PEFC model

Figure 3. Speedup of a massively parallelized PEFC model.

Much effort has been expended in the last 5 years upon development of numerical models with increasingly less restrictive assumptions and more physical complexities. Current development in PEFC modeling is in the direction of applying computational fluid dynamics (CFD) to solve the complete set of transport equations governing mass, momentum, species, energy, and charge conservation. 

Various forms of governing equations have been used in PEFC modeling, although all fall under the single-phase assumption. To clarify important subtleties with theoretical rigor, in this subsection we summarize a set of conservation equations and provide detailed comments of various terms that should be used. 

Table 1. Single-Phase PEFC Model Governing Equations with Source Terms Identified in Various Regions ...

Multidimensional thermal models were presented by many PEFC modeling groups. Maggio et al. performed the pseudo-three-dimensional simulations, neglecting the temperature gradient in the flow direction. However they only considered the overall heat source term as (Uo — In the three-... 

This is close to the simulated value of 2.57 W, including the irreversible, entropic, and Joule heat. The energy-conversion efficiency of this cell, defined as the ratio of the electric power (Jjvg VreiiA) to the total energy consumption IsvgVceuA + eat), is then calculated to be 40%. These simple calculations help demonstrate the validity and accuracy of a noniso-thermal PEFC model. 

Recently Meng17 developed a transient, multiphase, multidimensional PEFC model to elucidate the fundamental physics of cold start. The results showed the importance of water vapor concentration in the gas channels, which implies that large gas flow rates benefit cold-start performance. They also found that ice growth in the cathode catalyst layer during cold start was faster under the land than under the gas channels, and accumulated more at the interface between the cathode catalyst layer and GDL. 

In spite of the advances made in PEFC modeling, many challenges remain. First, there is a critical need to couple, in some computationally efficient way, the pore-level or particle-level submodels with the macroscopic cell-level models in order to take into account the effects of the microstmctures of CDL/MPL and CL. Second, further efforts are also needed to model the cold start, transient, and two-phase transport at the ceU level. At present, a framework of single fuel-cell modehng has been developed, but the physics is not yet completely understood. For example, ice formation within the catalyst layer and its impact on the electrochemical reaction need further study. The two-phase transport at the CL-MPL and MPL-CDL interfaces is not clearly understood at present. Third, current... 

Indeed, as = 1, vve have J2k k = (Hk k) = 0- This means, that Equations 1.31 are not independent any of these equations can be expressed as a linear combination of the others. Physically, Equations 1.31 determine the relative fluxes only to establish the reference frame for the fluxes, an independent closing relation has to be prescribed. In PEFC modeling, nitrogen flux at the cathode side is zero, and the condition = 0 is usually used as the closing relation. 

In gas dilTusion electrodes, it is usually assumed that the mass transport resistance caused by diffusion of dissolved oxygen is negligible, since the diffusion lengths in solution is small compared to the diffusion lengths in the gas phase. Under this assumption, Henry s constant can be absorbed as a constant factor in the exchange current density. This is a common practice in PEFC modeling. 

In PEFC modeling studies, a typical relative permeability expression for noncon-solidated sands is adapted to describe the phase transport in DM due to its simplicity [ r=( Snw) ]- However, recent work has indicated a slightly different relationship is more appropriate [42] ... 

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