Angelika Heinzel

Power Management Optimization of a Fuel Cell/Battery/Supercapacitor Hybrid System for Transit Bus Applications

In this paper, the optimization of a power management strategy of a fuel cell/battery/supercapacitor hybrid vehicular system is investigated, both offline and in real time. Two offline optimization algorithms, namely, dynamic programming and Pontryagins minimum principle, are first compared. The offline optimum is used as a benchmark when designing a real-time strategy, which is an inevitable step since the offline optimum is not real-time capable and is oriented only toward minimizing hydrogen consumption, which may result in the unnecessary overloading of the battery. The design and optimization of the real-time strategy makes use of a multiobjective genetic algorithm while taking into account, apart from hydrogen consumption, other important factors, such as the slow dynamics of the fuel cell system and minimizing the battery power burden. As a result, the real-time strategy is found to consume slightly more hydrogen than the offline optimum; however, it dramatically improves system durability.

PBI/H3PO4 Gel Based Polymer Electrolyte Membrane Fuel Cells Under the Influence of Reformates

High temperature polymer electrolyte membrane fuel cells (HT PEMFCs) offer tremendous flexibility when used as energy converters in stationary as well as mobile power devices. Coupling HT PEMFC stacks with fuel processors that use liquid as well as gaseous fuels to generate hydrogen rich gas is a promising prospect, which paves the way for a possible hydrogen economy. The current paper deals with the performance aspects of a 150 W el HT PEMFC stack, which potentially could be coupled to (i) a natural gas reformer, (ii) a propane reformer, or (iii) a methanol reformer. A 12 cell HT PEMFC stack with a total active area of about 600 cm 2 was operated in a test rack, and the results show that HT PEMFCs are principally suited for operation with reformates.

Development of a near-dead-ended fuel cell stack operation in an automotive drive system

During the past decade several important development steps, such as the 700 bar hydrogen storage or the freeze start capability, have brought fuel cell electric vehicles close to market introduction. Further drive system cost reduction by e.g. simplification of the fuel cell system architecture are intended for future fuel cell vehicle generations. Removing the anode recirculation loop and operating the fuel cell stack in the so-called near-dead-ended1 mode is one promising concept. Key experiments focussed on the fuel cell stacks anode side under vehicular load conditions and temperature levels have been performed successfully while maintaining fuel consumption constraints. The impact of cathode operating conditions on the liquid water accumulation and hydrogen concentration on the anode side has been investigated by simulation in order to optimize the operation of this lean anode concept.

Fuel cells and hydrogen technology

The energy technology of the future will have to meet a dual challenge: to deliver a secure and sufficient supply of energy to a growing world population, despite limited energy resources, and to curtail emissions that have a negative impact on the environment. Electricity and hydrogen are clearly evolving into the most important energy vectors of the future. Fuel cells fit in with this trend, as they are able to convert hydrogen efficiently into electricity and heat in an electrochemical reaction, their only by-product being water. Fuel cells can continue to operate as long as fuels are available. These systems achieve a high electrical efficiency of about 60%, even in the low power range of several hundred watts, in contrast to conventional power plants, which need to be dimensioned in the MW range to achieve high electrical efficiency values.

The influence of different pre-treatments of current collectors and variation of the binders on the performance of Li4Ti5O12 anodes for lithium ion batteries

In order to optimize the electron transfer between the Li4Ti5O12-based active mass and the current collector, the surface of aluminum foil was modified either by alkaline etching or by a carbon coating. The as-modified aluminum foils were coated with an active mass of Li4Ti5O12 mixed with polyvinylidene fluoride, sodium carboxymethyl cellulose, or polyacrylic acid as binders. Untreated aluminum and copper foils served as reference current collectors. The corrosion reactions of aluminum foil with the applied binder solutions were studied and the electrode structure has been analyzed, depending on the binder. Finally, the electrochemical performance of the prepared electrodes was investigated. Based on these measurements, conclusions concerning the electrical contact between the different current collectors and the active masses were drawn. The energy density of the Li4Ti5O12 electrodes cast on carbon-coated aluminum foils was significantly increased, compared to the corresponding electrodes with a copper current collector.

Methanol Utilisation Technologies

Oil and gas are raw materials—the availability of which is prognosticated to run short in the near future. The peak oil discussion is an example generally perceived as proof of this development to come.

Fluid Flow and Electrochemical Performance in Miniaturized HT-PEMFCs

In the current work, three types of flow field structures were analysed: i) Fluid flow patterns such as uniformity in gas distribution in various channels of these flow fields were studied using particle image velocimetry (PIV) technique, ii) Pressure drop across each flow field structure was measured using standard anemometers, iii) Impedance of 3 single cells built with these three types of flow fields, each with a commercially available Celtec-P 1000 based membrane electrode assembly (MEA) was examined, to ensure, adequate contact between various cell components using electrochemical impedance spectroscopy (EIS) technique, iv) 3Dcomputational fluid dynamics (CFD) simulations were performed using a commercially available software to ascertain oxidant distribution in these flow fields. Finally, v) Experiments were performed in a test stand with these three types of single cells, (with hydrogen and air as reactant gases) to gauge each cells overall performance. The active area of each cell was 27.6 cm². High temperature stable graphite compound based bipolar plates (FU 4369) from Schunk Kohlenstofftechnik GmbH, Germany were used in each cell.

Alkaline fuel cell with nitride membrane

The aim of this work is to fabricate patterned nitride membranes with Si-MEMS-technology as a platform to build up new membrane-electrode-assemblies (MEA) for alkaline fuel cell applications. Two 6-inch wafer processes based on chemical vapor deposition (CVD) were developed for the fabrication of separated nitride membranes with a nitride thickness up to 1 μm. The mechanical stability of the perforated nitride membrane has been adjusted in both processes either by embedding of subsequent ion implantation step or by optimizing the deposition process parameters. A nearly 100% yield of separated membranes of each deposition process was achieved with layer thickness from 150 nm to 1 μm and micro-channel pattern width of 1μm at a pitch of 3 μm. The process for membrane coating with electrolyte materials could be verified to build up MEA. Uniform membrane coating with channel filling was achieved after the optimization of speed controlled dip-coating method and the selection of dimethylsulfoxide (DMSO) as electrolyte solvent. Finally, silver as conductive material was defined for printing a conductive layer onto the MEA by Ink-Technology. With the established IR-thermography setup, characterizations of MEAs in terms of catalytic conversion were performed successfully. The results of this work show promise for build up a platform on wafer-level for high throughput experiments.

Portable Fuel Cells

Portable fuel cells are under development at present for remote off-grid applications, the first niche markets having notable numbers of pieces are military and leisure applications of the direct methanol fuel cell DMFC in the power range up to several hundreds of watts. Hydrogen-fuelled cells are not applicable yet due to missing refueling infrastructure. Different storage options are evaluated at present. Specially developed micro-fuel cells are an interesting alternative to batteries, particularly if extreme long and autarkic operation is required, but in fact the breakthrough of a marketable product has not yet been achieved.

Hochwärmeleitfähige Polymer-Compounds

Nach dem aktuellen Stand der Technik werden warmeleitfahige thermoplastische Compounds vermehrt fur Bauteile mit Entwarmungsaufgaben eingesetzt und haben somit metallische Bauteile zum Teil verdrangt. Einsatzbereiche finden sich in der Elektronik, Mechatronik aber auch in technischen Teilen in der Automobilindustrie, da bspw. die Verarbeitbarkeit im Spritzgiesverfahren mehr Freiheiten bei der Formgebung ermoglicht. Weiterhin besitzen warmeleitfahige Kunststoff-Compounds gegenuber metallischen Materialien eine wesentlich geringere Dichte und sie erlauben eine gezielte Einstellung der Materialeigenschaften durch die Variation der Fullstoffe und des Fullstoffanteils. Als Fullstoffe fur warmeleitfahige Kunststoffe haben sich organi-sche Fullstoffe (z.B. Graphit), metallische Fullstoffe (z.B. Kupfer) und keramische Fullstoffe (z.B. Bornitrid) durchgesetzt. Die Warmeleitfahigkeitswerte von kommerziell erhaltlichen Compounds liegen laut den Herstellerangaben zwischen 1 W/mK und 20 W/mK und somit um den Faktor 10 bis 100 uber dem von ungefullten Polymeren. Diese Werte konnten jedoch im Rahmen der hier vorgestellten Untersuchungen auf bis zu 30 W/mK gesteigert werden. Zum Erreichen solch hoher thermischen Leitfahigkeiten wurden bis zu 80 Gew.% an Fullstoffen in verschiedene Polymere eingebracht. Mittels einer Vielzahl an Versuchsreihen wurden neben der Warmeleitfahigkeit auch der Einfluss auf die mechanischen Kennwerte sowie die Verarbeitbarkeit der Materialien im Extrusions- und Spritzgiesprozess betrachtet. Durch den Spritzgiesprozess kommt es bei gefullten Compounds zur einer stromungsinduzierten Orientierung der Fullstoffpartikel im Bauteil. Mittels Raster-Elektronen- Mikroskop-Aufnahmen von verschiedenen Probekorpern konnte eine anisotrope Schichtstruktur nachgewiesen werden, die die Warmeleitfahigkeit signifikant beeinflusst und eine Differenzierung der Warmeleitfahigkeit in „through-plane“ und „in-plane“-Richtung erfordert.