Andreas Friedrich

Fuel Cells For Aircraft Applications

Although air transport is responsible for only about 2 % of all anthropogenic CO2 emissions, the rapidly increasing volume of air traffic leads to a general concern about the environmental impact of aircrafts. Future aircraft generations have to face enhanced requirements concerning productivity, environmental compatibility and higher operational availability, thus effecting technical, operational and economical aspects of in-flight and on-ground power generation systems. Today’s development in aircraft architecture undergoes a trend to a “more electric aircraft” which is characterised by a higher proportion of electrical systems substituting hydraulically or pneumatically driven components, and, thus, increasing the amount of electrical power. Fuel cell systems in this context represent a promising solution regarding the enhancement of the energy efficiency for both cruise and ground operations. For several years the Institute of Technical Thermodynamics of the German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt, DLR) in Stuttgart is engaged in the development of fuel cell systems for aircraft applications. In cooperation with Airbus several fuel cell applications within the aircraft for both ground and cruise operation were identified. In consequence fuel cell systems capable to support or even replace existing systems were derived. In this context, kerosene tank inertization and electrical cabin power supply including water regeneration represent the most promising application fields. The contribution will present the state of development discussing the following points: • Modeling of different system architectures and evaluation of promising fuel cell technologies (PEFC). • Experimental evaluation of fuel cell systems under relevant conditions (low-pressure, vibrations, reformate operation, etc.). • Fuel cell test in DLR`s research aircraft ATRA (A320) including the test of an emergency system based on hydrogen and oxygen with 20 Kilo Watts (kW) of electrical power and emission-free taxi ground operation with fuel cell system powering a nose wheel drive. DLR has developed the experimental motor glider Antares DLR H2 for testing of fuel cell technology under aviation conditions and for surveillance applications. The Antares DLR H2 is small, dismountable and has an experimental permit-to-fly. Thus both flight experiments and technical changes can be realized time- and cost-effectively. Equipped with fuel cell system, high power battery pack and all-electric drive train, the Antares DLR H2 is a flying test bench for fuel cells and fuel cell/battery hybrid systems. In September 2012 several long range flights have been successfully completed by this aircraft. In this context newly integrated low temperature fuel cell systems have been tested.

Local In-Situ Analysis of PEM Fuel Cells by Impedance Spectoscopy and Raman Measurements

An understanding of the processes inside of low temperature fuel cells on a local scale is required for an effective improvement strategy. For this purpose in situ Raman spectroscopy and local impedance spectroscopy is being developed. The contribution describes the modifications to the cell, and installations of additional devices and the experimental detection systems for integrating both methods into a single cell set up. First results to verify the combined results were carried out and are presented. In the case of the local impedance with segmented cells the additional effort for the simultaneous frequency analysis of all segments is described. The Raman signals of hydrogen, oxygen and water from the channels of the flow field are shown and first measurements of gas composition along the fuel path are analyzed.

Direct conversion of dimethyl ether in high-temperature polymer electrolyte fuel cells under stationary and dynamic conditions

The behavior of a polybenzimidazole-based high-temperature polymer electrolyte membrane fuel cell using dimethyl ether (DME) as fuel was investigated under stationary and dynamic load conditions. The power density was enhanced significantly with an increase of both operating temperature and anodic water stoichiometry. Likewise, the power density decreased with increasing DME stoichiometry. The characterization of the dynamic operation showed a strong qualitative similarity to low-temperature direct methanol fuel cells. The development of the cell voltage after a spontaneous change of cell current density could be assigned to the electrochemical oxidation of an intermediate species.

MONITORING REACTIONS IN ALKALINE DIRECT ETHANOL FUEL CELLS ASSEMBLED WITH NON-PT-CATALYST

This contribution shows how Raman spectroscopy can be used to pursue chemical reactions within fuel cells. For this, the oxidation of ethanol occurring in an alkaline direct ethanolic fuel cell was investigated. The analysis of a sequence of Raman spectra recorded during the reaction shows that ethanol was solely oxidized to acetate in a unique reaction.

Design-Considerations regarding Silicon/Graphite and Tin/Graphite Composite Electrodes for Lithium-Ion Batteries

An analytical model is proposed to investigate properties of composite electrodes that utilize more than one active material. We demonstrate how the equations can be applied to aid in the design of electrodes by comparing silicon-graphite and tin-graphite composite negative electrodes as examples with practical relevance. Based on simple assumptions, the results show how volume expansion tolerance and initial porosity are important factors for the achievable gravimetric and volumetric capacities as well as volumetric energy density. A Si-alloy/graphite composite electrode is used as an experimental system to corroborate the formulated analysis. Kinetic limitations are also addressed based on a novel heuristic approach.