A hydrodynamic energy recovery system for effecting mixing of gases within a headspace of a liquid-containment vessel containing a liquid enables mixing of the gases by converting energy of moving or sloshing liquid in a liquid-filled lower portion of the vessel into energy for moving the gases. The system includes (1) a hydro-driven actuator that is movably driven in response to the movement of the liquid in the vessel, (2) a mixing device driven by the actuator such that motion of the mixing device effects mixing of the gases contained in the headspace above the liquid, and (3) a motion converter for converting motion of the actuator into motion for driving the mixing device. The mixing device can be driven without the need to run electrical power within the vessel and without the transfer of motion through an external wall of the vessel.