Computational fluid dynamics modelling of a polymer electrolyte membrane fuel cell under transient automotive operations

A polymer electrolyte membrane (PEM) fuel cell is probably the most promising technology that will replace conventional internal combustion engines in the near future. As a primary power source for an automobile, the transient performance of a PEM fuel cell is of prime importance. In this thesis, a comprehensive, three-dimensional, two-phase, multi-species computational fuel cell dynamics model is developed in order to investigate the effect of flow-field design on the magnitude of current overshoot/undershoot and characteristics of current response when the cell is subjected to different voltage change patterns representing an automotive operation. The meshing strategy specific to PEM fuel cell modelling is studied in a systematic manner and employed in all analyses presented in this thesis. The predicted results compare very well with experimental data under both steady-state and transient operations. Two computational domains are used – the straight single-channel and practical-scale square cells with parallel, single-serpentine, and triple-serpentine flow-fields. The results from the straight single-channel cell suggest that the magnitude of current overshoot/undershoot increases with the voltage change rate. The behaviour of a current response curve is the result of complex interplay between water content at both sides of the membrane. It is also found that current overshoot/undershoot is amplified with the presence water flooding in the cell. The results from the square cell reveal that current overshoot/undershoot is caused by non-uniformity of local current density over the active area confirming the effect of flow-field geometry on transient response of the cell. By comparing the transient performance between the three flow-fields, a direct relationship between degree of water flooding in the cell and magnitude of current overshoot/undershoot has been found. A conclusion has been drawn which states that a cell with superior water removal ability will experience smaller current overshoot/undershoot

Meshing strategy for PEM fuel cells CFD modelling – a systematic approach

Typical PEM fuel cell models usually involve more than one million mesh elements making the computation very intense. This necessitates an effective way to mesh the computational domain with a minimum number of mesh points while, at the same time, maintaining good accuracy. The meshing strategy in each flow direction is investigated systematically in the current study and it has been found that mesh resolution in different directions has a different degree of influence on the accuracy of solutions. The proposed meshing strategy is capable of greatly reducing the number of mesh elements, hence computation time, while preserving the characteristics of important flow-field variable

A CFD investigation of effects of flow-field geometry on transient performance of an automotive polymer electrolyte membrane fuel cell

A three-dimensional, multispecies, multiphase polymer electrolyte (PEM) fuel cell model was developed in order to investigate the effect of the flow-field geometry on the steady-state and transient performances of the cell under an automotive operation. The two most commonly used designs, parallel and single-serpentine flow fields, were selected as they offer distinctive species transport modes of diffusion-dominant and convection-dominant flows in the porous layers, respectively. It was found that this difference in flow mode significantly effects membrane hydration, the key parameter in determining a successful operation. In a steady run, a serpentine flow field increased the averaged current density under the wet condition due to superior water removal, but this had a negative effect on the cell in the way that it caused membrane dry-out if dry reactant gases were used. The transient operation, on the other hand, seemed to favor the combination of a serpentine flow field and dry reactant gases, as it helped in the removal of product water and speeded up the transport of reacting species to the reactive site to find equilibrium at the new state with minimum time delay and current overshoot or undershoot, which is the most important aspect of a dynamic syste

Transient performance investigation of different flow-field designs of automotive polymer electrolyte membrane fuel cell using computational fluid dynamics

Transient performance of a polymer electrolyte membrane (PEM) fuel cell in terms of the time-dependent current density profile that responds to the varying cell potential is of critical importance for an automotive PEM fuel cell. A step change in cell potential is applied to the cell terminals to simulate a sudden change in load demand due to an engine startup or very high acceleration. The transient responses of the three most commonly used flow-fields, namely, parallel, single-serpentine, and interdigitated designs in terms of the magnitude of current overshoot and time taken to adjust to the new equilibrium state are compared. The results suggest the serpentine flow-field outperforms its two counterparts as it balances the satisfactory transient performance with an expense of acceptable pressure drop across the cell and hence it is the most appropriate design to be used in automotive PEM fuel cells