Research Of The Electrical And Thermal Dynamic Characteristics Of Proton Exchange Membrane Fuel Cell Stack

Proton exchange membrane fuel cell（PEMFC）is a promising alternative energy for portable devices especially automobiles due to its advantages of structural simplicity,low working temperature,high conversion efficiency and eco-friendly product.There’re frequent load changes during the operation of PMEFC.It leads to the fluctuations of output voltage,output electrical power and temperature.The lifetime and reliability of PEMFC will be rapid declined.Therefore,the investigation of electrical and thermal dynamic characteristics of PEMFC stack and the exploration of novel heat management are focused at present.This doctoral dissertation studies the electrical and thermal dynamic characteristics of PEMFC stack by dynamic theoretical model,load-varying experiements and numerical method,and proposes a cooling solution for PEMFC that combines phase change heat transfer of ultra-thin vapor chamber（UTVC）and forced-convection of ambient air.PEMFC stacks embedded with UTVCs are designed,fabricated and assembled,and the strengths and weaknesses of PEMFC-UTVC stacks are investigated by both theorectical and experiemental methods.Followings are the main work of this dissertation:Combining thermodynamic equivalent circuit theory,the electrical and thermal dynamic models of conventional liquid-cooled PEMFC stack and PEMFC-UTVC stack are built.The temperature response differential equation of conventional liquid-cooled PEMFC stack is a first-order differential equation about time.The corresponding transition time duration of stack temperature is a function of voltage response time constant and thermodynamic time constant of stack.By comparison,the temperature response differential equation of PEMFC-UTVC stack is a second-order differential equation about time.The corresponding transition time duration of stack temperature is a function of voltage response time constant,thermodynamic time constant of stack and thermodynamic time constant of UTVC.It’s revealed that the temperature variation of PEMFC-UTVC stack is more gentle than that of conventional liquid-cooled PEMFC stack.A serpentine symmetric cathode channel（SSCC）liquid-cooled PEMFC stack embedded with 80 temperature measuring points is designed and assembled.The evolutions output voltage,output electrical power,temperature,temperature uniformity after startup with different loading modes are observed and compared.It’s found that the undershoot of stack’s output voltage will be more prominent if the circuit current increases faster.When the SSCC liquid-cooled PEMFC stack starts up with one-step loading mode,there’s biggest overshoot of 11.19 V,and the recover duration is 57 s.The consistency of voltage and temperature distribution of SSCC liquid-cooled PEMFC stack is analyzed.It shows that the output voltages of cell units have significant differences in the response stage eventhough the steady voltages are almost the same.Further,the coolant flow field is the most important factor that determines the temperature distribution inside liquid-cooled PEMFC stack.The diffusion delay of H₂ and O₂ in gas diffusion layer is one of the important reasons that cause the output voltage undershoot of PEMFC stack.The influence factors of diffusion delay are numerically analyzed based on the dynamic gas diffusion equation.The results indicate that the liquid water saturation of gas diffusion layer is the most crucial factor.And the diffusion transition duration of O₂ is one order higher than that of H₂.Therefore,the diffusion delay of oxygen in cathode gas diffusion layer is the important influence factor that impact the dynamic performance of PEMFC stack.An electric-thermal coupling model of coolant and liquid-cooled PEMFC stack is built.The accuracy of coupling model is verified by experimental results SSCC liquid-cooled PEMFC stack.The influences of electrical potential response time constant and thermodynamic time constant of stack are researched using this coupling model.If the difference between the aforementioned time constants is too large,the PEMFC stack will suffering a longer transion stage.The electrical potential response time constant is the main factor that affects the stack output voltage transition duration while The thermodynamic time constant is the main factor that affects the stack temperature transition duration.The dynamic response characteristics of PEMFC-UTVC stack,conventional air-cooled PEMFC stack and PEMFC-metal stack are compared through experimental approach.It’s proved that PEMFC-UTVC stack is able to normally provide electrical power to external load and work over a long term.The results show that the temperature variation of PEMFC-UTVC stack is more gentle than that of conventional air-cooled PEMFC stack,and the temperature uniformity is better than PEMFC-metal stack.The impacts of loading strategy,cooling aintensity of condenser section of UTVC and inclination of stack are also experimental analyzed:The undershoot of PEMFC-UTVC stack’s output voltage will be more prominent if the circuit current increases faster;the stronger the cooling aintensity of UTVC’s condenser section,the better the dynamic performance of PEMFC-UTVC stack;the temperature uniformity of stack will be improved if the angle between gravity and medium back flow smaller than 90 degree.The effects of structural parameters of UTVC on the effective electrical resistance,effective thermal resistance,thermodynamic time constant of stack and dynamic heat dissipation portion through UTVC are investigated:the electrical resistance of the stack tested in the experiments will rise by 9.5 10Ωassembled with UTVCs,and the variation of stack effective electrical resistance is 10～10Ωwhile the structural parameters change from-50%to 50%which can be neglected;the crucial factors on effective thermal resistance,thermodynamic time constant of stack and dynamic heat dissipation portion through UTVC are porosity of wick,thickness of shell and thickness of wick layer,respectively.Based on the electrical and thermal dynamic model,the electrical efficiency and exergy efficiency of PEMFC-UTVC stack and conventional air-cooled PEMFC stack are compared.The electrical efficiency and exergy efficiency of PEMFC-UTVC stack can be higher at high working temperature,and they’re relative to the power ratio of air supply fan to cooling fan:larger the power ratio,higher the efficiency.The limiting factor of exergy efficiency of PEMFC stack is the power consumption of fans.The temperature of PEMFC-UTVC stack can be varied by regulating the power ratio of air supply fan to cooling fan with the electrical efficiency and exergy efficiency unchanged.