That thing is never going to be commercial viable. It doesn't have the energy density to be worthwhile, and comes with all the hassle that a moving mechanical system brings (maintenance etc). Plus I don't believe their 85% efficiency figure. That's ludicrously good.

Also most of the $40m/year saving of this battery is that it can respond in 100 ms which means they don't have to spend so much money buying electricity from other people when demand or supply changes suddenly.