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A quick evaluating method for automotive fuel cell lifetime
Pucheng Pei, Qianfei Chang, Tian Tang
State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China
art i cle info
ab st rac t
Article history:
Fuel cell vehicle commercialization and mass production are challenged by the durability
Received 10 February 2008
of fuel cells and could be promoted by accelerated lifetime evaluating methods. In this
Received in revised form
paper, an arithmetic equation of fuel cell lifetime is presented, which is relating with load
15 April 2008
changing cycles, start–stop cycles, idling time, high power load condition and the air
Accepted 16 April 2008
pollution factor. Basing on the practical data gathered from a fuel cell bus and the test
Available online 16 June 2008
results of a fuel cell stack in laboratory, the calculated lifetime fits the bus real running
Keywords:
Fuel cell durability
Accelerated test
Lifetime evaluating method
Potential lifetime prediction
Open circuit voltage
lifetime very well. It is shown that the automotive fuel cell lifetime mightily depends on
driving cycles, and the potential lifetime in different operating mode can be effectively
predicted by using this method with about 300 h test time. The test results also show that
the effect of start–stop cycling on fuel cell lifetime can be almost ignored if the stack open
circuit voltage is dispelled quickly after fuel cell stops operating. It is worthwhile that from
this quick lifetime-evaluating method we can find many possible directions to improve fuel
cell durability.
& 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
reserved.
1.
Introduction
The proton-exchange membrane fuel cells (PEMFCs) with the
advantages of low-operating temperature, high current density, high potential for low cost and volume, fast start-up
ability, and suitability for discontinuous operation become
the most promising and attractive candidate for electric
vehicle power [1–3]. However, some major technical issues
are still to be solved for the wide-spread marketing of FC
generators into the transportation area. Economical viability
depends notably on improving the durability and the reliability of these new embedded generators. Fuel cell lifetime
requirements vary significantly, from 5000 h for car applications up to 20 000 operating hours for bus applications [4–7].
However, practical experiments through a long time will
spent much cost and often get lifetime results too late to
follow the fuel cell technique progress. Therefore accelerated
evaluating method for fuel cell lifetime is necessary. The
method should shorten test time and be used to predict fuel
cell potential lifetime [8–10].
It is widely accepted that there are two stages associated
with the lifetime studies of fuel cells. The degradation sources
with respect to various material selections and different
operation conditions are identified in the first stage and in the
second stage, mathematical models are developed to predict
the lifetime of fuel cells, in which expressions of the aging
phenomena and aging effects are incorporated into performance models through constitutive relations [10]. But in
most literatures, the lifetime study of fuel cells remains in
the first stage, with mostly experimental characterizations
presented [11–13].
The reason why lifetime of automotive fuel cell is shorter
than that of stationary fuel cell is the complex operation
conditions undoubtedly, and so the effects of driving cycles,
start–stop, high power load condition and idling condition
should be seriously considered. The load on fuel cell stack
frequently changes during running especially in vehicular
Corresponding author. Tel.: +86 10 62789134; fax: +86 10 62789699.
E-mail address: pchpei@tsinghua.edu.cn (P. Pei).
0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2008.04.048
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application, which will accelerate the fuel cell degradation
[14].
Many studies on fuel cell long-term aging were conducted
to evaluate fuel cell durability. Xie carried out a 2000-h fuel
cell durability test and investigated two types of MEAs [15].
Wilkinson and St-Pierre took modified urban transit authority
(UMTA) driving cycles to do their tests on a 8-cell stack and a
20-cell stack [11]. Liu carried out a study on a small fuel cell
under cyclic current loading conditions, simulating the real
road driving conditions for automotives, and established a
phenomenological durability model to describe the aging
process and cell performance at different time nodes [10]. Lee
investigated the performance degradation of a fuel cell that
exposed to repetitive on/off cycles [16].
In this paper, lifetime expression basing on operating
conditions is studied. The effects of load changing cycles,
start–stop cycles, idling cycles and high power condition on
fuel cell durability are separately researched and the results
show us the potential direction to prolong fuel cell lifetime.
2.
Formula for fuel cell lifetime
Many FC lifetime tests have shown the performance degradation is linear with time. So if we get the linear rate and know
the life end point, then the lifetime can be doped out on the
performance aging line. The lifetime can be given by
Tf ¼
DP
,
rd
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prolonged by better powertrain configuration in FC vehicles?
We make a calculate formula of vehicular FC available
lifetime Tf as following:
Tf ¼
DP
,
kp ðP01 n1 þ P02 n2 þ P03 t1 þ P04 t2 Þ
where P01 ; P02 ; P03 and P04 are performance deteriorate rates
resulted in by large-range load change cycling, start–stop
cycling, idle condition and high power load condition
separately, measured in laboratory; n1, n2, t1 and t2 are load
changing cycle times, start–stop cycle times, idle time and
high power load time per hour, gained from vehicular driving
cycle.
3.
Parameters confirm and formula validation
3.1.
Parameters n1, n2, t1 and t2 in driving conditions
In last two years, one of our demonstrating fuel cell buses run
on a fixed route everyday, and it completed 43 000 km until
now. From data gathered in all range trial, we get
n1 ¼ 0.99 cycles/h,
t1 ¼ 13 min/h
and
n1 ¼ 56 cycles/h,
t2 ¼ 14 min/h in average. Fig. 1 shows the fuel cell system
power in 1 h driving cycle of this fuel cell bus.
(1)
where rd is the fuel cell performance decay rate; DP stands for
the limited decreased value of fuel cell performance from
beginning to the lifetime end according to its definition.
It is often not completely the same for the fuel cell lifetime
test results between in laboratory and on road, by reason of
the air quality difference and operating conditions. Then the
lifetime formula can be written by
Tf ¼
DP
.
kp rd
(3)
(2)
Here kp is the accelerating coefficient.
Different driving mode always results in a different lifetime
range even for the same fuel cell. How much difference
between different driving cycles and how many hours
Fig. 2 – Fuel cell bus trial results.
Fig. 1 – Fuel cell bus real driving cycle on road.
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Fig. 3 – Stack test on driving cycle in laboratory.
Fig. 4 – Performance change in start–stop cycles.
3.2.
The limited performance decreased value DP and the
accelerate coefficient kp
Fig. 2 is the fuel cell bus trial results with average speed of
32 km/h. In order to protect our fuel cells, at the first 1 min of
the trial, big electric current is not allowed to use. So 0.7 V is
the limited voltage by our definition. And at the first time of
the trial, the electric current of 0.7 V is the limited electric
current by our definition. After the fuel cell bus ran 35 000 km,
about 1100 h, with the cell voltage decreased by 10% at the
limited electric current, the fuel cell stack power became
decreasing faster. So we define the time when the cell voltage
decreases by 10% from rated power condition as the fuel cell
life end time, where DP ¼ 10%.
To get kp, driving cycle test in laboratory was performed on a
100-cell PEM fuel cell stack with 280 cm2 effective area in every
cell, which is the same as used in the above fuel cell bus, and
the driving cycle taken in this test is the same as the fuel cell
bus. Fig. 3 is the results of 100 h stack test. Result shows the
performance decay rate rd is 0.00534%/h. Comparing it with the
voltage decreasing rate in Fig. 2, or using Formula 2, therefore
we get the accelerating coefficient as kp ¼ 1.72.
Of course, for fuel cells operated in a certain driving cycle,
the lifetime can be calculated from Formula 2 basing on
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Fig. 5 – Idling test cycles.
Fig. 6 – Performance change in idling cycles.
hundreds hours driving cycle test like this. But this method
cannot meet our demands to predict lifetimes of fuel cells
operating in different driving cycles.
3.3.
Parameters P01 ; P02 ; P03 and P04 gained in laboratory
Different operation conditions, such as styles of start–stop,
idling current and load changing rate, will lead to different
lifetime results. To evaluate fuel cell lifetime simplified, the
same fuel cell stack and the same operation conditions were
taken as in the fuel cell bus. In the bus, five fuel cell stacks are
used and in the laboratory, one same fuel cell stack is used.
3.3.1.
Start–stop cycling
The fuel cell stack start–stop cycling test was carried out in
the strict process—start-up, idling 1 min at constant current
of 10 mA/cm2, stop, purging hydrogen by nitrogen gas, waiting until the stack voltage falls to zero, and then to the next
cycle. Every ten start–stop cycling tests later, the fuel cell
stack performance was recorded. We spent 80 h for this test
item and get the fuel cell performance deteriorating history.
As it is shown in Fig. 4, the cell voltage decays 0.00196% in
every start–stop cycle, then p02 ¼ 0:00196%=cycle. Please note
that if the test cycle number is not enough, the decay rate
precision will be much poor.
3.3.2.
Idling condition operation
When the fuel cell works under idling cycle, the power is not
big, so the water which made by the hydrogen and oxygen’s
reaction is not much. When the gas flows in the channels, the
MEA will be easy to be dry. In our fuel cell bus, the idling
voltage must be set lower in order to prolong fuel cell lifetime.
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Fig. 7 – Load changing cycles test mode.
Fig. 8 – Performance change in load changing cycles.
We set the idling condition at cell voltage no more than 0.9 V
in the bus.
In this test, we set idling current density of 10 mA/cm2, and
made an automatic program in which the fuel cell performance is recorded after every 15 min idling time, shown in
Fig. 5. In the test period, we kept the operating temperature
no more than 60 1C, the air stoichiometric ratio 2.5 and
hydrogen stoichiometric ratio 1.2.
Considering of the fuel cell bus operates day and rests
night, we performed the test 5 h everyday from morning to
afternoon with one start and one stop. Fig. 6 shows 50 h test
result, in which the fuel cell performance gets almost full
recovery at every beginning, with a little decay rate beyond
retrieve. From this figure, we get the voltage decay rate
P03 ¼ 0:00165% 0:00196%
¼ 0:00126%=h.
5
It is significative that although we took test in irregular way
for 10 h after 25 h, the following test results show the same
changing rate as the former test. To enhance the decay rate
accuracy, it is important to keep test process regularly and
strictly.
3.3.3.
Load changing operation
Load changing test cycle was set as Fig. 7, where the load
changes from idling condition to rated power condition, and
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Fig. 9 – Performance change in high power load cycles.
Fig. 11 – Voltage changing history after stop.
Fig. 10 – Performance deterioration related with operating
factors.
the fuel cell performance is measured after every 200 loading
cycles. Everyday we took 30 min idling time to warm fuel cell
to above 40 1C, and then carried out 2000 loading cycles.
Fig. 8 shows all test points shape in orderliness with decay
rate of 0.0000606%/cycle. Subtracting the effects of idling
factor and start–stop factor, we gain:
0:00196% þ 0:00126% 30=60
2000
¼ 0:0000593%=cycle.
P01 ¼ 0:0000606% This test item can be completed in about 80 h. In Fig. 8, the
fuel cell performance tends to steady after 1000 loading
cycles. So the performance decay rate may be gotten in much
shorter time if we perform the test continually after start up
without mid shutdown.
3.3.4.
High power load operation
The steady high power load cycling test was carried out in a
strict process also, in which the first step is start up and then
warm up to above 40 1C, the second step is to load high power
in steady, and make a performance test every 15 min. The
high power load condition was set according with the limited
voltage in fuel cell bus.
Test was performed for 5 h with 30 min warming time and
one start–stop cycle everyday. Fig. 9 shows all days test points
shape orderliness, and then the voltage decay rate contributed to high power load cycles can be written:
0:00196% þ 0:00126% 30=60
5
¼ 0:00147%=h.
P04 ¼ 0:00199% 3.4.
Lifetime calculation
Basing on all above parameters, the fuel cell lifetime
calculated by Formula 3 is TfE1000 h. Compared this result
with the real fuel cell bus running lifetime of 1100 h, they are
approximative. So Formula 3 is available and can be used to
predict fuel cell lifetime in every operating mode.
Maybe different fuel cell stacks work in different operations, but every automotive fuel cell life can be evaluated in
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Fig. 12 – Performance change in start–stop cycles with aftertreatment.
5.
Formula 3. In different vehicle driving cycle, n1, n2, t1 and t2
may be different.
4.
Analysis and discussion
The effect of every operating condition factor on fuel cell
lifetime can be showed in Fig. 10. The load change cycling and
the start–stop cycling are the main factors contributing to fuel
cell performance decay. One third of deterioration is resulted
in by start–stop cycling and 56% is by load change cycling.
Modifying start–stop cycling and load change cycling or
decreasing their times, the fuel cell lifetime will be prolonged
undoubtedly.
In the period of start–stop cycling, the fuel cell deterioration
is related with the high open circuit voltage [17]. It was found
that the stack often maintains open voltage for more than
25 min, especially with voltage of above 0.8 V/cell for 5 min
after every start–stop cycling, shown in Fig. 11.
It is delectable that the effect of start–stop cycling on
the fuel cell lifetime is much less, which almost can be
ignored, while the fuel cell voltage remainder was dispelled
betimes after every start–stop cycle. Fig. 12 shows the
performance decreasing rate is 0.0000234%/cycle, much lower
than the above decreasing rate of 0.00196%/cycle. In this
operating mode, the fuel cell lifetime can be predicted as
TfE1500 h, prolonged 50% more than the former normal
operating mode, with just being modified the stop aftertreatment.
From Formula 3, if the fuel cell load-changing time
decreases, the fuel cell lifetime will be increased. This can
be realized in fuel cell—battery hybrid vehicles and there
must be an optimized driving cycle. Furthermore, if fuel cell
vehicle runs in unpolluted environment, the fuel cell lifetime
will be further prolonged.
Conclusions
An accelerated evaluating method for automotive fuel cell
lifetime was brought up, and the calculated fuel cell lifetime
and data recorded from the practical fuel cell bus shows the
same result. Conclusions can be drawn from above as
followings:
(a) It is reasonable to define the cell voltage decrease of 10%
at a constant current as the fuel cell life end.
(b) The lifetime formula including factors of start–stop
cycling, idling cycling, load change cycling and steady
high power load cycling seems feasible.
(c) Using the evaluating method described in this paper the
fuel cell lifetime could be predicted basing on no more
than 300 h test in laboratory.
(d) The effect of start–stop cycling on fuel cell lifetime can be
ignored when the stack voltage is dispelled betimes after
fuel cell stops operating.
(e) Fuel cell installed in vehicles could run much longer in
clean environment without any pollution.
Acknowledgments
This work was financially supported by the National High
Technology Research and Development Program of China.
The authors would like to express their sincere gratitude to
Shanghai Shen-li High Tech Company for providing fuel cell
stacks.
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