Towards Full-Stack Scale Simulation of PEFC System Transient Control

Overview We will show results of computer simulations on full-stack scale behavior of the PEFC stack system, which also take into account models of auxiliary systems including hydrogen injectors, recirculation pumps, exhaust valves and radiators of coolant. Our simulations focus on transient internal states of the PEFC stack system under driving conditions of FCVs. In particular, we assumed transient loading profiles in similar to automotive uses, and various controlling ways of the stack system (control of injector and/or exhaust valves, recirculation pump and coolant flowrate, for example). Results of simulations show detailed distribution of gas species concentration, temperature, liquid water and electrochemical reaction current. We can discuss possibilities of hydrogen or oxygen local starvation and effect of temperature difference between stacked cells. We also try to investigate the behavior of condensed liquid water. Finally it is discussed how we utilize these results for developments of control systems of PEFC stacks. Numerical Modeling and Results Simulations are performed by our own simulation software for PEFCs, which can deal with full-stack PEFC systems for automotive uses (up to 100s cells) under transient operating conditions. The most important feature of the software is the employed macroscopic model comprehensively dealing with all the relevant transport phenomena coupled with electrochemical reactions. In anode and cathode channels and porous structures in GDL and MPL, mass transport of gas and liquid phase are solved by considering the two-phase fluid dynamics with taking into account the effects of phase changing. Here mass transport is coupled with heat and chemical species transport equations. Those all transport phenomena are coupled with electrochemical reactions in the MEA. Transport of chemical species and water through the MEA are also considered. In the software, compared with ordinary fully-resolved CFD-based models, the computational cost is greatly reduced by use of macroscopic engineering models. For example, two-phase fluid flow is solved by Darcy’s law on the coarse-grained mesh instead of solving Navier-Stokes equation. In order to recover appropriate flow and pressure distribution in channels, the distribution of the equivalent hydraulic parameter in flow channels is estimated by detailed CFD in advance, and mapped to the coarse mesh. Many engineering models are employed in order to consider various transport phenomena, like water up-take into electrolytes and effective transport of oxygen from gas phase to reaction cites in catalyst layers, etc. Electrochemical reactions are modeled by Butler-Volmer equation, in the manner of lumped parameter models. The detail of computational models of a cell stack system (300 cm2 reaction area in each cell) are followings. The numerical model for anode system includes transient boundary conditions for an injector and an exhaust system with an ejector valve. An interconnection between inflow and outflow manifolds by recirculation pump is also taken into account by adding interconnecting computational elements representing volume of pipes and recirculation pump with forced flow conditions. Recirculation of coolant is also considered in the same way, in addition with modeled effects of the radiator. The transient behavior of the stack system is calculated under transient loading profiles in similar to automotive driving conditions, where the following elements of stack system conditions are taken into account; controlled hydrogen and air feeding, purging of gas and liquid water, recirculation pumps and coolant flow. Simulation results show behaviors of transient internal states of the PEFC stack system under driving conditions. Possibilities of starvation of hydrogen and oxygen at high loading current are ones of interesting results. Temperature distribution between stacked cells and dependency on coolant flow control are also discussed. We also try to work on accumulation and purging of liquid water and effects on cell performance. These discussions can provide useful information for developments of efficient control system for PEFC stack, aiming to avoid possible drawbacks on durability and cost issues.