Utilizing Fuel Cell and Supercapacitors for
Automotive Hybrid Electrical System
Phatiphat Thounthong*, Stéphane Raël and Bernard Davat, Member, IEEE
Institut National Polytechnique de Lorraine (INPL), ENSEM, GREEN, CNRS
2, Avenue de la Forêt de Haye, 54516 Vandœuvre-lès-Nancy, France
*E-mail: Phatiphat.Thounthong@ensem.inpl-nancy.fr
for this purpose [6]. Then the very fast power response of
supercapacitors can be used to complement the slower power
output of the fuel cell to produce the compatibility and
performance characteristics needed by hybrid automotive
system as shown in Fig. 1.
Abstract— This paper presents a new control algorithm for
utilizing PEM fuel cell and supercapacitors for automotive
system. A PEM fuel cell is operated as a directional main power
source connected to 42 V dc bus (PowerNet) by boost converter,
and supercapacitors is controlled as a fast bidirectional auxiliary
power source connected to dc bus by a 2-quadrant dc/dc
converter. The system employs a 500 W PEM fuel cell and a
supercapacitive storage device, composed of six components
(3,500 F, 2.5 V, 400 A) associated in series. The system control
structure is realized by analogical for current loop at high
dynamics and digital control (dSPACE) for voltage loop and
algorithm. The proposed control, illustrated by experimental
results, avoids speedy transition of fuel cell current and is based
on the power sharing demanded at the dc bus between the main
and auxiliary sources.
iSuperC
PEM
Fuel Cell
2H 2
O2
Motor
Converter
Motoring
Regenerative
Breaking
I. INTRODUCTION
Electronics
loads
Figure 1. Fuel cell and supercapacitors hybrid system.
or the moment, automotive hybrid electrical system has
been developed for drastically cleaner and more
economical vehicles. Hybrid electrical cars, such as the Honda
Insight and Toyota Prius, were especially tested by U.S.
Department of Energy (DOE) and showed the fuel saving [1].
Fuel cell has been manifestly developed to become the main
source in many applications. The fuel cell transit bus, which
has been designed and developed by DOE, has been
acknowledged as a zero emission vehicle. Its only emission is
in fact water vapour [2].
One of the main weak points of fuel cell is its slow
dynamics [3-5]. In fact, the dynamics of fuel cell is limited by
the hydrogen delivery system, which contains pumps and
valves, and in some cases a reforming process. In particular, a
step electrical load will imply vast variation of the voltage of
42 V dc automotive distribution bus, because the main source
has slow dynamic response.
Moreover, the automotive system has problem when
starting electrical motor, which demands from the dc bus high
energy in short time.
To solve these problems, the system must have a fast
auxiliary source, to supply high transient energy. The new
high current supercapacitor technology has been developed
0-7803-8975-1/05/$20.00 ©2005 IEEE.
iL
2 H 2O
Index Terms— Automotive, Fuel Cell, Hybrid Electrical
System, Polymer Electrolyte Membrane, Supercapacitors
F
SuperCapacitors
iFC
Compared with batteries, supercapacitors have at least two
orders of magnitude higher specific powers, and much longer
lifetime. Because they are capable of millions cycles, they are
virtually free of maintenance. Their great rated currents enable
fast discharges and fast charges as well. Their quite low
specific energy, compared to batteries, is in most cases the
factor that determines the feasibility of their use in a particular
high power application [7].
By nature, fuel cell has slow dynamics. If it is operated in
nearly steady state condition in order to avoid high dynamics,
the key advantage is that the mechanical stretch of fuel cell is
prevented, and therefore the fuel cell stack damage will not
happen.
This paper presents automotive system having a PEM
(Polymer Electrolyte Membrane) fuel cell as main source and
supercapacitors as auxiliary source. It especially details the
new control algorithm.
The experimental results are composed of two parts; the
first one shows PEM fuel cell characteristics when connecting
with converter to dc bus; the second one shows hybrid
characteristics for different situations at dc bus in order to
authenticate system operation.
90
steady state conditions, and supercapacitors are functioning
during lacking energy from main source, transient energy
delivery or transient energy recovery.
The control principle of the hybrid structure is presented in
Fig. 4. The main point of this control is to regulate dc bus
voltage vBus with the following constraints: fuel cell electrical
power must be kept within an interval [PFCMin, PFCMax],
supercapacitive storage device voltage must be kept within an
interval [vSuperCMin, vSuperCMax], and fuel cell current slope must
be limited to a maximum absolute value. In previous work [9],
one has tried in hybrid system built with battery as a main
source and supercapacitors as an auxiliary source to control
currents in the different parts of the system (battery,
supercapacitors and load).
One of the problems, which appear in such a control, is the
presence of dead time operation while the system changes of
operating mode (from steady state to a sudden recovery state,
for example). For new conception, the hybrid system control
presents vBus regulation through the power delivered by the
fuel cell and the supercapacitors [10], and the current
references are a consequence of the power demand. More
precisely, the dc bus voltage controller (PI controller)
generates a power reference, called PBusREF as delineated in
Fig. 4. This signal is limited in level and rate of change
(slopped limitation), to create fuel cell power reference
PFCREF, and then it obtains fuel cell current reference iFCREF1.
The difference between the two previous power references
gives supercapacitors power reference PSuperCREF, and one of
the three supercapacitors current references, that is to say
iSuperC3. This signal defines supercapacitors modes of
operation: normal (charge from fuel cell) if iSuperC3 is zero,
recovery (charge from dc bus) if it is positive, and discharge if
it is negative. The two other supercapacitors current
references, iSuperC1 and iSuperC2, are generated by fuel cell
current controller2 (I controller) and supercapacitors voltage
controller (P controller), respectively. The hybrid control
algorithm as explained hereafter does the choice among these
three references.
Firstly, during normal operation, when iSuperC3 is zero,
supercapacitors are charged by the fuel cell up to the voltage
level vSuperCNormal, which is within the previously defined
interval [vSuperCMin, vSuperCMax]. To meet this target, fuel cell
current controller2 is supplied with fuel cell rated current as
reference, iFCRated, corresponding to fuel cell rated power.
Supercapacitors voltage controller is supplied with vSuperCNormal
as reference. The hybrid control algorithm leads to select the
minimum value between iSuperC1 and iSuperC2 if supercapacitors
voltage is less than vSuperCNormal (that is to say if iSuperC2 is
positive), zero otherwise.
Note that during this operation, charging current has to be
limited in rate of change, in order to avoid instability due to a
too fast increasing current, which would be seen as a peak
load by the system.
Note also that each transition in the normal mode begins
with the initialisation of the integrator of fuel cell current
controller2.
II. HYBRID SYSTEM STRUCTURE
A. Fuel Cell Converter
Fuel cell operates at a directional current, and at low
voltage. Thereby, the fuel cell converter, presented in Fig. 2,
is selected as a boost converter used to adapt the low dc
voltage delivered by fuel cell, which is around 12.5 V at rated
power, to the 42 V standard automotive dc bus.
The fuel cell converter is composed of a high frequency
inductor L1 (72 µH), an output filtering capacitor C1 (0.702
F), a diode D1 and a main switch S1. Switch S2 is a shutdown
device for test bench security to prevent the fuel cells stack
from short circuit in case of accidental destruction of S1, or of
faulty operation of the regulator. Taking into account the low
voltage, we choose power MOSFETs for S1 and S2 [8].
B. 2-Quadrant Supercapacitors Converter
The supercapacitors are connected to the dc bus by means
of a 2-quadrant dc/dc converter, as shown in Fig. 3. L2
represents the inductor used for energy transfer and filtering.
The inductor size is classically defined by switching
frequency and current ripple.
Supercapacitors size is defined by dc bus energy
requirements deduced from hybrid power profile. The
supercapacitor current, which flows across the storage device,
can be positive or negative, allowing energy to be transferred
in both directions.
Finally, the converter is driven by means of complementary
pulses, applied on the gates of the two MOSFET S3 and S4.
L1
Automotive
iD ( t )
iFC ( t )
D1 DC Bus: 42V
iS ( t )
A PEM Fuel Cell
(23 Cells)
12.5-23 V, 500W
+
v IN ( t )
iC ( t )
S2
Security
Shutoff
+
C1
-
vBus ( t )
-
S1
Boost Converter
Main Switch
Figure 2. Fuel cell boost converter.
Automotive
DC Bus: 42V
S3
D3
S4
D4
+
C1
vBus ( t )
-
L2
iSuperC ( t )
Supercapacitors
+
vSuperC ( t )
Figure 3. 2-quadrant supercapacitor converter.
C. Hybrid System Control Principle
Because fuel cell is supplied with gas through pumps,
valves and compressors, it has large time constants (several
seconds). Consequently, it cannot correctly respond to fast
increasing or decreasing power loads, and may be damaged
because of repetitive stepped power loads. For this reason, the
fuel cell, in the hybrid system, is only operating in nearly
91
I
~
vBusREF Soft Start
:42V
Rate of
DC Bus Voltage
Change
Controller
~p
BusREF
-
PI
~
iFCREF 1
~p
FCREF
÷
limited
FC Current
Controller1
~
d
PID
-
Fuel Cell
Processor
H
~
vBus
PWM
S1
FC
Converter
~
iFC
-
~p
SuperCREF
~
vSuperCMea
~
iFCMea
-
FC Current
iSuperC 3
Controller2 ~
iSuperC1
I
SuperC Voltage Controller
~
iSuperC 2
-
P
~
iSuperCREF
Hybrid Control
Algorithm
Control Signal
~
vSuperCREF
÷
~
Rate of
Change
~
iFCREF 2
~
iFCMea
~
vFCMea
~
vFC
SuperC Current
Controller
S3
1
-
limited
0
S4
~
vSuperC
SuperC
Converter
~
iSuperC
~
vSuperCREF
~
vSuperCMea
Analogue
dSPACE
Figure 4. Hybrid system control structure.
III. HYBRID SYSTEM IMPLEMENTATION
Secondly, when one of the two limitations (rate of change,
and level) on fuel cell power reference is working, a non-zero
iSuperC3 signal is generated, which can be positive or negative,
depending on power condition at dc bus. Therefore, in the
case of discharge mode, characterised by a fast transient
increasing load, or by a power load greater than PFCMax, the
current reference iSuperC3 become negative in order to transfer
the lacking energy to the dc bus. Supercapacitors voltage
controller is supplied with vSuperCMin as reference, and the
hybrid control algorithm leads to select the maximum value
between iSuperC3 and iSuperC2 if supercapacitors voltage is greater
than vSuperCMin, zero otherwise.
On the other hand, in the case of recovery mode (transient
fast decreasing load, or power load less than PFCMin), the
current reference iSuperC3 become positive. Supercapacitors
voltage controller is supplied with vSuperCMax as reference, and
the hybrid control algorithm leads to select the minimum
value between iSuperC3 and iSuperC2 if supercapacitors voltage is
less than vSuperCMax, zero otherwise.
In the two cases (iSuperC3 is positive or negative), the
reference and integrator of fuel cell current controller2 are set
to zero for prevision of the next normal operation as shown in
Fig. 4 by named Control Signal.
Finally, note that iSuperC3, which is sensitive to dc bus
disturbance (noise immunity), has to be filtered before
sending to hysteresis switch in order to define hybrid system
modes of operation.
In addition, Fig. 5 presents hybrid system implementation
in Matlab/Simulink block diagram for dSPACE interfacing
card. The hybrid system communicates with operator by
ControlDesk from computer screen, and contacts with
converter (vBus, iFCREF1, and so on) by DAC and ADC of
dSPACE interfacing card.
Note that motor controller is realized for becoming load at
dc bus. It does not relate with the hybrid control algorithm,
which is independent from load current.
Figure 5. Hybrid system context diagram.
92
ways. Firstly, fuel cell works at constant fuel flow
corresponding to the maximum available current of 50 A. In
this case, the fuel cell has always enough hydrogen and
oxygen. Secondly, the fuel flow varies depending on current
reference.
IV. EXPERIMENTAL RESULTS AND DISCUSSION
Fig. 6 shows the simplified diagram of the PEM fuel cell
system used for this research. Constructed by the Zentrum für
Sonnenenergie und Wasserstoff-Forschung (ZSW), Ulm,
Germany, the fuel cell stack is composed of 23 cells of 100
cm2. It is supplied with pure hydrogen (stored under pressure
in bottles) and air from a compressor. Additionally, the
hardware test bench system structure is presented in Fig. 7.
Hydrogen
from bottle
Flow controller
Cathode
Anode
Pressure
controller
Humidifier
FC Stack
Electric
heater
Air from
compressor
Flow controller
Pressure
controller
Excess
Excess
Heat exchanger
CH4: Current Reference [10A/Div]
CH2: Current Response [10A/Div]
Time: 10ms/Div
Figure 6. Simplified diagram of the 500 W PEM fuel cell system.
SAFT Supercapacitors
Fuel Cell Current Controller1
Figure 8. Current response to a 10-40 A step (40 A = Rated current).
Fuel Cell Converter
SuperC Current
controller
500 W PEM Fuel Cell
Control Panel
dSPACE
Interfacing Card
Figure 7. Hybrid system test bench [11].
CH1: DC bus voltage [10V/Div]
CH3: Current Response [10A/Div]
Time: 1s/Div
A. Fuel Cell Converter Testing with an Ideal Power Supply
Extensive testing is performed using an ideal 12.5V power
supply, which has the same rated voltage as the fuel cell, in
order to confirm that the boost converter can operate correctly
and to compare fuel cell characteristics with ideal power
supply.
Fig. 8 shows the input current response with a stepped
current command. It shows that current response has high
dynamics with optimum response by current controller (PID).
Moreover, Fig. 9 presents the voltage loop test with a load
disturbance. This result shows that the regulation of the fuel
cell converter correctly works.
Figure 9. Converter response to a stepped load disturbance.
As shown in Fig. 10, it can be seen that dynamic response
of fuel cell current is different from Fig. 8. There are two
ways to explain this phenomenon. Firstly, by electrical way,
the electrical fuel cell model is different from an ideal power
supply. Secondly, by physical way, the phenomenon is due to
the slowness of its thermodynamic operation.
Fig. 11 shows the effect of mechanical problems. It can be
seen from fuel cell voltage that it drops lower than on Fig. 10.
This means that its fuel supply and delivered electrical current
do not coincide. Fuel flow is not enough for converter current.
This condition of operating is hazardous for the fuel cell stack.
These tests clearly confirm the slowness and complex
model of the fuel cell system [12].
B. Fuel Cell Converter Testing with a PEM Fuel Cell
When fuel cell is operated, its fuel flow is controlled by
fuel cell processor, which receives current demand from
current reference as shown in Fig. 4. To present the fuel cell
characteristics, the test bench is operated in two different
93
CH4: Current Reference [10A/Div]
CH2: Fuel Cell Current [10A/Div]
Time: 10ms/Div
CH4: Current Reference [10A/Div]
CH2: Fuel Cell Current [10A/Div]
Time: 10ms/Div
CH1: Fuel Cell Voltage [2.5V/Div]
CH3: Fuel Cell Current [10A/Div]
Time: 100ms/Div
CH1: Fuel Cell Voltage [2.5V/Div]
CH3: Fuel Cell Current [10A/Div]
Time: 100ms/Div
Figure 10. Fuel cell current response for a 10-40 A step
at constant fuel flow for 50 A.
Figure 11. Fuel cell current response for a 10-40 A step
at variable fuel flow.
Voltage loop test without slopped limitation is presented on
Fig. 12. Compared with the current response of Fig. 9, it also
shows the slowness of the fuel cell response. The fuel cell
current points out high overshoot and delay time.
C. Hybrid System Test Bench
As storage device, hybrid system utilizes six SAFT
supercapacitors (capacitance: 3500 F, rated voltage: 2.5 V,
rated current: 400 A, series resistance: 0.8 mΩ) connected in
series. The technical specifications are as follows:
PFCRated = 500 W,
PFCMin = 50 W,
PFCMax = 530 W,
iFCRated = 40 A,
vSuperCNormal = 13 V,
vSuperCMin = 8 V,
vSuperCMax = 15 V.
Power slope of fuel cell is 500 W.s-1 (around 8.5 A.s-1)
while current slope of fuel cell for charging supercapacitors is
limited to 4 A. s-1.
While operating with the supercapacitors, the fuel flow is
not anymore fixed to a 50 A current but adapted to the value
of the delivered current to improve the efficiency of the
system.
CH1: DC bus Voltage [10V/Div]
CH2: Fuel Cell Voltage [5V/Div]
CH3: Fuel Cell Current [10A/Div]
Time: 1s/Div
Figure 12. Fuel cell converter response to a stepped load disturbance
at variable fuel flow.
Fig. 13 shows transient responses to a stepped load which
corresponds nearly to 10-40 A step of the fuel cell current.
One can observe that the dc bus voltage is well regulated, and
that fuel cell current smoothly increases with a slope of 8.5
A.s-1. Furthermore, during transient state, supercapacitors
transfer energy back to the dc bus in order to compensate the
energy, which is not supplied by the main source.
94
DC Bus Voltage [V]
31.5
21.0
10.5
Fuel Cell Current [A]
0.0
40.0
30.0
20.0
10.0
SuperC Voltage [V]
0.0
13.2
13.0
12.8
10.0
SuperC Current [A]
DC Bus Voltage [V]
Fuel Cell Current [A]
SuperC Voltage [V]
SuperC Current [A]
42.0
0.0
-10.0
-20.0
-30.0
42.0
31.5
21.0
10.5
0.0
40.0
30.0
20.0
10.0
0.0
14.0
13.0
12.0
11.0
30.0
20.0
10.0
0.0
-10.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
time [s]
Figure 13. Hybrid responses to a stepped load.
3.5
4.0
0
25 30 35 40 45 50
time [s]
Figure 14. Supercapacitors charge from 12 V to 13 V.
Fig. 14 presents hybrid characteristics during normal
operating mode, through supercapacitors charge from 12 V to
13 V. The dc bus has a constant load of about 10 A delivered
by the main source. At the beginning of charge at t = 3 s,
iSuperCREF = iSuperC1. It can be observed that fuel cell current
slope is approximately 4 A.s-1, which is lower than the
previous 8.5 A.s-1, necessary condition for stability.
Furthermore, during the charging process the fuel cell
delivered its rated current. Besides, the transition of iSuperCREF
from iSuperC1 to iSuperC2 occurs (end of charge) at time t = 25 s,
for a supercapacitors voltage of nearly 13 V, because of the
use of a high proportional gain (200) for supercapacitors
voltage controller.
Fig. 15 presents transient responses of the hybrid system to
an excessive load. Before this test, supercapacitors voltage is
equal to vSuperCNormal = 13 V. The supercapacitors compensate
the main source during both transient state and steady state,
because of fuel cell current slope and fuel cell power
limitations. During the first interval, beginning at t = 4 s, the
current delivered by the main source slowly increases (with a
controlled slope) up to its maximum value, the lacking energy
being delivered by supercapacitors during t = 7.8 s to 18 s.
Then, the sudden decrease of the power load at t = 18 s leads
to a recovery mode for supercapacitors, in order to allow a
slow controlled decrease of the fuel cell current.
5
10
15
20
55
60
Fig. 16 corresponds to a sudden recovery of energy on the
dc bus. This energy is recovered by the supercapacitors while
a slow decreasing of the main source current is performed. In
this example, fuel cell never delivered less than 50 W in order
to maintain fuel cell converter operating in continuous current
mode.
V. CONCLUSION
The main objective of this work is to propose a new method
of controlling an automotive dc bus supplied by a hybrid
source using supercapacitors as auxiliary source, in
association with a PEM fuel cell as main source, knowing that
this kind of electrical source is not able to supply energy
during fast transitions of load because of current slope
limitation, during peak loads because of power limitation, and
during recovery because of only positive current delivering.
The experimental results with a 500 W PEM fuel cell
confirm the slow dynamic response of the system, due to both
thermodynamic and mechanical phenomena. Results carried
out by means of a hybrid system test bench, which uses a
storage device composed of six SAFT 3500 F supercapacitors
connected in series, have shown the possibility to improve the
transient performance of the system and validate the proposed
control principle.
95
400
Load Power [W]
800
600
400
200
DC Bus Voltage [V]
0
42.0
31.5
21.0
10.5
Fuel Cell Current [A]
0.0
40.0
30.0
20.0
10.0
0.0
50.0
SuperC Current [A]
SuperC Current [A]
Fuel Cell Current [A]
DC Bus Voltage [V]
Load Power [W]
1000
0.0
-50.0
-100.0
200
0
-200
-400
42.0
31.5
21.0
10.5
0.0
30.0
20.0
10.0
0.0
50.0
25.0
0.0
-25.0
-50.0
0
2
4
6
8
10
12
14
16
18
20
22
24
0
time [s]
5
10
15
20
25
30
35
time [s]
Figure 15. Hybrid system response when overloading.
Figure 16. Hybrid system response when recovering.
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[3]
[4]
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