I always wonder how the thermal efficiency of these whole system compares to something as mechanical as pumped hydro.

Feels like many developments dance around the fact no one has a good way to actually *generate* the H2 cleanly.

Best I’ve seen is a gallium compound electrolysis, ignoring so called “blue” (black) hydrogen from natgas.

This project will use water electrolysis powered by renewable electricity for clean hydrogen generation:

https://www.energy.gov/lpo/advanced-clean-energy-storage

As of 2017 the US had about 250 GWH of pumped-storage capacity and the world had about 1600 GWH: https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...

So this one project can store more energy than all American pumped-storage projects. Pumped-storage is more efficient but it takes much more mass because gravity is such a weak force.