| High temperature fuel cell systems are considered an attractive solution for distributed power generation. However, there are several issues that must be resolved before SOFCs can be used widely. Currently, high temperature fuel cells are being deployed as base-load electric generators in conjunction with intermittent energy sources such as solar or wind. Complementing the fuel cell performance with power tracking capability will be extremely useful in addressing local power demand changes or the variability of the intermittent renewable sources. The main challenge of load following in SOFC systems is to operate them within the safe operating conditions. Transient operation of SOFC results in spatial temperature fluctuations. Such fluctuations can cause thermal fatigues that reduce efficiency and stack's lifetime. Consequently, controlling SOFC spatial temperature plays an important role in minimizing fuel cell thermal stresses and fatigue. In this research controlling spatial temperature variations as well as load following ability for three different planar configurations in various details (to avoid repetition) have been studied. It is first shown that under ideal actuation conditions, a controller can provide significant power following capability with minimal thermal variations, both in time and along the length of the cell. Then the emphasis is placed on control design in the presence of non-ideal actuation. This includes the effects of fuel processing delays, cathode inlet thermal delays, and parasitic power associated with the blower supplying air to the cathode. The controllers used, based on H-infinity, and energy-to-peak minimization synthesis, are applied to a dynamic model of planar SOFC stack. The results indicate that many of the problems associated with realistic and imperfect actuation can be addressed with relatively standard control synthesis modifications, but fuel flow delays can compromise power following significantly. Next, a strategy which relies primarily on partial internal reformation for power following addresses many of the difficulties associated with reformer delays. Both co-flow and counter-flow designs are considered and advantages and disadvantage of each are then reviewed. Finally the effect of uniform versus non-uniform internal reformation on spatial temperature profile and also the possibility of designing a temperature controller on a cross-flow configuration have been investigated. |
