Future Highly Efficient Engines with Solid Oxide Fuel Cell–Gas Turbine Coupling: System Modeling Study and Comparison of Directly and Indirectly Coupled SOFC–GT Systems

This study investigates hybridization of a solid oxide fuel cell with a gas turbine (SOFC–GT) for application in an ATR 72 regional aircraft. Several challenges hinder its viability, including the low gravimetric power density of SOFC stacks and stringent heat integration constraints. A steady-state model sweeps the cell voltage, overall pressure ratio (OPR), and a bounded turbine inlet temperature (TIT). This study introduces a new corrected power-share metric. This metric accounts for operating-point-dependent SOFC power density. It also enables weight-relevant comparisons. We analyze two types of coupling: direct and indirect. In the direct coupling, SOFC cooling fixes the core airflow and a TIT ceiling imposes a minimum power share. In the indirect coupling, a bypass decouples SOFC and gas turbine operation, incurring an efficiency penalty. We compare two heat-integration architectures: preheating with SOFC cathode exhaust versus low-pressure turbine (LPT) exhaust. Results show that direct coupling achieves efficiencies above 65% at high-corrected power shares, whereas indirect coupling offers greater operational flexibility but lower efficiency. Cathode exhaust preheating improves feasibility and outperforms LPT recuperation by more than 15% efficiency at low-to-mid-corrected power shares. However, LPT recuperation attains higher peak efficiency only at high-corrected power shares and within a narrow OPR window, which is limited by recuperator pinch.