It's not so bad. Less than pumped hydro and conventional batteries and at grid scale. Combined those are big pluses as pumped hybro can't be built anywhere and batteries are not suitable for grid scale.
The question is, what are the life cycle costs? If it's on par with pumped hydro, then we are talking.
The question is, what are the life cycle costs? If it's on par with pumped hydro, then we are talking.