Study On The Effects Of Operating Parameters On The Dynamic Response And Energy Efficiency Of PEMFC

The Proton Exchange Membrane Fuel Cell(PEMFC)is energy conversion devices by converting hydrogen energy directly into electric energy,which is used to power all or part of electric vehicle propulsion.The vehicle PEMFC is operated frequently under variable load conditions,which can short the lifetime of PEMFC.Hence,it is important to analyze the effects of relative humidity and operating temperature on the dynamic response and energy efficiency of PEMFC to extend the PEMFC lifetime.The dynamic heat transfer(DHT)model and the energy efficiency(GE)model were proposed.The DHT model was based on the mass conservation of water and gas components and the water transfer model along the hydrogen ion conduction direction was established,which was clarified the quantitative relationship between the water content and the operating temperature and relative humidity,and considered the effects of the current density on the water flow.The GE model was based on the original energy efficiency equation using the coupling between relative humidity and reactant flow rate to derive the specific formula for fuel utilization,thus improving the GE calculation model.The combined effect of relative humidity,operating temperature and load amplitude on the steady-state and dynamic response characteristics was analyzed.Secondly,the experimental test system was built and its working principle was described.The experiments were conducted on single cells under different operating conditions,which were included at different operating temperatures(50 ~oC,60 ~oC and70 ~oC)with symmetric and asymmetric humidification(50%and 100%relative humidity).The reliability of the model was verified by three performance evaluation methods,namely polarization curves,power density curves and dynamic response curves.Finally,the simulation and experimental results were analyzed in detail,and results can be obtained:(1)The effects of operating temperature on the steady-state and dynamic responses are similar,and the distribution of the PEM average water content is more uniform as the operating temperature increases,while the distribution trends of the gas components of the anode and cathode catalytic layers are opposite to the distribution trends of the average water content.The errors between the DHT model and the experimental polarization curves are 2.18%and 3.58%when the symmetric humidification is 100%and the operating temperatures are 60 ~oC and 70 ~oC,respectively.(2)There is a difference between the effects of relative humidity on the steady-state and dynamic responses.Increasing the relative humidity in the steady-state condition is more helpful to improve the water balance processes in the hydrogen ion conduction direction.While increasing the anode relative humidity has little effect on the current density distribution.When the relative humidity and the loading amplitude are coupled in the dynamic condition,increasing the anode side relative humidity,the difference between the maximum and minimum of the average water content across the membrane increases,the average water content shows a slight downward trend.When the anode relative humidity is 50%and 100%,the differences before loading and after loading are 2.9 and 3.1,3 and 4.2 respectively.And current density distribution is more inhomogeneous So it can be learned that increasing the anode side relative humidity during loading does not play a positive role in improving the water balance processes in the hydrogen ion conduction direction.It can be used as a basis for the design and control of water management in the loading process.(3)Based on the results of steady-state and transient studies,the effects of different anode relative humidity on the variation of hydrogen and water vapor concentrations in the flow channel at an operating temperature of 60 ~oC and the trend of energy efficiency were analyzed and explained.The energy efficiency increases with the increase of relative humidity.So the optimal performance is observed as a result of the relative humidity is 100%and operating temperature of 60 ~oC,when the power density increases to 110.9 m W/cm~2and the maximum energy efficiency around46.88%can be obtained.