Biofuels, Electrofuels, Electric or Carbon-free?: A review of current and emerging Sustainable Energy Sourcing for Aviation (SESA)

Abstract

Climate neutrality is becoming a core long-term competitiveness factor within the aviation industry, as demonstrated by the several innovations and targets set within that sector, prior to and especially after the COVID-19 crisis. Ambitious timelines are set, involving important investment decisions to be taken in a 5-years horizon time. Here, we provide an in-depth review of alternative energy sourcing technologies for aviation revealed to date, which we classified into three main categories, namely liquid fuels (biofuels, electrofuels), electric aviation (all electric and hybrid), and carbon-free options (hydrogen-based, solar-powered). For liquid fuels, 10 pathways were reviewed, for which we supply the detailed process flow picturing all input, output, and co-products generated. The market uptake and use of these co-products were also investigated, along with the overall international regulations and targets for future aviation. As most of the inventoried pathways require hydrogen, we further reviewed six existing and emerging carbon-free hydrogen production technologies. Our review also details the five key battery technologies available (lithium-ion, advanced lithium-ion, solid-state battery, lithium-sulfur, lithium-air) for aviation, as well as the possible configuration schemes for electric propulsion (parallel electric hybrid, series electric hybrid, all electric, partial turboelectric and full turboelectric) and reflects upon the inclusion of hydrogen-powered fuel cells with these configurations. Our review studied these three categories of energy sourcing pathways as modular technologies, yet these still have to be used in a hybridized fashion with conventional fossil-based kerosene. This is among others due to an aromatics content below the standardized requirements for biofuels and electrofuels, to a too low energy storage capacity in the case of batteries, or a sub-optimal gas turbine engine in the case of cryogenic hydrogen. Yet, we found that the latter was the only available option, based on the current and emerging technologies reviewed, for a long-range aviation completely decoupled of fossil carbon. The various challenges and opportunities associated with all these technologies are summarized in this study.