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Comment by tombert | Hacker News Reader

I am speaking completely out of ignorance on this, so someone with a bit more familiarity with both physics and chemistry might be able to clarify.

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From what I understand, one of the biggest issues with trying to replace commercial cross-country jets [1] with electric alternatives is due to the fact that there isn't enough energy density in a battery to make that possible, and as a result, airplanes are a huge polluter towards climate change.

Would it make more sense to try and make hydrogen jets, at least if we can make the hydrogen relatively efficiently?

[1] I know jets really only make sense with fuel, replace "jet" with "big airplane"

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Elemental hydrogen has this problem: it is hard to store. You need low temperatures, high pressure, and corrosion resistant containers. But there is one weird trick: if you bind hydrogen to carbon atoms, then you get best of both worlds. The fuel remains energetic and yet is relatively unreactive outside of combustion and is easy to store. Perfect for aviation.

I wonder what efficiencies look like for making hydrocarbons from atmospheric CO2 and water.

Not great, not terrible [https://en.wikipedia.org/wiki/Fischer%E2%80%93Tropsch_proces...]

Not price competitive with fossil fuels, but then not much is - hard to beat ‘mega joules for free*’ as it were.

Not disagreeing but just reminding.

It’s hard to compete with “let everyone else in the world pay for the true cost of this energy”.

That’s what makes fossil affordable and why we need carbon taxes right this second.

Not wonderful, since atmospheric CO2 is so dilute. You need either a hefty carbon tax or a ban on oil extraction to make the economics work. https://en.wikipedia.org/wiki/Synthetic_fuel

Our current spike in natural gas and oil prices is something of a dry run for what carbon taxes would look like. So far it doesn't seem like the approval ratings of the governing party can survive it.

Isn't that basically what trees do? Solar-powered, too!

H2 is not hard to store in liquified form, LH2. You just need good insulation. Earth is excellent insulation.

For aviation, sticking on a carbon that outweighs your hydrogens by almost 8x gives up a huge efficiency advantage. The efficiency advantage overcomes inconvenience of handling cryogenic LH2. My bet is the tanks will be nacelles slung under the wings alongside the engines.

Came across this recently:

https://en.m.wikipedia.org/wiki/Powerpaste

It might pan out and reduce issues with hydrogen leakage

The powerpaste is magnesium oxide. One magnesium atom weights around as much as 24 hydrogen atoms.

Thus for every hydrogen molecule you have to carry around 13 times the weight in magnesium oxide. Aka: This stuff is very heavy. Not exactly the thing you want on planes.

How heavy is the h2 pressure tank relative to the h2?

Depends how big it is. If you make the tank twice the size, it weighs 4 times as much, but carries 8 times the hydrogen.

Short answer: yes, hydrogen jets are probably the future. Electrochemical batteries fundamentally lack the required mass to energy ratio necessary for air travel. But there's a lot of challenges to actually powering an aircraft via hydrogen, and it's probably not going to happen for a while.

The long answer:

* The question is moot until we have widespread decarbonization of the energy grid. Currently, our hydrogen mostly comes from steam reformation - splitting methane, CH4, into 2H2 + CO - which emits carbon dioxide. Once our electrical production is carbon-free, then we can think of producing hydrogen and powering vehicles with that. Electrolysis can serve as a carbon-free source of hydrogen, if the electricity is produced from a carbon-free source. There are more efficient forms of hydrogen production, like thermochemical hydrogen production with heat provided by nuclear reactors.

* Once carbon-free hydrogen production becomes available, then there's the question of building hydrogen powered vehicles. Hydrogen is the most energy dense fuel by unit of mass, except for nuclear fuels, but it's about half the energy density by unit of volume compared to hydrocarbons. Substituting fossil fuels with hydrogen is probably more feasible in container ships than aircraft. Hydrogen containment, be it pressurized or liquid-cryogenic, scales better at higher capacities as there is a better volume to surface-area ratio. Aircraft wings (long and skinny) are basically the worst shape for hydrogen containment. Past examples of hydrogen powered aircraft stored it in the fuselage [1], which would sacrifice cargo and passenger space.

* Lastly, for aircraft there's also the question of making engines that don't produce greenhouse gases. High-temperature hydrogen gas turbines will also produce nitrous oxides, which are greenhouse gases. This can be mitigated by running turbines at lower temperatures, but that reduces efficiency. There's also the idea of using hydrogen fuel cells to produce electricity to run turbines, but fuel cells and electrical motors don't have as good power to weight ratios as combustion turbines.

I'm much more optimistic about converting maritime transport to hydrogen fuels than aircraft. That said, the military is very interested in hydrogen planes. One of the limiting factors in modern naval warfare is fuel to fly planes. With hydrogen powered planes, a carrier could produce hydrogen with its nuclear reactors giving carriers an effectively unlimited source of aviation fuel. If they figure it out, hopefully the tech propagates to civil air transport.

1. https://en.wikipedia.org/wiki/Tupolev_Tu-155

The natural form for aviation fuel is liquified hydrogen, LH2, sidestepping worries about embrittlement. LH2 is so much more efficient as an avaition fuel, despite problems handling ultra-cold liquids, that kerosene-powered aircraft will be simply unable to compete anywhere LH2 craft are in use. The rate of collapse in value of kerosene aircraft will be limited only by the rate that we can build out new or retrofitted LH2 aircraft.

It is not clear whether existing aircraft or designs can be usefully retrofitted for LH2. Existing wing tanks are, in any case, useless for LH2. You need much more room for LH2 tankage, and you might not want a big LH2 tank inboard; there are safety reasons for the fuel in current aircraft being out in the wings. So, maybe the LH2 is carried in nacelles slung under the wing, next to the engines. We will need to build up a great deal of aerogel manufacturing capacity to insulate them.

It is possible that existing wing tankage can be used to carry NH3 to inject, in small proportion, to minimize NOx pollution. Given a bit of surplus N, the N prefers to make N2, and the O prefers to make H2O. E.g., existing natural-gas plants inject NH3 to reduce their NOx output.

For shipping, anhydrous ammonia is probably a better choice of fuel. Existing engines can burn it, so you need only new room-temperature ammonia tanks and new plumbing.

> I know jets really only make sense with fuel, replace "jet" with "big airplane"

‘Jets’ could make sense even without fuel if you consider that jet propulsion [2] is the generation of thrust using a fast-moving stream of fluid. Today’s jet engines are already turbofans [3] where only part of the propelled air passes through the fuel combustion chamber.

[2]: https://en.wikipedia.org/wiki/Jet_propulsion

[3]: https://en.wikipedia.org/wiki/Turbofan

Airbus has dangled the hydrogen jet concept, but hydrogen presents a lot of problems (low density, cryogenic, leaks through solid metal causing embrittlement).

It probably makes more sense to just synthesize longer hydrocarbons.

> It probably makes more sense to just synthesize longer hydrocarbons.

Sorry, ignorance on my end is showing again; wouldn't a longer hydrocarbon still emit CO2 as a by-product since carbons are part of its makeup? Or does a longer hydrocarbon mean that there's a higher hydrogen-to-carbon ratio and therefore the pollution is less horrible?

It would mean adding carbon. In order to be carbon neutral that carbon would need to extracted from the carbon cycle.

Yes, but to make the hydro-carbon, you need to put carbon in. If you extract that from air you’re carbon-neutral.

You can also be carbon-neutral by making that cycle larger: extract carbon from trees, create fuel, burn it, and let new trees you planted convert the CO2 back into wood.

Embrittlement is a product of high-pressure high-temperature storage. LH2 has its own difficulties, but they don't worsen with time.

Not my area of expertise either, but I gather that hydrogen is difficult to store in quantity, requiring large heavy pressure vessels, and it has relatively low energy density. Both of which count against using it for commercial flight.

No one would bother with heavy, high-pressure hydrogen containment for large aircraft, so that is a moot point. The clear choice for aircraft is LH2, where it is fantastically more efficient than hydrocarbons.

>airplanes are a huge polluter towards climate change.

This is true on a relative basis: flying across the US puts out a lot of CO2 for a single person.

But on an absolute scale, it's pretty tiny: 2% of total CO2, 12% of transportation CO2.

Just the CO2 numbers aren't giving you the right picture, as the other effects multiply the emissions effects of air travel on warming (for example radiative forcing).

That's also why an air travel emissions factor is used by policymakers when talking about its effects.

(Also, US transportation emissions in other sectors, especially road, are enormous in comparison to other countries, for context on the relative comparisons: https://www.statista.com/statistics/1201189/road-transport-s...).

Jet engines can burn hydrogen.

Then would it make sense for Delta or United or KLM or something to start investing in retrofitting their planes to hydrogen? I would think that governments could give tax incentives to do so, and as a result make planes substantially less horrible for the environment.

Interesting idea. Supply chain issues aside I don’t know if the current gas tanks could be retrofitted for hydrogen. Maybe fuel cells would be easier. Could just shoot water out of its little plane tush as its flying.

Is there enough O2 at plane levels for fuel cells?

Sorry, it would not remotely make sense. Planes store liquid fuel in the wings. They are engineered from square one to optimize for this design. Hydrogen requires either cryo or high pressure, neither are going to fit in the wing spaces. Even if you could physically make it work, you'd have to re-certify the plane, which just wouldn't happen.

A hydrogen plane would have to be designed from the ground up.

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