Simulation of mass and heat transfer in an evaporatively cooled PEM fuel cell

Evaporative cooling is a promising concept to improve proton exchange membrane fuel cells. While the particular concept based on gas diffusion layers (GDLs) modified with hydrophilic lines (HPILs) has recently been demonstrated, there is a lack in the understanding of the mass and heat transport processes. We have developed a 3-D, non-isothermal, macro-homogeneous numerical model focusing on one interface between a HPIL and an anode gas flow channel (AGFC). In the base case model, water evaporates within a thin film adjacent to the interfaces of the HPIL with the AGFC and with the hydrophobic anode GDL. The largest part of the generated water vapor leaves the cell via the AGFC. The transport to the cathode side is shown to be partly limited by the ab-/desorption into/from the membrane. The cooling due to the latent heat has a strong effect on the local evaporation rate. An increase of the mass transfer coefficient for evaporation leads to a transport limited regime inside the MEA while the transport via the AGFC is limited by evaporation kinetics

3-D simulation of water and heat transport processes in fuel cells during evaporative cooling and humidification

Evaporative cooling is a promising concept improve the efficiency and reduced costs of polymer electrolyte fuel cells (PEFCs) using modified gas diffusion layers with hydrophilic and hydrophobic lines. This concept has been demonstrated to simultaneously achieve cooling and membrane humidification in experiments. We have developed a 3-D numerical model of such an evaporative cooling cell to address remain questions from the experiments

Modeling of the slip spectrum along mature and spontaneously forming faults in a visco-elasto-plastic continuum

Earthquakes, which develop along tectonic faults, represent a serious hazard in increasingly populated areas. A more effective mitigation of related risks requires a better physical understanding of earthquakes and faulting. This is a scientific challenge as these natural phenomena operate on a wide range of time and length scales, which vary from milliseconds to million years and from several grains to tectonic plate boundaries. Since this range is much wider than the availability of observations, numerical modeling is an important scientific tool to cross the different scales. The main objective of this thesis is to advance an existing modeling approach to provide insights into the relationships between earthquakes and large-scale and long-term tectonic processes. This continuum-based, seismo-thermo-mechanical (STM) modeling approach couples the evolution of velocity, pressure and temperature to elastic, viscous and plastic deformation. We apply this modeling approach to investigate the relationship between tectonic parameters (seismogenic zone width and subduction velocity) and seismicity pattern in subduction zones. We provide a new explanation for so called supercycles, which is based on the stress transfer through several large earthquakes along wide seismogenic zones. Furthermore, we demonstrate the importance of a long enough observational record to detect cause-effect relationships. The main part of the thesis is the advancement of the STM modeling approach to overcome its major limitation in resolving earthquakes in time and space. We propose and implement a new invariant reformulation of the laboratory-based rate-and state-dependent friction (RSF). This new STM-RSF formulation is combined with a modified adaptive time stepping scheme, which extends the range of resolved time-scales down to milliseconds. In a model setup of a mature strike-slip fault zone, we demonstrate the capability to simulate -- in an accurate and stable manner -- the entire earthquake cycle, including slow tectonic loading, nucleation and dynamic rupture propagation of earthquakes, and postseismic deformation. In addition, we identify a viscosity threshold, below which earthquakes cannot nucleate. We explain first-order physical controls for this threshold. The key advantage of the improved STM-RSF modeling approach lies in the coupled simulation of earthquakes and fault evolution. Deformation spontaneously localizes into fault zones. Both the degree of complexity of the evolving fault system and the short-term slip mode depend on the characteristic slip distance and dynamic pressure, which implies links between the processes on largely different time scales. Dynamic pressure also varies between earthquakes, which leads to a dynamic nucleation size and, hence, variable slip modes on the same fault. In addition, we discuss the grid-size dependency of the localization process. Finally, we present the first steps towards applying the developed STM-RSF methodology to large-scale tectonic problems. These steps include an adaptive treatment of the free surface approximation, implementation of cohesion and a transition from long-term to short-term modeling in a subduction zone setting

Advanced characterization of polymer electrolyte fuel cells using a two-phase time-dependent model

For polymer electrolyte fuel cells (PEFC) to become competitive, their operation in transport applications requires optimized performance, durability and costs. An indispensable component of this optimization is a detailed time-dependent characterization of PEFCs. For this purpose, several dynamic single-cell PEFC numerical models have been developed in the last 15 years. Here we present a new modeling approach with an advanced material parameterization of all governing time-dependent processes. This new modeling approach is based on the 1-D steady-state two-phase, five-layer PEFC model with a comprehensive material parametrization [1]. We further developed this approach by including the time-dependency of electron, proton, heat, dissolved water, gas and liquid water transport. Implementation in COMSOL allows for accurate spatio-temporal resolution and flexible model setups. We develop an improved electrochemical spectroscopy method by applying a sinusoidal perturbation of varying amplitude to the cell voltage. In contrast to (small-signal) electrical impedance spectroscopy, our model numerically solves for the nonlinear time-dependent response of the fuel cell, which enables us to retrieve information from large signals. We compute the response spectra at different operating points not only for electrical current density but also for dissolved water, liquid water, temperature and gas concentration and their gradients. These nonlinear response spectra serve for the development of improved model-based characterization techniques and fuel cell diagnostics. Acknowledgements: Financial support from the Swiss Federal Office of Energy (SFOE contract number: SI/501764-01) is gratefully acknowledged