Wind turbine nacelle components, simplified.
A nacelle, in the context of a wind turbine, is the box-like cover for the gear box, drive train, generator, and other components of the turbine. The nacelle, which sits on top of the tower and is connected to the rotor, has a housing of fiberglass that protects the internal components from the environment.
The nacelle cover is fastened to the main frame, which also supports all the other components inside the nacelle. The main frames are large metal structures that must be able to withstand large fatigue loads.
On top of the nacelle of some off-shore wind turbines is a platform capable of supporting service personnel and their tools, winched down to the platform from a helicopter hovering above it. Wind turbine rotors are stopped, feathered, and locked before personnel are dropped down to or picked up from the platform.