Mass-Based Optimization Studies for Sizing and Optimal Control of Hybrid Fuel Cell-Battery System for Commercial Airliners

As the contribution of the global aviation industry to CO$_2$ and non-CO$_2$ emissions has been increasing at the rate of 4.5\% per year, it has been raising an urgency in the industry for developing and implementing sustainable solutions to reduce the environmental impact and meet global climate goals. In order to remediate these issues batteries and fuel cell integrated systems are some alternative propulsion technologies currently being explored in the aviation sector. Integrating these systems however requires careful sizing and optimal control of the systems to minimize their mass, volume, or cost. In this study, a mass-based sizing and optimal control framework, using Dymos and OpenMDAO, was developed to size integrated fuel cell-battery systems for commercial airliners. It was used to optimize the mass of the powertrain system at provided constraints for the battery and fuel-cell systems, as well as operational power-profile requirements. An optimized split ratio for an integrated powertrain was also determined to define how to split the power between the fuel cell and the battery to meet the continuous power requirement profile. As a part of the study, optimization studies were performed for propulsion power time series of different short-, medium-, and long-haul airliners at multiple operating ranges with different gravimetric efficiencies of the hydrogen storage tank, specific powers of the fuel cell stack, and energy densities of the battery system. The study then presents a comparative analysis of the optimization results and discusses the suitability of the integrated powertrain system for different aircraft types and ranges in terms of take-off mass and retained passenger capacity. Results show that take-off mass is most sensitive to battery energy densities above 1 kWh/kg and fuel cell specific powers above 2 kW/kg, with optimal power split ratios shifting toward fuel cells at longer ranges and with improved hydrogen tank gravimetric efficiencies (45\% vs 15\%). Passenger capacity retention generally decreases with range but improves for larger aircraft, with fuel cell-battery systems showing greater viability for long-haul wide-body aircraft compared to short-haul regional jets.