Improving the performance of solid oxide fuel cell systems: Dissertation

Solid oxide fuel cell (SOFC) systems can provide power production at a high electrical efficiency and with very low emissions. Furthermore, they retain their high electrical efficiency over a wide range of output power and offer good fuel flexibility, which makes them well suited for a range of applications. Currently SOFC systems are under investigation by researchers as well as being developed by industrial manufacturers. The first commercial SOFC systems have been on the market for some years now, with the help of government subsidies. However, the performance, cost and durability of SOFC systems must be improved in order for them to be competitive in the open market. For this purpose, it is important to evaluate the technical feasibility and performance of possible SOFC system designs and the balance of plant components. Furthermore, the operating environment of the costly SOFC stack must be controlled rigorously in order to attain both high efficiency and long lifetime. The instrumentation and the peripheral equipment of SOFC systems must be simplified in order to decrease system costs, although without sacrificing the system's operational reliability. This thesis focuses on the design and operational aspects of SOFC systems. The top-level targets for the research work were (i) to identify a feasible system design and a set of components which would enable utilization of the high electrical efficiency potential, above 45%, of SOFC technology and (ii) to find solutions which would improve and simplify the operation of SOFC systems. This work presents a novel SOFC system design capable of reaching an electrical efficiency of 45%. The design utilizes the so-called anode off-gas recycling concept, which improves the system's self-sufficiency by eliminating the need for an external water supply during normal operation. The technical feasibility of the system design was validated by constructing a proof-of-concept prototype and performing long-term experiments with it. The proof-of-concept prototype was further used to investigate several specific operational challenges of the systems, including (i) prevention of carbon formation in the fuel system, (ii) quantification of the effects of stack leakages, (iii) protection of the anode from damage during system start-up cycles and (iv) control of the SOFC stack temperature. The solutions found for these challenges are readily implementable and could promote commercialization of SOFC systems by simplifying system design and improving their operational capabilities.