Decarbonising Aviation
Long runway required
Aviation represents about 3.5% of global warming effect in total - about 2-2.5% of CO2 emissions at around a GT, but additional impact through release of nitrous oxide and water vapour. Aviation is particularly tricky to decarbonise for a number of reasons. Firstly, energy density is critical because the mass has to be lifted into the air, so it is difficult to electrify like road transport, or have point-source carbon capture like cement. Secondly, fuel represents a large portion of airlines’ costs at something like 20-30% of operating expenses (it swings around). Lastly, the airline industry operates at very low margins, which reduces its capacity to make large investments in decarbonisation. Global profits for commercial airlines was something like $30bn pre-COVID. So, with emissions around 1GT, airlines would need to spend a third of their profits on offsetting at $10 per tonne to become carbon neutral, even if high-quality offsets were available at that price and volume. Which they are not.
For this piece, I plugged into this episode of Climate Now with the Steve Cosnka Executive Director of the Commercial Aviation Alternative Fuels Initiative (CAAFI) and this episode of Catalyst with Dan Rutherford, director of maritime and aviation programmes at The International Council on Clean Transport (The ICCT). Both CAAFI and The ICCT have lots of further content on the topic.
The 3% Problem: Whilst aviation has seen dramatic efficiency improvements, at about 2% per year for the industry, the industry is growing about 5% per year.
So, despite efficiency improvements, aviation emissions have increased by about a third over the last decade (pre-COVID):
The airline industry’s flagship climate program is the Carbon Offsetting and Reduction for International Aviation (CORSIA), which is committed to carbon-neutral growth from 2020. The industry is also aiming for 50% reduction of emissions by 2050.
Three paths to decarbonising aviation:
Sustainable Aviation Fuel (SAF) - liquid hydrocarbons that are drop-in fuels, i.e. can be mixed with fossil kerosene, slot into existing infrastructure and run with engines (although even better with new engines)
Hydrogen - either compressed gas or liquid
Electrification
SAFs are the only pathway currently contributing and only at a tiny scale - about 0.05% of aviation fuel used in 2019. However, with the Green Deal the EU is mandating that 5% of aviation fuel is SAF by 2030 and 63% by 2050 with a submandate for 28% for e-fuels by that time - full proposal here for the highly motivated. (A draft proposal is pushing to increase those 2050 targets to 100% SAF with 65% being e-fuels.)
As well as decarbonisation benefits of SAF, it also offers improvements on the co-pollutants.
SAF can be obtained by a number of pathways:
Upcycling waste oils, e.g. cooking oil, animal fats, etc. Low cost, but also low availability. Full life-cycle emission reductions of 40-50% vs fossil (produced by existing refiners, e.g. Total and Philips 66, and Neste)
Biofuels - from municipal solid waste (MSW) or cellulosic biofuels, greater scale potential, greater decarbonisation potential at around 80% reduction vs fossil, but currently more nascent and more expensive (companies include Fulcrum, Red Rock) Note that the Energy Transition Commission suggested that SAFs should be prioritised for use of sustainable biofuels given the relative difficulty of decarbonising it vs other sectors.
Electro fuels / e-kerosene / power-to-liquid - synthesising green hydrogen and CO2 (either from flue gas or DAC), most nascent, most expensive, theoretically unlimited scalability but would imply an incredible demand for clean electricity, which currently is better used for replacing fossil electricity generation and electrifying other sectors (companies include Ineratec and LanzaJet) [the opportunity cost of using clean electrons is a key consideration when weighing different climate solutions and will continue to be until we crack affordable fusion, deep rock geothermal or other superabundant zero carbon electricity, i.e. for decades]
Lastly - it is possible to use energy crops like corn, soy or palm but a dangerous path that should be avoided due to the risk of increased tropical deforestation.
SAF cost: generally about x2 the price of fossil fuel today, but electrofuels significantly higher premium (think x5).
SAF quality: jet fuel has a number of characteristics besides the energy content which have been used in the general engineering of the airplane systems. It is used not only as a fuel, but as a lubricant, heat-transfer mechanism, etc. Means that different SAFs will have different blending limits (currently limited at 50% limit, but industry working to remove that limit). The 7 different internationally approved technical pathways with blending limits can be found here.
Hydrogen: has the advantage of being energy dense by mass, which is especially helpful when you need to lift the fuel off the ground, but is x3-4 less energy dense by volume than fossil jet fuel, which means that additional storage is required. The low volumetric density means that hydrogen is unlikely to be viable for long-haul flights, but should be viable for flights <4000km, which represent about two thirds of emissions. The most prominent start up in this area is ZeroAvia.
Electrification: challenging for anything over very short haul because of the weight of batteries - (currently) x40-50 time less energy dense than jet fuel, plus, unlike fuel which gets burned off through the flight, the weight penalty is carried through the flight with batteries. Example companies working in the space include Wright Electric (focussing on 100 seater planes for 1 hr flights, electrifying one engine at a time), Ampaire (working on small, hybrid aircraft) and Eviation (starting with 9-seater pure electric). For further information on electric aviation, the National Renewable Energy Lab released this report last year. Indeed, they only see electric/hybrid commercial aviation appearing in the 2040s and 2050s.
eVTOLs - Stands for electric vertical take-off & landing - unlikely to be able to make a meaningful dent in aviation emissions. Aviation is really a mass-transit mode of transport. eVTOL companies (e.g. Joby, Lilium, Archer) are really opening up a new, super-short-haul segment, rather than decarbonising existing demand.