6 in axial direction and enters the system of tube bundles 6 via an inlet as indicated by arrow 4 and exits the tube bundles 6 through outlet as indicated by arrow 5 to thereby define an open secondary cooling circuit the tube bundles 6 act as heat exchanger between the inlet 4 and outlet 5 to transfer heat from air circulating in the closed primary cooling circuit of the heat source 1 to wind sweeping through the system of tube bundles 6 the heated air from the heat source 1 enters the system of tube bundles 6 via an outlet 7 is cooled there and flowed back through convection and or a fan 8 to the heat source 1 reference numeral 10 designates the transition zones between the primary cooling circuit and the secondary cooling circuit realized by the heat exchanger in the form of tube bundles 6 fig2 shows a cross section of the wind power installation and depicts in more detail the arrangement of the tube bundles 6 in the nacelle 2 the tubes of the tube bundles 6 have a cylindrical configuration and are suitably formed with unillustrated surface enlarging structures to enhance the cooling efficiency examples.