Generating electricity with iron as fuel would be suboptimal, that's like using electricity to turn co2 back into coal, and then using said coal to generate electricity again. Using iron in coal power plants would only male sense if transporting it from somewhere where IRS more efficient to produce iron is economically viable - say use solar in Spain to regenerate iron and then use it further up north where solar isn't as good.

The problem being solved is the bursty nature of green energy, we are trying to store solar energy generated during the day for usage at night or store excess wind energy for still days.

We are not trying to transfer green energy to less windy/less sunny places, the grid can take care of that.

Right, but in that case you'll get way better efficiency out of pretty much any other proposed solution: gravity, momentum, battery solutions are all more efficient than this for storing energy from renewables.

Yeah, or in the article’s case (where you’re brewing beer), you might as well use a solar furnace to boil water directly with sunlight. You’ll have a hard time beating that efficiency with other methods. If you have huge water storage tanks anyway then keeping the water hot overnight is not a problem either.

Batteries are expensive. Iron is abundant and cheap, and these seem much less difficult/costly to build than to create a dam, for example.

It also seems like it can make use of existing infrastructure and save money there.

I don’t think the expense of other solutions justifies the abject inefficiency of this one though.

Seasonality of solar is huge in northern latitudes. Using iron to store energy for months is an interesting prospect, although it needs to be kept dry so you can't just leave big heaps of it outdoors.

At first glance it seems reasonably transportable, but I'm worried about how you prevent it rusting in the air. Maybe it would be useful to blend it with biodiesel for that purpose.

Well, firstly we do store grain and sand and many other things in massive quantities.

But secondly, wind power makes a lot more sence in northern latitudes.

Limit the air exposure.

I imagine methane synthesis will beat some thinglike this out. Pipeline infrastructure exists, is convenient, etc.

The advantages I see of this process are: Ease of storage, re-use of the coal plants, ease of fuel generation.

Methane seems nice for the re-use of gas fired plants. I do think its gaseous nature has more downsides than upsides on the other two factors. There's also a gas-leak hazard with methane that doesn't exist here.

If the capital requirements are low enough, I could see a 40% efficient energy battery for smoothing out peaks being nicer than more complicated storage schemes.

Chemical batteries will likely eat the smoothing market.

For fuel that can be transported, methane is a winner. In addition to the distribution infrastructure, there's an awful lot of residential use, with lots of appliances that don't need replacement.

Or maybe add some oils ?

Iron is here used as an energy storage system. That is the whole point. You store excess energy from wind and solar power e.g. in iron to use later.

Yes for short term batteries are better but for say seasonal storage or when power is generated far away from where it is used then metal is better.

This is still going to be less efficient than an actual battery though, right? I'm all for alternative renewable energy storage solutions, but this one seems pretty poor.

The principle can be effective, I've not seen anyone seriously consider iron however.

Yes, exactly as with coal. What is the problem with that? Railroad and ships are really efficient at moving large chunks of cargo cheaply.