[ncmncm]

Li-ion is the most expensive storage.

https://news.ycombinator.com/item?id=31485229

Energy storage, chemical methods aside, is freshman-level physics: pump against pressure, lift something heavy, draw down against buoyancy -- E = fx, E = mgh -- using processes already used at industrial scale in billions of cases. The only open question is what exact scheme will end up cheapest. But you don't need to know that to start building. You might switch over to building something cheaper, later, but anything already built still works.

(But NB: Energy Vault is the Theranos of storage. Steer clear!)

The Black Sea is deep, making underwater methods attractive. Romania also has high mountains, making conventional pumped hydro also practical. The reservoir for pumped hydro need not encompass a watershed, unlike regular hydro, so can be minimally disruptive, environmentally.