Hybrid electric aircraft are developed to reduce emission and save energy. However, the thermal efficiency of turbo-electric aircraft and the endurance of battery-electric aircraft are both limited, respectively. Aircraft powered by fuel cells can achieve long endurance, low emission, and fuel consumption reduction. Solid oxide fuel cell hybrid electric jet engines without turbines for unmanned aerial vehicles was proposed in our previous work in which compressors are powered by fuel cells instead of the turbines. The hybrid electric jet engine combining the merits of a turboelectric engine and a fuel cell powered engine. To avoid component malfunctions and engine performance deterioration, determination of safe operation zone is necessary.
In this study, the performance analysis model of the hybrid electric jet engine is built and the main conclusions are as follows. The off-design performance of the hybrid electric jet engine is achieved by adjusting the mass flow of fuel injected into the reformer. The safe operating zone of the hybrid engine is not restricted by turbine inlet temperature. Under low fuel flow and low air flow, too low reforming temperature zones or too low SOFC open voltage zones exist. Additionally, the unbalanced energy zone exists under high fuel flow and low air flow. The power produced by the SOFC is over the one consumed by the compressor. In the safe zone, the hybrid engine has a characteristic of high specific thrust (837.6 N/kg.s-1 ) and high thermal efficiency (70.43%) with high rotational speed, vice versa.
Keywords safe operation zone, solid oxide fuel cell, turbine-less jet engine, unmanned aerial vehicles, hybrid electric