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

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

Evaporative cooling (EC) is a promising concept to increase the power density and reduce the complexity of polymer electrolyte fuel cells (PEFCs) by using gas diffusion layers (GDLs) modified with hydrophilic lines (HPIL). While this concept has been demonstrated in recent experiments by use of a thermal test cell, a detailed understanding of the simultaneous cooling and humidification is missing. To close this gap, we have developed a non-isothermal, two-phase continuum model of an EC cell. This 3-D model consists of a membrane electrode assembly (MEA) with an anode GDL with of one HPIL. The MEA is sandwiched by flow channel plates that consist of one gas flow channel (GFC) and liquid water channel on the anode side and two GFCs on the cathode side. In the base case model, water evaporates mostly within a thin film at the anode GFC/HPIL interface and – to a smaller degree – at the interfaces of the HPIL with the hydrophobic anode GDL. The largest part of the generated water vapor leaves the cell through the anode GFC and only about a tenth reaches the cathode side. The membrane humidification varies on the anode side being the highest below the HPIL and ribs and the lowest below the anode GFC, and decreases towards the cathode side. The temperature drop due to latent heat of evaporation is the largest along the interface between the HPIL and anode GFC. As our model essentially simulates the first contact surface between the HPIL and anode GDL in gas flow direction, it represents an extreme case in comparison to the experiments in that higher local evaporation rates and heat fluxes and lower membrane humidification levels are simulated. The role of evaporation kinetics on the results is analyzed by varying the evaporation rate constant over several orders of magnitude. While a plateau is reached for the water vapor flux via the anode GDL and towards the cathode side at a sufficiently large evaporation rate constant, a much larger rate constant would be necessary to reach a transport limited regime in the anode GFC. This is shown to be challenging to resolve numerically due to increasingly sharper gradients within a thinner film along the borders of the HPIL. Future studies could couple this continuum model with a pore-scale simulation in combination with ex-situ experiments to improve the simulation of the two-phase flow in the modified GDL, which includes the upscaling of the Hertz-Knudsen-Schrage equation to the continuum level

3-D simulation of heat and water transport in PEFCs during evaporative cooling and humidification

Funded by the Swiss Competence Center for Energy Research (SCCER Mobility) and the Swiss Federal Office of Energy SFOE (contract number SI/501764-01).Evaporative cooling is a promising concept to increase the power density and reduce the complexity of polymer electrolyte fuel cell systems (PEFCs) by using gas diffusion layers (GDLs) modified with hydrophilic lines (HPL) [1]. While this concept has been demonstrated in experiments [2-3], a quantitative understanding of evaporative cooling and humidification is missing. Here we simulate the heat and water transport processes in part of a non-operating evaporative cooling cell using a 3-D macro-homogeneous model, which consists of a mem-brane electrode assembly (MEA) with one HPL in the anode GDL, sandwiched by flow chan-nel plates. In the base case simulation, water evaporates mostly in the HPL in contact with the gas flow and, to a smaller degree, with the hydrophobic part of the GDL. Almost all of the generated water vapour leaves the cell through the anode gas channel. The resulting membrane humidification (l) varies on the anode side being the highest below the hydrophilic line and ribs and the lowest below the gas channel, and decreases towards the cathode side. Evaporating rates are partly limited by the water evaporation transfer coefficient at values typically adopted for simulating evaporation in PEFC models. In contrast, at higher transfer coefficients, evaporation rates reach a plateau and, hence, become transport limited. The next step is to simulate the increasing humidification with more hydrophilic lines, and an oper-ating cell including electro-osmotic drag

Experimental parameter uncertainty in PEM fuel cell modeling

Predictability of proton exchange membrane fuel cell (PEMFC) models has suffered from significant uncertainty in material properties with experimental data on several transport coefficients being scattered over orders of magnitude. In this study, we determine the most critical transport parameters for which a more accurate experimental characterization is required to enable reliable performance prediction. First, we incorporate a comprehensive set of material parameterizations from the literature into a recently developed macro-homogeneous two-phase membrane-electrode assembly model. This computational model demonstrates the large spread in performance prediction resulting from the scattered experimental data. Membrane transport properties induce the largest spread in the fuel cell performance curve: the diffusivity of dissolved water, the protonic conductivity and the electro-osmotic drag coefficient. Second, we conduct extensive forward uncertainty propagation analyses. These include a global sensitivity analysis in which a broad range of operating conditions and material properties is covered. By introducing the concept of condition numbers to fuel cell modeling, we measure the propagation of uncertainty through the model explicitly. We list the parameters with the highest impact on predicted fuel cell properties. These are the membrane hydration isotherm, the electro-osmotic drag coefficient, the membrane thickness and the diffusivity of dissolved water. In conclusion, the most critical model parameters are the membrane transport properties, which suffer from the largest scatter in available experimental data. This calls for a better experimental characterization of the ionomer to enhance the predictability of PEMFC models. Particularly, a more precise parametrization is required for the interplay between the different water transport mechanisms and protonic conductivity. Acknowledgements: This work was supported by the Swiss National Science Foundation (project no. 153790, grant no. 407040_153790); the Swiss Commission for Technology and Innovation (contract no. KTI.2014.0115); the Swiss Federal Office of Energy; and through the Swiss Competence Center for Energy Research (SCCER Mobility)

Mass Transport Limitations of Water Evaporation in Polymer Electrolyte Fuel Cell Gas Diffusion Layers

Facilitating the proper handling of water is one of the main challenges to overcome when trying to improve fuel cell performance. Specifically, enhanced removal of liquid water from the porous gas diffusion layers (GDLs) holds a lot of potential, but has proven to be non-trivial. A main contributor to this removal process is the gaseous transport of water following evaporation inside the GDL or catalyst layer domain. Vapor transport is desired over liquid removal, as the liquid water takes up pore space otherwise available for reactant gas supply to the catalytically active sites and opens up the possibility to remove the waste heat of the cell by evaporative cooling concepts. To better understand evaporative water removal from fuel cells and facilitate the evaporative cooling concept developed at the Paul Scherrer Institute, the effect of gas speed (0.5–10 m/s), temperature (30–60 °C), and evaporation domain (0.8–10 mm) on the evaporation rate of water from a GDL (TGP-H-120, 10 wt% PTFE) has been investigated using an ex situ approach, combined with X-ray tomographic microscopy. An along-the-channel model showed good agreement with the measured values and was used to extrapolate the differential approach to larger domains and to investigate parameter variations that were not covered experimentally.ISSN:1996-107

Evidence-based policymaking

Volume 1 introduces you to the concepts of evidence-based policymaking. We discuss the role of science in the policymaking context. You will learn about methods and tools to improve the science-policy dialogue