A review of hydrogen aircraft propulsion systems: recent advances and environmental perspectives

The present work performs a review for using hydrogen in aircraft propulsion systems analyzing challenges and opportunities with the two main driveline architectures: direct combustion of hydrogen and fuel cells. First, the capability of hydrogen aircraft to become more energy efficient than conventional aircraft are discussed on system level, by extending previous review work. Then, challenges for hydrogen combustion and ways to limit emissions by lean direct injection and micromix combustion are discussed. Polymer electrolyte membrane (PEM) and solid oxide fuel cells are reviewed and the outlook for high temperature PEM fuel cells and challenges with per- and polyfluoroalkyl substances (PFAS) emissions are discussed. Dual fuel aircraft and flexible combustion are discussed as ways to provide a transition to a hydrogen economy. Additionally, hybrid configurations and new cycles that simplify hydrogen integration are reviewed. Finally, recent promising results on water emissions and contrail formation for hydrogen combusting aircraft are discussed.