Storing hydrogen is substantially harder than storing stuff like methane, or CO2, or even O2.

Hydrogen's atoms are so small they try to pass through the crystalline lattice of metals, making the hydrogen-enriched layer which is more fragile [1]. Your regular polymer hoses are much more permeable to hydrogen.

Hydrogen gas burns with entirely infrared flame, not visible to human eyes [2].

Hydrogen, even highly compressed, is still very lightweight, so storing significant amounts requires either very high pressures, or liquefaction. Liquid hydrogen requires an uncomfortably low temperature of 20K, but starts to freeze into solid state at 14K.

Hydrogen is the most efficient rocket fuel, there's no denying. For all other fuel applications, it's pretty problematic.

[1]: https://www.osti.gov/servlets/purl/1727338

[2]: https://spinoff.nasa.gov/spinoff1997/ps1.html

Which is why the current proposals to significantly expand the global per annum hydrogen production via green solar hydrogen production are looking to transport Ammonia (Three Hydrogens and a Nitrogen) across the planet for use in large baseload generators.

Less storage problems, more options (higly compressable gas, or pure liquid, or "liquor" (dissolved)), and no mucking about with fuel cells.

Just straight up movement of energy (with losses to be sure) from places with an excess of sunlight to those that are short.

What's the benefit of ammonia vs. turning hydrogen into methane via carbon capture?

Carbon capture is hard, nitrogen is truly abundant. That is unless you happen to have a source of highly concentrated CO2 at hand. Which is certainly true now, but won't be in the endgame. Methane (or something with an even lower H-per-C) will surely be part of the picture, but it can't be the mainstay. I'd expect hydrolizer capacity to cluster around storage opportunities (like salt caverns) within convenient grid range from energy sources, large ammonia plants to continually work from that hydrogen buffer and higher-value H2 consumers to opportunistically tap into the same buffer whenever their "other ingredient" is available. That "other" could be metal ores, iron oxide returning from an oxidization energy cycle or CO2 from biomass burning or waste incineration. But there's always hydrogen as a first step, and the (centralized within grid range) hydrogen storage as the buffer that solves the dispatchability problem.

Fun observation: If some of the hydrocarbons created this way make it into plastics that isn't passed back into incineration (or a better form of recycling) you'll have true carbon-capture for the fraction of the carbon that came from biomass. Without we trying.

Thanks for the detailed reply

This. Is there even one power2anythingelse that does not have hydrogen as its first step? Hydrogen storage will be important, as the input queue for whatever step comes next so that only the hydrolysis capacity has to be scaled for being able to take in supply peaks and idling the rest of the day/week/year. But hydrogen storage won't be involved at all anywhere in transportation or distribution of energy (with a few possible exceptions in large very high energy density requirement applications, I wouldn't completely rule out hydrogen aircraft being less infeasible than e-fuel powered aviation at scale)

Hydrogen is far more energy dense than batteries. For example: for a 800km truck, the difference can be as much as 2 tonnes. https://www.umicore.com/en/newsroom/news/fuel-cells-battery-...

Also, there are ways of storing hydrogen in metal hydrides at room temperature and pressure. This makes a LOT of sense for people who may want to have an option of buying stored energy - such as those who live in apartments without access to the terraces where solar panels can be deployed.

If these metal hydride storage is used similar to how changeable batteries are used, it is conceivable that you could drive up to a place like a fuel station just to change out your metal hydride storage in a matter of a couple of minutes and perhaps even get additional supplementary units packed at the back of the vehicle if they intend to go on a long journey.

https://www.frontiersin.org/articles/10.3389/fenrg.2021.6161...

In terms of cost, it is also now possible to use Boron Nitride for storing hydrogen - very cheaply and very safely.

https://www.geelongmanufacturingcouncil.com.au/2022/07/innov...