The fuel cell part is a bit dodgy. It would make more sense to include a little hydrogen bottle on the tether if extra H2 is needed to top of the balloon. And

The fuel cell part is a bit dodgy. It would make more sense to include a little hydrogen bottle on the tether if extra H2 is needed to top of the balloon. And there's no point to storing power in the air like that - it's better fed to the grid when the sun is shining to be stored at the grid level. Generating hydrogen just isn't a very efficient way to store power.