Research On PEMFC Water-heat Management And Performance Optimization

With the advantages of high energy density,fast starting speed,strong endurance,efficient energy conversion and clean emission,the proton exchange membrane fuel cell(PEMFC)is considered to be one of the most promising future vehicle power sources.At high current density,the amount of water and heat produced by PEMFC will greatly increase,which makes the problem of water-heat management an important factor restricting the improvement of cell performance.In order to further optimize PEMFC water-heat management and improve the cell performance,the multi-scale,multi-dimensional and multi-phase numerical models are established in this paper.Main research results are as follows:1.A mathematical model for PEMFC water-heat management is proposed,and the Bruggeman equation describing oxygen diffusion in the gas diffusion layer(GDL)is modified,and the reliability of the modified model is verified.Results show that the modified model in this paper can more accurately simulate the actual working conditions of PEMFC during concentration polarization.2.A two-dimensional model of cathode porous media area for PEMFC is established based on the proposed mathematical model,and the distribution characteristics of liquid water in porous media are explored.Results show that the liquid water saturation and oxygen concentration in porous media predicted by the traditional Bruggeman equation are both higher.The increase of current density leads to the accumulation of liquid water and the increase of oxygen mass transfer resistance.The reaction rate decreases during concentration polarization,and the generation rate of liquid water slows down accordingly.The farther distance away from the inlet is,the larger the liquid water saturation will be.The growth rate of liquid water and the decrease rate of oxygen concentration under rib are larger than those under channel.The liquid water accumulates much more significantly under rib than under channel at a common working voltage of around 0.55 V.As GDL porosity decreases,liquid water saturation under channel increases almost linearly while the change of liquid water saturation under rib exhibits a parabolic trend.On the whole,liquid water is slightly saturated when porosity is greater than 0.65.Smaller GDL pore size will considerably increase ’flooding’ risk,and thinner GDL thickness is beneficial to the removal of liquid water.3.A three-dimensional full-cell model with cooling channels for PEMFC is established based on the proposed mathematical model.The influences of different cooling patterns on the performance of PEMFC are explored,and the best cooling patterns are found.Results reveal that different gas flow patterns have roughly the same effect on the temperature distribution of the membrane in the case of the cooling water counter-current flowing.Under the counter-current gas flow pattern,pattern 3 is the best cooling pattern,that is to keep the cooling water flow direction consistent with the cathode gas flow direction.Under the co-current gas flow pattern,pattern 6 is the best cooling pattern,that is to keep the cooling water flow direction consistent with the gas flow direction.Meanwhile,the temperature distribution of the cathode and the anode is basically symmetrical at the interface of the cathode gas diffusion layer and the cathode catalyst layer.Additionally,the optimal cooling patterns need to ensure the cooling water co-current flowing,and it has obvious advantages of system integration and cost reduction.4.The influences of different operating conditions on the cell performance are explored,and the cell performance is optimized through experiments.Results show that increasing operating pressure is beneficial to the oxygen diffusion,and alleviate problems of the outlet ’lack gas’ and ’flooding’.As cathode inlet humidity increases,liquid water first accumulates at the cathode outlet,and offset to the middle of the cell;as anode inlet humidity increases,liquid water first accumulates at the cathode inlet and shifts to the cathode outlet.The optimal operating parameters of PEMFC in this study are as follows:the operating temperature is 80℃;the operating pressure(gage pressure)of cathode and anode is 70 kPa and 80 kPa respectively;the inlet humidity of cathode and anode is 80%and 100%respectively.