Experimental Study on Voltage Uniformity of 14.4 kW PEMFC Stack Single Cell

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.