[dredmorbius]

The proposal is clearly for energy storage, not as a primary energy source.

To that extent, it resembles other synfuel concepts. The principle difference being that iron-as-energy-storage entails reduction rather than synthesis, in the chemical sense, for hydrocarbon synfuels.

There's a lot to be said for options which provide long-term, "shelf-stable", environmentally-benign, high-volume energy storage with convenient storage, handling, and utilisation characteristics. I've looked with interest on petroleum-analogue hydrocarbon synthesis (Fisher-Tropf) and alcohol (Sabattier) processes for some years. Both have long (multi-decadal, approaching a century) of established use. Yes, the overall process is lossy (as little as 15% net energy recovery), but there are applications for which there are very few alternatives: powered heavier-than-air flight, marine transport, mobile use, off-grid primary or back-up power systems, heating, and industrial applications.

I think I'd made abundantly clear that the abundance question is pedantry.

[rsaesha]

The challenge of using synthesized chemicals for energy storage exists for quite some time indeed. For aviation and marine there may be no other option outside of synthetic fuels, I agree.

Can this scale up? Or is this a small scale only solution?

Transportation. How much energy would a truck be able to move? How does it compare to a tank truck? Weight is absolutely relevant here.

Also, production. Consider that reducing iron is measured by millions of tons per year per plant, and right now, it's done burning it with plain old coal.

Seems very odd to me the subtitle of the news is 'carbon free fuel'. That alone is a massive bullshit indicator, but I digress.

How something that may have a 10% global energy recovery efficiency could beat a pure redox power storage solution? This question has been avoided so far.

[dredmorbius]

Germany and South Africa have both operated coal-to-liquids (Fisher-Tropsch) at industrial scale. Germany during WWII, South Africa from the 1940s or 1950s onwards (I'm not certain if it's still in process). Both nations had ample coal reserves but little petroleum.

I became aware of the prospect of synthesis from captured CO2 + hydrogen (from electrolysis) from a US Naval Research Lab study around 2015. Those papers had ... misleadingly-truncated citations, dating back only to the 1990s. It turns out that hydrocarbon synfuels were first proposed in the 1960s, by M. King Hubbert and studied at Brookhaven National Labs and M.I.T.

Google had an X Project devoted to the idea as well, though ran into insurmountable cost barriers.

Scaling seems to be a major concern, though the process does work at experimental scales, and produces usable fuel. It seems worth continued research based on the potential advantages, even if costs remain higher than fossil fuels. (The USNRL research suggested "competitive" costs, particularly for in situ military fuel generation, notably in aircraft carrier task groups which have ample supplies of nuclear energy, but need fuel for aircraft.)

Battery storage has numerous limitations: low energy density by both volume and weight, and the fact that whilst fuel burns off during flight (and accounts for 50% or more of take-off weight), batteries don't. In the case of metal-air batteries (iron and aluminium have both been proposed), as the redox reaction progresses, the battery gains mass as oxygen from the atmosphere is bonded to it. This poses problems for flight, and even ground-based transport tends not to work well with batteries at large scale.