For medium duration storage (days to weeks):

https://en.wikipedia.org/wiki/Compressed-air_energy_storage

https://en.wikipedia.org/wiki/Metal%E2%80%93air_electrochemi...

For long duration storage: hydrogen, ammonia.

Not the person you're replying to but pumped hydro is the most primitive I know of.

Also: why must the highly variable renewables share the grid with highly variable demand, use them to make hydrogen (only when the sun is shining/wind is blowing), feed that into a totally separate power plant, simple to manage - if renewables really get cheap who cares if it's inefficient.

Hydrogen will be one storage medium, mostly stored underground where geology favors it. Tanked anhydrous ammonia will be common, with more ordered from tropical solar farms when local tankage runs low. Underground and underwater compressed air will also be common. Liquified air might be.

Not the OP but dams and resevoirs is what he is talking about, I assume. Pump water uphill while the sun is out, let it feed back downhill and generate power overnight. It is century old stuff and it is very much validated to work.

There are numerous practical methods.

In addition to pumped hydro, you can use flywheels or even lifting heavy weights really high up.

Probably neither of those will find substantial use.