Methodological framework for Life Cycle Assessment of sustainable aviation (SA) systems.

A comprehensive framework is proposed for Life Cycle Assessment (LCA) in the field of commercial aviation (passengers and cargo), capable to ensure transparency and comparability when evaluating the overall environmental performances of four emerging aviation systems, i.e., biofuels, electrofuels, electric, and hydrogen. The projected global revenue passenger kilometer (RPK) and is suggested as the functional unit for two timeframes representing near-term (2035) and long-term (2045), and for two segments, namely domestic and international. To solve the difficulty of comparing liquid fuels and electric aviation, the framework proposes a methodology to translate projected RPK into energy requirements for each of the studied sustainable aviation systems. Generic system boundaries are defined with their key activities for all four systems, with the biofuel system being sub-divided into two categories to distinguish whether it stems from residual or land-dependent biomass. The activities are grouped in seven categories: (i) conventional (fossil-based) kerosene activity, (ii) conversion processes from feedstock supply (to fuel or energy production for aircraft operation), (iii) counterfactual uses of constrained resources and displacement effects associated to co-products management, (iv) aircraft manufacture, (v) aircraft operation, (vi) additional infrastructure needed, and (vii) end-of-life management (aircraft and batteries). Considering applying regulations, the framework also includes a methodology to handle: (i) hybridization (the use of more than one source of energy/propulsion system to power an aircraft), (ii) the mass penalty affecting the number of carried passengers in some of the systems, and (iii) impacts stemming from non-CO tailpipe emissions - aspects that are currently neglected in most LCA studies. The proposed framework builds upon the most recent knowledge in the field; however, some choices are dependent on upcoming scientific advances concerning e.g., tailpipe emissions at high altitude and their environmental impacts, new aircraft configuration, etc., and are subject to significant uncertainties. Overall, this framework provides a guideline for LCA practitioners addressing new energy sources for future aviation.