Research Papers: Alternative Energy Sources

A Study on the Capability of Backward Swept Blades to Mitigate Loads of Wind Turbines in Shear Flow

[+] Author and Article Information
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

Xiaocheng Zhu

School of Mechanical Engineering,
Shanghai Jiao Tong University,
800 Dongchuan Road,
Minhang District,
Shanghai 200240, China
e-mail: zhxc@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 Advanced Energy Systems Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received December 11, 2017; final manuscript received January 18, 2019; published online February 18, 2019. Assoc. Editor: Ryo Amano.

J. Energy Resour. Technol 141(8), 081201 (Feb 18, 2019) (11 pages) Paper No: JERT-17-1698; doi: 10.1115/1.4042716 History: Received December 11, 2017; Revised January 18, 2019

The vertical wind shear is one of the major sources of fatigue loads on the blades of a horizontal axis wind turbine. Traditionally, the active individual pitch control system is used to alleviate the cyclic load fluctuations, which requires sensors and actuators. In this paper, a passive load control technique by using backward swept blades is explored of its potential capability of mitigating load variations in shear wind. An advanced aeroelastic model based on free wake vortex lattice model (FWVLM) and geometrically exact beam theory (GEBT) is developed for the study. Both the aerodynamic and structural solvers are able to account for the three-dimensional (3D) shape effects of the undeformed and deformed swept blades. The NREL 5-MW reference wind turbine is analyzed by adding backward sweep to the baseline blade. Comparisons are made between the baseline and swept blades for the time-varying root moments and rotor moments under sheared inflow. Different amounts of tip sweep are discussed. An effective reduction in the amplitude of the flapwise root moment variations is obtained, while the edgewise root moment is less influenced. Reduced mean values of the rotor yaw and tilt moments are also achieved, at the cost of increased blade root torsion moment.

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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

Configuration of a cantilever beam subjected to a bending moment at the tip

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Fig. 5

The deformation of the beam under different bending moments

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Fig. 6

Schematic of the 45-deg curved cantilever beam loaded with a tip force

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Fig. 7

Deformed 45-deg cantilever beam under different tip forces

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Fig. 8

The shear wind velocity profile

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Fig. 9

Illustration of backward-swept and straight blades

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Fig. 10

Cyclic variation of AOA at different radial positions in shear wind

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Fig. 11

AOA distribution along the span at different azimuthal angles in shear wind

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Fig. 12

Cyclic variations of the aerodynamic loads on the blade in shear wind inflow: (a) axial force and (b) tangential force

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Fig. 13

Steady-state torsional deformations of the flexible straight blade and the backward swept blade with a tip sweep of 5 m, under rated uniformed inflow

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Fig. 14

Rotor power production for the straight blade and backward swept blade with initial aerodynamic twist distribution, under rated uniformed inflow

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Fig. 15

Pretwist for the swept blade (5 m tip sweep) and baseline straight blades

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Fig. 16

Comparison of root moments of straight and swept blades in shear wind: (a) flapwise moment, (b) edgewise moment, and (c) torsion moment

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Fig. 17

Comparison of rotor moments in shear wind for rotors adopting the straight and swept blades: (a) tilt moment and (b) yaw moment

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Fig. 18

Comparison of the cyclic tip twist variations (subtract the average value) of the baseline blade and the swept blade in shear wind

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Fig. 19

Illustration of the section twist deformations of the swept blade at different azimuth angles in shear wind: (a) Azimuth = 0 deg (maximum inflow speed) and (b) Azimuth = 180 deg (minimum inflow speed)

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Fig. 20

Comparison of the cyclic effective AOA variations at 50 m radius of the baseline blade and the swept blade in shear wind

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Fig. 21

Root moment variations of swept blades with different tip sweeps under shear wind condition: (a) root flapwise moments, (b) root edgewise moments, and (c) root torsion moments



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