Modeling Grid-Connected Hybrid Electric Vehicles Using ADVISOR
NREL/CP-540-30601Modeling Grid-Connected Hybrid Electric Vehicles Using ADVISORTony Markela and Keith WipkeaNational Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401aABSTRACTThe overall system efficiency of a hybrid electric vehicle is highly dependent on the energy management strategy employed. Inthis paper, an electric utility grid-connected energy management strategy for a parallel hybrid electric vehicle is presented.ADVISOR was used as a modeling tool to determine the appropriate size of the hybrid components and the energy managementstrategy parameter settings. Simulation results demonstrated that with this strategy it is possible to achieve double the fueleconomy of a comparable conventional vehicle while satisfying all performance constraints. In addition, the final vehicle designprovides an all-electric range capability in excess of 20 miles.IntroductionHybrid electric vehicles are under development today byvarious manufacturers. These vehicles are currentlymarketed as one way to improve the efficiency of ourtransportation system and to help reduce our dependence onand consumption of foreign petroleum. Engineers at theNational Renewable Energy Laboratory (NREL) with thesupport of the US Department of Energy (DOE) havedeveloped a software tool to help engineers in theautomotive industry make educated design decisions duringthe early stages of development of new hybrid electricvehicles. Typically, there are multiple designs that maymeet or exceed the perceived demands of the customer. Thissoftware package, called ADVISOR, allows users to quicklymove through the initial stages of vehicle design.ADVISOR has been built in the Matlab/Simulinkcomputing environment and is freely available via theInternet (http://www.nrel.gov/transporation/analysis). Theprogram evaluates the performance of a vehicle in acombined backward-forward facing approach (1). On a timebasis, the program calculates what is required from eachcomponent, working backwards through the vehicle fromthe wheels to the powerplant, in order for the vehicle tofollow the desired speed trace. As the requirements arepassed from one component to the next, performance limitsare enforced. On the forward path, the performance of thedownstream components is updated based on limitsenforced in upstream components. This approach simplifiesthe calculation process and eliminates the need to iterativelysolve at each time step. The disadvantage in this approach isthat it is difficult to generate true control algorithms that canbe carried directly to a finished product.Existing hybrid electric designs can be broken into threebasic categories, 1) series, 2) parallel, and 3) combinedseries/parallel (2). The vehicle is characterized by theconnection of the various components within the vehicleand the energy flow pathway. A series hybrid consists of apowerplant providing electricity (i.e. internal combustionengine (ICE)/generator combination, or fuel cell system) toa battery pack. The vehicle is then propelled by an electricdrive motor. The powerplant is not coupled directly to thewheels and can run in its most efficient operating region. Ina parallel hybrid, the powerplant (ICE) and the electricmotor can both provide power to the driveline in parallel.This design provides a direct mechanical path for powerdelivery between the engine and the wheels. A combinedseries/parallel hybrid, like the Toyota Prius, exhibits someof the characteristics of both parallel and series hybrids.The vehicle configurations can then be grouped into subcategories by the vehicle energy management strategy. Acommon energy management strategy employed today is acharge-sustaining strategy. In this case, the state of charge(SOC) of the battery pack will be maintained by the onboard powerplant as necessary. An alternative approach is acharge-depleting or grid-connected strategy (3,4). Thisstrategy relies mainly on grid electricity to charge thebattery pack while the vehicle is not in use. It attempts tofully utilize the capabilities of both the battery pack and theon-board powerplant. While in use, the vehicle will use thebattery pack and electric motor alone to propel the vehiclewhen it is most efficient to do so. The on-board powerplantis used as the primary power source only when it would notbe an effective use of battery power (i.e. high-speedoperation) or to maintain the state of charge of the batterypack. The advantages of this strategy include its ability touse off-peak electricity and to provide emissions freeoperation for extended periods. It is possible that a largeportion of normal driving could be covered all electricallywith this approach. A major drawback of this strategy is thecost and weight penalties incurred due to a large electricdrive system.NREL recently participated in the Hybrid Electric VehicleWorking Group (HEV/WG) by providing modeling supportin the analysis of the potential of hybrid electric vehicles,including grid-connected hybrids. The HEV/WG focused on3 vehicle classes; small car, mid-size car, and sport utilityvehicle and 4 vehicle designs; conventional (CV) andparallel hybrids with minimal (HEV0), 20 miles (HEV20),60 miles (HEV60) of all-electric range capability (5). TheCopyright 2001 IEEE. Reprinted from “Sixteenth Annual Battery Conference on Applications and Advances: Proceedings of the Conference 9-12 January2001, Long Beach, California.” This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEEendorsement of any of NREL’s products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish thismaterial for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writingto pubs-permissions@ieee.org. By choosing to view this document, you agree to all provisions of the copyright laws protecting it.
HEV/WG was lead by the Electric Power Research Institute(EPRI) with representation from government, the autoindustry, the utility industry, and academia.depleting and a charge-sustaining strategy. Figure 1 showsgraphically how this biasing is applied based on batterypack state of charge while simulating vehicle operation overeight Urban Dynamometer Drive Schedules (UDDS). Asthe SOC falls, usage of the engine increases while usage ofthe electric motor decreases, thus reducing the rate ofdecrease of the SOC.As a result of NREL’s participation in the HEV/WG severalmodel enhancements were made to ADVISOR to providethe capability to model grid-connected hybrid electricvehicles. This paper will provide an overview of a gridconnected energy management strategy as modeled usingADVISOR 3.0. It will also highlight the flexibility of such avehicle. It should be noted that the results published here arebased on knowledge obtained through participation in theHEV/WG. However, the results presented in this paperconstitute an entirely separate study with a smaller scope.Table 1: Energy Management Strategy ParametersNamecs lo soccs hi soccs electric launch spd lom/scs electric launch spd him/s6040302000200040006000800010000Electric Launch Speed (m/s).Vehicle Speed (mph)--This biasing within the strategy is achieved through anengine on/off state computer and logic to determine theamount of power to request from the engine when it is on.The main parameters used to implement this control logic inADVISOR for charge-depleting hybrid electric vehicleshave been detailed in Table 1.120001SOC (--)Nmcs min trq fracThe energy management strategy of a hybrid electricvehicle is extremely important. It defines how and whenenergy and power will be provided or consumed by thevarious components within the vehicle. In a grid-connectedhybrid electric vehicle the strategy will attempt to bias theenergy flows towards battery pack usage while the packexhibits a high state of charge. As the state of charge of thepack begins to fall, the strategy will bias the energy usagemore towards the engine in order to maintain state of chargein the pack and to prevent pack damage and reduced cyclelife. This strategy has characteristics of both a charge-Descriptionlowest desired SOC ofbattery packhighest desired SOC ofbattery packload applied to the engineto charge/discharge thebatteries based on SOCfraction of maximumengine torque above whichengine must operate ifSOC cs lo socvehicle speed below whichvehicle attempts to run allelectrically at low SOCvehicle speed below whichvehicle attempts to run allelectrically at high SOC--cs charge trqEnergy Management Strategy0.50Motor Torque (Nm) Engine Torque (Nm)Units--020004000600080001000012000150100Engine On15cs electric launch spd loEngine May Be Off105cs hi 80.91State of Charge (--)12000100Figure 2: Vehicle Electric Launch Speed EnergyManagement Strategy0-10020cs lo soc500cs electric launch spd hi250200040006000Time (s)800010000Figure 2 provides a graphical representation of the electriclaunch speed control strategy parameter application. Basedon the current SOC and vehicle speed the engine state canbe determined. Above the solid line the engine will be onwhile below the solid line the vehicle will attempt to run allelectrically.12000Figure 1: Vehicle Operation Using Grid-ConnectedEnergy Management Strategy on Repeated UDDS2
The following basic engine state computer has beenimplemented in the charge-depleting strategy in ADVISOR:Table 2: Vehicle AssumptionsNameAerodynamic drag coefficientCoefficient of rolling resistanceFrontal areaWheel radiusVehicle glider mass1Average electrical accessory loadAverage DC/DC converter efficiencyAverage air conditioning load (SC03 only)Engine must be on if,1) motor/battery power is insufficient to meet thedriver demandEngine can be off if,1) vehicle speed is less than electric launch speed2) driveline torque demand is negative (i.e.deceleration event)Units--m2mkgW%WTable 3: Performance ConstraintsAttributeGradeability@ 50 mph for15 min.@ 30 mph for30 min.Acceleration0-60 mph50-70 mphZEV RangeTop SpeedTrace missCycle chargesustaining SOCFigure 3 depicts the engine load modification strategygraphically. When the engine is on, the torque requested ofthe engine by the driveline may be modified based on thebattery pack SOC to provide more or less power whichenforces battery charging or discharging, respectively. Thestrategy employed is a simple linear model which requestscs charge trq when SOC cs lo soc, –cs charge trq whenSOC cs hi soc, and interpolates at SOC’s within thisrange. This load is in addition to the driveline load. Finally,a minimum engine torque fraction may be enforced if theSOC falls below the cs lo soc setting.ValueSpecial Conditions7.2 %1)2)1)2)7.2 %9.5 s5.1 s40 miles 90 mph 2 mph 20%initial SOC charge sustaining SOCfinal SOC 20%initial SOC charge sustaining SOCfinal SOC 20%1) initial SOC charge sustaining SOCcomposite of city and highway operationUDDS, HWFET, US06, and SC032UDDS, HWFET, US06, and SC03In Table 3, the terms initial SOC, final SOC, and chargesustaining SOC have been introduced. The initial SOC isthe state of the battery pack at the start of the test and thefinal SOC is the state at the end of the test. The chargesustaining SOC is the SOC at which the vehicle whendriven over a typical drive cycle will start and end at thesame state. In this analysis the charge-sustaining SOC wasbetween 0.2 and 0.3 depending on the drive cycle.Wide Open Throttle (WOT)cs min trq frac * WOTEngine gn Process13The following steps define the design process employed:1) Select baseline components2) Resize components as necessary based onperformance constraints3) Optimize control strategy parameters for fueleconomy and electric range on drive cycles.Engine SpeedFigure 3: Engine Load Modification Strategy, 1)Drivetrain Load, 2) Modified Load @ SOC cs lo soc, 3)Modified Load @ SOC cs hi soc, 4) Modified Load @SOC cs lo socTable 4: Hybrid Vehicle Base ComponentsVehicle Assumptions and ConstraintsIn this study the performance constraints and the vehicleassumptions will be the same as those used in the study bythe HEV/WG for the mid-size vehicle which were based onthe conventional vehicle. Table 2 details the basic vehicleassumptions while Table 3 defines the performanceconstraints used in this SOR FilenameFC CI67 emisMC PM49Battery PackESS 45NIMH Ovonic1TX 5SPD CIDescriptionVolkswagen 1.9 L TDI (67 kW)Honda EV Plus 49 kWPermanent Magnet Motor5-speed manual geared for CIenginesOvonic 45 Ah NiMH ModuleVehicle glider mass is equal to vehicle curb weight minus powertrainmass.2UDDS Urban Dynamometer Driving Schedule; HWFET HighwayFuel Economy Test; US06 high speed aggressive driving cycle, SC03 extreme thermal load driving schedule. These cycles are used or will beused in federal procedures in the near future to quantify the fuel economyand emissions performance of vehicles.3
In this study the components detailed in Table 4 wereselected from the ADVISOR database as the baselinecomponents. The 1.9 L Volkswagen engine was selectedbecause it is a fairly modern diesel engine with a highresolution data set collected by Oak Ridge NationalLaboratory. The Honda EV Plus motor was selectedbecause it carries with it a high degree of confidence. Hondaengineers using ADVISOR contributed this data set. It isappropriate technology (permanent magnet) for thisapplication since the vehicle will operate as an electricvehicle during a large portion of typical driving. Finally, theOvonic 45 Ah NiMH modules were selected based on theirperformance specifications of 67 Wh/kg and 550 W/kg asquoted by Ovonic (3). The high specific energy of thesemodules should lead to significant all-electric range for asmall weight penalty.If an electric drivetrain efficiency of 250 Wh/mi is assumedthen to achieve 40 miles of all electric operation, a packwith 10 kWh of useful energy is required. Assuming thatonly 75% of the pack capacity is usable on a daily basisthen a 13.3 kWh pack results. Using the Ovonic 45 Ahmodules this results in a pack consisting of 22 modulesconnected in series. This pack, capable of 88 kW at 50%SOC, will increase the base vehicle mass by 185 kg. Thispack is considerably larger than those used in productionhybrids today (Toyota Prius 1.8 kWh, Honda Insight 0.9kWh). It is likely that this pack will cost 5-10 times that ofpacks in current hybrids simply based on rated capacity.Drive Cycle Operation and Motor SizeIn this study, the motor size was primarily defined by thedrive cycle operation from a high SOC. Secondly, it shouldbe matched to the battery pack capabilities. For the currentFederal Test Procedure standards, the vehicle must not missthe speed trace by more than 2 mph at any time. It wasassumed that these standards would carry over to theadditional drive cycles that compose the SAE J1711Recommended Practice (6). For this vehicle to follow all ofthe acceleration events below the vehicle electric launchspeed in the US06 cycle it required an electric motor of 73.5kW. To achieve this power level it was assumed that theelectronic controls of the motor could be modified to allowshort duration operation of 50% higher than its continuousoperating region. As a result, it was not necessary to scalethe base electric motor. This effectively allows the electricmotor to operate at higher load fractions and thus higherefficiency a majority of the time.These base components may not be just the right size toprovide the desired vehicle performance but they exhibit thestate of the art performance characteristics desired in thisstudy. Using ADVISOR the base components will belinearly scaled as necessary to satisfy the design objectives.To determine the appropriate component sizes for thisvehicle many performance aspects must be evaluatedsimultaneously. These include, Acceleration Gradeability Drive cycle operation from a high SOC (EV mode) Drive cycle operation from a low SOC (hybridmode) Electric rangeDrive Cycle Operation and Engine SizeWith the electric components sized, the minimum enginesize to satisfy grade, acceleration, and drive cyclerequirements was determined. A small engine is importantto allow significant gains in operating efficiency while incharge-sustaining mode. In this analysis, operating in thecharge-sustaining mode on the US06 cycle was the activeconstraint that determined the engine size. A 38 kW enginewas required to maintain the state of charge of the batterypack. The mass and torque capability of the base engine wasscaled linearly to satisfy this requirement. In combinationwith the electric components, this engine size produced avehicle that exceeded the grade and accelerationperformance constraints.Electric Range and Battery Pack SizeIn this study all electric range (AER) is assumed to endwhen the engine first turns on during a drive cycle.3 Inaddition, a second parameter, defined as EV miles, will becalculated. This second parameter is the sum of all milesdriven with the engine off. All of these miles can not becounted as emissions free miles because at some point afterthe end of the AER, the engine is on. Once the engine hasbeen used to propel the vehicle, it has provided kineticenergy to the vehicle. Some of this energy will at somepoint during the cycle be collected via regenerative brakingand stored in the battery for future use. In addition, some ofthe battery energy may have come directly from the enginevia the charge maintenance algorithm. As a result, thisenergy stored in the battery can be associated at least in partto engine operation and emissions production. Thus thepropulsion energy can no longer be considered emissionsfree even though the vehicle is propelled electrically.Table 5 summarizes the final component sizes based onenforcement of all of the active vehicle performanceconstraints.3This assumption is based on current test procedures of Society ofAutomotive Engineers (SAE J1711, 1999) and the California AirResources Board (CARB, 1999).4
attempt to modify the engine load to maintain the state ofcharge in the battery pack while preventing inefficientengine operation. The cs charge trq parameter is slightlymore flexible because it provides the ability to discharge thepack as desired and it is functional at all states of chargerather than just below the low SOC setting. Thecs min trq frac parameter is useful for preventing very lowengine load points. Therefore, cs min trq frac was set to0.1 (i.e. 10% engine load). To provide charge-sustainingoperation on the US06 drive cycle, cs charge trq was set to10 Nm. However, it was also determined that to provideacceptable charge-sustaining operation on the SC03 cycle,the cs charge trq parameter would need to be 25 Nm. Itwas assumed that the on-board vehicle computer would beintelligent enough to adjust this charge maintenanceparameter based on knowledge that the air conditioningsystem is in use.Table 5: Final Vehicle Component CharacteristicsParameterEngine Peak PowerMotor Peak PowerBattery Pack Capacity4Battery Pack Power5Vehicle MassVehicle Test ametric Study on Energy Management StrategyWith the vehicle components defined, the next step was toevaluate the energy management strategy options. This wasaccomplished using ADVISOR’s built-in parametric studycapabilities.First, desirable values of cs charge trq and cs min trq fracwere determined. Refer to Table 1 and Figure 3 fordescriptions of these parameters. Both of these parameters151581.528.321.815.2cs electric launch spd lo (m/s)cs electric launch spd lo (m/s)83.634.810531.62518.511.9 8.660.50.679.3107570.7572.8UDDS Fuel Economy (mpgge)68.55.3All Electric Range (mi)00.477.10.70.8cs hi soc (--)0.900.41Figure 4: All Electric Range after Completion of 60Miles of UDDS Operation150.50.60.70.8cs hi soc (--)0
Energy Management Strategy The energy management strategy of a hybrid electric vehicle is extremely important. It defines how and when energy and power will be provided or consumed by the various components within the vehicle. In a grid-connected hybrid electric vehicle the strategy will attempt to bias the energy flows towards battery pack .
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Keywords: hybrid electric vehicle simulation, hybrid vehicles, modeling and simulation, physics-based modeling Cite This Article: Essam M. Allam, “Study Power Management of Hybrid Electric Vehicle Using Battery Model Simulation.” Advances in Powertrains and Automotives, vol. 1, no.
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