Modeling Of Mass Transport In PEMFC Stack With Metallic Bipolar Plate

The proton exchange membrane fuel cell(PEMFC)shows great potential as a promising candidate of alternative energy-conversion device due to its zero emission,low operating temperature,low noise and high power density.PEMFCs are also considered to be used in a wide range of applications including stationary power systems,automobiles and other power generation units.As the main product of electrochemical reaction in PEMFC,the accumulation of liquid water will affect the performance of the stack,which can cause the trouble for gas to enter the interface of the catalytic layer and decrease the reaction rate.At the same time,the liquid water in the flow channel will increase the flow resistance of the flow field.The pressure distribution will become more inhomogeneous,or the energy consumption of the air compressor will be increased.So the output efficiency of the stack is reduced.At present,the basic research on the accumulation and discharge of liquid water in the flow field and its influence on the reaction gas supply are not yet mature.It is urgent to break through especially in the study of the mechanism and modeling of mass transport when the gas diffusion layer(GDL)is compressed.As a result,there is no effective way to design the flow field and assembly compression.In this paper,the mechanism and modeling of mass transport in PEMFC stack with Metallic Bipolar Plate is studied.Firstly,the model of assembly is established considering the compression between metallic bipolar plates and GDL.It is also considered that compression influence on the internal mass transfer mechanism in GDL.Then,the behavior of liquid water in gas channel is analyzed by experiment and simulation method.The influence of liquid water on reaction gas distribution is obtained.The flow distribution is analyzed.In order to obtain a macroscopic scale mass transport model,it is promoted which is coupled with the influence of liquid water and shape error.Finally,based on the model above and operating conditions of the vehicle,the flow field and assembly conditions of the metallic bipolar plate are designed.A fuel cell stack with good performance is obtained by the process of prototype manufacturing,assembly and performance test.The research of this paper is mainly focused on the following aspects:1)Mass transfer modeling and analysis in GDL of PEMFCBased on the Winkler contact model,it is derived that the influence model of clamping force on GDL compression deformation.The correctness of the model is verified by finite element simulation.With considering the influence of the compression state of the internal pores in GDL on the mass transport,the Darcy seepage model of the liquid water and the oxygen diffusion model with Bruggeman correction are established.Using the GDL mass transport model,the effects of the flow channel and GDL geometric parameters,the assembly clamping force and the operating conditions on the discharge of liquid water and the oxygen partial pressure under the ribs are analyzed.It provides the base for flow field design and GDL matching.2)Analysis of liquid water discharge in gas channelThe VOF method was used to establish the two-phase flow model in the gas channel.And the behavior of liquid water discharge in the gas channel is summarized.Based on the law of liquid water discharge obtained by the simulation model,the adhesive force balance of the droplet on the surface of GDL is analyzed.The effect of the liquid water discharge process on the additional flow resistance is derived.The visualization experiment platform of fuel cell is built to observe the liquid water transport in the flow channel by high speed camera system.The model is verified by the experimental results.3)Modeling of mass transport in PEMFCAimed at the geometric characteristics of stamped ultra-thin metallic bipolar plates,a shape factor is proposed to describe the flow resistance.Combining the formula for the pipe resistance and the configuration analysis,the flow network model and its analytical solution can be obtained to predict the flow distribution.The mass transport model is established by coupling the flow network model,additional flow resistance correction and shape error correction.It is discussed that the influence of flow field structure and operating conditions on the distribution uniformity of reaction gas.It provides guidance for the design of flow field.4)Flow field design,prototype manufacturing and experimental researchof fuel cell stackThe flow field of the metallic bipolar plate for vehicle is designed by using the mass transport model.The metallic bipolar plate which is satisfied the design tolerance requirement is produced through stamping process.The prototype of fuel cell stack is completed according to assembly matching requirements.The evaluation of design is done by the fuel cell stack test of the output performance.The reliability of the design is verified.In this paper,the characteristic of mass transport in PEMFC stack with metallic bipolar plate is studied in detail.The mass transport model is established.The flow field design and prototype experiment are carried out based on the research.The model and analysis method established in this paper will provide a theoretical basis for the design of metallic bipolar plates and the matching of GDL assemblies.