Yes, compressed natural gas in underground caverns is cheap enough for seasonal energy storage.
But of course, you meant “storage that can be efficiently filled using electricity”. That’s a difficult question. In theory, thermal energy storage using molten salt or hot sand could work, and maybe a sufficiently cheap flow battery chemistry is possible. In theory, much better water electrolysis and hydrogen fuel cells are possible; there just currently aren’t any plausible candidates for that.
But currently, even affordable 14-hour storage is rather challenging.
Yes, compressed natural gas in underground caverns is cheap enough for seasonal energy storage.
But of course, you meant “storage that can be efficiently filled using electricity”. That’s a difficult question. In theory, thermal energy storage using molten salt or hot sand could work, and maybe a sufficiently cheap flow battery chemistry is possible. In theory, much better water electrolysis and hydrogen fuel cells are possible; there just currently aren’t any plausible candidates for that.
But currently, even affordable 14-hour storage is rather challenging.