A pneumato-mechanical regenerative power source converts mechanical rotational power at an input/output shaft to stored potential energy, and vice versa, by coupling the input/output shaft to a ball screw via a planetary gear system, which translates a ball nut in cooperation therewith, which in turn translates a piston that provides for either compressing or expanding an associated energy storage gas. A first roller-clutch mechanism provides for locking the carrier of the planetary gear system when storing energy, and releasing the carrier when releasing stored energy, wherein upon release, the ball screw is coupled to the input/output shaft via an one-wayoverrunning clutch, and the carrier may be locked to limit the release of stored energy.