Innovative design of large-scale horizontal-axis wind turbine blades

Abstract

One of the critical issues associated with the development of large-scale wind turbine blades is weight that, according to square/cubic law, grows with the cubic power while the energy extracted from with that increases with the square power of the blade length. Considering the slender nature of the large-scale composite blades, the conventional 45-degree off-axis fiber angle of the bi-axial (BX) and tri-axial (TX) skins in the longitudinal direction along the blade span is not optimized. Lower cost/weight as well as higher stiffness and strength can be achieved using non-crimp fabrics (NCF) skins with shallow off-axis fiber angles of less than 45 degrees while meeting the ultimate strength failure, buckling stability analysis, stiffness (tip deflection) and fatigue life design requirements. Novel shallow-angled symmetric and asymmetric skins are proposed and evaluated via their implementation to a 5 MW wind turbine blade, for the off-axis fiber angles of 45, 35 and 25 degrees. For the symmetric configuration, shallow-angled skins are applied to both the pressure and suction sides of the blade, while, for the asymmetric configuration, only the pressure side is implemented with a shallow-angled skin, keeping the conventional 45-degree-angled skin for the suction side. The use of shallow-angled skins improved the blade bending stiffness and strength, accompanied with a moderate reduction in the torsion stiffness (i.e. 6.3% reduction in the 1st torsion mode frequency). At shallow-angles of 35 and 25 degrees, the asymmetric skins demonstrated better buckling resistance than the symmetric skins due to the 45 degree off-axis fiber angle in the suction side skin under compression. For the symmetric skin, the buckling resistance of the blade can be enhanced by thickening the foam core at the blade region where buckling failure occurs. The increased bending stiffness and strength allowed to reduce the blade weight up to 8-11% and 13-14% by decreasing the thickness of the spar caps of the blade with 35 and 25 degree shallow-angled skins, respectively, eventually lowering the overall cost of energy (COE). The wind turbine blades must survive an operation life of 20 years. The predicted fatigue life as per existing method, recommend by IEC 61400-1 international specification and the Germanischer-Lloyd (GL) regulations, and using novel micro-mechanics based method, namely MMFatigue, was also increased for the blade with shallow-angled skins. The fatigue life of blades can also be enhanced by reducing the incident wind induced fluctuating loads using an adaptive blade design with bend-twist towards feather direction. The term ‘bend-twist coupling (BTC)’ describes the dynamic relationship between bending and torsion deformations of an adaptive blade. Under the influence of aerodynamic loads the blade twists towards as it bends, which causes a reduction in the angle of attack, thus, directly reducing the incident wind loads. The BTC magnitude depends on the amount of unbalance that can be generated in a composite laminate. The parametric study on BX and TX skins, demonstrated that the highest coupling can be achieved when all 03-unbalances (i.e. ply-angle, ply-material and ply-thickness unbalances) exist simultaneously in the skin layup, and computed coupling magnitude is 0.73 and 0.57 for the unbalanced BX and TX laminates. The observed optimal angle that generated maximum coupling is 25o for both BX and TX laminate. For fatigue load mitigation, the BTC towards feather is then implanted to the TX skin of a 5MW variable speed and collective-pitch controlled wind turbine rotor adaptive blades, fully coupled aero-structure analyses are performed for the incident turbulent winds ranging from 7 m/sec to 23 m/sec. The results shows that, in general, there is a reduction in the fatigue load and collective pitch demand across the spectrum of applied stochastic winds. For the ply-thickness unbalance being used along with the ply-angle and ply-material unbalances, the computed reduction in fatigue load increases by 2.8% at 7m/sec, 1.6% at rated wind of 11.4 m/sec, and 2-2.9% at winds beyond rated. And the reduction in pitch-actuator duty achieved is in a range of 15.4-19.9%, 0.8-8.7% and 0.4-5.5% at wind speeds of 5 m/sec, 19 m/sec and 23 m/sec, respectively. The vulnerability of the large-scale wind turbines with flexible blades to the dynamic instabilities has been increased. The “classical flutter” is expected to be one of the principle design driver for the future large-scale pitch-regulated turbines. It refers to a violent aero-elastic dynamic condition caused by the coupling of the bending and torsion vibrations, causing exponentially rising high-amplitude vibrations and leading to a catastrophic failure. The flutter performance of the large-scale blades with implemented “shallow-angled skins” and “bend-twist coupled using unbalanced skins” design concepts, is evaluated, due to variation in their bending and torsional stiffnesses. The time-domain aero-elastic simulation of the rotor for “run-away” conditions are performed to observe it response at the onset of the flutter. The flutter participant modes are identified from aero-elastic eigenvalue analyses. A moderate decrease in the blade flutter limit up to 9.2-10.3% and 4.2-5.2% is computed for the symmetric and asymmetric shallow-angle skins. For the bend-twist coupled blades, the flutter limits decrease, when the variation stiffness effect are not included, and vice-versa. The results showed that flutter does not pose a severe problem the 5MW blade with “shallow-angled skins” and with “bend-twist coupled design concept”.