[DoctorOetker]

would you mind explicitly listing the 4 conversions?

I see:

1) generation 2) storage efficiency (energy while storing divided by energy upon release)

what are the other 2 you had in mind?

[jillesvangurp]

1) generate hydrogen 2) store hydrogen in iron oxide 3) discharge hydrogen again 4) convert it into something useful (electricity, heat, movement, etc.).

All those steps lose energy. And there's stuff that happens in between involving pipes, leaky valves, tanks, compression, etc.

[DoctorOetker]

this is not compressed hydrogen storage, anyway your 2,3,4 are the same as my round trip storage efficiency 2