摘要：Proton exchange membrane fuel cell (PEMFC) stack is superimposed by multi single cells in series.Many factors such as material,process and flow field distribution may lead to the performance degradation of single cells.The service life of the stack depends on the single cell with the lowest voltage.Therefore,the single-cell voltage uniformity is an important factor affecting the fuel cell life.The existing voltage uniformity evaluation methods can not fully reflect the voltage consistency of fuel cells.To address this,experiments for single-cell voltage uniformity were conducted on 14.4 kW PEMFC stack test platform to explore the influences of stack current,flow field distribution and the spatial location on the single-cell voltage fluctuation rate.In the experiments,constant frequency and amplitude was adopted as the loading control strategy,the interval of loading was set as 20 ms,and the loading current amplitude was 2 A.When the steady state current was increased from 10 to 180 A with a step of 10 A and the transient state current was dynamically loaded from 47 to 112 A,the voltage fluctuation rates of single cell were tested respectively.Firstly,the data of steady state current loaded from 10 to 180 A were processed according to the statistical principle of discarding the samples at the high and low end.The functional relation between the voltage fluctuation rate and current of single cells were respectively Fourier trinomial and Gaussian binomial before (after) eliminating the end data of single cell.Then,as the voltage fluctuation rate of single cell can not be used for analyzing the voltage amplitude difference and stack fault,an improved evaluation method for voltage uniformity was proposed,which combines the voltage fluctuation amplitude of the single cell,variation ratio,minimum distance and mean deviation of voltage.The voltage fluctuation amplitude of the single cell was used to infer the fluctuation range of stack voltage.The proportion of the cells with large voltage deviation to all single cells was described by the variation ratio.The minimum distance represents the degree of the lowest voltage of the single cell offsetting from the average voltage of the stack.The mean deviation of voltage indicates the fluctuation range of stack voltage.Finally,by using the example of the voltage distribution of 75 single cells when the stack was dynamically loaded from 47 to 112 A,a comparison was made between the traditional voltage fluctuation rate of single cell and the improved evaluation method of voltage uniformity.Although the traditional voltage fluctuation rate of single cell is 0.77,it is difficult to determine the deviation degree of the stack voltage from the voltage fluctuation rate.The improved voltage uniformity evaluation method,however,can overall evaluate the voltage uniformity according to the voltage fluctuation amplitude of the single cell,variation ratio,the minimum distance and mean deviation of the stack voltage.In the comparison experiments,the values of the above parameters are 40.00,0.04,14.33 and 2.60 mV respectively,which indicates that the voltage amplitude difference of the stack is relatively large,and there is accidentally a small number of single cells with large voltage fluctuation.The degree of the lowest voltage of the single cell offsetting from the average voltage of the stack is small.These suggest that the overall uniformity of the stack is good.%质子交换膜燃料电池(proton exchange membrane fuel cell,PEMFC)电堆由多个单电池串联叠加而成,材料、工艺和流场分布等因素可能引起单电池性能衰减,电堆运行寿命取决于电压最低的单体电池,电堆中单体电池电压均衡性是影响燃料电池寿命的重要因素.为了解决现有单体电压均衡性评价方法不能全面准确地反映燃料电池电压一致性的问题,探究电堆电流、流场分布和空间位置对单体电压波动率的影响,以14.4 kW PEMFC电堆测试平台为基础进行了单体电压均衡性实验研究.实验分别测试稳态时电堆电流以10 A为步长、从10 A加载到180 A和暂态时从47 A动态加载到112 A的单体电压波动率,加载策略采用恒频恒幅加载控制策略,负载加载的时间间隔为20 ms,电流加载幅值为2A.首先,运用统计学“掐头去尾”的原则处理电堆稳态电流从10 A加载到180 A的数据,剔除端部单电池前(后)单体电压波动率与电流,分别得到Fourier 3项式和Gaussian 2项式函数关系.然后,针对单体电压波动率无法分析电堆电压幅差和故障的缺点,提出将单体电压波动幅值、异众比率、最小距和电压平均差相结合的改进电压均衡性评价方法.该方法以单体电压波动幅值推断电堆电压波动范围,并通过异众比率判断较大电压偏差电池占所有单电池数目的比例,使用最小距表示电压最低的单体电池偏移电堆平均电压的程度,利用电压平均差评价电堆整体波动大小.最后,以电堆从47 A动态加载到112 A时的75片单体电压分布为算例,对传统单体电压波动率与改进的电压均衡性评价方法做对比实验.研究结果表明:虽然传统的单体电压均衡性评价方法测得的波动率为0.77,但很难从电压波动率上反映出电堆各单体电池电压波动程度;改进的电压均衡性评价方法可从电堆单体电压波动幅值、异众比率、最小距和电压平均差多方面综合评价电压的均衡性,其对比实验值分别为40.00、0.04、14.33 mV和2.60 mV,电堆电压幅差较大;电压波动较大的单电池的数量很少,电压最低的单体电池偏移电堆平均电压的程度较小,电堆电压整体波动较小,电堆整体均衡性较好.
