> I’m pretty sure that the arbitrage possibility is made use of by building batteries to even out the duck curve.

You are absolutely correct, which is why these assumptions that building out lots of solar will result in free or even negatively priced energy make no sense.

Currently these arbitrage possibilities are infrequent enough that they can't be exploited profitably. There just aren't enough super sunny days where an absolute excess of power is generated that would make it possible for, say, a battery farm to get built out to capture that storage. But everyone knows storage will be essential as renewable energy makes up more of overall power generation, and at that point it will make sense for power producers to save up their excess "sunny day" energy to sell it at other times.

If we're at the point where power producers are saving up excess sunny-day energy to sell at other times, then we've solved renewable energy's infamous base-load problem.

Exactly, that's my whole point. Once renewables become a large majority of overall power production, it will be essential that we have adequate storage to account for the variability of renewables. Thus, the original idea that started this comment thread that once we get tons of renewables that energy will be free or negative (e.g. "road transport becomes nearly free") doesn't make any sense. Wholesale prices only go negative for a relatively small percentage of the time currently due to lack of storage and government subsidies.