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.