Don Lancaster has a bit to say about this at
http://tinaja.com/glib/energfun.pdf
There’s been some web noise lately over compressed air
vehicles. It is obvious they never talked to anyone in the
fire service who dearly would love to use compressed air for
such tasks as rescue saws, spreaders, rapid cutters, PPV vent
fans, and such. But are unable to do so because of the lousy
energy density and appalling inefficiency of compressed
air. Despite years of careful engineering.
Because of the law of diminishing returns, typical fire
departments have elected not to go from 150 to 300 Bar on
their airpack systems. Higher pressures are beyond the pale.
He also includes a table of energy densities and it shows compressed air as pretty poor, about on par with lead acid batteries (but he doesn't list the pressure he's assuming).
It doesn't sound like such a great idea to me.