Solid oxide fuel cells SOFCRoll single cell and stack design and development

Abstract

This study has focused on the implementation of a stack system for a novel design of solid oxide fuel cell (SOFCRoll). The issues affecting the commercialization of SOFCs are mainly based on durability and cost. The new design offers a number of advantages over the existing designs; it seeks to retain the specific advantages of both the tubular (high unit strength, no sealing problems) and planar arrangements (high power density). This design also aims to achieve low manufacturing cost by utilizing a cheap, easily scalable production technique: tape casting, together with co-firing all components, in one single step. In this study aspects of the design and operation of SOFCRoll stacks were studied particularly those affecting the single cell test reproducibility such as pre test quality control and scale up issues such as bundle and stack gas distribution. Initially the performance of single cells was characterized and the variation of their power output with temperature was observed. The maximum power, 0.7W at 800°C was achieved with a high silver content. The OCV and total resistance of this cell were 0.93V, 0.30Ω respectively. A standard pre-test quality control and current collection technique was introduced. At 800°C reproducible performance of 0.5W power obtained, average OCV was 0.935V and average series and polarization resistances of 0.18Ω and 0.19Ω was achieved respectively. Once single cell reproducibility was achieved, the design and operation of a 5 cell SOFCRoll bundle was investigated. A FLUENT CFD model was used to optimize the gas distribution in the five cell manifold design. The value of the model as a design tool was demonstrated by the comparison of 3 different gas manifold designs. The final manifold design M3 achieved 2.5W which is consistent with the 0.5W per a cell target. This manifold was then used as the basis for the development of a 25 cell stack which was built and tested. The 25 cell stack testing results were down to 0.35W per a cell. The performance drop highlighted the problem of fuel cell manufacturing reproducibility and also the importance of introducing reproducible manufacturing tequniques. That been the case for single cell manufacturing reproducibility issue, the fundamental concern for performance drop remains a design issue. To optimize the SOFCRoll design and to assist with the development program a single-cell CFD model was developed using FLUENT. The model was validated by comparison with data from experimental measurements for the single cell. The model work was used to predict the geometrical effect of the SOFCRoll tubular and the spiral gas channel configuration and current collector configuration. Results indicate the outlet gas flow velocity is higher around the spiral, near the gas inlet (the gas interring the cell preferentially flows around the spiral) therefore, velocity decrease as the gas moves along the cell. The lowest outlet velocity is registered opposite to the gas inlet, thus creating non-uniform gas distribution. The current density distribution is not uniform and is affected primarily by reactant flow distributions along the cell and possible current collection issues particularly around the spiral part of the cell.