Range Extender Vehicle Concept Based on High

Range Extender Vehicle Concept Based on High
2014 Ninth International Conference on Ecological Vehicles and Renewable Energies (EVER)
Range Extender Vehicle Concept Based on High
Temperature Polymer Electrolyte Membrane Fuel
Cell
Dave Dickinson
Mounir Nasri
German Aerospace Center (DLR)
Institute of Vehicle Concepts
Pfaffenwaldring 38-40
70569 Stuttgart, Germany
Email : dave.dickinson@dlr.de
German Aerospace Center (DLR)
Institute of Vehicle Concepts
Pfaffenwaldring 38-40
70569 Stuttgart, Germany
Email: mounir.nasri@dlr.de
Abstract— Battery electric vehicles that would be suitable for
urban traffic as well as for longer distances will be equipped with
a range extender (REX). In this range extender vehicle concept,
the powertrain is driven mainly by the high performance li-ion
battery added by a HT-PEFC (Polymer Electrolyte Membrane
Fuel Cell). The on-board fuel cell range extender serves as an
additional energy source, which charges the high performance
battery during the trip especially in a long distance trip.
On the basis of existing system models, such as fuel cell and
battery, a complete vehicle model is been developed with the help
of the AlternativeVehicles library [1], especially the thermal
model has been extended and parameterized. For the thermal
coupling between the high-temperature fuel cell and the
hydrogen storage, a high temperature cooling circuit is modeled.
The battery temperature control is represented in the model by
means of a low-temperature cooling circuit. Considered the
challenges that are now placed on modern cars, several scenarios
such as cold start and warm-up phase are been created. For these
scenarios, suitable operating strategies can be developed and
integrated into the overall vehicle model. Based on various
driving cycles, the thermal and electrical behavior of the overall
system is investigated.
Keywords—High temperature fuel cells; on-board
recharge; range extender; alternative drive concept;
hybrid electric vehicle; total vehicle simulation.
I.
INTRODUCTION
To achieve the set by the European Commission
emission reduction of 60 % compared to 1990 in the
transport sector, alternative drive concepts with lower
emission are required. According to the European
Commission's Paper on Transport 2011, the proportion of
the conventional operated fuels cars should be halved and
reduced in cities by 2050 to zero [2].
A promising solution is a battery electric range
extender (REX) vehicle, which is suitable for urban
traffics as well as for longer distances, in which a high
temperature polymer electrolyte membrane fuel cell (HT-
978-1-4799-3787-5/14/$31.00 ©2014 IEEE
PEFC) is integrated as a REX unit (figure 1). This is an
attractive way, which combines both technologies (li-ion
battery and fuel cell) either permanently or on demand and
also promotes the introduction of e-mobility as a
sustainable traffic concept.
Fig. 1: Architecture of the high temperature fuel cell range extender
vehicle
On the basis of existing system models the range
extender powertrain is modeled and the matching thermal
management
concept
using
the
own
DLRAlternativeVehicles modelica library [1] is developed.
To determine the thermal and electrical requirements of
different driving cycles for the entire system, the
powertrain of an existing test vehicle called HyLite [3] is
been replaced by the smart electric drive vehicle model.
The technical data of the modeled vehicle are summarized
in table 1.
Parameter
Value
Curb weight
Battery mass
Loading weight (fuel cell system,
hydrogen tank and periphery)
840 kg
175 kg
140 kg
Maximum engine performance
Driving resistance (ABCCoefficient)
Drag cW
Frontal area A
55 kW
[A=77.88; B=1.222;
C=0.4475]
0,36 m2
2,08 m
Table 1: Vehicle data used for the model parameterization
II.
OVERCOME COMMUTER ROUTE AND COMPARISON OF
BOTH TECHNOLOGIES
In the figure 2 the typical commuting distances from
home to work in Germany are shown and these values
can be up to about 50 km [4].
The first aim of this publication is to overcome this
round trip without refueling using the described battery
electric vehicle high temperature fuel cell (HT-PEFC)
REX. The second objective is to increase the efficiency
of the battery in a low ambient temperature on an
effective utilization of the thermal coupling between the
battery and the HT-PEFC.
To know the basic idea of a REX-vehicle closer
according to the trend of both technologies, the Li-ion
battery system and the HT-PEFC system will be
compared. The comparison is focused on the energy
density and based on the basis of available products in
the market. In the comparison, it includes the volume and
weight of the HT-PEFC-tank, however without the
observance of the peripheries. In the same case, the
battery’s peripheries were not considered. In figure 4, the
gravimetric energy densities from both are shown.
Fig. 2: Commuting distance in Germany
In this paper, a real commuter route between the cities
Stuttgart-Lampoldshausen-Stuttgart (Germany) is used,
which is driven over the rural road. The velocity profile,
which is recorded by means of a mobile GPS device in a
real trip, is shown in the figure 3. The outward and return
journey lasted without intermission about 3 hours 15
minutes. The total distance is approximately 192 km and
the height difference is about 350 m.
Stuttgart-Lampoldshausen-Stuttgart drive cycle
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
50
100
150
Mass (kg)
Fig. 4: Comparison of the gravimetric energy density
It is clear that the HT-PEFC has a clear advantage in
comparison to the battery. In figure 5 the volumetric
energy densities are displayed.
0.26
50
2000
4000
6000
8000
10000
12000
600
Stuttgart-Lampoldshausen-Stuttgart elevation curve
400
200
0
0
Battery gravimetric energy density
HT-PEFC + hydrogen tank gravimetric energy density
0.4
100
0
0
Elevation (m)
0.45
2000
4000
6000
Time (s)
8000
10000
Fig. 3: Stuttgart-Lampoldshausen-Stuttgart drive cycle
Volumetric energy density (kWh/l)
Velocity (km/h)
150
Gravimetric energy density (kWh/kg)
0.5
0.24
Battery volumetric energy density
HT-PEFC + hydrogen tank volumetric energy density
4 kg H2 tank capacit
3 kg H2 tank capacity
0.22
2 kg H2 tank capacity
0.2
0.18
0.16
1 kg H2 tank capacity
0.14
12000
0,63 kg H2 tank capacity
0.12
0
50
100
150
Volume (l)
200
Fig. 5: Comparison of the volumetric energy density
250
300
Here is also seen: from a certain tank storage capacity
of the HT-PEFC (over about 0.9 kg), the HT-PEFC
system has a higher volumetric energy density compared
to the Li-ion battery system. However the fuel cell
technologies are nowadays still immature, which affect a
poor cost effectiveness of the system including the
peripheral equipment.
As been clarified, the background of this idea is
therefore to take a benefit of the gravimetric and
volumetric energy density of the fuel cell system to
extend the range of electric vehicles and combine it with
the advantageous dynamic performance of the battery.
Additional advantage of this idea is to use the waste heat
of the HT-PEFC for tempering the battery and possibly to
warm the vehicle cabin.
III.
IMPORTANT COMPONENTS OF THE RANGE EXTENDER
POWERTRAIN
With N is the number of cells, U0 = 1.23 V and I is the
battery current.
B. Battery
Serial to the fuel cell system the batteries are
connected. A series 3 parallel of 31 Li-ion-battery-cells
have been established. The final charge voltage of the
cell is 4.0 V; the maximum charge voltage is at 391 V.
the maximum discharge rate can be up to a maximum of
200 A. The net energy capacity at room temperature is
17.6 kWh.
The AV library provides two different available
battery models, map-based and impedance-based model
[7]. The map-based battery model is been used here for
the modeling of single cells. With the help of the maps
from data sheets, the appropriate values for the internal
resistance and open-circuit voltage can be specified as a
function of state of charge, current and temperature [8].
C. Thermal Circuit
A. Fuel Cell System
The high temperature fuel cell system consist of 120
cells. The cells are connected in series with each other.
The maximum power is 6 kW. The schematic structure of
the HT-PEFC is shown in the figure 6. The hydrogen
capacity of the tank is about 0.905 kg.
The cooling system for the range extender drivetrain
consists of a high-temperature and low-temperature
cooling circuit and is implemented in the simulation
environment Dymola. The high-temperature cooling
circuit represents the thermal cycle of the fuel cell
system and consists of the HT-PEFC, pump, hightemperature radiator and the associated lines (figure 7).
Fig. 6: Schematic structure of the HT-PEFC
For the modelling of the fuel cell a map-based model
is used [5]. This model is suitable for system simulation
[1], in which a high computation speed is required. The
polarization curve is taken from measurements and data
sheets. The validation is done by a comparison between
the simulation results and the manufacturer's
measurement results. For the thermal simulation the
setting of the thermal mass is very important, since the
power loss leads to a change of the temperature of the
thermal mass. The thermal dissipation of the fuel cell is
proportional to the voltage difference from open circuit
voltage (U0) and the actual voltage (UStack) [6] and is
calculated as follows:
Ploss = (N * U0 – Ustack) * I
(1)
Fig. 7: The high-temperature cooling circuit
The working temperatures of the HT-PEFC are
between 100 - 160 °C. As a coolant, TEG (Triethylene
Glycol) is been used. TEG is a member of a homologous
series of dihydroxy alcohols. It is a colorless, odorless
and stable liquid with high viscosity and a high boiling
point and especially for the use of
temperatures between +100 °C and +285 °C.
operating
The tempering of the battery is represented in the
model by means of a low-temperature cooling circuit
with the coolant mixture of water-glycol. For
performance and durability reasons, the battery should
be operated only in a certain temperature window: From
operating temperatures of 50 °C, the battery life is
reduced and a loss of efficiency is observed. At very low
temperatures, below about 10 °C, the performance and
the efficiency of the battery decrease also significantly.
As can be seen in figure 8, the battery circuit, the
electrical components, the battery and the electric
machine (EM), are connected with a low-temperature
cooler (LT-HEX), which transfers the excess heat loss to
the environment. The valves (valve 1, 2 and 3) are used
depending on the current operating state for the supply
and to shut down some cooling circuit branches: If rapid
heating of the battery is required in the absence of
engine heat, only the valve 2 is opened. To dissipate the
heat of the battery, only the valve 1 will be opened.
Fig. 9: The overall vehicle model implemented in Dymola/Modelica
IV.
OPERATING STRATEGIES
For the following thermal scenarios, especially cold
start, high operating temperatures and operation strategies
will be developed.
A. Cold Start
The following figure 10 shows the temperature
progress of both main elements during a cold start at a
minus 10 °C ambient temperature for a sample short
drive of Stuttgart-Lampoldshausen drive cycle. In this
sample trip, the HT-PEFC is been preheated nearly at the
beginning of the trip. As the temperature of the HT-PEFC
reaches 100 °C, it begins to work and produces heat;
therefore the temperature increases significantly
afterward (figure 10).
120
HT-PEFC temperature
Battery temperature
100
Fig. 8: The low-temperature cooling circuit
Temperature (°C)
80
60
40
20
0
Figure 9 shows the overall vehicle model, which
includes 5 main subsystem namely the powertrain
model, the thermal model, the controller module, the
drive cycle (driver model) and the boundary conditions
model.
-20
-40
0
500
1000
1500
Time (s)
2000
2500
Fig. 10: HT-PEFC and battery temperature progress during the trip
Stuttgart-Lampoldshausen
As can be seen in figure 10, it takes about 35 minutes
until the fuel cell reaches an operating temperature of
B. Operating Phase
The main task during the warm-phase operation is to
keep the temperature of the fuel cell within the
recommended operating temperature range. So that the
upper limit temperature 165 °C is not exceeded, the
thermostat (valve 1 in Figure 2) opens at 160 °C.
The high-temperature radiator is included with the
circulation and issued the excess heat to the outside air.
Alternatively, this heat can also be used for heating the
interior. The air mass flow through the high-temperature
radiator is controlled so that the refrigerant inlet
temperature of the fuel cell is around 160 °C. The
performance of the battery and the HT-PEFC at the
operating phase during a sample trip of StuttgartLampoldshausen is been shown in figure 11. The HTPEFC is been preheated at the first 2400 seconds and
then been turned on afterwards (at a long distance trip)
till the hydrogen tank is been depleted. The continuous
net power to charge the battery during a long distance trip
is 5 kW.
40
Battery performance
HT-PEFC performance a REX unit
100
Battery SoC without a REX unit
SoC (%)
To improve the efficiency of the battery at low
temperatures, a 2.7 kW PTC heater has been
implemented in the battery model. The PTC heater heats
the battery up to 10 °C and then is been switched off
automatically.
minus 10 °C. The average speed is 60.5 km/h and can be
up to 123 km/h. These two boundary conditions (low
ambient temperature and quite high driving speed)
degrade the reliability of the battery and thereby decrease
the current carrying capacity. Therefore, the simulation
was interrupted after 7800 s (about 2 hours trip), as also
the SoC was already close to zero.
50
0
0
2000
3000
4000
5000
6000
7000
8000
7000
8000
Range without a REX unit
100
50
0
0
1000
2000
3000
4000 5000
Time (s)
6000
Fig. 12: Battery’s SoC progression and electric vehicle range without
a REX unit
In the second simulation, the HT-PEFC was installed
into the overall vehicle simulation as a REX unit. The
same result parameters as the first simulation are shown
in figure 13. To draw a precise comparison, the exact
boundary conditions as well as in the first simulation,
were used. The electric car now can manage to overcome
the commuter route and the simulation runs until the
desired goal.
200
20
SoC (%)
150
0
Power (kW)
1000
150
Range (km)
about 100 ° C by heating it up with a 600 W PTC heater.
The preheating time depends on some parameters; in this
case the fuel cell stack has a mass of 22 kg and a thermal
capacity of 450 J / kg K.
Battery SoC without a REX unit
Battery SoC with a REX unit
100
50
-20
0
0
2000
4000
6000
8000
10000
12000
8000
10000
12000
-40
-60
-80
0
500
1000
1500
2000 2500
Time (s)
3000
3500
4000
Fig. 11: Battery and HT-PEFC performance during an operating phase
V.
VARIANTS CALCULATION OF THE OVERALL SYSTEM
At first, an electric vehicle without a REX unit was
simulated to overcome the previously introduced
commuter route (figure 3). The figure 12 shows the result
of the first simulation. The ambient temperature here is
Range (km)
200
150
Range without a REX unit
Range iwith a REX unit
100
50
0
0
2000
4000
6000
TIme (s)
Fig. 13: Battery’s SoC progression and electric vehicle range with a
REX unit
In the final simulation, the overall model of the
second simulation was supplemented additionally with
the thermal coupling between the battery and HT-PEFC,
which has already been presented in the Chapter III.
After the PTC heater heated the battery up to 10 ° C,
additionally a thermal coupling took a progress in the
simulation, which leads the loss heat from the fuel cell
into the vehicle cooling circuit. As a result, the battery
was further heated up to its optimum operating
temperature, about 40-50 °C (figure 14). As can be seen,
the temperature of the fuel cell is also maintained in the
manufacturer's recommended operating range, between
100-160 °C.
180
160
140
Temperature (°C)
120
100
80
60
40
20
Battery temperature
HT-PEFC temperature
0
-20
0
2000
4000
6000
Time (s)
8000
10000
12000
Fig. 14: Temperatures of the battery and the HT-PEFC during a trip of
Stuttgart-Lampoldshausen-Stuttgart
To see the extra effect of this strategy more
accurately, the SoC and heat losses of the battery from
the second and third simulation were represented and
compared in figure 15. It turns out that there is an
improvement in the efficiency of the battery at the third
simulation.
SoC (%)
1
Battery SoC without thermal coupling HT-PEFC
Battery SoC with thermal coupling HT-PEFC
VI.
With the DLR-AlternativeVehicles modelica library
[1] a complete range extender (REX) powertrain was
modeled using a high temperature fuel cell and thermal
circuit, which are integrated into the total vehicle model.
With the generated model various scenarios were
investigated, such as driving of an electric vehicle with
and without Range Extender unit, the behavior of the
thermal cycle and the benefit impact of the thermal
coupling between the high temperature fuel cell and the
li-ion battery.
The thermal and electrical overall system behavior
was examined during a trip of a real commute route
Stuttgart-Lampoldshausen. It could be shown that it is
possible to overcome the commute route with the aid of
the construed range extender, meanwhile the original
subcompact electric vehicle (smart electric drive) is not
capable. Generally the range can be extended up to
38 %. Apart from that, the waste heat of the high
temperature fuel cell could be forwarded to the vehicle
thermal cycle for tempering the battery. As a further
benefit, it leads to an increasing of the li-ion battery’s
discharge efficiency during trip.
The vehicle cabin is been planned as a next step, as to
create an interior space model and to integrate into the
overall vehicle model. With this overall thermal vehicle
model, the further investigations of the energy and
thermal management of fuel cell vehicles can be
performed and the suitable operating strategies for the
heating of the vehicle cabin can be developed. By using
an appropriate operating strategy, the average 4.5 kW of
power consumption [9] to heat the subcompact vehicle
cabin can possibly completely be covered by the waste
heat of the fuel cell.
0.5
REFERENCES
[1]
0
0
2000
4000
6000
8000
10000
12000
[2]
Waste heat (kWh)
0.8
0.6
Battery waste heat without thermal coupling HT-PEFC
Battery waste heat with thermal coupling HT-PEFC
[3]
0.4
[4]
0.2
0
0
CONCLUSION
2000
4000
6000
Time (s)
8000
10000
12000
Fig. 15: SoC and the heat loss of the battery with and without a
thermal coupling to the HT-PEFC
[5]
[6]
J. Ungethüm, D. Hülsebusch, H. Dittus und T. Braig, Simulation
of alternative powertrains in Modelica, ASIM Conference, Ulm,
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H. E. Frederick. and P. Treffinger, Hylite technology platform
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[8] Dickinson, Dave and Shitole, Manikprasad, Use of a high
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[7]
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Germany.
[9] H. Grossmann, Car air conditioning, Berlin, Heidelberg,
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