Compressed air is notoriously low-efficiency when you do it like in ye olden days, by venting the compression heat to the environment. A-CAES means capturing th

Compressed air is notoriously low-efficiency when you do it like in ye olden days, by venting the compression heat to the environment. A-CAES means capturing that heat, storing it separately and transferring it back into the decompression stream in discharge. Yes, this means that there will be some trickle discharge loss when using the storage scheme for long duration, but heat storage is a happy square-cube law thing, at a certain scale it even become viable for seasonal storage. A-CAES usually claim about 70% round trip efficiency. But you are right in that this number really isn't all that important in the renewables age: when we are anywhere close to getting to 100% renewables on a point of median supply and median demand, production capacity (conversion capacity from sunshine and air movement) at times when sun coincides with wind will so far outpace demand that any energy sink will do that can still pay a positive rate. We're a long way from fully renewable were I live, and some days I see two thirds of turbines stopped in nice wind. It's really all about capex per W and per Wh. Admittedly, A-CAES isn't necessarily excellent in this right now, but it might scale quite nicely with routine. Recently there was a gravity storage scheme linked here on hn about lowering mining refuse back into old mines to discharge, and digging it back up to charge, which comes with the curious property that it never really reaches a point of saturated capacity: in theory you could keep digging new tunnels forever when energy surplus keeps coming in.