Charakterisierung verschiedener Designs autarker Direkt-Methanol-Brennstoffzellen-Systeme für portable Anwendungen

Most people, who use portable electric devices such as laptops, are experiencing a lack of energy in batteries for portable applications. To compensate or substitute batteries, fuel cells have been suggested for several decades. Among fuel cells, polymer electrolyte fuel cells including direct methanol fuel cells are the most probable type due to the simplicity of systems. Even if the systems are simple compared to other fuel cells, they are still quite heavy and large to carry. In this work, first of all, the reference system, which has each component to carry out a single function, is reviewed for steady state analysis and dynamic behaviour with simulations. Second, the models for steady state and dynamic analysis are validated with experiments at various environmental conditions. A fuel cell system was installed in a climate chamber, and operated autonomously - without additional water supply. Water accumulation rate and feasibility envelope for autonomous operation are compared with simulation results. Dynamic behaviour of the reference system is investigated with dynamic current load as disturbance to controllers such as temperature, concentration or water recovery. Third, to increase faradaic efficiency of systems, dynamic concentration control algorithms are employed. The layout of the reference system is modified to build a two-mixer system with an in-line static mixer to adjust concentration quickly. A further modified design, separate tank system, which is equipped with separate tanks for water and methanol solution respectively, is examined. The separate tank system is able to increase or decrease the methanol concentration immediately. In the experiment, the two-mixer system and the separate tank system are found out to have higher faradaic efficiency than the reference system. However, the additional pump and mixer make the system heavier and bigger than the reference system. Fourth, for a simple and compact system, anode and cathode outlet are integrated into a mingled outlet process to one combined heat exchanger in the system. But it loses a significant amount of gaseous methanol in a gas-liquid separator after mingled outlet, which decreases fuel efficiency. To reduce methanol loss, a dynamic concentration controller was implemented into the highly-integrated system. The highly-integrated system is equipped with an integrated separator which combines mixer and separator. In spite of its compact size, the highly-integrated system is revealed to have higher efficiency than the mingled-outlet system due to dynamic concentration control. To sum up the five different designs, two highly efficient systems and two integrated systems are investigated systematically with models and validation. These simulation and experimental results can guide to design optimal systems for high efficiency or compact size as portable power sources.Heutzutage ist die Verwendung tragbarer elektronischer Geräte, wie Laptops, sehr verbreitet. Dort eingesetzte Batterien weisen jedoch immer noch einen sehr niedrigen Energiegehalt auf. Um dies zu kompensieren, wurden seit mehreren Jahrzehnten Brennstoffzellen als Alternative vorgeschlagen. Wegen der Einfachheit des Systemaufbaus eignen sich hauptsächlich Polymer-Elektrolyt-Brennstoffzellen. Obwohl deren Systemaufbau im Vergleich zu anderen Brennstoffzellen einfach ist, sind die Systeme jedoch immer noch groß und schwer. In der vorliegenden Arbeit wurde als erster Schritt ein Referenzsystem, bei dem jede Komponente eine einzelne Funktion erfüllt, anhand von Simulationen im Hinblick auf das stationäre und das dynamische Verhalten untersucht. Die Modelle für die stationären und dynamischen Analysen konnten anhand von Experimenten bei verschiedenen Umgebungsbedingungen validiert werden. Ein Schwerpunkt lag dabei auch auf autarkem Betrieb, insbesondere hinsichtlich des Betriebs ohne externe Wasser zu-und abfuhr. Die Wasserakkumulationsrate und der dazugehörige autarken Betriebsbereich konnten von erfolgreich in den Simulationsergebnissen reproduziert werden. In einem weiteren Teil wurden verbesserte Regelalgorithmen entwickelt, um die Faradaysche Effizienz der Systeme zu erhöhen. Auch wurde das Layout des Referenzsystems geändert zu einem Zwei-Mixer-System mit einem Inline-Mixer, um Konzentrationen schnell anpassen zu können. Ein weiteres, modifiziertes Design, das separate Tanksystem, ausgestattet mit separaten Tanks für Wasser und für Methanollösung, wurde entworfen und untersucht. Das separate Tanksystem ist in der Lage, die Konzentration des Methanols nicht nur sofort zu erhöhen sondern auch zu verringern. Experimentelle Ergebnisse zeigten, dass das Zwei-Mixer-System und das separate Tanksystem höhere Faradysche Wirkungsgrade aufweisen als das Referenzsystem. Schließlich wurden für ein besonders kompaktes System (vermischte Ausganssystem) die Anoden- und Kathodenausgänge zusammengeschaltet und über einen gemeinsamen Wärmeübertrager und Gas-Flüssigkeits-separator geleitet. Um den Verlust an Methanol im Gas-Flüssigkeits-separator zu verringern, wurde daher ein dynamischer Konzentrationsregler für das hoch-integrierte System entwickelt. Dieses hoch-integrierte System wurde zusätzlich mit einem integrierten Separator ausgestattet, der die Funktionen eines Mixers und eines Separators verbindet. Dieses System zeigt wegen der dynamischen Konzentrationskontrolle trotz seiner kompakten Größe eine höhere Effizienz als das System ohne integrierten Separator. Die hier erarbeiteten Simulationen und experimentellen Ergebnisse können als Leitlinien dienen, um zukünftige portable oder auch mobile Brennstoffzellensysteme mit hoher Effizienz oder kompakter Größe zu entwerfen

Effect of Gravity and Various Operating Conditions on Proton Exchange Membrane Water Electrolysis Cell Performance

Water electrolysis is an eco-friendly method for the utilization of renewable energy sources which provide intermittent power supply. Proton exchange membrane water electrolysis (PEMWE) has a high efficiency in this regard. However, the two-phase flow of water and oxygen at the anode side causes performance degradation, and various operating conditions affect the performance of PEMWE. In this study, the effects of four control parameters (operating temperature, flow rate, cell orientation, and pattern of the channel) on the performance of PEMWE were investigated. The effects of the operating conditions on its performance were examined using a 25 cm2 single-cell. Evaluation tests were conducted using in situ methods such as polarization curves and electrochemical impedance spectroscopy. The results demonstrated that a high operating temperature and low flow rate reduce the activation and ohmic losses, and thereby enhance the performance of PEMWE. Additionally, the cell orientation affects the performance of PEMWE owing to the variation in the two-phase flow regime. It was observed that the slope of specific sections in the polarization curve rapidly increases at a specific cell voltage

Performance Comparison of Proton Exchange Membrane Water Electrolysis Cell Using Channel and PTL Flow Fields through Three-Dimensional Two-Phase Flow Simulation

Water electrolysis technology is required to overcome the intermittency of renewable energy sources. Among various water electrolysis methods, the proton exchange membrane water electrolysis (PEMWE) cell has the advantages of a fast response and high current density. However, high capital costs have hindered the commercialization of PEMWE; therefore, it is important to lower the price of bipolar plates, which make PEMWE expensive. In addition, since the flow field inscribed in the bipolar plate significantly influences the performance, it is necessary to design the enhanced pattern. A three-dimensional two-phase flow model was used to analyze the two-phase flow and electrochemical reactions of the PEMWE anode. In order to compare the experimental results with the simulation, experiments were conducted according to the flow rate, and the results were in good agreement. First, as a result of comparing the performance of the channel and PTL (porous transport layer) flow fields, the channel flow field showed better performance than the PTL flow field. For the channel flow field, the higher the ratio of the channel width-to-rib width and the permeability of PTL, the performance got better. In the case of the PTL flow field, with the increased capillary pressure, the performance improved even if the PTL permeability decreased. Next, the direction of gravity affected the performance only when the channel flow field was used, and the X+ and Z+ directions were optimal for the performance. Finally, increasing the inlet flow rate could reduce the difference in performance between the channel and PTL flow fields, but the pressure drop gradually increased

Increasing Fuel Efficiency of Direct Methanol Fuel Cell Systems with Feedforward Control of the Operating Concentration

Most of the R&D on fuel cells for portable applications concentrates on increasing efficiencies and energy densities to compete with other energy storage devices, especially batteries. To improve the efficiency of direct methanol fuel cell (DMFC) systems, several modifications to system layouts and operating strategies are considered in this paper, rather than modifications to the fuel cell itself. Two modified DMFC systems are presented, one with an additional inline mixer and a further modification of it with a separate tank to recover condensed water. The set point for methanol concentration control in the solution is determined by fuel efficiency and varies with the current and other process variables. Feedforward concentration control enables variable concentration for dynamic loads. Simulation results were validated experimentally with fuel cell systems

Minimizing Specific Energy Consumption of Electrochemical Hydrogen Compressor at Various Operating Conditions Using Pseudo-2D Model Simulation

With the increased usage of hydrocarbon-based fossil fuels, air pollution and global warming have accelerated. To solve this problem, renewable energy, such as hydrogen technology, has gained global attention. Hydrogen has a low volumetric density and thus requires compression technologies at high pressures to reduce storage and transportation costs. Techniques for compressing hydrogen include using mechanical and electrochemical hydrogen compressors. Mechanical compressors require higher specific energy consumption than electrochemical hydrogen compressors. Here, we used an electrochemical hydrogen compressor as a pseudo-two-dimensional model focused on electroosmotic drag, water back-diffusion, and hydrogen crossover flux at various temperatures, polymer electrolyte membrane thicknesses, and relative humidity conditions. To date, there have been few studies based on various operating conditions to find the optimal conditions. This study was conducted to determine the optimal parameters under various operating conditions. A numerical analysis demonstrated that the specific energy consumption was low in a specific current density section when the temperature was decreased. At the above-mentioned current density, the specific energy consumption decreased as the temperature increased. The polymer electrolyte membrane thickness yielded similar results. However, according to the relative humidity, it was confirmed that the higher the relative humidity, the lower the specific energy consumption in all of the current density sections. Therefore, when comparing temperatures of 30 °C and 80 °C at 145 A/m2, operating at 30 °C reduces the specific energy consumption by 12.12%. At 3000 A/m2 and 80 °C, the specific energy consumption is reduced by 11.7% compared to operating at 30 °C. Using N117 compared to N211 at 610 A/m2 for polymer electrolyte membranes can reduce specific energy consumption by 10.4%. Using N211 in the 1500 A/m2 condition reduces the specific energy demand by 9.6% compared to N117

Performance Analysis of Polymer Electrolyte Membrane Water Electrolyzer Using OpenFOAM®: Two-Phase Flow Regime, Electrochemical Model

In this study, an electrochemical model was incorporated into a two-phase model using OpenFOAM® (London, United Kingdom) to analyze the two-phase flow and electrochemical behaviors in a polymer electrolyte membrane water electrolyzer. The performances of serpentine and parallel designs are compared. The current density and overpotential distribution are analyzed, and the volume fractions of oxygen and hydrogen velocity are studied to verify their influence on the current density. The current density decreases sharply when oxygen accumulates in the porous transport layer. Therefore, the current density increased sharply by 3000 A/m2 at an operating current density of 10,000 A/m2. Maldistribution of the overpotential is also observed. Second, we analyze the behaviors according to the current density. At a low current density, most of the oxygen flows out of the electrolyzer. Therefore, the decrease in performance is low. However, the current density is maldistributed when it is high, which results in decreased performance. The current density increases abruptly by 12,000 A/m2. Finally, the performances of the parallel and serpentine channels are analyzed. At a high current density, the performance of the serpentine channel is higher than that of the parallel channel by 0.016 V

Comparative Analysis of On-Board Methane and Methanol Reforming Systems Combined with HT-PEM Fuel Cell and CO2 Capture/Liquefaction System for Hydrogen Fueled Ship Application

This study performs energetic and exergetic comparisons between the steam methane reforming and steam methanol reforming technologies combined with HT-PEMFC and a carbon capture/liquefaction system for use in hydrogen-fueled ships. The required space for the primary fuel and captured/liquefied CO2 and the fuel cost have also been investigated to find the more advantageous system for ship application. For the comparison, the steam methane reforming-based system fed by LNG and the steam methanol reforming-based system fed by methanol have been modeled in an Aspen HYSYS environment. All the simulations have been conducted at a fixed Wnet, electrical (475 kW) to meet the average shaft power of the reference ship. Results show that at the base condition, the energy and exergy efficiencies of the methanol-based system are 7.99% and 1.89% higher than those of the methane-based system, respectively. The cogeneration efficiency of the methane-based system is 7.13% higher than that of the methanol-based system. The comparison of space for fuel and CO2 storage reveals that the methanol-based system requires a space 1.1 times larger than that of the methane-based system for the total voyage time, although the methanol-based system has higher electrical efficiency. In addition, the methanol-based system has a fuel cost 2.2 times higher than that of the methane-based system to generate 475 kW net of electricity for the total voyage time

A robust methanol concentration sensing technique in direct methanol fuel cells and stacks using cell dynamics

The electrochemical behaviour of direct methanol fuel cells (DMFCs) is sensitive to methanol concentration; thus, to avoid external sensors, it is a promising candidate to monitor the concentration of methanol in the fuel circulation loop, which is central to the efficient operation of direct methanol fuel cell systems. We address this issue and report on an extremely robust electrochemical methanol sensing technique that is not sensitive to temperature, cell degradation and membrane electrode assembly (MEA) type. We develop a temperature independent empirical correlation of the dynamic response of cell voltage to step changes in current with methanol concentration. This equation is successfully validated under various operating scenarios at both the single cell and stack levels. Our sensing method achieves an impressive accuracy of ±0.1 M and this is expected to increase the reliability of methanol sensing and simplify the control logic of DMFC systems

Numerical Study of the Action of Convection on the Volume and Length of the Flammable Zone Formed by Hydrogen Emissions from the Vent Masts Installed on an International Ship

International ships carrying liquefied fuel are strongly recommended to install vent masts to control the pressure of cargo tanks in the event of an emergency. However, the gas emitted from a vent mast may be hazardous for the crew of the ship. In the present study, the volume and length of the flammable zone (FZ) created by the emitted gas above the ship was examined. Various scenarios comprising four parameters, namely, relative wind speed, arrangement of vent masts, combination of emissions among four vent masts, and direction of emission from the vent-mast outlet were considered. The results showed that the convection acts on the volume and length of an FZ. The volume of an FZ increases when there is a reduction in convection reaching the FZ and when strong convection brings hydrogen from a nearby FZ. The length of the FZ is also related to convection. An FZ is elongated if the center of a vortex is located inside the FZ, because this vortex traps hydrogen inside the FZ. The length of an FZ decreases if the center of the vortex is located outside the FZ, as such a vortex brings more fresh air into the FZ