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Iron-to-rust cycle tapped by scientists for safe renewable energy transport
(interestingengineering.com)
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this post was submitted on 10 Jul 2026
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Ok so aluminum is still a better candidate than iron powder since it doesn't use hydrogen. I know I'm in the hydrogen sublemmy arguing against the use of hydrogen here, but green hydrogen uses a ton of electricity and it's an extra step that costs some energy efficiency, so why not cut out the need for green hydrogen and use electricity directly to smelt aluminum oxide into aluminum metal?
Aluminum carries 31MJ/kg of energy vs irons 7MJ/kg.
They also don't address how the iron oxide is reduced back it iron metal. I'm assuming they use DRI for that (Direct Reduced Iron) where you pelletized the oxide and pass hot hydrogen over it in an oven. You still need to sinter the pellets to allow for optimal gas contact around each pellet along with internal gas passage, which means you do need to grind it down after you reduce it back to metal. I don't know where your numbers are coming from.
Where did you find info on reducing a iron oxide powder without sintering directly? Is that a fluidized bed process? AFAIK there's either DRI pellets, or you do a vortex lance method the Chinese pioneered but that produces liquid iron and liquid slag.
Source for claim of 1% mass loss per cycle:
https://research.tue.nl/en/publications/combustion-of-micron-sized-iron-particles-in-a-drop-tube-reactor/
I think the deciding factor between iron and aluminum comes down to exhaust filtration.
Iron oxide in the exhaust can be collected cheaply with a combination of centrifugal and electrostatic forces and it tends to retain is grain size. Aluminum oxide is lighter, more electrically resistive, and tends to break down into too-small nanoparticles.
Yeah, you'd only need to sinter pellets to pump hydrogen through them in a static pile. A fluidized bed reactor can reduce the iron oxide powder as is, and keeping the temperature lower prevents sticky sintering at the cost of taking longer to complete.
That source only covers losses due to iron evaporation during combustion, it doesn't cover reduction losses back into iron or what the regenerated iron's particle size is, which tells us what milling energy is needed to get back down to sub micron iron. That's the inefficient part of the energy loop, along with reduction efficiency with green hydrogen.
You need to understand that whatever method is being used for reducing your powder back into iron metal is going to be a mature technology already used for steelmaking. If green energy scientists somehow found a new method for reducing iron into metal it would be global breaking news in the steelmaking industry. That's the extraordinary claim you need to support with sources: how it gets regenerated and at what efficiency, does it need to be milled and to what starting and finishing particle size, consuming what energy, CAPEX, and OPEX? What's the total loop efficiency?
I really think the overall efficiency is going to be terribly low once you do the math on all steps in the cycle...