A typical approach to the synthesis/design optimization of energy systems is to only use steady state operation and high efficiency (or low total life cycle cost) at full load as the basis for the synthesis/design. Transient operation as reflected by changes in power demand, shut-down, and start-up are left as secondary tasks to be solved by system and control engineers once the synthesis/design is fixed. However, start-up and shut-down may be events that happen quite often and, thus, may be quite important in the creative process of developing the system. This is especially true for small power units used in transportation applications or for domestic energy supplies, where the load demand changes frequently and peaks in load of short duration are common. The duration of start-up is, of course, a major factor which must be considered since rapid system response is an important factor in determining the feasibility of solid oxide fuel cell (SOFC) based auxiliary power units (APUs). Start-up and shut-down may also significantly affect the life span of the system due to thermal stresses on all system components. Therefore, a proper balance must be struck between a fast response and the costs of owning and operating the system so that start-up or any other transient process can be accomplished in as short a time as possible yet with a minimum in fuel consumption. In this research work we have been studying the effects of control laws and strategies and transients on system performance. The results presented in this paper are based on a set of transient models developed and implemented for the components of a 5 kW net power SOFC based APU and for the high-fidelity system which results from their integration. The simulation results given below are for two different start-up approaches: one with steam recirculation and component preheating and the second without either. These start-up simulations were performed for fixed values of a number of system-level parameters (e.g., fuel utilization, steam to methane ratio, etc.) and were used to generate sufficient information to permit the development of appropriate control strategies for this critical operating point. These strategies are based on a balance between fuel consumption and response time. In addition, energy buffering hardware was added to the system configuration in order to minimize the effect of transients on fuel cell stack performance and lifetime.

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