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Research Papers

Impact of Blade Flexibility on Wind Turbine Loads and Pitch Settings

[+] Author and Article Information
Xiaocheng Zhu

School of Mechanical Engineering,
Shanghai Jiao Tong University,
800 Dongchuan Road,
Minhang district,
Shanghai, 200240, China
e-mail: zhxc@sjtu.edu.cn

Jinge Chen

School of Mechanical Engineering,
Shanghai Jiao Tong University,
800 Dongchuan Road,
Minhang District,
Shanghai, 200240, China
e-mail: jingechen@sjtu.edu.cn

Xin Shen

School of Mechanical Engineering,
Shanghai Jiao Tong University,
800 Dongchuan Road,
Minhang District,
Shanghai, 200240, China
e-mail: shenxin@sjtu.edu.cn

Zhaohui Du

School of Mechanical Engineering,
Shanghai Jiao Tong University,
800 Dongchuan Road,
Minhang District,
Shanghai, 200240, China
e-mail: zhdu@sjtu.edu.cn

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received July 11, 2018; final manuscript received December 13, 2018; published online January 8, 2019. Assoc. Editor: Yves Gagnon.

J. Sol. Energy Eng 141(4), 041002 (Jan 08, 2019) (13 pages) Paper No: SOL-18-1315; doi: 10.1115/1.4042315 History: Received July 11, 2018; Revised December 13, 2018

Along with the upscaling tendency, lighter and so more flexible wind turbine blades are introduced for reducing material and manufacturing costs. The flexible blade deforms under aerodynamic loads and in turn affects the flow field, arising the aeroelastic problems. In this paper, the impacts of blade flexibility on the wind turbine loads, power production, and pitch actions are discussed. An advanced aeroelastic model is developed for the study. A free wake vortex lattice model instead of the traditionally used blade element momentum (BEM) method is used to calculate the aerodynamic loads, and a geometrically exact beam theory is adopted to compute the blade structural dynamics. The flap, lead-lag bending, and torsion degrees-of-freedom (DOFs) are all included and nonlinear effects due to large deflections are considered. The National Renewable Energy Laboratory (NREL) 5 MW reference wind turbine is analyzed. It is found that the blade torsion deformations are significantly affected by both the aerodynamic torsion moment and the sectional aerodynamic center offset with respect to the blade elastic axis. Simulation results further show that the largest bending deflection of the blade occurs at the rated wind speed, while the torsion deformation in toward-feather direction continuously increases along with the above-rated wind speed. A significant reduction of the rotor power is observed especially at large wind speed when considering the blade flexibility, which is proved mainly due to the blade torsion deformations instead of the pure-bending deflections. Lower pitch angle settings are found required to maintain the constant rotor power at above-rated wind speeds.

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Figures

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Fig. 1

Schematic of rotor coordinates and the lifting surface model

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Fig. 2

Schematic of velocity and force decompositions of a blade element

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Fig. 3

Curved beam in the reference and deformed configurations

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Fig. 4

Aerodynamic performance curves of the NREL 5 MW RWT: (a) output power, (b) pitch angle, and (c) rotor speed

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Fig. 5

Coordinate system for defining blade deformations

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Fig. 6

Tip displacement responses in rated condition, with or without gravity, centrifugal force, and torsional moment applied: (a) out-of-plane deflections, (b) in-plane deflections, and (c) twist deflections

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Fig. 7

Comparison of rotor power production and shaft thrust with different loads applied on the flexible blade

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Fig. 8

Aerodynamic center offset with respect to elastic axis of the NREL 5 MW blade: (a) chordwise offset of aerodynamic center with respect to elastic axis and (b) typical section and aerodynamic force

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Fig. 9

Comparison of tip twist and out-of-plane deflections between different AC offsets with respect to SC under the rated condition

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Fig. 10

Comparison of rotor power production between different AC offsets with respect to SC under the rated condition

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Fig. 11

out-of-plane and in-plane bending deflections of the blade at wind speed of 15 m/s

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Fig. 12

Comparison of aerodynamic force between rigid and bending-bending flexible blades: (a) axial force and (b) tangential force

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Fig. 13

Bending-bending-torsional deflections of the blade under wind speed of 15 m/s, with twist DOF considered

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Fig. 14

Impact of blade bending-bending-torsion flexibility on AOA under wind speed of 15 m/s

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Fig. 15

Comparison of aerodynamic force between rigid and bending-bending-torsion flexible blades: (a) axial force and (b) tangential force

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Fig. 16

Steady-state aeroelastic deflections at 55 m radius of the blade across the operating wind speeds: (a) out-of-plane deflection, (b) in-plane deflection, and (c) torsional deflection

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Fig. 17

Comparison of aerodynamic force along the span of rigid and flexible blades: (a) axial force and (b) tangential force

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Fig. 18

Reduction of rotor power due to blade flexibility

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Fig. 19

Power production under different pitch angle at 15 m/s wind speed

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Fig. 20

Comparison of the AOA for rigid blade, and flexible blades with or without pitch angle correction, under wind speed of 15 m/s

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Fig. 21

Comparison of the aerodynamic forces for rigid blade, and flexible blades with or without pitch angle correction, under wind speed of 15 m/s: (a) axial force and (b) tangential force

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