That's exactly the appeal of flow batteries - for gigantic capacities, all you need is big tanks. Working your example - google tells me that zinc-iodine elect

That's exactly the appeal of flow batteries - for gigantic capacities, all you need is big tanks. Working your example - google tells me that zinc-iodine electrolyte gets you about 200 Wh per liter. Therefore you need about 400 million cubic meters of storage for the capacity you suggest. To estimate the cost of this, I looked at reservoirs. The largest drinking reservoirs in the world are in Qatar, where they have built 5 tanks of 436,000 cubic meters each. Therefore we'd need about 200 of those facilities. The cost was around 5 billion dollars, so our total cost would be around a trillion dollars. This is obviously a lot! But not unimaginable - the government borrowed over 300 billion pounds in the 2020 fiscal year alone just to pay for Covid. In practice you'd need considerably less than 90 days of continuous power, because the wind does blow and the sun does shine even in winter. All you really need is tanks to buffer the difference in renewable capacity between winter and summer, which is certainly not 100%. And the system can be built up incrementally over a long period of time, and still yield value - we don't need to spend a trillion dollars all at once. And in the worst case - you can erode the fraction of load taken on by renewables with nuclear (doesn't look so expensive now eh?). Obviously take all these numbers with a grain of salt, this is a back-of-the-envelope calculation built on another back-of-the-envelope calculation (in particular, I haven't included the cost of the electrolyte). The point is merely to show that it's not orders of magnitude outside our ability.