Search

Hybrid electric aircraft offer the potential to decrease emissions from air travel. A new hybrid concept is proposed for a fuel cell-battery hybrid aircraft. In contrast to existing hybrids, the proposed concept puts a battery directly on the AC phases of the motor, which together with a suitable switching circuit superimposes the DC voltage from the battery on the AC voltage of the motor phase providing a voltage boost depending on the battery voltage, which can be used during a high-power demand flight phase. The system is also capable of recharging the battery during flight. The necessary switching architecture was developed and modeled in MATLAB/Simulink to verify the concept and an experimental setup was built for demonstrating the functionality. Simulation and experimental results showed a very good agreement which is very promising for the proposed new hybrid topology., publishedVersion

To test single electronic components and complete hybrid-electric drive trains for airplane and other mobile applications under realistic operating conditions a climate chamber was set up at the Institute for energy conversion and storage. The chamber can simulate climatic conditions corresponding to flight altitudes up to 5500 m with temperatures from -15 to 40 ��C, pressures between 0.5 and 1 bar(a) and relative humidities from 10 to 100 %. A continuous stream of fresh air can be supplied to the chamber in order to facilitate measurements on fuel cells. The chamber is being used primarily for the characterization of electric motors, power electronics, power distribution systems and compressors., submittedVersion

Providing electrical energy with a reduced CO2 footprint and in a sustainable way is a significant challenge for the future. Therefore the research community is intensively studying more effective ways to provide electricity to consumers. In this respect a hybrid power plant, a combination of SOFC and gas turbine, is highly attractive and a research prototype is being developed at the German Aerospace Center. The hybrid power plant consists of a pressurized solid oxide fuel cell (SOFC) and a small gas turbine. It has the possibility to provide electrical energy with high electrical efficiencies. In the hybrid power plant the SOFC will be operated at elevated pressure which enhances the electrical power output of the SOFC. The increased SOFC cathode pressure is maintained by the gas turbine compressor. Pressure differences between the two electrode volumes lead to mechanical stresses and may lead to failure of the SOFC. The durability of the SOFC stack related to pressure differences between anode and cathode compartment was investigated with a planar two layer SOFC stack at operating temperature of 850 °C. The tests were carried out with stationary pressure changes to determine operational limits and potential reasons for failure. The testing procedure and the results will be explained and discussed.

Solid Oxide Fuel Cells (SOFC) and gas turbines can be coupled directly to form efficient hybrid power plants providing electricity based on various fuels. To ensure stable and efficient system operation the different operating characteristics of the major components SOFC and gas turbine require careful consideration of component design and operating strategy. The DLR undertakes efforts to erect and operate a pilot hybrid power plant in the range of 30 kW of electrical power. Technical feasibility is to be demonstrated and the operating and design parameters for proper system operation are to be determined. The SOFC system pressure management is considered very important from SOFC perspective. Pressure variations originating from normal gas turbine operation might harm or even destroy the SOFC stack. If considering start-up or shut-down procedures or flame-out the situation becomes even more critical. Since very little information is available on acceptable pressure differences for SOFC stacks, a test rig for differential pressure test was set up and operated. Several stacks have been pressurized up to their mechanical failure level with overpressure on anode and cathode side and in stationary and transient operation. The presentation will illustrate the general concept of the test rig and summarize the test results as well as identify potential reasons for pressure sensibility and stack failure.

Fuel cells offer high electrical efficiencies and the solid oxide fuel cell is particularly interesting because easily accessible fuels containing hydrocarbons like natural gas and reformates can be used. Nevertheless the desired high efficiency and high fuel utilisation lead to strong gradients in gas composition along the cell which can in turn lead to lower power densities and increased local degradation. To better understand the local distribution and its effects a combined experimental and modelling study was done. Segmented planar anode-supported cells were characterised in a setup with 4 x 4 segments and a detailed two-dimensional model representing this setup was developed and validated. The model was calibrated for hydrogen/nitrogen mixtures with various water contents as well as for reformate gases and internal reforming of methane. A very good agreement between the model and the experiments was achieved. Measurements with CO/CO2 were done to validate the CO oxidation in the model. Here the influence of very high fuel dilution could also be observed that led to a plateau-like behaviour in the current-voltages curves. This behaviour was attributed to the oxidation of nickel at the anode. Methane reforming leads to an additional variation in gas phase species along the flow path. A great part of the reforming process takes place at the nickel contact mesh and not at the anode of the cell which could be seen in comparison to experiments with a non-catalytic contact mesh. It was also shown that a dilution of the fuel with water or nitrogen can lead to a more homogenous distribution within the cell depending on control parameters. The model was used to asses the influence of cell parameters. A variation of the gas channels did not show a significant influence. A variation of the electrode thickness showed an increase of power density for thinner electrodes. The model was also used to asses the effect of the segmentation in comparison to a non-segmented cell. A non-segmented cell shows a more even distribution of voltage and a greater variance of current density while the gas composition along the flow path is similar for segmented and unsegmented cells. The validated model can be used further to estimate optimal and critical operating parameters. Through a variation of fuel flow rate and temperature the influence on gas composition along the cell and differences in fuel distribution at the side and in the middle of the flow field could be seen as well as its influence on cell performance. Higher load and higher temperatures lead to a stronger decrease of fuel along the cell. The segments located at the side of the cell showed a different performance than those in the middle, due to an unequal distribution of the fuel into the flow field. This influence increases with lower flow rates. During these experiments a correlation between poor local performance and electrolyte defects as well as local nickel oxidation at the anode was observed. It was also shown that extreme local conditions can lastingly damage the cell and local distributions have to be taken into account for stack development., Festelektrolytbrennstoffzellen haben hohe elektrische Wirkungsgrade und bieten zudem den Vorteil, dass Kohlenwasserstoffe z. B. aus Erdgas oder Reformatgasen als Brenngase verwendet werden können. Diese Brenngase haben den Vorteil, dass die Verfügbarkeit hoch ist. Allerdings führen die erwünschten hohen Wirkungsgrade und Brenngasnutzungen zu starken Gradienten in der Brenngaszusammensetzung entlang der Zelle, was wiederum zu niedrigen Leistungsdichten und erhöhter lokaler Degradation führen kann. Um die lokale Verteilung und ihre Auswirkungen besser zu verstehen, wurde in dieser Arbeit ein kombinierter Ansatz aus Experimenten und Modellierung gewählt. Segmentierte, planare, anodengetragene Zellen wurden in einem Aufbau mit 4 x 4 Segmenten charakterisiert und ein detailliertes zweidimensionales Modell dieses Aufbaus entwickelt und validiert. Das Modell wurde für Wasserstoff/Stickstoff Mischungen mit verschiedenen Wassergehalten sowie für Reformatgase und interne Reformierung von Methan kalibriert. Eine gute Übereinstimmung zwischen Modell und Experiment wurde erreicht. Messungen mit CO/CO2 wurden durchgeführt um die CO Oxidation im Modell zu validieren. Dabei konnte auch der Einfluss einer starken Verdünnung des Brenngases beobachtet werden, was zu einem Plateau in der Strom-Spannungskurve führte. Dieses Verhalten wurde auf die Oxidation von Nickel an der Anode zurückgeführt. Die interne Reformierung von Methan führt zu einer zusätzlichen Änderung der Gaszusammensetzung entlang des Strömungswegs. Ein Großteil der Reformierung findet am Nickelkontaktnetz und nicht an der Anode selbst statt. Dies konnte im Vergleich mit Versuchen mit einem nicht-katalytischen Kontaktnetz beobachtet werden. Es wurde außerdem gezeigt, dass eine Verdünnung des Brenngases mit Wasser oder Stickstoff je nach Kontrollparameter zu einer gleichmäßigeren Verteilung entlang der Zelle führen kann. Das Modell wurde weiterhin verwendet, um den Einfluss verschiedener Zellparameter zu untersuchen. Eine Änderung der Größe der Gaskanäle zeigte keine signifikanten Auswirkungen. Die Änderung der Elektrodendicke führte zu einer höheren Leistungsdichte für dünnere Elektroden. Das Modell wurde außerdem verwendet, um den Einfluss der Segmentierung im Vergleich mit einer nicht-segmentierten Zelle zu untersuchen. Eine nicht-segmentierte Zelle weist eine gleichmäßigere Verteilung der Spannung und einen stärkeren Gradienten in der Stromdichte aus, während die Gaszusammensetzung entlang des Strömungsweg für segmentierte und nicht-segmentierte Zellen ähnlich ist. Das validierte Model kann für weitere Untersuchungen der optimalen und kritischen Betriebsbedingungen verwendet werden. Durch die Variation der Brenngasflussrate und der Temperatur wurde der Einfluss auf die Gaszusammensetzung entlang der Zelle, Unterschiede in der Gasverteilung am Rand und in der Mitte des Strömungsfeldes, sowie der Einfluss auf die Zellleistung sichtbar. Eine größere Last und höhere Temperaturen führen zu einer stärkeren Abnahme des Brenngases entlang der Zelle. Die Segmente an der Seite der Zelle zeigten aufgrund einer ungleichen Gasverteilung ein anderes Verhalten als die in der Mitte. Dieser Unterschied nimmt mit sinkenden Durchflüssen zu. Während dieser Versuche wurde ein Zusammenhang zwischen niedriger lokaler Leistung, Elektrolytdefekten und lokaler Nickeloxidation in der Anode beobachtet. Es wurde außerdem gezeigt, dass extreme lokale Bedingungen wie lokal negative Spannungen die Zelle dauerhaft schädigen und lokale Verteilungen von Strom, Spannung und Brenngaszusammensetzung bei der Stackentwicklung berücksichtigt werden müssen.

The continuously increasing demand for electrical energy requires the development of power plants with high efficiencies and low emissions. A hybrid power plant consisting of a solid oxide fuel cell (SOFC) system coupled with a gas turbine (GT) promises to fulfill these requirements and also has the advantage of offering a wide range of applications from several 10 kW to multi MW. Indirectly and directly coupled systems are possible, mainly distinguished by the higher operating pressure and reduced heat transfer losses of the directly coupled SOFC system. Since the directly coupled systems with elevated SOFC operating pressure promise to outperform the system efficiency of the indirectly coupled systems [1-2], the DLR has been investigating the corresponding fundamentals and requirements of a directly combined fuel cell and gas turbine power plant for several years. An elementary kinetic SOFC stack model was developed and experimentally validated [3] in a wide range of operating conditions. This model was integrated into an existing model of a gas turbine [4]. The model is used to carry out stationary hybrid power plant simulations in order to investigate how variations in operating strategy and component characteristics influence the performance of the hybrid power plant as well as the operating conditions of the SOFC. Results show that varying electrical SOFC power while keeping electrical gas turbine power constant strongly influences the overall electrical efficiency of the hybrid power plant. The experimental and simulation results are to be used to adapt the existing models from stationary to transient regime. This allows for future development of operation and control strategies. A pilot power plant is to be designed and commissioned to validate the transient model accordingly. [1] Henke, M., Willich, C., Westner, C., Leucht, F., Leibinger, R., Kallo, J., & Friedrich, K. A. (2012). Effect of pressure variation on power density and efficiency of solid oxide fuel cells. Electrochimica Acta, 66, 158–163. [2] Zhang, X., Chan, S. H., Li, G., Ho, H. K., Li, J., & Feng, Z. (2010). A review of integration strategies for solid oxide fuel cells. Journal of Power Sources, 195(3), 685–702. [3] M. Henke et al., Modeling and Experimental Study of Pressurized Solid Oxide Fuel Cells, ModVal 9, Campus Sursee, Switzerland 2012 [4] T. Panne et al., Themodynamic Process Analyses of SOFC/GT Hybrid Cycles, 5th IECEC, St. Louis, Missouri, USA, 2007

With the increase in installed renewable power in Europe and especially in Germany, the need for energy storage becomes urgent. One interesting option due to its high energy density is the generation of hydrogen from renewable electricity using electrolysis. Produced hydrogen can either be sold as fuel for mobile applications or it can be stored in existing infrastructure. One major storage capability is the natural gas pipeline network in which a huge amount of energy can be stored and distributed based on already available infrastructure. Stored hydrogen could be converted into electricity on demand. Low temperature fuel cells are ideally suited for operation with pure hydrogen but not suited for hydrogen/natural gas mixtures. In these conditions high temperature SOFC with high efficiency and fuel tolerance are the much better choice. DLR has been investigating fuel cells and fuel cell systems for many years - ranging from small portable devices to large aircraft onboard power supply systems in the 30 kW range. In stationary fuel cell systems hybrid power plants combining fuel cells and gas turbines are one major interest of the institute. This publication focuses on the effect of hydrogen rich natural gas mixtures on pressurized SOFC systems. Rising hydrogen contents reduce system efficiency for several reasons. E.g. internal reforming acts as a heat sink, with less natural gas to reform, the cells have to be cooled in another way. This leads to higher parasitic losses due to higher air supply. Traditional system control requires information on the heating value of the fuel gas, which is set manually. With changing hydrogen content in the fuel, online information for fuel flow control becomes important. In this context we will discuss several control possibilities from a system perspective. All results presented are based on simulation results for a 100 kW class pressurized SOFC system published earlier [1], which was developed for the hybrid power plant project at DLR.

The increasing demand for electrical energy requires the development of power plants with high efficiencies and low emissions. Hybrid power plants consisting of an SOFC system coupled with a gas turbine promise to fulfil both requirements reaching electrical efficiencies above 60% [1]. They can also be used for a large range of applications from several 10 kW to multiple MW and can use natural gas from the existing grid. The German Aerospace Center (DLR) is aiming at the demonstration of the stable operation of a hybrid power plant in the 50 kW class in which the SOFC will be operated at elevated pressures. For the realisation of the power plant the effects of pressure on SOFC has to be understood, therefore the behaviour of SOFC at pressures ranging from 1 to 8 bar with varying fuel gas composition (H2/N2 mixture and reformate gases with different methane content) and different temperatures was examined experimentally in a specially designed setup [2]. Current-voltage characteristics and gas analysis at the anode were performed to examine the influence of pressure on OCV, power density at constant voltage and constant current as well as on gas composition at the cell for different fuels. Power density increases of up to 70% were found. Simulations were used to better understand the experimentally obtained results.

Für eine optimale Leistungsfähigkeit von Brennstoffzellen und möglichst geringe Degradation ist eine homogene elektrochemische Aktivität und Temperatur über den gesamten Bereich der Elektroden erwünscht, da eine inhomogene Stromdichte- und Temperaturverteilung zu einer verringerten Nutzung der Reaktanden führt, was sich in einem erniedrigten Wirkungsgrad niederschlägt. Auch die Langzeitstabilität von Zellkomponenten kann durch die ungleichmäßige Verteilung der elektrischen und thermischen Eigenschaften negativ beeinflusst werden. In-situ diagnostische Methoden können wertvolle Informationen liefern, um das Betriebsverhalten von SOFC-Zellen zu optimieren und Strategien zur Verminderung der Degradation zu entwickeln. Trotz der beträchtlichen experimentellen Schwierigkeiten, die mit der hohen Betriebstemperatur verbunden sind, ist es in den letzten Jahren gelungen, neuartige Analyseverfahren, wie ortsaufgelöste Messtechniken und bildgebende Verfahren, auch für die SOFC zu entwickeln und einzusetzen. Der Vortrag gibt einen Überblick über in-situ diagnostische Methoden, die beim DLR Stuttgart für die Untersuchung von SOFC-Zellen entwickelt wurden und eingesetzt werden. Es handelt sich dabei zum einen um ortsaufgelöste Messungen mit Hilfe segmentierter Zellen, wobei Strom-Spannungskennlinien, Impedanzspektroskopie, Gaschromatographie und Temperaturmessungen lokale Informationen über elektrochemische und thermische Eigenschaften der Zelle liefern. Mit dem aufgebauten Messsystem können realsystemnahe Effekte zeitlich und ortsaufgelöst gemessen und miteinander korreliert werden. Das Messsystem kann zur Aufklärung der Brenngasumsetzung entlang des Strömungswegs und zur Identifizierung der damit möglicherweise verbundenen korrosiven Bedingungen eingesetzt werden. Daneben kann es zur Optimierung des Strömungsdesigns und zur Untersuchung möglicher Effekte der Homogenisierung der Leistungsdichte- und Temperaturverteilung herangezogen werden. Zum anderen wurde kürzlich beim DLR ein Teststand mit einem transparenten optischen Zugang zum Anodengasverteiler aufgebaut, wodurch mittels Laser-Ramanspektroskopie die Konzentrationen von Gasspezies entlang eines Gaskanals mit hoher Ortsauflösung bestimmt werden können. Es werden beispielhaft Ergebnisse beider Analysenverfahren vorgestellt und die Potenziale sowie Grenzen der unterschiedlichen Techniken diskutiert.

The fuel flow rate that is fed to a cell as used in a stack as well as temperature has a significant impact on the distribution of voltage, current density and gas composition along the cell. To examine this influence measurements with H2/N2 = 50/50 and 3 % H2O were performed at 800°C, 750°C and 700°C for four different anode flow rates in a spatially resolved setup. Current, voltage and gas composition was measured along the flow path. Declining local performance and critical local operating conditions were shown experimentally for small fuel supply.

The operational behavior of SOFC short stacks at elevated pressures up to 7 bar was examined. The experiments were performed on planar, anode-supported 5-cell short stacks. First, the pressure-dependency of performance at steady state at different relevant operating conditions is shown, and then temperature variations and gas composition change as well as their effects on pressurized SOFC are presented.

High electrical performance and long lifetime are key requirements that must be fulfilled for a successful introduction of fuel cells into the market. For achieving a high efficiency, a high fuel utilization is required. This requirement, on the other hand, results in strong concentration gradients at the anode where the fuel is successively diluted by reaction products during the reaction process and, hence, in an inhomogeneous distribution of electrochemical performance and temperature. Inhomogeneous distributions of electrochemical and thermal properties such as local power density and local temperature might detrimentally affect both efficiency and long-term durability through thermo-mechanical stress and degradation phenomena. In order to optimize the operational behavior of fuel cells and minimize cell degradation the application of advanced diagnostic methods by monitoring cell characteristics under real operating conditions can provide detailed information about the spatial distribution of the electrical, chemical and thermal cell properties. DLR has developed spatially resolved diagnostic techniques with a segmented cell arrangement where different techniques such as IV characteristics, impedance spectroscopy, gas chromatography and temperature measurement are involved [1, 2]. The segmented cell setup allows a largely extended insight into fuel cell processes both for increasing the fundamental understanding and for optimizing cell and flow field design. The obtained data can be used for mathematical modeling and model validation and for predicting physical, electrochemical and fluid mechanical properties [3, 4]. Recently, imaging techniques such as optical microscopy and Raman spectroscopy have been additionally adopted for in-situ observation of fuel cells. The paper gives an overview of in-situ diagnostic methods applied at DLR Stuttgart for the determination of local effects and the identification of critical operating conditions during technically relevant SOFC operation. Exemplary results from experimental investigations are presented.

In order to optimise the operational behaviour of fuel cells and minimise cell degradation it is very helpful to use in-situ and ex-situ analytical methods. The application of advanced diagnostic methods by monitoring cell characteristics under real operating conditions provides valuable information to be used for the development of degradation mitigation strategies. The application of these methods in SOFC development is significantly limited mainly due to the experimental problems that are associated with the high operating temperature. However, an increasing effort in developing and implementing non-traditional analytical methods such as spatially resolved measurements and imaging techniques for SOFC development has been made in recent years. The presentation gives an overview of in-situ diagnostic methods that are applied at DLR Stuttgart for the study of SOFCs. It includes spatially resolved measurements with an experimental segmented cell configuration where different techniques such as IV characteristics, impedance spectroscopy, gas chromatography and temperature measurement are involved. The investigation by means of segmented cells aims at the determination of local effects and the identification of critical operating conditions during technically relevant SOFC operation. Recently, a new test setup with transparent optical access has been built up allowing for microscopic observation of processes within the cell as well as for application of in-situ laser Raman spectroscopy to determine highly resolved concentrations of gas species along a flow channel. Examples of analytical studies by applying these diagnostic methods are presented and potentials and limitations of the different techniques are discussed.