Comment not as great as it sounds (Score 5, Interesting) 307
I worked on a project doing exactly this about 5 years ago. The company, called SustainX, i believe is gone and disbanded. There are probably others too.
They basically took a giant marine diesel engine and modified it become an air compressor in storage mode, and an expander in retrieval mode. They had solved some of the technical challenges of doing it in a thermodyanically efficient way. Something about isothermal and adiabatic. I forget the details but they have some elaborate mechanism for *both* the compression and expansion of the gas to extract a lot more usable energy.
The big problem was storage. They used a giant tank array for their test system. They really wanted to deploy it globally but it turns out there's only a few locations in the entire word that have suitable geology for underground storage. So since tanks were relatively expensive and the geology wasn't prevalent, the company didn't sell anything and folded.
Cool idea and very technically savvy company though.