Water management in PEM fuel cells; A numerical CFD study using Volume-Of-Fluid (VOF) Method

Abstract

A fuel cell functions on the electrochemical principle of simultaneous reduction and oxidation (redox) reactions producing an electric current for powering devices. The fuel cell type under focus in this thesis work is of the Proton Exchange Membrane Fuel Cell or Polymer Electrolyte Membrane Fuel Cell (PEMFC) variety. In this type, hydrogen (H2) is used as fuel with an air/oxygen (O2) supply to produce electricity and water as products, making it a clean & sustainable source of energy with a plethora of applications.

This thesis work intends to understand the behaviour of the water product as it traverses across the flow channels designed to transport it away and out of the fuel cell by using the same oxygen supply (air) used as fuel. The level of water present in the electrolyte membrane and the bipolar plate flow channels in the fuel cell is extremely important to the performance of the fuel cell. Therefore, this management of the optimum hydration levels is fundamental to the optimal power performance. This region of focus in the study of PEMFC water management is an exciting avenue for research and possible optimization of hydration levels in the PEM fuel cell.

This thesis work utilizes existing literature to provide a valid simulation model as groundwork to elaborate on the same, due to the unpredictable nature of computer simulations. The simulation results from this thesis work is corroborated with the theoretical physical laws to validate the accuracy of the results.

Since the water product formed has to be taken away by air at a suitable to prevent both flooding and drying out of the fuel cell, the dynamics of the flow of the air and water phases in the flow channels assigned are dominated by complex physics. Since, the observation techniques are limiting in understanding the problem, the complexities and behaviour patterns must be studied with numerical simulations which model a flow which incorporates all the relevant factors in the flow of the constituent phases, their interactions, the physics of their interfaces and their movement behaviour in a region with transient flows at changing pressures and temperatures. These challenges can be effectively solved with the help of a CFD simulation. Therefore this thesis work utilizes the Volume Of Fluid (VOF) Method in simulating this two-phase problem for studying the phase interface and the scales of the problem at hand are within the range of this algorithm (both spatially and temporally), additionally investigating the effect of contact angles.

The results from the simulation show that the rate of the water transport across the channels is highly dependent on the inlet velocity of the air phase, the geometry and arrangement of the water inlet - pore size and number, and the relative humidity of the inlet air.