Model-based analysis for the thermal management of open-cathode proton exchange membrane fuel cell systems concerning efficiency and stability

In this work we present a dynamic, control-oriented, concentrated parameter model of an open-cathode proton exchange membrane fuel cell system for the study of stability and efficiency improvement with respect to thermal management. The system model consists of two dynamic states which are the fuel cell temperature and the liquid water saturation in the cathode catalyst layer. The control action of the system is the inlet air velocity of the cathode air flow manifold, set by the cooling fan, and the system output is the stack voltage. From the model we derive the equilibrium points and eigenvalues within a set of operating conditions and subsequently discuss stability and the possibility of efficiency improvement. The model confirms the existence of a temperature-dependent maximum power in the moderate temperature region. The stability analysis shows that the maximum power line decomposes the phase plane in two parts, namely stable and unstable equilibrium points. The model is capable of predicting the temperature of a stable steady-state voltage maximum and the simulation results serve for the design of optimal thermal management strategies.Postprint (author's final draft

Temperature control of open-cathode PEM fuel cells

Proper temperature control of Proton Exchange Membrane (PEM) Fuel Cells is a crucial factor for optimizing fuel cell performance. A robust temperature controller is required for optimal water management of PEM fuel cells. This paper describes a model-based characterization of the equilibrium points of an open-cathode fuel cell system as the baseline for proper controller design, highlighting the relation between fuel cell temperature, humidification and performance. Phase plane analysis of the nonlinear model versus a linearized model around different points of operation shows the potential of approximating the nonlinear system behavior with a linear model. The methodology for the system analysis presented in this paper finally serves for the development of control schemes using robust control techniques. The designed controller is validated in simulation with the nonlinear plant model.Postprint (published version

Study of hydrogen purge effects on performance and efficiency of a PEM fuel cell system

Presentado al V Congreso Nacional de Pilas de Combustible celebrado en Madrid del 21 al 23 de noviembre de 2012.[EN]: Experimental analysis and CFD modeling is used in this work to analyze system efficiency related to hydrogen purge based water management in an open-cathode PEM fuel cell system. Excess water in a dead-ended anode decreases hydrogen concentration at the active catalyst surface and thus causes fuel cell performance losses. Purging the anode with hydrogen removes water and nitrogen that diffused through the membrane but also means wasting energy and thus decreasing overall system efficiency. Experiments with a 100W open-cathode stack have revealed that the need for a hydrogen purge strongly depends on the operation conditions and the state-of-health of the fuel cell and therefore the decision to perform a purge has to be evaluated online. A dynamic 2D CFD model of a single cell within the stack is used to investigate water distribution and transport within the cell before, during and after performing a purge at different operating conditions, linked to cell performance. Moreover, the model is capable of studying water transfer dynamics across the membrane and along the channel, including liquid water saturation. Altogether, the presented experimental and modeling work helps to improve the understanding of water transport in a PEM fuel cell and thus facilitates the development of strategies for increasing system efficiency and optimizing the water management by properly controlling the hydrogen purge.[ES]: Análisis experimental y modelado CFD se utiliza en este trabajo para analizar la eficiencia del sistema relacionado con la gestión del agua basada en purgas de hidrógeno en un sistema cátodo-abierto de pilas de combustible tipo PEM. El exceso de agua en el ánodo con la salida cerrada disminuye la concentración de hidrógeno en la superficie del catalizador y por lo tanto provoca pérdidas de rendimiento. Purgar el ánodo con hidrógeno elimina el agua y el nitrógeno que difunde a través de la membrana, pero también significa una pérdida de energía y por lo tanto disminuye la eficiencia global del sistema. Experimentos con un stack de 100W han revelado que la necesidad de una purga de hidrógeno depende fuertemente de las condiciones de operación y el estado de salud de la pila de combustible y por lo tanto la decisión de realizar una purga tiene que ser evaluado en línea. Un modelo dinámico CFD de 2 dimensiones de una sola célula dentro de la pila se utiliza para investigar la distribución y el transporte del agua dentro de la célula antes, durante y después de realizar una purga en diferentes condiciones de funcionamiento, ligados al rendimiento de la célula. Por otra parte, el modelo es capaz de estudiar la dinámica de transferencia de agua a través de la membrana y a lo largo del canal, incluyendo la saturación de agua en estado líquido. En total, el trabajo experimental y de modelado ayuda a mejorar la comprensión de transporte de agua en una pila de combustible tipo PEM y por lo tanto facilita el desarrollo de estrategias para aumentar la eficiencia del sistema y la optimización de la gestión del agua para controlar la purga de hidrógeno de manera adecuada.This work is partially funded by the project of CICYT DPI2011-25649 MICINN.Peer Reviewe

Experimental and model-based analysis for performance and durability improvement of PEM fuel cells

Increasing global energy demand, growing carbon emissions and the depletion of fossil fuel sources are some of the most important driving forces for the development of sustainable energy solutions. Proton Exchange Membrane (PEM) fuel cells have been demonstrated to be a potential candidate for clean energy conversion in a wide range of applications reaching from highly dynamic transportation systems to stationary systems. Despite their benefits, such as high efficiency and wide operating range, PEM fuel cells must still meet or exceed the technological advantages, such as durability and cost, of conventional power systems in order to be truly competitive. Thus, current research is focused on improving these aspects. This doctoral thesis combines experimental and model-based studies in order to improve performance and durability of PEM fuel cells, that work without external humidification, as demanded by recent government-supported research programs. Improved performance and durability can be obtained by proper system control. The key factor for the development of successful control strategies is adequate thermal and water management considering their interconnections. Therefore, this work investigates the important links between performance, efficiency and lifetime with respect to fuel cell temperature and humidification. The experimental evaluation of temperature-related and purge-related effects shows the great potential of improving the system performance by proper thermal management. In-situ and ex-situ experiments, such as electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), gas chromatography (GC), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized in order to explore short-term and long-term effects of operation modes on performance and durability. To provide a better understanding of the experimentally observed phenomena and their different dynamics with respect to the development of efficient controllers, mathematical models have been derived. The dynamic models allow for relating electrode structure to the cell voltage transient behavior during changes in fuel cell temperature and humidification, including important phase change and ionomer sorption dynamics of water. The experimentally validated, model-based analysis provides recommendations of proper operating conditions and catalyst structure, such as optimal fuel cell temperature and adequate pore-size-distribution, in order to improve the PEM fuel cell performance. The modular character and inherent adaptability of the models has been successfully demonstrated in the study of water transport in a high temperature PEM fuel cell stack. It is shown how mathematical modeling can improve the interpretation of experimental results and provide insight into experimentally non-observable interactions. In conclusion, the presented laboratory and model-based work, including the developed experimental and mathematical tools, contribute to current international research targets for advancing sustainable energy solutions.La creciente demanda mundial de energía, el crecimiento de las emisiones de dióxido de carbono y el agotamiento de las fuentes de combustible fósiles son algunos de los factores más importantes para el desarrollo de soluciones basadas en energies sostenibles. Las pilas de combustible de tipo Proton Exchange Membrane (PEM) han demostrado ser un candidato potencial para la conversión limpia de la energía en una extensa gama de aplicaciones, desde los sistemas de transporte altamente dinámicos hasta sistemas estacionarios. No obstante sus beneficios, tales como una alta eficiencia y un amplio rango de operación, las pilas de combustible PEM todavía deben cumplir o superar las ventajas tecnológicas de los sistemas de energía convencionales, como son su durabilidad y coste, con el fin de ser verdaderamente competitivas. Por lo tanto, la investigación actual se centra en la mejora de estos aspectos. Esta tesis doctoral combina estudios experimentales y estudios basados en modelos físicos con el fin de mejorar el rendimiento y la durabilidad de las pilas de combustible PEM que trabajan sin humidificación externa, tal y como exigen recientes programas de investigación apoyados por los gobiernos. La mejora del rendimiento y de la durabilidad se puede obtener por control apropiado del sistema. El factor clave para el desarrollo de estrategias de control exitosas es la gestión adecuada de la temperatura y del agua y sus interconexiones. Por lo tanto, este trabajo investiga los vínculos importantes entre el rendimiento, la eficiencia y la vida útil con respecto a la temperatura de la pila de combustible y su humidificación. La evaluación experimental de los efectos relacionados con la temperatura y las purgas de hidrógeno muestra el gran potencial para mejorar el rendimiento del sistema pila mediante una gestión térmica adecuada. En esta tesis se emplean experimentos in-situ y ex-situ, tales como la espectroscopía electroquímica de impedancia (EIS), la voltametría cíclica (CV), la cromatografía de gases (CG), la espectroscopia de fotoelectrones emitidos por rayos X (XPS), la difracción de rayos X (XRD) y la microscopía electrónica de barrido (SEM) con el fin de explorar los efectos a corto plazo y a largo plazo de los modos de operación sobre el rendimiento y la durabilidad de una pila PEM. Para proporcionar una mejor comprensión de los fenómenos observados experimentalmente y sus diferentes dinámicas para el correcto desarrollo de controladores eficientes, se derivan modelos matemáticos dinámicos. Los modelos permiten relacionar la estructura de los electrodos con el comportamiento transitorio del voltaje durante los cambios de temperatura y de humidificación de la pila de combustible, incluyendo las dinámicas importantes del cambio de fase y de adsorción y desorción del agua. El análisis basado en modelos validados experimentalmente proporciona recomendaciones de las condiciones de funcionamiento y de la estructura del catalizador, tales como la temperatura óptima y la distribución de tamaño de poros apropiada, con el fin de mejorar el rendimiento de la pila de combustible PEM. El carácter modular y la adaptabilidad inherente de los modelos propuestos se demuestra con éxito en el estudio de transporte de agua en un stack de pilas de combustible PEM de alta temperatura. Se muestra como el modelado matemático puede mejorar la interpretación de los resultados experimentales y proporcionar información sobre las interacciones que experimentalmente no son observables. En conclusión, el trabajo de laboratorio y el basado en modelos que se presenta en esta tesis doctoral, incluyendo las herramientas experimentales y matemáticas desarrolladas, contribuyen a la consecución de los actuales objetivos internacionales de investigación que deben permitir aportar mejoras en las soluciones basadas en energías sostenibles

Electrode structure effects on the performance of open-cathode proton exchange membrane fuel cells: A multiscale modeling approach

In this paper we present a new dynamic multiscale model of an open-cathode Polymer Electrolyte Membrane Fuel Cell (PEMFC). The model describes two-phase water transport, electrochemistry and thermal management within a framework that combines a Computational Fluid Dynamics (CFD) approach with a micro-structurally-resolved model predicting the water filling dynamics of the electrode pores and the impact of these dynamics on the evolution of the electrochemically active surface area (ECSA). The model allows relating for the first time the cathode electrode structure to the cell voltage transient behavior during experimental changes in fuel cell temperature. The effect of evaporation rates, desorption rates and temperature changes on the performance of four different electrode pore size distributions are explored using steady-state and transient numerical simulations. The results are discussed with respect to water management and temperature control.This work is partially funded by the national project MICINNDPI2011-25649, as well as by the 7th Framework Programme of the European Commission in the context of the Fuel Cells and Hydrogen Joint Undertaking (FCH JU) through the project PUMA-MIND FP7 303419.Peer Reviewe

Water transport study in a high temperature proton exchange membrane fuel cell stack

A study of water transport in a high temperature phosphoric acid doped polybenzimidazole (PBI) membrane fuel cell stack is reported. Tests with different stoichiometries of dry cathode and different humidity levels of anode are performed. It is found that water transport across the membrane electrode assembly (MEA) is noteworthy and that water vapor partial pressure on the anode outlet is almost always higher than on the cathode outlet, even when using dry hydrogen. The water transport is a strong function of current density but it also depends on stoichiometry and humidity level. In a series of tests with dry nitrogen on one side and humid nitrogen on the other side, the membrane's water permeability coefficient is determined to be 2.4 × 10-13 mol s-1 cm-1 Pa-1 at 160 °C which is more than an order of magnitude higher than the values previously reported in the literature. Also, the results indicate that the permeability coefficient might be relative humidity dependent and could even be somewhat higher than the value reported here, but further investigation is needed. The experimental findings are reproduced and explained with a 2D steady state computational fluid dynamics (CFD) model. Internal water transport profiles across the membrane and along the gas flow channels are presented and discussed.This work is partially funded by the project of CICYTDPI2011-25649 MICINN. Finally, the authors highly appreciate the support of the Institut de Robòtica i Informàtica Industrial in enabling a research stay of Dario Bezmalinović at the Fuel Cell Laboratory in Barcelona.Peer Reviewe

Study of hydrogen purge effects on performance and efficiency of a PEM fuel cell system

Experimental analysis and CFD modeling is used in this work to analyze system efficiency related to hydrogen purge based water management in an open-cathode PEM fuel cell system. Excess water in a deadended anode decreases hydrogen concentration at the active catalyst surface and thus causes fuel cell performance losses. Purging the anode with hydrogen removes water and nitrogen that diffused through the membrane but also means wasting energy and thus decreasing overall system efficiency. Experiments with a 100W open-cathode stack have revealed that the need for a hydrogen purge strongly depends on the operation conditions and the state-of-health of the fuel cell and therefore the decision to perform a purge has to be evaluated online. A dynamic 2D CFD model of a single cell within the stack is used to investigate water distribution and transport within the cell before, during and after performing a purge at different operating conditions, linked to cell performance. Moreover, the model is capable of studying water transfer dynamics across the membrane and along the channel, including liquid water saturation. Altogether, the presented experimental and modeling work helps to improve the understanding of water transport in a PEM fuel cell and thus facilitates the development of strategies for increasing system efficiency and optimizing the water management by properly controlling the hydrogen purge.Peer ReviewedPostprint (published version

Experimental characterization and identification of the voltage losses in an open cathode PEM fuel cell stack

For the past 20 years remarkable progress has been made in PEM fuel cell materials, component design, production, and system power density im-provements. However, there is still a lot to be done in the field of fuel cell system control, which makes it essential to understand the different physical phenomena within a working fuel cell and how they need to be controlled in order to improve efficiency, operating range and durability. This experimental study analyses the effects of ambient conditions, through the use of an environmental chamber, on a Horizon® H-100 20 cell stack, 22cm2 active area, open cathode, where the only active control mech-anism employed is a single fan that both, cools and provides the oxygen needed for the reaction. All the other control mechanisms are disconnected and a constant dry pure hydrogen flow rate of 1.8 SLPM is supplied to the stack. The objective is to isolate and determine the dif-ferent voltage losses with respect to the ambient conditions and currents.Postprint (author’s final draft

A multi-timescale modeling methodology for PEMFC performance and durability in a virtual fuel cell car

The durability of polymer electrolyte membrane fuel cells (PEMFC) is governed by a nonlinear cou-pling between system demand, component behavior, and physicochemical degradation mechanisms, occurring on timescales from the sub-second to the thousand-hour. We present a simulation methodol-ogy for assessing performance and durability of a PEMFC under automotive driving cycles. The simu-lation framework consists of (a) a fuel cell car model converting velocity to cell power demand, (b) a 2D multiphysics cell model, (c) a flexible degradation library template that can accommodate physi-cally-based component-wise degradation mechanisms, and (d) a time-upscaling methodology for ex-trapolating degradation during a representative load cycle to multiple cycles. The computational framework describes three different time scales, (1) sub-second timescale of electrochemistry, (2) minute-timescale of driving cycles, and (3) thousand-hour-timescale of cell ageing. We demonstrate an exemplary PEMFC durability analysis due to membrane degradation under a highly transient load-ing of the New European Driving Cycle (NEDC).Peer ReviewedPostprint (author's final draft

Water transport study in high temperature proton exchange membrane fuel cell stack

A study of water transport in a high temperature phosphoric acid doped polybenzimidazole (PBI) membrane fuel cell stack is reported. Tests with different stoichiometries of dry cathode and different humidity levels of anode are performed. It is found that water transport across the membrane electrode assembly (MEA) is noteworthy and that water vapor partial pressure on the anode outlet is almost always higher than on the cathode outlet, even when using dry hydrogen. The water transport is a strong function of current density but it also depends on stoichiometry and humidity level. In a series of tests with dry nitrogen on one side and humid nitrogen on the other side, the membrane's water permeability coefficient is determined to be 2.4 × 10-13 mol s-1 cm-1 Pa-1 at 160 °C which is more than an order of magnitude higher than the values previously reported in the literature. Also, the results indicate that the permeability coefficient might be relative humidity dependent and could even be somewhat higher than the value reported here, but further investigation is needed. The experimental findings are reproduced and explained with a 2D steady state computational fluid dynamics (CFD) model. Internal water transport profiles across the membrane and along the gas flow channels are presented and discussed.Peer ReviewedPostprint (author’s final draft