hydrogen definitely not the only way for long distance sea, nuclear would just make so much more sense. and for place travel it also such the same as batteries, first of all its an explosive gas and second we only get less than 20%. hydrogen is just not a good solution to anything other than being a byproduct in the natural gas industry.
Though it may never prove viable, hydrogen from electrolysis of water creates no emissions other than oxygen. While large ships may be able to use nuclear power directly, the risks of a tenfold increase in small floating nuclear reactors in civilian hands are not trivial. Hydrogen provides a way to keep power generation centralized, secure, and efficient while also distributing it where needed. But this assumes an extreme excess of power, something only possible with nuclear or large-scale renewables, which may never come to pass in sufficient quantities or without even bigger problems.
I don't disagree that nuclear propulsion has a long way to go before becoming safe enough for civilian use and might require extra security and regulation forever, but we need to do something other than dumping bunker fuel waste into the sea and atmosphere even coubting in several terrorist attacks and accidents it might still end up killing less people using nuclear. As you say going with hydrogen and by extension ammonia, which is probably the only viable way to fuel ships, will require unthinkable amounts of renewables which is mostly intermittent energy meaning hydrogen electrolysis plants will have to be ramped up and down as the wind blows or the sun shines. this makes the efficiency calculations even worse for an already super wasteful undertaking. i guess we could potentially use nuclear but why waste 80-90% of the energy created by converting and then using hydrogen ? My opinion is that we need to throw much more money at making nuclear fission safer.
I think most of the heat and fuel will get wasted anyway. A nuclear-powered cargo ship is going to be closer to a nuclear-powered submarine than a nuclear-powered aircraft carrier, in terms of the number of people being supported and the needs beyond propulsion. That means running the reactor below its optimal fuel efficiency, and then intentionally discarding a portion of its heat most of the time to retain some spare capacity for demand spikes and emergencies. Of course, small-scale nuclear reactors are an active area of research, but AFAIK the results haven't been promising yet. So it's an avenue, but not the only one.
Running a reactor full-bore on shore, pumping ocean water, desalinizing it, electrolyzing it to hydrogen, cryogenically separating nitrogen from air, and combining the result to produce ammonia, might look really inefficient in isolation, but might be efficient enough when compared against the alternative. When the costs of logistics, personnel, and capital are accounted for, it might even come out ahead. That having been said, the amount of waste oxygen this would produce is immense, and could create risks of its own. Ammonia is also more likely to produce NOx when combusted in air than hydrogen is, so that's not great, either.
Nuclear would not make sense at all for long distance sea travel. Naval reactors require highly enriched fuel in order to be compact enough, so they could never be widespread on civilian ships for fear of proliferation. They're also extraordinarily expensive and require a specialized crew, and their power output is overkill for a cargo ship (but they can't really be scaled down much further).
The only real synthetic alternative is ammonia which brings it's own host of problems and potential for malicious use. while your arguments are valid, i think we can overcome most of the problems you mention with more research. at some point we'll need to make the hard decision to either give up on the environment or go nuclear and accept the extra cost of safeguarding it. It simply seems there's no other realistic alternative.
Maersk launch a commercial 172-metre container ship running on run entirely on green methanol a bit over a year ago and has 25 more new green methanol ships in the build pipeline.
Australian companies have already contracted to supply green methanol sourced from solar farms after their build and trial of methanol shipping (tugboats) in Singapore.
> It simply seems there's no other realistic alternative.
Biofuels are carbon neutral. They require land, but a single engineered biofuel crop could grow on land that other crops couldn't. It's a promising use for the Canadian tundra that's melting.
Sails are very promising - a cargo ship would need enough fuel to maneuver in port and to generate electricity en route, but for most of the trip it could rely on (computer-controlled) sails.
Neither of these options are mature today, but they're a lot closer than nuclear-powered cargo ships.
