It's not just liquid vs gas.

Hydrogen has a couple of "unique" properties that make it especially disadvantageous.

Namely: It'll diffuse through just about anything on a fairly short time scale if it isn't kept as a liquid at cryogenic temperatures, and it causes almost all metals to become brittle.

I think hydrogen is basically a dead-end as energy storage, except in a few niche applications where it's produced as a byproduct and can be consumed immediately.

Steel production, where the hydrogen is a chemical reagent, might be one of the few exceptions where hydrogen is really the best choice.

Not to mention liquid H2 has quite a bit lower energy density than liquid fossil fuels. Many rockets have used kerosene in their first stages for this reason. Not the end of the world for aircraft, but certainly another major disadvantage.

But the energy density per mass is still better than Li-ion batteries.

Li-ion batteries are improving at a faster rate, and you can use your gravitational energy to recharge the battery.

How does that work? You run your propellor in reverse like a wind turbine?

This doesn't sound completely ridiculous but is there even any proof of concept for that? The gravitational potential energy doesn't seem all that high - most airplane fuel is burnt maintaining height, not gaining it - to justify the complexity of recovering some small fraction of it. Still, I once thought the same about regenerative braking in electric cars.

Hydrogen is a storage mechanism; there'd be a way to reclaim your energy either way

It's also prone to explode, and is a potent (indirect) greenhouse gas if it leaks into the atmosphere. Agreed that it will have some niche applications though. As well as the byproduct situation, there are likely situations where batteries wouldn't be feasible due to insufficient capacity or too great of weight. Aerospace maybe?

Hydrogen (and helium) are too light to stay in the atmosphere and are lost once released.

How is hydrogen a greenhouse gas (and what do you mean by "indirect") ?

I think it'd be an "indirect" GG because it's electricity intensive to obtain it, and that likely comes from a carbon emitting source. The same argument is often made about electric vehicles.

In this case it's about how it interacts with OH radicals in the atmosphere, meaning other GHG stay in the atmosphere longer. So it's not a greenhouse gas, but it makes others hang around longer.

> The reaction of hydrogen with OH radicals has a further side-effect of reducing the availability of OH radicals with potential impacts on the build-up of greenhouse gases.

https://assets.publishing.service.gov.uk/government/uploads/...

Well you don't typically get hydrogen from H2O, you get it from CH4. So your byproduct isn't oxygen, it is carbon. Unlike electric vehicles, it doesn't get greener as the grid changes to wind/solar/nuclear/hydro. There's just more hydrogens per hydrocarbon than water. Not to mention dealing with oxidation.

> Namely: It'll diffuse through just about anything on a fairly short time scale if it isn't kept as a liquid at cryogenic temperatures, and it causes almost all metals to become brittle.

Both claims are wildly wrong. Neither actually happens except at high temperatures and only with certain metals.

Could you elaborate about “wildly wrong”?

I am not an expert on this, but random internet searches such as [0] indicate that common materials, like high strength steel, are susceptible to both diffusion and embrittlement at room temperature

[0] https://www.energy.gov/sites/prod/files/2014/03/f10/pipeline...

Those tests seem to be at >200˚C if I’m not mistaken. Hardly room temperature. Regardless, the main trick is to avoid using certain types of high-strength steel, line exposed surfaces with polymers, or using composites where ever possible. The problem is for the most part solved.

So you're admitting the problem exists, which is my entire point.

It's hardly a theoretical-only problem, either:

https://en.wikipedia.org/wiki/Eastern_span_replacement_of_th...

In the same sense all chemicals have handling issues. In practice, it is basically solved.

That's probably due to them using a vulnerable alloy, and was likely exposed to hydrogen while at a high temperatures. This is nothing like a gas tank filled will hydrogen.

What's a high temperature though? -50C?

Well into the hundreds of degrees C.

That's why some bet on ammoniac.

It astonishes me how far behind aviation is behind surface transport when it comes to sustainable fuels. They still use leaded gas!

It seems you would try methane before you try hydrogen:

https://energynews.us/2013/08/26/midwest/could-natural-gas-f...

Petroleum-based liquid fuels contain mixed entity hydrocarbons that match a specification; if you have the perfect feedstock and markets for the fractions you don't use this is cheap. Worst-case you have to synthesize them with Fischer-Tropsch chemistry

https://en.wikipedia.org/wiki/Fischer%E2%80%93Tropsch_proces...

which is such a PITA (normally a chem E would be delighted that you can use iron as a catalyst, but it works so poorly that a desperate search of the other 91 elements found just 2 that sorta-kinda work)

The issue there is you are sticking C's onto the end of a chain and you have to deal with a network of reactions that produce products you couldn't care less for, such as petroleum jelly that gums up your catalyst, sticks up your downrisers, etc.

Sustainable motor fuels tend to be single entities such as 1-butanol, dimethyl ether, etc. You might need to blend something in for low temperature starting, but it's possible for a single entity fuel to be synthesized with decent yield.

General av is seen as a backwater that is barely hanging on and couldn't possibly get the lead out. The USAF has done biofuels trials, but commercial av is spooked to try anything that could leave passengers up in the air.

> It astonishes me how far behind aviation is behind surface transport when it comes to sustainable fuels. They still use leaded gas!

It's important to qualify this: jet fuel contains no lead, and constitutes the vast majority of commercial aviation, and hence of emissions.

Jet fuel itself is kerosene, with some additives that aren't superb in raw form, but nothing so durable in the environment as lead.

A sibling comment does a fine job of explaining why avgas still contains lead, and to be sure, a plan to phase that out except where absolutely necessary would be welcome.

> General av is seen as a backwater that is barely hanging on and couldn't possibly get the lead out.

The issue is infrastructure. Modern engines run fine on unleaded fuel, but the little airstrips in the middle of nowhere all have 100LL and/or jet fuel, so that's what pilots use.

I know a few pilots who are prefer to use unleaded in their Rotax engines to prevent spark plug fouling, and they have to jump through some pretty ridiculous hoops to get gas, while their buddies just fill up with whatever is on the field.

General aviation fuel is leaded but jet fuel does not contain lead.

The issue is that while cars only have ~15 year lifetime, small general aviation aircraft have ~50 year lifetimes.

Not particularly relevant to the lead debate though as the engines are replaced after 1,500 - 2,000 hours and retrofit Diesel engines running Jet A are available on lots of aircraft (but people don’t buy them as they’re expensive and heavy)

The problem with LNG is that it's still a fossil fuel, still cryogenic, and that methane itself is a potent GHG. The first problem you could solve using synthetic methane, but the latter two still drives you towards LH2 as being the better option.

Why not both? ;)

Airbus is experimenting with liquid hydrogen: https://www.airbus.com/innovation/zero-emission/hydrogen/zer...

A lot of the concepts for hydrogen aircraft use liquid hydrogen though.

It's hard to store, transport, and handle though, so I don't think it has much of an advantage.

Synthetic jet fuel will need hydrogen as an input chemical. This is not an "either-or" question.

I didn't say we don't need hydrogen though. I just said it makes more sense to use liquid fuels rather than hydrogen in aircraft.

That falls under the “maybe” category. Synfuel are not easy to make, and will add a substantial layer of extra complexity to an already complex process. If it can’t be done cheaply then it might not happen or make sense. Hydrogen is not an alien substance BTW. It’s possible that we figure out hydrogen before synfuels get cheap.