An unplanned, no warning shutdown of a smelter would be bad - it ruins the rather large and expensive carbon electrodes used to melt the aluminium. Probably not so good for the crucibles and other equipment that might get frozen up.

Planned shutdowns are another matter entirely.

In terms of instantaneous interruptible load, wood pulping mills are king. There are some enormous wood chippers in NZ's north island that can be remotely turned on and off by one of the generating companies.

some marketing speak here.

http://energyforindustry.co.nz/experience/interruptible-load...

Also, just about everyone's hot water cylinder in New Zealand is remote controlled by the power company (and has been since the 50's) Allows instant shedding of quite a lot of load in the short term without any visible impact to customers.

They can't just turn the whole plant off; but we were taught they can just be left with a trickle charge to keep them in liquid state.

The large energy cost is in driving the reaction forward.

Oh right, you can adjust the speed of the reaction throughout the day to adjust to market fluctuations.