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Fearlessleader85 t1_iy91spo wrote

Reply to comment by haircut50cents in Rolls-Royce successfully tests hydrogen-powered jet engine by je97

For airplanes, the energy intensive nature of it doesn't matter quite so much. We can produce it on the ground using alternatives, but up in the air is where energy density matters, both per unit volume and mass. Hydrogen has mass energy density in spades, which does matter.

There's definitely some barriers, but if we can get it to work, it opens some massive doors for fighting climate change.

It's possible, even likely, that an algae based biofuel could be a lower hanging fruit, but before that gets to be carbon neutral, it has to scale to support the full supply chain. Hydrogen doesn't have that limit. We could produce it cleanly, even with non-firm power options like solar and wind. Then from day one, it's carbon neutral.

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haircut50cents t1_iy9crlr wrote

Hydrogen is great per mass but horrible per volume. That's why it has to be cooled down past liquid into slush. Even then the tanks needed are enormous compared to jet fuel used now. Would require a complete redesign of airplanes - maybe larger lifting body aircraft or flying wings.

That would require a more space at airports. Maybe the supply chain is there but rebuilding all the worlds airports? That's a mess.

I do like biofuels because of the mass and volume energy storage and they drop into the supply chain really well.

The best way to stay carbon neutral is always to not build new stuff to replace working stuff. True for cars and I would bet even more so for airplanes.

Interesting discussion though, thank you!

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Fearlessleader85 t1_iy9e3g8 wrote

I was also thinking of much larger lifting bodies, like flying wings. I don't think you'd need to rebuild airports completely, but renovations would for sure be needed. But in that change, you could get some significant benefits, like higher altitude flights with greater ease.

But i disagree with your claim of reusing things rather than building new. For most things like cars, the carbon use of the fuel or energy quickly outstrips the corbon footprint of production. So, continuing to use that refrigerator from 1956 that keeps on chugging is churning out more carbon every few years than building a new fridge. Replacing a car that gets 20 mpg with a new one that gets 30-35 mpg has a very rapid "carbon payback".

If efficiency is more or less constant, sure, keep using the old thing as long as possible. If new versions produce significant improvements, replacement is the best option. The math for these decisions isn't that complex.

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