Effects of fuel utilization on performance of SOFC/gas turbine combined power generation systems

Combined cycles using Solid oxide fuel cells (SOFCs) are expected to provide very high efficiency. The SOFCs are combined with a Gas turbine (GT) or a Gas turbine combined cycle (GTCC). The major SOFC design parameters greatly impact the combined cycle efficiency because the SOFC still produces a majority of the electricity in the combined cycles. In this paper, the influence of the SOFC’s fuel utilization on the efficiency of the combined cycles is carried out using parametric analysis. It is demonstrated and validated that an optimal fuel utilization exists. Four types of SOFC/GT and SOFC/GTCC combined cycles are analyzed. Each combined cycle is found to have an optimal fuel utilization, which is always lower than that of the SOFC-alone system. The main reason is that the turbine inlet temperature rises and thus GT or GTCC power increases with decreasing fuel utilization because of the increased remaining fuel after the cell stack. The value decreases as the power share of the turbomachinery part increases. The peak efficiencies of the SOFC/GT and SOFC/GTCC were predicted to be over 72 % and 76 %. The optimal fuel utilization corresponding to peak efficiency was 0.8 for the SOFC-alone system, 0.7 for the SOFC/GT system, and 0.6 for the SOFC/GTCC system.