Full text of DOI:10.1016/S0378-7753(01)00965-X

Journal of Power Sources 105 (2002) 58–65
Fuel economy and life-cycle cost analysis of a fuel cell hybrid vehicle
Kwi Seong Jeong*, Byeong Soo Oh
Department of Mechanical Engineering, Chonnam National University, 300 Youngbong-Dong, Pukgu, Kwangju 500-757, South Korea
Received 2 July 2001; accepted 28 September 2001
Abstract
The most promising vehicle engine that can overcome the problem of present internal combustion is the hydrogen fuel cell. Fuel cells are
devices that change chemical energy directly into electrical energy without combustion. Pure fuel cell vehicles and fuel cell hybrid vehicles
(i.e. a combination of fuel cell and battery) as energy sources are studied. Considerations of efficiency, fuel economy, and the characteristics
of power output in hybridization of fuel cell vehicle are necessary. In the case of Federal Urban Driving Schedule (FUDS) cycle simulation,
hybridization is more efficient than a pure fuel cell vehicle. The reason is that it is possible to capture regenerative braking energy and to
operate the fuel cell system within a more efficient range by using battery.
Life-cycle cost is largely affected by the fuel cell size, fuel cell cost, and hydrogen cost. When the cost of fuel cell is high, hybridization
is profitable, but when the cost of fuel cell is less than 400 US$/kW, a pure fuel cell vehicle is more profitable. # 2002 Elsevier Science
B.V. All rights reserved.
Keywords: Fuel cell hybrid vehicle; Life-cycle cost; Fuel economy; Battery
1. Introduction
Pure fuel cell vehicles and fuel cell hybrid vehicles that
consist of a fuel cell and battery are being studied. Con-
siderations of efficiency, fuel economy and characteristics of
power output in hybridization of fuel cell vehicles are
necessary. Hybridization of a fuel cell enables the size of
fuel cell to be reduced by using a battery, when power
demand is high, such with higher loads or acceleration, and
allows the fuel cell system to be operated more efficiently. In
addition, the initial cost of vehicle manufacturing is less
because the cost of the expensive fuel cell can be reduced.
When the power demand is low, the fuel cell provides the
required power. The use of a battery allows fast start-up of
the fuel cell and allows capture of regeneration energy. The
disadvantages of hybridization are complexity of the vehicle
system, weight increase, complexity of the control system,
and extra battery cost.
Gasoline and diesel fuels are used in present vehicle
engines. These engines are operated by combusted working
fluid. The internal combustion engine has been used for
more than 100 years for vehicle power. The environmental
problem caused by vehicles and the limitation of fossil fuel
are demanding new engines with new fuels. The most
promising vehicle engine that can overcome the problems
of present internal combustion is the hydrogen fuel cell.
Fuel cells are devices that convert chemical energy
directly into electrical energy without combustion. Using
a fuel cell as an automotive engine, there is no Carnot
limitation and the efficiency increases to about twice that
of an internal combustion. Because fuel cells have no
moving parts, fuel cells are very quiet and reliable.
Hydrogen, methanol, natural gas and gasoline can be used
as fuels in fuel cells, but the best option is hydrogen in a
polymer electrolyte membrane fuel cell (PEMFC). Because
such fuels, except hydrogen, contain carbon, it is impossible
to avoid exhaust gas of carbon dioxide. When hydrogen is
used, the exhaust is pure water only and there is no need for a
fuel processor.
In this study, the vehicle, fuel cells, battery and motor
model and the control strategy are established and computer
simulation is developed to the examine overall vehicle design.
The vehicle cost and the life-cycle cost of the fuel cell hybrid
vehicle are analyzed by using the results of the simulation.
2. Configuration of fuel cell hybrid vehicle
* Corresponding author. Tel.: þ82-62-530-0676; fax: þ82-62-530-1689.
E-mail address: ujungks@yahoo.co.kr (K.S. Jeong).
Fig. 1 shows the configuration of the fuel cell hybrid
vehicle which consists of a fuel cell system, an assistant
0378-7753/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 7 7 5 3 ( 0 1 ) 0 0 9 6 5 - X

K.S. Jeong, B.S. Oh / Journal of Power Sources 105 (2002) 58–65
59
Table 1
Vehicle parameters
Glide mass (kg)
600
0.2
2.0
Drag coefficient
Frontal area (m2)
Vehicle wheel base (m)
Cargo/passenger weight (kg)
2.755
136
and the cost of the vehicle. Start-up to normal operation
takes between a few minutes and 30 minutes. According to
many studies, because of the need for a fuel processor, the
estimated cost of fuel cell vehicle using gasoline or methanol
is much higher than that of a vehicle fueled by hydrogen.
When hydrogen is used as a fuel, a fuel processor is not
necessary, the exhaust gas is pure water, start-up time and
response to load change are fast, and efficiency is increased.
on the other hand, a much higher cost for infrastructure is
needed.
Methods of hydrogen storage on the vehicle are liquid
hydrogen, compressed hydrogen, metal hydride, and hydro-
gen absorbed in carbon nanotubes. The energy density of
liquid hydrogen is high. To store hydrogen in a liquid state, it
is necessary to maintain it at ꢀ253 8C at ambient pressure.
Thus, a highly insulated liquid hydrogen tank is needed. A
quarter of the chemical energy of hydrogen itself is con-
sumed in the liquefaction process. Metal hydride is safest,
but it is very heavy and much time is consumed to store
hydrogen. Carbon nonotube are at the developmental stage.
In the case of compressed hydrogen, the stored energy
density is low and energy is consumed in compression.
Table 2 shows the compressed hydrogen storage character-
istics used in this study [2].
Fig. 1. Fuel cell hybrid vehicle system.
power battery, and a driving system which includes a motor
and a control system. The fuel cell system comprises a fuel
supply system, an air supply system, a humidification system
to operate the fuel cell more efficiently, and a thermal
management system to control the operation temperature
and to use the heat of the fuel cell.
The voltage of the fuel cell is usually lower than that of the
vehicle. Increase of the fuel cell voltage can be made
possible by increasing the stack number, but a dc to dc
converter or a dc to ac inverter is more general. The direct
vehicle wheel-drive system consists of a motor and a speed
reducing transmission. A controller regulates the fuel cell
operation conditions, such as temperature, pressure, humi-
dification, battery state-of charge (SoC), charge-discharge
current, boost converter and hybrid power output.
2.1. Vehicle model
The forces of vehicle driving are rolling resistance, air
drag, accelerating force and climbing force. The force
needed in vehicle driving is the sum of these forces and
the power needed in vehicle driving can be written as
2.2. Fuel cell model
Fuel cells are devices that convert chemical energy
directly into electrical energy. In general, the characteristics
of a fuel cell are represented by current–potential curves.
The characteristics of a fuel cell differ according to the
operating conditions. A fuel cell system consists of a fuel
supply system, an air supply system, a water management
system, and a fuel cooling system. Part of the fuel cell power
output is used to drive auxiliary systems. The polarization
curve model uses the current–potential curve including
auxiliary driving power.
2
1
2
Pd ¼ ðma þ CRmg þ mg sin y þ raCDAFv Þv
(1)
where Pd is the power needed by vehicle, m the total mass
of vehicle, a the acceleration of vehicle, CR the coefficient
of rolling resistance, g the gravity constant, y the angle of
gradient, ra the density of air, CD the drag coefficient, AF the
formal area of vehicle.
On vehicle driving, there are power losses in many vehicle
components. The required power of the fuel cell and battery
includes these component losses as well as the auxiliary
power for cooling, air supply, fuel supply, head lights etc.
The main losses in a fuel cell hybrid vehicle are largely
motor and controller losses, transmission losses, breaking
losses, and dc–dc boost converter losses. The vehicle para-
meters [1–3] used in this study are shown in Table 1.
Hydrogen, methanol and gasoline can be used as fuels in
fuel cells. Gasoline and methanol can be supplied by the
present infrastructure for vehicles. With hydrogen, however,
a fuel reformer is needed to produce hydrogen from gasoline
or methanol. A fuel reformer increase both the complexity
The power–efficiency curve is used in modeling fuel cell
systems. The power–efficiency model does not consider the
characteristics of current–potential and the auxiliary system
in detail, but uses the relationship between fuel cell power
Table 2
Compressed hydrogen tank (5000 psig)
Specific energy (HHV) (Wh/kg)
Energy density (HHV) (Wh/l)
Fuel tank mass (kg)
2630, 6.7% H2
780, 20 kg H2 (m3)
50
Stored hydrogen mass (kg)
3.35

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K.S. Jeong, B.S. Oh / Journal of Power Sources 105 (2002) 58–65
Fig. 2. Fuel cell system power rate versus efficiency.
Table 3
Fuel cell system
3. electrical energy storage that is generated from the fuel
cell at low load;
4. fuel cell power assist at higher load;
5. main energy supplier when fuel cell system operates at
low load.
Fuel cell system energy efficiency at peak (%)
Power density (W/l)
60
250
Specific power (W/kg)
250
Durability (h)
3000
Various types of battery are under development. The
factors in selecting a battery for vehicles are specific
power, specific energy, life-cycle and cost. Table 4 shows
the characteristics of various types of battery. This paper
uses an advanced lead-acid battery for the vehicle simula-
tion model. Details of this battery [3,5,6] are given in
Table 5.
and efficiency. The relation between fuel cell system power
and efficiency has been the subject of many studies. At low
load, the fuel cell system efficiency is very low because of
the operation of the air compressor or blower to supply air,
and operation of the cooling system and the humidifying
system. The fuel system efficiency is high at medium load.
At high load, the efficiency is again low because fuel cell
efficiency decreases as the fuel cell current increases. The
power–efficiency model is shown in Fig. 2. This study uses
this power–efficiency model [2,4]. The present technology
of fuel cells is summarized in Table 3.
Peukert expressed the relation between the rate of battery
discharge and capacity as follows:
In ꢁ t ¼ constant
where I is the current, t the time, and n the constant.
This equation can be rewritten as
(2)
(3)
2.3. Battery model
Ci ¼ Coeff ꢁ Iexp
The role of the battery in a fuel cell hybrid vehicle can be
summarized as follows:
where C is the battery residual capacity, exp ¼ 1 ꢀ n, Coeff
the constant coefficient.
This study used a Hawker Genesis lead-acid battery for
which the Coeff is 16.28 and exp is ꢀ0.225 [7].
1. energy source of vehicle electrical devices;
2. regenerative braking energy storage;
Table 4
Rechargeable battery option
Technology
Specific
Energy
Specific
Power
Initial cost
(US$/kW)
Life-cycle
energy (Wh/kg)
density (Wh/l)
power (W/kg)
density (W/l)
Advanced lead-acid
Nickel–metal hydride
Lithium–polymer
35
80
71
200
220
150
150
412
220
315
100
955
600
445
200
180
450
400
500–1000
1000
155
90
1000
Sodium–nickel chloride
Nickel–cadmium
50

K.S. Jeong, B.S. Oh / Journal of Power Sources 105 (2002) 58–65
61
Table 5
The battery is charged between 20 and 80% of the fuel cell
maximum power. When the SoC is below 0.4, the battery is
charged above 0.8, battery charging is stopped.
The efficiency of the motor and the motor controller is
affected by speed and torque. The efficiency is assumed to be
0.78 in this paper. When regenerative energy is recovered at
deceleration and brake operation, the efficiency is assumed
to be 0.5.
Parameters of advanced lead-acid battery
Number of cell per module
Voltage per module (V)
Capacity (C/20) (Ah)
6
12
12
Mass of single module (kg)
4.785
When a fuel cell hybrid vehicle drives on the road, the
battery charge-discharge current changes. If the current at
unit time, is known, the variation in battery SoC, can be
determined, i.e.
3. Simulation results
For a fuel cell system of 30 kW and a battery power of
45 kW, the vehicle power requirement, battery power and
fuel cell power output are shown for a FUDS cycle in Fig. 3.
The fuel cell operates only when the required vehicle power
is 6–30 kW. The battery operates only when the required
vehicle power is lower than 6 kW. Both the fuel cell and the
battery operate when the required vehicle power is more than
30 kW. When the vehicle is decelerating and braking,
regenerative energy is recovered and stored.
DCi
Ci
IiDt
IiDt
DSoC ¼ ꢀ
¼ ꢀ
¼ ꢀ
3600Ci
3600 ꢁ Coeff ꢁ Iexp
(4)
(5)
At time t, the SoC is
Z
SoC ¼ SoCinial
þ
DSoC dt
The efficiency of charge-discharge is associated with the
loss due to the battery internal resistance, and actual mea-
sured data are used.
The fuel economy of a fuel cell hybrid vehicle changes
with operation condition, control method, etc.The hybridi-
zation ratio defined as
2.4. Hybrid control methods
hybridratio
maximumvehiclepowerꢀmaximumfuel cellpower
¼
(6)
Methods of fuel cell hybrid control are based on battery
SoC. The vehicle simulation mode is Federal Urban Driving
Schedule (FUDS). When the required vehicle power is lower
than 20% of the fuel cell maximum power, the battery
supplies all the driving power of the vehicle and the fuel
cell shuts off. When the required vehicle power is greater
than the fuel cell maximum power, the battery supplies
surplus power.
maximumvehicle power
The simulation results for the fuel of economy of fuel cell
hybrid vehicles are shown Fig. 4. As battery power increases
to 25 kW, fuel economy is increased. This is possible
because regenerative braking energy can be recovered and
stored in the battery. At low loads, it is possible that the fuel
cell operates more efficiently by operation the battery when
Fig. 3. Power demand, battery power and fuel cell power as a function of time for FUDS cycle.

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K.S. Jeong, B.S. Oh / Journal of Power Sources 105 (2002) 58–65
Fig. 4. Comparison of fuel economy for fuel cell and fuel cell hybrid vehicles.
the fuel cell efficiency is low. The reasons that fuel economy
is lower when the hybridization ratio is 0.2 rather than 0.33
are (i) sometimes the fuel cell cannot be operated efficiently
because the battery is so small and (ii) the battery charge-
discharge efficiency is low because the battery current is
large. When the hybridization ratio is more than 0.33, the
fuel economy is decreased. The reason for this is that the
time of charge-discharge and the current from the battery
are increased because the battery is the main vehicle power
source. When the battery power is 55 kW and the fuel cell
power is 20 kW, the efficiency is lower than that for a pure
fuel cell vehicle.
today. The fuel cell cost will be decreased through reduction
of platinum loading, improvement of stack performance, and
mass production. It is forecast that the fuel cell cost will be
decreased to 200 US$/kW in 1 or 2 years. The fuel cell cost
must be lower than 50 US$/GJ in order to compete with
internal combustion engines [8].
4.2. Initial cost of fuel cell vehicle
The assumed price of the fuel cell vehicle component is
given in Table 7. The vehicle is calculated based on these
data. Fig. 5 shows the initial cost of fuel cell hybrid vehicle.
If the fuel cell cost is high, hybridization can reduce the
initial cost of the vehicle. If the fuel cell cost is lower than
50 US$/kW, how ever, hybridization will increase the initial
4. Vehicle and life-cycle costs
The life-cycle cost is the sum of the initial vehicle cost and
the maintenance cost. The initial vehicle cost is affected by
the fuel cell cost and battery cost. Because the fuel cell cost
is presently more than 1200 US$/kW, hybridization of the
fuel cell vehicle can reduce initial vehicle cost.
Table 6
Material costs for a present-day fuel cell stack
Material mass
(kg/kW)
Material cost
(US$/kW)
The maintenance cost is affected greatly by the hydrogen
fuel cost. If fuel economy is increased by hybridization of
the fuel cell vehicle, the life-cycle cost is decreased. Because
the initial vehicle cost is settled at the time of manufacturing,
development of fuel cell technology and performance cannot
affect life-cycle cost. Because the hydrogen fuel cost can be
decreased by increase of demand and development of man-
ufacturing technology, how ever, the life-cycle cost can be
decreased.
Membrane
Electrode
Catalyst
0.025
0.082
0.016
3.3
120
31.16
243.2
825
Bipolar plate
End plate
Plastic frame
Total
0.12
0.105
3.684
0.24
0.105
1219.705
Table 7
Initial cost of fuel cell vehicle components
4.1. Fuel cell cost
Car body (US$)
10000
Battery (US$/kWh)
50
5000
4000
Motor and controller (US$)
Fuel, air supply and water management (US$)
The PEM fuel cell cost is 1000–2000 US$/kW at present.
Table 6 summarizes the material cost for a fuel cell stack

K.S. Jeong, B.S. Oh / Journal of Power Sources 105 (2002) 58–65
63
Fig. 5. Fuel cell vehicle cost as a function of hybridization ratio.
cost of the vehicle because battery cost increases and the
vehicle control system is complex.
chemical industry. An infrastructure for supplying hydrogen
for vehicles is not in place. To use hydrogen as a fuel in fuel
cell vehicles, an infrastructure for production and supply
must be established. The cost of hydrogen is very high. If
many fuel cell vehicles are produced, however, the hydrogen
cost will be decreased through the mass production of
hydrogen. If hydrogen fueled vehicles become more than
4.3. Hydrogen fuel cost
Hydrogen production methods are natural gas steam
reforming, electrolysis of water, and by product of the
Fig. 6. Life-cycle cost of fuel cell hybrid vehicle.

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K.S. Jeong, B.S. Oh / Journal of Power Sources 105 (2002) 58–65
Fig. 7. Base line for fuel cell vehicle hybridization
50,000, the estimated cost of hydrogen is 20–30 US$/GJ in
California [9,10].
numbers, the estimated cost of the fuel cell system is about
200–400 US$/kW. If this estimate is realized, hybridization
of the fuel cell vehicle is not necessary on economic
grounds.
4.4. Life-cycle cost
Life-cycle cost includes vehicle cost, used hydrogen fuel
cost, and the battery cost. Some assumptions must be made
in the calculation of life-cycle cost. The number of battery
charged-discharged is 300 in a year, and battery life is 3.3
years. Vehicle life is 10 years. The vehicle travels 15,000 km
in a year. The hydrogen consumed is calculated based on the
above simulation result.
5. Conclusions
The fuel economy and life-cycle costs for a pure fuel cell
vehicle and a fuel cell hybrid vehicle have been compared.
In the case of FUDS cycle simulation, hybridization is
more efficient than a pure fuel cell vehicle. This is because it
is possible to capture regenerative braking energy and to
operate the fuel cell system within a more efficient range
when using a battery. Fuel economy is decreased, however,
when battery power is small because of charge-discharge
losses. Fuel economy is affected by driving condition,
control method, etc.
Fuel cell vehicle cost is reduced by hybridization when
the fuel cell cost is high. Life-cycle cost is largely affected
by the fuel cell size, fuel cell cost, and hydrogen cost. When
the cost of the fuel cell is high, hybridization is profitable,
but when the cost of the fuel cell is smaller than 400 US$/
kW, a pure fuel cell vehicle is more profitable.
Many challenges have still to be met before a fuel cell
power system can achieve the cost, performance and relia-
bility in order to guarantee successful commercialization of
fuel cell vehicles. Fuel cell research will provide improve-
ments in both performance and cost. Clearly, fuel cells can
be adopted widely as cleaner and more efficient automotive
engines.
Fig. 6 shows the life-cycle cost based on fuel cell and
hydrogen costs. If the fuel cell cost is as high as 1250 US$/
kW, hybridization can reduce the life-cycle cost. If the fuel
cell cost is 50 US$/kW, then hybridization increases the life-
cycle cost because it increases the initial vehicle cost.
Hybridization of a fuel cell vehicle must to be considered
in both economical and technical terms. Fig. 7 presents a
standard of judgment for whether hybridization is neces-
sary or not with variation in fuel cell cost and hydrogen
cost, based on life-cycle costs. If the fuel cell cost is high,
hybridization is profitable because the initial vehicle cost
is decreased. If there are more than 50,000 hydrogen
vehicles, the estimated hydrogen cost is about 30 US$/GJ
in California. If the fuel cell cost is about 550 US$/kW,
at the early stage of commercialization, hybridization is
economical. If the fuel cell cost is lower than 400 US$/kW, a
pure fuel cell vehicle is profitable because the initial fuel
cell cost has a stronger influence on the life-cycle cost than
hydrogen cost. If fuel cell vehicles are produced in large

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65
[4] National Renewable Energy Laboratory, Advisor-Advanced Vehicle
Simulator, 1999.
Acknowledgements
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vehicle applications, J. Power Sources 61 (1996) 33–34.
[6] W. Luttrell et al., Integration of Fuel Cell Technology into the
Virginia Tech 1999, Hybrid Electric Future Car.
This work was supported by the Brain Korea 21 Project in
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[7] Valerie Johnson, Matthew Keyser, Testing, Analysis, and Validation
of a Hawker Genesis Lead Acid Battery Model in ADVISOR.
[8] Per Ekdunge, Monica Raberg. The fuel cell vehicle analysis of energy
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