How about when the sun don't shine and the wind don't blow?
> Hydropower is popular in the Nordics, I believe.
Popular, but insufficient. Only Norway could ostensibly be run on hydro.
> Overproduction of both wind and solar is also feasible.
But is it economically viable?
> Winter production of solar is not nothing
Yeah, it pretty much is in the Nordics.
> Scaling up the number of collectors will still get you more production.
Nope. Zero times a large number is still zero.
> Having summer electricity be nearly free can create business opportunities, whereas high winter prices will reduce demand from industrial consumers.
Sure, business opportunities for end users. But is it sustainable?
With too much supply renewables make no money and other producers have to make all their money in the winter months.