Wind Turbine Airfoils and Rotor Wakes - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 396

as calculated for two wind regimes: all winds from the prevailing direction, and wind flow
equally distributed in all directions.
A typical case, s 0 /U = 0.15 and crosswind spacing of 3 diameters, gives a wake
loss estimate of 10 percent to 12 percent for this idealized array. As shown in Figure 4-26,
extensive field experience in California wind power stations indicates that array losses are
typically about 10 percent, but can vary from 2 percent to almost 30 percent depending on the
terrain, the concentration of turbines, and the wind turbulence.
Another factor is introduced by array interference which can be as significant as the
direct energy loss, although it is less quantifiable. This is the extra turbulence generated
by upwind rotors, which will increase the turbulence to which downwind units are exposed
[ e.g. , Kelley 1989]. This so-called generated turbulence can affect the operation of a
downwind turbine by reducing its fatigue life, increasing the probability of catastrophic
failure caused by strong gusts, and by exciting blade vibrations and unfavorable control
system responses. This is likely to increase the operating and maintenance costs of these
machines and possibly cause premature wear-out and/or failure.
Physical Factors Controlling Wake Interference
It is clear that the dominant parameters are the downwind distance between units
(usually defined in terms of turbine diameters and normally between 6D and 12D ), the
amount of power extracted from the wind stream by the turbine units (defined in terms of
the power coefficient and normally about 0.40, maximum) and the turbulence in the wind
stream (both ambient and generated). The qualitative effects of these parameters are
described below.
Downwind Spacing
Close spacing is always undesirable, but this must be viewed in the context of
increasing the total energy production of the site and utilizing the directional nature of
seasonal wind patterns. Optimized spacing may call for arrays that are not orthogonally
symmetrical, but are oriented with respect to both prevailing and local wind condi-
tions. Typically, the principal energetic flows are from a prevailing direction, so that an
average crosswind direction can be defined from which energetic flows are quite rare. This
permits closer spacing crosswind than downwind, as pictured in Figure 4-21.
Power Extracted
The higher the output of a turbine, expressed in terms of its power coefficient, the
greater is the downstream wake interference. Interference, as a percent of potential energy
production, is approximately independent of wind speed, provided the wind is not very
strong or light. In the case of very strong winds (above the turbine's rated speed), upwind
turbines operating at rated power will be shedding excess wind power by stall or pitch
control. This in turn reduces rotor thrust and the axial induction factor (see Chapter 5), so
the retarded speed in the wake may still exceed rated speed. Thus downwind units will also
operate at rated power, and there will be no array loss. For very light winds, modest axial
inductions will retard the wake speed enough so that downwind units will experience winds
below the cut-in speed and will not operate. In this case, the interference percentage will
be very large.