Composite wind turbines -  passive aerodynamic control

Glass reinforced plastics have been used extensively in the construction of blades for large wind turbines. In some machines they are used to provide lightly stressed aerofoil profiles over a main load-bearing steel spar or as a cladding material for wood. In other designs, the GRP provides the main load bearing structure. Carbon fibre materials, although highly suitable in terms of mechanical loads, are not normally considered owing to cost. For large machines, blades fall into three groups: tape wound, filament wound and hand laid-up. The figure shows an outline of each of the main types of design.

Wind turbine blade designs

The design requirements for a typical rotor blade are as follows:

• Low amplitude, high cycle fatigue properties (10⁷-10⁸ cycles)
• Stiffness and strength to accommodate 30-year extreme wind conditions.
• Adequate stiffness and structural damping to avoid resonance and to ensure aeroelastic stability

Passive aerodynamic control

The main forces acting on the blade of a wind turbine are primarily aerodynamic and centrifugal. Depending on the type of machine, these may lead to blade bending or axial tension. To achieve power control via blade twisting therefore requires coupling between bending and twisting or between axial extension and twisting. In a vertical axis wind turbine (see figure below), with bending/twisting coupling, centrifugal forces give rise to twist whose magnitude remains constant as the rotor turns. Also generated are aerodynamic forces which act radially inwards on an upwind blade, but radially outwards on a downwind blade. These forces give a blade twist whose magnitude varies cyclically as the rotor turns.

Forces acting on vertical axis wind turbine

One means of achieving coupling of this type is using a 'mirror' type lay-up where the laminate angle is of opposite sign above and below the chord (see figure below).

Mirror lay-up

The magnitude of coupling is determined by the materials of construction, particularly the degree of anisotropy and the details of the aerofoil geometry. The figure below shows the twisting curvature/ bending curvature ratio for a two-layer laminate (q f). In general the maximum coupling is obtained when the thicker lamina is orientated at 20° to the longitudinal axis.

Contours of the ratio of twisting curvatures (relative to bending curvature)

Given the example of q = 20, f = - 20 the ratio of curvatures is 0.5. For a blade of length 5 m, chord 1 m, the bending curvature is 1.53 /m for a strain of 0.2%. This is equivalent to a maximum end to end twist of 3.8 . Similar contour plots can be determined for stiffness properties. The figure below shows the effective bending stiffness as a percentage of the value obtainable when all the fibres are placed along the longitudinal axes.

Contours of bending stiffness (relative to stiffness at 0º laminate)

For a given laminate a judgement can be made as to whether or not the stiffness is sufficient to prevent dynamic instabilities. The figure below shows calculations indicating the effect of twist on turbine performance.

Effects of twisting on wind turbine performance

 The - 3º curve indicates a 3º twist due to centrifugal forces plus a cyclically varying twist of 2º due to the aerodynamic force. The influence of twist on performance is clearly indicated.