Direct air capture of CO2

A system, comprising:

• a pair of substantially vertically oriented concentric tapered tubular structures having a first, larger diameter end positioned at or near an ocean surface and a second, smaller diameter end positioned at an ocean depth, the pair of tubular structures including an inner tube that defines an inner, substantially cylindrical pathway for air to travel in a first substantially axial direction and an outer shell having a shell diameter that is larger than an inner tube diameter of the inner tube, an inner surface of the outer shell and an outer surface of the inner tube defining a substantially annular pathway for air to travel in a second substantially axial direction substantially opposite the first substantially axial direction;
• and
• one or more bifans each rotatably mounted at a corresponding axial location along a central vertical axis of the pair of substantially vertically oriented concentric tapered tubular structures, each bifan comprising:
• an inner set of fan blades disposed in the substantially cylindrical pathway defined by the inner tube;
• an outer set of fan blades disposed in the substantially annular pathway between the inner tube and the outer shell;
• and
• a motor configured to operate in a motor mode to drive the bifan and a generator mode to generate electricity when driven by air flowing through either the inner set of fan blades or the outer set of fan blades of the bifan;
• wherein air enters via an intake pathway comprising one of the inner, substantially cylindrical pathway or the outer, substantially annular pathway and is compressed substantially isothermally as the air travels through the intake pathway to the opposite end of the intake pathway, at the ocean depth, the movement of air through the intake pathway being facilitated at least in part by those blades of the plurality of bifans disposed in the intake pathway;
• and the compressed air expands substantially isothermally and returns to the ocean surface via the other of the inner, substantially cylindrical pathway or the outer, substantially annular pathway, said pathway comprising a return pathway, the rising and expanding air driving the one or more bifans by impinging on those blades of the bifans disposed in the return pathway.