Research on the in-plane temperature distribution in a PEMFC stack integrated with flat-plate heat pipe under different startup strategies and inclination angles

•Flat-plate heat pipes are applied as cooling devices in PEMFC stack.•Temperature field goes through transition stage and rebalance stage during startup.•Fluctuation in rebalance stage is smaller with step-wise startup.•Gravity affects the temperature uniformity of PEMFC with flat-plate heat pipes.•In-plane temperature evenness is improved under different orientations. Thermal behavior plays an important role in the application of a proton exchange membrane fuel cell (PEMFC) stack. Flat-plate heat pipe (FPHP) is proposed as an alternative cooling device in this study. Compared with conventional liquid-cooled PEMFC stack, PEMFC-FPHP stack needs less parasitic power for auxiliary system and provides a uniform temperature field. A PEMFC-FPHP stack was fabricated, assembled and tested. The experimental result shows that FPHP successfully delivers heat from the stack to surroundings. A more even in-plane temperature uniformity of cell unit is observed as load increases. The transient and stable in-plane temperature field of cell unit are influenced by startup strategy and gravity. In addition, direction of improvement for future work is given.