There are many ideas in the pipeline for grid scale storage to smooth day-to-day renewable intermittency, but we also need seasonal storage. We need the ability to draw energy from storage for extended periods (weeks or months), to cover low energy production during winter.

The seasonal energy storage problem is one that humanity (and, really, life in general) has always faced. Many of our traditional food products are methods of preserving energy from the good times to tide us over in the bad times. We have traditionally adapted our activities for the season. I would imagine that part of the solution will be re-introducing seasonality to industry (e.g. making aluminium mostly in the summer).