| Fuel cells, and in particular solid oxide fuel cells (SOFC), have the potential to meet the requirements of a next-generation aeronautical power source: low emissions, high efficiency, and fuel flexibility. However, the design of an SOFC system requires a determination of the operating pressure that will be used, and knowledge of how each component in the system is affected by the entire range of possible operating pressures. In aeronautical applications, this range must include operation at the pressure in the atmosphere at the altitude of flight. Thus, to design SOFC systems for most aeronautical applications, the behavior of the SOFC at pressures from 101 kPa (sea level) to 7 kPa (60,000 ft) must be known.;Unfortunately, there are no existing data on SOFC operation at these pressures. Furthermore, the theory, at best, may be able to predict SOFC performance at these conditions but it has never been tested, or at worst the theory itself remains largely uncertain. Either way, experimental work is required to accurately determine the effects of sub-atmospheric pressure on SOFC operation.;Through experimentation, this work determines the effects of sub-atmospheric pressure on SOFC performance not only through bulk measurements (e.g., current and voltage) but also through careful examination of all the processes occurring in an operating SOFC. The fuel cell is tested in a facility specially designed and constructed for this work.;These experiments show that while changes in pressure in the range of 7 kPa to 99 kPa did not affect the ohmic conduction processes, it greatly affects the activation and mass transfer processes. The results show that the observed changes in activation processes conform closely to those predicted by the Butler-Volmer theory. Observed changes in mass transfer process are attributed to a variation in the governing processes themselves as pressure was lowered: from that governed by a degree of convective Darcy flow to that governed primarily by Knudsen flow.;This work provides the first published data of an SOFC operating at sub-atmospheric pressures. Designers of aircraft-based SOFC power systems now have the experimental data needed to help determine the ideal design pressure for these systems. |
