Wind Turbine Airfoils and Rotor Wakes - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 381

Rotor power was calculated using a computer model derived from the original PROP
code for predicting wind turbine aerodynamic performance [Wilson et al. 1976]. A later
version of this is the PROP93 computer code [McCarty 1993]. Power train losses were
estimated using the following general loss model [Spera and Janetzke 1981] and deducted
from the rotor power:

P PT = - aP G , R - ( b + s ) P R

(6-5a)

For the Gedser Wind Turbine:

a » 0.055
P G , R = 200 kW
b » 0.040
s » 0.050

(6-5b)

where

P PT = power-train loss (kW)
a, b = empirical constants from tests on the 200-kW Mod-OA wind turbine
P G,R = rated power of the generator (kW)
s = slip in the generator
P R = rotor power (kW)

The dashed power curve in Figure 6-4 is the result of calculations based on two-
dimensional airfoil data and Prandtl tip- and hub-loss models. The solid power curve
illustrates how the correlation between the calculated and measured peak powers of a fixed-
pitch rotor can often be significantly improved by modifying lift and drag curves in
accordance with Equations (6-3) and (6-4). This is an important achievement, since the
accurate determination of this peak power controls the design of the gearbox and generating
equipment and is thus a primary driver of the cost of a fixed-pitch turbine.
An alternative set of empirical equations for modeling lift and drag coefficients in the
pre- and post-stall regimes has recently been developed, extending the Viterna-Corrigan
model [Spera 2008, Fig. 2-16].

Airfoil Aerodynamic Requirements

There are evidently many engineering requirements entering into the selection of a wind
turbine airfoil. These include primary requirements related to aerodynamic performance,
structural strength and stiffness, manufacturability , and maintainability. Requirements
related to other rotor characteristics like electromagnetic interference, acoustic noise
generation, and aesthetic appearance are generally assumed to be of secondary importance.
Here we refer only to aerodynamic aspects, although we note that the critical wind turbine
performance and reliability features associated with aeroelastic behavior (changes in angle
of attack caused by blade deflections) introduce a strong coupling between aerodynamic and
structural requirements.

Lift and Drag Requirements

The usual assumption, historically established in airplane lifting surface theory, is that
high lift and low drag are desirable for an airfoil, and that the lift-to-drag ratio (often
abbreviated as L/D ) is a critical consideration. For wind turbine rotors this point of view