Modeling and Analysis of a PEM Fuel Cell System for a Quadruped Robot
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The polymer electrolyte membrane fuel cell is a prospective alternative energy conversion device that has environment-friendly characteristics. It has not only electrochemical reaction but also coupled thermal fluid characteristics. Chemical reaction of the reactants, mass transport of products, heat generation and heat transfer from catalyst layer to the end plate occur during operation of the fuel cell. In particular, water management and heat management play an important role in safe and successful operation of the Polymer Electrolyte Membrane (PEM) fuel cell system. Therefore, the understanding of the dynamic characteristics of the PEM fuel cell stack is crucial to ensure accurate analysis and effective design of the PEM fuel cell system. Generally, computational methodology is an essential tool for investigating interrelated physical parameters and dynamic behavior. The dynamic model for the PEM fuel cell system was developed considering temperature and two-phase effects in the Advanced Propulsion Research Laboratory at Auburn University. The model developed and compared with experimental results. Moreover, the two cell stack model is used to analyze static and dynamic behaviors under different operating conditions that include operating temperature, relative humidity of air, stoichiometric ratio, partial pressure of reactants in addition to various geometric parameters that include thickness and porosity of the gas diffusion layer (GDL) and the thickness of the membrane under the ramp and step current loads. The analysis of the 20-cell stack is also performed to investigate the temperature effect between cells within the stack before a multi cell stack is constructed. The analysis consists of static and dynamic behavior for understanding temperature distribution and voltage difference due to the temperature effect. Finally, the 100-cell stack is constructed and its characteristics are analyzed. However, stack model alone has a limitation for estimating real net power from the fuel cell system without consideration of auxiliary components such as an air processing system and a thermal circuit. Therefore, each component is modeled for the 100-cell PEM fuel cell system. Various comparative studies showed that proper operating conditions are critical for improved performance and two phase effects which both affect concentration of reactants, thus should be considered for the design and optimization of the stack. Moreover, appropriate design and sizing of the air supply system and thermal circuit components could reduce parasitic power and increased efficiency of the fuel cell system. In the multi cell stack model, the temperature distribution affected not only concentration of the reactant but also proton conductivity of the membrane. Consequently, it caused the change in concentration overpotential, ohmic overpotential and cell voltage.