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Iron-to-rust cycle tapped by scientists for safe renewable energy transport (interestingengineering.com)
submitted 1 month ago by to c/hydrogen@piefed.social
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...One of the challenges of clean energy is transporting and storing massive amounts of solar and wind power over long distances...

According to the new study, pulverized iron powder could act as a highly efficient, recyclable clean energy storage material...

The concept taps into a beautifully simple chemical loop. When iron powder is combusted, it releases intense heat and turns into iron oxide. In other words, it rusts. To reset it, scientists use green hydrogen generated from excess renewable energy to strip away the oxygen, reducing the rust back into pure iron powder. No carbon dioxide escapes into the atmosphere. The cycle simply repeats...

As it burns similarly to fossil fuels, energy giants wouldn’t need to rebuild their infrastructure from scratch...

The expensive components already in place, such as the steam cycles, massive turbines, heavy generators, and local grid connections, could be fully preserved...

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[–] [S] 1 point 1 month ago (1 child)

The whole mass of iron powder does not need to be re-ground after every use. From what I've read so far, about half a percent experiences sintering and needs to be re-ground, and another half a percent breaks down into too fine of dust and needs to be filtered out of the exhaust, sintered, and then reground. The efficiency penalty of reprocessing the fuel is about 0.1% - 1 kWh of reprocessing for every 1 MWh of generation.

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  • [–] 1 point 1 month ago* (last edited 1 month ago) (1 child)

    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.

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  • [–] [S] 1 point 1 month ago (1 child)

    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.

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  • [–] 1 point 1 month ago*

    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...

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