Fuel Cell/Battery Hybrids: An Overview of Energy … Cell/Battery Hybrids: An Overview of Energy...

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

2 9th Ulm Electrochemical Talks

Content

• Objectives/Background• Motivation for using energy storage in fuel cell

vehicles • Analysis tool used – ADVISORTM

• Sample results – Performance – Fuel Economy

• Summary


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.


Fuel Cell/Energy Storage Hybridization

Energy Storage

Fuel Cell

Fuel Cell

EnergyStorage


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


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)


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


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


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

http://www.nrel.gov/transportation/analysis


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%


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


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


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

10

15

20

25

Trac

tion

Pow

er (k

W)

Time (s )

Traction Power Characteristics for first 200 Seconds of FTP for typical Vehicle


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%


Fuel Economy Depends on Driving Cycle and Degree of Hybridization

34

39

44

49

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


Key Benefit of Hybridization Is Fuel Economy Increase for Urban Cycles

0

2

4

6

8

10

12

14

16

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


Typical System Efficiency Characteristics from Fuel Cell Model

0

10

20

30

40

50

60

70

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

7

8

9

0 1 2 3 4 5 6 7

Ancillary system

Fuel cell stack (polarization curve)


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%.


Advantages of Downsizing Tied to Fuel Cell Efficiency Characteristics

0

10

20

30

40

50

60

70

0 20 40 60 80 100Percentage of Peak Power (%)

Syst

em E

ffici

ency

(%)

Curve 1 - Peak Efficiency@ 10% Full Power



0

2

4

6

8

10

12

14

16

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


Drive Cycle Power Output Histogram Helps Explain Hybridization Benefits

0

5

10

15

20

25

30

35

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


Fuel

Cel

l Sys

tem

Effi

cien

cy (%

)


Benefit of Downsizing Fuel Cell as a Function of Peak Efficiency Position

40

45

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


Fuel

Cel

l Sys

tem

Effi

cien

cy (%

)


Potential Active Roles for Supplementing Fuel Cell Operation

Curve 2 - Peak Efficiency @ 25% Full Power

0

10

20

30

40

50

60

70

0 20 40 60 80 100Percentage of Peak Power (%)

Syst

em E

ffici

ency

(%)


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.


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

http://www.ctts.nrel.gov/BTM

http://www.ctts.nrel.gov/BTM

http://www.ctts.nrel.gov/analysis

http://www.ctts.nrel.gov/analysis

Fuel Cell/Battery Hybrids: An Overview of Energy … Cell/Battery Hybrids: An Overview of Energy...

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