Dynamic bolt forces during cold starts and drying processes of a PEM fuel cell

TL;DR: This study analyzes dynamic bolt forces during PEM fuel cell cold starts and drying processes, revealing significant force increases during cold starts and decreases during drying, with residual water content in membranes qualitatively derivable from tie rod forces.

Abstract: When assembling a fuel cell stack, the individual cells are attached to each other via a clamping system. The clamping system must provide high contact pressures so that the cells are sealed gas-tight and the electrical contact resistance between the cell layers and the bipolar plates is kept low. Due to swelling of the membranes and temperature expansion, the cells expand or contract during operation. For optimum functionality, the clamping system must be designed in such a way that sufficient contact pressure is always present, even at various operating points. In this study, the forces in the tie rods of the clamping system and the expansion of a stack during a cold start and during a drying process after shutdown are analyzed. It is found that the contact forces increase significantly during a cold start and the stack expands. During drying after a shutdown, it is observed that the forces decrease and the stack contracts. It is finally found that residual water content in the membranes can be qualitatively derived from the forces in the tie rods. From this, criteria are formulated when a drying process should be optimally terminated to avoid a complete dry-out of the membranes.