Simulation and Validation of 3D Complex PEM Flow Fields

Proton Exchange Membrane fuel cells oxidize hydrogen and reduce oxygen, generating only water and an electric current. This makes them a viable green power source for many applications. Gases are pumped into a cell’s bipolar plates, and then travel through the gas diffusion layers before reacting at the catalyst/membrane boundary. Water is generated at the cathode reaction, and at high current density operation, water exists as a liquid which must be carried away by air flow, or else precious reaction sites will be blocked from the oxidant. This effect is called mass transport loss, and engineers minimize it by designing efficient gas flow channels built into the bipolar plates. In 2015, Toyota found a way to drastically reduce this effect using a complex flow-field designed in 3 dimensions, while most existing PEM fuel cells use 2D channels with a constant cross section. The design decreases the contact area of the bipolar plate, which allows for very efficient water removal. The design also has ‘secondary’ flow channels that are raised above the cell’s membrane, so liquid water can exist without hindering the reduction reaction. Computational Fluid Dynamics (CFD) software can be used to simulate the multi-species flow and diffusion across the porous media, and the multiphase water activity in a PEM fuel cell using finite-element analysis. Two different cases of fuel cells were simulated using the ANSYS Fluent PEM module with the same inlet and operating conditions. One case used a 3D complex flow field while the other used conventional straight, rectangular flow channels. After comparing the results, it appears that 3D flow fields with secondary channels allow for slightly enhanced performance at high current densities despite almost no difference in liquid saturation. More pressure was required to drive 3D flow, suggesting the performance increase may be due to forced convection. Boundary conditions like inlet humidity and velocity, which greatly effect water removal, may not be optimized for 3D flow, despite using standard stoichiometry. Real world studies need to be conducted on 3D flow fields in order to understand the mechanisms that remove water.