Nitrogen front evolution in purged polymer electrolyte membrane fuel cell with dead-ended anode

In this paper, we model and experimentally verify the evolution of liquid water and nitrogen fronts along the length of the anode channel in a proton exchange membrane fuel cell operating with a dead-ended anode that is fed by dry hydrogen. The accumulation of inert nitrogen and liquid water in the anode causes a voltage drop, which is recoverable by purging the anode. Experiments were designed to clarify the effect of N-2 blanketing, water plugging of the channels, and flooding of the gas diffusion layer. The observation of each phenomenon is facilitated by simultaneous gas chromatography measurements on samples extracted from the anode channel to measure the nitrogen content and neutron imaging to measure the liquid water distribution. A model of the accumulation is presented, which describes the dynamic evolution of a N-2 blanketing front in the anode channel leading to the development of a hydrogen starved region. The prediction of the voltage drop between purge cycles during nonwater plugging channel conditions is shown. The model is capable of describing both the two-sloped behavior of the voltage decay and the time at which the steeper slope begins by capturing the effect of H-2 concentration loss and the area of the H-2 starved region along the anode channel

Nonlinear predictive control for durability enhancement and efficiency improvement in a fuel cell power system

© . This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/In this work, a nonlinear model predictive control (NMPC) strategy is proposed to improve the efficiency and enhance the durability of a proton exchange membrane fuel cell (PEMFC) power system. The PEMFC controller is based on a distributed parameters model that describes the nonlinear dynamics of the system, considering spatial variations along the gas channels. Parasitic power from different system auxiliaries is considered, including the main parasitic losses which are those of the compressor. A nonlinear observer is implemented, based on the discretised model of the PEMFC, to estimate the internal states. This information is included in the cost function of the controller to enhance the durability of the system by means of avoiding local starvation and inappropriate water vapour concentrations. Simulation results are presented to show the performance of the proposed controller over a given case study in an automotive application (New European Driving Cycle). With the aim of representing the most relevant phenomena that affects the PEMFC voltage, the simulation model includes a two-phase water model and the effects of liquid water on the catalyst active area. The control model is a simplified version that does not consider two-phase water dynamics.Peer ReviewedPostprint (author's final draft

Study on the key factors allowing the PEM fuel cell systems large commercialization: fuel cell degradation and components integration

PEM Fuel Cells are expected to gradually substitute internal combustion engines as electrical and co-generation power sources thanks to high efficiency, low operating temperature, fast startup time and favourable power-to-weight ratio. However, while PEMFCs have achieved significant progresses in the last decade, their short lifetime and high cost still continue to impede large-scale commercialization. The first subject of the present work had been the study of the PEM fuel cells degradation mechanisms with the aim of: a) find out the most relevant phenomena concerning the fuel cell lifetime, b) testing some methods able to promptly detect the degradation mechanisms and, mostly, c) find out the mitigation strategies able to increase the fuel cells lifetime. At the end of the research three mitigation strategies had been developed and tested: cell voltage monitoring, the current modulation and the stack shunt. According to the tests results all these mitigation strategies, if adopted all together, can effectively led to doubling the fuel cells lifetime. In parallel to the fuel cell lifetime increase, a deep investigation on system components integration had been conducted. Following this principle, the system cost has been considerably reduced mostly thanks to the DC-DC converter integration with the stack and the coolant circuit simplification. The prototypes realized during this work has been taken as example for the production of new fuel cell power systems with increased lifetime at lower cos

Effect of Anode Purge on Polymer Electrolyte Membrane Fuel Cell Performance

Polymer Electrolyte Membrane Fuel Cells (PEMFC) are promising power generating devices that use an electrochemical reaction to convert the energy from hydrogen fuel into usable electricity. One cell produces a small voltage so many cells are combined in series in order to produce a useful voltage, this configuration is referred to as a stack. Hydrogen is supplied to the anode of the stack in amounts greater than the electrochemical reaction requires to guarantee that enough hydrogen is available for every cell in the stack and to provide enough pressure throughout the cell flow channels for good mass transfer. For reasonable fuel efficiency, the anode outlet gas containing unconverted hydrogen is recycled (or recirculated) back to the anode inlet. PEMFC performance is highest when pure hydrogen fuel is supplied, however, nitrogen at the cathode will permeate through the membrane and accumulate in the anode gas with recirculation. Nitrogen buildup dilutes the hydrogen gas which adversely affects fuel cell performance at the anode. Also, in practical applications hydrogen-rich gas produced from reformed methane, called reformate, is used as the fuel. Reformate contains impurities such as, nitrogen, carbon dioxide, carbon monoxide, and sulfur compounds. This thesis will focus on trace levels of carbon monoxide entering in the hydrogen fuel stream, and the impact of contaminant build-up due to anode recirculation. Carbon monoxide adsorbs readily onto the platinum catalyst sites, called poisoning, thus decreasing PEMFC performance. In efforts to minimize the buildup of impurities and crossed over nitrogen, a portion of the anode outlet gas is periodically and continuously purged to the exhaust. How often the outlet gas is purged depends on a variable called the purge fraction. The purpose of this research is to study the effect of purge fraction on PEMFC performance, measured by the average cell voltage, for a Hydrogenics 10 cell stack. The operating parameters used for testing and the experimental apparatus were designed to mimic a Hydrogenics 8kW Hydrogen Fuel Cell Power Module. A pump connected between the anode outlet and anode inlet form the anode recirculation loop. In Phase 1 of the test program the effect of purge in the absence of carbon monoxide was studied to see if hydrogen dilution from nitrogen crossover and accumulation would cause significant cell voltage degradation. In Phase 2 the effect down to 0.2 ppm carbon monoxide was evaluated. The results showed that nitrogen buildup, in the absence of carbon monoxide, did not significantly penalize the cell performance in the range of purge fractions tested. However, for the same purge fraction but with as little as 0.2 ppm carbon monoxide present, the voltage loss was significant. A discussion of the effect of purge on the impurity concentration and the associated cell voltage degradation is detailed with particular emphasis on carbon monoxide poisoning

Effect of reactant gas flow orientation on the current and temperature distribution in self-heating polymer electrolyte fuel cells

Fuel cell polarisation performance is typically reported under controlled/constant temperature conditions, as a sign of robust metrology. However, in practice, fuel cells self-heat as they generate current; which varies the temperature across the polarisation curve and affects performance. More detail regarding the internal cell operation can be gleaned by current and temperature distribution mapping. For the case of an unheated cell, ‘self-heating’ increases the cell temperature and improves performance, resulting in a ‘voltage recovery’ and a more homogeneous current and water distribution. For actively heated cells, a reduced current is observed in regions of high temperature and low humidity. The positioning of the gas manifolds also has a decisive impact on performance by affecting the reactant concentration, humidity and water distribution. Counter- and cross-flow orientations in a self-heating cell were studied, with a counter-flow orientation with air flowing with gravity producing the most uniform temperature distribution

Scale-up and study of the BioGenerator

For the time being transfer from the fossil fuel powered electricity generation technologies to renewable sources is facing a great deal of challenges, because of their intermittent nature. Efficient ways of electricity storage are essential to make it happen, and our electro-biotechnology - the BioGenerator - may be a potential solution, due to its uniqueness consisting in employing iron oxidizing microorganisms. This work presents a scale-up of the BioGenerator from 1W to 300W capacity in a stepwise manner. It involved the design, study and scale-up of the airlift bioreactor from 1.4 to 600 L, and electrochemical cells with different catholyte flow patterns from 4x4 cm (1.6 W) to a stack of 20x20 cm cells (271 W). An impact of different operating regimes and the catholyte characteristics on the electrochemical cell performance was studied. Based on the experimental results collected over the course of this Ph.D. research project, the largest and most advanced system to date - 10 kW BioGenerator - was designed and currently is under construction. Oxygen mass transfer and microbial dynamics in the large-scale bioreactors (400 and 600 L) were studied and extraordinary resilience of L. ferriphilum dominated culture was observed. It was found that it takes ~5.5. days for the bacteria to recover and resume their iron oxidizing ability even after 5 months of starvation. An array of commercially available proton exchange membranes was tested in terms of their suitability for the use in the BioGenerator, and Selemion HSF was found to be the best amongst them. The straight forward techniques for the synthesis of Nafion- and polyvinyl alcohol (PVA)-based membranes were proposed. The proton conductivity, water transport and ferric ion diffusion through the synthesized membranes were measured. Testing in the Fe3+/H2 electrochemical cell revealed that the most promising, in terms of both performance and economics, amongst them is a phosphorylated PVA membrane. A mathematical model describing the operation of the BioGenerator system was developed and successfully tested during validation experiments. The data predicted was in a fairly good agreement with the experimental

Modeling and control of PEM fuel cells

Aplicat embargament des del moment de la defensa fins al 5 de juliol de 2019.In recent years, the PEM fuel cell technology has been incorporated to the R&D plans of many key companies in the automotive, stationary power and portable electronics sectors. However, despite current developments, the technology is not mature enough to be significantly introduced into the energy market. Performance, durability and cost are the key challenges. The performance and durability of PEM fue! cells significantly depend on variations in the concentrations of hydrogen and oxygen in the gas channels, water activity in the catalyst layers and other backing layers, water content in the polymer electrolyte membrane, as well as temperature, among other variables. Such variables exhibit intemal spatial dependence in the direction of the fuel and air streams of the anode and cathode. Highly non-uniform spatial distributions in PEM fuel cells result in local over-heating, cell flooding, accelerated ageing, and lower power output than expected. Despite the importance of spatial variations of certain variables in PEM fuel cells, not many works available in the literature target the control of spatial profiles. Most control-oriented designs use lumped-parameter models because of their simplicity and convenience for controller performance. In contrast, this Doctoral Thesis targets the distributed parameter modelling and control of PEM fuel cells. In the modelling part, the research addresses the detailed development of a non-linear distributed parameter model of a single PEM fuel cell, which incorporates the effects of spatial variations of variables that are relevant to its proper performance. The model is first used to analyse important cell intemal spatial profiles, and it is later simplified in arder to decrease its computational complexity and make it suitable for control purposes. In this task, two different model order reduction techniques are applied and compared. The purpose of the control part is to tackle water management and supply of reactants, which are two major PEM fuel cell operation challenges with important degradation consequences. In this part of the Thesis, two decentralised control strategies based on distributed parameter model predictive controllers are designed, implemented and analysed via simulation environment State observers are also designed to estímate intemal unmeasurable spatial profiles necessary for the control action. The aim of the first strategy is to monitor and control observed water activity spatial profiles on both sides of the membrana to appropriate levels. These target values are carefully chosen to combine proper membrane, catalyst layer and gas diffusion layer humídification, whilst the rate of accumulation of excess liquid water is reduced. The key objective of this approach is to decrease the frequency of water removal actions that cause disruption in the power supplied by the cell, increased parasitic losses or degradation of cell efficiency. The second strategy is a variation of the previous water activity control strategy, which includes the control of spatial distribution of gases in the fuel and air channels. This integrated solution aims to avoid starvation of reactants by controlling corresponding concentration spatial profiles. This approach is intended to prevent PEM fuel cell degradation due to corrosion mechanisms, and thennal stress caused by the consequences of reactant starvation.A pesar de los avances actuales, la tecnología de celdas de hidrógeno tipo PEM no está suficientemente preparada para ser ampliamente introducida en el mercado energético. Rendimiento, durabilidad y costo son los mayores retos. El rendimiento y la durabilidad de las celdas dependen significativamente de las variaciones en las concentraciones de hidrógeno y oxígeno en los canales de alimentación de gases, la humedad relativa en las capas catalizadoras, el contenido de agua de la membrana polimérica, así como la temperatura, entre otras variables. Dichas variables presentan dependencia espacial interna en la dirección del flujo de gases del ánodo y del cátodo. Distribuciones espaciales altamente no uniformes en algunas variables de la celda resultan en sobrecalentamiento local, inundación, degradación acelerada y menor potencia de la requerida. Muy pocos trabajos disponibles en la literatura se ocupan del control de perfiles espaciales. La mayoría de los diseños orientados a control usan modelos de parámetros concentrados que ignoran la dependencia espacial de variables internas de la celda, debido a la complejidad que añaden al funcionamiento de controladores. En contraste, esta Tesis Doctoral trata la modelización y control de parámetros distribuidos en las celdas de hidrógeno tipo PEM. En la parte de modelización, esta tesis presenta el desarrollo detallado de un modelo no lineal de parámetros distribuidos para una sola celda, el cual incorpora las variaciones espaciales de todas las variables que son relevantes para su correcto funcionamiento. El modelo se usa primero para analizar importantes perfiles espaciales internos, y luego se simplifica para reducir su complejidad computacional y adecuarlo a propósitos de control. En esta tarea se usan y se comparan dos técnicas de reducción de orden de modelos. El propósito de la parte de control es abordar la gestión de agua y el suministro de reactantes, que son dos grandes retos en el funcionamiento de las celdas con importantes consecuencias para su vida útil. En esta parte de la tesis, dos estrategias de control descentralizadas, basadas en controladores predictivos de modelos de referencia con parámetros distribuidos, son diseñadas, implementadas y analizadas en un entorno de simulación. Estas tareas incluyen también el diseño de observadores de estado que estiman los perfiles espaciales internos necesarios para la acción de control. El objetivo de la primera estrategia es monitorear y controlar perfiles espaciales observados de la humedad relativa en las capas catalizadoras para mantenerlos en niveles apropiados. Estos niveles son escogidos cuidadosamente para combinar la correcta humidificación de la membrana y las capas catalizadoras, reduciendo la velocidad de acumulación de agua líquida. El objetivo clave de este enfoque es disminuir la frecuencia de las acciones de remoción de agua dentro de la celda, ya que estas acciones causan interrupción en la potencia suministrada, aumento de las cargas parasitarias y disminución de la eficiencia. La segunda estrategia es una variación de la estrategia anterior que considera adicionalmente el control de la distribución espacial de los gases en los canales del ánodo y cátodo. Esta solución integrada tiene como objetivo evitar la ausencia local de reactantes mediante el control de perfiles espaciales de concentración de gases. Este enfoque pretende prevenir la degradación de las celdas debido a mecanismos de corrosión. Los resultados muestran un mayor rendimiento de la celda considerando los enfoques de control de perfiles espaciales propuestos en esta tesis, en comparación con técnicas de control que ignoran dichos perfiles. Además, la característica descentralizada de los esquemas de control, combinada con el uso de modelos reducidos dentro de los controladores predictivos, tiene un impacto positivo importante en el rendimiento general del control.Postprint (published version

Characterizing Performance of a PEM Fuel Cell for a CMET Balloon

We present the design of a multi-cell, low temperature PEM fuel cell for controlled meteorological balloons. Critical system design parameters that distinguish this application are the lack of reactant humidification and cooling due to the low power production, high required power mass-density and relatively short flight durations. The cell is supplied with a pressure regulated and dead ended anode, and flow controlled cathode at variable air stoichiometry. The cell is not heated and allowed to operate with unregulated temperature. Our prototype cell was capable of achieving power densities of 43 mW/cm2/cell or 5.4 mW/g. The cell polarization performance of large format PEM fuel cell stacks is an order of magnitude greater than for miniature PEM fuel cells. These performance discrepancies are a result of cell design, system architecture, and reactant and thermal management, indicating that there are significant gains to be made in these domains. We then present design modifications intended to enable the miniature PEM fuel cell to achieve power densities of 13 mW/g, indicating that additional performance gains must be made with improvements in operating conditions targeting achievable power densities of standard PEM fuel cells

Simplified mathematical model of proton exchange membrane fuel cell based on horizon fuel cell stack

This paper presents a simplified zero-dimensional mathematical model for a self-humidifying proton exchange membrane (PEM) fuel cell stack of 1 kW. The model incorporates major electric and thermodynamic variables and parameters involved in the operation of the PEM fuel cell under different operational conditions. Influence of each of these parameters and variables upon the operation and the performance of the PEM fuel cell are investigated. The mathematical equations are modeled by using Matlab–Simulink tools in order to simulate the operation of the developed model with a commercial available 1 kW horizon PEM fuel cell stack (H-1000), which is used for the purposes of model validation and tuning of the developed model. The model can be extrapolated to higher wattage fuel cells of similar arrangements. New equation is presented to determine the impact of using air to supply the PEM fuel cell instead of pure oxygen upon the concentration losses and the output voltage when useful current is drawn from it