Wind Turbines
Summary
- Classification Details
- General Performances
- Aerodynamic Controls
- Selected References
Classification Details
Wind turbines are the most recent technology used for wind energy conversion. Rudimental wind mills have been used in Europe for centuries. The actual systems consists of a mast, a number of freely rotating blades, a power generation system, and various controls (Hansen-Butterfield, 1993).
We will discuss horizontal-axis wind turbines (HAWT), which make up most of the
systems in operation today. These machines can be classified according to the
position of the mast (upwind or downwind), to the blade articulation (rigid or
teetering), aerodynamic control system (stall or pitch regulated), and number of
blades.
Wind turbines have some peculiar aerodynamic problems. The flow is unsteady
most of the time, with combined effects of wind shear, atmospheric turbulence,
gusts, yaw mis-alignment, tower shadow.
Three-dimensional effects are also interesting. It has been known for some
time that two-dimensional airfoil data are unsuitable for predicting the
aerodynamic performances on stall-regulated turbines.
The rotation of the blades delays the static stall, because the centrifugal pumping
results in Coriolis acceleration terms that induce favourable pressure gradients. The
result is that boundary layer separation is delayed.
Fig. 1 below shows an example of 2D wind tunnel data, as compared with data taked at
specified span sections on a rotating blade. Stall does not seem to occur. The normal
force coefficient Cn is actually increasing toward large values at large angles of
attack.
Similar effects appear on thrust and torque coefficients (one example shown in
Fig. 2). This problem makes it difficult to predict the peak power.
Airfoil selection is important in optimizing the rotor performance (which in the
general case is quite a complicated problem.) Latest research shows that stall-regulated
turbines work best with thin outboard sections with limited CLmax and smooth
static stall. Inboard sections must be thick (well above 30%, it turns out).
The peak power from a wind turbine can be controlled by letting the blades stall
(stall regulated machines), or by pitch adjustment (pitch controlled machines), to
allow for the best inflow angles. One obvious difference is that the pitch regulated
machines need sophisticated mechanical installations, but the performances do have
their own impact.
By letting the blades stall, there is the problem of predicting both the unsteady
loads (dynamic stall), and three-dimensional rotating effects.
General Performances
Figure 1: Normal Force Coefficient
Figure 2: Thrust Coefficient
Aerodynamic Controls
Related Material
Selected References