDD: viscous dissipative device; TMD: tuned mass damper.



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Energy harvesting from human motionThe potential of harvesting energy from human activi-ties has been reviewed by Starner and Paradiso (2004).The available energy for harvesting from differenthuman activities is summarized in Figure 11. As can beseen from Figure 11 there are many sources we can har-vest energy from human, including walking, armmotion, finger motion, breathing, blood pressure, andso on. It should be noted that most of them are on thesmall scale, less than 0.5 W, except for energy harvest-ing from walking. Examples of successfully harvestingenergy over 1 W from human motion have beenreported (Li et al., 2008; Rome et al., 2005) and will bereviewed in the following sections.

Energy harvesting from ocean waveAnother important renewable energy source is theocean wave energy, which has attracted many research-ers due to its great potential. The worlds ocean wave

energy has the potential of approximately 8000-80000TWh every year (Boud, 2003). According toElectric Power Research Institute (EPRI, 2011), thetotal available wave energy resource along the U.S.continental shelf edge, is estimated to be 2,640 TWh/yrwhile the total extractable wave energy resource yields

a total extractable resource along the U.S. continentalshelf edge of 1,170 TWh/yr, which includes 250 TWh/yr for the West Coast, 160 TWh/yr for the East Coast,60 TWh/yr for the Gulf of Mexico, 620 TWh/yr forAlaska, 80 TWh/yr for Hawaii, and 20 TWh/yr forPuerto Rico. Theoretically, if all the extractable energycan be harvested, it can feed 1/4 of the nations need.

Generally, ocean wave energy extraction technolo-gies are utilized to convert the kinetic energy from sur-face waves into electricity or make it available directlyfor other purposes. Modern research on wave energyextraction technologies began in earnest following theoil crises of the early 1970s (Drew

et al., 2009; Falca

o,

2010). Broadly, there are three main types of waveenergy technologies: (1) float-type wave energy conver-ters, including point absorber (Figure 12-a), attenuatorand terminator, which use a float, buoy, or pitchingdevice to tap the oscillating force of the waves togenerate electricity; (2) oscillating water column(Figure 12-b), in which water enters a chamber andforces the trapped air though an opening connected toa turbine; and (3) overtopping device, with a reservoirabove mean water level from which wave water flowsthrough one or more conventional low-head hydraulicturbines. The energy converting unit, known as powertake-off (PTO), is the part in the ocean wave harvesterto convert the wave kinetic energy into electricity.PTOs are commonly adopted in the forms of: hydrau-lic (Henderson, 2006; Cargo, et al. , 2011), linear gen-erator (Mei, 2012), rotational turbine (Delaure, 2003).

Transducers

Traditionally, the vibration energy is dissipated intoheat waste by the damping elements of the systems.

(a) (b)

Figure 12. A diagram of point absorber type ocean energy harvester (PowerBuoy of Ocean Power Technologies, Inc) and oceanenergy harvester (LIMPETof Voith Hydro Wavegen Limited).

Figure 11. Available energy from human activities (Starner andParadiso, 2004).

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Rather than dissipating the vibration energy into heatwaste by damping elements, the transducers in vibra-tion energy harvesting system can convert the mechani-cal energy into electric energy. Various transducershave been investigated for the vibration energy harvest-ing, including piezoelectric materials (Galhardi et al.,2008; Sodano and Inman, 2004), linear and rotationalelectromagnetic motors (Rome et al., 2005), electro-static generators (Mitcheson et al., 2004), and dielectricgenerators (Kornbluh et al., 2002). Among these trans-ducers, the piezoelectric materials and electromagneticmachines have more potential for large-scale vibrationenergy harvesting. In some situations, where the vibra-tion mitigation of the primary structure is concerned,piezoelectric materials and electromagnetic motors canalso serve as actuators simultaneously for the purposeof active or regenerative vibration control, and thus,

the power flow is bidirectional, as shown in Figure 2.Although both piezoelectric materials and electro-

magnetic motors have been used in vibration energyharvesting, they have different features. Piezoelectricmaterial is a force- or stress-induced transducer, whileelectromagnetic motor is a velocity-induced transducer.Hence, piezoelectric material is more suitable for vibra-tion with large force and small deformation (due to lim-ited strain range). Electromagnetic motor is preferredin the situations where vibration has large velocity oramplitude. So far, electromagnetic motors have beenfound more as the transducers for large-scale energy

harvesting. They have been used to harvest energy fromvehicle suspension and buildings. On the other hand,piezoelectric materials have larger energy density (Chenet al., 2006) and are more suitable for the applicationswhere the space or weight is a concern. In addition,electromagnetic motor usually produces a low voltage,while piezoelectric materials normally generate a veryhigh voltage, so they have different requirements onpower electronic circuits.

Piezoelectric vibration harvestersPiezoelectric material is one of the most widely usedsmart materials. It can deform when certain voltage is

applied on the surface. On the other hand, it can gener-ate voltage or charges on its surface when a force orpressure is exerted on it. While the first characteristichas been used for actuator applications, such as piezo-electric stacks or benders, the second characteristic canbe used for sensor applications, such as accelerometers,microphones, load cells (Mirzaeifar et al., 2008), or toharvest energy from vibration. The governing electro-mechanical equations for piezoelectric materials can beexpressed by equation (2)

S D = s d d e s E 2

where S and s are strain and stress, respectively; s iscompliance; D is electric displacement (charge per unitarea); E is electric field (volts per unit length); d is piezo-

electric coefficient; and e is dielectric constant.When used as energy harvester, the piezoelectric

material can work in d 31 or d 33 mode, as shown inFigure 13, where t p is the thickness of piezoelectricmaterials or the distance between electrodes in thepolarization direction and A is the area of conductiveelectrodes. The 31 mode is usually seen in piezoelectricfilm, where the electric field is perpendicular to thedirection of mechanical strain; the 33 mode oftenappears as piezoelectric stacks where both electric fieldand strain are in the poling direction. When working asenergy harvester, the piezoelectric transducer can be

modeled as an alternating current (AC) voltage sourcewith a capacitor in series, as shown in Figure 14(a).Based on Thevenins and Nortons theorem, it can alsobe modeled as an AC current source with a capacitorin parallel, as shown in Figure 14(b).

When the piezoelectric material is open circuit, therewill be no charge displacement and D is equal to 0.From the relation between electric field and strain inequation (2), we can get the open-circuit voltage gener-ated by the piezoelectric material expressed by equation(3)

V oc = E t p = d

s t pe = g s t p 3

Figure 13. Two modes of piezoelectric materials when used for vibration energy harvesting.



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where g is the voltage constant, and related with thepiezoelectric coefficient d by equation (4)

d = e g 4For the second modeling, the short-circuited current

can be obtained by shoring the electrodes of piezoelec-

tric material. In this case, E is 0, and the electric displa-cement can be obtained from equation (2) as

D = d s 5Hence, the short circuit current is obtained as equa-

tion (5), which is in proportion to the derivative of thestress

i sc = _ D A = d _s A 6Or a short circuit electric charge proportional to thestress

Q sc = d s A 7Many types of piezoelectric materials with different

properties can be used as the transducers (Galhardi etal., 2008). The most popular piezoelectric materials arelead zirconate titanate (PZT) ceramic and polyvinyli-dene fluoride (PVDF) polymer. PZT ceramic has largeelectromechanical coupling factors, typically k31 =0.34 and k33 = 0.69 ( k31 is the factor for electric field

in direction 3 and longitudinal vibrations in direction 1;k33 is the factor for electric field in direction 3 and long-itudinal vibrations in direction 3), which means it isable to convert 34% and 69% of mechanical energy inthe piezoelectric material into electric energy. PVDF ismore flexible and sensitive (Shenck and Paradiso, 2001;

Starner and Paradiso, 2004); however, the electrome-chanical converting coefficient is much smaller, k31 =0.12 and k33 = 0.15. Single crystal piezoelectric materi-als have also been used for its high energy density,high-energy converting efficiency, and large operationaltemperature range (Badel et al., 2006). In general, thepiezoelectric materials have relatively small strain,which prevents their direct application in large-amplitude vibrations. On the other hand, this is alsoone of the advantages of piezoelectric materials, insituations where small deformation is preferred, forexample, energy harvesting from human walk (Shenckand Paradiso, 2001; Starner and Paradiso, 2004), wherelarge deformation may have effect on walking gait andcause discomfort to the walker.

Since one piezoelectric ceramic wafer can only gener-ate limited amount of power due to small deformation,the output power and the efficiency could be improvedby stacking them together or group them into arrays.Research & Development Center of JR East Group(East Japan Railway Company, 2008) developed apiezoelectric arraybased power-generating floor,which has been tested at Tokyo Stations MarunouchiNorth Exit (Figure 15). This power-generating floorcould harvest 10,000 W s/day, which can lighten a 100W bulb for about 80 min, that is to say, it harvests aver-age power of 5.6 W in 24 h. However, after the thirdweek of the experimental period (a total of 800,000 peo-ple passing), production of electricity decreased due toa degradation in durability. Also using stack configura-tion, Antaki et al. (1995) developed a regenerative shoesusing piezoelectric ceramic, for the power supply of artificial organs. By arranging the piezoelectric materi-als in stack and applying force magnification

Figure 15. Power generating floor: (a) piezoelectric energy harvesting stacks and (b) experiment in Tokyo Stations MarunouchiNorth Exit (East Japan Railway Company, 2008).

Figure 14. Modeling of piezoelectric materials as energyharvesters: (a) voltage source and (b) current source.

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mechanism, an average of 0.85 W energy is claimedduring the normal walk of a 75 kg human, withoutcausing much discomfort to the walking gait.

Another important configuration to induce stress orstain to piezoelectric material is realized by mounting itto cantilever beam with a proof mass at the end (Armset al., 2005; Ferrari et al., 2006; Johnson et al., 2006),as shown in Figure 16, with the piezoelectric materialworking in bending mode. Based on this configuration,Arms et al. (2005) developed piezoelectric material based energy harvester for a wireless sensor with lowpower consumption. With a 250 g proof mass attachedto a 50-mm-long cantilever beam, the energy harvesterdelivered up to 2 mW under low-level vibration condi-tions. In this configuration, the cantilever beam oper-ates at the first natural frequency. The resonance canbe used to magnify the vibration and hence improvethe energy harvesting efficiency.

Piezoelectric materials are also used directly without

stack or cantilevered configuration. For example,Shenck and Paradiso (2001) built energy harvestingshoes with its removable insole embedded with piezo-electric materials and can generate electricity when it isflattened or bended during human walk. However, theaverage energy harvested by this device is very low (1.8mW), with a peak power of 60 mW. By replacing thetraditional straps of the backpack with the ones madeof the PVDF, Granstrom et al. (2007) developed aregenerative backpack, which can harvest energy at theaverage rate of 45.6 mW.

Electromagnetic transducersBesides piezoelectric materials, electromagnetic motorsare also often used in vibration energy harvesting, espe-cially when the vibration magnitude is large.Electromagnetic motor can act as an actuator and aharvester at the same time, capable of bidirectionalpower flow. The relative motion between the stator andmover of the motor can induce a voltage em in the coils,which is proportional to the relative velocity of statorand mover v

em = k ev

8

where k e is the back electromotive voltage coefficient of the electromagnetic motor. Meanwhile, the current flowinside the motor coil will induce an electromotive forceproportional to the current, which serves as the damp-ing force for the mechanical system

f d = k t i 9where k t is back electromotive force constant of electro-magnetic motor.

The electromagnetic motor can be modeled as vol-tage source in series with the inherent inductance andresistor of the motor. If the inductor Lm of the motor issmall and the two electrodes of the electromagneticmotor are shunted with a resistor R, the electromotiveforce will appear as an ideal viscous damping force

f d = k ek t

R + Rmv 10

where Rm is the resistance of coils of the electromag-netic motor. And in this case, the electric energy isdissipated by the resistor into heat waste (Palomera-Arias, 2005). Pure resistance load also provides amethod to measure and estimate the potential amountof energy in the energy harvesting system (Gupta etal., 2006), although the practical loads are not alwayspure resistive. On the other hand, when the electro-magnetic transducer is used as passive vibration dam-per, the vibration performance can be furtherimproved by shunting the damper with resistor, capa-

citor, and inductor network (Fleming, 2002; Hagoodand Flotow, 1991; Hollkamp, 1994). Rather than dis-sipating the electric energy into heat waste, we canreplace the resistor with a charging circuit and energystorage device to store the electric energy. The above-mentioned analysis and modeling are for linear elec-tromagnetic motors. Similar relations can be obtainedfor the rotational electromagnetic motors with per-manent magnets.

Actually, electromagnetic motors have been usedmore often in large-scale vibration energy harvesting.Energy recovery from vehicle suspension is such anexample. Instead of dissipating the vibration energyinto heat waste using shock absorbers, the energy canbe harvested, meanwhile reducing the vibration (Boldeaand Nasar, 1997; Gupta et al., 2006; Nakano et al.,2003; Zuo et al., 2011c).

Direct drive linear electromagnetic generator. For a tubularlinear motor, when the coil moves at a velocity perpen-dicular to the magnetic field of flux intensity Br , theinduced open-circuit voltage and the maximum powerin the coil (short circuit) can be obtained as (Zuo et al.,2011c)

Piezoelectric material

Proof mass

Structure

Figure 16. Piezoelectric material working in bending modewhen mounted on a cantilever beam.



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V = 2 p Br v z Dc Ac

ffiffiffi3p d 2 11

P = p 2 s B2r v

2 z Dc Ac

2 ffiffiffi3p 12

where d is the diameter of the wire; Dc is the averagediameter of the coils; Ac is the cross-sectional area of the coil and wire, respectively; and s is the electricconductivity of the wire material. We see that the out-put voltage is inversely proportional to the square of the wire diameter, and the maximum power will onlydepend on the total volume ( D c Ac ) of conductingmaterial in the coils. Equation (12) indicates that thepower is proportional to the B2r and v

2 z , thus suggests

two ways to increase the power, namely, increasingthe magnetic flux intensity B and amplifying themotion v z .

Zuo et al. (2011c) developed a linear energy har-vester prototype and showed that average power of 16W can be achieved by one harvester at suspension velo-city of 0.25 m/s, using axial magnets and spacers. Ahigh magnetic conductive steel casing, which movestogether with the magnet assembly without introducingeddy current effect, is added to further increase themagnetic flux density. Tang et al. (2011) investigatedon the parameters of the magnet thickness/diameter

ratio and coil thickness of the linear energy harvesterand proposed a configuration using both radial andaxial magnets to achieve significantly higher powerdensity, as shown in Figure 17.

Bose Corporation (Rani, 2005) spent over 20 yearsto develop a unique active-controlled vehicle suspen-sion system with high-speed linear electromagneticmotor, which can significantly improve the passen-gers comfort and the maneuverability of the vehicle.Together with regenerative switching power amplifiersof bidirectional power flow ability, the linear electro-magnetic motors are also used to recover the part of vibration energy, and thus, a reduction of the powerconsumption in the active control by 1/3 has been

claimed (Rani, 2005). Chen and Liao (2012) devel-oped a self-powered and self-sensing magnetorheolo-gical (MR) damper, where the power source of MRdamper is the integrated linear electromagnetic motor,as shown in Figure 18. In addition, several researchersalso developed linear electromagnetic eddy currentdampers (Ebrahimi et al., 2008; Palomera-Arias,2005; Zuo et al., 2011a). Though energy harvesting isnot explored in the literature of eddy current dampers,

some idea therein can be extended for the linear elec-tromagnetic harvester design.

Rotational electromagnetic motors. Although linear elec-tromagnetic motors have the advantage of being easilyand reliably integrated into most existing vibration sys-tems without the requirement for transmission mechan-ism, their efficiency is relative low and their size is stilllarge, because of the relative low vibration velocity.Hence, rotational electromagnetic motors, includingdirect current (DC) and AC permanent magnet motors,are adopted in vibration energy harvesting. Appropriatemotion transmissions are needed to convert the linearmotion into rotational motion, which we will discuss insection Motion and magnification mechanisms.

Using the rotational motor as a generator, Romeet al. (2005) developed a backpack-driven energy har-vesting system (Figure 19), which can generate powerup to 7.4 W with little extra metabolic energy. Thisdevice also harvests the energy from normal humanwalk, and it is much more efficient compared with theenergy harvesting shoes (Shenck and Paradiso, 2001)or the backpack (Granstrom et al, 2007) based onpiezoelectric transducers, as we mentioned earlier.Electromagnetic motor is also used in vibration energy

Figure 18. Self-powered MR damper (Chen and Liao, 2012).MR: magnetorheological.

Figure 17. Linear energy harvesters with radial and axialmagnets and highly magnetic conductive casing.

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harvesting from building structures (Scruggs, 1999,2004; Tang and Zuo, 2011b). For example, Scruggs(1999, 2004) analyzed the possibility of developing thiskind of regenerative actuator and presented a regenera-tive force actuation (RFA) network consisting of multi-ple actuators distributed throughout a structuralsystem to reduce the vibration of the structure, wheresome actuators can harvest the mechanical energy fromthe vibration, while others reinject a portion of thatenergy back into the structure at other location toreduce the vibration.

Motion and magnification mechanisms

As seen from equations (3) and (8), the stress in thepiezoelectric material and the relative velocity betweenthe movers and stators of electromagnetic transducersare important for the power output of the piezoelectricand electromagnetic transducers, respectively.Therefore, there is an opportunity to improve theenergy harvesting performance by designing the motionmechanism to magnify the force or the vibration velo-city or changing one type of motion to another. Itshould be noted that by adding these mechanisms, wewill introduce extra dynamics into existing system,which should be taken into account in the systemdesign and analysis.

Motion and force magnificationMotion magnification mechanism can be used toincrease the efficiency of the electromagnetic motor based energy harvesting system. It may have the sameeffect when increasing the motor constants ke and ktsince when a magnification mechanism with the ratio

of M is adopted, the induced electric voltage will be M times

em = Mk ev 13At the same time, the back electromotive force will

be M times compared with the one without mechanism

F d = Mk t i 14From equations (13) and (14), we can find that the

motion magnification M has influence on both theinduced voltage and dynamics. In this section, we willreview different mechanisms, which have the potentialto be used in energy harvesting system.

Gears are very convenient in magnifying or reducingthe rotational speed or force. In energy harvesting fromvibration, it is used to magnify the displacement andvelocity in order to improve the output power and effi-ciency. An electromagnetic motor with gears can bedriven at a velocity much higher than the input, mean-while providing larger damping force. Donelan et al.(2008) and Li et al. (2008) adopted geared electromag-

netic motor in their energy harvesting system fromhuman walking, as shown in Figure 20. They designeda knee-mounted device, resulting little effect on humanwalking gait during the harvesting. The control systemprovides the power generation engagement or disen-gagement commands, where the energy harvestingfunction is activated only at the end of the swing phasewhen knee flexor muscles act to brake knee motion,based on the measured knee kinematics during a gaitcycle. It harvested 4.8 6 0.8 W in experimental tests.

Rather than magnifying the motion for electromag-netic transducers, force magnification has been used inpiezoelectric materialbased harvester. As shown inFigure 21 (Antaki et al., 1995), the piezoelectric ceramic

Figure 19. Regenerative backpack, harvesting energy from human walk, 7.8 W (Rome et al., 2005).



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to transfer the linear motion of vibration into rotationalmotion. Several mechanisms have been proposed, andsome prototypes have been built to obtain harvesterswith rotational electromagnetic motors, including links,screws, rack and pinions, and fluids.

One of the mechanism is motion links. Gupta et al.(2006) proposed to use level mechanism in a regenera-

tive shock absorber, which consists of a geared rota-tional motor and a level, resulting in six resolutions of the motor to one of the level. This configuration cannot only change the relative linear motion into rota-tional motion, but also can magnify the motion result-ing in a high efficiency. It was tested on a small all-terrain vehicle (125 kg) and harvested up to a peakpower of 88.8 W when passing 4 $ 3 4$ bars, which is21% in efficiency.

Traditionally, ball screw mechanism is used to trans-form the rotational motion of the electromagneticmotor into linear motion, resulting in linear actuator.In energy harvesting from vibration, researchers use itinversely. Kawamoto et al. (2008) proposed an electro-mechanical actuator consisting of rotational electro-magnetic motor and ballscrew mechanism, as shownin Figure 23(a). A prototype is also built. The ball screw transfers the linear motion into rotational motionand then drives the electromagnetic motor. Also usingballscrew mechanism, Zhang et al. (2007) conducted afull-vehicle experiment to test the vibration perfor-mance and feasibility for energy harvesting. Cassidy etal. (2011) designed an electromagnetic transducer withballscrew mechanism for energy harvesting from

large-scale civil structures, for which the power levelscan be above 100 W for excitation frequencies below 1Hz, as shown in Figure 23(b). Applying of ballscrewmechanism to the system will also introduce extradynamics, which needs to be fully investigated. Forexample, Cassidy and Scruggs (2012) treat the

Coulomb friction force introduced by ballscrew as anequivalent linear viscous term based on statistical line-arization. A nonlinear modeling of the system is alsodiscussed by Cassidy (2011), based on experimentaldata, where the hysteretic effect is modeled as a non-linear spring over wide range of frequencies. Similarballscrew mechanisms based on ballscrew can befound in passive vibration control of vehicle suspension(Suda et al. (2000); Suda et al. (2003)).

Rack and pinion and relevant modified systems alsohave potential in linear to rotational motion transfor-mation. It is used in the study by Rome et al. (2005) to

transfer the vertical movement of the mass into therotation. In practical application, the gear backlashbetween the rack and pinion is always not negligible,posing uncertain factor to the dynamics. Zuo et al.(2011; R&D100 Award) designed a regenerative vehicleshock absorber with rackpinion mechanism, as shownin Figure 24. Together with bevel gears, the mechanismmakes the regenerative shock absorber compact and isretrofittable to the conventional vehicles. Tang andZuo (2011b) also used rackpinion mechanism to con-vert the oscillation of the building to the rotation of thegenerator, as shown in Figure 25. Both energy harvest-ing and vibration control are achieved at the same time.Choi et al. (2009) used the rack and pinion mechanismto transfer the linear motion of the shock absorber of avehicle into rotation to drive the generator, as shown inFigure 26. An integrated electrorheological (ER) shockabsorber, the typical energy consumption of which is20 W, is controlled and driven only using the harvestedenergy. The weight of a passing vehicle engages aratchet to drive the flywheel and generator. The uniquefeature of this system is that the electromagnetic gen-erators only rotate in one direction and can directlyobtain DC voltage without using rectifiers.

Figure 23. Vibration energy harvesters using ballscrew mechanism: (a) regenerative shock absorber (Kawamoto et al., 2008) and(b) large structural vibration energy harvester (Cassidy et al., 2011).PM: permanent magnet.

Figure 22. Piezoelectric multilayer stacks and forceamplification mechanism (Xu et al., 2011).



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Penamalli (2011), Phillips (2011) and Wang et al .,(2013) utilized rackpinion mechanism to harvest thevibration energy from railway tracks. Their energy har-vesting systems have unique mechanism, which is toconvert the bidirectional vibration into unidirectionalrotation of the generator; thus, efficiency and lifetimeof the harvester can be improved significantly. Thedesign and prototype with motion rectifier built byWang et al., (2013) is shown in Figure 27.

The ballscrew and rackpinion mechanisms men-tioned earlier convert the bidirectional vibration intothe alternating rotation of the electromagnetic motor.However, there are many problems. For example, dueto the inertia, the shock absorber investigated in thestudy by Kawamoto et al. (2008) had bad vibration

performance at high frequency when active vibrationcontrol method was used. The alternating directions of the motor and gears will wear out the gears easily. So,Li, and Zuo et al . (Zuo, et al, 2011, Li et al, 2012) alsoproposed to use a mechanical motion rectifier, as shownin Figure 28, which can change the bidirectional vibra-tion into smooth one-directional rotational motion of the motor, resulting in high efficiency and more robust-ness. Besides, the electromagnetic motor will provideDC voltage if DC motor is utilized.

In addition to the above-mentioned links, screws,and rackpinions, fluid is another mechanism for

Figure 24. Regenerative shock absorber using rack-pinion mechanism (R&D100 Award, 2011).

Figure 26. Self-powered ER damper with rack and pinionmechanism (Choi et al., 2009).ER: electrorheological.

Figure 25. Energy harvesting from buildings with TMD usingrackpinion mechanism (Tang and Zuo, 2011b).TMD: tuned mass damper.

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motion transmission. Anderson et al. (Chandler, 2009)designed a regenerative shock absorber by adopting

turbine inside the hydraulic shock absorber. The displa-cement and velocity are magnified and transmitted tothe turbine-driven electromagnetic motor when thefluids flow through the turbine. Energy was claimed upto 1 kW when tested on a heavy truck or up to 10%improvement in fuel efficiency. However, the road con-dition for the experimental tests is not given, and thedynamics and the vibration mitigation performance of the truck were not considered. First, the mechanicalforce exerted by the passing vehicle compresses the airin a vessel. Then, the compressed air drives the genera-tor continuously, and thus, energy is harvested indir-

ectly from the traffic bumps. In ocean wave energyharvesting, one popular power take off (PTO) mechan-ism is to use wave energy to drive pumps that pressur-ize a hydraulic or pneumatic fluid and then drive arotational generator via a hydro or air turbine.

Energy harvesting circuits and power management

In energy harvesting system, the power electronic cir-cuits have four main functions: (1) to regulate the ACharvested power to DC with a voltage suitable for the

load or energy storage device; (2) to enhance the har-vesting efficiency. In most situations, the vibration levelalways varies, resulting in the low efficiency of circuitwith fixed parameters optimized for certain vibrationlevel. Power electronic circuits with controllable para-meters are able to improve the energy efficiency byadaptively changing certain parameters according tothe vibration level or external load (Ottman et al., 2002,2003); (3) to control the vibration, which is a specialissue when semi-active or active control is adopted; and(4) to manage the power flow.

Typically, the power electronic circuits in vibrationenergy harvesting consist of rectifier, DCDC conver-ter, and energy storage device or grid tie inverters, as

shown in Figure 29. We will focus on the AC/DC andvoltage regulation parts of the circuit, which are partic-

ularly associated with vibration energy harvesting.

ACDC convertersThe electricity generated by the vibration energy har-vesting system is usually AC, with varying frequencyand amplitude. It cannot power the electronic devicesor feed the power grid directly. Traditional rectifier,which consists of four diodes, changes the AC gener-ated by the transducer into DC. However, due to thelarge parasitic power consumption and the forward vol-tage drop of the diodes (0.61.5 V for a normal silicon

diode and 0.20.4 V for a Schottky diode), the efficiencyis low. Several literatures have proposed alternativeschemes for rectifying the current with higher efficiency.For example, Dallago et al. (2008) presented an activevoltage doubler ACDC converter for piezoelectricenergy harvesting systems. Their simulation result showsan improvement as high as 94% in the efficiency.

Synchronous rectifier has high efficiency, especiallyin low-voltage applications, since the forward voltagedrop is less than traditional diode bridge rectifiers(Mohan et al., 1995). It is also used for the purpose of improving the efficiency in vibration energy harvesting

(e.g. Han et al., 2004). The power loss in traditionalrectifier is proportional to the product of its forwardvoltage drop V and its forward conduction current I .The synchronous rectifier is composed of controlledswitches such as power metal oxide semiconductor fieldeffect transistors (MOSFETs) or power bipolar junc-tion transistors (BJTs), and it appears as a resistor interms of the power loss. From the currentvoltage rela-tions of these two rectifiers (Figure 30), we can con-clude that under certain current level, the synchronousrectifier has higher efficiency. However, it should benoted that synchronous rectifier is an active device.The utilization of this should be carefully considered.On the other hand, the synchronous rectifier has great

(a) (b)

Figure 27. Railway energy harvester with motion rectifier and flying wheel (Wang et al., 2013): (a)3-D modeling of the design; (b)full-size prototype.



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used either to boost or to reduce the voltage to therange of the energy storage device. Step-up DCDCconverter (boost converter) (Cao et al., 2007; Cao andLee, 2006; Kazimierczuk and Massarini, 1997), step-down DCDC converter (buck converter) (Ottmanet al., 2002, 2003), and flyback converter (Sodano et al.,2004) have been widely used.

The topologies of the step-up DCDC converterand step-down DCDC converter are shown in Figure32. The capacitors C1 and C2 serve as buffers tosmooth the voltage. The switching frequency of theDCDC converter is usually much higher than the oneof the vibration. Hence, the input and output can becontrolled in real time through the electronic switchusing pulse width modulation (PWM). The output of the DCDC converters can be adjusted by the dutycycle of PWM.

Controlled by the duty cycle of the switch, the step-up DCDC converter can work in either discontinuousor continuous modes. The difference between thesetwo modes is that the current of the inductor L falls to0 or not.

Traditionally, the DCDC converter is applied afterthe rectifier. Considering that the parasitic power lossdue to the forward voltage drop of the rectifier diodesis considerable, Dwari and Parsa (2010) proposed directAC to DC converters by adopting voltage boost beforethe diode rectifier, as shown in Figure 33, which will

reduce the current flow through the diode by increasingthe voltage, and hence improve the efficiency. Actually,this type of circuit has been used by Kim and Okada(2002), and the function was to eliminate the dead zonedue to the voltage drop of the diodes. Tang and Zuo(2011b) investigate the force control capability of this

circuit in energy harvesting from tall buildings, wherethe damping force provided by the energy harvesting iscontrolled by the duty cycle of the switch S.

Cao and Lee (2006) and Cao et al. (2007) used boostconverters to step up the low voltage generated by anelectromagnetic microgenerator. Meanwhile, feed-forward and feedback circuits (Figure 34) are adoptedto adjust the duty cycle to keep the output voltage sta-ble, despite the input voltage (vibration level) andexternal load change. Kim et al. (2007) used a buckconverter to transfer the electricity of high voltage gen-erated by piezoelectric ceramics to lower output voltage

for the load. Similar applications can be found inpiezoelectric materialbased energy harvester, sincepiezoelectric materials generate high voltage.

Ottman et al. (2002, 2003) pointed out that thepower electronic circuits have influence on the energyharvesting efficiency. The energy harvesting efficiencyis relevant with the duty cycle of the buck converterand derived the analytical optimal duty cycle for thepiezoelectric materialbased energy harvester with buckconverter. Similarly, Zuo and Tang (2009) analyzed theoptimal duty cycle of electromagnetic motorbasedenergy harvester with booster converter, with takingthe voltage drop of the diodes and inherent resistanceand inductance of the motor into account.

Other voltage regulators include the double voltagecircuit proposed by Kim and Okada (2002), which sim-ply doubles the output voltage, and four-state chargepump proposed by Han et al. (2004) to boost the out-put voltage, where the capacitors are charged in paral-lel and discharged in series and controlled by a switcharray. Guyomar et al. (2005) investigated a nonlinear

Figure 32. DCDC converter: (a) step-up DCDC converter (booster converter) and (b) step-down DCDC converter (buck converter).DC: direct current.

Figure 31. Regenerative H-bridge motor/generator driver.



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bidirectional power flow can be borrowed from thehybrid vehicles, fuel cells, or battery energy storage.

One simple way to achieve bidirectional power trans-fer is to use two independent unidirectional converters.However, bidirectional converters are more compact

with lower component count, and tend to be more effi-cient. According to whether the circuit uses transformerto realize isolation between the two power sources, thebidirectional DC-DC converters can be categorized intotwo types, isolated and non-isolated.

Non-isolated bidirectional DC-DC converters gener-ally have advantages of simple structure, high effi-ciency, low cost, and high reliability. The non-isolatedbidirectional DC-DC converters that are derived frombasic unidirectional topologies such as half-bridge con-verter, Cuk converter, and SEPIC converter have beensummarized in literature (Du et al., 2010, Li et al.,

2013). A basic half-bridge bidirectional topology basedon traditional unidirectional buck converter is shownin Figure 35-a. In this circuit, the converter operates asa buck converter when power is flowing from the highvoltage side (left hand side) to the low voltage side(right hand side), and as a boost converter when oper-ating in the reverse direction. In large power applica-tions, switch voltage stress and switch loss become aconcern in circuit topology designs. Three-level (TL)bidirectional DC-DC converters have been investigatedfor fuel-cell/battery hybrid power systems (Figure 35-b,Du et al. (2010). The Split-Pi topology (Figure 35-c,Maclaurin et al., 2011) is a boost converter followed bya buck converter. It is symmetric and which means itcan work as either a boost or a buck converter in bothdirections by applying appropriate control on theswitches.

Isolated DC-DC converters utilize a transformer toprovide galvanic isolation to protect sensitive circuitsfrom noise or high voltage and achieve high voltageconversion ratio. In order to feed the transformer, theDC current must be converted into AC current beforethe transformer and then rectified to DC current againafter the transformer, as shown in Figure 36.

Basic topologies can be used to construct isolatedbidirectional converters by adding a transformer in

between. Non-isolated bidirectional converter is goodon price. However, isolated full-bridge topologies canachieve much better conversion ratio (over 100) thannon-isolated topologies with inductor. To minimize thetransformer size, weight and cost, the frequency of the

AC current should be as high as possible(Jalbrzykowski and Citko, 2008). However, the fre-quency increase is limited by the transistor conductionand switching losses. Due to the energy loss in thetransformers, isolated bidirectional DC-DC convertersmay be applied in large scale energy harvesting systems,such as tall buildings and ocean wave energy harvestingsystems.

Vibration control algorithms

Quite different from small-scale vibration energy har-

vesting, in the large-scale vibration energy harvesting,the vibration performances of the primary structuresare always concerned. Often, the researchers try tomake dual use of the existing vibration control mechan-ism for energy harvesting. Hence, the vibration energyshould be extracted while providing better vibrationmitigation of the primary structure or at least maintain-ing the vibration suppression of the primary structure.The circuits with potential to control the damping forceof the systems have been reviewed in section Vibrationcontrol. On the other hand, researches have lookedinto different control algorithms to control the relevant

circuits in order to control the vibration. There are twotypical implementations of the control algorithms. Oneis first designing the full active control algorithm andthen put the constraints of the circuit to the feedbackforce, shown in Figure 37(a) as an example. The otherone is to include the constraints due to the energy har-vesting circuit when designing the controller. This con-troller using this method may be formed as a bilinearmatrix inequality (BMI) or linear matrix inequality(LMI) problems (Scruggs, 2007, 2010). Besides, manyperformance objectives can be considered in the sameframework (Scruggs et al., 2012). Giorgetti et al. (2006)proposed another method for the semi-active suspen-sion control, where the quarter-car model and the

(a) (b) (c)

Figure 35. Isolated bidirectional DC-DC converters (Li et al ., 2013) (a) half-bridge structure, (b) neutral point clamped three-level,and (c) Split-Pi bidirectional converter topology.



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constraints of the semi-active control is modeled into ahybrid dynamical systems and Modeling PredictiveControl (MPC) is used for the controlled design. It isfound that the explicit MPC control law is the same asthe clipped LQG control when the predictive horizon isequal to one and MPC control can provide better per-formance than the clipped LQG control if it has morepredictive horizon. Similar observation was also foundby Huang et al. (2011).

On the other hand, instead of harvesting and storingthe energy, the harvested electricity is further used toimplement active vibration control, resulting self-powered active control (Nakano et al., 2003; Nakanoand Suda, 2004). Although little energy for externalusage is obtained, the vibration of the primary struc-ture is suppressed significantly. For example, Scruggsand Lindner (1999) investigated on the feasibility of self-powered active control on harvesting energy frombuildings using simple velocity and displacement algo-rithm. Tang and Zuo (2010) confirmed the feasibilitywith linear quadratic Gaussian (LQG) control. Theregular rectifier can be used in semiactive control. In

active vibration control with energy harvesting, wherethe power flow is bidirectional, switch-based control isusually used (Nakano and Suda, 2004). For example,Nakano et al. (2003) analyzed the possibility of self-powered active vibration control using linear elec-tromagnetic motor and concluded that active vibrationcontrol without consuming external energy can beachieved under suitable conditions derived from energybalance analysis. Their experiment test verified that thevibration mitigation performance of the self-poweredactive control is significantly improved over the passivevibration mitigation approaches. Instead of storing theelectricity in capacitors, Jolly and Margolis (1997)stored the energy temporarily in inductor and by alter-natively storing and releasing the energy controlled byswitches, realizing a self-powered active control system.Similarly, Zheng et al. (2008) analyzed the performanceand energy by dividing the whole system into two oper-ation modes controlled by switches, namely, electricmotor mode and regenerative braking mode. In electricmotor mode, the optimal ride comfort is obtained byactive control; in regenerative braking mode, the

Figure 37. Implementations of vibration control in vibration energy harvesting system: (a) using clipped control (Tang and Zuo,2010) and (b) multiobjective energy harvesting problem (Scruggs et al., 2012).

Figure 36. Schematic of isolated bidirectional converter.

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system can harvest energy from the vibration andimprove the ride comfort performance meanwhile.

Challenges and future directions

Small-scale energy harvesting has been investigated byresearchers for more than a decade, resulting in awealth of articles and interesting applications in wire-less sensors and electronic devices. Large-scale energyharvesting at 1W -100 kW or more promises a moremeaningful solution to the energy crisis and a self-power active or semiactive vibration control. However,more challenges exist in large-scale vibration energyharvesting, and many questions remain to be answered.In this paper, we first review the power potential fromlarge-scale vibration energy harvesting from vehicle sus-pensions, civil structures, rail way tracks, ocean waves,and human motions. We survey the piezoelectric, lin-

ear electromagnetic and rotational electromagnetictransducers, and different motion and transmissionmechanisms. Power electronic circuits and control stra-tegies are also reviewed.

Efficiency is always a concern in vibration energyharvesting, which requires more efficient transducer,motion mechanisms, and novel power electronic cir-cuits. The efficiency of piezoelectric materialbasedenergy harvester has been widely investigated using newmaterials and developing novel physical or geometricalconfigurations. However, the optimization of electro-magnetic motor receives little attention. Significant effi-

ciency improvement can be achieved if the backelectromotive force coefficient k e can be increased.Motion mechanism is also very important to improvethe efficiency. Partial power is lost when the kineticenergy is transformed into electric energy, such as eddycurrent or friction. Another important power loss isparasitic voltage drops or the inherent resistances of theelectronic components or the power consumption bythe controller. Techniques to reduce this voltage dropand the parasitic power consumption need to be furtherinvestigated. A fundamental challenge is that large-scalevibration is very irregular at time-varying frequencyand at low, alternating velocities, which makes efficientand reliable energy conversion difficult and limits theoptions for efficient power takeoff technology. Thenovel mechanical motion rectifier that converts irregu-lar oscillatory vibration into regular unidirectional rota-tion is worth of attention.

Vibration control is another particular challengeassociated with large-scale energy harvesting. Quite dif-ferent from the small-scale energy harvesting where thevibration of the host structure is not a concern, the pri-ority in most large-scale vibration harvesting is the pro-tection of the mechanical systems and the humanoccupants or passengers during the vibration. Thismakes many techniques in small-scale energy harvesting

impractical here. For example, mechanic resonance canbe used to magnify the vibration and hence improve theoutput power of the harvester in small-scale applica-tion. However, it becomes difficult in regenerative vehi-cle suspensions. Hence, the ability of simultaneousvibration control and energy harvesting should be con-

sidered in every component of large-scale vibration har-vesting, and advanced control algorithms are required.For example, the motion mechanism should be efficientin driving and driven modes, the power electronics shouldbe able to take energy from the mechanical systems andalso to pump energy into it also, and the control algo-rithms should regulate the electrical output voltage andcontrol the regenerated electrical current or voltage at thesame time. Since the large-scale energy harvesting is not just a design problem, multiple disciplinary and system-level approaches should be taken, which involves struc-ture dynamics and vibration, mechanical design, power

electronics, materials, and control.

Acknowledgements

The authors wish to thank the assistance from many gradu-ates and undergraduates in the Energy Harvesting andMechatronics Research Lab at Stony Brook University.

Funding

This study was funded by the National Science Foundation(NSF grant CMMI1031038), New York State EnergyResearch and Development Authority (NYSERDA contracts

15761 and 25537), Department of Transportation (RITA/UTRC grant), University Transportation Research Center,Region II (Faculty Development Minigrant), SUNY ResearchFoundation (Technology Accelerator Fund), and industry.

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