[zahlman]

I don't think I understand the idea here properly.

When storing energy, the idea is to split water into hydrogen and oxygen, and then let the hydrogen recombine with the oxygen from iron oxide... to make water again. Meanwhile the oxygen from the original water is just released (since it's everywhere anyway)? That doesn't really seem to me like "storing hydrogen", since you just get the water back that you already had. Rather, it's using the energy to deoxidize rust.

Then on the recovery side, why use this steam process? Apparently (because the thermodynamics work out so that this whole thing has efficiency > 0) you get energy out of the process of putting the oxygen back into the iron. So why not just, well, burn (i.e. rust) the iron directly? What exactly is the dissociation and re-combination of the steam accomplishing?

[usrusr]

> "So why not just, well, burn (i.e. rust) the iron directly? What exactly is the dissociation and re-combination of the steam accomplishing?"

Direct use of reduced iron as an energy carrier has been discussed here:

https://news.ycombinator.com/item?id=24996153

Oxodizing iron gives rather low intensity heat, comparable, iirc, to burning lignite. That might be good for some combined cycle applications (but still a logistics nightmare?), but it's not a drop-in replacement for anything. Hydrogen on the other hand is good for an extremely wide range of applications, from fuel cells to e-fuel production (and all kinds of other chemical processes) to flying Neil Armstrong to the moon.

To succeed in decarbonization we will need an entire "cache hierarchy" to solve the intermittency problem, a single storage solution will never be enough. Batteries to make hydrolyzers able to run around the clock during high availability seasons, hydrogen storage to make converters further down the pipeline run continuously during surplus seasons and not only on the best days.

What will definitely not get us to decarbonization is any of the following three approaches:

(a) building enough production capacity that we don't need storage (most production would be idle most of the time for lack of a buyer)

(b) focusing on one kind of storage (same problem again, now with conversion capacity)

[kortex]

> Rather, it's using the energy to deoxidize rust.

Exactly, really what they are storing is electrons.

Rusting the iron directly releases heat, which limits your efficiency to the delta-T of the process. Reducing steam to hydrogen and then converting the hydrogen in a fuel cell allows higher efficiency to produce electricity.