Originally Posted by Grecko
Underground storage would only work if you had geothermal heating available to heat the air it might make sense, but if you didn't forget it.
It has been done for quite a while and has considerable potential for economy if the primary compression is powered by offpeak grid, solar, wind, or some other periodically low cost supply.
The heat lost in the storage is a toss up, intercoolers are needed between stages anyway. The energy lost through cooling in storage is a loss but the mass flow is the same from atmosphere all the way to the burners and that is what matters. These systems use recuperators to recover heat from the exhaust so they have the best of both worlds, high density and high temperature in to the burners without having to use inlet chillers. Look up compressed air energy storage, it's an interesting concept but to claim the same numbers for this air car are a bit too much of a stretch.