I just discovered the new Twitter feed from Volker Quaschning, a Professor for renewable energy systems at the Berlin University of Applied Sciences HTW Berlin.
I recommend following that feed.
In one of his Tweets Quaschning points to a blog post he wrote in May.
There he simulates running electricity in Germany in 2040 with 80 percent from wind and solar (the balance coming from other renewable sources, like hydro and biomass).
He concludes that there is a need to store about 5% of yearly demand for the occasional winter week with slow wind and small solar production.
And that could be done already today with existing gas infrastructure, which is able to store about 200 TWh of energy, much more than needed.
Of course it also makes sense to have batteries for home users. They will need batteries for their electrical vehicles anyway. But the really big capacity will come from synthetic fuel, most likely gas, though quicklime might be an interesting alternative.
[...] can be used in the existing gas infrastructure to store around 120 TWh in Germany, which beats the 30 TWh storage capacity necessary to run a renewable-only electricity system in Germany by a large margin. This is one of the obvious [...]
[...] German existing gas infrastructure could handle storage of up to 200 TWh, which is much more than the about 30 TWh an electricity system of 100% renewable would need. But [...]