Off-Design Performance Analysis of a Solid Oxide Fuel Cell–Gas Turbine Hybrid System Based on Optimal Design Conditions

The off-design performance of gas turbine–fuel cell hybrid systems, resulting from variations in operational conditions, remains a major challenge for their stable and optimal operation. This study aims to identify the optimal design point of a gas turbine–fuel cell combined cycle and to evaluate its behavior under off-design conditions, thereby quantifying performance degradation and system stability at varying loads. To this end, a multi-objective optimization was conducted, focusing on maximizing thermal efficiency, exergetic efficiency, and net power output, and the optimal point was selected as the design reference. Results indicated that at the optimal point, the thermal efficiency reaches approximately 65%, the exergetic efficiency about 57%, and the net power output around 33.4 MW. Subsequently, the system's off-design performance was analyzed using the gas turbine performance map while maintaining the fuel cell parameters at their optimal values. Findings reveal that reducing the rotational speed to 50% of its nominal value decreases the exergetic efficiency to approximately 50% and the net power to about 62.2 MW, whereas the highest efficiency occurs near the nominal speed. Moreover, the variations in power and efficiency remain consistent and free of unrealistic fluctuations.