Inductive Charging of Ultracapacitor Electric Bus - EVS-24

The World Electric Vehicle Journal, Vol 2, September 2008

© 2008 WEV Journal, pp. x-x

1© 2008 WEV Journal, pp. x-x

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Battery-electric buses are widely recognized as themost environmentally benign of all the alternatively-fueled public transportation options, even whenemissions from the power plants that provide theelectricity to recharge the batteries are included in theanalysis [1]. This is true of both regulated (“criteria”)emissions and greenhouse gas emissions. Electricpropulsion is also the only vehicle propulsion technologythat can directly utilize clean and renewable energysources such as solar and wind power. Battery electricbuses have additional benefits such as quiet operationand zero tailpipe emissions, highly desirable attributesfor urban, campus, and medical center applicationsinvolving substantial pedestrian activity and/or noise-and emission-sensitive populations.

In addition to the well-documented environmentalbenefits, battery electric buses also have lower fuel costsand offer the potential for favorable maintenance costsas compared with diesel and other fuels. A recentFederal Transit Administration report to Congressindicates that battery electric buses require only aboutone-third the fuel cost per mile than the next cheapestpropulsion strategy (catenary electricity for trolleybuses), and only about one-quarter the fuel cost asdiesel propulsion; even greater fuel cost savings accrue

when compared with other alternative fuels [2]. Thesame study also states that “No fuel considered in thisstudy has lower maintenance costs than diesel . . .Electric-drive buses have fewer drive train andtransmission maintenance requirements but mayrequire costly battery replacements. Maintenance costsvary widely for electric drive buses . . .” [3].

Given a battery electric bus’s favorable environmentalbenefits, low noise, zero local emissions, low fuel cost,and potential for low maintenance costs, it is somewhatsurprising that they have not enjoyed widespreadcommercialization. Battery electric buses have not beensuccessfully deployed in most applications because thevarious shortcomings of the battery-electric bus aresignificant. Virtually all of the associated problems,however, are directly related to deficiencies in batterytechnology:

1. Low range between recharges;2. Relatively low battery cycle life expectancy

necessitates at least one replacement of the expensivebattery system during the service life of the bus;

3. Poor operational reliability of electric busesprimarily arises from battery systems beingundercharged, out of balance, or containing deterioratedcells that interfere with the proper function of theremaining healthy cells;

4. Substantial maintenance required to keepbattery systems in top operational form (e.g., frequentload testing, cell balancing, and cell replacement);

5. Reduced capacity of most batterychemistries during periods of cold ambient

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Paul Griffith,* Dr. J. Ronald Bailey,* Dan Simpson*6

Many public transit agencies are exploring ways in which alternative fuels can be utilized to reduce our nation’sdependency on imported oil and to avoid the environmental impacts associated with the combustion of petroleumproducts. Of all the alternative fuel options, electricity is among the most desirable in terms of affordability, availability,security, and sustainability. Unfortunately, limitations in battery technology result in electric vehicle operating rangesthat are much lower than those of internal combustion-powered vehicles. Although opportunity charging is one strategyfor increasing the driving range of electric buses by means of a series of brief recharges during normally scheduledmidday layovers, it has yet to be embraced by the transit industry, in part because of the difficulties involved in havingdrivers connect and disconnect the bus from the charger at regular intervals throughout the day. Roadway-mountedinductively-coupled power transfer systems effectively remove this constraint by automating the charge process, therebyobviating the need for driver intervention. Ultracapacitor energy storage systems offer advantages over battery systemsfor such applications.

KKeeyywwoorrddss:: Ultracapacitor, Opportunity Charging, Inductive Charging, Transit, Electric Bus.

*Advanced Technologies for Transportation Research Program University of Tennessee at Chattanooga615 McCallie Avenue, Chattanooga, TN 37403Fax:+1-423-425-5464

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temperatures;6. Inaccurate state-of-charge gauges (imagine

driving an automobile containing an undersized fueltank that did not have an accurate fuel gauge).

Although recent battery developments have producedincremental improvements in battery energy density,thereby enabling modest improvements in vehiclerange, such advances have generally come at increasedcost and do not remedy the other shortcomings notedabove (new concerns have also arisen such as somerelating to safety). Modified operational strategies,such as increased frequency inductive charging, canimprove daily range capabilities without relying onbattery advances. But the disadvantages associatedwith available and near-term battery technologiespersist, representing a considerable impediment toefforts to expand the applicability of battery electrictransit.

Aside from batteries, two other electricallyrechargeable energy storage systems exist that mayhave applicability to pure-electric buses: flywheelsystems and ultracapacitors. Of these, ultracapacitorsare farther along in development with products alreadyhaving been installed in a number of buses, albeit ascomponents of hybrid-electric and diesel-electricsystems rather than in pure-electric buses.

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Ultracapacitors (also referred to as supercapacitors)are electrochemical capacitors that have unusually highenergy densities when compared with commoncapacitors; they are of particular interest in automotiveapplications for hybrid vehicles and as supplementarystorage for battery electric vehicles [4]. Although theyexhibit numerous advantages as compared with tractionbatteries, their gravimetric energy density is only about10% that of lead-acid batteries, heretofore renderingthem unsuitable as a primary energy storage elementin electric vehicles. A Ragone plot illustrating energydensity vs. power density for various energy storagedevices is shown in Figure 1 [5].

Energy density, however, may be the only parameter

in which batteries exhibit an advantage overultracapacitors. Areas of advantage for ultracapacitorsas compared with battery systems include the following:

1. Very high rates of charge and dischargeresulting in fast recharge times, adequate accelerationpower in relatively low energy systems, and excellentacceptance of regenerative braking;

2. Little degradation over hundreds ofthousands of cycles resulting in exceptional cycle life;

3. Essentially maintenance-free operation;4. Excellent reliability over a wide

temperature spectrum and harsh environments;5. Low toxicity of materials used;6. High cycle efficiency (95% or higher).

As a result, ultracapacitors would be an excellentsubstitute for batteries in pure electric vehicles if notfor their relatively poor energy density. This soleshortcoming could be circumvented, however, in anapplication that entailed high frequency, short duration,automated recharging. Certain public transitapplications demonstrate favorable operationalcharacteristics, particularly those involving low-speedshuttle operations in which the bus traverses arelatively short loop, returning to the same stops on afrequent, recurring basis. The installation at one ofthose stops of a roadway-mounted, automatic,inductively-coupled charging station (which does notrequire driver intervention to accomplish recharge)would complete the essential attributes of a pureelectric vehicle system that might effectively utilizeultracapacitors in lieu of batteries for its sole energystorage medium.

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Energy storage characteristics of interest to theelectric bus industry include specific energy (the energycapacity of a battery per unit weight), cost, and cyclelife. These parameters are summarized in Table 1 forthe battery chemistries and ultracapacitors presently inuse with (and now under consideration for) electricbuses [6].

The reader may note from Table 1 that althoughultracapacitors have a high acquisition cost per kWh ofrated capacity as compared with batteries, theirextremely long cycle life more than compensates,resulting in relatively low life-cycle costs (~$0.03/mi ascompared with ~$0.24/mi for the low-cost lead-acidbattery). Coupled with their high reliability andvirtually maintenance-free operation (occasional serviceonly required for the integral cooling fans), anultracapacitor-powered pure electric bus promises torealize the full cost savings potential of electric buses.

In order to charge a lead-acid battery system at a 60-kW rate, 60-kWh of capacity is required in order toavoid exceeding the 1C charge rate [7]. (Fast chargingtends to result in accelerated heating of batteries, a



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Figure 1. Ragone Plot

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condition that should be minimized because elevatedbattery temperature can result in accelerated internalcorrosion and decreased battery longevity.) Arepresentative 60-kWh lead-acid battery system has apower rating of approximately 300 kW. By comparison,a 2.6-kWh rated ultracapacitor system has a powerrating of approximately 2,700 kW. The nearly ten-foldimprovement in power rating attendant to theultracapacitor system means that frequent rechargescan occur without excessive heat generation and thattotal recovery of regenerative braking energy ispossible, unlike with a typical battery system which isestimated to accept only about 50% of the availableregenerative braking energy.

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Although opportunity charging is one strategy forincreasing the driving range of pure electric buses bymeans of a series of brief recharges during normallyscheduled midday layovers, it has yet to be embraced bythe transit industry, in part because of the difficultiesinvolved in having drivers connect and disconnect thebus from the charger at regular intervals throughoutthe day. An inductively-coupled charging systemeffectively removes this constraint by automating thecharge process, thereby obviating the need for driverintervention.

An inductive charge system is a contactless powertransfer system that allows electrical energy to besupplied to buses without any electrical or mechanicalcontact. Each system consists of two parts (a primaryand secondary) that are magnetically coupled similar to

a conventional transformer. The primary (stationary)side of a 60-kW system consists of a 70-kW track supplyand two 30-kW primary coils imbedded in the street atthe opportunity charge station (typically a bus stop at ornear the end of the line). The secondary (vehicle) sideconsists of two 30-kW pickups and rectifiers installedonboard the vehicle. Unlike a conventional transformer,where primary and secondary coils are tightly coupled,the inductive charge system is a loosely coupled system.Power can be transferred across air gaps of severalcentimeters. Having no physical contact, operation inharsh environments becomes possible. Power transferis not affected by concrete, asphalt, or other non-permeable materials.

Major components for a typical 30-kW system aredepicted in Figures 2 and 3. A 60-kW systemcontemplated for bus applications involves the use oftwo stationary primary coils and two vehicle-mountedsecondary pickups per bus.

When an electric bus pulls into the charging station,the positioning system checks whether the bus iscorrectly positioned over the charging platform. Ifproper positioning is confirmed, a signal is sent to thetrack supply to automatically start the chargingprocess. If the bus is not correctly positioned, the tracksupply will not switch on. Other objects, such as people,animals, cars, trucks, or other buses cannot cause theactivation of the charging station. Termination of thecharging process can be executed after a pre-programmed time (e.g., 2 minutes), by the bus driver, orautomatically before the bus leaves the chargingplatform.



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Lead-Acid Ni-Cd Zebra Li-Ion UltracapacitorSpecific Energy ~35 Wh/kg ~50 Wh/kg ~107 Wh/kg ~110 Wh/kg ~3 Wh/kgAcquisition Cost1 ~$120/kWh ~$740/kWh ~$700/kWh ~$1,000/kWh goal ~$33,240/kWhCycle Life2(nameplate cycles)3 ~500 NPC ~1,200 NPC ~1,000 NPC >1,000 NPC >1,000,000 NPCBattery Life-Cycle Cost4 ~$0.24/mi ~$0.62/mi ~$0.70/mi ~$1/mi goal ~$0.03/mi

Table 1. Battery and Ultracapacitor ComparisonSources: Exide, Saft, MES-DEA, Santa Barbara MTD, Maxwell

1 Battery pricing is dependent on volume and currency exchange rates in the case of imported products.2 Cycle life based on field experience and is highly dependent on duty cycle, operating temperature, charge profile, maintenance, etc.3Nameplate cycle: The process of one complete or multiple partial discharges of a battery equaling the battery’s rated energy (kWh)capacity, and subsequent recharge(s).4 Assuming 1.0 DC kWh/mi energy usage rate for a 6.7-m (22-ft) electric bus; excluding differences in maintenance costs.

Figure 2. Major Components for Typical 30-kW Inductive Charge System, Block Diagram

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Four candidate electric bus configurations of varyinglengths were evaluated for potential deployment on fourcandidate routes at a major US university campus withrespect to the prospects for successful utilization ofinductive charge technology [8]. The smaller buses havethe lowest energy use requirements and thereforerequire the least time connected to the charge systemto replenish the energy expended during each loop.Although the most promising route has a high energyconsumption rate per mile driven because of its highstop-start frequency, its short loop length results inrelatively short recharge times required at the end ofeach loop (less than two minutes for a 22-ft busoperating without air-conditioning). The operatingparameters for a 22-ft. electric bus running without air-conditioning on the most favorable route are presentedin Table 2.

It is believed that a 1-minute, 20-second recharge atthe conclusion of each nine-minute loop does notrepresent an unreasonable layover period. Given anominal 1.24-kWh energy consumption expectation foreach loop, an ultracapacitor system with a higherenergy capacity is desirable in order to account fornormal variations in duty cycle and driver performance,as well as for energy capacity degradation over time.Assuming a 30% variance between average energyconsumption and maximum consumption, and a 20%capacity loss, an ultracapacitor system with an energycapacity of approximately 1.24-kWh x 1.3 x 1.2 = 1.93kWh would be required. (An additional safety margin isbuilt into the energy use rate estimation because anultracapacitor-powered bus is expected to have greaterregenerative braking energy recovery than the batterybuses upon which the empirical data is developed.)

It should be noted that this analysis assumes that only

one charging station is installed on the route. Ifmultiple charging stations were deployed it wouldenable one or more of the following: a longer route, asmaller ultracapacitor system, and/or shorter rechargetimes at each station.

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Critical to the success of the contemplated project isthe identification of a well-integrated ultracapacitorsystem. One candidate system would appear to be theHeavy-duty Transportation Module (“HTM”)manufactured by Maxwell Technologies. Maxwell’s390V HTM module is depicted in Figure 4 and issummarized in Table 3 [9].

In order to achieve the 1.93-kWh deliverable energyrequirement identified in the previous section, sevenMaxwell 390V HTM modules would be required (7 x 282Wh = 1.97 kWh). These seven modules would yield asystem mass of 1,155 kg (2,541 lbs.), or about one-halfthe weight of a lead-acid battery system of appropriatecapacity to accept a 60-kW recurring charge.

Assuming six recharge events per hour (one every tenminutes), ten hours of bus service per day, five days perweek, and 50 weeks per year, the ultracapacitor systemwould experience a total of 15,000 cycles per year. Thisamounts to approximately 200,000 cycles of theultracapacitor over a 12-year bus life expectancy, well



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Figure 3. Major Components for Typical 30-kW Inductive Charge System, Installed

Loop Length Loop Duration Energy Use Rate Energy Used Charge Time Required1(mi) (min) (kWh/mi) (kWh) (min sec)1.3 9 0.95 1.24 1 min 20 secTable 2. Operating Parameters for 22-ft Electric Bus w/o A/C on Campus Route

1Assumes 95% round-trip energy efficiency through ultracapacitor.

Figure 4. Maxwell 390V HTM Module

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short of the 1,000,000 projected cycle life and suggestivethat ultracapacitor reliability would be extremely highduring the operational life of the bus.

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Although the ultracapacitor system underconsideration has adequate energy to propel the bus onthe route circuit, it is unlikely that the limited rangewould be adequate for deadhead travel and/or travel tomaintenance facilities. If the vehicle cannot be storedand maintained in close proximity to the route, therange limitation could be overcome through the use of atrailer containing a small battery system. The SantaBarbara MTD has used such a system to power anelectric bus between its overnight facility and the routelocation (Figures 5 and 6). Upon reaching the route, thetrailer is uncoupled and the bus performs its servicewithout the trailer; upon conclusion of service, thetrailer is again connected to the bus whereupon itpowers the bus back to the depot and the batteries inthe trailer are recharged in the conventional overnightmanner.

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Battery electric buses are widely regarded as the mostenvironmentally benign of all the alternatively-fueledpublic transportation options, and offer additionalbenefits such as quiet operation, zero-tailpipe emissions,low fuel costs, and the potential for favorablemaintenance costs. They have not, however, enjoyedwidespread commercialization because of a number ofoperational and maintenance shortcomings that are theresult of battery technology deficiencies. Althoughrecent battery developments have produced incrementalimprovements in energy density, high costs andpersistent maintenance issues associated with thesetechnologies continue to limit the successful deploymentof battery electric buses.

The use of ultracapacitors in lieu of batteries in a pureelectric bus would appear to circumvent most of theshortcomings associated with electric buses, namelybattery system reliability and battery maintenancerequirements. Furthermore, ultracapacitors are moreeffective at capturing regenerative braking energy,thereby improving the efficiency of the system. The onlyrelative disadvantage of ultracapacitors is their lower



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Nominal Operating Voltage 390 VdcMaximum Operating Voltage 394 VdcSurge Voltage 406 VdcEnergy Available

(to ½ nominal voltage) 282 WhMaximum Continuous Current 150 AMaximum Intermittent Current 950 ACycle Life (to 98% DOD) 1,000,000Operating Temperature Range -40C to +65C (-40F to +117F)Maximum Ambient Operating

Temperature +50C (+122F)Environmental Protection IP65 (dust tight, water-jet protection)Mass 165 kg (363 lbs)Dimensions

(with fan shroud assembly)1200 mm L x 740 mm W x 299 mm H (47.3” x 29.2” x 11.8”)

Cost $12,500 Table 3. Maxwell 390V HTM Module

Figures 5 and 6. Battery Trailer - Rear and Front Quarter-Panel Views

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energy density which results in reduced vehicle rangebetween recharges; this can be mitigated, however, bythe use of an automated inductive charging systeminstalled in the roadway at one or more bus stops. Iftravel to storage or maintenance facilities cannot beaccomplished with the ultracapacitor system, a trailer-mounted battery system could be utilized. In additionto improved vehicle reliability, an ultracapacitor electricbus would have considerably lower life cycle costs thanits battery-powered counterpart as a result of reducedmaintenance costs and lower energy storage systemcosts.

RREEFFEERREENNCCEESS

[1] P. Griffith. Status of U.S. Battery-Electric BusOperations. Proceedings of Electric Vehicle Symposium22, Advanced Transportation Technology Institute(ATTI), December 2006.[2] U.S. Department of Transportation, FederalTransit Administration. Alternative Fuels Study: AReport to Congress on Policy Options for Increasing theUse of Alternative Fuels in Transit Vehicles. December2006.[3] Ibid.[4] http://en.wikipedia.org/wiki/Supercapacitor[5] Ibid.[6] Griffith, op. cit.[7] Approximately five times the overnight rate.[8] P. Griffith. Preliminary Technical Assessment ofInductively Charged Electric Bus for Application atUniversity Campus. Advanced TransportationTechnology Institute (ATTI), January 26, 2007.[9] Maxwell Technologies, Doc. # 1011285 Rev.2

AAUUTTHHOORRSS

Paul Griffith, National ProjectsManager, ATTRP, University ofTennessee at Chattanooga,[email protected]. Mr. Griffithhas spent twenty-seven years inengineering research and development

activities with a number of organizations in thetransportation and aerospace industries. He wascofounder and President of both the Santa BarbaraElectric Transportation Institute and the SantaBarbara Electric Bus Works, and has also worked withthe Santa Barbara MTD, Patec Corporation, Oerlikon-Contraves of Switzerland, and General DynamicsCorporation, Convair Division. Mr. Griffith received hisBachelor of Science degree in Mechanical Engineeringfrom the University of California at Santa Barbara. Heis past Chair of the National Electric VehicleInfrastructure Working Council, is the author ofnumerous technical reports and articles coveringelectric-bus and other alternative fuel technologies, andholds five U.S. and foreign patents for aerospaceproducts currently under manufacture in Europe.

Dr. J. Ronald Bailey is the GuerryProfessor in the College of Engineeringand Computer Science at the Universityof Tennessee at Chattanooga.Immediately prior to joining UT

Chattanooga, Dr. Bailey served four years as F.W. OlinProfessor and Dean of the College of Engineering atFlorida Institute of Technology. Dr. Bailey also servedas a member of the faculty at N.C. State University forten years where he taught and served as principalinvestigator for several research projects. At theUniversity of Texas at Arlington, Dr. Bailey served asHead of the Mechanical & Aerospace Engineering, Deanof the College of Engineering and Vice President for fouryears. Dr. Bailey’s experience in the private sector ishighlighted by his twelve years with IBM Corporationwhere he held a number of engineering andmanagement positions, each with increasingresponsibility, culminating in the position of productmanager for the Industrial Sector Division’s Boca RatonDevelopment Laboratory. Dr. Bailey earned bachelorsand master’s degrees in engineering from NorthCarolina State University where he was inducted intothe honorary societies of Pi Tau Sigma, Tau Beta Pi, andPhi Kappi Phi. He earned a Ph.D. from the Institutefor Sound and Vibration Research at the University ofSouthampton, England.

Dan Simpson serves as the ChiefResearch Scientist with the AdvancedTechnologies for TransportationResearch Program (ATTRP) at theUniversity of Tennessee at Chattanooga(UTC). Previously, Mr. Simpson heldthe position of Director of Laboratories

at the Florida Institute of Technology where he wasresponsible for managing multiple budgets, a technicalservices group, engineering projects, technicaloperations and technical support for 160 Laboratories,including the university machine shop and electronicservices. Mr. Simpson also has 10 years of professionalengineering and management experience as amanufacturing, project, industrial, and design engineerin the aerospace industry working for Boeing, MartinMarietta, NASA and Delevan Gas Turbine products. Mr.Simpson has an undergraduate degree in IndustrialTechnology from the University of Idaho and continuingprofessional education with a number of universities,colleges, private businesses and the military.



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Inductive Charging of Ultracapacitor Electric Bus - EVS-24

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