Numerical Simulation of PEFC Stack System for Design of Configuration and Control of Anode Systems

Overview Performances of cell stacks are simulated, focusing on driving state of anode system. This simulation technique takes overall transport phenomena in PEFC fuel cell systems, such as transport of gas species and liquid water in both anode and cathode channels and across MEA, thermal balance, electrochemical reactions and current distributions. Injector and exhaust systems are modeled by boundary conditions, mimicking actual driving conditions. Relation between cell performances and driving state of anode system, for example, consumption of hydrogen, build-up of liquid water and cross-over nitrogen and purging by control of injector and/or ejector can be discussed based on simulated results. Over-all performance of anode system is one of crucial issue for durability and cost-reduction of fuel cell systems. These results show good applicability of this kind of simulations on design of PEFC stack system. Numerical Modeling and Results Simulation is performed by our own simulation software for PEFCs, which is capable of simulating full-stack fuel cell system for fuel cell vehicles (up to 400 cells) under transient operating conditions. An important feature of this software is that macroscopic models are applied for microscopic phenomena in the MEA such as electrochemical reactions and proton/water transport. Heat transfer and fluid of liquid-gas two phase fluid with phase changing are also taken into account. These models are coupled with multi-dimensional heat and mass transport equations, including effective parameters for gas diffusivity and permeability in the GDL and equivalent hydraulic diameter in the flow channel. This software can also simulate cathodic degradation by carbon corrosion reactions. Performances of up to 400 cells stacks are simulated. A numerical model of anode system is constructed by adding transient boundary conditions for an injector and an exhaust system with an ejector. These transient boundary conditions represent open / close timings of injector and ejector under actual driving conditions. Distributions and consumptions of hydrogen, build-up of cross-leak nitrogen and condensate liquid water can be shown under constant (up to 30A) loading. Simulated results show difference of cell potential between of stacked cells, and relation between performance distributions and balances of transport phenomena in stack systems. Effects of configuration of stack systems are also shown. Based on these results, design of stack systems and control schemes of anode system can be discussed. Those discussion can provide useful information for improving durability of stacked cells by avoiding depletion of hydrogen, and / or examination of control schemes which can efficiently utilize hydrogen fuel.