Basic study on electric load following operation of small distributed solid oxide fuel cell system

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The solid oxide fuel cell (SOFC) adopts oxygen ion conducting ceramic materials, such as yttria-stabilized zirconia (YSZ), as the electrolyte, and its operating temperature is very high as 1200-1300 K, leading to an advantage that very high overall efficiency can be achieved by combining it with some suitable bottoming cycle, such as the gas turbine, that can utilize its exhaust gas. Its research and development, therefore, have been actively promoted for middle to large-scale electric power sources. In addition, it is now also expected as the power source for the small distributed co-generation systems, because its efficiency is expected to reach about 50% by itself without the bottoming cycle by utilizing the internal heat generation for the steam reforming reaction of the fuel. The SOFC, however, has a problem in durability of the ceramics used as its cell materials, because its operating temperature is very high and cell temperature fluctuation induces the thermal stress to the cer … More amics. The cell temperature distribution in it, therefore, should be kept as constant as possible. In the case of the small distributed co-generation systems, however, the variable load operation through the control of the average current density in the cell is necessary, and this operation usually causes the cell temperature fluctuation. Considering this fact, aiming at the establishment of the variable load operation of the SOFC, we investigate the relation between the average current density and the temperature distribution in the co-flow and counter-flow type planar SOFC single cells by numerical simulations. It is made clear from the simulations that the change of the temperature distribution can be suppressed to sufficiently low level and the variable load operation can be realized for both types of cells by properly regulating the air utilization. To compensate the very fast load change and to smoothen the power fluctuation, we propose to install an electric double layer capacitor between the SOFC and the load, and its effect is confirmed through numerical simulations. In addition, an experimental circuit using an electric double layer capacitor is composed, and the experimental results are compared with the simulation results. As a result, although there exists a little difference on the transient response, it is made clear that the experimental circuit operates almost according to the simulation. This indicates the validity of the capacitor control method proposed in this study. Less