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Optimization of Fuel Cell Vehicle FuelOptimization of Fuel Cell Vehicle Fuel Economy
October 2004
A i RAymeric RousseauPhil Sharer
Rajesh AhluwaliaArgonne National LaboratoryArgonne National Laboratory
Sponsored by Lee Slezak (U.S. DOE)
This presentation does not contain any proprietary, confidential, or otherwise restricted information
Fuel Cell Vehicle Fuel Economy OptimumOptimum
Study Scope
Hybridization Degree
Energy Storage TechnologyEnergy Storage Technology
Control Strategy
i Perspectives
2
Fuel Cell Vehicle Fuel Economy OptimizationOptimization
St d S Study Scope
Hybridization Degree
Energy Storage Technology
Control StrategyControl Strategy
Perspectives
3
FreedomCAR FCV Energy Storage Proposed Goals Spring 2003 Proposed Goals Spring 2003
FreedomCAR Goals Low Power High Power Characteristics Units Energy Storage Energy Storage
Pulse Discharge Power (10s) kW 25 50 M R P l (5 ) kW 30 60Max Regen Pulse (5s) kW 30 60Total 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 -30°C (TBD kW for TBD min.) kW 5 5Cold start at 30 C (TBD kW for TBD min.) kW 5 5Calendar Life Yrs 15 15 Max Weight kg 40 65 Max Volume liters 32 50 Production Price @ 100k units/yr $ 500 1,000 M i O ti V lt Vd </ 440 </ 440Maximum Operating Voltage Vdc </= 440 max </= 440 maxMinimum Operating Voltage Vdc >/= 0.5 x Vmax >/= 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
4
p
Structure of the Study
Hybridization HotCurrent FCHybridizationDegrees
FUDS
ESSSUV Mid-Term FC FHDS AmbESSTechnologies
C t lCar Future FC US06 ColdControlStrategies
5
Fuel Cell HEV ConfigurationFuel Cell HEV Configuration
DC Link
6
Major Assumptionsj p Vehicle and Performance
– Mid‐size SUV (Explorer, Durango, Blazer)
– Target 0‐60 mph acceleration in 10.2 s g p
– 55 mph at grade of 6.5% continuous (a least 20 minutes)
– Top speed of 100 mph
Fuel Cell System Requirements y q
– 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
7
Detailed Models Necessary for l hRealistic Behavior
(%)
ffici
ency
Fuel Cell System Efficiencyyste
m E
ff
Fuel Cell System Efficiency is Not a Monotonic Function of Power Demand
l Cel
l Sy
Fuel
8
Power Demand (kW)
Design-Specific FC System Modeling R i d t A C t I tRequired to Assess Component Impact
Electric Hydrogen
Process Water
Coolant
Electric Motor
Hydrogen Tank
PEFC
Humidified Air
Humidifier Heater
PEFCStack
Humidified Hydrogen
Demister
Radiator & Condenser
Fan
Compressor/Motor/Expander
AirExhaust
Water TankCondensate
Pump
9
Small Differences in Components Can Have Large System ImplicationsSystem Implications
(%)
ffici
ency
ys
tem
Eff
l Cel
l Sy
Fuel
10
Power Demand (kW)
Design-Specific Models Required for Realistic FC Cycle Efficiency
64.5
cy (%
)
FUDS Cycle
63 5
64
ency
(%)
Effic
ienc
63
63.5
Cyc
le E
ffici
em
Cyc
le E
62
62.5
Cel
l Sys
tem
Cl S
yste
m
61
61.5Fuel
CFu
el C
ell
11
61FC HEV 140kW FC HEV 120kW FC HEV 100kW FC HEV 80kW
F
Fuel Cell Vehicle Fuel Economy OptimizationOptimization
St d S Study Scope
Hybridization Degree
Energy Storage Technology
Control StrategyControl Strategy
Perspectives
12
Increase in Hybridization Degree Can Lead to Decrease in Fuel Economy
65
mpg
e)
50*37.5*
25*FUDS Cycle
60
. (m
pge)
e Eq
. (m
12.5*
55
Gas
olin
e Eq
.G
asol
ine
50
el E
cono
my
Gco
nom
y
0*
40
45Fue
Fuel
Ec
13
*Hybridization Degree
40FC HEV 160kW FC HEV 140kW FC HEV 120kW FC HEV 100kW FC HEV 80kW
Because the Regen Energy Increase is Nullified by the FC Efficiency Decrease
84.590 FC system effPercentage regen braking
FUDS Cycle
y
64.0 63.660.9 63.0
72.2
62.270
80
38.550
60
20
30
40
0
10
20
14
0FC HEV 140kW FC HEV 120kW FC HEV 100kW FC HEV 80kW
Hybridization Results
• Key benefit of hybridization is fuel economy increase for FUDS h k i b kiFUDS thanks to regenerative braking
• Increasing the hybridization degree is interesting until theIncreasing 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 toFor Li‐ion, it is better to limit the ESS power to 40kW to preserve FC system efficiency while capturing most available regen energy
15
Fuel Cell Vehicle Fuel Economy OptimizationOptimization
St d S Study Scope
Hybridization Degree
Energy Storage Technology
Control StrategyControl Strategy
Perspectives
16
Optimum Hybridization Degree d h h l
62.5
63
ge)
Depends upon the ESS Technology
mpg
e) FUDS Cycle
61.5
62
ne E
q (m
pg
Li-ione Eq
. (m
60
60.5
61
my
Gas
olin
Li ionNiMHUltracap
Gas
olin
e
59
59.5
60
el E
cono
mco
nom
y
58
58.5
140 120 100 80
Fue
Fuel
Ec
17
Fuel Cell Power (kW)
NiMH and Ultracap have lower specific power than Li-ionpower than Li ion
Relative comparison of vehicle test mass for each energy storage technology (Reference Li-ion)
1 10
1.08
1.09
1.10
NiMH
Ultracap
1.05
1.06
1.07
Rel
ativ
e M
ass
1 02
1.03
1.04
R
The fuel economy penalty due to mass increase is lower for a low hybridization degree
1.02140 120 100 80
Fuel Cell Power (kW)
18
hybridization degree
NiMH and Ultracap allow better regenerative braking recovery at low hybridization degreebraking recovery at low hybridization degree
FUDS Cycle - Comparison of regenerative braking energy recovered
100Due to the
90
95ve
ene
rgy
)
Due to the need to size the ultracapf Z60 f
75
80
85
e of
rege
nera
tivre
cove
red
(%) Li-ion
NiMHUltracap
for Z60 for energy
65
70
Perc
enta
ge
Small ess strategy ; SOCtarget = 0 5
60140 120 100 80
Fuel Cell Power(kW)
19
Small ess strategy ; SOCtarget = 0.5
The SOC varies more for the Li-ion 6Ah, decreasing the maximum charge power
3.5
4x 104 Comparison of Maximum Charge Power
Li-ionNiMH
2
2.5
3
tery
Pow
er
0.5
1
1.5Bat
t
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90
Battery SOC
20
Energy Storage Technology Results
• Optimum hybridization degree depends on energy storage technology
• Specific power and specific energy characteristics are key• 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 ltracapacitors achie e best res lts atboth NiMH and ultracapacitors achieve best results at very low hybridization degrees
21
Fuel Cell Vehicle Fuel Economy OptimizationOptimization
St d S Study Scope
Hybridization Degree
Energy Storage Technology
Control StrategyControl Strategy
Perspectives
22
Default Control Strategy Maximizes Fuel Cell System Usey
23
Control Strategies Options Considered 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
Mi f l ll d d 15kW• Min fuel cell power demand = 15kW
Use the battery as main power source– SOCtarget = 0.7
SOC 0 5– SOCtarget = 0.5
With min fuel cell power demand = Pwheel + P(SOC)
24
Impact of fuel cell power min on battery power battery power
25
30Pminfc=0
25
30Pminfc=15kW
10
15
20
25
10
15
20
25
-5
0
5
-5
0
5
-20
-15
-10 veh spdfc pwr kWess pwr kW
-20
-15
-10 veh spdfc pwr kWess pwr kW
640 645 650 655 660 665 670 675 680-25
640 645 650 655 660 665 670 675 680-25
1 – Battery provides 2 – FC provides more 3 – Regen amount in
25
1 – Battery provides more power during a longer period
2 FC provides more power to recharge the battery
3 Regen amount in unchanged
Impact of fuel cell power min on battery SOCbattery SOC
75Im pact o f Min FC Power on SO C window
Pfcm in=0
73
74
P fcm in 0P fcm in=5kWPfcm in=15kW
71
72
SO
C (%
)
69
70
Bat
tery
S
67
68
26
0 200 400 600 800 1000 1200 140066
T im e(sec)
Increasing the min fuel cell power demand leads to fuel economy penaltyy p y
58 Example of 80kW FC58 Example of 80kW FC57.2
57.157.5
(mpg
)
57.257.157.5
(mpg
)
56.5
56.5
57
Com
paris
on
56.5
56.5
57
Com
paris
on
55 5
56
uel E
cono
my
55 5
56
uel E
cono
my
55
55.5Fu
0kW 5kW 15kW55
55.5Fu
0kW 5kW 15kW
27
Because the increase in regen energy is nullified by the decrease in fuel cell efficiency
84.50 85.3088.10100
Reference Control Strategy (SOC=0.7, Different Pfcdmd, 80kW FC)
62.2 61.9 60.770
80
90
50
60
70
20
30
40
FC system effPercentage regen braking
0
10
20
0kW 5kW 15kW
28
0kW 5kW 15kW
Summary Table – Example of 80kW fuel cell system (SOCtarget = 0 7)cell system (SOCtarget = 0.7)
Units 0kW 5kW 15kWMech. Braking
Energy Loss Wh 106 100 76Fuel Cell
Wh 1818 1839 1906Energy Loss Wh 1818 1839 1906Difference Wh 14.9 57.7
29
Control Strategies Options d dConsidered
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
Mi f l ll d d 15kW• Min fuel cell power demand = 15kW
Use the battery as main power source– SOCtarget = 0.7
SOC 0 5– SOCtarget = 0.5
With min fuel cell power demand = Pwheel + P(SOC)
30
Impact of target battery SOC on p g ybattery power
30
SOC = 0.7; Pminfc=15kW
veh spdfc pwr kW
kW30
SOC = 0.5; Pminfc=15kW
veh spdfc pwr kW
kW
20
ess pwr kW
20
ess pwr kW
0
10
0
10
-10 -10
640 645 650 655 660 665 670 675 680
-20
640 645 650 655 660 665 670 675 680
-20
2 – Battery provides 3 – FC does not need 1 – Increased regen
31
2 – Battery provides more power during a longer period
3 FC does not need to recharge the battery
1 Increased regen amount
A smaller target SOC (0.5) leads to fuel economy benefits
59.360
Impact of initSOC and Pfc min on FE - 80kW fceconomy benefits
57.3
58.1
58
59
on (m
pg)
56.557
58
y C
ompa
riso
55
56
Fuel
Eco
nom
y
54
F
SOC = 0.7; Pfcmin=0
SOC = 0.5; Pfcmin=0
SOC = 0.7; Pfcmin=15kW
SOC = 0.5Pfcmin=15kW
32
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
Mi f l ll d d 15kW• Min fuel cell power demand = 15kW
Use the battery as main power source– SOCtarget = 0.7
SOC 0 5– SOCtarget = 0.5
With min fuel cell power demand = Pwheel + P(SOC)
33
US06 Cycle Could Benefit From Using More The Battery
57.5
80kW
More The Battery
mpg
e)
56.5
57
on (m
pg) 80kW
100kW FC120kW FC
e Eq
. (m
55.5
56
Com
paris
oG
asol
ine
54.5
55
Econ
omy
Cco
nom
y
53 5
54
54.5
Fuel
EFu
el E
c
34
53.5FC main source - SOC = 0.7 ESS main source - SOC = 0.7
Use more the battery
Control Strategy Results
• For the same control strategy, it is possible to increase losses by increasing the regenerative braking due to fuellosses 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 todemand, 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
35
too much, whereas it is the opposite for US06
System Approach is Needed to Achieve Optimum Fuel EconomyOptimum Fuel Economy
• Key benefit of hybridization is fuel economy increase for FUDS thanks to regenerative brakingthanks 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 lowerIncreasing hybridization degree and SOC window can lower fuel economy
‐ Minimizing SOC target is a good way to increase the regenerative brakingregenerative braking
36