Holger Janßen

Design and Experimental Investigation of a Heat Pipe Supported External Cooling System for HT-PEFC Stacks

A 10-cells HT-PEFC stack with an active cell area of 200 cm2 has been built up and tested regarding the temperature distribution from cell to cell and over the active area since not every cell is cooled. Measurements with artificial reformate as a fuel show that the vertical temperature distribution over the active area is sufficiently small, with a maximum of 5.1 K at 550 mA cm−2. Also the temperature gradient from cell to cell is sufficiently small with 10.7 K at 550 mA cm−2. As a result it can be mentioned, that the heat pipe supported external cooling is well suited to cool HT-PEFC stacks with large active areas in reformate operation.Copyright

Development of direct methanol fuel cell systems for material handling applications

Direct methanol fuel cells (DMFCs) are attractive for various applications, above all, however, as replacements for batteries or accumulators. They may be used in different power classes. A market analysis indicated that the use of a DMFC energy system in the kW class had the best chance of commercial realization if applied in forklift trucks for material handling in large distribution centers or warehouses. An advantage of such energy systems is that there is no need for the relatively time-consuming recharging of the lead-acid batteries, nor is it necessary to have spare batteries available for multishift operation. This calls for DMFC energy systems that are capable of replacing the existing Pb accumulators in terms of space requirements and energy. However, this requires considerable improvements to be made in terms of power and stability over time of DMFC systems and, in comparison to their present status, an increase of overall efficiency. Recent cost analyses for the overall system; for example, show that for the DMFC stack, a durability of at least 5000 h must be achieved with an overall efficiency for the DMFC system of at least 30%, with the constraint that the system can be operated in a waterautonomous manner up to an ambient temperature of 35 °C. As part of a joint R&D project with industrial partners, two systems were constructed and each subjected to long-term testing for 3000 and more than 8000 h, respectively, with realistic load profiles from driving cycles. In this test, the stack from the first system, DMFC V 3.3―1, displayed an aging rate of approximately 52 μ V h ―1 at a current density of 100 mA cm ―2 . This corresponds to a performance degradation of 25% over a period of 3,000 h. The DMFC V 3.3―2 system, a modified and optimized version of the first system, also underwent long-term testing. In this case, the aging rate of the stack was only approximately 9 μ V h ―1 at a current density of 100 mA cm ―2 . The system has thus been operated to date for more than 8000 h under realistic load profiles.

Stack Concepts for High Temperature Polymer Electrolyte Membrane Fuel Cells

Stationary and mobile applications are attractive applications for high temperature polymer electrolyte membrane fuel cells (HT-PEMFC). Examples for stationary applications are electricity and heat generation for households or uninterruptible power supply (UPS) systems. In the case of mobile applications the on board power supply with auxiliary power units (APUs) and backup power for the recreation area are most promising. In comparison to the low temperature PEMFC (operated at 60–80 °C) the operating temperature of a HT-PEMFC, which is between 120 and 200 °C, allows a more effective co-generation of power and heat. The higher temperature also leads to an improved tolerance to fuel impurities and a simpler system design [1]. Therefore applications where hydrogen is generated with reformer systems are ideally suited for a HT-PEMFC due to the increased CO tolerance [1, 2]. The majority of these demands have electric power requirements between some hundred Watt and the low kW range.

Hochtemperatur-Polymerelektrolyt-Brennstoffzellen

Phosphorsaure ist ein thermisch stabiler und preiswerter Elektrolyt fur die HT-PEFC. Weiterhin ist sie ungiftig, besitzt einen sehr geringen Dampfdruck (geringe Verluste durch Gasstromung) und benotigt aufgrund der hohen Betriebstemperatur kein aktives Management von flussigem Wasser. Jedoch andert sich die Leitfahigkeit als Funktion des Wassergehaltes. Demzufolge andert sich der Widerstand mit den Betriebsbedingungen. Diese Anderung kann durch die Wahl eines geeigneten Flowfield-Designs der Brennstoffzelle gunstig beeinflusst werden. Die Medienfuhrung hat auch einen wesentlichen Einfluss auf die Stromdichteverteilung von Zellen bei Betrieb mit CO-haltigem Brenngas. Vorteilhaft ist eine Fuhrung der Gase im Gegenstrom mit gleichzeitiger Gleichstromfuhrung des Kuhlmediums. Grose Gradienten in der Stromdichteverteilung bedingen eine beschleunigte Alterung. Aufgrund der Betriebstemperatur von ca. 160 °C stellt die HT-PEFC nur geringe Anforderungen an die Reinheit von Wasserstoff als Brenngas. Es konnen CO-Gehalte bis 3 Vol.-% am Brenngaseingang toleriert werden. Geringe Methan-Konzentrationen (bis 1 Vol.-%) haben keinen Einfluss auf die Leistung.

Cooling methods for high temperature polymer electrolyte fuel cell stacks

One promising future application for a high temperature polymer electrolyte fuel cell (HT-PEFC) stack coupled with a reformer is an auxiliary power unit (APU) for mobile applications using diesel or kerosene which is also used for the main engine. Despite of the high efficiency of a HT-PEFC, the stack has to be cooled during operation. Hence, this work focuses on the investigation of different cooling strategies regarding the complete system, the use of heat transfer oil as cooling medium is fixed in this contribution. In detail, three cooling methods to maintain operating temperature in stacks with more than 1 kW electrical power and large active areas (> 200 cm2 per cell) were analyzed. In the first method heat transfer oil flows through the stack in internal channels that are located on the backside of the cathode-side bipolar plate. In the second cooling arrangement the oil flows through capsuled cooling cells, which are arranged between every third electrochemical cell. For the third cooling method the excellent heat conducting properties of heat pipes are used. Outside the stack, the heat is removed by heat transfer oil from the overlapping heat pipes. These three methods were evaluated experimentally and with CFD simulations. In this paper the detailed measurements of the temperature distributions are presented containing the overall result that all cooling methods are applicable to maintain the temperatures of large HT-PEFC stacks during the operation in an APU system.Copyright

Proton dynamics of phosphoric acid in HT-PEFCs: Towards “operando” experiments

High Temperature Polymer Electrolyte Fuel Cells (HT-PEFCs) have been studied with quasielastic neutron scattering, which gives access to the proton diffusion in the fuel cell on local lengthand timescales. So far, the different components such as the proton conducting membrane and the electrode layers have been studied separately. Here we show that also operating fuel cells can be investigated and the proton diffusion can be measured under real working conditions. The proton diffusion during power production is compared to that “at rest” but at elevated temperatures.

Stackentwicklung Hochtemperatur-Polymerelektrolyt-Brennstoffzellen

Hochtemperatur-Polymerelektrolyt-Brennstoffzellen-Stacks sind pradestiniert, Bordstrom fur Luftfahrtanwendungen bereitzustellen. Aus verfahrenstechnischer Sicht ist diese Technologie vorteilhaft, da auf der einen Seite eine vereinfachte Systemtechnik mit nicht erforderlicher Gasbefeuchtung und effizienter Warmeein- und auskopplung und auf der anderen Seite die direkte Verwendung des Brennstoffs Kerosin (in Verbindung mit geeigneter Brenngaserzeugung) steht. In diesem Kapitel werden auf der Basis von technischen und regulativen Vorgaben mogliche und bevorzugte Konzepte fur den Aufbau, die Auslegung und den Betrieb von Stacks fur die Anwendung im Luftfahrtbereich dargestellt und diskutiert. Fur die Hauptkomponenten Membran-Elektroden-Einheit und Bipolarplatte wird ein Schwerpunkt auf die Verwendbarkeit alternativer Materialien in Bezug auf die Anwendung gelegt. Die Beschreibung moglicher Stackkonzepte umfasst thematisch die Bereiche Spanntechnik, Dichtung, Stacktemperierung sowie Stromungsmechanik. Abschliesend folgt noch ein kurzer Blick in die Zukunft, der die notwenige Weiterentwicklung der Zell- und Stacktechnik in Bezug auf eine notwendige Erhohung der Performance und Leistungsdichte anreist.

Design and Experimental Investigation of a Heat Pipe Supported External Cooling System for HT-PEFC Fuel Cell Stacks

A 10-cells HT-PEFC stack with an active cell area of 200 cm2 has been built up and tested regarding the temperature distribution from cell to cell and over the active area since not every cell is cooled. Measurements with artificial reformate as a fuel show that the vertical temperature distribution over the active area is sufficiently small, with a maximum of 5.1 K at 550 mA cm−2. Also the temperature gradient from cell to cell is sufficiently small with 10.7 K at 550 mA cm−2. As a result it can be mentioned, that the heat pipe supported external cooling is well suited to cool HT-PEFC stacks with large active areas in reformate operation.Copyright