Closed versus open cycle energy recovery from solid oxide fuel cells

A computer model of a solid oxide fuel cell (SOFC) was implemented with a number of operational options to evaluate the best use of the cell waste heat. The simplest solution for heat recovery is represented by adopting a pressurized fuel cell with the eventual expansion of the exhaust stream in a gas turbine. Other alternatives feature the adoption of various closed cycle recovery systems: a standard steam cycle, an inter-refrigerated helium cycle, and an advanced binary cycle. Assuming a fuel energy input of 100 MW, the overall efficiency of a hybrid cell was computed, which was much higher than that of a small capacity (100 kW) existing plant (67 versus similar to 53 per cent). Such high efficiency hybrid SOFC systems were taken as the reference to evaluate the performance of alternate options. Reclaiming the cell waste heat by means of a subcritical steam cycle yields a performance similar to that of a hybrid system. Similar results are obtained resorting to a closed helium cycle. Advanced binary cycles, using either potassium or cesium as working fluids, were extensively investigated in view of achieving a superior overall performance. The hot, clean stream exhausted by the fuel cell is recognized as an ideal heat source for a binary plant. The SOFC-binary cycle system was fully optimized yielding top efficiencies similar to 74 per cent and an additional power output of similar to 45 per cent of the cell rating (30 per cent in the case of the hybrid system). The excellent efficiency of a binary cycle combined with the large amount of oxygen that is available in the cell exhaust flow suggests that a supplementary firing of the fuel could improve the overall merit of the system. Burning 75 per cent of the additional natural gas, for example, reduces the total efficiency to a level of that of a hybrid system but makes available an extra power 1.35 times greater than the fuel cell capacity. Pushing further this concept, the fuel cell could become an auxiliary equipment of a binary cycle.