Hydrogen Fuel Cell Aircraft for Regional Travel

Licentiate thesis, 2024

With ever increasing travel demand, with some projections estimating a two-fold increase of passenger volume in 2050, the efficiency gains made in conventional aircraft designs are simply not keeping up with the pace of increasing emissions. A possible way to de-carbonize regional air travel is through the use of fuel cell aircraft, which are electric aircraft with liquid hydrogen as its energy-carrier.

Hydrogen carries roughly three times the energy per mass compared to conventional jet-fuel, but is of much lower density and therefore needs large storage volumes. The liquid hydrogen is a cryogenic, and therefore requires specialized insulated pressure vessels, which adds weight. Additionally, due to the low system specific power of fuel cells, the total aircraft weight will increase compared to a turboprop counterpart. On the flip-side, a fuel cell system has a comparatively high efficiency, and can therefore offset the negative performance aspects brought on by the system.

In this thesis, methods for conceptually designing and simulating the mission performance of regional fuel cell aircraft are presented. These produce representative airframes and propulsive systems that then can be used for studying the aircraft’s performance, in areas such as the impact of choices made in the cryogenic storage design.

In the first appended paper, a regional fuel cell aircraft was sized for Nordic market requirements and its cryogenic storage evaluated in two different types of flight operation. For the conventional design mission, a tank with moderate ventilation pressure and high insulation layer count struck the best balance between weight and boil-off losses. For the return-without-refuel mission, the tank with a high ventilation pressure and high insulation count minimized boil-off losses and outperformed the lighter tanks for groundhold times in excess of 2 hours.

The second paper covers the integration of sizing and mission methods into SUAVE. This includes a routine for sizing the cooling system using a conceptual design heat-exchanger code. Additionally, the procedure for performing redesigns of existing turboprop aircraft is described. An ATR 42 is redesigned for fuel cell propulsion which requires airframe resizing due to increased weight and the cryostorage. For 6 m3 of fuel the take-off weight increased by 6% and exhibits a 5.5% MAC centre-of-gravity shift between full and empty conditions.