Airlines emitted 900 million tonnes of CO2 in 2020
Burning jet fuel at a higher altitude has more of an effect on the climate than burning it at ground level - also emitting nitrogen oxides, black carbon and water vapour. 3x more climate damage than CO2 alone
In terms of CO2 alone, air travel is around 12% of emissions in the transport sector
Who pays for the increased cost of travel if hydrogen fuel is used, carbon offsets are bought, etc.
“Drop-in” so can be used in planes currently in service
Energy dense (liquid hydrocarbons)
Allow for same flight ranges
Less polluting
Bad news
Supply is very low
Cost is very high (3-4x cost of conventional jet fuel/jet-A) 2/gallon
4 Buckets of alternative fuels
Waste fats, oils, or greases: expected to reduce impacts by 40-50%. Could supply maybe 2% of overall jet fuel use
Advanced biofuels: cellulostic fuels, municipal solid wastes, exhaust gases from steel plants (to alcohol to jet fuel). More expensive but supply is much, much larger if tech and CapEx is overcome. Something like 80% reduction in life-cycle greenhouse gases
Electrofuels/synthetic fuels: Basic idea is you capture carbon from a source (air/landfill), generate hydrogen from renewable electricity, synthesise jetfuel from that. E-fuel, syn-fuels, e-kerosene. Huge supply but expensive. $5 euros/litre. 99% reduction in gases. Nearly infinite supply.
Crop-based biofuels: soy, corn, or palm. Generally considered a bad idea as this may indirectly trigger deforestation in the tropics. May be net negative for this reason and may produce more emissions than base jet fuels
Cheap options have limited supply, need to reduce costs of expensive options
Most cost for cheap fuels is for the feed stock, where most cost for expensive options is capital cost, the upfront cost of the facility after which the cost drops for operations.
Hydrogen
Airbus is currently looking at 3 different designs
Very energy dense on a mass basis but not a volume basis
Further Research
Airbus hydrogen designs
Hydrogen is not very heavy, which is a good thing for fuel. It does, however, require 4-5x more space than conventional jet fuel.
Most fuel on an aeroplane is stored in the wings - changing storage areas can affect the centre of gravity.
There is a ‘mass penalty’ when using batteries in flights, as they do not reduce weight (like fuel does as it is burned through the flight).
Flight physics are very punishing for electric aircraft