None of your suggestions allow for a movement in space. I haven't actually ran the calculations^ myself, but most of your "complaints" are wrt to energy efficiency. And that won't matter (too much), if we overbuild production capacity. Upthread someone claims that mines could benefit from hydrogen simply because they are not permitted to produce any more CO2. That would make this niche a realistic target, where you need to move energy.
And yes, in that scenario I would absolutely expect all your alternative suggestions to be deployed already. But we are also just starting to have excess production at all.
^ I also don't think those are possible right now, as the number of electrolysis apertures would need to increase by orders of magnitudes (plural)
A cryogenic pressure vessel that's not affected by hydrogen embrittlement is hardly portable, cheap, or energy dense. Maybe if you're using a vehicle that's otherwise going to use bunker fuel, sure, you've got space for a reasonable hydrogen tank.
If we're speaking from a practical perspective not green-washing, using a methane fueled engine in an electric motor / generator setup is going to be way more efficient and flexible than some insane setup where you've got cryogenic hydrogen tanks hanging out a rock quarry.
[edited to add this:]
Let's distinguish between "pull more carbon from the ground and put it in the air" and "pull carbon from the air and make methane with it". Any H2 system today is going to use a hydrocarbon feedstock. Any "we'll crack water and save the H2" story is a pure fiction. Any process that relies on some theoretical "we've got spare energy so we're going to use that to crack water, then compress the everloving hell out of those H2s and then chill the crap out of that" could just as easily be "make methane using spare energy and spare CO2" likely with substantially less "spare" energy.
[edit]
There are, I think two questions:
1) will we have enough energy to do “split water and use H2”?
2) how is the energetic ratio between “compress and move the H2” on the one side and “filter CO2 out of the atmosphere, split that and integrate the H from 1) into the COs from here”?
I think 1) will come with a storage and movement demand anyhow (at least short term and for steel making), so there is a feed source for funding (that is not just state subsidies) for the former. Whereas 2) fundamentally requires the CO2 removal from the atmosphere. And I do not see a clear funding source (besides state subsidies) for _that_ problem. (So I think we may mostly disagree on the existence of “spare CO2” as a cheap feeding source)
Do you have any comparison of the physical limits of energy required for those processes? As I said, a lot of my thoughts still rely on “intuition” and that’s always reasonably dangerous. But for me “move H2” sounds like a solvable problem. Yes probably not for longterm, cheap and pressurized storage, but that’s also not required. If you have “pressurized and short” and “longterm and plentiful” that would be fine too.
At "low pressures" (5-30x normal sea level atmospheric pressures) H2 needs roughly 2.5meters cubed to store 1kg of H2.
Apparently it's "normally" stored at 350 to 700 bar (700x sea level air pressure). Compressing it that much requires a bunch of specialized equipment and a lot of energy.
It needs to be stored in complex pressure vessels at -160C or below[2]. I'd hate to have this stuff in big pipelines in a giant network, when utilities still can't quite do normal pressure natural gas correctly[3].
It leaks through just about everything, and lots of "normal" things that it leaks through it also destroys it while leaking through it[4].
A hydrogen economy would require trillions of dollars of infrastructure to work. A methane economy can just work -- it's basically what's already in use to move natural gas around.
So -- I can't point to an existing technology that'd be 100% ideal for a zero net pollution (overall, not just at the point of use) for a mega truck or similar big utility vehicle running a strip mining operation. But H2 really seems like a non-starter once you look at the details. I think maybe better batteries, hot-swapped batteries, hybrid methane generators plus batteries, etc, is a start, and it can be done today. I personally would take a 60% improvement now over a maybe 90% improvement later but probably never.
There's lots of carbon floating around; you can collect it from industrial processes (cement / steel making). You can pull it out of the air or water if you really do have enormous amounts of "spare" energy, which realistically, you probably don't.
And yes, in that scenario I would absolutely expect all your alternative suggestions to be deployed already. But we are also just starting to have excess production at all.
^ I also don't think those are possible right now, as the number of electrolysis apertures would need to increase by orders of magnitudes (plural)