It has been invented, the majority are in China and Japan if I remember rightly.

Synthetic methane and other hydrocarbons are also ideas that are floated. But those need hydrogen as an input anyway. They also need a source of carbon, which is too difficult to extract from the atmosphere. So using the hydrogen directly may be more feasible.

Short-range aircraft will probably switch to batteries in the coming decades, because the prototypes are already flying[1] and battery and electric motor tech is ever-increasingly ubiquitous and mature.

[0] https://en.wikipedia.org/wiki/Greenhouse_gas_emissions#Aviat...

[1] https://www.cbc.ca/news/canada/british-columbia/vancouver-el...

It's not just long haul flights out of reach of battery powered aircraft. Even a regional flight like Seattle to San Francisco cannot be feasibly done with batteries. This is fundamentally due to the energy density of lithium ion batteries, no amount of engineering can change the chemical limitations at hand: https://en.m.wikipedia.org/wiki/Energy_density#/media/File%3...

As far as I understand, the problem is not just the speed, but the thermal management. IIRC the impact on battery life is not as bad if cooled properly.

> and a plane needing a new set of batteries after a few hundred flights due to aggressive wear from fast charging is unacceptable.

That's an economics question. How much does it cost to get that battery replaced and charged over its lifetime versus the alternative.

> This is fundamentally due to the energy density of lithium ion batteries, no amount of engineering can change the chemical limitations at hand

That's the beauty of battery technology, you can substitute. Cobalt a problem? Fine we can make batteries with out it. Lead-acid not suitable, fine here's lithium ion. Lithium ion not good enough, find we'll find a better battery.

Hydrogen however is just that: hydrogen. Notice how low it is on the energy per volume scale. You can't substitute it out for anything. The volume requirement is what's going to kill you with hydrogen.

Well, then get back to us when you find that battery. Unfortunately, simply finding a new battery chemistry that has an order of magnitude better energy density is easier said than done.

> Hydrogen however is just that: hydrogen. Notice how low it is on the energy per volume scale.

You're looking at uncompressed hydrogen. Hydrogen at 700 bar has several times better energy density than lithium ion batteries, and over 1000x as much energy by mass. And lower mass means you need less fuel since the craft is lighter.

Will point out takes a decade now to produce a blue print aircraft. So if we started designing one now it would be flying when 2033? China rolled out a high speed rail network in ten years. You could do that in the US and other places.

Notable point. The complaints I hear about high speed rail in Europe is tickets cost more than airplane tickets and the trains are always full. The explanation is the ergonomics and logistics of passenger aircraft sucks.

H2 has a daily boil off 5 times greater than LNG. https://www.sciencedirect.com/science/article/pii/S277265682...

Not only that, liquid H2 has worse energy density than LNG. So again, you're spending more units of energy to comparatively more less units of energy.

You will need bigger tanks but the fuel is much lighter. It is a solvable problem.

>You can liquefy hydrogen on the spot. You don't need to move LH₂ around.

Why are you liquefying H2 if you are using it right away and or not transporting it? Liquefaction is to optimize for storage. You're all over the shop.

Actually, there are many carbon free fuels. It is hard for any to use as jet fuel. An example fuel that has much better properties than hydrogen is for instance sodium. Making a sodium fuel cell does not need any rare earth metals such as platinum, sodium is solid at room temperature, does not produce any fumes, better energy density than liquid hydrogen, the resulting NaOH sodium hydroxide can be recycled into sodium (with hydrogen as a byproduct) by the Castner process. Hard to say, if this could use the water vapor present in the atmosphere, if it did, you would also have to think about what to do with the caustic hydroxide solution. Probably, that would be neutralized quickly in the atmosphere but at ground level or above cities could "raise eyebrows" even though burning leaded petroleum in aviation still seems to be a thing...

Of course, for almost all uses, the volumetric efficiency is important because you have to create bigger tanks, which if they have to contain liquid hydrogen, are usually heavy and costly.