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Comment by daveguy | Hacker News Reader

Why aluminum smelters in particular? The process has a wide range of parallelism? It seems like most industrial processes would adjust more on demand.

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The way you separate aluminum from alumina is by electrolysis. Like you guessed, it is highly parallel. It is as simple as having hundreds of pots, each with huge carbon electrodes, dumping megawatts of power into the molten matrix to extract the metal. And it consumes an obscene amount of power to do so.

https://www.youtube.com/watch?v=fa6KEwWY9HU&feature=youtu.be...

https://theconversation.com/the-trouble-with-aluminium-7245

This is also the primary reason that aluminum recycling is so beneficial. It is dirt cheap to remelt already refined aluminum vs. smelting fresh ore, even though aluminum itself is an exceptionally common element.

Yes, but aluminium smelting has certain constraints - namely, the molten metal cannot solidify[1]. This restricts your ability to quickly spin up and down smelters, but they are still potentially a very useful part of an energy smoothing strategy.

[1] https://en.wikipedia.org/wiki/Aluminium_smelting

Sure. But the thermal mass of an in-use pot is significant, and that buys you time. You can read more about the details here:

http://www.aluminiumtoday.com/contentimages/features/Oyeweb....

It's a well-done summary, I think you'll find it interesting. The tl;dr is that power interruptions up to ~4-5 hours do not cause significant trouble.

Huh, interesting. I read the whole thing and understood at least half of it ;).

Are the damages primarily thermal? I couldn't figure it out from that report, although it seemed to somewhat hint that they were. I.e. if you could keep the cells warm (with more insulation or otherwise), would they still get damaged by the shutdown?

Your assumption is correct. The vast majority of the damage comes from thermal stressing of the refractory materials that make up the crucible of the furnace. If there were better insulators available, so that the rate of heat loss through the furnace was minimized, it would be a huge improvement to the industry, both in energy saved during production, and greater amounts of time available after shutdown before thermal effects wreck the furnace.

I remember reading that in aluminum smelting, compared to other industrial processes, the energy cost is an especially large part of the overall costs. So when energy costs are abnormally low, smelting aluminum could go from bad to OK to wildly profitable.

You're right. A key sentence from the second link I posted:

> the embodied energy (all the energy used to make the material) for aluminium is 211 GJ per tonne, compared to 22.7 GJ per tonne for steel.

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