Ultracapacitor technologies and application in hybrid and electric vehicles

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2010; 34:133–151Published online 17 December 2009 in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/er.1654

REVIEW

Ultracapacitor technologies and application in hybridand electric vehicles

Andrew Burke�,y

Institute of Transportation Studies, University of California-Davis, Davis, CA, U.S.A.

SUMMARY

This paper focuses on ultracapacitors (electrochemical capacitors) as energy storage in vehicle applications and thus evaluatesthe present state-of-the-art of ultracapacitor technologies and their suitability for use in electric and hybrid drivelines of varioustypes of vehicles. A key consideration in determining the applicability of ultracapacitors for a particular vehicle application is theproper assessment of the energy storage and power requirements. For hybrid–electric vehicles, the key issues are the useableenergy requirement and the maximum pulse power at high efficiency. For a Prius size vehicle, if the useable energy storage isabout 125Wh and needed efficiency is 90–95%, analysis shown in this paper indicate that vehicles can be designed using carbonultracapacitors (both carbon/carbon and hybrid carbon) that yield high fuel economy improvements for all driving cycles andthe cost of the ultracapacitors can be competitive with lithium-ion batteries for high volume production and carbon prices of lessthan $20kg�1. The use of carbon/carbon devices in micro-hybrids is particularly attractive for a control strategy (sawtooth) thatpermits engine operation near its maximum efficiency using only a 6kW electric motor. Vehicle projects in transit buses andpassenger cars have shown that ultracapacitors have functioned as expected and significant fuel economy improvements havebeen achieved that are higher than would have been possible using batteries because of the higher round-trip efficiencies of theultracapacitors. Ultracapacitors have particular advantages for use in fuel cell powered vehicles in which it is likely they can beused without interface electronics. Development of hybrid carbon devices is continuing showing energy densities of 12Whkg�1

and a high efficiency power density of about 1000Wkg�1. Vehicle simulations using those devices have shown that increasedpower capability in such devices is needed before full advantage can be taken of their increased energy density compared withcarbon/carbon devices in some vehicle applications. Energy storage system considerations indicate that combinations ofultracapacitors and advanced batteries (Whkg�14200) are likely to prove advantageous in the future as such batteries aredeveloped. This is likely to be the case in plug-in hybrids with high-power electric motors for which it may be difficult to limit thesize and weight of the energy storage unit even using advanced batteries. Copyright r 2009 John Wiley & Sons, Ltd.

KEY WORDS: ultracapacitor; hybrid vehicles; fuel economy; energy density

1. INTRODUCTION

It is well recognized that the future developmentand successful marketing of hybrid and electric

vehicles of various types are highly dependent onthe performance and cost of the energy storagetechnologies available. There seems to be highconfidence that the performance and cost of the

*Correspondence to: Andrew Burke, Institute of Transportation Studies, University of California-Davis, Davis, CA, U.S.A.yE-mail: [email protected]

Received 6 October 2009

Accepted 8 October 2009Copyright r 2009 John Wiley & Sons, Ltd.

other mechanical and electrical components inelectric and hybrid drivelines are suitable forvehicle applications, but there remains consider-able uncertainty regarding the energy storagetechnologies. Whether a particular energy storagetechnology is suitable for use in a particular typeof vehicle depends both on its characteristics andthe requirements for energy storage in the vehicledesign. This paper is concerned with both therequirements for energy storage in various types ofelectric and hybrid vehicles and the characteristicsof the energy storage devices being developed. Thispaper focuses on ultracapacitors (electrochemicalcapacitors) as energy storage in vehicle applica-tions and thus evaluates the present state-of-the-art of ultracapacitor technologies and its suitabil-ity for use in electric and hybrid drivelines ofvarious types of vehicles. Comparisons are madeof vehicle fuel economy performance using ultra-capacitors and advanced batteries (primarilylithium) and applications identified in whichultracapacitors could be used in place of or withbatteries to further reduce the energy use (fuel and/or electricity) of the vehicles. Cost considerationsare included in the comparisons of the variousultracapacitor and battery energy storage systems.

2. VEHICLE ENERGY STORAGECONSIDERATIONS

From a vehicle performance point-of-view, energystorage requirements are defined in terms of thepeak power (kW) and energy storage capacity (Whor kWh). The vehicle designer is also interested inthe weight and volume of the energy storage unitwhich follows once the energy and power densitiesof the technologies are known. It is important torecognize that the energy capacity and the peakpower refer to the useable energy capacity and theuseable peak power from the energy storage unitfor the particular application of interest. By‘useable’ is meant the ‘quantity’ that can beutilized from the storage unit consistent with othersystem constraints such as the effect of round-tripefficiency on peak power, depth of discharge onenergy capacity and cycle life, and maximumcharge voltage on energy capacity, cycle life and

safety. These further considerations in most casesresult in storage unit performance that is signifi-cantly less than one would infer from the name-plate ratings given by the manufacturer of thebatteries or ultracapacitors.

A second difficulty in quantifying the peakpower and energy requirements for hybrid–electricvehicles is that the useable power and energy re-quirements can be highly dependent on the controlstrategy linking operation of the engine and elec-tric drive system. In the case of a charge-sustaininghybrid, the useable energy required can vary from100–300Wh depending on how often and at whatpower level the engine is used to recharge the en-ergy storage unit [1–3]. In the case of the plug-inhybrids, the peak power requirement depends onthe blending strategy of the electric motor andengine when the vehicle is operating in the ‘all-electric’ or charge-depleting mode [4,5]. If ultra-capacitors and batteries are used together in eitherplug-in hybrid or electric vehicles, the strategyutilized for the load sharing between the two en-ergy storage units has a large effect on the powerrequirements for each of them.

3. ULTRACAPACITOR SYSTEMS

3.1. Device characterization

An ultracapacitor unit in a vehicle consists ofmany cells in series and possibly also in parallelmuch as is the case for batteries. In most cases, anumber of cells are combined into modules forconvenience of assembling the ultracapacitor packfor the vehicle. Nevertheless, the cell character-istics are the key factors in determining theultracapacitor unit performance. For ultracapaci-tors, the primary performance characteristics arecapacitance C (Farad) and resistance R (O). To areasonable approximation, the usable energystored in the ultracapacitor cell is given by

EðWhÞ ¼ 1=2 C V2r ð3=4Þ=3600

where Vr is the rated voltage of the cell.The above equation assumes that the cell is

discharged between its rated voltage Vr and 1/2 Vr.The rated voltage is the maximum voltage at whichthe cell should be used and in practice is usually

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somewhat less than the rated voltage at which thecell is tested to determine its rated energy density(Whkg�1). Derating the cell voltage is done tomaximize the cycle life of the ultracapacitor unit inthe vehicle. The useable pulse power capability [6]of the cell is given by

PmaxðWÞ ¼ 9=16 ð1� EFFÞ V2r =R

where EFF is the electrical efficiency of the pulse.

EFF ¼ Vpulse=3=4 Vr

Often the power capability of a cell is calculatedfrom the relationship V2

r =4R, which corresponds toan efficiency of 50%. This is not a useable effi-ciency for electric and hybrid vehicle applications.More practical efficiencies are 75–80% for electric(battery powered) and 90–95% for hybrid vehicleoperation. In either case, the key cell performancecharacteristic for determining its maximum pulsepower is its resistance R. The maximum usableconstant power for the cell can be determined fromits Ragone curve (Whkg�1 vs Wkg�1). Test datafor typical ultracapacitor cells indicate that theenergy density for a constant power discharge to1/2Vr at a power density equal to that for a 95%efficient pulse results in a 10% decrease in energydensity from the specified energy density of the cell(Wkg�1 5 200–300). Hence the useable pulsepower capability of a cell is significantly higherthan its constant power capability.

Ultracapacitor cells from various manufacturersworldwide have been tested at UC Davis [7–9]. Thetest results are summarized in Table I and test datafor a particular cell, the Maxwell 3000F device, aregiven in Table II. The performance of this cell istypical of commercially available carbon/carboncells with a useable energy density of 4.2Whkg�1

and a pulse power of 994Wkg�1 for 95% effi-ciency. As discussed in a later section of the paper,higher energy density and higher-power carbon/carbon cells are being developed, but they are notyet fully commercialized.

3.2. System sizing considerations

The weight and volume (kg and L) of anultracapacitor pack can be estimated with goodconfidence if the characteristics of the cells to beused in the pack are known from previous testing.

First it is necessary to calculate the kg and L ofthe cells. This can be done using the simplerelationship

WcellðkgÞ ¼ðWhÞstored=ðWhkg�1ÞcellVcellðLÞ ¼ðWhÞstored=ðWhL�1Þcell

The packaged weight and volume of the cellsare then calculated based on assumed values forthe packing factors (pfw for weight and pfv forvolume) for the modules.

Wmodules ¼Wcell ðkgÞ=pfw

Vmodules ¼Vcell=pfv

Reasonable values for the packing factors arepfw5 0.7 and pfv5 0.6.

The peak pulse power density (Wkg�1)cell at anyefficiency EFF can be calculated from the cell weight

ðW kg�1Þcell ¼ f9=16 ð1� EFFÞ V2r =Rg=wcell

where wcell is the weight of an individual cell (kg).The number Ncell of cells in the unit is de-

termined by dividing the system voltage Vsystem bythe useable rated voltage Vru of the cells.Ncell 5Vsystem/Vru rounded off to the nearest in-teger. As noted previously, it is common practice toset Vru slightly less than the rated voltage Vr of thecell in order to maximize the cycle life of the unit.Derating the cell voltage increases the number ofcells by the ratio Vr/Vru and decreases the useableenergy density and power density by the ratio(Vru/Vr)

2. For example, for Vr 5 2.7, Vru 5 2.5, thecell count would be increased by 8% and theenergy and power densities decreased by 17%.

4. VEHICLE APPLICATIONREQUIREMENTS

The energy storage requirements vary a great dealdepending on the type and size of the vehicle beingdesigned and the characteristics of the electricpowertrain to be used. Energy storage require-ments for various vehicle designs and operatingstrategies are shown in Table III for a mid-sizepassenger car. Requirements are given for electricvehicles and both charge-sustaining and plug-inhybrids. These requirements can be utilized to sizethe energy storage unit in the vehicles when the

ULTRACAPACITOR TECHNOLOGIES AND APPLICATION 135


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Table I. Summary of the performance characteristics of ultracapacitor devices.

Device V ratedC(F)

R(mO)

RC(s) Whkg�1*

Wkg�1

(95%)yWkg�1

match. imped.Wgt.(kg)

Vol.(L)

Maxwellz 2.7 2885 0.375 1.08 4.2 994 8836 0.55 0.414Maxwell 2.7 605 0.90 0.55 2.35 1139 9597 0.20 0.211ApowerCapy 2.7 55 4 0.22 5.5 5695 50 625 0.009 —Apowercapy 2.7 450 1.4 0.58 5.89 2574 24 595 0.057 0.045Ness 2.7 1800 0.55 1.00 3.6 975 8674 0.38 0.277Ness 2.7 3640 0.30 1.10 4.2 928 8010 0.65 0.514Ness (cyl.) 2.7 3160 0.4 1.26 4.4 982 8728 0.522 0.38Asahi Glass (propylenecarbonate)

2.7 1375 2.5 3.4 4.9 390 3471 0.210(estimated)

0.151

Panasonic (propylenecarbonate)

2.5 1200 1.0 1.2 2.3 514 4596 0.34 0.245

EPCOS 2.7 3400 0.45 1.5 4.3 760 6750 0.60 0.48LS Cable 2.8 3200 0.25 0.80 3.7 1400 12 400 0.63 0.47BatScap 2.7 2680 0.20 0.54 4.2 2050 18 225 0.50 0.572Power Sys. (activated carbon,propylene carbonate)y

2.7 1350 1.5 2.0 4.9 650 5785 0.21 0.151

Power Sys. (graphitic carbon,propylene carbonate)y

3.3 1800 3.0 5.4 8.0 486 4320 0.21 0.15

3.3 1500 1.7 2.5 6.0 776 6903 0.23 0.15Fuji Heavy Industry-hybrid (AC/graphitic Carbon)y

3.8 1800 1.5 2.6 9.2 1025 10375 0.232 0.143

JSR Micro (AC/graphiticcarbon)y

3.8 1000 4 4 11.2 900 7987 0.113 0.073

2000 1.9 3.8 12.1 1038 9223 0.206 0.132

�Energy density at 400Wkg�1 constant power, Vrated �1/2 Vrated.yPower based on P5 9/16�(1-EF)�V2/R, EF5 efficiency of discharge.zExcept where noted, all the devices use acetonitrile as the electrolyte.yall device except those with y are packaged in metal containers, these devices are in laminated pouches.

Table II. Test data for the 3000F Maxwell device.

Current (A) Time (s) Capacitance (F) Resistance (mO)

Constant current discharge data 2.7V–050 153.4 2869 Not calculate100 76.7 2883 Not calculate200 38 2900 0.375300 25 2885 0.333400 18.4 2886 0.40

Constant power discharges data 2.7–1.35V

Power (W) Wkg�1� Time (s) Wh Whkg�1

63 115 135.3 2.349 4.27102 186 82.7 2.332 4.24201 365 40.8 2.278 4.14301 547 26.5 2.216 4.03400 727 19.4 2.156 3.92500 909 15.1 2.097 3.81

�weight of device, 0.55 kg.

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characteristics of the energy storage cells areknown. In some of the vehicle designs consideredin Table III, ultracapacitors are used to providethe peak power rather than batteries. The objectiveof this paper is the evaluation of ultracapacitortechnology (present and future) to assess whetherthese vehicle applications of ultracapacitors ap-pear to be feasible and attractive.

For ultracapacitors, the key issue is the mini-mum energy (Wh) required to operate the vehiclein real world driving because the energy densitycharacteristics of ultracapacitors are such that thepower and cycle life requirements will be met inmost cases if the unit is large enough to meet theenergy storage requirement.

5. ULTRACAPACITOR TECHNOLOGIES

There are a number of approaches being pursuedto develop high energy density, high-power capa-citors suitable for use in vehicle applications.These approaches are identified in Table IV alongwith the basic chemistry/physics [10,11] of theenergy storage mechanisms, the materials used inthe active electrodes, cell characteristics, and theirpotential performance (energy density, power

density, etc.). Each of the capacitor types isdescribed briefly in the following sections.

5.1. Double-layer capacitors (carbon/carbon)

Most of the electrochemical capacitors currentlyon the market are termed electric double-layercapacitors (EDLC). Energy storage in double-layer capacitors results from charge separation inmicroscopically thin layers formed between asolid, conducting surface and a liquid electrolytecontaining ions. The dominant electrode materialis microporous, activated carbon [12,13]. Thedouble-layer is formed in the micropores of thehigh surface area carbon material. Either anaqueous or organic electrolyte can be used.Photographs of several commercially availabledevices are shown in Figures 1–3.

The performance of an electrochemical capa-citor is simply related to the characteristics of theelectrode material and the electrolyte used in thedevice. The relationship for the energy density(Whkg�1) can be expressed as

Whkg�1 ¼ 1=8 ðFg�1Þ � V20=3:6 ð1Þ

where F g�1 is the specific capacitance of theelectrode material and V0 is the cell voltage which

Table III. Energy storage unit requirements for various types of electric drive (mid-size passenger cars).

Type of electric drivelineSystem

voltage (V)Useable energy

storage

Maximum pulsepower at 90–95%efficiency (kW)

Cycle life(number of

cycles)

Useabledepth-of-discharge

Electric 300–400 15–30 kWh 70–150 2000–3000 Deep 70–80%Plug-in hybrid 300–400 6–12 kWh battery 2500–3500 Deep 60–80%

100–150Wh ultracapacitors 50–70Charge-sustaining hybrid 150–200 100–150Wh ultracapacitors 25–35 300K–500K Shallow 5–10%Micro-hybrid 45 30–50Wh ultracapacitors 5–10 300K–500K Shallow 5–10%

Table IV. Technology approaches for the development of high energy density electrochemical capacitors.

Technology typeElectrodematerials

Energy storagemechanisms

Cellvoltages

Energy density(Whkg�1)

Power density(kWkg�1)

Electric double-layer Activated carbon Charge separation 2.5–3 5–7 1–3Advanced carbon Graphite carbon Charge transfer or

intercalation3–3.5 8–12 1–2

Advanced carbon Nanotube forest Charge separation 2.5–3 Not known Not knownPseudo-capacitive Metal oxides Redox charge transfer 2–3.5 10–15 1–2Hybrid Carbon/metal oxide Double-layer/charge transfer 2–3.3 10–15 1–2Hybrid Carbon/lead oxide Double-layer/faradaic 1.5–2.2 10–12 1–2



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depends primarily on the electrolyte used in thedevice. The weight of the materials in the cell otherthan carbon are neglected in Equation (1). As

shown in Table I, the energy density of presentlyavailable carbon/carbon devices using an organicelectrolyte is 4–5Whkg�1. The carbons in thesedevices have a specific capacitance of about100F g�1 [12]. Large improvements in the energydensity of the carbon/carbon devices depends ondeveloping carbons with higher-specific capaci-tances of 150–200F g�1 and electrolytes that cantolerate higher voltages of 3–3.5V. These materialimprovements would result in cell energy densitiesof greater than 10Whkg�1.

The power characteristics of a cell are propor-tional to V2

0=R where R is the DC resistance of thedevice. Estimation of the resistance of a cell, in-cluding the contribution of ion diffusion in themicropores and the effects of current transients inthe electrodes, is not simple as shown in [14,15].However, a first approximation for the resistancecan be written as

R ¼ 2=3 t� relectrol:=Ax1rcontact=Ax ð2Þ

where t is the electrode thickness, relectrol’ is theresistivity (O cm) of the electrolyte, rcontact’ is thecontact resistivity (O cm2) of the carbon coating onthe metal current collector and Ax is the geometricarea of the electrode.

The key factor in achieving high power cap-ability is reducing the cell resistance. As shown inTable I, most of the presently available carbon/carbon cells have relatively low resistance withpower capability of about 1000Wkg�1 for 95%efficient pulses. A few of the cells have a powercapability in excess of 2500Wkg�1. It can be ex-pected that even higher power density capabilitywill be possible with the higher-specific capaci-tance carbons as that will permit the use of thinnercarbon coatings in the electrodes.

5.2. Pseudo-capacitors

In an EDLC, the active ions in the electrolyte arenot transferred onto or into the solid electrodesurface. If the ions in the double-layer aretransferred to the surface and combine with atomson the surface, the mechanism is termed ‘pseudo-capacitance’ [[14], Chapters 10–11]. Redox reac-tions are good examples of this process and metaloxides are good candidates for use in the electro-des of pseudo-capacitive devices. Equations (1)

Figure 1. Maxwell 3000F and 650F capacitors.

Figure 2. The NessCap 3000F capacitor.

Figure 3. The Batscap 2700F capacitor.

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and (2) can be used to estimate the characteristicsof these devices, but the specific capacitance of theelectrode materials used are significantly higherthan the microporous carbons. Research [11] isbeing done on devices using pseudo-capacitance,but such devices are not presently commerciallyavailable. No proto-types have yet been tested thatexhibited both energy density410Whkg�1 andpower density42000Wkg�1, 95% efficiency.Achieving high cycle life (4200 000 cycles) utiliz-ing pseudo-capacitance is also a concern.

5.3. Hybrid (asymmetric) capacitors

This category of electrochemical capacitors refersto devices in which one of the electrodes ismicroporous carbon and the other electrodeutilizes either a pseudo-capacitance material or aFaradaic material like that used in a battery. Thesedevices are often referred to as asymmetriccapacitors. The charge/discharge characteristicsof the hybrid capacitors have features of adouble-layer capacitor (a linear voltage vs timefor a constant current charge/discharge) and thatof a battery (voltage limits fixed by the potential ofthe battery-like electrode). As indicated in Table I,the energy density of the hybrid capacitorsutilizing intercalation carbon (graphite) in one ofthe electrodes is significantly higher than that ofthe carbon/carbon double-layer capacitors. How-ever, even though the power density of thosedevices is relatively high (about 1000Wkg�1,95%), the power capability has not increasedproportional to the increase in energy density.

6. ENERGY STORAGE COSTCONSIDERATIONS

Reducing the present high cost/price of EDLCs isa key issue in achieving high market penetration inthe future especially of mid-size and large devices.There are many applications for which EDLCs arepresently precluded or even seriously consideredbecause they remain too expensive even thoughtheir selling price has decreased significantly inrecent years. The cost of manufacture of anyproduct is closely tied to volume with the costdecreasing rapidly with increased volume up to

relatively high production rates. Potential sales ofEDLCs are in the many millions of units per year,so automated production facilities are necessary toreduce the unit costs to levels at which the largemarkets can develop. Semi-automated productionfacilities presently exist in a number of companiesfor EDLCs of all sizes. In fact, productioncapabilities exceed sales volumes for most devicesand that is the reason the price of devices hasdecreased markedly in recent years. It is commonto speak of the price of devices in terms of centsper Farad (cents F�1) or $Wh�1 stored. It is easierto interpret the price information on the cents F�1

basis as it does not concern the cell voltage or whatfraction of energy stored can be used in aparticular application. For example, for a 10Fdevice, if the price is quoted as 10 cent per Farad,the device cost would be $1. Similarly, a 2500Fdevice would cost $25 at 1 cent F�1.

The cost to manufacture an EDLC (carbon/car-bon) device depends on the material and productioncosts. At the present time, material costs are high.The cost of carbon suitable for use in EDLCs can beas high as $100kg�1 with the average price being inthe $30–$50kg�1 range. The cost of the electrolytesolvent is also high in the range of $ 5–10L for bothpropylene carbonate and acetonitrile. The ionic saltsthat dissociate in the solvent into the positive andnegative ions that move into and out of the double-layer in the microporous carbon to store the energyare also expensive being $50–$100kg�1. As theanalysis of EDLCs is straightforward, material costscan be calculated [16,17] with good accuracy. Theresult of a typical costing exercise is shown inTable V. Note the strong dependency of the cen-tsF�1 and $Wh�1 unit costs for the device on theunit material costs. Presently the price of EDLCs ishigh because both the material and manufacturingcosts are high. With more automated productionand reduced material costs, it is anticipated that theprice of ECCs in high volume can be in the range of1–2centsF�1 for small devices and 0.25–0.5 cen-tsF�1 for large devices like those needed for vehicleapplications.

EDLCs cannot compete with batteries in termsof $Wh�1, but they can compete in terms of$ kW�1 and $ unit�1 to satisfy a particular vehicleapplications. Both energy storage technologies



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must provide the same power and cycle life andsufficient energy (Wh) for the application. Theweight of the battery is usually set by the systempower requirement and cycle life and not theminimum energy storage requirement. Satisfyingonly the minimum energy storage requirementwould result in a much smaller, lighter batterythan is needed to meet the other requirements. Onthe other hand, the weight of the EDLC is de-termined by the minimum energy storage require-ment. The power and cycle life requirements areusually easily satisfied. Hence, he EDLC unit canbe a more optimum solution for many applicationsand its weight can be less than that of the batteryeven though its energy density is less than one-tenth that of the battery.

Consider the example of a charge-sustaininghybrid like the Prius. If the energy stored in theEDLC unit is 125Wh and that in the battery unitis 1500Wh, the unit costs of the capacitors andbattery are related by

ð$Wh�1Þcap ¼ 0:012ð$ kWh�1Þbat

The corresponding capacitor costs in terms ofcents F�1 and $ kWh�1 are given by

ðcents F�1Þcap ¼ 0:125 � 10�3�ð$ kWh�1Þbat�V2cap

ð$ kWh�1Þcap ¼ 9:6 � 104ðcents F�1Þcap=V2r

and in Table VI for a range of battery costs.The results shown in Table VI indicate that for

the charge-sustaining hybrid application, EDLCcosts of 0.5–1.0 centsF�1 are competitive with li-thium battery costs in the range of $500–700kWh�1.Note also that the $kW�1 cost of the EDLCs areabout one-fourth those of the batteries.

7. COMPARISONS OF ULTRACAPACITORSAND BATTERIES

As discussed in [18,19], cells and modules ofseveral lithium-ion battery chemistries have beentested in the laboratory at UC Davis. Theperformance characteristics of the lithium-ion batteries are summarized in Table VII. Alsoshown in the table are the characteristics of other

Table VI. Relationships between capacitor and battery costs.

Battery cost$ kWh�1

Batterycost� $ kW�1

Ultracapcost cents F�1

Vcap 5 2.6

Ultracapcost cents F�1

Vcap 5 3.0

Ultracap costy

$ kWh�1

Vcap 5 3.0

Ultracapcost $ kW�1

Vcap 5 3.0

300 30 0.25 0.34 3626 7.3400 40 0.34 0.45 4800 9.6500 50 0.42 0.56 5973 11.9700 70 0.59 0.78 8320 16.6900 90 0.76 1.0 10 667 21.31000 100 0.84 1.12 11 947 23.9

�Battery 100Whkg�1, 1000Wkg�1.yCapacitor 5Whkg�1, 2500Wkg�1.

Table V. Material costs for a 2.7V, 3500F capacitor.�

CarbonElectrolyte ACN

Device unit costsF g�1 gCdev.�1 $ kg�1 $L�1 $ kg�1 salt Total mat. $ $ kg�1 $Wh�1 $ kW�1 Ct. F�1

75 187 50 10 125 17.0 29 6.4 29 0.48120 117 100 10 125 15.5 26 6 26 0.4475 187 5 2 50 3.6 6.0 1.3 6 0.10120 117 10 2 50 2.5 4.2 0.93 4.2 0.070

�4.5Whkg�1, 1000Wkg�1 95% eff.

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batteries for comparison. The energy and powerdensities of the various batteries vary over a widerange and indicate clearly the trade-offs betweenenergy and power capabilities in various batterydesigns. The consequences of these trade-offs formeeting energy storage requirements will bediscussed in the next section where combinationsof batteries and ultracapacitors are considered.

The power to energy ratio (P/E) of the batteries isalso given in Table VII. The power and energycharacteristics of batteries and ultracapacitors arecompared in Table VIII. The P/E ratio for thecapacitors is at least a factor of ten higher thanthat of batteries with the factor increasing ingeneral as the energy density of the batteriesincreases.

Table VII. Performance characteristics of various batteries.

Battery wgt kg/Ah kg�1 R (mO) Whkg�1 Wkg�1 90% Wkg�1 80% P/E 90% P/E 80%

ChemistryIron phosphateEIG 15/0.424 2.5 115 897 1585 7.8 13.8A123 2.1/0.07 12 88 1132 2000 12.9 22.8K2 2.5/0.082 17 86 682 1205 7.9 14.0

Lithium titanateAltairnano 12/0.34 2.2 70 693 1225 9.9 17.5Altairnano 3.8/0.26 1.1 35 2260 4020 64.5 115EIG 11/0.44 1.9 43 620 1100 14.4 23.8

Li(NiCo)O2

EIG 18/0.45 3.0 140 913 1613 6.5 11.6GAIA 42/1.53 0.48 94 1677 2965 17.8 31.5Quallion 1.7/0.047 70 170 374 661 2.2 3.9Quallion 1.3/0.043 59 144 486 860 3.4 6.0

NiMt hydridePanasonic. HEV 6.5/1.04 11.4 46 393 695 8.5 15.1EV 65/11.5 8.7 68 87 154 1.3 2.3

Lead-acidPanasonic HEV 25/11.4 7.8 26 146 258 5.6 9.9EV 60/21.0 34 89 157 2.6 4.6

Zn-AirRevolt Technology 10/--- 450 200 0.5–1.0 1–2

Pmax 5Eff. (1- Eff.) (Voc)2/R, P/E5 (Wkg�1)/Whkg�1.

Table VIII. Comparisons of the energy and power characteristics of ultracapacitors and batteries.

Device technologyNominal

cell voltage Whkg�1 Wkg�1 90% P/E 90% P/E 80%

Carbon/carbon supercapacitors 2.7 5 2500–5000 500–1000 1000–2000Hybrid carbon supercapacitors 3.8 12 1635 140 280Lithium-ion batteriesIron phosphate 3.25 90–115 700–1200 8–13 14–23Lithium titanate 2.4 35–70 700–2260 10–65 18–115NiCoMnO2 3.7 95 1700 19 34

3.7 140 500 3.5 6.23.7 170 400 2.4 4.3

Ni Mt hydride HEV 1.2 46 400 8.6 15Lead-acid HEV 2.0 26 150 5.6 10Zn-air 1.3 450 200–400 0.5–1.0 0.9–1.8



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8. COMBINATIONS OFULTRACAPACITORS AND BATTERIES

It has been recognized for many years thatcombining ultracapacitors and batteries wouldsignificantly reduce the stress on the batteries invehicle applications in which the batteries aresubject to high current pulses in both charge anddischarge. It is further recognized that to gainmaximum advantage from this arrangement wouldrequire the use of interface electronics to controlthe currents from the battery. There has been somelaboratory testing of this arrangement [20,21], butlittle work directly with vehicles. In general,experience to date has been that if batteries wereavailable that could meet both the energy andpower requirements of the vehicle design, thedesigners chose to use batteries alone even thoughthey realized batteries plus ultracapacitors wouldhave some advantages. In other words, designerswill not select a battery/capacitor combinationunless there are clear, large advantages to do so.As discussed in the following paragraphs, this maybe the case when one considers the use of advancedbatteries (Whkg�14200) in PHEVs and EVs.

In PHEVs and EVs, it is desirable for the bat-tery to be sized by the energy needed to sustain aspecified all-electric range. In that case, the weightand volume of the battery pack would follow di-rectly from its energy density (Whkg�1, WhL�1).This means that battery technologies with highenergy density will be strongly favored. However,the batteries must also be able to meet the powerrequirements of the large electric motors usedin the PHEVs and EVs. Unfortunately batteriesdesigned to attain maximum energy density in

most cases require a sacrifice in power capability asshown in Table VII. As a consequence, for somevehicle designs the battery will be sized by the powerrequirement and not the energy requirement result-ing in a larger and more expensive battery thanwould be the case if the battery had a higher powerdensity. Design options using batteries of variousenergy densities are shown in Table IX for a PHEVwith all-electric ranges of 10–40miles. The effect ofelectric motor size (50, 70kW) on the requiredpower densities are also shown in Table IX. Notethat for the shorter all-electric ranges and a batteryenergy density of 200Whkg�1, the power densitiesrequired exceed by a considerable margin those ofthe batteries shown in Table VII. In those cases, itmakes sense to consider combining batteries withultracapacitors. This design option is shown inTable X. Note that the combination of the200Whkg�1 battery and the carbon/carbon ultra-capacitors results in the lowest weight energy sto-rage unit for all the PHEV ranges even for a 50kWelectric motor. It can be expected that the weightadvantage of the combination will be even larger forbatteries with an energy density greater than200Whkg�1. These results indicate that combiningbatteries and ultracapacitors will become increas-ingly advantageous as designers consider using themore advanced batteries with higher energy density.

9. COMPUTER SIMULATIONS OF HYBRIDAND ELECTRIC VEHICLES

Simulation of the operation of hybrid vehiclesequipped with various alternative powertrainsand energy storage technologies (nickel metal

Table IX. Battery sizing and power density for various ranges and motor power.

Battery 200Whkg�1 100Whkg�1 70Whkg�1

Rangemiles

kWh�

neededkWhy

stored kgy50 kW

kWkg�170 kW

kWkg�1 kg50 kW

kWkg�170 kW

kWkg�1 kg50 kW

kWkg�170 kW

kWkg�1

10 2.52 3.6 18 2.78 3.89 36 1.39 1.94 51 0.98 1.3715 3.78 5.4 27 1.85 2.59 54 0.92 1.30 77 0.65 0.9120 5.04 7.2 36 1.39 1.94 72 0.69 0.97 103 0.49 0.6830 7.56 10.8 54 0.93 1.30 108 0.46 0.65 154 0.32 0.4640 10.1 14.4 72 0.69 0.97 144 0.35 0.49 206 0.24 0.34

�Vehicle energy useage from the battery: 250Whmiles�1.yUseable state-of-charge for batteries, 70%; weights shown are for cells only.

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hydride and lithium-ion batteries and ultracapa-citors) can be performed using the UCDavis versionof Advisor. The following alternative hybrid power-train arrangements have been modeled:

a. Single-shift, parallel (Honda)b. Single-planetary, dual-mode (Toyota/Prius)c. Multiple-planetary, dual-mode (GM)d. Multiple-shaft, dual-clutch transmission (VW

and Borg-Warner)e. Series–range extended EV (GM Volt)

The results of the simulations for selected casesare discussed in this paper, but more completeresults are given in [22–24]. This paper will focuson the use of ultracapacitors for energy storageand will be concerned primarily with the fueleconomy gains that can be achieved utilizing asawtooth control strategy that optimizes theefficiency of the engine. Most of the simula-tions are performed using the single-shaft, parallelarrangement because that arrangement is closestto the standard driveline and leads to satisfactoryvehicle operation even when the ultracapacitorenergy storage is depleted.

The sawtooth strategy has essentially twomodes–charge depleting (operation in the electricmode with the engine off) and recharging (engineon at relatively high power to recharge the ultra-capacitor or battery). In the charge-depletingmode, system efficiency is maximized by relying onthe electric drive, which is inherently efficient; inthe recharge mode, the engine runs in the mostefficient region (torque and speed) of the engine

map. In this mode, the engine both recharges theultracapacitors and provides power to drive thevehicle. The ultracapacitors are also rechargedduring regenerative braking. With these twomodes, the engine can be run at its most efficientstates while keeping the energy storage within agiven SOC range. The electric drivelines and ul-tracapacitor units utilized in the simulations forvarious designs are shown in Table XI. Ultra-capacitor units were envisioned using both thecarbon/carbon and hybrid carbon technologies.

Simulations of mid-size passenger cars using theultracapacitors in micro-hybrid, charge-sustaining,and plug-in hybrid powertrain designs were per-formed using the Advisor vehicle simulation pro-gram. All the powertrains were in the same vehiclehaving the following characteristics: test weight1660 kg, Cd 5 0.3, AF 5 2.25m2, RRCF5 0.009.The engine map used in the simulations was for aFord Focus 2L, 4-cylinder engine. The rated en-gine power was 120 kW for the conventional ICEvehicle and the micro-hybrid and 110 kW forthe charge-sustaining and plug-in hybrids. All thehybrids use the single-shaft approach similar to theHonda Civic hybrid. The same induction electricmotor map was used for all the vehicle designs.

The simulation results are summarized inTable XII for a conventional ICE vehicle and eachof the hybrid designs. Results are given for fuelusage in terms of both L 100 km�1 and mpg forvarious driving cycles. It is clear from Table XIIthat large improvements in fuel usage are pre-dicted for all the hybrid powertrains using ultra-capacitors for energy storage. The simulation

Table X. Storage unit weights using a combination of batteries and ultracapacitors for various all-electric ranges and70 kW power.

Whkg�1 5 200 100 70

Rangemiles

Ultracap(kg)�

Battery(Kg)y

Combination(kg)

Battery(kg)

Combination(kg)

Battery(kg)

Combination(kg)

10 20 18 38 36 56 51 7115 20 27 47 54 74 77 9720 20 36 56 72 92 103 12330 20 54 74 108 128 154 17440 20 72 92 144 164 206 226

�The carbon/carbon ultracapacitor unit stores 100Wh useable energy.yWeights shown are for cells only; packaging into modules not included.



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results will be discussed separately for each hybriddesign.

9.1. Micro-hybrids

The results for the micro-hybrids are particularlyinteresting and surprising, because of the large fueleconomy improvements predicted. These improve-ments were about 40% on the FUDS and ECE-EUDcycles and 20% on the Federal Highway and US06cycles using the carbon/carbon ultracapacitor units.The improvements were significantly less using the

hybrid carbon units because of their lower round-tripefficiencies. In the micro-hybrid designs, the ratedengine power used was the same as that in theconventional ICE vehicle in order that the perfor-mance of the hybrid vehicle when the energy storagein the ultracapacitors is depleted would be the same asthe conventional vehicle. The ultracapacitors wereused to improve fuel economy with only a minimalchange in vehicle acceleration performance. Thecontrol strategy used was to operate on the electricdrive when possible and to recharge the ultracapaci-tors when the engine was operating. As shown in

Table XI. Ultracapacitor units for hybrid vehicle applications.

Vehicle design

Ultracapenergy stored

(Wh)

Ultracappeak power

(kW)

systemvoltage(V)

No. ofcells Capacitance (F)

Max.power 90%eff. (kW)

Weight (kg)/volume (L)

cells unit�

Micro-hybrid 30 6 48Carbon/carbon 18 2550 410 6/4.6 9/9.2Hybrid carbon 14 2000 5.7 2.8/1.8 4.3/3.6Charge sustaininghybrid

150 35 200

Carbon/carbon 80 2865 450 30/23 46/46Hybrid carbon 65 2000 26 13/9 20/18Plug-in Hybrid12 kWh bat.y

200 45 300

Carbon/carbon 120 2200 4100 40/30 61/60Hybrid carbon 84 2000 36 18/12 28/24

�Packaging factors: weight, 0.65 volume, 0.5.yEnergy density of the battery in the Plug-in hybrid—200Whkg�1, Vehicle electric use 156Whkm�1.

Table XII. Summary of the vehicle fuel economy simulation results using ultracapacitors for various driving cycles.

Driveline typeEnergy

storage typeVoltage and

weight cells (kg)EM Peak(kW)

L 100 km�1mpg�1

FUDS HWFET US06 ECE-EUDC

ICE baseline 10/23.8 6.9/34.4 9.6/24.7 9.7/24.6Micro-HEV Lead-acid/ultracaps 48

Carbon/carbon 6 kg 6 5.7/41.7 5.3/44.7 7.8/30.6 5.9/40.2Hybrid carbon 3 kg 6 7.3/32.8 6.3/38.0 8.9/26.7 7.1/33.4

Charge sustaininghybrid

Ultracaps 200

Carbon/carbon 30 kg 35 5.4/43.8 5.0/47.9 7.1/33.6 5.5/43.2Hybrid carbon 13 kg 35 5.8/40.9 5.2/45.8 8.0/29.9 5.8/41.3

Plug-in hybrid. 12 kWh Li battery(200Whkg�1) andultracaps

300 70 kW with45 kW

from capsCarbon/carbon 40 kg 45 5.5/43.2 5.0/47.7 7.0/33.9 5.5/42.9Hybrid carbon 18 kg 45 5.8/41.2 5.2/46.2 7.9/30.2 5.8/41.2

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Figure 4, this resulted in a large improvement inaverage engine efficiency from 19% in the ICE vehicleto 30% in the micro-hybrid even though the electricmotor had a peak power of only 6kW.

Additional computer simulations were made forhigher motor power (up to 12 kW) and larger ul-tracapacitor energy storage (up to 50Wh). It wasfound that the improvements in fuel economy wereonly marginally greater. Using a motor power of3 kW reduced the fuel economy improvement onthe FUDS by more than 50%. Note fromTable XII that the fuel economy improvementsusing the carbon/carbon ultracapacitors were for

all the cycles greater than those using the hybridcarbon devices. This was the case because theround-trip efficiencies for the carbon/carbon unitswere 95–98% and those of the hybrid carbon unitswere 75–90% for the various driving cycles. Asnoted previously, the hybrid carbon devices hadhigher energy density, but even though their powerdensity for 95% efficiency was relatively high(1050Wkg�1), it was not proportionally higher—that is twice as high—as the carbon/carbon deviceswith lower energy density. These results showclearly that it is essentially to develop high energydensity ultracapacitors with proportionally higherpower density; otherwise their use in vehicle ap-plications will be compromised.

9.2. Charge-sustaining hybrids

The fuel economy simulation results for charge-sustaining hybrids are also shown in Table XII for amid-size passenger car using both carbon/carbonand hybrid carbon ultracapacitors. Using thecarbon/carbon ultracapacitor unit, the fuel savingsare about 45% for the FUDS and ECE-EUD cyclesand about 27% for the Federal Highway and US06cycles. These improvement values are higher thanfor the micro-hybrid, but not by as large a factor asmight be expected. The prime advantage of thehigh-power electric driveline in the charge-sustain-ing hybrid is that it yields large fuel economyimprovements even for high power requirementdriving cycles like the US06. The fuel economyimprovements using the hybrid carbon ultracapa-citor unit are not much less (5–10%) than those withthe carbon/carbon unit even though the round-tripefficiency of the hybrid carbon unit is only 85–90%compared with 98% for the carbon/carbon unit. Asthe weight/volume of the hybrid carbon unit isrelatively small �43% of that of the carbon/carbonunit, it appears that the charge-sustaining hybridapplication is a better one for the hybrid carbontechnology than the micro-hybrid application.

9.3. Plug-in hybrids

The plug-in hybrid vehicle studied is one thatutilizes a high energy density battery (200Whkg�1)and ultracapacitors that would provide two-thirdsof the power to a 70 kW electric motor. The electric

Figure 4. A comparison of engine efficiencies for aconventional ICE vehicle and a micro-hybrid on the

FUDS cycle using carbon/carbon ultracapacitor.



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energy use of the vehicles in the charge-depletingmode (engine off) is assumed to be 156Whkm�1

resulting in a charge-depleting range of 60 km(38 mi.) for 80% DOD of the battery. The fueleconomy results shown in Table XII are for vehicleoperation in the charge-sustaining mode after theenergy battery (12kWh) has been depleted. Asexpected in this mode, the operation of the plug-inhybrid vehicle is essentially the same as previouslydiscussed for the charge-sustaining hybrid. Hencethe hybrid carbon ultracapacitor unit would alsobe suitable for the plug-in hybrid. The combinedweight of the cells in the battery and hybrid carbonultracapacitors would be 78kg for a plug-in hybridwith an all-electric range of about 40miles. Thecombined weight using the carbon/carbon ultra-capacitors would be 100kg. Using a high-powerlithium-ion battery with an energy density of100Whkg�1 without ultracapacitors, the weightof the battery cells alone would be 120kg. Hencefor plug-in hybrids combining a battery withultracapacitors is an attractive design option.

9.4. Series–range extended EV

This hybrid vehicle is essentially an electric vehiclewith on-board electricity generation via an engine-powered generator or a fuel cell for range extension.In the case of the GM Volt, it is intended to beutilized as a plug-in hybrid with much of theelectricity used provided by a relatively large battery.It could, however, be used as a charge-sustaining

hybrid using a smaller battery. Battery-poweredand series hybrid vehicles are modeled/simulatedat UC Davis using SIMPLEV, which is a vehiclesimulation program developed at the IdahoNational Engineering Laboratory [25]. Compo-nent efficiency maps are available in SIMPLEV fora wide variety of engines, electric, and lithium-ionbatteries. The available control strategies permitthe simulation of series hybrids as plug-in andcharge-sustaining hybrids. Simulations were per-formed using the SIMPLEV program for serieshybrid vehicles for comparison with battery-powered (BEV) and parallel hybrid vehicle results[26]. The electric drivelines of the series hybridvehicles would be the same as that of the battery-powered vehicles except that the batteries storedonly 40% of the energy in the BEVs. The engine/generator power was selected such that the vehicleshad acceptable steady gradeability on generatorelectricity alone. Ultracapacitors could be used inthe plug-in, series hybrids if the batteries wereunable to provide the peak power to the electricmotor. As discussed previously, this would becomemost likely if the plug-in range was relatively shortand/or high energy density batteries of modestpower density were being used in the vehicle.In those cases, the ultracapacitors would greatlyreduce the power demands on the battery and leadto less stress on the battery and longer cycle life.

Simulation results for series hybrids are sum-marized in Table XIII for a mid-size passenger carand SUV.

Table XIII. Summary of the vehicle characteristics and simulation results.

VehicleTest weight

(kg)Engine/generator

(kW)Battery(kWh)

FUDS(mpg)

Highway(Mpg)

Mid-size carSeries HEV� 1830 40y 10 40 47CS HEV 1640 36 44Conventional ICE 1640 20 32Mid-size SUVSeries HEV� 2150 55z 13.3 29 31CS HEV 1910 28 32Conventional ICEy 1910 16 25

�All-electric range of 30miles, lithium-ion batteries–120Whkg�1.yElectric motor power 105 kW.zElectric motor power 145 kW.yAll the vehicles have the same acceleration performance (0–60mph in 9 s).

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As plug-in hybrids, the series hybrids have anall-electric range of about 30miles if the batteriesare discharged to 80% of their rated capacity. Thesimulation results indicate that the fuel economyof the series hybrid is slightly higher than that ofthe parallel charge-sustaining hybrid when bothare operated in the charge-sustaining mode. Theengine/generator was sized such that the serieshybrids were full-function vehicles. Thus, all thehybrid vehicles including the series hybrids in theall-electric mode have performance equivalent tothe conventional ICE vehicle.

9.5. Fuel cell vehicles

Ultracapacitors can be used to meet the peak powertransients in vehicles powered by a fuel cell. This wasinitially done by Honda in their ‘FCV’ fuel cellvehicle [27]. The capacitors were used to load levelthe fuel cell during accelerations and to recoveryenergy during regenerative braking. In the Hondasystem, the capacitors were used without interfaceelectronics. Connecting the fuel cell and capacitorsin parallel without electronics functions better thanis the case with batteries because the V vs I curve ofthe fuel cell is inherently more sloped than that of abattery. Hence as the ultracapacitor is dischargedand its voltage decreases, the fuel cell will providehigher power which either drives the vehicle orrecharges the capacitors. Hence the system isinherently self-controlling to a large extent.

A fuel cell connected in parallel with ultra-capacitors in a mid-size SUV (test weight 1960kg) hasbeen simulated using a special version of the SIM-PLEV program available at UC Davis. In the simu-lation, the fuel cell is modeled as a battery whose V,I curve is independent of SOC. The rest voltage andresistance are based on the V, I curves of state-of-theart fuel cells (Ballard or UTC) [28]. The resistance ofthe equivalent battery is specified to yield a fuel cellwith the desired rated power at a selected cell voltage.In this analysis, the cell voltage for rated power wastaken to be 0.65Vcell�1. The fuel cell power used was50kW for the vehicle with capacitors and 100kW forthe vehicle without capacitors. The energy stored inthe capacitors was 300Wh and the system voltagewas 300V. The electric motor had a power of 100kWfor both vehicles.

The simulation results for the power profiles ofthe fuel cell and fuel cell/capacitor-powered ve-hicles are given in Figures 5 and 6. The presence ofthe capacitors significantly reduces the power de-mand on the fuel cell even with the capacitorsconnected in parallel. The maximum fuel cellpower on the FUDS was 30.2 kW with the capa-citors and 51.9 kW in the case of the fuel cell alone.The average fuel cell power was the same for bothcases. As expected, the capacitors provide most ofthe power when their voltage is relatively high andthe fuel cell provides high power when the capa-citors become significantly discharged. The fuelcell rapidly recharges the capacitors when thepower demand is reduced. If interface electronicsare used to control the current from the fuel cell, itis possible to maintain the fuel cell operation at anear constant power [29]. This is probably thepreferred arrangement of fuel cell and ultra-capacitors, but it is more expensive. In their pre-sent fuel cell vehicle, the Clarity, Honda utilizes alithium-ion battery and interface electronics toload level the fuel cell [30].

10. VEHICLE PROJECTS USINGULTRACAPACITORS

There have been a number of demonstrationprojects of hybrid–electric vehicles using EDLCs[31–38]. Most of these projects have involvedtransit buses and trucks, but a few have involved

Figure 5. Fuel cell alone on the FUDS cycle.



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passenger cars. In some instances, the capacitorshave been used in conjunction with batteries—lead-acid or nickel cadmium. In all cases, theultracapacitors used were of the carbon/carbontype.

10.1. Transit buses

In the United States, the company must active inutilizing EDLCs in hybrid—electric powertrainsfor buses and large trucks has been the ISECorporation in San Diego, CA [31,32]. ISE hasdeveloped a 360V capacitor unit consisting of 1442600F Maxwell cells connected in series (seeFigure 7). The weight and volume are 114 kg and189L, respectively. The unit stores 0.325 kWh ofenergy (.245 kWh useable). In a transit bus, two of

the units are used in series resulting in a voltage of720V and energy storage of 0.650 kWh. The peakpower capability of the combined unit is over300 kW. ISE utilizes this EDLC unit with a225 kW electric motor in series hybrids usinggasoline and diesel engines and hydrogen fuelcells. As the capacitor unit stores only about0.5 kWh, it can provide power only during vehicleacceleration and recover energy during brakingand the engine or fuel cell must provide all thepower during cruise and high climbing. ISE hasbuilt over 100 buses using the EDLC energystorage units for transit companies in SouthernCalifornia. The buses are in daily revenue service.Fuel economy records indicate that the hybridbuses using ultracapacitors achieve 25–30% betterfuel economy than the diesel-powered buses andconsistently better fuel economy than hybrid busesusing batteries [31]. The measured round-tripefficiency of the ultracapacitor unit was 94%.

10.2. Passenger car

A recent passenger car project involving ECCs in ahybrid–electric driveline is discussed in [33,34].The project was termed ‘SUPERCAR’ and wasfunded by the European Community (EC). It wasa joint project between EPCOS, the EDLC

Figure 6. Fuel cell and ultracapacitor in parallel withoutinterface electronics on the FUDS cycle.

Figure 7. The ISE ECC unit (360V, .325 kWh).

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developer, and Siemens VDO, the vehicle inte-grator. The parallel hybrid passenger car (VWGolf) combined an EDLC module and lead-acidbattery into a 42V, 10 kW (peak) electric drivelinewith a 66 kW engine. The vehicle was tested onboth the chassis dynamometer and the road. Thetests showed a 16–18% improvement in fueleconomy compared with the standard ICE car.

The capacitor unit consisted of 24 3600F cellsconnected in series (see Table I for the cell char-acteristics). The rated voltage of the unit is 60Vand its capacitance and resistance are 150F and8mO, respectively. The unit is shown in Figure 8.

Additional passenger car projects are discussedin [38–40].

11. SUMMARY AND CONCLUSIONS

This paper is concerned with both the require-ments for energy storage in various types ofelectric and hybrid vehicles and the characteristicsof the energy storage devices being developed. Thepaper focuses on ultracapactors (electrochemicalcapacitors) as energy storage in vehicle applica-tions and thus evaluates the present state-of-the-art of ultracapacitor technologies and theirsuitability for use in electric and hybrid drivelinesof various types of vehicles.

A key consideration in determining the applic-ability of ultracapacitors for a particular vehicleapplication is the proper assessment of the energystorage and power requirements. For hybrid–electric vehicles, the key issues are the useableenergy requirement and the maximum pulse power

at high efficiency. For a Prius size vehicle, if theuseable energy storage is about 125Wh and nee-ded efficiency is 90–95%, the test data and analysisshown in this paper indicate that vehicles can bedesigned using carbon ultracapacitors (both car-bon/carbon and hybrid carbon) that yield high fueleconomy improvements for all driving cycles andthe cost of the ultracapacitors can be competitivewith lithium-ion batteries for high volume pro-duction and carbon prices of less than $20 kg�1.The use of carbon/carbon devices in micro-hybridsis particularly attractive for a control strategy(sawtooth) that permits engine operation nearits maximum efficiency using only a 6 kW electricmotor. Vehicle projects in transit buses andpassenger cars using ultracapacitors have shownthat the ultracapacitors have functioned as ex-pected in the vehicles and significant fuel economyimprovements have been achieved that arehigher than would have been possible using bat-teries because of the higher round-trip efficienciesof the ultracapacitors (lower losses in chargingand discharging the energy storage unit).Ultracapacitors have particular advantages foruse in fuel cell powered vehicles in which it islikely they can be used without interfaceelectronics.

Carbon/carbon ultracapacitors are presentlyavailable from a number of companies. The useableenergy density of these devices is about 4.5Whkg�1

and the power density of a 95% efficient pulse isabout 1000Wkg�1. A limited number of carbon/carbon devices with a high-efficiency power densityof greater than 2000Wkg�1 have been tested.Development of hybrid carbon devices is continuingshowing energy densities of 12Whkg�1 and a high-efficiency power density of about 1000Wkg�1.Vehicle simulations using those devices have shownthat increased power capability in such devices isneeded before full advantage can be taken of theirincreased energy density compared with carbon/carbon devices in some vehicle applications. Energystorage system considerations indicate that combi-nations of ultracapacitors and advanced batteries(Whkg�14200) are likely to prove advantageous inthe future as such batteries are developed. This islikely to be the case in plug-in hybrids with high-power electric motors for which it may be difficult to

Figure 8. The EPCOS 60V ECC module (150F, 56Wh).



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limit the size and weight of the energy storage uniteven using advanced batteries.

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DOI: 10.1002/er

Ultracapacitor technologies and application in hybrid and electric vehicles

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