Development Of Multi-Physics Transient PEMFC System Model And Analysis Of Water And Heat Transfer

Owing to the advantages of high energy conversion efficiency,zero emission,and low operating noises,proton exchange membrane fuel cell（PEMFC）is widely acknowledged as one of the most promising energy conversion devices.As pointed out by the U.S.Department of Energy（DOE）,cost,performance,and durability are the three main technical challenges before the large-scale commercialization.Startup from subzero temperatures,performance heterogeneity inside stacks,and water and heat management at the system level not only affect output performances of PEMFC applications but also significantly influence the product durability.To comprehensively investigate aforementioned research issues,the study developed multi-scale and multi-physics coupling simulation models under multi-operating conditions,including the one-dimensional transient fuel cell model,the pseudo two-dimensional stack model considering uneven flow distributions,and the dynamic system model integrating all auxiliary subsystems.Effects of structural designs on cell performances under cold start conditions,performance heterogeneity of stacks,coupled heat and mass transport mechanisms inside the whole system,and assisted cold start strategies at the system level are investigated.The research can be summarized as follows:（1）Effects of membrane electrode assembly designs on cold start performances and ice formation phenomena.A transient multi-phase fuel cell model is developed,considering coupled heat and mass transport mechanisms,super-cooled water freezing mechanism,and ice formation in the cathode channel.Influences of structural properties（such as hydrophobicity of porous layers and membrane thickness）,startup temperature,heat transfer coefficients between cell and surroundings,and super-cooled water freezing rates are investigated under the maximum power startup mode.Decreasing the micro-porous layer（MPL）hydrophobicity facilitates the removal of super-cooled water,which consequently reduces the ice formation in catalyst layer（CL）.After the cell successfully survives the startup process,the hydrophobicity of MPL strongly affects the overall performances.Besides,hydrophobicity differences among CL,MPL,and gas diffusion layer（GDL）should be carefully chosen to avoid possible water flooding phenomena.A thinner membrane is beneficial to the improvement of membrane hydration,and it is more sensitive to the variations of MPL hydrophobicity.（2）Performance heterogeneity of PEMFC stack and optimizations.To simultaneously consider uneven flow distributions as well as electrochemical reactions,phase changes,and transport processes inside every fuel cell,a comprehensive stack model is developed based on the integration of a pseudo two-dimensional multiphase stack sub-model and a flow distribution sub-model.Differences between the uniform flow assumption and the authentically non-uniform distribution are quantitatively investigated,and effects of operating conditions（such as current density,stoichiometry,and pressure）,manifold structural parameter,and stack configurations are studied.Results show that the uniform assumption not only overestimates stack output performances but also underestimates single cell voltage variations and temperature variations.Even though the total amount of air seems abundant,it is still possible for some fuel cells to suffer from the local reactant starvation owing to uneven flow distributions,making it necessary to supply sufficient air for the stack.A larger manifold cross-sectional area leads to more uniform reactant distributions among the stack.Compared with the Z-type configuration,the U-type configuration is able to operate normally with smaller manifold cross-sectional areas,and it improves the performance heterogeneity.（3）Coupled water and heat transport mechanisms at the system level.A comprehensive dynamic system model is developed,including a pseudo two-dimensional transient stack sub-model,a one-dimensional transient membrane humidifier sub-model,a one-dimensional electrochemical hydrogen pump sub-model,an air compressor model with proportion-integral-derivative control,and a ribbon-tubular fin radiator model.To ensure the reliability of integrated system model,all sub-models have been validated against the experimental data from literature or author’laboratory under various operating conditions.Effects of operating conditions（such as current density,temperature,mass flow rate and pressure）and structural designs（such as gas flow pattern,effective transport area）are investigated.It is found that the system is likely to suffer from membrane dehydration when the expected stack temperature reaches 70℃or the operating current density is relatively small(0.5 A cm^-2),causing significant performance deterioration.The co-current flow pattern contributes to better water utilization of the whole system,but the counter-current flow pattern significantly improves the temperature distribution uniformity inside fuel cells.Decreasing the operating temperature difference between stack and memrbane humidifier does not effectively solve the membrane dehydration problem.Increasing the temperature of dry air flowing into humidifier improves the air humidification.Increasing the cathode stoichiometry is disadvantageous under membrane dehydration conditions because it leads to more generated water being purged rapidly.However,increasing the compressor pressure ratio is beneficial to the enhancement of electrochemical reactions inside stack as well as the water utilization of whole system.（4）Assisted cold start strategies at the system level.Based on the integrated transient system model,assisted startup strategies such as reactant gas heating method,stack heating method,and coolant heating method are quantitatively compared.The output performances,stack inlet reactant relative humidity,ice generation rate,temperature distribution,and cold start duration are comprehensively investigated.It is concluded that investigating the reactant gas heating method at the stack level overestimates the cold start survivability because it is unable to consider the disadvantageous moisture coupled with hot reactant gases at the authentic system level.The advantages of reactant heating effect are not able to compensate for the disadvantages of increased stack inlet relative humidity,which may lead to expedited cold start failure.For stack heating method,the successful startup of middle cells can be regarded as a crucial index for the successful cold start of whole stack.Increasing the heating power results in the reduction of startup duration as well as the significant increase of temperature differences,indicating that improving the thermal conductivity of fuel cell materials is of great importance.For coolant heating method,the ice formation can even be avoided if the coolant inlet temperature is kept at 10°C.However,the power consumption for heating coolant is extremely large,and it is unrealistic to merely rely on the stack output power to satisfy the power requirement,implying that secondary power sources are necessary.