Development of Planar Type SOFC Stacks Operable Under Rapid Starting

Toho Gas Co. Ltd. and Sumitomo Precision Products Co. Ltd. have been jointly developing a SOFC system using scandia-stabilized zirconia (ScSZ) electrolyte cells. Especially, we focused the scandia tetragonal zirconia polycrystalline (Sc-TZP) electrolyte, because the Sc-TZP electrolyte has good mechanical and electrical properties, therefore high reliability and power generation characteristics are expected. We have been developing the 1kW SOFC system using Sc-TZP electrolyte cells as proof of concept since 2002. The 1kW SOFC combined heat and power (CHP) system was installed in The 2005 World Exposition, Aichi, Japan (EXPO2005), and the system successfully operated during about six months. During the demonstration, some troubles caused by balance of plant (BOP) system and controlling system, and these experiences are useful to our system development. The target of our developing system is a small-scale commercial CHP application and target power range is below 10kW class. To apply such a small-scale commercial use, the rapid start up is very attractive for customers in Japanese market. In this study, we have been developing the rapid starting system. To shorten the start up time, reducing the volume of cell stack and strengthening the cell are developed in parallel. Because heating capacity is very affected factor to determine the start up time. To reduce the volume of cell stack, the improvement of cell performance is very attractive. For the electrolyte-supported type cell, the electrical conductivity of electrolyte material is very important factor on the cell performance. On the other hand, to realize the rapid start up system, the mechanical strength of electrolyte is also important factor, because in the rapid start up conditions, large temperature distribution may be easily occurred, and it leads the cell broken. The relation between electrical conductivity and mechanical strength is trade off in the electrolyte material, and then we focused the electrolyte in the range from 4mol% to 7mol%, and demonstrated that these materials have good combination of electrical and mechanical properties. To estimate the suitable composition, the mechanical strength of electrolyte from room temperature to 1073K that is the maximum operating temperature of our system were investigated. And piston on ring (POR) method was also investigated to estimate the strength of actual electrolyte sheets. Part of this work was performed as R & D program of New Energy and Industrial Technology Development Organization (NEDO).