Instead of drilling deep, there is also an intersting case for storing cheap solar energy as hat in piles of dirts in the summer to power turbines in the winter: https://austinvernon.site/blog/standardthermal.html

The Berlin one also had some promising news today: https://www.adlershof.de/en/news/successful-tests-on-undergr...

We have to see if and when any of them goes into production, but the technology seems very interesting