What do you mean it works best when there's less load? Solar works best when the sun is shining, and that is when the demand for electricity is highest (especially in places with air conditioning).
Energy intensive industries were only built that way because it matches the output profile of electric grids pre-renewables. You usually had a coal or nuclear power station that delivered the same amount of electricity 24/7.
Energy intensive industries are already (in Germany at least) adapting to match their demand with the electricity spot price. There's nothing inherently that difficult about an aluminum smelter matching its highest production rates to when electricity is cheapest.
The aluminum industry has some weird physical constraints.
Alcoa has put out some engineering-heavy white papers on what actually happens and how the price and availability of power works in that industry. I can't find them at present but they were revelatory to me.
In brief, you can't just arbitrarily turn up or down your aluminum operation. At some point the giant molten pots of aluminum freeze up and destroy your capital equipment.