04 Fakultät Energie-, Verfahrens- und Biotechnik
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Item Open Access Messung und Modellierung der effektiven Wärmeleitfähigkeit von Dämmstoffschüttungen für vakuumwärmegedämmte Warmwasserspeicher(2022) Lang, Stephan; Spindler, Klaus (apl. Prof. Dr.-Ing.)Das Ziel dieser Arbeit ist es, hinsichtlich Dämmwirkung und Kosten geeignete schüttfähige Wärmedämmstoffe für doppelwandige Warmwasserspeicher mit Vakuumwärmedämmung zu identifizieren und eine Vorhersage über die effektive Wärmeleitfähigkeit in Abhängigkeit von Luftdruck und Temperatur treffen zu können. Zu diesem Zweck werden expandierte Perlite unterschiedlicher mittlerer Korngrößen und Mischungen aus expandierten Perliten und pyrogener Kieselsäure untersucht. Schwerpunkt der Untersuchungen ist die Ermittlung der effektiven Wärmeleitfähigkeit bei unterschiedlichen Luftdrücken und Temperaturen. Die effektive Wärmeleitfähigkeit wird in einer eigens entwickelten Versuchsanlage, nach einem stationären Messprinzip, bei Luftdrücken zwischen 0,001 mbar und Atmosphärendruck von (960 ± 20) mbar sowie bei Probenmitteltemperaturen zwischen -5 °C und 90 °C bestimmt. Die maximale relative Messunsicherheit der Versuchsanlage beträgt < 8 % bei den geringsten und < 3 % bei den höchsten gemessenen effektiven Wärmeleitfähigkeiten. Reine feinkörnige expandierte Perlite mit Dichten der Schüttungen ≥ 182 kg/m³ erreichen bei Luftdrücken ≤ 0,1 mbar und allen gemessenen Probenmitteltemperaturen die geringsten effektiven Wärmeleitfähigkeiten. Bei einer Probenmitteltemperatur von 48 °C liegen diese bei ≤ 4,84 mW/(m·K). Mischungen aus einem vergleichsweise grobkörnigen expandierten Perlit mit einer sehr feinkörnigen und feinporigen pyrogenen Kieselsäure können hingegen, bei technisch einfacher zu handhabenden Luftdrücken von > 1 mbar, z. T. geringere effektive Wärmeleitfähigkeiten erreichen als reine expandierte Perlite. Mischungen dieser Komponenten werden in dieser Arbeit erstmals in Form einer losen Schüttung bzgl. ihrer effektiven Wärmeleitfähigkeit untersucht. Bei einer Probenmitteltemperatur von 48 °C werden in diesem Luftdruckbereich effektive Wärmeleitfähigkeiten dieser Mischungen von > 12,17 mW/(m·K) gemessen. Anhand der gemessenen effektiven Wärmeleitfähigkeiten sowie weiterer Stoffeigenschaften, werden vollständig prädiktive analytische Modelle der effektiven Wärmeleitfähigkeit entwickelt. Das Modell für Mischungen gilt für ein ausgewähltes Stoffpaar, während das Modell für expandierte Perlite für beliebige ungemahlene expandierte Perlite verwendbar ist. Es handelt sich nach Kenntnis des Autors um das erste vollständig prädiktive analytische Modell der effektiven Wärmeleitfähigkeit für expandierte Perlite, welches auch die Kopplung von Gas- und Festkörperwärmeleitung berücksichtigt und für welches lediglich drei einfach und kostengünstig zu messende Größen bestimmt werden müssen. Diese Größen sind der volumengewichtet gemittelte Korndurchmesser, die mittlere Korndichte sowie die Dichte der Schüttung des expandierten Perlits. Aus den Messwerten der effektiven Wärmeleitfähigkeit wird ein Zusammenhang von volumengewichtet gemitteltem Korndurchmesser zur Kopplung von Gas- und Festkörperwärmeleitung deutlich, der für das Modell für expandierte Perlite verwendet wird. Mit den Modellen ist es nun möglich, ohne entsprechende Messungen, die effektive Wärmeleitfähigkeit von Mischungen aus einem expandierten Perlit und einer pyrogenen Kieselsäure sowie für beliebige ungemahlene expandierte Perlite, mit zufriedenstellender bis hoher Genauigkeit vorherzusagen.Item Open Access Untersuchung des thermohydraulischen Förderverhaltens einer Thermosiphonpumpe bei unterschiedlichen Beheizungsarten(2019) Bierling, Bernd; Spindler, Klaus (apl. Prof. Dr.-Ing.)In der vorliegenden Arbeit wird das thermohydraulische Förderverhalten einer zweiphasigen Thermosiphonpumpe bei unterschiedlichen Beheizungsarten untersucht. Grundlage für die experimentellen Untersuchungen ist ein Prüfstand, der konstante Versuchsbedingungen bietet. Dies wird durch die kontinuierliche Messung des Dampf- bzw. Kondensatmassenstroms, die Vorwärmung des Arbeitsmediums auf nahe Siedetemperatur sowie die strömungsoptimierte Gestaltung des Reservoirs und des Förderrohreinlaufs erreicht. Der Einfluss der relativen Heizlänge auf die Förderfähigkeit der Thermosiphonpumpe wird untersucht, indem ein vertikales Förderrohr punktuell, teilflächig und flächig beheizt wird. Als Arbeitsmedium wird demineralisiertes Wasser verwendet. Der Prüfstand ist zur Umgebung hin offen. Nahezu über den gesamten untersuchten Bereich gilt: Je kleiner die relative Heizlänge, desto höher ist der geförderte Flüssigkeitsmassenstrom. Bei punktueller Beheizung weist die Förderkennlinie einen untypischen Verlauf mit einem lokalen und absoluten Maximum auf. Das instationäre Förderverhalten der Thermosiphonpumpe bei punktueller Beheizung wird messtechnisch untersucht. Ein Schwerpunkt der vorliegenden Arbeit ist die Messung der Strömungsgeschwindigkeit des flüssigen Arbeitsmediums in der horizontalen Zulaufstrecke zur Quantifizierung des instationären Strömungsverhaltens. Eine Frequenzanalyse des gemessenen statischen Druckes in der horizontalen Zulaufstrecke gibt Aufschluss über die Periodizität des instationären Strömungsverhaltens. Ebenfalls werden durch die Ermittlung der Strömungsform im Förderrohr Rückschlüsse auf das Förderverhalten sowie die Förderfähigkeit gezogen. Durch das instationäre Förderverhalten treten Strömungen entgegen der eigentlichen Förderrichtung auf. Mit dem Ziel der Verminderung bzw. Verhinderung dieser Rückströmungen wird der Einbau diverser Düsen und Venturirohre sowie eines Rückschlagventils in der horizontalen Zulaufstrecke untersucht. Die Messergebnisse zeigen, dass das Förderverhältnis durch diese Rohreinbauten in der Zulaufstrecke gesteigert werden kann. Aus den Erkenntnissen der Untersuchungen hinsichtlich der Beheizungsart und der Verminderung der Rückströmungen geht ein neues Beheizungskonzept für Diffusions-Absorptionskältemaschinen (DAKMs) in Form eines Plattenwärmeübertragers mit nachgeschaltetem Förderrohr hervor. Dies ermöglicht neben einer kompakten Bauweise sowie der Entkopplung von Wärmeübertragung und Förderung zur Nutzung verschiedener Wärmequellen die Beheizung der Thermosiphonpumpe bei niedrigen Temperaturen. Zudem wird ein hohes Förderverhältnis im Vergleich zu herkömmlich angetriebenen DAKMs erreicht.Item Open Access Multistep reactions of molten nitrate salts and gas atmospheres(2024) Steinbrecher, Julian; Thess, André (Prof. Dr.)Dissertation zur Untersuchung der Stabilität von Nitratsalzschmelzen unter verschiedenen atmosphärischen Bedingungen und Temperaturen.Item Open Access Perovskite chromite-based fuel electrode for solid oxide cells (SOCs): towards the understanding of the electrochemical performance(2023) Amaya Dueñas, Diana María; Friedrich, K. Andreas (Prof. Dr. rer. nat.)The current energy transition is a key driver for the continuous development of fuel cells and electrolyzers due to the rapid growth of the clean energy demand and the need to overcome the intermittency of the power supply of renewable energy sources, such as wind and solar energy. In this regard, solid oxide cells (SOC) are promising systems that allow to overcome such fluctuations: they convert renewable electrical energy into chemical energy in the form of hydrogen and valuable fuels and chemicals, while they can also repower the grid by converting fuels and hydrogen into electrical power. This feature in reversibility has attracted the interest among Power-to-X technologies, which can be exploited by operating SOCs in fuel cell (SOFC), electrolysis (SOEL) and reversible (rSOC) modes. Nevertheless, SOCs are not yet a mature technology due to limitations on the performance of their electrolyte and electrodes. Typical fuel electrodes made of Ni-based cermets are in contact not only with hydrogen, but also with reactants such as natural gas, biogas, steam and carbon dioxide, leading to important operation issues related to high temperatures and poisoning tolerance, which significantly detriment the performance of these systems. Due to the urgent need for the development of sustainable SOC systems in clean energy scenarios, this thesis aims to cover the Ni cermets issues related to SOCs operation, such as nickel agglomeration, nickel migration, structural cell damage and carbon deposition. Therefore, with the motivation to propose alternative fuel electrode materials to the state-of-the-art Ni cermets, formulations of perovskite chromite-based fuel electrodes were investigated in different SOC operating conditions. Firstly, different perovskite compositions were investigated by X-ray diffraction (XRD) to ensure the desired phase. With these crystal structure characterizations, the lanthanum-chromite perovskite with Ni doping (LSCrN) was selected as candidate fuel electrode material with the compositions La0.7Sr0.3Cr0.85Ni0.15O3-δ (L70SCrN) and La0.65Sr0.3Cr0.85Ni0.15O3-δ (L65SCrN). These materials were synthetized by the glycine-nitrate combustion method and ceramic powder morphology was characterized by scanning electron microscopy (SEM). An experimental protocol for the cell manufacturing process was designed and the electrolyte-supported-cells (ESCs) were produced by screen-printing, drying and sintering processes. ESCs were tested in different operating SOC modes: fuel cell (SOFC), steam electrolysis (SOEL), steam and carbon dioxide co-electrolysis (co-SOEL), as well as in reversible mode (rSOC) and even in dry carbon dioxide electrolysis operation. In situ electrochemical characterizations were performed by evaluating the voltage - current response and the electrochemical impedance spectroscopy (EIS). In parallel, the exsolution of nickel particles from the produced LSCrN ceramic powders was investigated by means of temperature programmed reduction (TPR), X-ray spectroscopy (XPS) and XRD techniques. It was shown that the introduction of A-site deficiency promoted the reduction of metallic nickel particles on the perovskite surface. The particle distribution was found to be dependent on the temperature, the atmosphere and the overpotential. In co-SOEL operation, cells with the developed L65SCrN electrode showed a comparable performance to the ones with state-of-the-art Ni cermets, e.g. - 0.8 A·cm-2 at 1.32 V and 860 °C. The long-term stability (~ 1000 hours) suggested that under strongly reducing atmospheres, such as in SOEL at 860 °C, the L65SCrN electrode suffered from accelerated performance degradation due to an alteration of the transport properties. Nonetheless, it was found that a decrease in operating temperature (below 830 °C) could be a suitable strategy to mitigate this durability issue. These findings are related to a gain in performance of the perovskite electrodes against the state-of-the-art Ni electrodes at temperatures between 770 °C and 830 °C, possibly due to lower reaction energy barriers. These outcomes were used as basis for a scale-up analysis from the cell level up to the system level, i.e. up to the MW scale, by analyzing a real case application of SOEL-based systems for hydrogen production. This analysis suggested that the implementation of perovskite electrodes in SOEL systems, together with a decrease of the system operating temperature, would lead to a significant reduction of the number of cells in the stacks and hence of the system components, simplifying the system layout. Additionally, the required amount of Ni raw material would also be significantly decreased, which would mitigate future supply chain issues that the mineral market may experience in the upcoming years. This study paves the way for future alternative electrode development for SOC applications while suggesting potential benefits at the system scale.Item Open Access Influence of natural convection on melting of phase change materials(2019) Vogel, Julian; Thess, André (Prof. Dr.)Latent heat storage could play an important role in bridging the gap between supply and demand of sustainable energy sources. However, the numerical models for natural convection dominated melting that are needed for storage system design are not sufficiently validated, due to a lack of suitable experiments. A novel validation experiment for the melting of a model phase change material (n-octadecane) by heating from two vertical opposite sides was developed. The phase state and the velocities in the liquid phase were measured using shadowgraphy and Particle Image Velocimetry. Interior and boundary temperatures were measured with thermocouples. The performed experiments delivered space and time-resolved data of the relevant quantities including an error analysis. Two numerical models for natural convection dominated melting were developed with the commercial fluid flow solver ANSYS Fluent: a first detailed model with variable material properties allows volume expansion of the phase change material into an air phase with the volume of fluid method. A second simplified model assumes constant material properties and models buoyancy with the Boussinesq approximation. Due to similar results, the simplified model was selected to reproduce the experiment in a 3D simulation including mechanical and thermal boundary effects. The simulated velocities were found to be higher as in the experiment, but the liquid phase fraction and temperatures, which are more relevant to the design process, agreed well. In a numerical parameter study, the simplified model was used to investigate melting in rectangular enclosures with various dimensions. The influence of natural convection on heat transfer was assessed with the introduced convective enhancement factor, which was defined as the ratio of the actual heat flux to a hypothetical heat flux by conduction. The study was extended with experimental data for three different values of driving temperature difference. Correlations for the liquid phase fraction in dimensionless form were derived to predict similar melting processes for a large parameter range. This enables the consideration of natural convection in the design of latent heat storage systems without expensive and time-consuming numerical analyses.Item Open Access Modellierung und Analyse der thermo-fluiddynamischen Vorgänge in Schaltschränken unter Berücksichtigung von Wärmestrahlung und Entropieproduktion(2019) Frank, Alexander; Spindler, Klaus (apl. Prof. Dr.-Ing.)Durch die fortschreitende Miniaturisierung der Elektronikkomponenten sind thermische Probleme eine Hauptursache für den Ausfall von Schaltschränken in der Fertigungstechnik. Um dieser Problematik zu begegnen, befasst sich die vorliegende Arbeit mit der Modellierung und Simulation von thermo-fluiddynamischen Vorgängen in Schaltschränken. Zu diesem Zweck wird die Open Source CFD-Bibliothek OpenFOAM angewendet und weiterentwickelt. Die turbulente Luftströmung im Schaltschrank wird mit dem SST-Modell modelliert, wobei zwei unterschiedliche Produktionsterme für Auftrieb untersucht werden. Neben Randbedingungen für die Klimatechnik werden Wandfunktionen für erzwungene Konvektionsströmungen implementiert. Zur Berechnung der Wärmestrahlung wird ein Sichtfaktormodell verwendet. Die hierfür erforderlichen Sichtfaktoren werden mit Hilfe eines neu-entwickelten Monte-Carlo-Verfahrens ermittelt. Um die Genauigkeit und die numerischen Eigenschaften der Sichtfaktormatrix zu verbessern, wird ein Glättungsverfahren implementiert. Zur Validierung der Teilmodelle werden Testfälle aus der Literatur verwendet, wobei eine gute Übereinstimmung erzielt wird. Durch einen Vergleich mit Temperaturmessdaten, die an verschiedenen Positionen in einem Laborprüfstand erfasst werden, wird das Gesamtmodell verifiziert. Es wird dabei sowohl der Betrieb bei freier Kühlung als auch der Betrieb mit Klimatechnik untersucht. Die maximale Abweichung zwischen Messungen und Simulationen liegt im Bereich von 3.6 K. Es zeigt sich, dass beim untersuchten Schaltschrank bei freier Kühlung ca. 50 % des Wärmestroms von den elektronischen Bauteilen durch Strahlung übertragen wird. Um den Schaltschrank-Betrieb energetisch zu optimieren, wird neben den Strömungsgrößen die lokale Entropieproduktion im Schaltschrank untersucht. Die Gleichungen für die lokale Entropieproduktion durch irreversible Wärmeleitung und Dissipation mechanischer Energie werden der Literatur entnommen und in OpenFOAM implementiert. Die Gleichungen für die Entropieproduktion durch Wärmestrahlung werden für das Sichtfaktormodell hergeleitet und ebenfalls implementiert. Anhand von Betriebssituationen mit und ohne Klimatechnik werden Optimierungspotentiale aufgezeigt und dadurch der praktische Nutzen der Methodik demonstriert. Es zeigt sich, dass die lokale Entropieproduktion einen tiefen Einblick in die Strömungs- und Wärmeübertragungsprozesse ermöglicht und dadurch wertvolle Informationen liefern kann.Item Open Access Experimental analysis of the influence of elevated operating pressures on Solid Oxide Cell stacks during steam, co- and CO2 electrolysis(2022) Riedel, Marc; Friedrich, K. Andreas (Prof. Dr. rer. nat.)Item Open Access Modeling and simulation of closed low-pressure adsorbers for thermal energy storage(2019) Schäfer, Micha; Thess, André (Prof. Dr. rer. nat.)Closed low-pressure adsorption systems can be applied for thermal energy storage. Their performance is determined by the mass and heat transport processes in the adsorber. Therefore, thorough knowledge of these transport processes is required for further storage development. The present thesis contributes to this by providing detailed models of closed low-pressure adsorbers and by conducting simulations over a broad range of parameters and configurations. The focus is on adsorbers of larger scale (length L = 0.1 . . . 1 m) and on the discharging process. As the adsorption pair, binderless zeolite 13X with water is examined. The models are developed in a stepwise manner from pore to storage scale. The Finite-Difference-Method is implemented to numerically solve the models. Simulations are conducted for defined reference cases as well as over a broad range of geometric and process parameters. The reference cases are analyzed in detail to gain a better understanding of the transport processes. Furthermore, the results are analyzed with respect to two particular modeling aspects: equilibrium assumptions and rarefaction effects (e. g. slip effect). With respect to the application, the discharging performance is analyzed in terms of thermal power and a defined discharging degree. Both the adsorber and the adsorbent configurations are varied. In addition, the effect of the discharging conditions is evaluated. Finally, one exemplary charging process is examined. The detailed analysis of the reference cases reveals that the mass and heat transport and the adsorption processes are strongly coupled and can only be understood in their interaction. For onedimensional adsorber configurations, that is the mass and heat transport are in the same direction, the discharging process is generally limited by the heat transport. This leads to insufficient thermal power and unsuitable discharging durations of up to one year. In contrast, for two-dimensional adsorber configurations, that is the mass and heat transport are in perpendicular directions, the discharging process can be limited either by the mass or heat transport or by the adsorption. The limitation depends on the configuration of the adsorber and adsorbent. Moreover, the twodimensional adsorber configurations can provide sufficient thermal power. With respect to the modeling, it is found that the assumption of a uniform pressure distribution is applicable for one-dimensional adsorber configurations. In contrast, for two-dimensional configurations, no equilibrium assumptions can be applied in general. However, for powder adsorbent it is always valid to assume local adsorption equilibrium. Regarding the rarefaction effects in twodimensional adsorber configurations with honeycombs and granules, the slip effect is relevant for small channel and particle diameters (d = 1 mm). For adsorbers with powder adsorbent, the reduction of the effective heat conductivity due to the rarefaction effect becomes relevant. With respect to the application, the variation of the adsorber configuration shows that the volumetric thermal power generally decreases with increasing adsorber length. Furthermore, the power decreases with increasing width between the parallel heat exchanger plates in the adsorber. Regarding the adsorbent configuration in two-dimensional adsorber configurations, it is found that the volumetric thermal power can be optimized by variation of the channel or particle diameter. Interestingly, the optima for peak and mean power do not coincide. In addition, the discharging degree is found to strongly depend on the discharging conditions in terms of discharging temperature and volume flow of the heat transfer fluid extracting the heat from the adsorber. In general, the discharging degree decreases with increasing discharging temperature. Similarly, the discharging degree decreases with increasing volume flow of the heat transfer fluid. Finally, the analysis of an exemplary charging process revealed that the pressure in the adsorber can increase significantly (> 50%) due to the desorption.Item Open Access A metal hydride air-conditioning system for fuel cell vehicles(2020) Weckerle, Christoph; Thess, André (Prof. Dr.)High-pressure tanks are the established hydrogen storage technology for automotive systems. However, around 15% of the lower heating value of hydrogen is required for compression up to a pressure of 700 bar. Since this energy is available onboard but has been wasted so far, an “open” cooling system based on metal hydrides (MHs) is a promising way to utilize the potential energy of compressed hydrogen. The thesis presents the systematic investigation of a first-of-its-kind system and a demonstration of the extent to which this energy can be transformed into useful cold. For this purpose, an experimental setup is built that consists of two novel plate reactors coupled to a polymer electrolyte membrane fuel cell (FC). The reactors with an optimized heat transfer characteristic and an average heat transfer distance in the MH bed of 0.44 mm are filled with around 1.5 kg of Hydralloy® C2 (Ti0.98Zr0.02Mn1.46V0.41Cr0.05Fe0.08), which is thermodynamically characterized in the temperature range of 0-50 °C. The functional demonstration at an electrical power of 5 kW shows that the FC operation is not affected by the alternately H2-desorbing reactors with a half-cycle duration of 145 s. Hydrogen is absorbed at a pressure of 35 bar and a continuous flow rate is released at an FC backpressure of 4.1 bar. Under reference conditions for an ambient temperature of 30 °C and a cooling temperature of 20 °C, around 45% of the as-yet-unexploited potential energy of hydrogen at 700 bar can be utilized by generating a cooling effect. A novel operation optimization of time-shifted valve switching increases the performance by more than 50% compared to the case without its implementation. Based on reference conditions, extensive performance investigations are performed while varying the key influencing parameters: the electrical FC power and the operating temperatures. The variation of the electrical FC power between 1.8 and 7.9 kW results in a maximum average cooling power of 807 W at an electrical power of 7 kW, reaching a specific cooling power of 564 W kgMH-1 referred to the MH mass of a single reactor. The performance decreases with rising ambient temperatures (varied in the range of 24.3-42.3 °C) and decreasing cooling temperatures (varied in the range of 13-25.4 °C) due to increased thermal losses and reduced half-cycle times. To further improve the performance, the plate reactor is numerically investigated and optimization recommendations are given. The validated model shows that an increase of the cooling power is obtained by reducing the distance of the hydrogen gas transport, the porosity of the MH bed and the FC backpressure. For this optimized system design, related to the maximum obtainable cooling power of 18.3% of the electrical FC power, cooling efficiencies above 60% are feasible even in harsher operating conditions. As an innovative “hydrogen pressure transducer”, the system can be transferred to all applications where a hydrogen pressure difference is available.Item Open Access Physical modelling of DMFC performance heterogeneities and the recovery of reversible cathode degradation(2022) Fischer, Marie-Dominique; Friedrich, K. Andreas (Prof. Dr.)Direct methanol fuel cells (DMFC), which are an alternative power source to batteries and diesel engines, exhibit a great potential for a locally heterogeneous cell performance. The DMFC anode is fed with a liquid methanol-water mixture while the cathode is supplied with air, which results in an even more complex fluid management in comparison with the structurally similar polymer electrolyte fuel cell (PEMFC) operated with hydrogen as fuel. The transfer of water and methanol from the anode through the polymer electrolyte membrane to the cathode side is an important factor for limits in the cell performance. The crossover of water from the anode to the cathode side, where water is also produced in the electrochemical reaction, increases the risk of liquid accumulation in the porous layers of the cathode and thus mass transport limitations in the cell. Methanol crossover leads to the formation of a mixed potential in the DMFC cathode, and the resulting high overpotential increase the development of oxide species on the platinum catalyst surface. These processes lead to a reduction of the cell performance, which is partially reversible. In this work, a physics-based DMFC model in 2D is developed in order to study the local cell performance along the channel with a focus on the two-phase flow as well as humidity-related properties of the ionomer. The model features a spatial resolution of the catalyst layers, which enables the examination of the local conditions' impact on the electrochemical reactions and on effects at the membrane interface. The model is verified against experimental data from a macro-segmented DMFC single cell for two different humidity levels in the cathode. The validation not only comprises the local cell performance, but also mass transport and the ohmic resistance of the membrane. Simulation results for the cell performance under varying operating conditions are shown in comparison with corresponding experimental data, proving the predictiveness of the model. The transient model is further used to study the processes inside the cell during the recovery of reversible degradation effects in the cathode. The formation of platinum oxide species during DMFC operation and their reduction during a refresh sequence including an OCV phase as well as a phase with air starvation is simulated and explored with respect to the local conditions inside the cathode catalyst layer. Moreover, the simultaneously occurring spontaneous evolution of hydrogen in the DMFC anode is examined. Several variations of the air stop sequence are simulated and evaluated with regard to their effectiveness in recovering the temporary performance losses within the DMFC cathode.