One thing that hasn't been mentioned in siblings yet: the bulk of the money, more if you lean further towards capacity in the capacity vs throughput decision, i

One thing that hasn't been mentioned in siblings yet: the bulk of the money, more if you lean further towards capacity in the capacity vs throughput decision, is in the excavation. This is not only a "forever" investment, it's also money spent on the local economy. This isn't the case at all for battery cost, unless you happen to be the world center of the battery business, all the way from mine to recycling. Another aspect, completely unrelated and I'm not sure hydrostor already makes that part of the design (but it could totally be introduced later, without invalidating any of the excavation investment): some of the energy stored in an A-CAES system is stored as heat. When you do need heat, for a district heating system, for a swimming pool site or whatever, you can decouple some of the heat from the pressure storage. Worst case some of the joules repurposed end up missing in discharge, but it's also possible that they are simply joules not lost to cycle inefficiency. And if you happen to need cooling (datacenter on site?), at the time of discharge you can just keep some of the stored heat untapped, substituting with energy from the warm end of the coolant cycle that you want to freeload on the A-CAES. Compared to other waste heat/cold coupling schemes, at hydrostor pressure levels you would get considerably higher heat/cold deltas to work with. Huge potential, and with the reservoir shaft having very few site requirements, coupling opportunities should be plentiful (as compared to e.g. opportunities that only ever arise in remote valleys)