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A U.S. Department of EnergyOffice of Science LaboratoryOperated by The University of Chicago

Argonne National Laboratory

Office of ScienceU.S. Department of Energy

Optimization of Fuel Cell Vehicle Fuel Economy

Sponsored by Lee Slezak (U.S. DOE)

Aymeric RousseauPhil Sharer

Rajesh AhluwaliaArgonne National Laboratory

2Pioneering Science andTechnology

Office of ScienceU.S. Department

of Energy

Fuel Cell Vehicle Fuel Economy Optimization

Study Scope Hybridization Degree Energy Storage Technology Control Strategy Perspectives



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FreedomCAR FCV Energy Storage Proposed Goals Spring 2003

FreedomCAR Goals Low Power High Power Characteristics Units Energy Storage Energy Storage

Pulse Discharge Power (10s) kW 25 50 Max Regen Pulse (5s) kW 30 60 Total Available Energy kWh 1.5 3 Round Trip Efficiency % >90 >90 Cycle Life Cyc. TBD (15 year life equiv.) TBD (15 year life equiv.)Cold-start at -30C (TBD kW for TBD min.) kW 5 5 Calendar Life Yrs 15 15 Max Weight kg 40 65 Max Volume liters 32 50 Production Price @ 100k units/yr $ 500 1,000 Maximum Operating Voltage Vdc /= 0.5 x Vmax Maximum Self Discharge Wh/d 50 50 Operating Temperature C -30 to +52 -30 to +52 Survival Temperature C -46 to +66 -46 to +66



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Structure of the Study

SUV

Car Future FC

HotCurrent FC

Mid-Term FC

HybridizationDegrees

FUDS

FHDS

US06 Cold

AmbESS

Technologies

ControlStrategies



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Fuel Cell HEV Configuration

DC Link



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Major Assumptions Vehicle and Performance

- Mid-size SUV (Explorer, Durango, Blazer)- Target 0-60 mph acceleration in 10.2 s - 55 mph at grade of 6.5% continuous (a least 20 minutes) - Top speed of 100 mph

Fuel Cell System Requirements - Fuel cell should be sized to provide a least power for top speed and grade

performance- FCS must have 1-s transient response time for 10% to 90% power.- FCS should reach maximum power in 15 s for cold start from 20C ambient

temperature and in 30 s for cold start from -20C ambient temperature Power Requirements (based on PSAT simulations)

- 160kW peak power for 0-60 mph acceleration- Minimum fuel cell power of 80kW for achieving speed at 6.5% grade

Default: tight SOC control, lithium-ion, FUDS



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Detailed Models Necessary for Realistic Behavior

Fuel Cell System Efficiency is Not a Monotonic Function

of Power Demand

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Design-Specific FC System Modeling Required to Assess Component Impact

Demister

Electric Motor

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PEFCStack

Compressor/Motor/ExpanderAir

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Small Differences in Components Can Have Large System Implications

Power Demand (kW)

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Pioneering Science andTechnology


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Design-Specific Models Required for Realistic FC Cycle Efficiency

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Increase in Hybridization Degree Can Lead to Decrease in Fuel Economy

*Hybridization Degree

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FC system effPercentage regen braking

FUDS Cycle

Because the Regen Energy Increase is Nullified by the FC Efficiency Decrease

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Hybridization Results

Key benefit of hybridization is fuel economy increase for FUDS thanks to regenerative braking

Increasing the hybridization degree is interesting until the additional gain is nullified by the decrease in fuel cell efficiency

For Li-ion, it is better to limit the ESS power to 40kW to preserve FC system efficiency while capturing most available regen energy

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Optimum Hybridization Degree Depends upon the ESS Technology

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NiMH and Ultracap have lower specific power than Li-ion

The fuel economy penalty due to mass increase is lower for a lowhybridization degree

Relative comparison of vehicle test mass for each energy storage technology (Reference Li-ion)

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NiMH and Ultracap allow better regenerative braking recovery at low hybridization degree

Small ess strategy ; SOCtarget = 0.5

FUDS Cycle - Comparison of regenerative braking energy recovered

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Due to the need to size the ultracapfor Z60 for energy

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The SOC varies more for the Li-ion 6Ah, decreasing the maximum charge power

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Comparison of Maximum Charge Power

Li-ionNiMH

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Energy Storage Technology Results

Optimum hybridization degree depends on energy storage technology

Specific power and specific energy characteristics are key to optimum fuel economy

For Li-ion a higher hybridization degree is necessary while both NiMH and ultracapacitors achieve best results at very low hybridization degrees

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Default Control Strategy Maximizes Fuel Cell System Use

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Control Strategies Options Considered

Use the fuel cell as main power source- SOCtarget = 0.7

- Min fuel cell power demand = 0 (Default Control)- Min fuel cell power demand = 5kW - Min fuel cell power demand = 15kW

- SOCtarget = 0.5- Min fuel cell power demand = 0- Min fuel cell power demand = 15kW

Use the battery as main power source- SOCtarget = 0.7- SOCtarget = 0.5

With min fuel cell power demand = Pwheel + P(SOC)

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Impact of fuel cell power min on battery power

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veh spdfc pwr kWess pwr kW

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1 Battery provides more power during a longer period

2 FC provides more power to recharge the battery

3 Regen amount in unchanged

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Impact of fuel cell power min on battery SOC

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Increasing the min fuel cell power demand leads to fuel economy penalty

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Because the increase in regen energy is nullified by the decrease in fuel cell efficiency

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Reference Control Strategy (SOC=0.7, Different Pfcdmd, 80kW FC)

FC system effPercentage regen braking

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Summary Table Example of 80kW fuel cell system (SOCtarget = 0.7)

57.714.9WhDifference190618391818Wh

Fuel Cell Energy Loss

76100106WhMech. Braking

Energy Loss

15kW5kW0kWUnits

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Impact of target battery SOC on battery power

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2 Battery provides more power during a longer period

3 FC does not need to recharge the battery

1 Increased regen amount

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SOC = 0.7; Pfcmin=0

SOC = 0.5; Pfcmin=0

SOC = 0.7; Pfcmin=15kW

SOC = 0.5Pfcmin=15kW

Impact of initSOC and Pfc min on FE - 80kW fc

A smaller target SOC (0.5) leads to fuel economy benefits

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FC main source - SOC = 0.7 ESS main source - SOC = 0.7

80kW100kW FC120kW FC

US06 Cycle Could Benefit From Using More The Battery

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Control Strategy Results

For the same control strategy, it is possible to increase losses by increasing the regenerative braking due to fuel cell efficiency

Rather than increasing the minimum fuel cell power demand, minimizing the target SOC is a better way to increase the regenerative braking

1 - Low SOC should be targeted to increase regen capture2 Optimum control strategy philosophy depends upon driving cycle: For FUDS, it is better not to use the battery too much, whereas it is the opposite for US06

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System Approach is Needed to Achieve Optimum Fuel Economy

Key benefit of hybridization is fuel economy increase for FUDS thanks to regenerative braking

Optimum hybridization degree is energy storage technology dependant

Fuel cell system efficiency and regenerative braking trade-off is key to optimum fuel economy- Increasing hybridization degree and SOC window can

lower fuel economy- Minimizing SOC target is a good way to increase the

regenerative braking

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Aymeric Rousseau [email protected] Sharer [email protected] Ahluwalia [email protected]

Transportation website www.transportation.anl.govPSAT www.psat.anl.gov

Optimization of Fuel Cell Vehicle Fuel EconomyFuel Cell Vehicle Fuel Economy Optimization - Study ScopeFreedomCAR FCV Energy Storage Proposed Goals Spring 2003Structure of the StudyMajor AssumptionsFuel Cell HEV ConfigurationDetailed Models Necessary for Realistic BehaviorDesign-Specific FC System Modeling Required to Assess Component ImpactSmall Differences in Components Can Have Large System ImplicationsDesign-Specific Models Required for Realistic FC Cycel Efficiency

Fuel Cell Vehicle Fuel Economy Optimization - Hybridization DegreeIncrease in Hybridization Degree Can Lead to Decrease in Fuel EconomyBecause the Regen Energy Increase is Nullified by the FC Efficiency DecreaseHybridization Results

Fuel Cell Vehicle Fuel Economy Optimization - Energy Storage TechnologyOptimum Hybridization Degree Depends upon the ESS TechnologyNiMH and Ultracap have lower specific power than LI-ionNiMH and Ultracap allow better regenerative braking recovery at low hybridization degreeThe SOC varies more for the Li-ion 6Ah, decreasing the maximum charge powerEnergy Storage Technology Results

Fuel Cell Vehicle Fuel Economy Optimization - Control StrategyDefault Control Strategy Maximizes Fuel Cell System UseControl Strategies Options ConsideredImpact of fuel cell power min on battery powerImpact of fuel cell power min on battery SOCIncreasing the min fuel cell power demand leads to fuel economy penaltyBecause the increase in regen energy is nullified by the decrease in fuel cell efficiencySummary Table Example of 80kW fuel cell system (SOCtarget = 0.7)Control Strategies Options ConsideredImpact of target battery SOC on battery powerA smaller target SOC (0.5) leads to fuel economy benefitsControl Strategies Options ConsideredUS06 Cycle Could Benefit From Using More The BatteryControl Strategy Results

System Approach is Needed to Achieve Optimum Fuel EconomyContacts

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Documents

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peak power

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operating temperature

survival temperature