Iron as an inexpensive storage medium for hydrogen

This is a pretty elegant idea. It takes 826 kJ to split a mole of iron oxide (Fe2O3) and it takes 855 kJ to split 3 moles of water (H2O). So if you take H2 and blow over one mole of Fe2O3 you can strip the O3 for the cost of 826 kJ but then by burning the hydrogen in oxygen you get 855 kJ, for a net exothermic effect of 29 kJ, which is a rounding error. The opposite reaction requires 29 kJ, again negligible, there are probably bigger energy losses bringing the reactant mass at the required temperature (400 degrees C).

Unfortunately, I don't see this making any sense for large scale energy storage. Storage tanks for compressed hydrogen enjoy the square-cube law. The larger they are the less expensive they are proportional to the mass of hydrogen they hold.

With this iron oxide method, you need 27 tons of iron oxide for one ton of hydrogen. You can procure right now tanks that can hold 2.7 tons of hydrogen and weigh 77 tons empty [1], the ratio is 28 to 1. But the round-trip efficiency of the tank is virtually 100%. The efficiency of the iron-based storage is only 50%. The tanks are not very expensive.

I can't see the niche that this idea can apply to.

[1] https://www.iberdrola.com/press-room/news/detail/storage-tan...

Actual publication linked is very readable: https://pubs.rsc.org/en/content/articlelanding/2024/se/d3se0...

We have a cheap, stable, infrastructure-friendly, high-density storage formula for hydrogen. Or better, since the application here isn't hydrogen-specific but is simply looking to find a fuel-storage solution: energy storage.

It's hydrocarbons.

In this case, synfuel hydrocarbons as direct analogues of fossil-fuel based compounds of chain-lengths 1 (methane) to around a dozen or so (kerosene / aviation fuel, at a stretch, diesel fuel).

It stores forever (proved to 300 million years), it is drop-in compatible with extant infrastructure and equipment, it's infinitely miscable with present fuels, it doesn't leak out of storage, it doesn't embrittle metals (and in fact generally lubricates and protects them).

Yes, the round-trip storage efficiencies are low (as low as ~15--20% recovery based on thermal electrical generation, roughly the same as the solution named here), but that's in exchange for something that can readily provide weeks to months of storage capacity in a stable, low-risk form. Where you need storage that's long-term stable, dense, safe, and instantly dispatchable, your options are few.

The technology has been demonstrated in numerous experimental trials, and is similar to processes run at national scale for decades in Germany and South Africa. US-based research has been conducted at Brookhaven National Laboratory, M.I.T., and the US Naval Research Lab, amongst others. The stumbling block to date has been that fossil fuel prices are sufficiently low[1] that synfuels simply are not competitive presuming market-based mechanisms which fail to account for externalities and other market failures.

I've be aware of this for about a decade and have written about the technology, Fischer-Tropsch fuel synthesis, multiple times on HN:

<https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...>

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Notes:

1. A market failure of staggering proportions, as the under-pricing is on the order of a million-fold. See: Jeffrey S. Dukes, "Burning Buried Sunshine", <https://core.ac.uk/download/pdf/5212176.pdf> (PDF)

You really want to store excess energy as natural gas or jet fuel because of all the existing infrastructure. Especially since excess power is available at so many solar sites, we’re so good at transporting these fuels, and the cost of photovoltaic will keep going down

The vast, cheap power that photovoltaic will provide is a giant opportunity. Please review the links below

https://terraformindustries.com/

Terraform industries converting sun and air into natural gas:

https://techcrunch.com/2024/04/01/terraform-industries-conve...

The solar industrial revolution is the biggest investment opportunity in history:

https://caseyhandmer.wordpress.com/2024/05/22/the-solar-indu...

Is this same as rust batteries earlier?

https://www.scientificamerican.com/article/rusty-batteries-c...

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?

At scale, what I don’t get is this requires a lot of energy to kickstart the reaction (heating the iron ore to 400 degrees). Where is that energy coming from when energy production is constrained in winter.

Or would the plan be to slowly heat over fall?

Efficiency is quite low: 40%-60%.

I would say it's only worth it if the marginal cost of producing the hydrogen is close to 0.

This seems to be the most important problem to be solved for a green, future grid. So I’m happy there’s a new solution shown every other month.

It’s annoying that they always seem to contain some hand-wavy efficiency calculations. I think this one didn’t even consider the losses from hydrogen production? Is there a benchmark out there, of these long-term electricity storage solutions? Like: you get 1 MWh at 25 deg C, and 6 months later, it’s measured how much your system restores. Everything taken from the grid during storage for upkeep or kickstarting the process is subtracted as well.

This solution is better and no toxicity

https://www.pnnl.gov/news-media/baking-soda-solution-clean-h...

Why would you use hydrogen to extract the energy of the iron? Wouldn't it be more efficient to burn the iron directly? Likewise, is there no better way to reduce iron oxide to iron than by creating hydrogen first?

The energy density of the system is surprisingly high (in my modest perspective). It looks like 800kWh per ton of iron. Isn't it ~five times as much as the batteries we have in cars?

Despite the cycle losses, this seems like a good idea for easy to maintain storage. And they plan to use the heat as well, so that will step up efficiency.

Will be interesting to see how their campus power project works out in the next years.

Also, innovative “sausage safety test” for the Fe powder reacting with air (figure 6: https://doi.org/10.1039/D3SE01228J ).

For seasonal grid-level storage, I wonder if simple compressed hydrogen storage (around 350 bar) is the most reasonable solution. AFAIK doesnt require any high-tech materials, avoids most embrittlement caused by LH2 and boil-off rates are reasonable.

When I was a child, we would play a game called "the floor is lava". It's a simple game: you have to get around, but not touch the floor. Jumping on the furniture and such. Fine for a pastime, when you're small, but to get places, you use the floor.

A certain faction of the project to decarbonize the electrical grid likes to play a similar childish game: "nuclear power is lava". It causes them to come up with whimsical and absurd epicycles, which make no sense at all unless you're playing that game.

Seasonal storage of 2GWh? Please. A 2GW plant produces 2GWh every hour, with 90% uptime. And it doesn't involve losing more than 90% of the photovoltaic energy, I will eat my whole hat if the ray-to-electricity pipeline for this boondoggle exceeds 10% efficiency.

Can we please stop wasting time and effort, and invest in the buildout of a substantial nuclear fleet to provide baseline power?

Looks like a variant of iron-air battery project to me.

Is this the same idea as?

https://teamsolid.org/

or you can maybe skip the hydrogen intermediary on one end and burn the iron in an iron-air battery. then you get much higher efficiency. electrowinning of iron in alkali is also feasible, eliminating the hydrogen on both ends

production and conversion are inefficient compared to other sources of energy, as up to 60 percent of its energy is lost in the process.