A computational model is developed for simulations of transient behavior of PEFC stack under realistic driving conditions. This model is applied to simulations of internal states of cells stack (maximum output about 100kW) with an injector, an exhaust and a recirculation system. Transient internal states under pressure swing and purging process are calculated. The physical model is based on so-called 2D + 1D model, which takes into account overall transport phenomena in PEFC fuel cell systems, such as transport of gas species and liquid water, thermal balance, electrochemical reactions and current distributions. Injector and exhaust systems are modeled by boundary conditions. Transient internal states, including fuel flow distributions between stacked cells and cell performance change at purging process will be demonstrated. Possible applications of those simulations to designing fuel cell stack control system will be discussed.
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 electrochemical reactions and transport phenomena in the MEA. Heat transfer and fluid dynamics 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.
Computational models of cell stack (250 cm2 reaction area in each cells) are constructed. A numerical model of anode system is constructed by adding transient boundary conditions for an injector and an exhaust system with an ejector valve. An interconnect boundary condition between inflow and outflow manifolds are also introduced as a computational model of recirculation system. 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 calculated under transient (maximum output is about 100kW) loading.
Simulated results show transient behavior of the stacked cell system, for example,
- Build-up of cross-leak nitrogen and condensate liquid water under transient loading conditions, and their effects on the distribution of performances between stacked cells.
- Evolution of fuel flow distribution between stacked cells under pressure swing.
- Efficiency of purging process and change of performance before and after purging.
Those results can be calculated for various configurations of cells and driving conditions, and relations between stacked cell behaviors and those conditions can be discussed. Those discussions can provide useful information for improving durability of stacked cells by avoiding depletion of hydrogen, and / or examinations of control schemes which can efficiently utilize hydrogen fuel.