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Comment by thot_experiment | Hacker News Reader

I am very skeptical they will be able to get the efficiency numbers to a place where it makes sense in the timeframes they're throwing around unless oil goes up like 4x in price. Storing hydrogen is really hard and the related tradeoffs are much tougher than with a room temp liquid hydrocarbon. I imagine burning it in turbines also comes with some new issues but I know less about that. I think it's a decent path forward if we can figure it out, and I imagine the engine related problems are likely much more surmountable than the storage issues.

There's also the related fuel infra that will need to happen before this is viable. It's far away unless something truly drastic changes the calculus on fossil fuels. (which would be great)

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Storing hydrogen underground is, in fact, very easy and cheap. And, storing liquified hydrogen is likewise cheap. We can expect hydrogen to be produced on-site at large airports from power delivered over transmission lines, stockpiled against low-wind low-sun periods.

Once hydrogen-fueled aircraft enter a particular air freight market, kerosene-fueled craft will be unable to compete, because with LH2's much better mass-energy density, the fuel mass dispensed with may be replaced with paying cargo. Furthermore, hydrogen fuel produced on-site electrically will be cheaper per unit joule than mined, refined, and transported kerosene.

I am surprised to see designs with inboard LH2 tanks. I would expect to see LH2 tanks in wing nacelles, minimizing piping to engines.

Producing hydrogen is not very energy efficient. Electric power would have to get much cheaper to make hydrogen cost competitive with fossil fuels. While hydrogen has sightly higher mass-energy density, the tankage is larger and more complex so the total mass savings is limited and there's a significant drag penalty.

> While hydrogen has sightly higher mass-energy density

It’s not slightly, it’s three times as much, or more.

Per wikipedia [1], Hydrogen has an energy density of 120 MJ/kg vs 43 MJ/kg for kerosene. That’s if you burn it outright. If you put Hydrogen in a fuel cell, and the output of the reaction is liquid water rather than water vapor, then the energy density is 142 MJ/kg.

[1] https://en.wikipedia.org/wiki/Energy_density#In_chemical_rea...

The mass density of hydrogen is very much negated by the fact that liquefied hydrogen needs to be in a 700 bar pressurised tank whereas kerosene is happy at atmospheric pressure. The more valid comparison is "hydrogen in a hydrogen pressure vessel" to "kerosene in a tank".

I know cars are different, but the Toyota Mirai is actually heavier than the Tesla Model 3 even though it only needs a measly 5 kg of hydrogen to go as far as the Tesla.

They plan to use liquid hydrogen [1, page 2]

"FlyZero has concluded that green liquid hydrogen is the most viable zero-carbon emission fuel with the potential to scale to larger aircraft utilising fuel cell, gas turbine and hybrid systems. This has guided the focus, conclusions and recommendations of the project."

[1] https://www.ati.org.uk/wp-content/uploads/2022/03/FZO-AIN-RE...

I don't see why LH2 would need to be kept at 700bar. The STS LH2 tank was operated at ~1.6 bar.

And what happens if ATC directs the airplane to hold on the runway for a few hours? The space shuttle used cryogenic fuel, and after loading they had to continuously replenish the liquid hydrogen due to leakage and evaporation for several hours until launch. That can work for a rocket with a huge operating budget on a dedicated pad but I don't see that working at a busy commercial airport.

Jet fuel is much more stable. It can sit in simple fuel tank for months with no special cooling or containment measures.

I suppose they have to flare off the H2. Or the plane itself has the capacity on board to capture and cool boil off.

LH2 in tanks boils off and is vented. There will be different procedures for LH2 vehicles on hold than today. That might involve plugging in somewhere for active cooling while on hold. Or, it might just mean you load on enough LH2 to make up for losses while holding.

The practical merits of Jet-A fuel will be traded off against the value proposition of LH2 fuel. LH2 will win easily where volume is high and profits are greatest. So, marginal routes and uses will be the last to switch over.

The space shuttle, by the way, could not afford insulation on its external LH2 and LOX tank. We may assume that airliners' tanks will have vacuum- or aerogel-insulated tanks.

You keep saying this, but nobody keeps LH2 at 700 bar, or even 2 bar.

I have to conclude you are just making it up, each time, and after you have been repeatedly corrected.

Efficiency of electrolysis will only ever improve. In the meantime, it can be produced and banked preferentially during periods when power is cheapest.

>Storing hydrogen is really hard

Wouldn't this just be a low pressure insulated tank? The temperature would self regulate as the liquid H2 boiled off.

There are also issues like hydrogen embrittlement, tiny cracks, etc.

These are all solvable, but it's just expensive and hard.

Hydrogen embrittlement is a problem in high-pressure high-temperature tanks.

When the pressure is low, nothing is being driven into the metal.

I don't think they burn the hydrogen at all. Rather, it's turned into water and free electrons by some chemical process whose details I totally forgot.

One idea I've seen floating around would be having small scale nuclear plants near airports producing hydrogen from water and electricity (the reverse process of what happens in the plane turbines).

You’re talking about fuel cells, which are only used with the smallest (prop) design. The two larger jets are intended to have hydrogen turbines, burning hydrogen and producing water.

I love the idea of small modular reactors (SMRs) being used to produce the hydrogen from water. We also need a vast buildout of safe, clean, dependable modern nuclear reactors to provide the developing world with enough energy to make the demographic transition (via a higher standard of living) to better environmental practices.

I wonder if we'll just end up running jet engines on biodiesel for long-haul flights where batteries aren't viable.

Yes this has already been tested in existing commercial turbine engines and works fine. It's safe and won't require any changes to aircraft or aviation infrastructure.

The sticking point is the cost of manufacturing the fuel. It will only be economically viable with some major government intervention.

A sticking point for biofuels but in all likelihood one they share with hydrogen.

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