What is the expected efficiency of this?
Even if you use natural gas as a reheating element, the compressed air stores a significant amount of energy and is doing the majority of the work.
Efficiency / cost
I am worried that the cost is the driving factor here...
Goldman wouldn't be making that kind of an investment if they didn't have a handle on both parts of the equation. Indeed efficiency (dealing with losses from seepage) have been the nut that hasn't been cracked with compressed air storage; these guys seem to have an economical (efficient) way to deal with that problem. If true, then they really are worthy of Goldman's investment.