in the drawings , like reference numbers refer to like objects throughout . objects in the diagrams are not necessarily drawn to scale . fig1 shows a schematic representation of a wind turbine 2 with a first type of prior art cooling arrangement 122 , 3 in the interior of a canopy 24 mounted on a tower 21 . during operation of the wind turbine 2 , rotor blades 22 cause the hub 23 to rotate , so that an electrical machine in the interior of the canopy is caused to generate electricity . heat is generated during operation of the wind turbine 2 , for example in components 120 or modules 120 of the electrical machine , in a power converter 121 , etc . here , only a few such heat sources 120 , 121 are indicated , but it will be clear to the skilled person that heat may be generated by other components also . to cool these components 120 , 121 , a heat transfer fluid may be pumped through suitably placed conduits 122 or pipes 122 , and the heat transfer fluid may be cooled in an active heat exchanger unit 3 using techniques known in the field of refrigeration . the active heat exchanger unit 3 requires an appropriate power supply 30 . evidently , such a power supply 30 is associated with certain running costs . furthermore , as mentioned in the introduction , such an active heat exchanger unit 3 may be costly to manufacture and maintain . fig2 shows a schematic representation of the canopy 24 of a wind turbine 2 with a second type of prior art cooling arrangement 122 , 25 . here , heat generated by the heat - generating components 120 , 121 of the wind turbine 2 is transferred to a heat transfer fluid in conduits 122 or pipes 122 , and directed to a passive heat exchanger 25 mounted on the exterior of the canopy 24 . a heat dissipating structure of the heat exchanger , for example an arrangement of vertical fins with intervening spaces or gaps , is heated at the hottest region r in which the conduits 122 enter the heat exchanger . such a passive heat exchanger 25 generally comprises a housing supporting a plurality of heat - dissipating fins , and the conduits 122 may extend into the heat exchanger 25 and may be arranged to effectively transfer heat to the base of the fins , as will be clear to the skilled person . the passive heat exchanger 25 presents a relatively large surface area to the wind af w or airflow af w passing over the canopy 24 and through the heat exchanger 25 . however , the cooling capacity of the passive heat exchanger 25 is limited by its size . therefore , to cool a large wind turbine at all wind speeds , even around the critical threshold wind speed , the passive heat exchanger 25 would have to be correspondingly large . however , for the reasons given above , a too large passive heat exchanger 25 is impracticable , and very strong winds might even damage it . therefore , depending on the wind turbine construction , this type of passive heat exchanger 25 with its limited surface area might not be able to provide the necessary cooling during wind speeds around the threshold wind speed . fig3 shows a schematic representation of the canopy 20 of a wind turbine 2 with a cooling arrangement 1 according to a first embodiment . for simplicity , the heat - generating components and the conduits for a heat transfer fluid are not shown , but may be assumed to be as shown in fig2 above . here , the cooling capacity of a passive heat exchanger 10 is augmented by an additional cooling means or ventilation means , in this case , an air channel 11 arranged in the interior of the canopy 20 , with an air inlet 110 on the side of the canopy 20 , and an air outlet 111 arranged to open onto the passive heat exchanger 20 . the air channel 11 with its air inlet 110 and air outlet 111 is arranged to make use of the pressure difference in air pressure at different regions about the canopy , so that the air pressure at the air outlet 111 is lower than at the air inlet 110 . the air outlet 111 is arranged to open directly onto the heat exchanger 10 . in this way , airflow af pd is effectively drawn of its own accord , owing to the under - pressure at the channel outlets 111 , through the air channel 11 and onwards through openings between fins of the heat exchanger 10 . at wind speeds about the critical threshold wind speed , i . e . at times when the cooling requirements are greatest , the contribution of the additional air cooling provided by the airflow af pd through the air channel 11 and onto the panel 10 may be enough to ensure that the total cooling capacity of the cooling arrangement 1 is sufficient to cope with the maximum heat generated by the wind turbine components . fig4 shows a schematic representation of the canopy 20 of a wind turbine 2 with a cooling arrangement according to a second embodiment . this diagram also shows the fins 101 of the heat exchanger 10 . here , the passive heat exchanger 10 or panel 10 is mounted on the canopy 20 in such a way that it forms part of a heli - hoist platform 4 on top of the canopy 20 . the height of the panel 10 in this embodiment does not exceed 1 . 5 m above the height of the platform floor , since 1 . 5 m is the maximum allowable height according to the european aviation authorities . a helicopter may therefore safely hover above the platform while maintenance workers are lowered to or lifted from the platform 4 , for example by means of a motorised winch in the helicopter . any railings 40 or safety features such as warning lights 41 may be arranged to avoid any obstruction of an airflow af w over the panel 10 . fig5 shows a schematic representation of a wind turbine 2 with a cooling arrangement 1 according to a third embodiment . here , the canopy 20 , passive heat exchanger 10 and inlets 110 are shown from above . the cross - sectional view of the passive heat exchanger 10 schematically indicates a heat dissipating structure 101 arranged within a housing 100 , for example an aluminium housing 100 , with a plurality of vertical fins 101 separated by intervening spaces to maximise the area of the heat - dissipating structure 101 . in this embodiment , two air channels 11 are arranged within the canopy 20 . the diagram also shows a possible shape for the air channels 11 , in this case , the air inlets 110 are positioned to either side of a highest part of the canopy 20 , and the air outlets 111 are relatively wide , flaring towards the ends of the channels 11 to give a combined opening onto the base of the passive heat exchanger 10 . again , the air outlets 111 are arranged to open directly onto the hottest part of the heat exchanger 10 . the air channels 11 may open onto a part of the panel in which the conduits , transporting warm heat transfer fluid , enter the panel , for example at a lower region of the passive heat exchanger 10 . in the case where the cooling capacity of the passive heat exchanger 10 augmented by an airflow af pd ( arising on account of a pressure difference between the inlets 110 and the outlets 111 or an under - pressure at the outlets 111 ) is insufficient to cope with the heat given off by the wind turbine components , the cooling capacity of the cooling arrangement 1 may be augmented further by activating ventilators 112 arranged in the air channels 11 to generate an increased airflow af fan may be generated . the ventilators 112 may be activated by a signal 114 provided from a sensor 113 , for example a wind speed sensor 113 . at peak times , therefore , the cooling capacity of this hybrid cooling arrangement 1 , using wind airflow af w and augmented channel airflow af fan may reliably cool the heat transfer fluid of the cooling circuit to ensure optimal and sufficient cooling for the heat - generating components of the wind turbine . fig6 shows a schematic representation of the canopy 20 of a wind turbine 2 with a cooling arrangement 1 according to a fourth embodiment . again , the passive heat exchanger 10 or panel 10 is mounted on the canopy 20 in such a way that it forms part of a heli - hoist platform 4 ′ on top of the canopy 20 . however , in this realisation , the heli - hoist platform 4 ′ comprises a robust mesh 4 ′ or grid 4 ′ with many openings or holes to allow air to pass from above the canopy to a space underneath the platform 4 ′, providing an additional cooling airflow af pd . in this embodiment , ventilators 112 are also positioned in the space beneath the platform 4 ′, and may be used to generate an increased airflow as required . fig7 shows graphs of the cooling capacity cc pa of a prior art passive heat exchanger shown in fig2 , the cooling capacity cc 1 of a cooling arrangement according to the embodiment described in fig3 , and the heat loss hl ( in kw ) of the components of the wind turbine as a function of wind speed ws ( in m / s ). the maximum heat loss hl max depends of a wind turbine depends on various factors , for example the wind turbine dimensions , the efficiency of the electric machine , reactive power mode of the converter , etc . the graph shows a wind turbine heat loss curve hl wt . as the wind speed increases from 0 m / s , the heat loss of the wind turbine increases steadily , up to a certain maximum value hl max . beyond a certain wind speed ws th , the heat loss remains more or less at this maximum hl max . the cooling capacity of a cooling arrangement using a passive heat exchanger , as described in fig2 and 3 above , follows an essentially parabolic curve cc pa , cc 1 . the steepness of this curve will depend on the area of the passive heat exchanger . as indicated by the curve cc pa , the cooling capacity of a prior art passive heat exchanger that is not large enough is insufficient to cope with the peak cooling requirements . this ‘ insufficiency ’ is indicated by the intersection 60 of the first curve cc pa and the heat loss curve hl wt . even so , to the left and right of the threshold wind speed ws th , the passive heat exchanger has wasted cooling capacity . a larger passive heat exchanger might be able to provide sufficient cooling , but its physical dimensions would be impracticable for the reasons given above , and such a physically large design would be associated with correspondingly higher levels of wasted cooling capacity . a cooling arrangement , i . e . comprising a passive heat exchanger and a number of air channels for providing additional airflow af pd over the passive heat exchanger as described in fig3 above , may provide sufficient cooling capacity , as indicated by the curve cc 1 . the differences between the curves cc pa , cc 1 may be attributed solely to the additional cooling effect of that additional airflow af pd . however , some cooling capacity of the cooling arrangement is also ‘ wasted ’ here , as indicated by the crosshatched regions between the cooling capacity curve cc 1 and the heat loss curve hl wt . fig8 shows a graph of heat loss hl wt ( in kw ) of the components of the wind turbine as a function of wind speed ws ( in m / s ) and a graph cc 2 of the cooling capacity of the hybrid cooling arrangement of fig5 , which uses both passive cooling and fan - augmented active cooling in a hybrid cooling arrangement . the hybrid cooling arrangement 1 of fig5 is associated with less wasted cooling capacity , as indicated by the crosshatched regions . however , sufficient cooling of the wind turbine components is ensured by additional active cooling which may be applied whenever required . here , beyond a certain first wind speed ws lo , the active cooling is activated to cope with the peak cooling requirements , so that the ventilators in the air channels actively draw in air and direct it at the passive heat exchanger . this additional cooling may be maintained until the wind speed either drops below the first wind speed ws lo again or increases beyond a second , higher , wind speed ws hi , at which wind speed ws hi the cooling capacity of the passive heat exchanger is again sufficient to cool the heat transfer fluid . with such a hybrid design , cooling is sufficient , even at wind speeds about the threshold wind speed ws th , while the dimensions of the passive heat exchanger may be kept within practicable limits . although the present invention has been disclosed in the form of embodiments and variations thereon , it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope . for example , the passive heat exchanger could also comprise lateral elements that extend sideways away from the canopy to increase cooling capacity , whereby such lateral extensions may be are dimensioned so that these also comply with maximum allow height so that aviation regulations are complied with . accordingly , the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope , which is to be given the full breadth of the appended claims , and any and all equivalents thereof . for the sake of clarity , it is to be understood that the use of “ a ” or “ an ” throughout this application does not exclude a plurality , and “ comprising ” does not exclude other steps or elements . a “ unit ” or “ module ” may comprise a number of units or modules , unless otherwise stated .