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Research Papers: Gas Turbines: Turbomachinery

Design Considerations of Rotor Cone Angle for Downwind Wind Turbines

[+] Author and Article Information
Christian Kress

Laboratory for Energy Conversion,
Department of Mechanical and
Process Engineering,
ETH Zurich,
Zurich 8092, Switzerland
e-mail: kressc@ethz.ch

Ndaona Chokani, Reza S. Abhari

Laboratory for Energy Conversion,
Department of Mechanical and
Process Engineering,
ETH Zurich,
Zurich 8092, Switzerland

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 14, 2015; final manuscript received September 2, 2015; published online November 3, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(5), 052602 (Nov 03, 2015) (10 pages) Paper No: GTP-15-1334; doi: 10.1115/1.4031604 History: Received July 14, 2015; Revised September 02, 2015

Due to their potential to use light, flexible blades, downwind turbines are well suited for offshore floating platforms, for which there is a need to substantially lower the cost of wind-generated electricity. However, downwind rotors must operate in the presence of the tower's wakes with which are associated strong changes in flow incidence, and thus high fatigue loads. In order to guide the development of design rules for multimegawatt downwind turbines, a comprehensive experimental study has been conducted to better understand the characteristics of the unsteady rotor torque on downwind turbines. High-frequency measurements of the unsteady rotor torque on a model turbine that can be configured with rotors of different cone angles and operated either downwind or upwind in well-controlled flow conditions are conducted. The measurements show that in the case of the downwind turbine, the blade's passage through the tower's wake accounts for 56% to 61% of the variance of the rotor torque; the proportion of this unsteadiness is independent of the cone angle. For nonoptimum tip speed ratios (TSRs), the increase in unsteadiness is consistently less for downwind configurations than for upwind configurations. For the 5 deg-cone downwind configuration, the increase in rotor torque unsteadiness is 13–18% of the increase observed for the 5-deg-cone upwind configuration for nonoptimum TSRs. Thus from a design perspective, downwind rotor configurations offer above or below rated wind speed, a smaller increase in unsteadiness of the rotor torque compared to upwind turbine configurations. These characteristics differ from upwind turbines, on which broadband vortex shedding from the blade is the primary source of the unsteadiness, which may be reduced by increasing the rotor-tower clearance. It is suggested that given the strong periodic character of the blade's passage through the tower's wake, the turbine control system may be designed to reduce fatigue loads and there is a broader design space on downwind turbines that can be exploited for peak load mitigation by moderately adjusting the blade's stiffness.

FIGURES IN THIS ARTICLE
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Copyright © 2016 by ASME
Topics: Torque , Design , Rotors , Blades
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References

Figures

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Fig. 1

Contours of normalized axial velocity superposed with in-plane velocity vectors in a horizontal plane at 30% blade span with blade at bottom-most position for (a) downwind and (b) upwind configurations of commercial 2 MW wind turbine; unsteady simulations are from Ref. [8]

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Fig. 2

Bird's-eye view of ETH wind turbine test (WEST) facility

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Fig. 3

Photograph of (a) Hitachi HTW2.0-80 turbine [15] (courtesy of Wind Power Ltd.) and (b) model turbine

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Fig. 4

Side view of turbine model. The location of strain gauge application on the torque shaft is shown by the shaded area.

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Fig. 5

Time series of ensemble-averaged rotor torque over one rotor rotation for downwind and upwind configurations at TSRs of (a) TSRopt, (b) TSRopt − 10%, and (c) TSRopt + 10%; rotor cone angle is 5 deg

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Fig. 6

Time series of ensemble-averaged rotor torque over one rotor rotation for downwind and upwind configurations at TSRs: (a) TSRopt, (b) TSRopt − 10%, and (c) TSRopt + 10%; rotor cone angle is 0 deg

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Fig. 7

Time series of ensemble-averaged rotor torque over one rotor rotation for downwind and upwind configurations at TSRs: (a) TSRopt, (b) TSRopt − 10%, and (c) TSRopt + 10%; rotor cone angle is 10 deg

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Fig. 8

TSR versus wind speed characteristic of a variable-speed, variable pitch horizontal axis wind turbine; the characteristic is based on the NREL 5 MW reference wind turbine [17]

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Fig. 9

Amplitude spectrum and power spectral density of rotor torque for (a) downwind configuration and (b) upwind configuration; rotor cone angle is 5 deg and TSR is optimum

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Fig. 10

Amplitude spectrum and power spectral density of rotor torque for (a) downwind configuration and (b) upwind configuration; rotor cone angle is 0 deg and TSR is optimum

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Fig. 11

Amplitude spectrum and power spectral density of rotor torque for (a) downwind configuration and (b) upwind configuration; rotor cone angle is 10 deg and TSR is optimum

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Fig. 12

Amplitude spectrum and power spectral density of rotor torque without rotor while shaft is rotating

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Fig. 13

Effect of cone angle on contributions of specific frequencies to the torque variance. For downwind (a) and upwind (b) rotor orientations. Frequencies are frotor, fblade, and fvortex and broadband excitation.

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Fig. 14

Effect of cone angle on amplitude of rotor torque at frequency fBlade for downwind and upwind configurations

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