Study of Water Freezing and Thawing Process inside the PEMFC Stacks during Cold Start

Abstract

Cold start is a necessary function for PEMFC systems. In order to investigate the water freezing and thawing process inside PEMFC stack under subzero temperature, a 2-D transient model containing 40 cells has been developed. This model approximately estimates freezing and thawing time of each cell and shows the spatial distribution of water and ice for the purpose of analyzing the differences between single cells as well. As simulation examples, two processes under 260K ambient temperature have been studied in this paper and the results show as follows: (1)after the stack shut down, first of all, the outermost layer cell of the stack freezes before the inside cells and the freezing duration of each cells is almost the same, secondly, the freezing interval time of the outer cells is longer than the inside cells and the innermost cells almost freeze at the same time; (2) before the stack start up, heating up the membrane under two situations, one with the residual water inside and the other one has been purged after it shut down, the temperature rise of the stack with water is slower than that without water, this result indicates that the residual water would influence the cold start process.