Dynamic Modeling of Two-Phase Heat and Vapor Transfer Characteristics in a Gas-to-Gas Membrane Humidifier for Use in Automotive PEM Fuel Cells
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Due to the recent negative outlook on the availability of cheap, long term energy supplies from non-renewable resources, several technologies have been proposed and revisited as possible solutions to the current energy crisis. Along with alternative fuels, such as ethanol and biofuels, and clean energy resources, such as wind and solar power, Hydrogen proton exchange membrane (PEM) fuel cells have been seen as one of these options capable of replacing fossil fuels for power, especially in the automotive industry. However, because the technology is relatively new in comparison to combustion engines, there are many unique problems that must be overcome in order for it to be considered a viable substitute. One of the most concerning issues with the PEM fuel cell is its membrane water production and management. When the chemical reaction takes place in the fuel cell, liquid water is produced, which must be removed in order for the fuel cell to continue functioning and maintain a high level of performance. However, there are typically two situations that arise due to poor water management: flooding and dehydration. Flooding occurs under a heavy load (high power output) when the fuel cell generates an excess of water which blocks air channels within the gas diffusion media, causing poor performance. Conversely, dehydration occurs as a result of severe drying of the membrane due to imbalance between water take-up and back diffusion. This is also iii bad for fuel cell performance since the membrane needs to be humidified in order to maintain proton transfer. Thus, the humidifier has come to be seen by researchers as a regulation device, capable of supplying water to the membrane under many different electrical loads and input conditions. In this study, the background and current state-of-the-art of the membrane humidifier are discussed with special attention paid to heat and vapor transfer phenomena. In addition, a model is proposed that describes the operation of the humidifier and predicts its response to various inputs such as humidity, flow rate, and temperature. Lastly, several static and dynamic experiments are undertaken to simulate the real-world operations of the humidifier and were used for comparison with the results of the model.