Wind Turbines: Upwind or Downwind Machines?
Upwind Machines
Upwind machines have the rotor facing the wind. The basic advantage of upwind designs is that one avoids the wind shade behind the tower. By far the vast majority of wind turbines have this design.
On the other hand, there is also some wind shade in front of the tower, i.e. the wind starts bending away from the tower before it reaches the tower itself, even if the tower is round and smooth. Therefore, each time the rotor passes the tower, the power from the wind turbine drops slightly.
The basic drawback of upwind designs is that the rotor needs to be made rather inflexible, and placed at some distance from the tower (as some manufacturers have found out to their cost). In addition an upwind machine needs a yaw mechanism to keep the rotor facing the wind.
Downwind Machines
  Small downwind turbine (22 kW). You may notice that the rotor is "coning" away from the tower. Photograph Soren Krohn
© 1998 DWIA
Genvind 22 kW downwind turbine Downwind machines have the rotor placed on the lee side of the tower. They have the theoretical advantage that they may be built without a yaw mechanism, if the rotor and nacelle have a suitable design that makes the nacelle follow the wind passively. For large wind turbines this is a somewhat doubtful advantage, however, since you do need cables to lead the current away from the generator. How do you untwist the cables, when the machine has been yawing passively in the same direction for a long period of time, if you do not have a yaw mechanism? (Slip rings or mechanical collectors are not a very good idea if you are working with 1000 ampere currents).
A more important advantage is that the rotor may be made more flexible. This is an advantage both in regard to weight, and the structural dynamics of the machine, i.e. the blades will bend at high wind speeds, thus taking part of the load off the tower. The basic advantage of the downwind machine is thus, that it may be built somewhat lighter than an upwind machine.
The basic drawback is the fluctuation in the wind power due to the rotor passing through the wind shade of the tower. This may give more fatigue loads on the turbine than with an upwind design.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 23 July 2003
http://www.windpower.org/en/tour/design/updown.htm
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