Turbines are very common machines in the City. Some turbines have rotating parts, which can be seen from outside. It is not known, what the turbines are used for or what they are powered by. In Running Interference there is a room called Electrics, that is containing a turbine, so turbines might be some kind of generator.
As most turbines in Thief are producing an unconstant sound, it is conjecturable, that they are idling. Nevertheless, some turbines are producing a loud turbofan like sound.