Retrofitting a Turboshaft Engine to An Engine-Fuel Cell Hybrid: A Case Study

Distributed electric propulsion is a proposed method for reducing aircraft fuel consumption in response to increasing fuel prices and emission concerns. An efficient yet power-dense method of generating the electricity required for this type of propulsion is through a hybridized gas turbine generator featuring a recuperator and a high-temperature fuel cell system between the gas turbine compressor and burner. Previous studies have explored hybrid systems' advantages in purpose-built engine systems. However, what could be accomplished by converting an existing gas turbine engine to a turbine and fuel cell hybrid has not been investigated. Thus, this study aims to explore the possible performance improvements and tradeoffs associated with the conversion of an Allison Model 250 C-18 turboshaft engine through a thermodynamic cycle analysis. The total electric power output and overall thermodynamic efficiency of several different turbogenerator configurations have been determined, employing a combination of recuperator, fuel cell, and fuel cell bypass components. The results suggest that adding only a recuperator to the baseline engine provides up to a 23.9% increase in thermal efficiency at the cost of a 12.8% decrease in total power output, along with increased mass and volume. Hybridizing the recuperated engine with a 48.6kW solid oxide fuel cell system consisting of 500 10x10cm cells with a bypass improves the combined thermal efficiency by 43.6% and the total power output by 5.6% compared to the baseline engine at the cost of further increases in mass and volume. Notably, the system performance is highly sensitive to the pressure losses specified for the additional components for all the modified systems; 8% of additional pressure loss in the hot section of the turbogenerator will cause a 9.4% decrease in shaft power output and thermal efficiency, suggesting that minimizing the pressure losses associated with these components is essential for achieving the full potential of hybridized turbogenerator systems on aircraft.