Someone pointed me to a link where round trip efficiency was analyzed, with the claim that ammonia is better than hydrogen or methanol for energy storage. Apparently hydrogen compression takes a big hit, although I'm not clear if that's still the case if the hydrogen is burned in stationary turbines (that compressional energy could be recovered).

Perhaps this..

Ammonia—a renewable fuel made from sun, air, and water—could power the globe without carbon

https://www.sciencemag.org/news/2018/07/ammonia-renewable-fu...

Excerpt:

Ammonia's energy density by volume is nearly double that of liquid hydrogen — its primary competitor as a green alternative fuel and it is easier to ship and distribute. "You can store it, ship it, burn it, and convert it back into hydrogen and nitrogen,".

..he shows off one of the devices, about the size of a hockey puck and clad in stainless steel. Two plastic tubes on its backside feed it nitrogen gas and water, and a power cord supplies electricity. Through a third tube on its front, it silently exhales gaseous ammonia, all without the heat, pressure, and carbon emissions normally needed to make the chemical. "This is breathing nitrogen in and breathing ammonia out,".

Companies around the world already produce $60 billion worth of ammonia every year, primarily as fertilizer, and MacFarlane's gizmo may allow them to make it more efficiently and cleanly. But he has ambitions to do much more than help farmers. By converting renewable electricity into an energy-rich gas that can easily be cooled and squeezed into a liquid fuel, MacFarlane's fuel cell effectively bottles sunshine and wind, turning them into a commodity that can be shipped anywhere in the world and converted back into electricity or hydrogen gas to power fuel cell vehicles..

Hydrogen is a scalable storage technology, unlike batteries, and large projects are underway:

https://amer.mhps.com/world’s-largest-renewable-energy-stora...

You should actually read the linked article.

It's relatively sparse on technical details, but it does mention:

- Renewable hydrogen

- Compressed Air Energy Storage

- Large scale flow batteries

- Solid oxide fuel cells

Seems like hydrogen is not actually used as a storage mechanism. They convert to hydrogen on demand to power turbines.

They will be using sealed salt caverns to storage large quantities of gas cheaply.

Why are batteries not scalable?

Cost increases linearly with capacity, which is undesirable at scale. You want sublinear scaling. For example, to store a year’s worth of US electrical generation using batteries would cost approximately 500 trillion dollars.

1) Huge number, but why would you need an entire year of energy in reserve 2) If there was a desire to build that many batteries, cost would surely decrease exponentially as new mines, factories, etc. open up and competition increases.

By definition, batteries are scalable. Battery is a word that defines a method of scaling.

edit - For example; 'A battery of batteries of batteries of cells.'