Research On Closed-loop Water Management Technology Of Proton Exchange Membrane Fuel Cell Based On AC Impedance

Proton exchange membrane fuel cells have broad application prospects in the fields of energy and transportation due to their high efficiency and pollution-free.The performance of fuel cell stacks is deeply affected by the internal water content.Membrane drying and flooding faults caused by poor operating conditions can lead to a decline in the stacks’ performance and stability,as well as a sharp decline in their life.This difficulty is considered to be a substantial hindrance to the large-scale application and promotion of fuel cell stacks.Fuel cell water management is a technology for identifying and regulating the stack water content level during operation,which is of great significance to the improvement of the stack’s stability and durability.In this thesis,research on the online closed-loop water management technology of fuel cells based on AC impedance is carried out.The main contents are as follows:Firstly,an online AC impedance measurement module for fuel cells is established.According to the working principle of fuel cells and the process of water transfer within the stack,the requirements of a water management system are analyzed.To meet its application requirements,an online AC impedance measurement module is designed.The experimental results show that the error rates of impedance amplitude and phase measurements are within ±5 %,which proves the high measurement accuracy of this module.Secondly,an online identification method of the stack water content based on a single frequency point impedance phase angle is explored.AC impedance spectra show that the impedance phase angle in the middle frequency range has a strong ability to identify the stack’s membrane drying and flooding faults.The positive correlation between impedance phase angle and stack humidity is proved by the equivalent circuit,which proves the effectiveness and universality of impedance phase angle as a water content identification index.A fuel cell water content identification method based on single-frequency impedance phase angle is proposed innovatively.Experiments verify that this method can predict drying and flooding faults in advance.The identification time is less than 0.5 s,which ensures the real-time performance of the water management system.Finally,the closed-loop water content control strategy and fault recovery strategy of fuel cell are studied.The airflow rate and stack temperature are selected as the operating conditions to regulate the stack water content.The single frequency point impedance phase is used to analyze and evaluate the regulating characteristics of operating conditions on water content level.On this basis,the closed-loop water content control strategy and fault recovery strategy with different priorities are proposed.Experimental results show that the designed water management system can control the stack water content within the normal range under constant load and variable load,and can predict the flooding and drying states of the stack before the average cell voltage is abnormal.The fault recovery time is less than 30 s,which is of great significance to the improvement of fuel cell life.