Zero Carbon Emission Aviation Fuel Technology Review - The Hydrogen Pathway

Abstract

<div class="section abstract"><div class="htmlview paragraph">The commercial aviation currently accounts for roughly 2.5 % of the global CO<sub>2</sub> emissions and around 3.5% of world warming emissions, taking into account non CO<sub>2</sub> effects on the climate. Its has grown faster in recent decades than the other transport modes (road, rail or shipping), with an average rate of 2.3%/year from 1990 to 2019, prior to the pandemic. Moreover, its share of Greenhouse (GHG) emissions is supposed to grow, with the increasing demand scenario of air trips worldwide. This scenario might threaten the decarbonization targets assumed by the aviation industry, in line with the world efforts to minimize the climate effects caused by the carbon emissions.</div><div class="htmlview paragraph">In this context, hydrogen is set as a promising alternative to the traditional jet fuel, due to its zero carbon emissions. Furthermore, its high energy content makes it suitable for the aviation industry, especially in the short to medium haul flights niche, that currently accounts for around 43.8% of global aviation CO<sub>2</sub> emissions. Hydrogen fueled aircrafts might have fewer range limitations, compared with battery electric counterparts, currently restricted to smaller commuter flights, given the low energy density of the batteries. For long range flights, liquid fuels alternatives, such as sustainable aviation (SAF), still have a leading position in the short to medium term environmental agenda.</div><div class="htmlview paragraph">Hydrogen can be burned directly in (modified) gas turbine engines, in fuel cells, to generate electricity to power electric motors, or in hybrid-electric propulsion systems. Nevertheless, despite the environmental benefits, there are great challenges to make hydrogen a viable alternative to the fossil liquid jet fuel. One of the main hurdles is the fuel storage, associated with the much higher volume and storage system complexity required for (liquid) hydrogen, to provide the same amount of energy of liquid jet fuel. These fuel features require aircraft and engine design modifications, as well as a new fuel distribution infrastructure. Another major challenge is the full understanding of the non CO<sub>2</sub> related climate impacts of hydrogen combustion, such as H<sub>2</sub>O emissions at cruise altitudes, which interacts with soot and particles in the atmosphere, to form contrails. Finally, the H<sub>2</sub> cost, might be addressed to enable a fair competition with fossil jet fuel.</div><div class="htmlview paragraph">Currently, there is a great research effort, from both the government and academic sectors, as well as from the aircraft manufacturers, which includes the test of demonstration H<sub>2</sub> aircraft prototypes. This effort also might include policies to foster environmental friendly fuel alternatives, to make them cost competitive.</div><div class="htmlview paragraph">This work presents a review of the aviation hydrogen technology, with a focus on both the propulsion and onboard storage systems, as well as on the potential environmental benefits and the associated costs of the aviation hydrogen fuel pathway.</div><div class="htmlview paragraph">The review research has been supported on a wide search on the technical literature, by using up to date (mainly published in the last two years) articles, whitepapers and technical reports, available at specialized directories and scientific journals. The search has used key words, such as aviation sustainability, hydrogen for aviation propulsion, aviation environmental footprint (and Greenhouse emissions) reduction, as well as liquid and gaseous hydrogen storage.</div></div>