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Membrane moisture transfer in fuel cell humidifiers Cave, Peter William

Abstract

High system cost and large size are two barriers preventing mainstream commercial viability of proton exchange membrane fuel cells. These barriers can be addressed by (a) improving water management in the cell for a higher power density at lower cost and (b) reducing the cost and size of balance of plant components. The cathode reactant humidification system directly affects the water management of the cell and is one of the most expensive balance of plant components in the system. A thermodynamic model of a cathode gas to gas membrane humidifier was implemented. The model considers two-phase heat and mass transfer along two parallel, one-dimensional channels in counter flow operation. A single channel humidifier was constructed with the capability of measuring temperature and dew point temperature profiles along the channel. Dew point profiles and the effect of flow rate on the outlet dew point were measured at 30, 50, and 70°C isothermal cases with fully saturated wet-side inlet conditions. Water flux across the membrane was constant along the channel and the outlet dew point decreased with dryside flow rate for all cases. The effect of wet-side flow rate was minor. The experimental data were compared to the thermodynamic model using membrane diffusivity from four published correlations and three different techniques. The different diffusivity correlations affected model predictions of moisture flux by up to 86%, demonstrating that fitted membrane parameters must be used if modeling accuracy is expected. Calculating diffusivity at the average membrane water content of the two sides of the membrane was shown to be a better approximation than using an average relative humidity. Also, the model over-predicted the outlet dew point more at lower temperatures than at higher temperatures compared to the experimental data. Finally, a 5kW humidifier design was simulated at a pressure of 1 atm with 65°C fully-saturated and 25°C completely-dry conditions at the wet-side and dry-side inlets, respectively. Analysis revealed that modeling moisture transfer using humidity ratio as a driving force can lead to unphysical results under non-isothermal conditions with high moisture flux. Pressure drop estimations using an all-vapour, Darcy friction factor analysis were very low compared to measured data and a fitted flow restriction equation was suggested. Keywords: humidifier, membrane, fuel cell, water management, mass transfer, single phase

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