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Hydrogen is a better solution for ocean shipping, long-range airplanes, and industrial processes that need very high heat. Even Elon Musk admits the last one will be necessary. Fuel cells are not a good idea because they require expensive catalyst metals. Hydrogen, in the form of ammonia running in combustion engines, is just about the only decarbonized solution that work for the big container ships. Large scale ammonia engines have been successfully tested and may go into production in the next few years for these applications[1]. For trains, it's not a good idea because you can more easily supply power with overhead wire and you don't even need to deal with batteries.

[1]https://www.power-technology.com/news/man-ammonnia-engine-te...

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> For trains, it's not a good idea because you can more easily supply power with overhead wire and you don't even need to deal with batteries.

Rail networks with large stretches of non-electrified lines are extremely costly to upgrade, so battery-electric and hydrogen are the likely solutions unless there is massive investment in electrification. The US, Canada, Mexico, Brazil, Argentina and Australia are prime examples of this.

Currently CPKC is working on a number of HFC locomotives that are showing very promising results.

Also seasonal storage. Batteries are good for storing excess energy on Tuesday to use Wednesday, not so good at storing energy produced in July to use in December.

Solid Oxide fuel cells do not require expensive catalysts and they can also directly convert hydrocarbon fuels as well as ammonia…not just pure hydrogen. And they have about 30-50% improvement in efficiency over combustion processes.

What are the trades between synthesizing ammonia for combustion versus synthesizing methane?

Hydrogen is needed for routes with no electrification infrastructure. Think long journeys across the empty parts of the US. There really isn't any alternative to that.

> Hydrogen is a better solution for [...] industrial processes that need very high heat.

Why would that be, except maybe in rare cornercases? You can electrify almost anything, and it'll be more efficient and less polluting.

That depends on your ability to get access to electric. While you can imagine wires across the ocean (or at 35000 feet), it doesn't seem practical or cost effective to install them. However if you are in more populated areas odds are you already have electric close and so it is more cost effective to connect to the grid.

For transit use, a rule of thumb is if you have a bus every 10 minutes or less then it would be more cost effective in the long run to have wires to the bus (better than diesel or battery buses). However even when you have a bus every 10 minutes odds are you have enough other routes that don't have that frequency and so overall it isn't worth it even though in some places it would be.

Lots of lines are not electrified, because of the infrastructure cost of raising bridges for the overhead line. In some places it might be simpler to switch diesel electric train engines to hydrogen engines.

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