Experimental and simulative investigation of water transfer in membrane humidifiers operating with liquid and vaporous water

Abstract

Polymer electrolyte membrane (PEM) fuel cells require exact water management to achieve high performance and long service lifetimes. The water management is controlled by the active humidification of the incoming cathode gas. This is commonly achieved by a membrane humidifier as part of the balance of plant. Membrane humidifiers are passive components that transfer water from the cathode exhaust stream to the cathode inlet stream. The operating modes of a humidifier in a mobile fuel cell system vary between gas-to-gas or gas with a small amount of liquid water-to-gas transport. The change of operating modes depends primarily on the operating point of the fuel cell. The gas-to-gas transport mode is widely investigated, but experimental and simulative studies of gas with a small amount of liquid water-to-gas transfer in membrane humidifiers are still insufficient. So far, only our own experimental data from gas with a small amount of liquid water-to-gas transport measurement points have been published. This paper presents a new experimental data set that consists of 39 gas-to-gas and 86 gas with a small amount of liquid water-to-gas transport measurement points. The following boundary conditions for the experimental data are varied: Temperature, pressure, gaseous water, liquid water, and nitrogen mass flow rate. The data presented confirm previous work: That liquid water significantly enhances mass transport for all boundary conditions. Additionally, this paper presents a new 1D simulation approach using Modelica as the modeling language. An existing gas-to-gas humidifier model was extended to a 2-phase flow model by adjusting 2 fit factors. The experimental data presented here enable the validation. The model matches the overall experimental data set with a high accuracy of R² = 0.90. This shows a generic approach for a simple extension from a gas-to-gas humidifier model to a 2-phase flow model. Using this modeling approach will result in more accurate humidifier performance prediction and therefore more accurate water management control for the entire fuel cell system. Moreover, it shows the potential of actively using liquid water to enhance the humidifier’s performance.