The idea is obviously to use iron for energy storage. You grind the iron until it is a fine dust, and store the dust somewhere. When the time comes, you burn th

The idea is obviously to use iron for energy storage. You grind the iron until it is a fine dust, and store the dust somewhere. When the time comes, you burn the iron in a power plant. Modern coal power plants burn the coal in a fluid bed, so probably some retrofitting can be done, to switch from particles of coal to particles of iron. The idea is that while burning coal produces CO2, burning iron produces iron oxide Fe3O4, which is a form of rust. You then store that rust, and when you have spare electricity (for example during the summer, when solar panels generate more electricity than there is demand), you remove the oxygen and return to iron powder. The idea would be that this type of energy storage is cheaper than other round-trip chemical reactions, the most obvious being burning hydrogen and then electrolyzing water. The advantage of the hydrogen-based energy storage is that you don't need to store the output of the oxydation reaction, because water is all around us. The iron-based solution requires you to store lots of this rust, in large storage places. In many places storage is reasonably cheap, so this would not be such a big cost (but it would be an additional cost nonetheless). With hydrogen, people have run the numbers. It's not easy. Not easy by a mile. It's very difficult to produce hydrogen out of water at a cost below the cost of the natural gas that would produce the same electricity. Even if you add some subsidies to account for the zero-emissions, it's still very difficult to make this work. Why? Mainly because of capital costs. To make a plant that produces enough hydrogen to feed a typical 1GW power plant you need billions of dollars. And you need that for every 1GW natural-gas power plant that you want to convert to hydrogen. It's doable in principle, but it was never done in practice. Nothing remotely approaching that scale was ever done, and it's not clear anything like that can be done this decade. With iron, you need to replace the electrolysis with something close to an iron smelting furnace. We have lots of those furnaces around already, busy separating iron from ore. It's one of the toughest problems of the road to net zero: how do you make iron green? People say you can do it via green hydrogen, but then if you make the green hydrogen why would you not just use that for energy storage, rather than add one more step in your energy storage economy? So, unfortunately, although I like a lot the idea of finding a cheap chemical solution for the electricity storage problem, I'm not that optimistic that iron can provide it, at least not in this form.