PEM fuel cell multi-phase system
This thesis presents an experimental investigation into the feasibility of using a functionally thermal fluid to enhance the performance of a Proton Exchange Membrane (PEM) Fuel Cell. Specifically, a fluid was developed that utilised a liquid-solid phase change to enhance heat transport within the fuel cell. Increasing the convective heat transfer coefficient could permit the use of smaller volumetric flow rates and reduce pumping power. The objective of the thermal fluid was to create isothermal conditions across a fuel cell and to reduce parasitic loadings from pumps and other components to enhance the overall system performance. Additionally, the fluid could reduce the system size and component cost, and stabilise temperature fluctuations within the system. The thermal fluid that was developed constituted a mix of fine, Microencapsulated Phase Change Material (MicroPCM) particles suspended in a single-phase working fluid. For successful integration with the fuel cell, the microPCMs thermal and fluid properties, and their effectiveness in transferring heat, had to be fully characterised and understood. Research consisted of experimental investigations of the fuel cell, followed by microPCM development. Experimentation on the fuel cell stack revealed a requirement for thermal stability and reduction in parasitic load from the pumps. Quantitative characterisation and development of the microPCM properties involved state of the art equipment to measure the latent heat of fusion, melting and freezing points, surface morphology and viscosity of the microPCM slurry. The effects of repeated use of solid to liquid phase change particles upon melting and solidification were studied. This lead to the further development of microPCM particles and experimentally examined in a fuel cell system. The use of MicroPCM developed in this study balanced the improvement in thermal capacity of the fluid with the increase in pumping load, when compared to the use of water alone. The study suggested that with further development of the microPCM slurry, it has the potential to significantly increase the thermal capacity of the fluid and stabilise temperatures across the fuel cell, which in turn would results in improved stack performance and electrical conversion efficiency.