A Development of an Energy Storage System for Hybrid Electric Vehicles
Using Supercapacitor
Jin-uk Jeong, Hyeoun-dong Lee, Chul-soo Kim,
Hang-Seok Choi, Bo-Hyung Cho
Abstract
An energy storage system for improving performance of hybrid electric vehicles (HEV) is presented.
The hybrid power system consists of batteries and supercapacitors. The supercapacitor contributes to
the rapid energy recovery associated with regenerative braking and to the rapid energy consumption
associated with acceleration in electric vehicles. This power system allows the acceleration and
deceleration of the vehicle with minimal loss of energy and minimizes the stress of the main batteries
by reducing high power demands away from the batteries. It also leads to longer battery life by
extracting energy at a slower average rate. The total weight of hybrid system is lighter than that
applied only batteries, so that the efficiency of vehicle is increased.
In this paper, the equivalent model of supercapacitor is included. This model can be used in simulating
for automotive power systems, e.g. the voltage response and energy efficiency. The soft switching bidirectional DC-DC converter is used to connect the supercapacitor with the battery for the controlled
instantaneous electric power flow. The hybrid power system is analyzed and verified by experimental
results from the prototype system. Copyrightⓒ 2002 EVS19
Keywords: “DC-DC”, “HEV (hybrid electric vehicle)”, “regenerative braking”, “supercapacitor”,
“energy storage”, “Buck-Boost”.
1. Introduction
The supercapacitor can not only be charged and discharged more than one million times but also be
stored with ten times more energy than conventional electrolytic capacitors. Usually, the battery has
the limitation on a life cycle, and a prompt storage and consumption of stored energy. It is due to the
basic nature of chemical reaction of a battery. In contrast, the supercapacitor has the merits of a rapid
charge and discharge of energy and a longer life cycle, because of electrostatic nature of capacitor
rather than chemical reaction. Besides, a supercapacitor has extremely high capacitance with superior
durability and maintenance-free characteristic[1,2].
It is tendency that the applicable field of supercapacitor is gradually promoted by virtue of its merits.
Specially, the usefulness is more and more increased as an energy buffer in developing eco-friendly
and low/zero-emission vehicles, such as Electric Vehicle(EV), Hybrid Electric Vehicle(HEV) or Fuel
Cell Vehicle(FCV) etc. Figure 1 presents a simple block diagram that the supercapacitor is applied to
electric vehicle/hybrid electric vehicle system. As shown in Fig. 1, it can be expected that the
improvement of system efficiency and the life extension of the energy storage system by using
batteries as a main storage and supercapacitor as an assistant one. At this time, the supercapacitor is
responsible for the transient demand of power and the battery is responsible for the constant
requirement of energy[3,4].
In this paper, the electrical characteristic of supercapacitor is looked into and the electrical equivalent
model on the basis of the investigation is proposed. The validity of the proposed model is proved in
that the simulation results are fitted well to actual supercapacitor impedance characteristic over all test
frequency range. In order to control the energy stored in supercapacitor bank in real-time manner, the
storage system structure and specifications are investigated. And the modular configuration of bidirectional DC/DC converter is studied. With the proto-type of DC/DC converter and supercapacitor
bank, the experiments in which electric energy can be controlled through regulating the supercapacitor
bank voltage are conducted.
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Battery
MOT
INVERTER
Super-capacitor
ENG
DC/DC
CONVERTER
Motoring
Regenerative Braking
Figure 1: Electric vehicle/hybrid electric vehicle system using supercapacitors.
2. System Composition
2.1. The modeling of supercapacitor
Since supercapactor is mainly introduced to the applications requiring high-dynamic performance, the
voltage and current transient behavoir of the supercapacitor should be taken into account. Therefore,
for energy storage system design and its control, it is important to make an electrical equivalent circuit
for supercapacitor in detail.
Even if supercapacitor can be modeled as a conventional capacitor, it is insufficient to use the reduced
model using a resistance and a capacitance connected in series considering high-dynamic behavior. In
this paper, new equivalent circuit of supercapacitor, as shown in Fig. 2, is obtained for the
supercapacitor of PC2500TM of Maxwell Technologies, Inc(MTI). The specifications of PC2500TM are
listed in Table 1. According to voltage, load current, temperature, etc, the resistances and capacitances
in the proposed equivalent circuit are possibly varied. The modeling error caused by operating
condition, however, can be minimized by modifying the value of each element in the proposed
equivalent circuit.
Figure 3 shows the validity of the proposed equivalent circuit. The supercapacitor impedance
(magnitude and phase) is plotted in this figure, where dotted and solid curve are experimental and
simulated results, respectively. It can be seen that the simulated results are fitted well to actual
supercapacitor impedance characteristic over all test frequency region.
Table 1: Specifications of supercapacitor( PC2500TM ) [1]
parameter
capacitance
ESR @ 25oC DC
1kHz
voltage(continuous)
voltage(peak)
rated current
weight
volume
specification
2700
1
0.55
2.5
2.7
625
725
0.6
1380
unit
F
mΩ
mΩ
V
V
A
g
ℓ
ESR2
ESR1
EPR
Cs
Co
Figure 2: Proposed electrical equivalent circuit of supercapacitor.
Figure 3: Impedance characteristic of supercapacitor( PC2500TM ) [1].
2.2. Energy storage system using supercapacitor
2.2.1. System specifications
To control the energy stored in supercapacitor bank, it is need that the voltage of the supercapacitor
bank should be controlled. If not, the supercapacitor voltage depends on the battery voltage, so that
there is no possibility to control the energy stored in supercapacitor bank. Thus, DC/DC converter is
indispensable to regulate the bank voltage level. Moreover, because the current can flow to
supercapacitor when the supercapacitor is charged and the current can flow from supercapacitor when
the supercapacitor is discharged, the DC/DC converter has to have a bi-directional nature. Figure 4
shows the system configuration with battery pack and supercapacitor bank as an energy storage. The
DC/DC converter is on boost-mode operation as the inverter supplies traction power to the motor. On
the other hands, the DC/DC converter is on buck-mode operation as the regenerative energy come to
supercapacitor bank.
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Super-capacitor
40-69V
Battery
120-180V
Boost mode
(Motoring)
Inverter
Battery
120-180V
Inverter
Super-capacitor
40-69V
Buck mode
(Regenerating)
Figure 4: System configuration of the supercapacitor implemented.
Table 2: Specification of system.
item
supercapacitor
voltage
battery voltage
electric power
capacity
efficiency
operating
temperature
specification
40 ~ 69 Vdc
120 ~ 180 Vdc (nominal 150 V)
motoring :
PMot = 12kW, I Max = 80A, 5 sec
generating :
PGen = 9kW, I Max = -60A, 10 sec
(ref : battery voltage 150 V )
more than 90%
-30° ~ 85°C
From the required specification of Table 2, the number of supercapacitor cell is designed to be 30, on
the basis that the maximum voltage of each supercapacitor cell is 2.3V (92% of continuous voltage
rating), and the minimum voltage of each supercapacitor cell is 1.25V (50% of continuous voltage
rating). As a consequence, the capacitance of the supercapacitor bank used in this study becomes
2700/30=90F and equivalent series resistance (ESR) 1mΩ×30=30mΩ, to make the maximum stored
energy become 210kJ. The the total volume and weight of the supercapacitor bank is 18l and 22kg,
respectively.
2.2.2. The topology of bi-directional DC/DC converter
There can be lots of converter topology for realizing a bi-directional DC/DC converter; single-stage
buck/boost type and full-bridge type as a typical one. Full-bridge type topology has merits compared
to single-stage buck/boost type topology. 1) Electrical isolation between input and output is
guaranteed. 2) Higher boost ratio can be implemented. 3) System protection is possible when output
stage short take place. From these facts, full-bridge type topology is employed in this study, in spite
that the full-bridge type topology is somewhat bulky, requires more components rather than singlestage buck/boost type.
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Boost operation
Buck operation
Lf
Sc1
Sc4
Sb1
Sb4
Sa
Llk
Vc
Vb
120-
40 - 69V
180V
Super-capacitor
Battery
Sc3
Sc2
Ch
Sb3
Low-voltage side Current-fed type
High-voltage side
Sb2
Voltage-fed type
Figure 5: The bi-directional DC/DC converter(full-bridge type topology).
As shown in Fig. 5, the bi-directional full-bridge topology DC/DC converter is operated on boost
mode at the time electric power is supplied from supercapacitor stage (low voltage stage) to battery
stage (high voltage stage), and on buck mode at the time electric power is absorbed from battery stage
to supercapacitor stage. Because the supercapacitor stage of the DC/DC converter has low voltage
level, a current control is necessary in the cause of reducing current’s burden on semiconductors. At
the battery stage of the DC/DC converter, voltage control is necessary to match to DC bus voltage of
the inverter. Also, soft switching technique of zero voltage-zero current switching is applied to this
system for improving the DC/DC converter efficiency.
2.3. Bi-directional DC/DC converter with supercapacitor bank
2.3.1. The unit module design of bi-directional DC/DC converter
In the case that a 10kW DC/DC converter is designed as a single module, there are lots of difficulties
such as limitation on a selection of switching device, DC/DC converter volume, and so on. For this
reason, the multiple module type is more profitable, which could be supplying electric power to
inverter stage through parallel operation of each module. After considering various conditions such as
complexity of system, size, efficiency, and the required specification as listed in Table 2, the overall
DC/DC converter is made up of 6 unit module as shown in Fig. 6. Accordingly, the power capacity of
each module in boost and buck mode operation is 12kW/6 = 2kW and 9kW/6 = 1.5kW, respectively.
At the same time that the switching frequency of system is established by 60kHz, after considering the
switching device voltage, current capacity, and transformer size, etc. Table 3 shows specification of
converter’s main parts which is designed by this research.
Figure 6: Proto type of DC/DC converter unit module.
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Table 3: Power stage design of converter unit module
item
symbol
device
MOSFET
Sc1-Sc4, Sa
IXFN180N20
IGBT
Sb1-Sb4
SGL50N60RUFD
transformer
T1
PQ5050
inductor
clamp capacitor
filter capacitor
fuse
fuse
Lf
Ch
Cf
F1
F2
PQ5050
35uF/250V
220uF/400V, #2
100A/100V
25A/500V
note
180A, 200V
Rds(on):10mΩ
50A, 600V
Vce : 2.2V
Nb : 9 turns
Nc : 6 turns
N : 15 turns
Parallel Connection
Low Voltage Stage
High voltage stage
2.3.2. The design of supercapacitor bank
A supercapacitor energy storage unit will necessarily consist of many cells in series to attain the
required system voltage. The issue of importance in accessing the feasibility of the supercapacitor
bank is cell-to-cell voltage variability. At the moment the capacitors connected in series are charged,
each capacitor voltage can vary according to its capacitance. Therefore, the bigger the difference of
each capacitance is, the larger the voltage difference of each capacitor is. With the large deviation of
capacitance, the capacitor of small capacitance may be damaged due to the excessive voltage over
rating. The capacitance of each capacitor can be calculated from voltage measured when the voltage of
capacitor bank is reach to 56V. After charging and discharging several times to recognize a deviation
of capacitance in each capacitor, the capacitance of each capacitor, as shown in Fig. 7, is estimated.
In contrast to the case of pre-charge stage in which the voltage of each capacitor appears to function of
capacitance, Equivalent Parallel Resistance (EPR) decides voltage of each capacitor at steady state.
Internal EPR among the capacitors has big deviation. If the capacitor is used for a long times, the
capacitor having big internal EPR can be damaged to pass over rated voltage. Therefore, as connecting
on the terminal of supercapacitor with balancing resistor bigger than internal EPR, we can keep each
capacitor's voltage evenly. In this research, the balancing resistor is chosen as the result of considering
voltage balance and power consumption among the terminals of capacitor[5].
Figure 8 shows the supercapacitor bank, in which each capacitor is connected by using copper bus bar
to reduce the ohmic resistance of the connecting wire. Figure 9 shows an electric circuit diagram of a
supercapacitor with cell balancing resistor and a fault detection branch. Supercapacitor fault detection
branch has LED and driving resistor, so that the damaged cell is easily detected without a timeconsuming job to find out which a cell is broken down.
2.3.3. Efficiency estimation of DC/DC converter unit module
Although the calculation of DC/DC converter efficiency must consider various kinds of factors, the
efficiency can be calculated by considering conduction loss and switching loss of power switching
semi-conductor(MOSFET, IGBT) because the conduction loss and switching loss are mainly
responsible for the DC/DC converter loss.
The DC/DC converter have efficiency of 88.6% for boost mode operation in full load condition, if we
calculate efficiency of converter considering factors, such as conduction loss of rectifier diode,
conduction and switching losses of MOSFET(switching device). In buck mode operation, the
efficiency of DC/DC Converter is about 90% if we calculate efficiency of converter considering
factors, such as conduction loss of rectifier-diode, conduction and switching losses of IGBT(switching
device).
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3500
3000
capacitance
2500
2000
1500
1000
500
0
1
3
5
7
9 11 13 15 17 19 21 23 25 27 29
Figure 7: Estimation of capacitance per each capacitor cell.
(a)
(b)
Figure 8: (a) Supercapacitor bank. (b) Cell balancing and fault detection circuit hardware of
supercapacitor(An enlarged photograph of (a)).
Super
capacitor
Balancing
resistor
Figure 9: The cell balancing and fault detective circuit of supercapacitor.
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3. Experimental results and discussion
3.1. Basic boost/buck mode experiment of bi-directional DC/DC converter
3.1.1. Boost mode operation
In boost mode experiment, the supercapacitor bank in fully charged condition plays a role of an input
source and resistor load is connected to output terminal(high voltage stage)of bi-directional DC/DC
converter. Thus, electric power is supplied from supercapacitor to resistor load. The bi-directional
DC/DC converter performs a boosting control of supercapacitor voltage. Figure 10 shows the
experimental result of boost mode operation, where each waveform means the gating signal(Sc1Sc4,Sa) of each switch and current waveform( I lk ) at high voltage side of transformer.
3.1.2. Buck mode operation
In buck mode test, the bi-directional DC/DC converter supplies electric power to resistor load that is
connected to output stage(low voltage stage) of the DC/DC converter. External DC power supply is
introduced for this experiment and plays a role of high voltage input source. Figure 11 shows
voltage( Vlk ) and current waveform( I lk ) of the transformer. As shown in Fig. 11, we can confirm
Zero-Voltage Switching(ZVS) operation from experimental result.
Figure 10: An experimental result of boost mode operation.
Figure 11: An experimental result of buck mode operation.
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3.2. Experiment of electric power exchange between supercapacitor bank and battery
pack
After connecting supercapacitor bank(low voltage) and battery pack(high voltage) on both stages of
the bi-directional DC/DC converter, the test of bi-directional electric power flow is conducted. In this
experiment, the bi-directional DC/DC converter is in current control mode. The current flowing
to/from supercapacitor through boost inductor is controlled, so that the energy stored in supercapacitor
bank is charged/discharged. The DC/DC converter receives a starting signal and a current control
command from high-level controller.
Figure 12 and 13 show the experimental results for the boost and buck mode operation, repectively. In
both boost mode and buck mode experiment, the current command is 35A and maintains for 25
seconds. The negative sign of the current command means discharging supercapacitor bank(Fig. 12)
and the positive sign of that means charging supercapacitor bank(Fig. 13). Under full load condition,
the efficiency of the bi-directional DC/DC converter in boost mode and buck mode experiments is
measured as 86% and 88%, respectively, which corresponds well with the calculation results except a
small error of 2%.
Figure 12: Experimental waveforms of boost mode operation(discharge of supercapacitor bank).
Figure 13: Experimental waveforms of buck mode operation(charge of supercapacitor bank).
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4. Conclusion
This paper deals with an optimal design of energy storage system using supercapacitor and modular
development of bi-directional DC/DC converter. For a high-dynamic energy storage system design
and its control, an electrical equivalent circuit modeling of supercapacitor using passive elements is
performed. The proposed supercapacitor model can be used in the simulation of automotive power
systems with respect of a power delivery and energy consumption. The validity of the proposed model
is proved in that the simulation results are fitted well to actual supercapacitor impedance characteristic
over all test frequency range.
In order to control the energy stored in supercapacitor bank in real-time manner, the storage system
structure and specifications are investigated. And the modular configuration of bi-directional DC/DC
converter is studied. With the proto-type of DC/DC converter and supercapacitor bank, the
experiments in which electric energy can be controlled through regulating the supercapacitor bank
voltage are conducted. Currently, the algorithm deciding how much electric energy is taken from or
charged to the supercapacitor bank for parallel type hybrid electric vehicle is under development stage.
5. References
[1]
Maxwell. Ultracapacitors Data Sheets and technical information for PC2500TM, [Maxwell Publications].
[2]
Thomas Dietrich,“UltraCaps-A new Energy Storage Device for Peak Power Applications”, 18th
Internati -onal Electric Vehicle Symposium, Berlin, Germany, October 2001, paper PP244 [on CDROM]
[3]
B.J.Arnet, and L.P.Haines,“Combining Ultracapacitors with Lead-Acid Batt -eries”, 17th International
Electric Vehicle Symposium, Montreal, Canada, October 2000, paper 2B-2 [on CD-ROM]
[4]
Burke,A.F. and Miller,M.,“Update of Ultracapacitor Technology and Hybrid Vehicle Applications:
Passenger Cars and Transit Buses”,18th International Electric Vehicle Symposium, Berlin, Germany,
October 2001, paper PP044 [on CD-ROM]
[5]
S. Buller, E. Karden, D. Kok, R.W. De Doncker, “Simulation of Super capacitors in highly dynamic
Applications”, 18th International Electric Vehicle Symposium, Berlin, Germany, October 2001, paper
PP056 [on CD-ROM]
6. Affiliation
Jin-uk Jeong
R & D Division for Hyundai Motor Company and Kia Motors Corporation, 772-1,
Changduk-Dong, Whasung-Si, Kyunggi-Do, 445-850, Korea Phone: +82-31-369-7244, Fax:
+82-31-369-6299, E-mail: jinuk@hyundai-motor.com
He received a M.S. degree in electrical engineering from Kyung-pook National University,
Taegu, Korea, in 2001. Since 2001, he has been with Hyundai Motor Company as a Research
Engineer. His current research interests are control system for electric and hybrid electirc
vehicle.
Hyeoun-dong Lee
Phone: +82-31-369-7243, Fax: +82-31-369-6299, E-mail : dong@hyundai-motor.com
He received a M.S and Ph.D degrees in electrical engineering from Seoul National
University, Seoul, Korea, in 1995 and 1999, respectively. Since 1999, he has been with
Hyundai Motor Company as a Senior Research Engineer. His current research interests are
developments of electric motor drives, power conversions and their controls for electric and
hybrid electric vehicle.
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Chul-soo Kim
Phone: +82-31-369-7240, Fax: +82-31-369-6299, E-mail : kimcs@hyundai-motor.com
He was born in Korea in 1957, and received M.S and Ph.D degrees in mechanical
engineering from Korea Advanced Institute of Science & Technology, Korea, in 1982 and
1988, respectively. Since 1988, he has been with Hyundai Motor Company as a team
manager, and took a part of developing the EV, HEV and FCEV program. His current
research interests are developments of the pure, engine hybrid and fuel cell hybrid electric
vehicle.
Hang-Seok Choi
Seoul National University, Korea Phone: +82-2-880-1785, Fax: +82-2-878-1452,
E-mail: hangseok@snu.ac.kr, URL: http://pearlx.snu.ac.kr.
Hangseok Choi was born in Korea in 1970. He received the B.S. and M.S. degrees in
electrical engineering from the Seoul National University, Seoul, Korea, in 1997 and 1999,
respectively. He is presently working toward the Ph.D. degree at Seoul National University.
Bo-Hyung Cho
Phone: +82-2-880-1785, Fax: +82-2-878-1452, E-mail: bhcho@snu.ac.kr
Bo Hyung Cho (M’89-SM’95) received the B.S. and M.E. degrees in electrical engineering
from California Institute of Technology, Pasadena, and the Ph.D. degree from Virginia
Polytechnic Institute and State University (Virginia Tech), Blacksburg. From 1982 to 1995,
he was a professor in the Department of Electrical Engineering, Virginia Tech, Blacksburg,
Virginia. Since 1995, he has been joined as a professor the school of Electrical Engineering,
Seoul National University, Seoul, Korea.
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