Hyundai Supercapacitor Fuel Cell Hybrid Electric Vehicle
Soonil Jeon, Joonyong Lee, Sungjin Jeong, Taewon Lim
Abstract
A supercapacitor FCHEV(fuel cell hybrid electric vehicle) has three major advantages over a battery
FCHEV satisfying almost same packaging and cost requirements: higher energy efficiency, better
accelerating performance, and extended fuel cell life. For investigating these advantages through vehicle
tests, Hyundai Motor Company developed a prototype of supercapacitor FCHEV based on a SantaFe
SUV. In this paper, we introduce the configuration of this prototype, and show several chassis
dynamometer test data to reveal its fuel economy and dynamic performance related to supercapacitor’s
automatic charging and discharging characteristics.
Keywords: fuel cell, supercapacitor, ultracapacitor, HEV, efficiency.
1
Introduction
A Fuel Cell System can be hybridized with batteries or supercapacitors (or ultracapacitors) depending on
its design goals and constraints related to packaging and cost issues. Compared to a battery FCHEV,
HMC(Hyundai Motor Company) proved through in-house vehicle simulation that a supercapacitor
FCHEV is approximately 10% more energy efficient, can response 0.5~1.0 second more quickly to the
accelerating scenario from 60 to 100 kph, and can prolong fuel cell durability. These advantages over a
battery FCHEV come from several aspects as follows:
· Higher charging and discharging efficiency of supercapacitor itself under broad range of operating
temperature
· Removal of high voltage DC/DC converter (see Figure 1)
· Larger amount of recuperation energy during vehicle deceleration
· Faster dynamic response of supercapacitor
· Comparatively mild operation of fuel cell resulting in less air starvation
As disadvantages of a supercapacitor FCHEV, following three can be addressed.
· Limited power assisting due to lower energy density of supercapacitor
· Extra startup process and device in order to get supercapacitor voltage balanced with fuel cell
voltage
· Cost competitiveness of supercapacitor itself
For the purpose of investigating advantages and disadvantages above through vehicle tests, HMC
developed a prototype of supercapacitor FCHEV (see Figures 1 and 4) where a fuel cell is directly
connected to a supercapacitor without a high voltage DC/DC converter. In this paper, we briefly introduce
three processes targeting successful development of the prototype as follows: firstly, feasibility check in
Chapter 2 based on a small-scale supercapacitor and fuel cell system, secondly bench test in Chapter 3
based on a large-scale supercapacitor and fuel cell system, and lastly chassis dynamometer test in Chapter
4 to evaluate the developed prototype’s energy efficiency and dynamic performance under three standard
driving cycles: FUDS, HWFET, and JAPAN10-15.
2
Feasibility Check of Supercapacitor Fuel Cell Hybrid System
In order to check the feasibility of a supercapcitor and fuel cell bybrid system where these two power
sources are directly connected with each other not using a high voltage DC/DC converter as shown in
Figure 1, we used a small-scale supercapacitor (75V, 117F), a corresponding virtual fuel cell (12kW,
45~75V), and a virtual inverter (-20~20kW). The virtual fuel cell and inverter were realized by a
programmable electrical loader. Theoretically, in this hybrid configuration, the charging and discharging
of supercapacitor is automatically controlled by the fluctuation of main bus voltage following I-V curve of
fuel cell. For example, in case of negative change of main bus voltage (e.g. vehicle acceleration), the
supercapacitor is discharged. In a similar manner, in case of positive change of main bus voltage (e.g.
vehicle deceleration), the supercapacitor is charged.
Figure 1 conceptually shows three operating modes of a supercapacitor FCHEV:
· Mode 1: supercapacitor discharging mode or power assisting mode in case of vehicle acceleration
· Mode 2: fuel cell only mode in case of vehicle cruising
· Mode 3: supercapacitor charging mode using fuel cell and regenerative braking power in case of
vehicle deceleration
In order to check characteristics of Mode 1, we did a test shown in Figure 2 (a) where the step input of
inverter power (15, 0kW) was imposed. Here, inverter power equals to summation of fuel cell and
supercapacitor powers. During inverter power change from 0kW(idle) to 15kW(motoring), a
supercapacitor quickly supplies 11kW power assist in approximately 0.1 second while a fuel cell increases
powering slowly. Additionally, the supercapacitor charging mode by only fuel cell power can be seen after
inverter power change from 15kW to 0kW. In terms of voltage change, it can be observed that there is a
time delay between fuel cell and supercapacitor voltages. That is, the fuel cell voltage is followed by the
supercapacitor voltage. Figure 2 (b) is for checking characteristics of Modes 1 and 3. During inverter
power change from 15kW(motoring) to -15kW(generating), the supercapacitor suddenly switches to the
charging mode, where charging power decreases from -23kW (-8kW fuel cell + -15kW inverter) to 0kW.
Due to the energy limitation of supercapacitor related to its maximum voltage 75V, the supercapacitor
charging more than or equal to -15kW continues for only 10 seconds. In addition to that, dotted lines
shown in Figure 2 (upper four sub-figures) represent simulation results, which show a good agreement
with test data because of intrinsic linear characteristic of this hybrid system.
Figure 1: Operating modes of battery or supercapacitor FCHEV
(a) 15, 0kW
(b) 15, -15kW
Figure 2: Responses of fuel cell and supercapacitor to the step input of inverter power
3
Bench Test
The purposes of bench tests are, firstly to verify a large-scale supercapacitor and fuel cell system where
the supercapacitor for a prototype vehicle (see Table 1 and Figure 4) and a corresponding virtual fuel cell
are used, and secondly to develop control logics related to startup and power-limiting strategy during
vehicle operation. The bench tests targeting the first purpose can be classified into two tests: worst-case
charging and discharging test, and driving cycle test. In the first worst-case test, the step input of inverter
power (75, -50kW) or inverter current (300, -300A) is applied to this large-scale hybrid system in the
same manner described in Chapter 2. In the second driving cycle test, the simulated inverter current under
FUDS driving cycle (see Figure 3) through an in-house vehicle simulator is used as an input of
programmable electrical loader. Some knowledge from these two bench tests is summarized as below.
· Dynamic responses of supercapacitor: from 0 to 260A or -420A in 0.2~0.3 second, from 0 to
65kW or -70kW in 0.2~0.4 second.
·
·
Supercapacitor’s charging and discharging duration: 5.2~7.3 seconds in case of the step input of
inverter power (75, -50kW), 7.0~7.2 seconds in case of the step input of inverter current (300, 300A).
As shown in Figure 3, the supercapacitor and fuel cell are in charge of high (more than 0.02 Hz)
and low (less than 0.02 Hz) frequency parts of inverter current, respectively, which characteristic
may contribute to extending fuel cell life by smoothing current draw from fuel cell (also see
Figure 5).
Figure 3: Bench tested currents of fuel cell and supercapacitor under FUDS driving cycle
4
Chassis Dynamometer Test
For developing a prototype of supercapacitor FCHEV, HMC changed a battery FCHEV already developed
in 2002 whose configuration is shown in Figure 1 by replacing a battery and a high voltage DC/DC
converter with a supercapacitor. Also the corresponding wire-harness was modified. Table 1 and Figure 4
show the specification of this prototype and its photo. The supercapacitor of 270V and 15.7F is composed
of 108 cells (each 2.5V and 1700F). In order to validate the developed prototype of supercapacitor
FCHEV in terms of its energy efficiency, we measured the gasoline-equivalent fuel economy through
chassis dynamometer tests under three standard driving cycles: FUDS, HWFET, and JAPAN10-15. Table
2 summarizes the results, where fuel economy from a FCEV(fuel cell electric vehicle) and a battery
FCHEV where 30kW battery is implemented is included for comparison. In this comparison, state(or
health) of each fuel cell itself is almost same, and the fuel economy is the SOC-corrected value based on
SAE J2572 accounting for consumed electric energy from a battery or a supercapacitor. Especially, under
FUDS driving cycle, the fuel economy of supercapacitor FCHEV is 30.8% better than that of FCEV, and
11.2% better than that of battery FCHEV, which shows that our original target, 10% better fuel economy,
is accomplished. As described in Chapter 1, this better fuel economy comes from high efficiency of
supercapacitor itself compared to battery, reduction of power transfer loss by excluding a high voltage
DC/DC converter, and larger regenerative braking energy. Additionally, Figure 5 shows that fuel cell
current at the supercapacitor FCHEV is much smoother than that of the battery FCHEV, from which
characteristic, it is expected that fuel cell life can be prolonged by reducing its transient operation.
Table 1: Specification of supercapacitor FCHEV
Fuel Cell
Supercapacitor
Motor
Base vehicle
75kW PEM
270V, 15.7F
65kW AC
HMC Santa Fe
(a) Santa Fe supercapacitor FCHEV
(b) Supercapacitor packaging
Figure 4: HMC prototype of supercapacitor FCHEV
Table 2: Gasoline-equivalent fuel economy (km/liter) through chassis dynamometer tests
FUDS
HWFET
JAPAN10-15
FCEV
19.8
25.3
19.4
Battery FCHEV
23.3
25.2
21.4
Supercapacitor FCHEV
25.9
27.5
24.3
Figure 5: Comparison of fuel cell currents at battery FCHEV (red and thin line) and supercapacitor
FCHEV (blue and thick line) under FUDS driving cycle (chassis dynamometer test)
5
Conclusion
We investigated the automatic charging and discharging characteristics of a supercapacitor and fuel cell
hybrid system without an extra voltage converting device through small- and large-scale experiment
settings. The results show that dynamic response of supercapacitor from idle state to maximum charging
or discharging state is about 0.2~0.4 second, and its worst-case charging and discharging duration is
approximately 5~7 seconds. Also, it was tested under FUDS driving cycle that a supercapacitor is in
charge of high frequency part more than 0.02Hz of inverter current, and the other low frequency part is for
a fuel cell. Finally, we proved through chassis dynamometer tests that a Santa Fe based supercapacitor
FCHEV shows 11.2% better fuel economy than a baseline battery FCHEV under FUDS driving cycle. In
addition to that, we observed the possibility to increase fuel cell durability through its milder operation. In
order to prove this possibility, we are preparing some test procedures.
References
[1] Sangkwon Kim and Seoho Choi, Development of Fuel Cell Hybrid Vehicle by Using Ultra-Capacitors as a
Secondary Power Source, SAE world congress, 2005
[2] Seoho Choi, Soonwoo Kwon, et. al, Control of Power Distribution in Fuel Cell Hybrid Vehicle, EVS20, 2003
[3] Seoho Choi, Soonwoo Kwon, et. al, Design of Fuzzy Logic Power Distribution Controller for Fuel Cell Hybrid
Vehicles, Fisita, 2004
[4] Kichun Lee, Seoho Choi, et. al, Hyundai Santa Fe FCV Powered by Hrodrogen Fuel Cell Power Plant Operating
Near Ambient Pressure, SAE world congress, 2003
[5] Andrew Burke and Marshall Miller, Ultracapacitor and Fuel Cell Applications, EVS20, 2003
[6] Ardalan Vahidi, Anna Stefanopoulou, and Huei Peng, Model Predictive Control for Starvation Prevention in a
Hybrid Fuel Cell System, IEEE Proceedings of 2004 American Control Conference, 2004
Author
Soonil Jeon, Senior Research Engineer
Fuel Cell System Team
R&D Division for Hyundai Motor Company & Kia Motors Corporation
772-1, Jangduk-Dong, Whasung-Si Gyunggi-Do, 445-706, KOREA
Tel: +82 31 368 7480, Fax: +82 31 368 6787
Email: soonill@hyundai-motor.com
2003, Ph. D., Seoul National University
Joonyong Lee, Research Engineer
Fuel Cell System Team
R&D Division for Hyundai Motor Company & Kia Motors Corporation
772-1, Jangduk-Dong, Whasung-Si Gyunggi-Do, 445-706, KOREA
Tel: +82 31 368 7958, Fax: +82 31 368 6787
Email: the741@hyundai-motor.com
2004, BS, Korea University
Sungjin Jeong , Research Engineer
Fuel Cell System Team
R&D Division for Hyundai Motor Company & Kia Motors Corporation
772-1, Jangduk-Dong, Whasung-Si Gyunggi-Do, 445-706, KOREA
Tel: +82 31 368 6669, Fax: +82 31 368 6787
Email: sjjeong@hyundai-motor.com
2002, MS, Gwangju Institute of Science and Technology
Taewon Lim, Chief Research Engineer
Fuel Cell System Team
R&D Division for Hyundai Motor Company & Kia Motors Corporation
772-1, Jangduk-Dong, Whasung-Si Gyunggi-Do, 445-706, KOREA
Tel: +82 31 368 7080, Fax: +82 31 368 6787
Email: twlim@hyundai-motor.com
1991, Ph. D., The State University of New York