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Comment by shawnb576 | Hacker News Reader

I think the hydrogen-electric [1] development is more promising here because it has the potential to deliver long haul flights with no emissions or contrails.

While H2 isn't great for decarbonization at scale, in specific industries like aviation it might be applicable assuming it's made cleanly.

[1] https://www.zeroavia.com/

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What's the justification to expect that 25-50% of human climate impact will be from aviation by 2050?

Growing population? Lower cost of access to flight? Other industries decarbonizing and causing aviation impact to just be a larger fraction of overall impact?

IIRC it's a combination of air travel continuing to expand at the roughly 2.5% yearly rate it has been doing for the past decades, and other sectors of the economy successfully decarbonizing.

Isn't a contrail a condensation trail? In a hydrogen fuel cell the waste product is H2O so surely there is a contrail.

I believe contrail formation requires nucleation sites, which is normally provided by soot particles from the burning fuel.

It doesn’t in a really moist layer. Ice condensation nuclei are relatively rare (thus the silver nitride seeding idea), so a lot of times ice clouds form when a parcel of air gets supersaturated. This is pretty common.

Good point, thanks.

Even more, because all the energy is from "burning" H2 only, no Cs to burn. Whether or not visible contrails form is a separate question. Even transparent water is highly efective outside of the visible range.

If you burn the hydrogen in a turbine, you need to worry about this. If you use a fuel cell you can easily capture the water.

Presumably at the cost of the airplane getting heavier as it flies rather than lighter as today?

Depends on whether the plane currently takes off with any water that could instead be replaced mid-flight and/or if dumping the excess water at different temperatures/places could avoid the issues.

I would be shocked if the airplane has use for water in the same order of magnitude as the combusted fuel multiplier.

Jet-A has around 43 MJ per kg. An A320 burns around 2.5 tons of fuel per hour. Hydrogen is about 3x as dense on an energy basis, so call it 0.8 tons of hydrogen per hour. Hydrogen combustion to water has a mass ratio of around 9:1, meaning an A320 would be creating over 7 tons of water (~1500 gallons) per hour of cruise flight. The potable water tanks on an A320 are around 50 gallons or around 2 minutes of cruise flight (that’s potable water tanks only). It’s a different order of magnitude.

Having listened to both Boeing and Airbus recently, neither seem to see any alternatives to SAF for long haul though.

Hydrogen has it's challenges. I heard someone did the math on running Heathrow exclusively on it who came to the conclusion that you'd basically need 3-4 reactors to produce it.

Hydrogen production, though, is one of the workable applications of wind turbines.

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