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Comment by Hypx | Hacker News Reader

Fuel cells are too heavy, at least for the time being. Other than biofuels or synfuels, there's basically no alternatives. Aviation companies will have to pick one of those fuels.

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Fuel cells are better because they don't emit directly. Turns out high altitude water vapor is itself a powerful greenhouse gas. At least with fuel cells, you could collect the water and dispense it periodically as a liquid at lower altitudes. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/200...

But actually, batteries are better than people think. 1000km battery-electric range is feasible with current chemistries, and ~3000-5000km is feasible with metal-anode, maybe lithium-sulfur or other advanced chemistries plus extremely high performance structures and aerodynamics. Lithium-air batteries with these improvements could enable >10,000km flights.

H2O as an emission lasts for days, not centuries. It shouldn’t be that big of a deal.

You do realize a fuel cell is a hydrogen-air battery? If you’re going to bring up metal-air batteries, you might as well skip to the end and try to figure out fuel cells.

It lasts for days in the lower troposphere but years in the stratosphere. And kg for kg, H2O is a much stronger Greenhouse gas than CO2, so the long-term averaged effect is on the same order as CO2 (maybe higher, depending on conditions).

And sure, I guess you could argue lithium-air is a type of fuel cell (or that hydrogen fuel cells are a kind of inefficient battery), however lithium-air is much more energy dense (volumetrically) than hydrogen-air and both the "fuel" and the "exhaust" are solid and stay in the battery, which has operational advantages. (Although to be completely fair, hydrogen fuel cells are high TRL whereas lithium-air is still lab scale at best.)

> It lasts for days in the lower troposphere but years in the stratosphere

I addressed this in my other reply. Table 1 of the study you cited stated that the residence time of stratospheric water vapor is measured in days. I'm not seeing a lifetime of years in that study.

> however lithium-air is much more energy dense (volumetrically)

Maybe. Lithium-air absorbs oxygen when it is operating. Not sure how to calculated this though.

But are batteries more efficient for long distance planes?

Conversion losses may be lower, but a battery-powered plane doesn’t lose weight during flight. That makes the impact on landing harder, so planes will have to be made stronger and, thus, heavier.

And yes, that should make quite a difference. Reading https://en.wikipedia.org/wiki/Fuel_fraction, 40% of the take-off weight of long-haul flights is fuel. Not all of that will be gone at landing, but if 10% of fuel is reserve, such an airliner still will be a third lighter at landing time.

Also, commercial planes have turnaround times of about an hour, sometimes half of that (https://simpleflying.com/turnaround-time-importance/). I would think pumping in hydrogen would be easier to do in that time than charging batteries. Batteries could be replaceable, but that would add more weight.

but wont this just create another problem, the mass for the 'spent' hydrogen would be 9 times more than original because of Oxygen being much weightier (atomic mass: 16). so your fuel tank would basically get 9 times heavier. that will negate any benefit you get from higher energy density. also it may create storage problems because now you have to provision another tank for water storage.

I think they'll just handwave it and release it periodically.

Metal-air cells have the same problem. That’s why they were briefly considered for airplanes, and then abandoned once they realized that problem.

Releasing it periodically can be done at a slightly lower altitude and in a liquid phase, both of which should significantly reduce impact.

Batteries are way, way, way worse than you imagine. Their energy density would need to get >10x better, when 2x is a miracle.

Furthermore, without internal combustion, you are limited to ~350 knots.

i study this professonally. batteries are sufficient if we make other improvements as well. and not true about combustion being essential.

Do you have any links where I can read more about this?

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