Energy Storage Fuel Cell Vehicle Analysis

Ahmad Pesaran National Renewable Energy Laboratory Golden, Colorado, USA

NREL/PR-540-38143 August 2005 1 Acknowledgments

Co-Authors Tony Markel (NREL) Matthew Zolot (NREL) Sam Sprik (NREL) Harshad Tataria (GM/USABC) Tien Duong (DOE)

Support and Guidance FreedomCAR Energy Storage Technical Team USABC Technical Advisory Committee Ted Miller (Ford) Cyrus Ashtiani (DaimlerChrysler) Dave Howell (DOE)

2 Outline

• Study Objectives • Assumptions & Requirements • Analysis Approach •Results • Conclusions

3 Objective

Determine ESS Requirements for Fuel Cell Hybrid Vehicles for FreedomCAR Tech Teams

FreedomCAR Goals Fuel Cell Hybrid Battery Characteristics Units Pulse Discharge Power (x s) kW TBD Max Regen Pulse (y s) kW TBD Total Available Energy kW h TBD Round Trip Efficiency % >90 Cycle Life Cyc. TBD Cold-start at -30°C (TBD kW for TBD m in.) kW TBD Calendar Life Yrs TBD Max W eight kg TBD M ax Volum e liters TBD Production Price @ 100k units/yr $ TBD Maximum Operating Voltage Vdc TBD Minimum Operating Voltage Vdc TBD Maximum Self Discharge W h/d 50 Operating Tem perature °C -30 to +52 Survival Tem perature °C -46 to +66

Will these requirements will be different than ones for ICE-HEVs?

4 Vehicle Attributes for Analysis Forward-looking (What would be the vehicle characteristics when the fuel cell technology is ready and affordable?) •Midsize – Initial focus (popular) – Needs smaller fuel cell, thus lower cost • Extremely Lightweight – For the same increase in fuel economy, it is more cost effective to take weight out of the vehicle rather than using a larger fuel cell • Light weighting $/kg >> Larger fuel cell $/kW • Aerodynamic (relatively low drag coefficient)

5 Vehicle Assumptions Characteristics and Requirements

Characteristic Chevrolet Malibu Final NREL Assumptions Model Year 2004 Mid-size car for Mid-Size Car (similar to Basic Malibu Sedan)

Dimensions [mm] 4,783 x 1,460 x 2,700 4,749 x 1,416 x 2,656 (L x H x L-wheelbase) Curb Weight [kg] (calculated) 1437 kg TBD kg = 1060*0.6 + PT Mass Glider Mass [kg] 1030 (w/ driveline) 636 =1060*0.6 Weight Fraction (Front:Rear) [%] 63:37 50:50 Center-of-Gravity Height [m] 0.5 – estimated 0.45 Coefficient of drag 0.30 0.25 Frontal Area [m2] 2.0 (calculated) 2.0 Rolling Resistance Coefficient 0.008 – 0.009 0.0070 Range (composite City/highway) 756 km (320 miles) 500 km (minimum) Maximum Speed (FC only) Better than 160 km/hr (100 mph) 160 km/h (0-60 mph) 0-100 km/h Acceleration 11.0 seconds (estimate) 11 seconds Gradeability (FC only) Better than 6.5% Grade @ 65 mph 5.5% Grade @ 55 mph (88 km/h)

6 Roles of Energy Storage System (ESS) • Mostly likely 3 1. Regenerative braking capture 3 2. Traction assist during acceleration (FC slow ramp rate) 3 3. Traction assist during high power transients (down sizing FC) • Probably 3 4. Traction assist during fuel cell “warm” startup from idle ¾ Traction assist and power for ancillaries during fuel cell system “cold” startup & shutdown The combined FC and ESS hybridized system • Probably not must meet performance target requirements of – Sustained gradeability the vehicle – Electrical accessory loads - Acceleration (in steady state) - Top speed 3: assumed in this study - Grade sustainability

7 Other Assumptions 2010 FreedomCAR/DOE Targets

Fuel Cell Motor/Controller Assumption Description Units Value Assumption Description Units Value Specific Power (Motor and Controller) kW/kg 0.75 Fuel Type -- hydrogen Specific Cost (Motor and Controller) $/kW 1112 Fuel Cell Peak Efficiency % 60 Power Density (Motor and Controller) kW/L 3.53 Fuel Cell Efficiency at 25% Power % 60

Fuel Cell Efficiency at Rated Power % 50 Fuel Cell System Specific Power W/kg 500 Power Electronics Fuel Cell System Power Density W/L 500 Fuel Cell System Cost $/kW 10532 Assumption Description Units Value Fuel Cell System 10-90% Power Efficiency % 95 Transient Response Capability s 1 Specific Cost $/kW 5 Time from Start to Full Power Output Capability (20C) s 15 Hydrogen Storage Assumption Description Units Value H2 Storage Energy Density kWh/L 1.2 H2 Storage Specific Energy kWh/kg 1.5 H2 Storage Cost $/kWh 64 • Assumed 700 W constant accessory loads • Fuel cell is always on (i.e. no start/stop operation) • Fuel cell net power is zero at vehicle stop/idle (gross power >0) • At idle, hydrogen fuel consumption 0.3% of rated power consumption

8 Fuel Cell Efficiency Characteristics

These fuel cells have the SAME 70 net power

50 2010 Target used for initial analysis 40

30 Curve 1 - Peak Efficiency @ 10% Full Power Curve 2 - Peak Efficiency System Efficiency (%) System Efficiency 20 @ 25% Full Power Curve 3 - Peak Efficiency 10 @ 40% Full Power

0 0 20406080100 Percentage of Peak Power (%)

9 Analysis Approach using ADVISOR™

• Implemented an ESS control strategy (energy management) and regen capture strategy Energy • Simulated a range of FC/ESS configurations (Cases) Storage – Smallest fuel cell (Case1: sized for top speed) Fuel • Cases 2-4 : incrementally increased fuel cell size; ESS sized to Cell satisfy acceleration constraints – Fuel cell only case (Case 5) • Cases 5a-5c: ESS size increased to capture % of peak regen pulse power – Fuel cell plus ESS sized for max regen capture (Case 5a) • Analyzed ESS power (kW) profile and Energy (Wh) history over several profiles – Urban, Highway, US06 cycles, and European cycles – Acceleration and gradeability tests

10 Energy Management Strategy/Assumption Used

• Monitored changes in ESS and modified the fuel cell command to maintain ESS energy level •Included – Accounting of kinetic energy – Opportunity charge and discharge functionality (only take action if it makes the overall system efficiency better) – Monitored delta ESS energy to fuel-use ratio for correcting fuel economy • Used multiple parameters to manage the strength of various elements of control • A Design of Experiments was performed on each case to determine the best parameter settings • Two Regenerative Braking Energy Capture – A fraction of total possible – Deceleration rate-based

11 Deceleration Rate-Based Regenerative Braking Strategy

100%

90%

• Fractional split between 80%

driveline and friction brakes 70% defined as a function of 60% deceleration rate 50% 40% Friction Braking Driveline Braking 30% Regenerative Braking Strategy

Percent Energy Dissipation (%) 20%

100% 10%

90% 0% US06 HWFET UDDS HYZEM-Hwy HYZEM- HYZEM- 80% Urban ExtraUrban Friction Braking 70% Driving Schedule Driveline Braking 60%

50% • Given the current input 40% assumptions (below 1g all 30% Percentage of Total of Percentage driveline braking, above 3g all 20% friction braking) the driveline 10%

0% would try to recapture between 0 -1-2-3-470-90% of the available G rate (m/s^2)/(m/s^2) braking energy depending on the drive cycle. 12 Matrix of Vehicle Configurations Evaluated

Fuel Cell ESS • Case 1-4 varies fuel cell Name Description (kW) Regen (kW) Discharge(KW) size with “deceleration- FC sized for grade/top speed; decel regen Case 1 strategy; FC_FC50_P25 47000 34000 25000 based regen strategy” Case 1a Case 1 + 100% regen 47000 34000 25000 Case 1b Case 1 + 75% regen 47000 25500 25000 – ESS sized to satisfy Case 1c Case 1 + 50% regen 47000 17000 25000 Case 1f Case 1a + FC_FC50_P10 47000 34000 25000 acceleration performance Fuel cell - sized to 25% point; decel regen constraints Case 2 strategy; FC_FC50_P25 54250 34000 18000 Case 2a Case 2 + 100% regen 54250 34000 18000 • Case Xa-c with increasing Case 2b Case 2 + 75% regen 54250 25500 18000 Case 2c Case 2 + 50% regen 54250 17000 18000 regen power limits Case 2f Case 2a + FC_FC50_P10 54250 34000 18000 Fuel cell - sized to 50% point; decel regen • Case Xf differs from Xa by Case 3 strategy; FC_FC50_P25 61500 34000 12500 Case 3a Case 3 + 100% regen 61500 34000 12500 the fuel cell characteristics Case 3b Case 3 + 75% regen 61500 25500 12500 Case 3c Case 3 + 50% regen 61500 17000 12500 – Xa peak eff at 25% power Case 3f Case 3a + FC_FC50_P10 61500 34000 12500 (DOE goal) Fuel cell - sized to 75% point; decel regen Case 4 strategy; FC_FC50_P25 69000 34000 7500 – Xf peak eff at 10% power Case 4a Case 4 + 100% regen 69000 34000 7500 Case 4b Case 4 + 75% regen 69000 25500 7500 Power Requirements (FC-Only): Case 4c Case 4 + 50% regen 69000 17000 7500 Case 4f Case 4a + FC_FC50_P10 69000 34000 7500 • Accel 0-60 mph: 75 kW Case 5 Fuel cell only - no ess; FC_FC50_P25 75000 0 0 • Top speed 100 mph: 47 kW Case 5a Fuel cell only plus 100% ess 75000 36000 36000 Case 5b Fuel cell only plus 75% ess 75000 27000 27000 • Gradeability (55 mph @ 5.5%): 34 kW Case 5c Fuel cell only plus 50% ess 75000 18000 18000 Case 5f Case 5 + FC_FC50_P10 75000 0 0

13 Fuel Cell and Energy Storage System Discharge Ratings 120000 ESS Peak Discharge Power Fuel Cell Peak Power 100000

FC Only

80000

60000 Power(W) 40000

20000

1f 2f 3f 4f a 5f 3a 5 se 1 e e 2b e 2c se 3 e e e 4b e 4c e e e a a se 4a s C Case 2 as ase C Case 4a as a Case 5 Case 1aCase 1bCase 1cCas Case 2aCas C C Cas Case 3bCase 3cCas C C Cas C Cas Case 5bCase 5cCas 14 Preferred Usable Energy Window for EES 300

250

200

150

100 Max Energy Range (Wh) Energy Range Max

c c c c c 1b e 2 2b 3b 3 4b 4 5a 5b 5 e e 1 s e e 2 e e e e s s s s s se s s Case 1 Ca Case 3 Case 4 Case 5 Case 1aCa Ca Case 1f Case 2aCas Ca Case 2f Case 3aCa Case Case 3f Case 4aCa Ca Case 4f Ca Ca Case Case 5f 15 Braking Energy Losses Due to Charge Power Limits

40 40

UDDS Braking Loss 35 US06 Braking Loss 35 Charge Power Limit

30 30

25 25

US06 Fuel Fuel Consumption Consumption (Lit/100km) (Lit/100 : km) 20 • Case Case 5 5 (75 (75 kW kW FC FC only) only) = 4.22 =4.22 20 • Case Case 5a 5a (75 (75 kW kW FC FC + 36 + kW36 ES)kW =ESS) 3.38 = 3.38 • Case Case 5a 5b (75 (75 kW kW FC FC + 27 + kW27 ES)kW =ESS) 3.41 = 3.41 Power (kW) (kW) (kW) Power Power Power 15 • Case Case 5b 5c (75 (75 kW kW FC FC + 18 + kW18 ES)kW =EES) 3.45 = 3.45 Small EES 15 to capture Mid-size EES 50% Regen 10 10 Charge PowerCharge PowerCharge Power Limit Limit Limit (kW) (kW) (kW) % OpportunityOpportunityOpportunity% % % Loss Loss Loss Energy Energy Energy to capture Largest EES 75% Regen

Braking Losses (% of fuel cell only case) of fuel cell only case) of fuel cell only case) Braking Losses (%Braking Losses (%Braking Losses (% 5 to capture 5 100% Regen

(Energy Not Captured/ Total Braking Energy for Vehicle for Vehicle for Vehicle Not Not Not Total Total TotalCaptured/ Captured/ Captured/ Energy Energy Energy Available Available Available Braking Braking Braking (Energy (Energy (Energy with with with) ) ) FC Only FC Only FC Only 0 0 Case 5a Case 5b Case 5c 16 Fuel Consumption Results Adjusted City/HWY Composite 4.5 • Comparing Case 5 (FC only) to all others, demonstrates advantageComposite (adjusted) of having any ESS on-board • Very best fuel consumption for Case 1f FC Only 4 • Smallest fuel cell • ESS for 50% regen power • Fuel cell peak efficiency at 10% rated power

3.5 3 L/100km ~ 80 MPG ~ 0.8 kg H2/100 km

2.5 Fuel Consumption (L/100km) Fuel Consumption

1 a b a b a b a b c a b 1f 2 2c 2f 3 3c 3f 4 4 5 e 1c e 5 se s se e 3 se se 4 se 5c Ca Case 2 Case 3 Case 4 Cas Case 1Case 1Ca Case Case Case 2Ca Case Case Cas Ca Case Case Case Ca Case 4f Case 5Case Ca Case 5f 17 First Cost Summary using 2010 Targets 5000

4500

4000

3500

3000

2500

2000

• Fuel cell primary cost component

Powertrain ($) Cost 1500 Energy Storage (20 $/kW) • Costs decrease with decreasing fuel cell size Power Electronics (5 $/kW) 1000 Traction Motor ($12/kW) • Costs decrease with decreasing battery size within Hydrogen Storage (4 $/kWh) 500 a case Fuel Cell (32 $/kW)

0 f f f f 1 a c 1 2 b 3 b 4 a c 4 5 b e e 2 e 5 s e 1 e 1 e s e e 2 e 3 e 4 e 4 e s e e 5 s s s a s s s s s s s s Ca a ase 1b a C ase 2aa a Case a ase 3case 3fCase a ase 4b a Ca ase 5aa a C C Ca C C C Case 2cC Case 3aC C C C C Ca C C C Case 5cC 18 Choosing a Scenario Figure of Merit (Fuel Economy/Cost)

0.025 Best Cases Higher fuel economy, 0.02 lower cost

0.015

0.01

Figure of Merit (mpgge/$) Figure of Merit 0.005

Lower fuel economy, 0 higher cost f 1 b c 2 b b b a e 1a 1f e 3a 3f 4f 5b 5 s e e s se 3 e e e e e s se 2a s s ase 4 s ase 5 s s Ca a ase 1 ase 1 Ca a ase 2 ase 2c Ca a ase 3 a C ase 4 ase 4ca C a C C C Cas C C C Case 2f C C Case 3cC Case 4aC C C Case 5C Case 5cCa 19 How to determine ESS requirements from instantaneous power demands from cycles?

US06

Given typical ESS power profile, what would be the appropriate: • Discharge power (x s) • Regen power (y s) • Energy range (Wh)?

Example: Case 1 20 Analysis of ESS Power Profile Duration of Event

Avg. Regen Pulse Power

Peak Pulse Power

Duration at Peak Power 21 Power and Energy Categories

40 100% regen cases

75% regen cases 20 Rated Power (kW) 50% regen cases 10

0 50 100 150 200 250 300 Usable Energy Range (Wh) 22 Peak Power vs. Duration Data for Multiple Cycles

• Peak power events typically only last for short duration (artifact of study approach) • Discharge sized for acceleration • Charge sized for US06

23 Avg. Power Need vs. Duration Data for Multiple Cycles

• Power is average power of an event (energy/duration) • Duration is the time from 0 to 0 power • Acceleration performance sets discharge requirements • US06 cycle sets charge requirements

24 Proposed Recommendations for ESS for mid-size FCVs

(Smallest fuel cell with moderate ESS) (For a light-weight, aerodynamic, mid-size car)

Goal ESS for FCV (Specifications) (units) Pulse Discharge Power (12 s) kW 25 Max Regen Pulse (5 s) kW 20 Available Energy Wh 250

25 Conclusions

• Intelligent energy management strategy to capture and utilize regen energy in fuel cell hybrid vehicle is critical • In general, 25-30% improvement in fuel consumption from hybridization – As long as regen capture is maximized – regen strategy not critical – smallest fuel cell with moderate ESS was the most fuel efficient, but also the least expensive scenario • ESS with 200-250 Wh of usable range seems sufficient for assumed lightweight midsize fuel cell car • ESS with regen power of around 20 kW for 5s and discharge power of 25 kW for 10-15s appears to be sufficient for lightweight midsize fuel cell car • Planning to perform sensitivity analysis and investigate other opportunities provided by fuel cell operating strategies