Fuel cell startup from freezing temperatures is a requirement for automotive applications as many countries experience cold climate. It has been shown at Ballard Power Systems that an optimum MEA water content is necessary at startup in order to achieve fast freeze startup with no performance loss. In this paper we present stack models and designs for obtaining an appropriate MEA water content at startup by maximizing water migration from MEA to plate channels due to temperature gradients during natural cooling. Using this concept no external power is needed after fuel cell shut down from wet operation to achieve the intended MEA water content at startup. Experimental results for the MEA water content along the stack after shut down and natural cooling and for the stack performance during startup from freezing temperatures are presented for the new stack design.

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An infinitely long circular cylinder embedded horizontally in a porous medium at thesame temperature is suddenly heated to a conslQnl temperoture. The governing equations are soWed numericaJJyfor small and lJuge vabus of the Rayleigh number, and at small times, the resuJJs are found to be in very good agreement with the ana/ytica/ solution as obtained by Pop et al. fJI, who used the method of matched asymptotic expansions. A pwme region develops at the top of the cylinder, and two counterrotating vortices form adjacent to the sides ofthe cytinder. A novel feature ofthe results is that as convection becomes increasingly more dominant, a single hat ceU of fluid forms vertical/y above the cylinder and rapidly moves away from the cylinder as time increases. The hot etU offluid occurs at smoJJu times and distances from the surface of the cylinder as the Rayleigh number increases. The numerical results indicate that the heat and fluw penetrate farther upward into the porous medilJ as time increases, whereas the conservation of energy principle shows that the heat penetraUs infinitely into the porous media. At very lJuge times the numerical resuks show reasonable agreement, in the vicinity of the cylinder, with the steady state solution obtmntd by Ingham and Pop [21. which should only be considered as an inner solution to the problem.