Post on 14-May-2018
Fuel Cell/Battery Hybrids:An Overview of Energy Storage
Hybridization in Fuel Cell Vehicles9th Ulm Electrochemical Talks
Ulm, GermanyMay 17-18, 2004
Ahmad PesaranMatthew ZolotTony MarkelKeith Wipke
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Content
• Objectives/Background• Motivation for using energy storage in fuel cell
vehicles • Analysis tool used – ADVISORTM
• Sample results – Performance – Fuel Economy
• Summary
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Objectives
• Review previous studies on how the fuel cell and energy storage sizing and choices can affect efficiency and performance.
• Use ADVISORTM to show the fuel cell and energy storage system demands under drive cycle and performance tests.
• Support the FreedomCAR Energy Storage technical team in defining energy storage requirements for fuel cell vehicles.
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Fuel Cell/Energy Storage Hybridization
Energy Storage
Fuel Cell
Fuel Cell
EnergyStorage
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Fuel Cell HEV –Fitting the System in the Car
InverterFuel Cell
Air Conditioning Compressor
Radiator
Fuel Cell Cooling Pump
Air Compressor Controller
Charger
H2Fuel Tanks
Battery Pack
A/C Drive Motor
Air CompressorWater Reservoir
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Previous Studies
• Hybridization of a fuel cell vehicle with energy storage improves fuel economy and performance, and makes it practical (UCD, VTech, ANL, NREL). Hybridization:– Captures regenerative breaking.
– Improves transient response.
– Uses a smaller, lower cost fuel cell.
– Warms up the fuel cell or reformer.
• Some demonstration prototype fuel cell vehicles are hybrids:– Toyota FCHV – (NiMH batteries)
– Honda FCXV4 – (ultracapacitors)
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Optimization of Fuel Cell Vehicle Design Provides Insight into System Trade-Offs
• Derivative-free optimization algorithms are necessary for the complex design space of HEVs.
• The drive cycle influences the optimal degree of hybridization and control parameters.
• Fuel cell transient response capability is critical for a “pure” fuel cell vehicle.
• An optimized hybrid design can nullify the effects of fuel cell transient response.
• The fuel economy impact of gasoline reformer warmup may be substantial.
• The relatively small energy storage system can overcome the warmuplimitations of the reformer.
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
Minimum Power Maximum Power Charge Power Minimum Off TimeControl Parameter
Pow
er (k
W),
Tim
e (s
*10)
US06 VehicleComp. VehicleNEDC Vehicle1015 Vehicle
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Roles of Energy Storage System (ESS)
• Mostly likelyRegenerative braking captureTraction assist during acceleration (FC slow ramp rate) Traction assist during high power transients (downsizing FC)
• Probably Traction power during fuel cell startupFuel cell system startup and shutdown
• Not desirableSustained gradeabilityElectrical accessory loads(in steady state)
The combined FC and ESS hybridized system must meet performance target requirements of the vehicle:
AccelerationTop speedGrade sustainability
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ADVISORTM Tool Is Used for HFCV Simulations
• ADVISORTM = ADvanced VehIcle SimulatOR– Simulates conventional, electric, or hybrid vehicles (series,
parallel, or fuel cell).– Simulates various components (ES, FC) and drive cycles.
• ADVISORTM was created in 1994 to support DOE Hybrid Program research decisions.
• Available from www.nrel.gov/transportation/analysiswww.nrel.gov/transportation/analysis..• Downloaded by more than 8000 people around the world.
PEM Fuel CellStack
Humid.
WaterReservoir
Humid.
AirCompressor
Airintake
H2 from tanks
PressureRegulator
M
Airexhaust
Air in
Cooling water(de-ionized)
H2 in
WaterReservoir
ThermostatBy-Pass
Radiator
Legend
Fuel SystemAir SystemThermal System
Condenser
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Typical Vehicle Specifications and Performance Assumptions
Vehicle CharacteristicsAssumption Description Units mid-size SUV mid-size Car
Vehicle Description -- Rear wheel drive
mid-size SUVFront wheel drive
mid-size carBase Conventional Vehicle Mass kg 1865 1480Base Vehicle Glider Mass kg 1276 1074Cargo Mass kg 136 136Fuel Cell Vehicle Mass kg 1923 1553Aero. Drag Coef. -- 0.41 0.33Frontal Area m^2 2.6 2Rolling Resistance Coef. -- 0.012 0.009Wheel Radius (effective) m 0.343 0.314Vehicle Range mi (km) 300 (483) 300 (483)
Performance
Assumption Description Unitsmid-size
SUVmid-size
Car0-60 mph (0-97 kph) s <=11.2 <=1240-60 mph (64-97 kph) s <=4.4 <=5.30-85 mph (0-137 kph) s <=20.0 <=23.4Grade @ 65mph (105kph) for 20min. @ Curb Mass + 408kg % >=6.5 >=6.5Drive Cycle Tolerance mph (kph) <=2 (3.2) <=2 (3.2)SOC Balancing % <=0.5% <=0.5%
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Typical Fuel Cell and Hydrogen Storage Systems Assumptions
• Fuel Cell– Should be sized to provide at least top speed and grade
performance.– Must have 1- or 3-s transient response time for 10% to 90%
power.– Should reach maximum rated power in 15 s for cold start from
20°C and 30 s for cold start from –20°C ambient temperatures.– System efficiency of 60% at maximum and 50% at rated peak
power (DOE Technical Targets) .– Specific energy of 400-600 W/kg.
• Hydrogen Storage– Pure compressed hydrogen.
Assumption Description Units 2005 2010H2 Storage Energy Density kWh/L 1.2 1.5H2 Storage Specific Energy kWh/kg 1.5 2H2 Storage Cost $/kWh 6 4
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Typical Energy Storage System Assumptions
Assumption Description Units PbA NiMH Li-IonUltra-
capacitorEnergy Storage Energy Density Wh/L 75 100 190 5Energy Storage Specific Energy Wh/kg 35 55 100 4Energy Storage Energy Density W/L 1600 2000 2800 4500Energy Storage Specific Energy W/kg 550 1000 1300 3500Energy Storage Cost (power) $/kW $10.00 $40.00 $60.00 $15.00
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Regen Capture Is the Primary Contribution of EES to Fuel Cell Vehicle Efficiency
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0
T im e ( s )
Vehi
cle
Spee
d (k
m/h
)
B a g 1 B a g 2 B
W e ig h te d T o ta l = 0 .4 3 * ( B a g 1 + B a g 2 ) + 0 .5 7 * ( B a g 3 + B a g 4 )
20 40 60 80 100 120 140 160 180 200-10
-5
0
5
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25
Trac
tion
Pow
er (k
W)
Time (s )
Traction Power Characteristics for first 200 Seconds of FTP for typical Vehicle
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Maximum Fuel Savings Potential from Regen Energy Use
• Assumption: fuel cell power output is limited such that remaining peaks consume all regen energy.
• Assumed 100% efficiency for recovering regen energy.2UDDS19%
Highway7%
US0618%
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Fuel Economy Depends on Driving Cycle and Degree of Hybridization
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54
100 105 110 115 120 125 130 135 140 145 150Fuel Cell Power (kW)
Fuel
Eco
nom
y (m
pgge
)
Composite FE (Base Case) US06 FE (Base Case)City FE (Base Case) Highway FE (Base Case)
US06
City
Composite
Highway
(Fuel Cell + Energy Storage) combination provides total 140 kW for Max SUV Power Requirements
Fuel Cell
EnergyStorage
Fuel Cell
EnergyStorage
140 kW
Fuel Cell Only
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Key Benefit of Hybridization Is Fuel Economy Increase for Urban Cycles
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4
6
8
10
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18
100 110 120 130 140 150
FC Power (kW)
FE In
crea
se fr
om fu
ll si
ze F
C (%
)
City %FE increase Hwy %FE increase US06 %FE increase
Fuel Cell + Ultracapacitor ESSCity
US06
Highway
140 kW
Fuel Cell Only
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Typical System Efficiency Characteristics from Fuel Cell Model
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0 10 20 30 40 50 60 70 80 90 100
Percentage of Peak Net Power (%)
Fuel
Cel
l Sys
tem
Effi
cien
cy (%
)
Default Scenario @358K
0
1
2
3
4
5
6
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8
9
0 1 2 3 4 5 6 7
Ancillary system
Fuel cell stack (polarization curve)
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Fuel Cell System Efficiency Variability Could Affect FC-ESS Hybridization Outcome
Energy storage system characteristics will depend on the fuel cell system characteristics
Scaled Some of Analysis
NREL used a modified version of the GCtool hydrogen data in the plot. The modification is that the efficiency curve was scaled to provide an efficiency of 60% at 25% peak power (12.5kW). The peak efficiency was 62%-63%.
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Advantages of Downsizing Tied to Fuel Cell Efficiency Characteristics
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0 20 40 60 80 100Percentage of Peak Power (%)
Syst
em E
ffici
ency
(%)
Curve 1 - Peak Efficiency@ 10% Full Power
Curve 2 - Peak Efficiency@ 25% Full Power
Curve 3 - Peak Efficiency@ 40% Full Power
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2
4
6
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857463Fuel Cell Net Power (kW)
Perc
ent F
uel E
cono
my
Impr
ovem
ent (
%)
10% Peak, Cty/Hwy 25% Peak,Cty/Hwy40% Peak,Cty/Hwy Linear (10% Peak, Cty/Hwy)Linear (25% Peak,Cty/Hwy) Linear (40% Peak,Cty/Hwy)
Proposed theoretical FC efficiency curves for current analysis due to lack of data
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Drive Cycle Power Output Histogram Helps Explain Hybridization Benefits
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400 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 +
Discharge Power (kW)
0
10
20
30
40
50
60
70
Fuel
Cel
l Sys
tem
Effi
cien
cy (%
)
Highway Cycle
FC 10% P_Peak
FC 25% P_Peak
FC 40% P_Peak
Average Power 12.25 kW
% o
f Ope
ratio
n, H
ighw
ay C
ycle
02468
101214161820
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 +
Discharge Power (kW)
% o
f Ope
ratio
n, U
S06
Cyc
le
0
10
20
30
40
50
60
70
US06 CycleFC 10% P_PeakFC 25% P_PeakFC 40% P_Peak
Average Power 22.46 kW
Fuel
Cel
l Sys
tem
Effi
cien
cy (%
)
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Benefit of Downsizing Fuel Cell as a Function of Peak Efficiency Position
40
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50
55
60
65
70
75
96857463Fuel Cell Net Power Output (kW)
Fuel
Eco
nom
y (m
pgge
)
10% Peak, Cty/Hwy Combo 25%Peak, Cty/Hwy Combo40% Peak, Cty/Hwy Combo
05
101520253035404550
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 +Discharge Power (kW)
% o
f Ope
ratio
n, C
ity C
ycle
0
10
20
30
40
50
60
70
City CycleFC 40% 96kWFC 40% 85kWFC 40% 74kWFC 40% 63kW
Average Power 6.96 kW
Fuel
Cel
l Sys
tem
Effi
cien
cy (%
)
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Potential Active Roles for Supplementing Fuel Cell Operation
Curve 2 - Peak Efficiency @ 25% Full Power
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30
40
50
60
70
0 20 40 60 80 100Percentage of Peak Power (%)
Syst
em E
ffici
ency
(%)
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Concluding Observations • Vehicle performance specifications can significantly
influence ESS requirements.• Fuel cell operating characteristics can significantly
influence ESS requirements.• Regen capture is the key contribution of the ESS to fuel
cell vehicle efficiency. • A key benefit of hybridization is an increase in fuel
economy for urban cycles.• Maximizing fuel economy depends on drive cycles and
fuel cell system characteristics (efficiency curve).• Ultracapacitors can recapture a significant portion of the
regen energy and thus improve fuel economy. However, because of their limited energy storage capability, they provide less potential for fuel cell system downsizing.
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Acknowledgments
• This work is sponsored by U.S. Department of Energy’s Office of FreedomCAR and Vehicle Technologies.
• We appreciate the support and technical advice from the USABC/FreedomCAR Energy Storage technical team.
www.ctts.nrel.gov/BTMwww.ctts.nrel.gov/analysis