Characteristics of Energy Storage Devices in Piezoelectric Energy Harvesting Systems

StructuresJournal of Intelligent Material Systems andDOI: 10.1177/1045389X070789692008; 19; 671 originally published online Jul 10, 2007;Journal of Intelligent Material Systems and Structures M.J. Guan and W.H. LiaoCharacteristics of Energy Storage Devices in Piezoelectric Energy Harvesting Systems /cgi/content/abstract/19/6/671The online version of this article can be found at:Published by:can be found at:Journal of Intelligent Material Systems and Structures Additional services and information for /cgi/alerts Email Alerts: /subscriptions Subscriptions: /journalsReprints.nav Reprints: /journalsPermissions.nav Permissions:/cgi/content/refs/19/6/671 CitationsCharacteristics of Energy Storage Devices in PiezoelectricEnergy Harvesting SystemsM.J.G UAN AND W.H.L IAO*Smart Materials and Structures Laboratory,Department of Mechanical and Automation EngineeringThe Chinese University of Hong Kong,Shatin,N.T.,Hong Kong,ChinaABSTRACT:Using piezoelectric elements to harvest energy from ambient vibration has beenof great interest recently.As the power harvested from the piezoelectric element is relativelylow,energy storage devices are needed to accumulate the energy for intermittent use.In thisstudy,the energy storage devices considered include rechargeable batteries and super-capacitors.The traditional electrolytic capacitors are not considered due to their low energydensity.The charge/discharge efficiencies of the energy storage devices are of major concern.The equivalent circuit model of the energy storage devices is investigated.It is found that theleakage resistances of the energy storage devices are the dominant factor that influences thecharge/discharge efficiency in the piezoelectric energy harvesting systems.A quick test methodis proposed to experimentally study the charge/discharge efficiencies of the energy storagedevices.The experimental results verify our findings.Adaptability,lifetime,and chargingprotection circuit of the energy storage devices are also discussed.It can be concluded thatsupercapacitors are suitable and more desirable than the rechargeable batteries to store theenergy in the piezoelectric energy harvesting systems.Key Words:energy storage device,piezoelectric energy harvesting,charge/dischargeefficiency,leakage resistance.INTRODUCTIONW IRELESS sensor networks(WSNs)have drawn significant attention in monitoring plants, resources,and infrastructures(Liao et al.,2001). Traditional electrochemical batteries are usually used in powering the WSNs.But replacing batteries,due to their limited lifetime,is very inconvenient.The labor and cost associated with changing hundreds or thousands of batteries would be troublesome and expensive in maintaining the networks.Moreover,in many cases, the WSNs are installed in remote areas;therefore,it is difficult to retrieve for battery replacement.Without replacing batteries,the WSNs will stop working as soon as the power runs out.Therefore,it is desired that the WSNs can acquire energy from the ambient environ-ment to have a perpetual power supply.Considering the needs,for example,the health moni-toring of infrastructures,to harvest the power from the vibration environment would be a convenient means to supply power for the sensor network.While there are several options for harvesting the ambient vibrational power,piezoelectric materials have been of great interest and promise(Starner,1996;Umeda,et al.,1997; Elvin et al.,2001;Shenck and Paradiso,2001;Roundy et al.,2003;Guyomar et al.,2005;Kim et al.,2005; Ng and Liao,2005;Beeby et al.,2006;Shu and Lien, 2006).Several researchers have put their efforts in the piezoelectric energy harvesting area.Roundy et al. (2003)drove a bimorph PZT simulating the vibrations on a small microwave oven.The vibrating PZT has an average output power of250m W.Shenck and Paradiso (2001)developed a piezoelectric system that would harvest the energy during walking and used it to power a radio transmitter.An average output power of1.8mW was obtained.As the average harvested power is less,usually,some energy storage means are needed to accumulate the harvested energy for intermittent use.Most of the earlier research on power harvesting used the traditional electrolytic capacitors as energy storage devices (Starner,1996;Umeda et al.,1997;Elvin et al.,2001; Shenck and Paradiso,2001;Ng and Liao,2005).But due to the low energy density of traditional electrolytic capacitors,the output energy from the capacitor storage per discharge cycle is very limited.For example, if a1000m F capacitor is discharged from5to2V, which is a normal operating voltage range for many electronic sensors,it can only let a kind of sensor node,*Author to whom correspondence should be addressed.E-mail:whliao@.hkFigures3,4and6–8appear in color online:J OURNAL OF I NTELLIGENT M ATERIAL S YSTEMS AND S TRUCTURES,Vol.19—June2008671 1045-389X/08/060671–10$10.00/0DOI:10.1177/1045389X07078969ßSAGE Publications2008Telos Mote,developed by Berkeley,work in active mode for0.15s.The Telos Mote operates at an extremely low voltage of1.8V and also has the lowest working current among existing wireless sensor nodes(Jiang et al.,2005). It consumes a current of20mA in active mode and5m A in sleep mode.Therefore,rechargeable batteries are considered due to their higher energy density(Ottman et al.,2002,2003;Sodano2004,2005a).Sodano et al. (2005a)showed that a40mA nickel metal hydride (NiMH)rechargeable battery could be charged from a completely discharged state in less than one hour underthe vibration from a typical vibrating machine. However,they only showed the feasibility of using the rechargeable battery as energy storage device but did not evaluate the performances such as efficiency of the energy storage cells.Supercapacitors are alternative energy storage devices other than traditional electrolytic capacitors and rechargeable batteries.They are also named ultracapa-citor,double-layer capacitor(DLC),and electrical double-layer capacitor(EDLC).The energy density of supercapacitors is10–100times higher than that of traditional electrolytic capacitors.Supercapacitors could benefit many applications,from those involving short power pulses to those requiring low-power supplies of critical memory systems.So far,few researchers have looked into supercapacitors as energy storage devices in energy harvesting.Casciati et al.(2005)developed a vibrational energy harvesting device and mentioned using a supercapacitor for energy storage but did not provide any experimental results.Jiang et al.(2005) presented their implementation of a solar energy harvesting system and used a supercapacitor as the primary energy storage device.Their results showed that supercapacitors can be employed as energy storage devices in the energy harvesting systems.However, the performances of supercapacitors as the energy storage device were not evaluated.Rechargeable battery or supercapacitor,which one is preferred when used in the piezoelectric energy harvesting system?To address this issue,the character-istics of supercapacitors and rechargeable batteries as energy storage devices in piezoelectric energy harvesting will be studied and compared in this study. Charge/discharge efficiency,adaptability to piezoelectric harvesting circuit,lifetime,and charge protection circuit of these energy storage devices will be discussed by considering the piezoelectric energy harvesting systems.PIEZOELECTRIC ENERGY HARVESTINGA piezoelectric energy harvesting system can be basically divided into three parts:the energy source, the harvesting circuit,and the storage device.Energy SourceA harmonic vibrating piezoelectric element can be modeled as a sinusoidal current source i p in parallel with its internal capacitance C p as shown in Figure1.i pðtÞ¼I p sin!tð1Þwhere!and I p are the frequency and peak amplitude of the current source,respectively.The peak open-circuit voltage V oc of the source is related to I p by,V oc¼I p!C pð2ÞThe value of the peak open-circuit voltage depends on the type and size of piezoelectric element,vibration level and frequency.In general,V oc ranges from several volts to tens or hundreds of volts(e.g.,150V in Shenck and Paradiso,2001;67V in Ottman et al.,2002).While V oc was assumed by Ottman et al.(2002)to be relatively unchanged regardless of the external harvesting circuit, it should be noted that the vibrating structure can be modified to obtain different V oc even with the same excitation.Harvesting CircuitA basic energy harvesting scheme is shown in Figure 1.Since the source is alternating,a diode bridge rectifier is used as an AC–DC rectifier. As developed by Ottman et al.(2002),there exists an optimal rectifier voltage V rect_opt to harvest the energy for maximum power.The optimal rectifier voltage V rect_opt is one-half the peak open-circuit voltage V ocV rect opt¼V oc2¼I p2!C pð3ÞThe theoretical power harvested at a certain rectifier voltage V rect is,P theoretical¼V rect i esd¼V rectÁ2ðI pÀ!V rect C pÞð4Þwhere i esd represents the average current flowing into the energy storage device.From the previous studies (Ottman,2002),we know the efficiencies of theoreticalEnergystorage devicePiezoelectricelementFigure1.Basic piezoelectric energy harvesting circuit.672M.J.G UAN AND W.H.L IAOharvested power at different rectifier voltage levels can vary dramatically from5%to100%of the optimal harvested power.Since the power from the piezoelectric energy source is generally low,the rectifier voltage is strongly recommended to be at the optimal value.To harvest the power from varying exciting sources, another scheme is to use a two-stage harvesting circuit (Ottman,2003).The power from the piezoelectric element is firstly rectified to a temporary storage device whose voltage is kept at the optimal rectifier voltage,then transferred to the energy storage device through a converter.Guan and Liao(2007)have studied and compared the efficiencies of these two energy harvesting schemes considering the storage device voltages.In this study,we will explore the characteristics of energy storage devices in piezoelectric energy harvest-ing with the basic energy harvesting scheme.More discussions on the two-stage energy harvesting scheme are given in section‘Other Characteristics’.Storage DeviceThere are several types of energy storage devices including traditional electrolytic capacitors,recharge-able batteries,and supercapacitors.As discussed in the first section,traditional electrolytic capacitors may not work well as an energy storage device in the piezoelectric energy harvesting systems for WSNs due to their low energy density.In this study,rechargeable batteries and supercapacitors are considered.Rechargeable batteries mainly include nickel cadmium(NiCad),NiMH,and lithium ion/polymer(lithium)rechargeable batteries. Because the NiCad rechargeable batteries have memory effect,which is not suitable for shallow charging(however,energy harvesting is usually shallow charging),we will consider the NiMH and lithium rechargeable batteries in this study.CHARGE/DISCHARGE EFFICIENCYTheoretical AnalysisCharging and discharging rechargeable batteries or supercapacitors is never100%efficient.Charge/dis-charge efficiency,also called Coulombic efficiency or charging/discharging efficiency,is defined as,¼Discharge energyCharge energyð5ÞConsidering the low level of source energy in piezo-electric energy harvesting systems,the charge/discharge efficiencies of the energy storage devices are especially important.Based on the electrical characteristics of the energy storage devices,the charge/discharge efficiencies will be discussed.An equivalent circuit model for energy storage devices will be valuable to study the electrical characteristics.EQUIVALENT CIRCUIT MODELThe charge/discharge performances of rechargeable batteries or supercapacitors under various operating conditions are nonlinear and complicated.An equiva-lent circuit model to delicately describe their behaviors including self-discharge,overvoltage,and temperature effect is under study.The charge/discharge deficiency comes from the energy loss during the charge/discharge process.It is understood that the energy loss exists in two manners: internal series resistive loss and leakage loss.Therefore, an equivalent circuit model for energy storage devices as shown in Figure2is considered(Buchmann,2001). Figure2(a)and(b)describes the charge and discharge processes,respectively.In the circuit model,R int and R lea represent the internal series resistance and the leakage resistance,respectively;C s and V s represent the internal capacitance and the source voltage of the energy storage device respectively;V in and V out are the input/output voltage.The charge/discharge efficiency of the energy storage device is directly related to the energy losses due to the resistances R int and R lea.Therefore,the R int and R lea should be first explored.INTERNAL SERIES RESISTANCEThe internal series resistances of the energy storage devices such as rechargeable batteries and superca-pacitors are usually given in the specifications by manufacturers and also can be measured.The internal series resistance varies with the electrolyte type of different energy storage devices.For the supercapaci-tors,in particular,the internal series resistance is called equivalent series resistance(ESR)and ESR also varies with the cell capacity.The larger the cell capacity,the smaller the internal series resistance it has.Table1shows the internal series resistances of some energy storage devices measured by a precision impedance analyzer(40Hz–110MHz,Agilent4294A). The corresponding rated maximum ESR and internal resistances from manufacturers are also given for comparison.From Table1,we can find the manufacturer specifica-tions reliable for the internal series resistance.From the+_(a) Charged process(b) Discharged processsii−+sViiFigure2.Equivalent circuit model of the energy storage devices.Characteristics of Energy Storage Devices673specifications of several representative manufacturers,we find out that the internal series resistances of the common supercapacitors,with capacity from0.1F to3000F, range from30 to0.5m ;the internal series resistances of the common rechargeable battery cells of various capacity range from$50m to1 .LEAKAGE RESISTANCEThe leakage resistance of the energy storage device is an interesting issue but little understood since hitherto little work has been done on it.One kind of leakage resistance,the self-discharge resistance,has been studied.The self-discharge resistance refers to the leakage resistance under the self-discharge process. Self-discharge is a natural phenomenon of any recharge-able battery or supercapacitor.After fully charged, a rechargeable battery is usually maintained with a trickle charge to compensate for the self-discharge. For rechargeable batteries,several researchers used the self-discharge resistance as the leakage resistance (Salameh et al.,1992;Conway et al.,1997;Buchmann, 2001).For supercapacitors,we find the leakage resis-tance under other states is not equal but related to the self-discharge leakage resistance.In this part,we will study the leakage resistances of the supercapacitors under self-discharge and other states.At present,no quick test is available to measure the self-discharge of a rechargeable battery. Usually the self-discharge rate is described in capacity loss per month.For example,the self-discharge of the NiMH rechargeable battery is$30%per month(Buchmann,2001).The lithium rechargeable battery self-discharges$10%per month.The capacity of supercapacitors drops$35%per month.Based on the specifications of the self-discharge data from the manufacturers,we can estimate the average self-discharge resistance usingR lea self%V cellÂÁtQ cell %ð6Þwhere V cell and Q cell are the rated voltage and capacity of the cell; %is the self-discharge energy loss percentage in the time intervalÁt.From Equation(6), we find that the self-discharge resistance varies with the capacity of the cell.Table2gives the estimated self-discharge leakage resistances of the some commercially available energy storage cells.Conway et al.(1997)have analyzed the self-discharge mechanisms and proposed that in the charging condi-tion,electrochemical capacitors and batteries are in a state of high energy relative to that of the system in the discharging state.Hence there is a‘driving force,’corresponding to the free energy of discharge,tending to spontaneously diminish the charge if there exists some self-discharge mechanism(s).To find out the leakage resistances of super-capacitors under various states,experiments are conducted.The objective of experiments is to find out the leakage resistance under self-discharge and charging with certain levels of current on the super-capacitors.Suppose a supercapacitor with capacitance C s self-discharges from voltage V0,after timeTable1.Internal series resistances of the energy storage devices.Supercapacitor0.47F5.5V Supercapacitor1F5.5VPanasonic NF Panasonic SG NEC Tokin TK MAXCAP ELNA Philips Sam Young Rated max.ESR(at1kHz)(Ohms)303013307303030 Measured ESR(at1kHz)(Ohms)302912.816.4 1.810.814.98.4NiMH rechargeable battery Lithium rechargeable batteryEPT 110mAhBTY700mAhUSANCE700mAhPlantronic190mAhMotorola860mAhSamsung770mAhRated internal resistance(Ohms)0.200–10.200–10.200–10.200–10.200–10.200–1 Measured internal resistance(Ohms)0.1790.5400.1910.2900.2500.270Table2.Estimated self-discharge resistances of common energy storage cells.Energy storage device NiMH Lithium Supercapacitor Cell voltage 1.2V 3.6V 5.5VCell capacity30mAh500mAh30mAh500mAh0.47F20F Self-discharge leakage resistance96k 5.74k 8.5M 510k 15M 370k 674M.J.G UAN AND W.H.L IAOinterval Át ,its voltage decreases by ÁV ,then the self-discharge resistance can be calculated from Figure 2(b)by,R leaself%ÁtC s Âln V 0=ðV 0ÀÁV ½Þð7ÞSuppose a supercapacitor with capacitance C s is charged by current I in from voltage V 0,after time interval Át ,its voltage increases by ÁV ,then the leakage resistance under this charging process can be calculated from Figure 2(a)by,R leacha%V 0þðÁV =2ÞI in ÀC s ðÁV =Át Þð8ÞFigure 3shows the estimated results of the leakage resistances of several kinds of supercapacitors under various conditions.In Figure 3,‘MAX’,‘TK’,‘NEC’,‘SY’,‘ELNA’,‘PaSG’,‘Phi’,and ‘PaNF’represent eight supercapacitors from MAXCAP,TK,NEC Tokin,Sam Young,ELNA,Panasonic SG series,Philips and Panasonic NF series,respectively.Rated capacitances,voltages and ESRs of these eight supercapacitors are given in Table 1.From the results,we can find the leakage resistance under charging is always smaller than the self-discharge resistance.And as the charging current increases,the leakage resistance decreases.This can be understood based on Conway et al.’s (1997)analysis on self-discharge.The ‘driving force’under charging is larger than that when the energy storage cell is left alone.Therefore,the leakage current under charging is larger than the self-discharge current.The leakage resistance under charging is smaller than the self-discharge resistance.The study of leakage resistances of supercapacitors under various states could be interesting,but this is out of the scope of this study.In this study,the rangeof the leakage resistances is our major concern.From the experimental data,we can estimate the leakage resistances of supercapacitors under charging with a current no larger than 200m A will be in the range of 10–100%of the self-discharge leakage resistances depending on the levels of the charging current.POWER LOSS AND EFFICIENCYAs shown in Figure 2,the power losses of the internal series resistance during the charging and discharging processes are:P int cha ¼I in ÀV in R lea2ÂR int ð9ÞP int dis ¼I out þV out R lea2ÂR int ð10ÞThe power losses of the leakage resistance during the charging and discharging processes are:P lea cha ¼V 2inR lea ð11ÞP lea dis ¼V 2out R leað12ÞFrom the studies on the internal series resistances and leakage resistances,the leakage resistances are usually more than 100times the internal series resistances.Therefore,in Figure 2(a),the charge current i in is approximately equal to i int1.Similarly,in Figure 2(b),the discharge current i out is approximately equal to i int2.Therefore,the power losses of the internal series resistance R int during charging and discharging processesare approximately I 2in ÂR int and I 2out ÂR int ,respectively.Based on the range of the internal series and leakage resistances,we can plot the energy losses of the internal series resistance and leakage resistance under a range of the charge/discharge currents.The charge/discharge voltage is assumed to be 3V.The results are shown in Figure 4.From Figure 4,we can see that the energy losses of the internal series resistances greatly depend on the charge/discharge current while the energy losses of the leakage resistances are independent of the charge/discharge current.Basically,in higher current applications (0.5–10A),the energy loss due to the internal series resistance is much larger than the energy loss due to the leakage resistance.In lower current applications (1–200m A),the energy loss due to the internal series resistance is much smaller than that due to the leakage resistance.As stated in the previous section,the charge/discharge current is typically below 200m A in piezoelectric energy harvesting systems.Therefore,the leakage resistances are of the most importance in considering the efficiency of energy storage devices in piezoelectric energy harvest-ing systems.MAX TK NEC SY ELNA PaSG Phi PaNF123456SupercapacitorsL e a k a g e r e s i s t a n c e (M Ω)Figure 3.Leakage resistances of supercapacitors.Characteristics of Energy Storage Devices675The charge/discharge efficiency of the energy storage device can be calculated by,¼1ÀEnergy loss Charge energyð13ÞExperimental ValidationQUICK TEST METHODIn general,to test the charge/discharge efficiency of an energy storage device,a charger is connected to the storage device until fully charged.Then,a load is connected to the energy storage device to fully discharge it.The voltage and current passing through the device are monitored.Then Equation (5)is used to calculate the energy storage device’s efficiency.Monitoring all of the procedures and parameters is time-consuming and labor-intensive.Moreover,in piezoelectric energy harvesting,the source current usually is much lower than the current used in standard test.Thus using the full charge/discharge method to find out the charge/discharge efficiency in piezoelectric energy harvesting is not adopted in this study.As the charge/discharge efficiency of an energy storage device is almost the same during its total range of state of charge (SoC)except a bit lower near full SoC (Stevens and Corey,1996),we can test the charge/discharge efficiency within a small SoC range of the device.That is to say,we will charge and discharge the device between two close energy levels.Basically,the voltage of the device is an indicator of the energy stored in the device.However,the voltage–energy relations during charge and discharge processes are nonlinear for most energy storage devices.In this study,to overcome the nonlinear problem in testing,a method is applied to calculate the charge/discharge efficiencies of thesupercapacitors and rechargeable batteries.The sche-matic of this method is shown in Figure 5.During the charging process by the piezoelectric harvesting current,when the storage device’s voltage reaches a reference voltage V 0,we start calculating the time.After a certain time interval t 1,the voltage increases by ÁV .At the end of the interval t 1,we stop charging the cell and use a known current I dis to discharge the cell for a time interval t 2to voltage V 0ÀÁV .The discharging load is chosen to make the discharge current the same as the charge current from the piezoelectric element for a given excitation level.At the end of time interval t 2,we stop discharging the cell and start to recharge the cell to voltage V 0.The time interval used to recharge the cell to voltage V 0is denoted as t 3.When the voltage variation ÁV is small compared to V 0,we can useexp ¼Discharge energy Charge energy %I dis ÂV 0Ât 2P cha Âðt 1þt 3Þð14Þto calculate the charge/discharge efficiency.Based onEquation (4),the average charging power can be calculated by,P cha %V 0Á2ðI p À!V 0C p Þð15ÞThe method is based on the assumption that voltage–energy curve of the cell has a good repeatability during two consecutive charge/discharge cycles.EXPERIMENTAL SETUP AND RESULTSThe experimental setup is shown in Figure 6(a).The corresponding block diagram is shown in Figure 1.The components are described as follows.For the piezoelectric element,we use Mide quick pack QP20W,as shown in Figure 6(b),which was also used by Ottman et al.(2003)and Sodano et al.(2005b).The QP20W is a bimorph piezoelectric device that uses monolithicC e l l v o l t a g eFigure 5.Charge/discharge processes.110−101010Charge/discharge currentP o w e r l o s s (µW )Figure 4.Energy losses of the internal series resistances and leakage resistances.676M.J.G UAN AND W.H.L IAOpiezoceramic material embedded in an epoxy matrix. The epoxy shell makes it more robust than the raw monolithic material.One end of the QP20W is mounted on a shaker(B&K4810).The other end of the QP20W is clamped,thus allowing the excitation to be applied with the motion of the shaker.When the shaker moves,the QP20W will vibrate at a frequency and amplitude.In real applications,it is possible to modify the vibrating structure to have a specific frequency and amplitude.We use commercially available supercapacitors typed EDLC/NF from Panasonic Inc.They have a maximum voltage rating of5.5V/cell and a capacitance of0.47F. They are stacked coin typed with a diameter of21.5mmand a height of8.0mm.The rechargeable batteries used are a110mAh NiMH battery from EPT Battery Tech. Co.,Ltd with dimensions:diameter of14.5mm,height of50.5mm,and a190mAh lithium ion polymer battery from Plantronics Co.with area of20mmÂ35mm and thickness of 4.0mm,as shown in Figure6(c). The experimental conditions are given in Table 3. The reference voltages of NiMH and lithiumExperimental setup (a)(b)(c)QP20W quick pack Supercapacitor, NiMH and Lithium Piezoelectric elementShakerAC/DC rectifierEnergy storage deviceFigure6.Experimental setup and key components.Table3.Experimental conditions.Frequency (Hz)CapacitanceC p(nF)Open circuitvoltageV oc(V)ReferencevoltageÆmaximumvoltage variationV0Æ"V max(V)6038.024.00Supercapacitor:3.60Æ0.0517.50NiMH:0.88Æ0.0212.15Lithium:3.21Æ0.048.606.63Characteristics of Energy Storage Devices677。