A system to cool the air inside a nacelle (12) of a wind turbine including the heat generating components such as the drivetrain (14), the electrical generator (16), the converter (20), and the transformer (22). The system has an upper cooling circuit (40), with a reservoir (26) disposed below the nacelle (12). The reservoir (26) has at least one pair of annular chambers (28, 30) and a lid (36) that freely rotates about its axis and about the nacelle yaw axis (38). Coolant flows from a first annular chamber to a second annular chamber through a heat exchanger (17) in the nacelle (12), thereby carrying heat from the nacelle (12) to the second annular chamber. The system also has a lower cooling circuit (42), having a coil (48) that connects the first annular chamber to the second annular chamber and being exposed to the outside of the wind turbine.