I really like this one because it's fundamentally very low tech and doesn't require microprocessors or complex systems. Basically just a big spinning wheel with high mass and low friction which allow it to keep spinning until you put a load on it to generate electricity. Along with gravity systems like stored hydro, it's a great way to balance the load with intermittent generating systems like solar and wind.
https://interestingengineering.com/energy/china-world-largest-flywheel-energy-storage
World's largest flywheel energy storage system with 30 MW output connected to grid
RE