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Development of Fuel-Cell Hybrid System
ELSEVIER
Copyright © IFAC Mechatronic Systems, Sydney, Australia, 2004
IFAC PUBLICATIONS www.elsevier.comllocate/ifal.:
DEVELOPMENT OF FUEL-CELL HYBRID SYSTEM Tetsuhiro lshikawa, Shoichi Sasaki Uee/ric & Hl'hrid I't'hicle Engineering /)il', TOlDTA MOTOR CORPORA T/ON Kota Manabe, David Hermance Cm'ironll1en/a/ Engint'ering Dil'., TO),07>l TECHX/C.·/ /. CLVTER. I i s. I.. !XC. Hiroshi Yoshida
n ))'OT.·I (' OAlil/UN/CA nu\' S,),,\TE:I/S (.( JR/'(J/?I flO.\ Toyota Motor Corporation started the development of fuel cell vehicle in 1992, and the company began leasing a new generation fuel cell vehicle the FCHV(Fuel Cell Hybrid Vehicle) in December 2002. The FCHV's system is designed to improve the efficiency and aims for high responsiveness when the vehicle is in a transitional state. In much the same way as most electric vehicles and the gasoline powered hybrid "Prius", the energy the traction motor creates during braking can be used to regenerate the secondary battery. The fuel cell and traction motor inverter are connected directly, with the secondary battery connected through the DC/DC converter to the fuel cell in parallel. The efficiency of the FCHV has the ability to be approximately 3 times greater than the conventional internal combustion engine (ICE) powered vehicle. In 2003, Toyota developed the highly efficient next-generation FCH V. The system uses a lithium battery , the optimum power storage device for the characteristics of the fuel cell system, and a highly efficient half bridge convelter circuit. The change to a lighter secondary battery and the reduction of converter losses make it possible to improve the fuel economy by 4% over the current Toyota FCHV. This paper describes the circumstances of fuel cell vehicle development in Toyota and the overview of Toyota FCHV's system control and next-generation FCHV's system. CO!JFrigh/ © 2nO-llFAC Keywords: fuel-cell, hybrid, efficiency. power storage device Even as this fuel storage resea rch has continued. Toyota has pursued studies of di fferent types of highvoltage electrical systems for fuel cell automobiles. which are described below.
I. INTRODUCTION The entire world is c1amoring for environmental protection and demanding that the automobile industry make cleaner and more energy-efficient vehicles than it has to date. The fuel cell is a clean. highly efficient energy conversion device that generates electricity fr0111 hydrogen and oxygen and produces only water. For that reason, the making of a practical automobile that uses the fuel cell as a power source has long been anticipated.
...".
:fffJ:...,;:'WL " ......J
') DEVELOPMENT OF FUEL CELL VEHICLE IN TOYOTA
..
Fig.l. TOYOTA rCHV J. HYBRID SYSTEM OF TO'{OTA FCHV
Ever since TLlyota started the development of fuel cell vehicles in 1992, the company has searched for a method of carrying hydrogen fuel on board the vehicle. In 1996. Toyota developed a fuel cell automobile with a hydrogen storage device that used a hydrogen-abso rbing alloy, and in 1997. the company announced the world's first fuel cell automobile that employs a methanol reformer. Also, in recent years, Toyota became the first c,)mpany in the world to start limited leasing of the Toyota FCHV that uses a high-pressure tank as the hydrogen storage device and that incorporates the hyhrid technology developed for the Prius.( Fig.I)
The FCHV's system is designed to improve the efficiency and aims for high responsiveness when the vehicle is in a transitional state. In the same way as most electric vehicles. and as in the gasoline powered hybrid "Prius", the energy regenerated by the traction motor during breaking can be used to regenerate the secondary battery. The fuel cell and traction motor inverter are connected directly. with the secondary battery connected through the DC/DC converter to the fuel cell in parallel. The efticienc), of the FCH V has the ability to be approximately 3 times greater than the conventional internal combustion engine (ICE) powered vehicle. 31
]. 1
F(
The secondary battery, with its low power ratio, is configured in parallel with the fuel cell via a DC/ DC converter and provides a power assist when the fuel cell response is delayed or when the vehicle is driven under high loads. The secondary battery also absorbs the energy recovered by regenerative braking and acts as the power source for PEV operation under low loads. The hybrid control (electric power control) of the fuel cell and the secondary battery is accompli shed by controlling the output voltage from the DC/DC converter.
HI" S:V.I'tem Ol'erviell'
The electric power supply is a hybrid configuration with the fuel cell and a secondary battery. The output power from the fuel cell and the charging/discharging of the secondary battery are precisely controlled according to the operating state of the vehicle. A nickel-metal hydride battery provides superior energy to the FCHV as a pure electric vehicle (PEV) using only the secondary battery. thereby improving the fuel economy under lo lV load conditions . The power ratio of the fuel cell versus the secondary battery is shown in Fig. 2. Optimum fuel economy is achieved when the ratio is roughly from 40:60 to 80:20. As a result, the output power from the fuel cells was set to 90 kW and the output power from the secondary battery was set 21 kW, given that the fuel cell is also used to provide auxiliary power jilr the air conditioning, etc. good
)(
cu
""c
...
3.2 FCHV Composition The FCHV system composition is shown in Fig. 4. The FCHV system is divided into functionally distinct systems. The fuel cell system is the power source that provides the vehicle's drive power, while the hybrid system utilizes the output power from the fuel cell system with high efficiency _
-----_. _- -------- --- ------ - , Japanese 10-15 mode
';:'., 140 E o c o ~ 120
I
F UEL CELL SYSTEM
I 'If' . 7 7 'f71
The system is composed of the proton exchange membrane (PEM) fuel cell stack, the fuel supply -.; Optimum subsystem and cooling subsystem. ~ Power Distribution Hydrogen is supplied to the fuel cell stack from the 100 high pressure tank, its maximum fill pressure is 35MPa. In order to improve performance of the fuel oL ______________ cell , any surplus hydrogen remaining after the fuel o 20 40 60 80 100 cell reaction is returned to the supply side of the fuel TOVola Esl im allon Fe Power Ratio [%] cell by the circulation pump. Air is pressurized by the compressor and pumped to the stack through the Fe Power Ratio [%] = Fe max. Power Fe max. Power + Battery max. Power humidifier. The humidifier takes water vapor from the exhaust air of the stack and uses it to humidifY the incoming compressed air. The water pump Fig.2 .Power Ratio-Fuel Consumption Characteristics circulates coolant between the stack and rad iator. Also, control of auxiliary parts such as the air As shown in Fig _ 3, the fuel cell and the traction compressor, etc., is optimized according to the fuel inverter/motor are connected in series to improve cell output, so that the fuel cell stack operates with a efficiency during steady state mode; which occurs minimum loss of the auxiliary equipment. most frequent during vehicle operation.
I
1 -------- - - --------------
Fuelcen
Batte. . .
Hl'BRID S)STEM The Hybrid System is composed of the fuel cell system, the secondary battery, the DC/ DC converter, and the traction inverter/motor. The bas is of the vehicle's drive power is the output from the fuel cell. But when the fuel cell's output power is insufficient, drive power is assisted by the secondary battery. The fuel cell system efficiency , including the energy consumption of the auxiliary parts, such as the air compressor. decreases in the low power region . Therefore, in low load operation, the fuel cell and the auxiliary equipment operations are stopped and the vehicle runs as a PEV only with the power from the secondary battery .
~
I ~
_Ehctri:: power Mechani:alpow er
!mlIIGII
Fig.]. Conceptual Diagram of FCHV System
32
The hybrid unit sends the target current to FC control unit. The target current is determined using the motor requirement and fuel cell characteristic. At the same time, the Fuel cell's operating point is being continuously controlled by the DCIDC converter. The auxiliary systems of the fuel cell are controlled by the fuel cell control unit according to the fuel cell target current. Once it has been decided how much energy output the fuel cell must supply, the current is determined from the P-I characteristic. Next, the voltage V that corresponds with the current I is decided by I-V characteristic block. The fuel cell operational point is decided by the output voltage control of the fuel cell via the DC/ DC converter. When there is an alteration in the temperature, pressure etc . of the fuel cell stack, the P-I characteristics and the I-V characteristics are also periodically compensated.
so the region III which the fuel cell operates intermittently is smaller than the region of intermittent engine operation in the gasoline hybrid.
Region of intermittent Fe operation 60r-------------------------------~
Fuel cell system efficiency
?f!. 50 .... --V'1iiiiWSjji..lilllr..i-,.....-.... P.._. ._...~Miiillliiiijllii M ' iiiillMIiiI7iji ' _.iiiJi!.;:a;¥u..
• •
g40 .- fIJ----·
.~
CD
a;
20
Z
10
o
o
2
5
-
-'h~iiWne ettlfWf¥~.~~ + .••
------------~ -~ 10
15 20 Drive output[kWj
POWER STORAGE DEVICE
The determination of what sort of power storage device is best suited to the characteristics of a fuel 5 ce ll was approached from the two points of view 4 described below. Note that a comparison between ~ 3 the Toyota FCHV and a gasoline hybrid of the same U oVl 2 class is also shown for reference purposes.
Amount of SOC change
1
_.
The output response of the fuel cell is shown in Fig.6. The output response of the fuel cell is faster than that of the engine in the gasoline hybrid, so the amount of energy extracted from the power storage device is small.
-2
o
~i~:1j\~JV -
500
1000
,
The current technologies for power-type power storage devices include the lithium battery and the power capacitor, among others. The results of a study of different power storage devices are shown below
- ._-- - -- - . _ - - !
asoJine..engine _ _ . _ ___
i
8
10
POWER CAPACITOR
.
o ~~~----~~~~~~~-----~
6
Fuel cell HV --
Time rsecl Fig.8 Amount of Battery SOC Change During Operation In LA#4 Mode (Simulation)
·-- ---1
4
- _
12
14
Time rsecl Fig.6 Fuel Cell Response(at WOT) .Vel efficiencv
As shown in Fig.7, the fuel cell system exhibits better net efficiency than the gasoline hybrid throughout the range where the drive output is low,
34
!
________________.________._ .. __.____.____.____.J
-1
+ Secondary battery
35
(Fuel ce ll I-IVl
o
/ 'owe r response
2
30
For that reason, in cyclic operation, the time during which the fuel cell operates intermittently is shorter than for the gasoline hybrid, so the amounts of power that are stored and discharged are also smaller. Fig. 8 shows the provisional calculations for amount of change in the state of charge (SOC) when the system is operated in LA#4 mode. It can be seen that the range of SOC change for the fuel cell system is smaller than for the gasoline hybrid . These two observations led to the conclusion that a power type of power storage device is better suited to the FCHV system than is an energy type of device.
This section describes the optimulll characteristics for electrical power storage devices which are used together with the fuel cells.
o
25
Fig.7 Comparison of Net Efficiency of Fuel Cell and Engine (in Japan 10-15 Mode)
-I. I l~l'hrid POll"er Source Con/igurmion
Gasoline engine
- - - -. . . - . _ -
._".h. ------ _. .. -
30
i:
4. NEXT GENERATION FUE L-CELL HYBRID SYSTEM
-- - ----
i
oo
The key point in using a power capacitor is how large a capacitance to specify. For a high-voltage circuit configuration, a converter-less configuration like that shown in Fig. 9 is suitable, because it naturally takes advantage of the broad voltage range over which the capacitor can be used . However, in the FCHV, because the output from the fuel cell is determined only by the load and the voltage, this configuration does not allow the output from the power so urce to be controlled.
1500
(_Fuel Cell System _______ _________ _____________________ ,
System Main Relay
Air Compressor Inverter/Moto r Hydrogen Pump Inverter/Motor Coolant Pump Inverter/Motor
Fig.4. FCHV System Composition
3.3 HFhrid Control
In the EV control unit, the required power is determined by the accelerator sensor etc. And is then sent to the Hybrid control unit. The motor power allowance is received from the hybrid control unit, and the motor is driven in the restricted area. In the hybrid control unit, the available FC maximum power at that point in time, and the secondary battery maximum power, are calculated. This is the total motor power allowance.
In Fig.S, the outline of the fuel cell system control unit as well as the block outline of the Hybrid System's control unit is shown. This unit is composed of the base of the EV control unit, combined with the hybrid control unit, and the fuel cell control unit. As a result, the system modules can he replaced easil y as improvements are made. : .EV Control Diagram ..
.------~~
Inverter Vo ltage
angle
:. Hybrid Control Diagram ........ Motor power allowance C alculation of
Power Distribution
.. FC Control Diagram ........ ·· Air Control
,------i'O~~~ of Command of
Fe control
ydrogen flow
Hydrogen Control
ommand of
Fe coolant
Cooling Co ntrol
Fig.S. Hybrid Control Block Diagram
33
output was therefore set to 80 kilowatts and the lithium battery output to 31 kilowatts.
Drive motor Capacitor
good
-_._----------- - - - -- - - - - - - - - - - ,
~ 140
US LA#4 mode
-0
Fe auxiliary motor
.,... 11>
o
<.)
-.;
&! 100 Fe efficiency decrease
Fig.9.Capacitor System High-voltage Circuit Configuration
~
Fe Power Ratio [%]
Fe Power Ratio [%]
This section describes the different circuit configurations of the high-voltage electrical system used for the different hybrid power sources that are suitable for the fuel cell described in the previous section .
O-12S[kmIb) full
•
..
Rant;e where acceleration
'.
/'. . . /. perfonnance is-satisfactory
"-:::, .m
(shaded area)
%<">:~~.
maximum output from each fuel cell 85
90
95
HIGH-VOLTAGE SYSTEM CONFIGURATION POll'er capacitor system
As stated earlier, where a power capacitor is used, the high-voltage electrical system is con figured so that the power capacitor is connected in parallel with the fuel cell to take advantage of the wide voltage range..over which the capacitor can be used. Lithium batter\! system
Minimum capacitor capacitance where acceleration performance is satisfactory at
80
Fe max Power + Battery max Power
4.2 Next-generatiol1 FCHV High-voflaf,:e System
Best fuel economy point where acceleration performance is satisfactory at maximum output from each power source Capacitor capacitance at which operation is possible with repeated 60-90[km/h) full - - - --.. - Cap8citonapacitance-arwhich I ~ operation is ~ssible with repeated
~
Fe max Power
Fig.II.Fuel Cell Maximum Output and Li-ion Battery Output
o
/
Regeneration e decrease
100
ct
o L -_ _ _ _ _ __ _ _ _ __ _ _ _ _ _ o 20 40 60 80 100
In determining such capacitor specifications as the capacitance one must take into consideration such factors as the vehicle's acceleration and deceleration, the repetition of acceleration and deceleration, and the recovery of deceleration energy during cyclic operation. Fig.IO shows the results of a parameter study of vehicle acceleration and deceleration and the like in which the model was used . It shows that while the vehicle meets the acceleration performance requirements, when acceleration is repeated on a flat road , there is a lower limit that does not fall below the designed voltage of the motor. In the graph, the shaded area represents the region where the prescribed requirements are met. The point where the fu el econolllY is best is where the capacitor capacitance is 7 farads and the maximum output from the fuel cell is 95 kilowatts.
• •
8'g 120 >1.l
..s
15 --...
..s
Where a lithium battery is used as the power storage device, a converter becomes necessary, because the usable voltage range of the battery is limited. However, because the lithium battery has high power density, the configuration of the converter circuit can be changed from a full bridge type to a more efficient half bridge type. This is the situation shown in Fig. 12. The change to a half bridge converter requires that the usable voltage range of the fuel cell be separated frolll the usable voltage range of the lithium battery, but this is feasible as long as the number of battery cells is sixty or less. It can also be seen that even when one takes into account the increase in converter loss caused by the increase in the transformer ratio, the half bridge converter is better than the full bridge type.
105
Fuel cell maximum output [kW] ( ) Fuel economy index
Fig. 1O.F uel cell Maximum Output and Capacitor Output
UTHf(;'M BATTERY
The fuel economy characteristics of a system that uses a lithium battery when the output ratio of the lithiulll battery to the fuel cell is the parameter that is varied are shown in Fig.ll . The fuel economy is best \-vhen the ratio of fuel cell output to total system output is in the range from 65% - 85%. The fuel cell
35
Half Bridge Converter
good
Full Bridge Converter
11 0
Fuel economy ~
-~ --
105
~~
"0
..s>-. ~
C
100
0
u
[..tJ
-.; ::l
u..
~
100 ~
Con~~rter
95
efficjencL
With the lithium battery systems, which use converters, the fuel economy is good. because the fuel cell can be operated intermittently when the vehicle is stopped or when energy is being recovered during deceleration .
>-.
u
c
90
'u'"
4::
5. CONCLUSION
'-'
[..tJ
....
_________
'" '"c:;>
t::
80
0
U
O~LI------------------------------~~I 0 o 30 60 90 120 Number of Li-ion cells
Fig.12 Effic iency of Li-ion System FUEL ECONOMY CHARACTERISTICS Table I shows a comparison of the fuel economy characteristics of the high-voltage system con figurations described above. The lithium battery system shows better fuel economy than the current nickel-metal hydride battery system, indicating that a 4% improvement is possible. Because the power capacitor system can not operate the fuel cell intermittently, the auxiliary motor loss increases, as shown in Fig. 13 , so that the fuel economy is worse than that of the current system. Table 1. Comparison of Characteristics in Different System Ni-MH
Capacitor
Li-ion
Volume [L] (include converter & FC stack)
220
235
196
Weight [kg] (include convel1er & FC stack)
252
259
206
(I) TOYOTA FCHV on limited sale in December 2002, with its new fuel cell hybrid system featuring fuel cell and the secondary battery connected in parallel, both optimally controlled according to the state of the vehicle, was developed to improve fuel economy while also providing high responsiveness III vehicle transitional states. (2) In 2004, A high-voltage system was developed for the highly efficient next-generation FCHV. The system uses a lithium battery, the optimum power storage device for the characteristics of the fuel cell system, and a highly efficient half bridge converter circuit. (3) The change to a lighter secondary battery and the reduction of converter losses make it possible to improve the fuel economy by 4% over the current Toyota FCHV. (4) Because a system that uses a power capacitor does not allow the fuel cell to be operated intermittently, its fuel economy is I I % worse than that of the Li-ion system.
REFERENCES
I.
2. Fuel economy Inclex (Ni-MH=IOO)
100
93.7
104.2 3.
Fuel economy Improvement [%]
based
----'"
+4.2
--
based ~ + I1.3 Fuel Economy: @LA#4 mode
4.
3500 3000
:c~ '"'"0 ...J
Battery
.:.. ,;.
2500
::-- ::::
~
2000 1500
Fe uXlliary
otor
1000
Fuel cell gene tion
500 0
Il _______ Capacitor
Ni-MH
lj-ion
Fig.13. Comparison of Loss in Different Systems
36
S. Sasaki, T. Takaoka, H. Matsui, T. Kotani. "Toyota's Newly Developed Electric-Gasoline Engine Hybrid Power train System ", The 14th International Electric Vehicle Symposium, ( 1997). T. Matsumoto, N. Watanabe, H. Sugiura, T. Ishikawa. "Development of Fuel-cell Hybrid Vehicle", the 2002 SAE World Congress, (2002). Y. Hori, T. Teratani, R. Masaki. "Motor Technology for Automobile", The Nikkan Kogyo Shimbun. LTD (The Business & Technology Daily News). (2003). T. lshikawa, S. Hamaguchi. T. Shimizu, T . Yano, S. Sasaki, K. Kato, M. Ando, H. Yoshida. "Development of Next Generation Fuel-cell Hybrid System", the 2004 SAE World Congress, (2004) .
Copyright © IFAC Mechatronic Systems, Sydney, Australia, 2004
IFAC PUBLICATIONS www.elsevier.comllocate/ifal.:
DEVELOPMENT OF FUEL-CELL HYBRID SYSTEM Tetsuhiro lshikawa, Shoichi Sasaki Uee/ric & Hl'hrid I't'hicle Engineering /)il', TOlDTA MOTOR CORPORA T/ON Kota Manabe, David Hermance Cm'ironll1en/a/ Engint'ering Dil'., TO),07>l TECHX/C.·/ /. CLVTER. I i s. I.. !XC. Hiroshi Yoshida
n ))'OT.·I (' OAlil/UN/CA nu\' S,),,\TE:I/S (.( JR/'(J/?I flO.\ Toyota Motor Corporation started the development of fuel cell vehicle in 1992, and the company began leasing a new generation fuel cell vehicle the FCHV(Fuel Cell Hybrid Vehicle) in December 2002. The FCHV's system is designed to improve the efficiency and aims for high responsiveness when the vehicle is in a transitional state. In much the same way as most electric vehicles and the gasoline powered hybrid "Prius", the energy the traction motor creates during braking can be used to regenerate the secondary battery. The fuel cell and traction motor inverter are connected directly, with the secondary battery connected through the DC/DC converter to the fuel cell in parallel. The efficiency of the FCHV has the ability to be approximately 3 times greater than the conventional internal combustion engine (ICE) powered vehicle. In 2003, Toyota developed the highly efficient next-generation FCH V. The system uses a lithium battery , the optimum power storage device for the characteristics of the fuel cell system, and a highly efficient half bridge convelter circuit. The change to a lighter secondary battery and the reduction of converter losses make it possible to improve the fuel economy by 4% over the current Toyota FCHV. This paper describes the circumstances of fuel cell vehicle development in Toyota and the overview of Toyota FCHV's system control and next-generation FCHV's system. CO!JFrigh/ © 2nO-llFAC Keywords: fuel-cell, hybrid, efficiency. power storage device Even as this fuel storage resea rch has continued. Toyota has pursued studies of di fferent types of highvoltage electrical systems for fuel cell automobiles. which are described below.
I. INTRODUCTION The entire world is c1amoring for environmental protection and demanding that the automobile industry make cleaner and more energy-efficient vehicles than it has to date. The fuel cell is a clean. highly efficient energy conversion device that generates electricity fr0111 hydrogen and oxygen and produces only water. For that reason, the making of a practical automobile that uses the fuel cell as a power source has long been anticipated.
...".
:fffJ:...,;:'WL " ......J
') DEVELOPMENT OF FUEL CELL VEHICLE IN TOYOTA
..
Fig.l. TOYOTA rCHV J. HYBRID SYSTEM OF TO'{OTA FCHV
Ever since TLlyota started the development of fuel cell vehicles in 1992, the company has searched for a method of carrying hydrogen fuel on board the vehicle. In 1996. Toyota developed a fuel cell automobile with a hydrogen storage device that used a hydrogen-abso rbing alloy, and in 1997. the company announced the world's first fuel cell automobile that employs a methanol reformer. Also, in recent years, Toyota became the first c,)mpany in the world to start limited leasing of the Toyota FCHV that uses a high-pressure tank as the hydrogen storage device and that incorporates the hyhrid technology developed for the Prius.( Fig.I)
The FCHV's system is designed to improve the efficiency and aims for high responsiveness when the vehicle is in a transitional state. In the same way as most electric vehicles. and as in the gasoline powered hybrid "Prius", the energy regenerated by the traction motor during breaking can be used to regenerate the secondary battery. The fuel cell and traction motor inverter are connected directly. with the secondary battery connected through the DC/DC converter to the fuel cell in parallel. The efticienc), of the FCH V has the ability to be approximately 3 times greater than the conventional internal combustion engine (ICE) powered vehicle. 31
]. 1
F(
The secondary battery, with its low power ratio, is configured in parallel with the fuel cell via a DC/ DC converter and provides a power assist when the fuel cell response is delayed or when the vehicle is driven under high loads. The secondary battery also absorbs the energy recovered by regenerative braking and acts as the power source for PEV operation under low loads. The hybrid control (electric power control) of the fuel cell and the secondary battery is accompli shed by controlling the output voltage from the DC/DC converter.
HI" S:V.I'tem Ol'erviell'
The electric power supply is a hybrid configuration with the fuel cell and a secondary battery. The output power from the fuel cell and the charging/discharging of the secondary battery are precisely controlled according to the operating state of the vehicle. A nickel-metal hydride battery provides superior energy to the FCHV as a pure electric vehicle (PEV) using only the secondary battery. thereby improving the fuel economy under lo lV load conditions . The power ratio of the fuel cell versus the secondary battery is shown in Fig. 2. Optimum fuel economy is achieved when the ratio is roughly from 40:60 to 80:20. As a result, the output power from the fuel cells was set to 90 kW and the output power from the secondary battery was set 21 kW, given that the fuel cell is also used to provide auxiliary power jilr the air conditioning, etc. good
)(
cu
""c
...
3.2 FCHV Composition The FCHV system composition is shown in Fig. 4. The FCHV system is divided into functionally distinct systems. The fuel cell system is the power source that provides the vehicle's drive power, while the hybrid system utilizes the output power from the fuel cell system with high efficiency _
-----_. _- -------- --- ------ - , Japanese 10-15 mode
';:'., 140 E o c o ~ 120
I
F UEL CELL SYSTEM
I 'If' . 7 7 'f71
The system is composed of the proton exchange membrane (PEM) fuel cell stack, the fuel supply -.; Optimum subsystem and cooling subsystem. ~ Power Distribution Hydrogen is supplied to the fuel cell stack from the 100 high pressure tank, its maximum fill pressure is 35MPa. In order to improve performance of the fuel oL ______________ cell , any surplus hydrogen remaining after the fuel o 20 40 60 80 100 cell reaction is returned to the supply side of the fuel TOVola Esl im allon Fe Power Ratio [%] cell by the circulation pump. Air is pressurized by the compressor and pumped to the stack through the Fe Power Ratio [%] = Fe max. Power Fe max. Power + Battery max. Power humidifier. The humidifier takes water vapor from the exhaust air of the stack and uses it to humidifY the incoming compressed air. The water pump Fig.2 .Power Ratio-Fuel Consumption Characteristics circulates coolant between the stack and rad iator. Also, control of auxiliary parts such as the air As shown in Fig _ 3, the fuel cell and the traction compressor, etc., is optimized according to the fuel inverter/motor are connected in series to improve cell output, so that the fuel cell stack operates with a efficiency during steady state mode; which occurs minimum loss of the auxiliary equipment. most frequent during vehicle operation.
I
1 -------- - - --------------
Fuelcen
Batte. . .
Hl'BRID S)STEM The Hybrid System is composed of the fuel cell system, the secondary battery, the DC/ DC converter, and the traction inverter/motor. The bas is of the vehicle's drive power is the output from the fuel cell. But when the fuel cell's output power is insufficient, drive power is assisted by the secondary battery. The fuel cell system efficiency , including the energy consumption of the auxiliary parts, such as the air compressor. decreases in the low power region . Therefore, in low load operation, the fuel cell and the auxiliary equipment operations are stopped and the vehicle runs as a PEV only with the power from the secondary battery .
~
I ~
_Ehctri:: power Mechani:alpow er
!mlIIGII
Fig.]. Conceptual Diagram of FCHV System
32
The hybrid unit sends the target current to FC control unit. The target current is determined using the motor requirement and fuel cell characteristic. At the same time, the Fuel cell's operating point is being continuously controlled by the DCIDC converter. The auxiliary systems of the fuel cell are controlled by the fuel cell control unit according to the fuel cell target current. Once it has been decided how much energy output the fuel cell must supply, the current is determined from the P-I characteristic. Next, the voltage V that corresponds with the current I is decided by I-V characteristic block. The fuel cell operational point is decided by the output voltage control of the fuel cell via the DC/ DC converter. When there is an alteration in the temperature, pressure etc . of the fuel cell stack, the P-I characteristics and the I-V characteristics are also periodically compensated.
so the region III which the fuel cell operates intermittently is smaller than the region of intermittent engine operation in the gasoline hybrid.
Region of intermittent Fe operation 60r-------------------------------~
Fuel cell system efficiency
?f!. 50 .... --V'1iiiiWSjji..lilllr..i-,.....-.... P.._. ._...~Miiillliiiijllii M ' iiiillMIiiI7iji ' _.iiiJi!.;:a;¥u..
• •
g40 .- fIJ----·
.~
CD
a;
20
Z
10
o
o
2
5
-
-'h~iiWne ettlfWf¥~.~~ + .••
------------~ -~ 10
15 20 Drive output[kWj
POWER STORAGE DEVICE
The determination of what sort of power storage device is best suited to the characteristics of a fuel 5 ce ll was approached from the two points of view 4 described below. Note that a comparison between ~ 3 the Toyota FCHV and a gasoline hybrid of the same U oVl 2 class is also shown for reference purposes.
Amount of SOC change
1
_.
The output response of the fuel cell is shown in Fig.6. The output response of the fuel cell is faster than that of the engine in the gasoline hybrid, so the amount of energy extracted from the power storage device is small.
-2
o
~i~:1j\~JV -
500
1000
,
The current technologies for power-type power storage devices include the lithium battery and the power capacitor, among others. The results of a study of different power storage devices are shown below
- ._-- - -- - . _ - - !
asoJine..engine _ _ . _ ___
i
8
10
POWER CAPACITOR
.
o ~~~----~~~~~~~-----~
6
Fuel cell HV --
Time rsecl Fig.8 Amount of Battery SOC Change During Operation In LA#4 Mode (Simulation)
·-- ---1
4
- _
12
14
Time rsecl Fig.6 Fuel Cell Response(at WOT) .Vel efficiencv
As shown in Fig.7, the fuel cell system exhibits better net efficiency than the gasoline hybrid throughout the range where the drive output is low,
34
!
________________.________._ .. __.____.____.____.J
-1
+ Secondary battery
35
(Fuel ce ll I-IVl
o
/ 'owe r response
2
30
For that reason, in cyclic operation, the time during which the fuel cell operates intermittently is shorter than for the gasoline hybrid, so the amounts of power that are stored and discharged are also smaller. Fig. 8 shows the provisional calculations for amount of change in the state of charge (SOC) when the system is operated in LA#4 mode. It can be seen that the range of SOC change for the fuel cell system is smaller than for the gasoline hybrid . These two observations led to the conclusion that a power type of power storage device is better suited to the FCHV system than is an energy type of device.
This section describes the optimulll characteristics for electrical power storage devices which are used together with the fuel cells.
o
25
Fig.7 Comparison of Net Efficiency of Fuel Cell and Engine (in Japan 10-15 Mode)
-I. I l~l'hrid POll"er Source Con/igurmion
Gasoline engine
- - - -. . . - . _ -
._".h. ------ _. .. -
30
i:
4. NEXT GENERATION FUE L-CELL HYBRID SYSTEM
-- - ----
i
oo
The key point in using a power capacitor is how large a capacitance to specify. For a high-voltage circuit configuration, a converter-less configuration like that shown in Fig. 9 is suitable, because it naturally takes advantage of the broad voltage range over which the capacitor can be used . However, in the FCHV, because the output from the fuel cell is determined only by the load and the voltage, this configuration does not allow the output from the power so urce to be controlled.
1500
(_Fuel Cell System _______ _________ _____________________ ,
System Main Relay
Air Compressor Inverter/Moto r Hydrogen Pump Inverter/Motor Coolant Pump Inverter/Motor
Fig.4. FCHV System Composition
3.3 HFhrid Control
In the EV control unit, the required power is determined by the accelerator sensor etc. And is then sent to the Hybrid control unit. The motor power allowance is received from the hybrid control unit, and the motor is driven in the restricted area. In the hybrid control unit, the available FC maximum power at that point in time, and the secondary battery maximum power, are calculated. This is the total motor power allowance.
In Fig.S, the outline of the fuel cell system control unit as well as the block outline of the Hybrid System's control unit is shown. This unit is composed of the base of the EV control unit, combined with the hybrid control unit, and the fuel cell control unit. As a result, the system modules can he replaced easil y as improvements are made. : .EV Control Diagram ..
.------~~
Inverter Vo ltage
angle
:. Hybrid Control Diagram ........ Motor power allowance C alculation of
Power Distribution
.. FC Control Diagram ........ ·· Air Control
,------i'O~~~ of Command of
Fe control
ydrogen flow
Hydrogen Control
ommand of
Fe coolant
Cooling Co ntrol
Fig.S. Hybrid Control Block Diagram
33
output was therefore set to 80 kilowatts and the lithium battery output to 31 kilowatts.
Drive motor Capacitor
good
-_._----------- - - - -- - - - - - - - - - - ,
~ 140
US LA#4 mode
-0
Fe auxiliary motor
.,... 11>
o
<.)
-.;
&! 100 Fe efficiency decrease
Fig.9.Capacitor System High-voltage Circuit Configuration
~
Fe Power Ratio [%]
Fe Power Ratio [%]
This section describes the different circuit configurations of the high-voltage electrical system used for the different hybrid power sources that are suitable for the fuel cell described in the previous section .
O-12S[kmIb) full
•
..
Rant;e where acceleration
'.
/'. . . /. perfonnance is-satisfactory
"-:::, .m
(shaded area)
%<">:~~.
maximum output from each fuel cell 85
90
95
HIGH-VOLTAGE SYSTEM CONFIGURATION POll'er capacitor system
As stated earlier, where a power capacitor is used, the high-voltage electrical system is con figured so that the power capacitor is connected in parallel with the fuel cell to take advantage of the wide voltage range..over which the capacitor can be used. Lithium batter\! system
Minimum capacitor capacitance where acceleration performance is satisfactory at
80
Fe max Power + Battery max Power
4.2 Next-generatiol1 FCHV High-voflaf,:e System
Best fuel economy point where acceleration performance is satisfactory at maximum output from each power source Capacitor capacitance at which operation is possible with repeated 60-90[km/h) full - - - --.. - Cap8citonapacitance-arwhich I ~ operation is ~ssible with repeated
~
Fe max Power
Fig.II.Fuel Cell Maximum Output and Li-ion Battery Output
o
/
Regeneration e decrease
100
ct
o L -_ _ _ _ _ __ _ _ _ __ _ _ _ _ _ o 20 40 60 80 100
In determining such capacitor specifications as the capacitance one must take into consideration such factors as the vehicle's acceleration and deceleration, the repetition of acceleration and deceleration, and the recovery of deceleration energy during cyclic operation. Fig.IO shows the results of a parameter study of vehicle acceleration and deceleration and the like in which the model was used . It shows that while the vehicle meets the acceleration performance requirements, when acceleration is repeated on a flat road , there is a lower limit that does not fall below the designed voltage of the motor. In the graph, the shaded area represents the region where the prescribed requirements are met. The point where the fu el econolllY is best is where the capacitor capacitance is 7 farads and the maximum output from the fuel cell is 95 kilowatts.
• •
8'g 120 >1.l
..s
15 --...
..s
Where a lithium battery is used as the power storage device, a converter becomes necessary, because the usable voltage range of the battery is limited. However, because the lithium battery has high power density, the configuration of the converter circuit can be changed from a full bridge type to a more efficient half bridge type. This is the situation shown in Fig. 12. The change to a half bridge converter requires that the usable voltage range of the fuel cell be separated frolll the usable voltage range of the lithium battery, but this is feasible as long as the number of battery cells is sixty or less. It can also be seen that even when one takes into account the increase in converter loss caused by the increase in the transformer ratio, the half bridge converter is better than the full bridge type.
105
Fuel cell maximum output [kW] ( ) Fuel economy index
Fig. 1O.F uel cell Maximum Output and Capacitor Output
UTHf(;'M BATTERY
The fuel economy characteristics of a system that uses a lithium battery when the output ratio of the lithiulll battery to the fuel cell is the parameter that is varied are shown in Fig.ll . The fuel economy is best \-vhen the ratio of fuel cell output to total system output is in the range from 65% - 85%. The fuel cell
35
Half Bridge Converter
good
Full Bridge Converter
11 0
Fuel economy ~
-~ --
105
~~
"0
..s>-. ~
C
100
0
u
[..tJ
-.; ::l
u..
~
100 ~
Con~~rter
95
efficjencL
With the lithium battery systems, which use converters, the fuel economy is good. because the fuel cell can be operated intermittently when the vehicle is stopped or when energy is being recovered during deceleration .
>-.
u
c
90
'u'"
4::
5. CONCLUSION
'-'
[..tJ
....
_________
'" '"c:;>
t::
80
0
U
O~LI------------------------------~~I 0 o 30 60 90 120 Number of Li-ion cells
Fig.12 Effic iency of Li-ion System FUEL ECONOMY CHARACTERISTICS Table I shows a comparison of the fuel economy characteristics of the high-voltage system con figurations described above. The lithium battery system shows better fuel economy than the current nickel-metal hydride battery system, indicating that a 4% improvement is possible. Because the power capacitor system can not operate the fuel cell intermittently, the auxiliary motor loss increases, as shown in Fig. 13 , so that the fuel economy is worse than that of the current system. Table 1. Comparison of Characteristics in Different System Ni-MH
Capacitor
Li-ion
Volume [L] (include converter & FC stack)
220
235
196
Weight [kg] (include convel1er & FC stack)
252
259
206
(I) TOYOTA FCHV on limited sale in December 2002, with its new fuel cell hybrid system featuring fuel cell and the secondary battery connected in parallel, both optimally controlled according to the state of the vehicle, was developed to improve fuel economy while also providing high responsiveness III vehicle transitional states. (2) In 2004, A high-voltage system was developed for the highly efficient next-generation FCHV. The system uses a lithium battery, the optimum power storage device for the characteristics of the fuel cell system, and a highly efficient half bridge converter circuit. (3) The change to a lighter secondary battery and the reduction of converter losses make it possible to improve the fuel economy by 4% over the current Toyota FCHV. (4) Because a system that uses a power capacitor does not allow the fuel cell to be operated intermittently, its fuel economy is I I % worse than that of the Li-ion system.
REFERENCES
I.
2. Fuel economy Inclex (Ni-MH=IOO)
100
93.7
104.2 3.
Fuel economy Improvement [%]
based
----'"
+4.2
--
based ~ + I1.3 Fuel Economy: @LA#4 mode
4.
3500 3000
:c~ '"'"0 ...J
Battery
.:.. ,;.
2500
::-- ::::
~
2000 1500
Fe uXlliary
otor
1000
Fuel cell gene tion
500 0
Il _______ Capacitor
Ni-MH
lj-ion
Fig.13. Comparison of Loss in Different Systems
36
S. Sasaki, T. Takaoka, H. Matsui, T. Kotani. "Toyota's Newly Developed Electric-Gasoline Engine Hybrid Power train System ", The 14th International Electric Vehicle Symposium, ( 1997). T. Matsumoto, N. Watanabe, H. Sugiura, T. Ishikawa. "Development of Fuel-cell Hybrid Vehicle", the 2002 SAE World Congress, (2002). Y. Hori, T. Teratani, R. Masaki. "Motor Technology for Automobile", The Nikkan Kogyo Shimbun. LTD (The Business & Technology Daily News). (2003). T. lshikawa, S. Hamaguchi. T. Shimizu, T . Yano, S. Sasaki, K. Kato, M. Ando, H. Yoshida. "Development of Next Generation Fuel-cell Hybrid System", the 2004 SAE World Congress, (2004) .