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Current Collection and Stacking of Anode-Supported Cells with Metal Interconnects to Compact Repeating Units
... This value is very large and due to the purposefully chosen conservative loss characteristics (Fig. 2), equivalent to an area specific resistance (asr) of around 1.2 cm 2 . Our own ASE cells presently show an asr of approximately 0.6 cm 2 [16], so that half the amount of cells (i.e. 1200) could even- tually suffice to construct the 100 kW el sized stack, with a obvious enormous impact on cost. ...
A model for a 1000 kW class solid oxide fuel cell (SOFC) system running on biogas from a sewage sludge digestion plant was implemented in a process flow scheme using external steam reforming. The model stack consisted of planar anode supported cells operated at 800 degreesC displaying state-of- the-art electrochemical performance (0.15 W/cm(2) at 80% fuel utilisation). Real annual data from an existing sewage plant were used as input to the model. From the input of 43 m(3)/h biogas (63% CH4), equivalent to 269 kW (higher heating value, HHV), the SOFC stack was calculated to deliver 131 kW,l electricity (48.7%) using a steam-to-carbon ratio of 0.5. This would allow the sewage site to more than cover its own electrical needs, hence to depollute the waste stream at negative energy cost. In its current exploitation using a low efficient gas engine (130 M), the site is only approximate to50% self- sufficient. Special attention was given to the thermal balance of the stack. The stack developed heat (143 kW) could be balanced by endothermal reforming (78 kW) and by cathode excess air lambda (=3), allowing a temperature difference between stack inlet and outlet of 200 K. The case was compared to other fuel scenarios. Steam-added biogas behaves basically identically to steam-reformed methane. For partial oxidation of biogas or pure hydrogen feeding, electrical efficiency drops to under 43% while needs to be raised to 4.5 to maintain the 200 K thermal gradient over the stack.
Electrical output behaviour obtained on solid oxide fuel cell stacks, based on planar anode supported cells (50 or 100 cm2 active area) and metallic interconnects, is reported. Stacks (1–12 cells) have been operated with cathode air and anode hydrogen flows between 750 and 800 °C operating temperature. At first polarisation, an activation phase (increase in power density) is typically observed, ascribed to the cathode but not clarified. Activation may extend over days or weeks. The materials are fairly resistant to thermal cycling. A 1-cell stack cycled five times in 4 days at heating/cooling rates of 100–300 K h−1, showed no accelerated degradation. In a 5-cell stack, open circuit voltage (OCV) of all cells remained constant after three full cycles (800–25 °C). Power output is little affected by air flow but markedly influenced by small fuel flow variation. Fuel utilisation reached 88% in one 5-cell stack test. Performance homogeneity between cells lay at ±4–8% for three different 5- or 6-cell stacks, but was poor for a 12-cell stack with respect to the border cells. Degradation of a 1-cell stack operated for 5500 h showed clear dependence on operating conditions (cell voltage, fuel conversion), believed to be related to anode reoxidation (Ni). A 6-cell stack (50 cm2 cells) delivering 100 Wel at 790 °C (1 kWel L−1 or 0.34 W cm−2) went through a fuel supply interruption and a thermal cycle, with one out of the six cells slightly underperforming after these events. This cell was eventually responsible (hot spot) for stack failure.
A planar solid oxide fuel cell repeating unit, 50 cm2 in total active electrode size, consisting of an anode supported electrolyte cell bearing two 7mm holes for fuel and air injection, and contacted to two dense metal current collector plates via gas distribution layers, was constructed with the aim of measuring local current densities rather than the integral current over the full area. The cathode side was entirely segmented (i.e. cathode layer, gas distribution layer, metal current collector plate) into eight galvanically separated parts of ca. 6.5 cm2 each, with own current and potential leads. The element was characterised at 750–800 ◦C and different H2 fuel flows, by total and local current–voltage recording as well as by local electrochemical impedance measurement. The segment that incorporates the fuel injection hole for the whole cell always outperforms all other segments, the corner segments furthest away from the fuel injection perform least. Differences in local potential can be higher than 200mV. Polarizing one segment individually and recording the change in potential of the other segments reveals the different contributions of convection and diffusion on the flow field. Contrarily to small ideal single cells, total performance of such larger sized, stackable cells is decisively governed by the distribution fields and their weakest zones.
Planar SOFC stack technology based on a unique concept (SOFConnex™) uses structured gas distribution layers between unprofiled metal sheet interconnects and thin Ni-YSZ anode supported electrolyte cells. The layers are flexible both in material and designand allow to implement new configurations relatively simply; manifolding can be internal, external, or combined. Together with thin stack components, independent of the supplier, the SOFConnex™ stacking approach allows compact planar assembly with low cost potential and adequate power density. Different cell and flow designs have been realized. With a basic flow configuration, short stacks (50 cm2 cell active area) were assembled and tested, power density at 800°C reaching 0.5 W/cm2 at 0.7 V average cell voltage (1.5 kWe /L, 0.36 cm2 area specific resistance), for 65% fuel utilization and 35% lower heating value electrical efficiency. Short stacks were thermally cycled and operated with both hydrogen and syngas. Degradation was essentially Ohmic(confirmed from impedance spectroscopy on stacks) and at first mainly due to the cathode-electrolyte interfacial reaction, performance loss was subsequently strongly reduced after cathode replacement. Using multiple voltage probes with additional interconnects allowed to separately monitor current collection losses during polarization. With an improved design in terms of sealing, postcombustion control and flow field, stacks up to 1 kWe have been operated.
Reliability of SOFC stacks is a complex and key issue. This paper presents a simulation study including some degradation processes, namely, interconnect degradation and the anode reoxidation potential. Quantification for these phenomena has been included in a repeat element model to simulate stack degradation and study the influence of design and operating parameters on the degradation. Interconnect degradation is based on Wagner’s law for oxide scale growth, parameters applying to metallic interconnects used in planar SOFCs are used. Anode re-oxidation is modeled by thermodynamic equilibrium which allows identification of the operating conditions where the anode is likely to be re-oxidized. Simulations have been carried out for a large number of cases at different current density, fuel utilization and temperature, for 2 different stack designs (base-case and modified design). Using an appropriate criterion to express degradation, all these cases point to a clear trade-off between interconnect degradation and local temperature. The base case design is likely to be exposed to anode reoxidation.
A 3D simulation tool for solid oxide fuel cells is presented. The aim of this work is to predict current density, flow, temperature and concentration fields in order to compare and optimize repeat element geometry for a whole stack. A commercial CFD tool was used, solving mass, momentum and energy equations; whereas chemical kinetic equations are computed from external sub-routines. A steady-state case is presented, fed with hydrogen. The flow is laminar for both air and fuel. Radiative heat transfer is taken into account between inner surfaces. On boundaries, convective and radiative heat transfers are assumed at external surfaces between repeat element and oven. Due to the large range of dimensions (cells: 300 mum thick, gas channels: 1 mm height, whole cell: 80 mm x 80 mm) a fine mesh was needed. Data for conductivities and kinetics were estimated from experiments performed in- house. Simulation results are presented and compared to real repeat element test measurements for the current-potential characteristics.
A model for planar solid oxide fuel cell repeat elements and stacks has been developed. Distribution of concentrations, reaction rates and temperatures (both gases and solids) are computed as well as overall performance results. Specific experiments provide inputs to the model by a parameter estimation method. The modeling approach developed allows to compare several configurations. As the number of design parameters is large (from cell size, component thicknesses to gas flow configuration), the model is designed to change easily these parameters so as to explore as many cases as possible. This is particularly true for the flow configuration (inlet position, outlets) for which several options are considered. This model assists in choosing a configuration and allows to perform sensitivity studies in an efficient way (without having to produce a new mesh such as for CFD tools) or to be combined with an optimization tool. A first validation with experimental results, performed on a particular stack design, is presented. Issues of model accuracy and sensitivity to uncertain inputs are discussed.
Results on solid oxide fuel cell stacks tested at 800° C with H2 fuel and using planar Ni-zirconia anode supported cells, 80x80x0.2 mm in size, are presented. Modeling and numerical simulation is used to interpret observed results and develop improved designs. Where neccessary, the models are calibrated with additional experimental data. Emphasis is placed on the critical issue of nickel anode reoxidation, related to the fuel flow field. Consideration is also given to gradients of temperature and current density developing over the cells and predicted by the models; local current density could be validated by measurement. Flow distribution within stacks is also illustrated by both experimental and modeling results.
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- F Meschke
- W Meulenberg
- H Buchkremer
- R Steinbrech
- D Stover
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and D. StOver, in SOFC-VH, H. Yokokawa and S. C. Singhal, eds., PV 2001-16, pp.
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- R Ihringer
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- L Constantin
- J Van Herle
R. Ihringer, S. Rambert, L. Constantin, and J. Van herle, in SOFC-VH, H. Yokokawa
and S. C. Singhal, eds., PV 2001-16, pp. 1002-11, The Electrochemical Society
Proceedings Series, Pennington, NJ, (2001).