Research On Water Management And Hydrogen Recirculation System For Polymer Electrolyte Membrane Fuel Cell

In polymer electrolyte membrane fuel cell(PEMFC),the proten needs water to transfer from the anode to the cathode during the reaction.When the water content in the membrane is low,the ohmic loss voltage will increase,resulting in performance degradation.On the contrary,liquid water may occupy the pores in the catalytic layer,micro-porous layer or gas diffusion layer,and block the transport of reaction gases,reducing the fuel cell performance.Hence,to improve the performance of PEMFC,it is meaningful to study water management of PEMFC.Hydrogen recirculated system plays an important role in water management while improving hydrogen utilization in PEMFC system.The effects of hydrogen recirculation on the water management of PEMFC was investigated in this study.Firstly,based on the consistency of the current density,the influences of flow pattern,inlet gas relative humidity and hydrogen recirculation on the water and gas transport in the fuel cell were analyzed.And then,to meet the requirements of the wide working range and the large recirculation demand of the PEMFC system,the ejector was designed and optimized.Finally,a 10 k W PEMFC system test bench was built.The ejector was designed to investigate the water management and control strategy of the hydrogen circulation system in the PEMFC system.The main research contents and results are as follows:(1)A dynamic two-phase 1-D model of PEMFC is established to study the gasliquid transport process.During the operation,the transport of heat,water vapor and liquid water on both anode and cathode sides were investigated.It is found that under the low current density condition,the fuel cell has a high demand for relative humidity of inlet gases.However,under the high current density condition,the water production of PEMFC increases and the demand for relative humidity of inlet gases decreases.Since the flow rate of anode gas is lower than that of cathode gas,the liquid water on the anode side cannot be purged in time,which is more prone to water flooding.(2)The effect of water distribution on the consistency of current density in a single cell was investigated by single cell test and model.Firstly,the effects of flow pattern,air inlet humidity and hydrogen recirculation on the cell’s performance were tested on a single cell test bench.Then,a segment single cell model was built based on the onedimensional model.The standard deviation and maximum difference of current density were used to evaluate the consistency of fuel cell current density.It is found that it has a higher requirement for water management under the self-humidification condition.The combination of counter-flow and hydrogen recirculation can significantly improve the water distribution.(3)To meet the requirements of the wide working range and the large recirculation demand of PEMFC system,the structure design method was analyzed based on the design principle of the ejector,and then an ejector for an 80 k W fuel cell stack was designed and manufactured.It is found that the primary pressure and primary gas flow show a linear relationship.In the aspect of the ejector structure optimization,increasing the primary pressure,reducing the mixing chamber diameter and the angle of the diffusion chamber are helpful to improve the recirculated performance under the low power conditions.(4)A 10 kW PEMFC system test bench was built,and an ejector for this system was designed and manufactured.In ejector mode,the gas circulates in the anode pipe,the liquid water inside the anode can be purged in time,which can improve the water distribution and the stack’s performance.When the minimum voltage is lower than 0.6V,the maximum stack current raises from 130 A(dead-ended mode)to 240 A(ejector mode).The hydrogen consumption can be saved by 4.39% to 10% compared with the dead-ended mode.The pressure fluctuation generated by hydrogen purge and water purge will increase the recirculated flow rate,and the ejector has a good recirculation performance during the 10% to 100% stack rated power.(5)The influence of hydrogen circulation system on water management in cathode humidification condition and self-humidification condition was studied by the 10 k W PEMFC system test bench.In the cathode humidification system,since the ejector has no parasitic power consumption during the whole operation,the net power of the PEMFC system using the ejector is high,increasing the system’s net power by 1.09%to 25.46% during the whole working conditions.In the self-humidification system,the membrane is dry during the low current condition,which needs a large recirculation rate.Combined with the characteristics of the ejector and the pump,a scheme of the hydrogen circulation system with ejector and pump in parallel is proposed.Under the condition of current between 5 A and 80 A,adjusting the pump speed between 1000r/min to 4000 r/min can solve the concave phenomenon of polarization curve caused by membrane drying and reduce the single voltage fluctuation rate of 0.12% to 1.98%.Under current larger than 80 A condition,the net power of the PEMFC system increases from 0.06% to 1.39% by adopting the ejector.