Imagine a power plant (nuclear would be ideal -- it's a good base-load provider) built in the center of the ring. During peak hours, the plant sends juice over the wires as usual. Off peak, the plant feeds excess power into the ring's accelerator magnets. Instead of lofting a spacecraft into orbit, the ring magnets use the juice to accelerate a conductive body (say an iron slug) to the maximum speed mentioned in the article, 10 km/sec. Since Ek=mv2/2, a 10 kg slug traveling at 10000 m/sec will have a instantaneous kinetic energy of five hundred million Joules. The power plant then closes off the power to the ring magnets. Due to induction, the speeding slug will then cause a current to flow back through the circuit, converting its energy of motion into about 139 kW/h of instantaneous electrical energy. Figure 50% loss through the system and that's roughly 70 kW/h of tappable electrical power per slug -- enough power to meet an American home's electrical needs for several weeks!
Now imagine the entire ring filled with such slugs. At a diameter of 10km, the power ring would have a circumference of 31.4 km, give or take a few centimeters. Assume the slugs are of equal size (90 cm, say); with a 10 cm gap between slugs, the ring could hold 34, 906 slugs, making its total instantaneous electrical energy storage potential an astounding 2,443,400 kW/H (2.44 gW/h) -- enough stored energy to power a city of seventeen thousand homes for a month!
In addition, the power ring would (through losses) generate a good deal of heat energy. By careful design, the cooling system could make use of this "waste" heat to provide cogeneration energy for other uses.
Of course, if the levitation magnets fail or the physical structure of the ring is compromised, the slugs will impact the ring walls, depositing all 17,453,000,000,000 Joules into the surface of the earth — the equivalent of a nuclear explosion of a little over four kilotons. Ouch!
All this is just a thought experiment, of course. I am no kind of engineer and I may be overlooking one or more important objectons to the use of high-speed magnetic-levitation rings as power storage devices. (And I also may have the math wrong.) Still, it's an interesting idea, I think.
Why not just use it with compressed air? Maybe a little less eficient using compressors and wind turbines, but you're not risking a nuclear explosion.