Analysis of PEM Fuel Cell Stacks and Systems with Water/Thermal Balances

University dissertation from Department of Energy Sciences

Abstract: The Proton Exchange Membrane (PEM) fuel cell has been investigated for a long time, and is regarded as a new energy source for various applications, such as portable, stationary and automotive ones. However, due to the shortage of the infrastructure in the society, the high cost, the difficulty of the water/thermal management, etc., practical systems have not reached the general public, yet. Of these matters, the water/thermal management is a critical problem to obtain a high efficiency and durability of the systems. This is because the proton conductivity in the electrolyte made of membrane is strongly dependent on its humidity. Therefore, the systems are designed and operated to maintain high humidity inside the stack. However, high humidity may induce water condensation as well. The liquid water in the stack obstructs the reactant to reach the active sites, and reduces the performance of the stack. The water condensation leads to the increase of the heat loads in the radiator as well. Therefore, the analyses of the water/thermal balances, related to the operating and design parameters, in the components and at the system level are important. In these analyses, the study of the stack design to avoid large pressure drop and maldistribution is recently activated. However, the water/thermal balance at the system level has not deeply involved it yet. This thesis involves a study of the water/thermal balances at the component level and the system level and the interaction between them. One major concern of the current work is the analysis of the water/thermal balances in 100 kW class PEM fuel cell systems. The system consists of PEM fuel cell stack, compressor, intercooler, heater, humidifier, condenser and radiator. As a first step, the heat and mass balances in the intercooler and the design of the heat exchangers for it were conducted. As a second step, the heat and mass balances in the whole system were analyzed. The analysis was focused on the heat load in the heat exchangers related to the water recovery schemes. The study showed that the water recovery scheme without phase change would reduce the heat load in the heat exchangers and be able to simplify the system configuration. Another major concern is the analysis of the stack design concerning the pressure drop and the maldistribution. The considered design parameters were the size of manifold, the number and the turn of the flow channels on the bipolar plate. According to the study, it is found that the maldistribution can be eliminated by increasing the size of the manifold or reducing the number of flow channels in the bipolar plate. The water removing effect is also improved. However, in such cases, the pressure drop is large. The final part of the work analyses the influence of the stack design on the water/thermal balance in the whole system. The stack design influences the pressure drop and maldistribution. By involving the pressure drop in the stack, the heat load in the radiator is reduced about 5%. This reduced heat is mostly shifted to the condenser for the water recovery. It corresponds to the 10 % increase in the heat load in the condenser. The design of the radiator and condenser must consider such shift of the heat load due to the pressure drop in the stack. A future study is highlighted which will contribute to the introduction of PEM fuel cell system to the general public.

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