Wind Turbine Blades Fibre Orientation
Controlling the pitch angle of a wind-turbine blade in fluctuating wind-speeds, without the use of active control-systems, is a subject in wind-turbine technology that has received quite some attention in recent years. Since most of the modern wind turbine blades are constructed of Fibre Reinforced Plastics (FRPs), one can make use of the anisotropic mechanical properties of such a composite structure. Using this principle for governing aeroelastic deformation of an aerodynamically-shaped component, such as a wing or a blade, is known as aeroelastic tailoring.
Aeroelastic tailoring has been defined as “the (incorporation) of directional stiffness into an aircraft’s structural design to control aeroelastic deformation, whether static or dynamic, in such a fashion as to affect the aerodynamic and structural performances of that aircraft in a beneficial way”. This is a way to avoid typical design problems, such as the necessity to add material, hence weight, because certain criteria such as stiffness requirements, have to be met. The applicability of anisotropic stiffness properties to control stability is not limited to aircraft wings or wind turbines alone, nor can its only benefit be stability enhancement. Stiffness tailoring can be beneficial in many areas where advanced composite materials are (or might be) incorporated in flexible structures to ‘control’ structural performance.
For wind-turbine blades, aeroelastic tailoring can be attractive for two reasons: to achieve lower fatigue loads or to optimise energy output. To achieve lower fatigue loads, an immediate change in blade angle of attack is desired to reduce the (over)loads suddenly imposed on the blade due to gusts. Optimisation of energy output can be realised by adjusting the blade’s angle-of-attack for each wind speed, to obtain optimal torque.
This can be achieved by coupling torsion deformation with flapwise curvature of a wind-turbine blade. In this manner, a change in wind velocity, causing a change in bending moment and hence curvature, would result in a change in effective angle-of-attack. This is called passive pitch control, as opposed to active pitch control, which involves wind measurement systems and control equipment to change the pitch angle of the blade. It is evident that such systems will make the turbine more susceptible to failure; the blade will be heavier, increasing the fatigue loads at the blade root. Passive power optimisation by bending torsion coupling results in a simpler blade, bypassing the subsystems will increase reliability, requiring less maintenance.
Deformation control of a lift-generating composite surface through proper laminate design can be illustrated with the blades as in Fig. 1(a) and (b). Consider a blade with a balanced symmetrical laminate. Symmetry means that for each ply above the centreline of the laminate, there is an identical ply below the centreline (in material, thickness and fibre orientation angle). Balanced implies that for every ply with fibre orientation angle 
with respect to the span axis, a ply exists with a fibre orientation angle in the ? direction. The airflow generates a lift force, creating a bending moment. In a simplified representation, the upper side of the blade is under compression, and the lower side under tension. In this configuration, the deformation of the structure is orthotropic with respect to a chosen set of axes, a bending moment imposed on the surface by lifting loads will only produce wing bending and not twist.
If a symmetric laminate composition is used where some fibres are directed off-axis, without a counterpart in the reverse direction, this results in an unbalanced laminate. The skin laminate has asymmetric stiffness with respect to the reference axis. If the lay of the laminate on the other side of the neutral axis is mirrored by this laminate, this is known as Circumferentially Asymmetric Stiffness (CAS). The stresses in the laminate tend to follow the fibre direction, along the stiffest path, causing a resultant shear component. At the laminate level, a tension/shear or compression/shear coupling is generated. This causes a bending-torsion coupling at the construction level. So the blade will now not only bend, the mirrored lay-up will cause the blade to twist, either increasing the angle-of-attack (towards stall or wash-in) or decreasing the angle-of-attack (towards feathering or wash-out), dependent on the orientation of the fibres with respect to the span axis.
In the case of a helical lay-up, also known as Circumferentially Uniform Stiffness (CUS), this would cause tension–torsion coupling and can be used in helicopter blades, which are subjected to high centrifugal forces.
This is achieved through stiffness tailoring of the composite laminate and will introduce another design parameter. Wind-turbine design is already ruled by many requirements. Wind turbine blades must be able to resist instabilities such as overloads due to gusts, and to withstand a very high number of cycles (about 500×106). They have to be non-corrosive and light. A wind-turbine blade has a design life of about 25 years, which should be reached with the necessity of few inspections. Being placed in remote areas or at sea, they are hard to inspect, so a damage-tolerant design philosophy, based on regular service intervals, cannot be applied.
In common design with FRPs, it is usual to obtain the optimal strength, which implies that one fibre-orientation is placed in the primary-load direction and at least two additional fibre-orientation angles are implemented to provide for strength in other directions, commonly using a quasi-isotropic lay-up. In the present blade-design, the goal is not only to reach a required stiffness, strength and lifetime, but also a certain directional stiffness. Changing the fibre orientation out of the primary load direction will inevitably result in a higher blade-flexibility and matrix strains. The conception that high structural rigidity is essential must be released. The increased amount of flexibility might force the choice towards a downwind rotor configuration, to prevent the blade from hitting the tower. The large number of parameters make optimisation procedures quite extensive. Firstly, it has to be established whether a desired amount of coupling can be reached while maintaining adequate strength and endurance.
From an analysis of a single-cell monocoque cross-section model with symmetric unbalanced skin laminate lay-up by, based on a theory by, it was shown that a sufficient amount of bending-torsion can be obtained to improve the power output at every wind speed. The optimal blade-pitch angles were calculated for wind speeds between 5 and 25 m/s.
Finite-element analysis confirmed these calculations. The finite-element model was subsequently extended to a 3D, 3-celled blade model. For the present analysis, the ‘key’ change in wind speed from 5 to 13 m/s was used as a reference value to determine whether the calculated change in blade twist corresponded with the desired change in blade twist. In this preliminary model, to achieve the required circumferential axisymmetric stiffness, carbon fibres have been implanted at an orientation of 25° with respect to the span axis to provide for the axisymmetric stiffness. Glass fibres were placed perpendicularly to them, at an orientation angle of ?65°. These fibre orientations were implanted in the skin shells for the outer 60% of the blade, where bending-torsion coupling is most effective. The blade configuration is in fact similar to a conventional blade, which is usually built up with two load-carrying blade shells, usually glass/polyester, with spar webs between them to provide sufficient stiffness and to prevent buckling. Ribs are not commonly utilised in wind-turbine blades. The difference with the conventional blade is that, in the ‘coupled’ blade, stiff carbon fibres are placed in a single orientation: carbon fibres are uncommon in wind-turbine blades. The initial concept was to use carbon/glass [0/90] hybrid weaves to build up the skin laminate. The aerodynamic loads were calculated for the relevant blade profiles and angles-of-attack and implemented in the model. In Table 1, a description is given of the blade model as well as a summary of the results for a change in wind speed from 5 to 13 m/s.
Tags: wind turbine
