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Hybrid Lithium-ion Ultracap Energy Storage Systems for Plug-in Hybrid Electric Vehicles


Hybrid Lithium-ion/Ultracap Energy Storage Systems for Plug-in Hybrid Electric Vehicles
Farzad Ahmadkhanlou and Abas Goodarzi US Hybrid Corporation, 445 Maple Ave, Torrance, California, 90503, USA E-mail:farzad@ushybrid.com, abas@ushybrid.com

Abstract - Battery technologies to maximize power density and energy density simultaneously are not commercially feasible. The use of bi-directional DC-DC converter allows use of multiple energy storage systems, and the flexible DC-link voltages can enhance the system efficiency and reduce component sizing. In this paper we have conducted vehicle level study and modeling to quantify the benefit of bi-directional DCDC converter in hybrid energy storage systems for vehicles. The goal of this study is to reduce the overall cost of plug-in hybrid electric vehicle (PHEV) and demonstrate high power density and efficiency by hybrid energy storage system, including a Lithium-ion battery, an Ultracap, and two DC-DC converters. The simulation results show the PHEV with hybrid energy storage system has better performance over the conventional PHEV. The hybrid energy storage system allows the best utilization of Ultracap and battery technologies for both high power density and high energy density.

energy storage system is sized to provide the same amount of energy and the same peak power as the single battery system.

I.

INTRODUCTION
Figure 1, Hybrid energy storage system (HESS): Dual energy storage systems (ESS) with two DC-DC converters.

Plug-In Hybrid Electric Vehicles (PHEV) require high power density energy storage system (ESS) for hybrid operation and high energy density ESS for Electric Vehicle (EV) mode range. However, ESS technologies to maximize power density and energy density simultaneously are not commercially feasible. The use of bi-directional DC-DC converter allows use of multiple energy storage systems, and the flexible DC-link voltages can enhance the system efficiency and reduce component sizing. To reduce life cycle cost of PHEV we propose hybrid energy storage system with two bi-directional DC-DC converters. Two design alternatives have been investigated: (i) Conventional PHEV with single battery pack and (ii) Our proposed hybrid energy storage system with two DC-DC converters (Figure 1). A port truck with Gross Vehicle Weight Rating (GVWR) of 35000kg (77000lb) has been investigated in this study (Figure 2). This GVWR can also be considered as a class 8 heavy duty on road vehicle. Parallel hybrid power-train control strategy has been employed and two distinct drive cycles have been considered for simulation. In hybrid energy storage system, the Lithium-ion battery pack with high energy density is used to provide the required energy in EV mode with charge depleting strategy. This is the energy module of the hybrid energy storage system, denoted by E-ESS. The Ultracap with high power density is used with charge sustaining strategy. This is the power module of the hybrid energy storage system, denoted by P-ESS. The hybrid

Figure 2, Port Truck.

The simulation results show the hybrid vehicle with hybrid energy storage system has better performance and higher overall efficiency over the conventional PHEV. The hybrid energy storage system is also more cost effective and allows the best utilization of Ultracap and battery technologies for both high power density and high energy density. Furthermore, the vehicle performance and handling is not compromised as the battery is operating in low State of

978-1-61284-247-9/11/$26.00 ?2011 IEEE

Charge (SOC) after charge depletion operation mode. The battery energy capacity can be fully utilized, enhancing the EV only range for the size of battery. The battery system life cycle for the charge depletion battery is based on the charge cycles, while the power ESS life cycle is for power cycling at higher SOC band of operation. II. MODELING

TABLE I
DRIVE CYCLE STATISTICS

Parameter Average speed (mph) Max speed (mph) Distance (miles) Time (s) Idle time (s) Number of stops

Cycle 1 8.1 25 1.49 660 215 7

Cycle 2 6.6 18.46 2.27 1235 234 12

A. Simulation Modified (customized) version of Advanced Vehicle Simulator (ADVISOR) is used for modeling and simulation purposes. A control algorithm has been developed to distribute the load demand between the Ultracap and battery in EV mode. Parallel hybrid power-train control strategy has been employed for the HEV mode. B. Drive Cycles Two drive cycles have been considered for simulation. The specifications and statistics of these drive cycles are presented in Figure 3 and table 1.
40

C. Control Algorithm A 50 Ah battery pack with high energy density is used with charge depleting strategy (E-ESS). It provides 32.5 kWh energy. A 15 Farad Ultracap with high power density is used with charge sustaining strategy (P-ESS). For the energy battery, the highest and lowest desired SOC are set to 100% and 5% respectively. The initial SOC is set to be 100% for both energy and power ESS. There are three main modes of operation: 1-Motoring: The E-ESS is providing the average power while the P-ESS provides the extra required power depending on its SOC and available power. 2-Regeneration: Ultracap gets charged first then the energy battery 3-Idle: The energy battery charges the Ultracap if required III. SIMULATION RESULTS

speed (mph)

20

The single ESS and hybrid ESS are simulated separately. The simulation results for the EV range are presented in this section A. Drive Cycle 1, Hybrid Case The load demands for hybrid ESS configurations are presented in Figure 4. The E-ESS is providing the average power while the P-ESS provides the extra required power depending on its SOC and available power.
200 P-ESS E-ESS 150

0

0

100

200

300 400 time (sec)

500

600

700

20

15 speed (mph)
Power (kW) 100

10

50

5

0

0

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400

600 800 1000 1200 1400 time (sec)

-50

0

1000

2000 Time (s)

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Figure 3, Drive Cycles 1 and 2.

Figure 4, Load Demand for hybrid ESS

The energy battery (E-ESS) is in charge depleting operation mode and the Ultracap (P-ESS) is in charge sustaining mode. The SOC of the hybrid ESS systems is presented in Figure 5.
P-ESS E-ESS

Ultracap (P-ESS) voltage and current are presented in Figure 7. In this figure Vin and Iin are input voltage and current from ESS to DC-DC, Iout and Vout are input voltage/current coming out of DC-DC converter. Figure 8 depicts voltage and current of energy battery (E-ESS).
700 600 500 Voltage and Current 400 300 200 100 0 -100 Vin Iin V out Iout

1 0.9 0.8 0.7 0.6 SOC 0.5 0.4 0.3 0.2 0.1 0 0 1000

2000 Time (s)

3000

4000

-200

0

1000

2000 Time (s)

3000

4000

Figure 5, State of Charge (SOC)

Figure 7, Voltage and Current of Ultracap (P-ESS)

The total energy provided by hybrid ESS is depicted in Figure 6. As expected, the total energy is provided by energy battery (E-ESS) in the hybrid ESS. As the Ultracap (P-ESS) is in charge sustaining mode, its net energy is close to zero. The total energy provided by single battery configuration is very close to that of hybrid ESS.
Voltage and Current

700 600 500 400 300 200 100 0 Vin Iin Vout Iout

30 25 20

P-ESS E-ESS

Energy (kWh)

15 10 5 0 -5

0

1000

2000 Time (s)

3000

4000

Figure 8, Voltage and Current of Energy Battery (E-ESS)

0

1000

2000 Time (s)

3000

4000

Figure 6, Energy Consumption

Fast Fourier Transform (FFT) of the Ultracap (P-ESS) and battery (E-ESS) are depicted in Figure 9. At higher frequencies the amplitude of the energy battery (E-ESS) is diminishing and at lower frequencies it has the dominant amplitude.

Load statistics for Ultracap (P-ESS) and battery (E-ESS) are plotted in Figure 10. The energy battery (E-ESS) is providing a very narrow range of power, mainly a continuous power of 25kW. The power Ultracap, on the other hand, is providing a wide range of charging and discharging loads.
50 45 40 Power Amplitude (kW) 35 30 25 20 15 10 5 0 0 20 40 60 Freq (Hz) 80 100 P-ESS E-ESS

B. Drive Cycle 2, Hybrid Case The same hybrid ESS configuration has been simulated for drive cycle 2 as well. The load demands, SOC, energy, current and voltage results for hybrid ESS configuration are presented in this section (Figure 11 to Figure 17).
P-ESS E-ESS

150

100

Power (kW)

50

0

-50

-100

0

1000

Figure 9, Fast Fourier Transform (FFT) of power ESS and energy ESS

2000 3000 Time (s)

4000

5000

Figure 11, Load Demand for hybrid ESS

70 % P-ESS 60 % Percentage of Occurrences 50 % 40 %
SOC 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3

P-ESS

E-ESS

E-ESS

30 % 20 % 10 % 0 % -50

0.2 0.1

0

50 100 Power (kW)

150

200

0

0

1000

2000 3000 Time (s)

4000

5000

Figure 10, Load statistics of power ESS and energy ESS

Figure 12, State of Charge (SOC)

30 25 20 Energy (kWh) 15 10 5 0 -5 P-ESS E-ESS

700 600 500 400 300 200 100 0 Vin Iin Vout Iout

Voltage and Current

0

1000

2000 3000 Time (s)

4000

5000

0

1000

2000 3000 Time (s)

4000

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Figure 13, Total Energy

Figure 15, Voltage and Current of Energy Battery (E-ESS)

700 600

Vin

Iin

Vout

Iout

50 45 40 P-ESS E-ESS

500 Voltage and Current 400 300 200 100 0 -100 -200 -300 0 1000 2000 3000 Time (s) 4000 5000

Power Amplitude (kW)

35 30 25 20 15 10 5 0 0 10 20 30 Freq (Hz) 40 50

Figure 14, Voltage and Current of Ultracap (P-ESS)

Figure 16, Fast Fourier Transform (FFT) of power ESS and energy ESS

60 % P-ESS 50 % Percentage of Occurrences E-ESS

250 Hybrid 200 150 Single

40 %

30 %

Current (A)
-50 0 50 Power (kW) 100 150

100 50 0 -50 -100

20 %

10 %

0 % -100

-150

0

1000

2000 3000 Time (s)

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Figure 17, Load statistics of power ESS and energy ESS

Figure 19, Comparison of Battery Current in Single and Hybrid ESS configurations (Drive Cycle 2)

C. Compare Single and Hybrid ESS Configurations The current of energy battery (E-ESS) in hybrid ESS configuration and single battery system are plotted and compared in Figure 18 and Figure 19 for drive cycles 1 and 2 respectively. The energy battery in hybrid battery configuration shows lower RMS current for both drive cycles.
350 Hybrid 300 250 200 Current (A) 150 100 50 0
Figure 20, Load Demand for Hybrid and Single ESS

Single

-50 -100

0

1000

2000 Time (s)

3000

4000

Figure 18, Comparison of Battery Current in Single and Hybrid ESS configurations (Drive Cycle 1)

Peak, RMS, and average load demand for single and hybrid ESS are shown in Figure 20. In drive cycle 1, the peak and RMS load of energy battery (E-ESS) in hybrid ESS configuration are reduced 14% and 27% respectively. In drive cycle 2, the peak and RMS load of energy battery (E-ESS) in hybrid ESS configuration are reduced 7% and 31% respectively. The average power for single and hybrid ESS are the same for both drive cycles.

The total battery energy used for single and hybrid ESS configurations are presented in Figure 21. There is a slight increase (1-3%) in total energy in hybrid ESS configuration and that is due to lower final SOC of energy battery (E-ESS) comparing to the single ESS system. The simulation results show the hybrid vehicle with hybrid energy storage system configuration exhibits less stress on the energy battery and consequently better performance and higher overall efficiency over the regular PHEV with single ESS. The hybrid ESS is also more cost effective (life cycle cost) and allows the best utilization of Ultracap and battery technologies for both high power density and high energy density. Furthermore, the vehicle performance and handling is not compromised as the battery is operating in low SOC after charge depletion operation mode. The battery energy capacity can be fully utilized, enhancing the EV only range for the size of battery. The battery system life cycle for the charge depletion battery is based on the charge cycles, while the Ultracap life cycle is for power cycling at higher SOC band of operation.

efficiency, and provide flexibility in system design. DC-DC converter sizing has been modeled and determined as well. We recommend two 180kW bi-directional DC-DC converters with high efficiency (greater than 95%), high power density, and low cost to meet up to port truck or class 8 heavy duty vehicles driving at typical port drive cycles. REFERENCES
[1] Abas Goodarzi and Farzad Ahmadkhanlou, “Latest Trends in Energy Storage for Commercial Vehicles”, 2nd International Rechargeable Battery Expo (Battery Japan), Tokyo, Japan, March 2-4 2011. [2] Farzad Ahmadkhanlou, Abas Goodarzi, and Don Kang, “Plug-In Hybrid Electric Vehicle with Dual Battery System”, 25th World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium (EVS25), China, November 2010. [3] Toshihiko Furukawa, Abas Goodarzi, Nobukatsu Abe, and Harue Yanai, “The Energy-Conserving Hybrid Street Sweeper, a NewlyDeveloped Off-Road Vehicle Using an Environmentally-Friendly DLCAPTM Energy Storage System”, 25th World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium (EVS25), China, November 2010. [4] Gao, David Wenzhong; Mi, Chris ; Emadi, Ali "Modeling and Simulation of Electric and Hybrid Vehicles", Vol. 95, No. 4, Proceedings of the IEEE, April 2007 [5] S.K. Mazumder, C. Tan, and K. Acharya, Design of a radio frequency controlled parallel dc-dc all-SiC converter, IEEE Power Electronics Specialists Conference, June 2007, pp. 2833-2839. [6] S.K. Mazumder and S. Pradhan, Efficient and robust power management of reduced cost distributed power electronics for fuel-cell power system, ASME Journal of Fuel Cell Science and Technology, 2008. [7] Aymeric Rousseau, Neeraj Shidore, Richard Carlson, VRLA automotive batteries for stop & go and dual battery systems, Journal of Power Sources, Volume 144, Issue 2, 15 June 2005, Pages 411-417 [8] J. G. Kassakian, Automotive Electrical Systems –the Power Electronics Market of the Future, IEEE APEC 2000. [9] J. M. Miller and A. R. Gale, Hybrid Electric Vehicle Success Will Depend on Low Cost, Efficient Power Electronics Systems, PCIM, vol. 23, no. 11, November 1997.

Figure 21, Total energy used for Hybrid and Single ESS

IV.

CONCLUSIONS

In this paper we studied vehicle level performance, component optimization and hybrid control system for PHEV with hybrid energy storage system that consists of an Ultracap to provide power, a battery to provide energy, and two DC-DC converters. The EV portion of the drive cycle has been investigated. Hybrid energy storage system can reduce the peak and RMS power of the battery by at least 7% and 27% respectively, reduce battery initial and life cycle cost resulting in total cost saving of 10%-15%, increase the overall


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