This source also offers an option of $1 Trillion USD to do it with battery storage.
All of Europe. $1 Trillion USD. Oh, and that figure has already fallen by 1/3rd in reality and the article claims it should drop by half again.
And that seems to be assuming you only have wind power as input. The long lull periods that drive the high storage requirements are, as that article claims, caused by large high pressure air masses. High pressure systems like that often come with clear skies! Indeed, go look at weather history for that same 2015 period and you see that the skies were calm and clear, and precipitation was about half the "normal" amount for that time of year. While there is perfect correlation between a windless day and a night without sunlight, battery to get you through the night is trivial and solved far more cheaply than this article seems to understand. Enough battery to maintain 24 hour output for a solar farm is cheap enough to compete with fossil fuels. Long term, wind and solar do not correlate, so it's very rare to have long lulls in both at the same time.
So this article is leaving out important details and also is way more pessimistic than even it admits is true.
That also ignores that even in the "lulls", wind never seems to go to zero, so even in lulls, you can always just have more wind. Building 10x as much wind as you need is not as feasible as building 10x as much solar as you need though IMO.
Oh, and a very very very important fact: Renewable generation is almost entirely a one time cost, or one time every 30ish years on average. OPEX per kilowatt hour is dramatically lower than fossil fuels. In fact, today Europe imports 10 million barrels of crude oil a day, and at $100 a barrel (a number which will rise quite a bit in the coming months), Europe spends $1 Trillion every few years.
Europe's current energy spend is to buy an entire continent's worth of energy storage and just turn it into CO2 every few years. Every single day of crude oil import, Europe could instead pay for one of the Coire Glas model plants this article is doing the math with.
Storage is beyond feasible and will reduce energy costs.
Note: This article is about making wind energy constant over month long time scales, not about building enough storage to power Europe durably, so that explains some of it's misses, but also doesn't really explain much. The 2.1 TWh of storage it suggest would be enough to power all of Europe for 8 hours a day.