A quick evaluating method for automotive fuel cell lifetime
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], [2], [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], [5], [6], [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], [9], [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], [12], [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 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.
Section snippets
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 bywhere rd is the fuel cell performance decay rate; ΔP 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
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=56 cycles/h, n1=0.99 cycles/h, t1=13 min/h and t2=14 min/h in average. Fig. 1 shows the fuel cell system power in 1 h driving cycle of this fuel cell bus.
The limited performance decreased value ΔP 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
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
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
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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