Making fuel from solar and wind seems key to me. Because you need long term energy storage and batteries don't cut it.
But you're right that we definitely need something better than batteries for certain applications, particularly transport.
As a locally generated process input, H2 appears to make sense, say for steel production or yes, ammonia. But H2 is hard to store; best is to do so cryogenically which uses a lot of energy and is heavy. storig higher temperature H2 is more difficult (those crafty molecules are tiny!). The economics likely work for many industrial processes, but not for transport, much less grid power generation. The "fuel" part doesn't pencil out; I believe H2 fuel story are just a subsidy to the fossil fuel companies in order to get legislation passed.
Compared to other fuels Ammonia isn't especially energetic, but for a large vessel with a lot of space it can work. But it's really toxic and harder to manage safely than buker fuel or jet fuel. "Sustainable" doesn't mean just its production but its overall risk to human life. It lacks adequate energy density to drive something like a car much less a plane, even if you were willing to have something as nasty as NH3 in your fragile, weight-sensitive vehicle.
Don't take my word for it: Maersk, who is famously commissioning ammonia (and methanol) vessels, has publicly stated that these are not long term solutions but rather (they hope) a way to push the green agenda forward.
> What fuels do you think are sustainable?
I really don't think we know yet (for transportation). The fuels we have are highly energetic (in particular by weight) because a lot of energy went into them to generate those tight bonds. Transportation fuels need a lot of important factors: energy density at a system level (energy per weight including storage), general safety at STP, etc.
There have been some efforts to make ptroleum products from captured CO2, which might be a temporary bridge (by definition it's GHG neutral and can use renewable power) but burning these fuels produces other pollution beyond GHGs (thus are't sustainable) so they still need to be eliminated.
Fortunately batteries appear viable for the middle of the curve, most wheeled vehicles. But at the extrema (ships, planes, and rockets), we need something that performs closer to what we have today and I don't see anything on the horizon.
What I am concerned about is energy storage for power grids or microgrids.
Do you want to claim that hydrogen and ammonia can't work for this use case either, and that there is nothing viable? Implying that the most sustainable option is to just continue using fossils fuels.
What I think is that there a ways to make both hydrogen and ammonia and other options very viable not only for grid storage but also for transportation. What's holding it back is a lack of critical thinking. This is because most people's views are dictated by social circumstances and then they subconsciously find ways to rationalize them. In other words, it's hard to get people on board with solving new problems related to using different energy sources, and so executives will invent reasons it can't work.
What sort of fuel do you think makes sense to pursue for long term (months) energy storage?
I don't think you've reached your opinion on the basis of adequate reasoning.