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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
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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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3Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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4Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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5Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
Fuel Cell HEV Configuration
DC Link
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6Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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7Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
Detailed Models Necessary for Realistic Behavior
Fuel Cell System Efficiency is Not a Monotonic Function
of Power Demand
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Power Demand (kW)
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8Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
Design-Specific FC System Modeling Required to Assess Component Impact
Demister
Electric Motor
Hydrogen Tank
Humidifier Heater
PEFCStack
Compressor/Motor/ExpanderAir
Exhaust
Radiator & Condenser
Water Tank
Process Water
Humidified Air
Humidified Hydrogen
Coolant
Condensate
Fan
Pump
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9Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
Small Differences in Components Can Have Large System Implications
Power Demand (kW)
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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
Design-Specific Models Required for Realistic FC Cycle Efficiency
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FC HEV 140kW FC HEV 120kW FC HEV 100kW FC HEV 80kW
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FUDS Cycle
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Pioneering 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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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
Increase in Hybridization Degree Can Lead to Decrease in Fuel Economy
*Hybridization Degree
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FC HEV 160kW FC HEV 140kW FC HEV 120kW FC HEV 100kW FC HEV 80kW
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FUDS Cycle
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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
64.0
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FC HEV 140kW FC HEV 120kW FC HEV 100kW FC HEV 80kW
FC system effPercentage regen braking
FUDS Cycle
Because the Regen Energy Increase is Nullified by the FC Efficiency Decrease
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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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Pioneering 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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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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Fuel Cell Power (kW)
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Li-ionNiMHUltracap
Optimum Hybridization Degree Depends upon the ESS Technology
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FUDS Cycle
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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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)
1.02
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Fuel Cell Power (kW)
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Ultracap
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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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Fuel Cell Power(kW)
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Due to the need to size the ultracapfor Z60 for energy
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19
Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
The SOC varies more for the Li-ion 6Ah, decreasing the maximum charge power
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90
0.5
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Battery SOC
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Comparison of Maximum Charge Power
Li-ionNiMH
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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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Pioneering 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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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
Default Control Strategy Maximizes Fuel Cell System Use
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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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24
Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
Impact of fuel cell power min on battery power
640 645 650 655 660 665 670 675 680-25
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veh spdfc pwr kWess pwr kW
640 645 650 655 660 665 670 675 680-25
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veh spdfc pwr kWess pwr kW
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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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
Impact of fuel cell power min on battery SOC
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P fcm in=0P fcm in=5kWPfcm in=15kW
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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
Increasing the min fuel cell power demand leads to fuel economy penalty
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Example of 80kW FC
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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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30
Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
Impact of target battery SOC on battery power
640 645 650 655 660 665 670 675 680
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SOC = 0.7; Pminfc=15kW
veh spdfc pwr kWess pwr kW
640 645 650 655 660 665 670 675 680
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SOC = 0.5; Pminfc=15kW
veh spdfc pwr kWess pwr kW
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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31
Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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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32
Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
53.5
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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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Use more the battery
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Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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35
Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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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36
Pioneering Science andTechnology
Office of ScienceU.S. Department
of Energy
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