>So why hasn't this come up much before?

My guess is volumetric inneficiency. This will be an alternative to pumped hydro, not batteries. But this has none of the conveniences of pumped hydro - nature already built the holding tank and you only need a pump and generator.

Storing all the uncompressed CO2 will require a massive structure for relatively little energy storage.

The other large aspect is that the technology has the same major issue as pumped hydro when compared to lithium-ion batteries. They don't give a clear return of investment.

Long duration general means that the intended target has a long charge cycle, and the longer the charge cycle is, the longer it will take to get an return of the investment. In contrast, lithium-ion batteries (with solar) is used with a daily charge cycle which means that calculating a return of investment is almost trivial. Every day the sun go down, demands goes up, batteries are discharged. Long duration storage tend to be combined with wind, and so you discharge during days of calm weather.

My understanding is that storage is not tightly coupled to a specific power source. Pumped hydro, or any kind of storage, is just another knob that the electrical grid operators have at their disposal to make sure load matches generation.

>But this has none of the conveniences of pumped hydro - nature already built the holding tank and you only need a pump and generator.

Only if you're in an area with favourable geography, of course...