> Granted a flywheel costs less per unit mass and compressed air is cheap, but on a capacity and fabrication basis? Maybe. I don't think I'd call it a shoo-in though and wouldn't invest in it myself.

Its power and energy density is next to nothing in the industry.

Fabrication, cost on the market? Even most basic flywheels like one used as industrial UPSes can be used right away for grid storage, and be more or less competitive with their high round-trip efficiency.

This just shows that how low a commercial opportunity for energy storage as such is. You need truly monstrous daily variations in energy price to make people put money it it.