On the Evolution of the Integral Time Scale within Wind Farms

Huiwen Liu,Yaqing Jin,Leonardo Chamorro,Imran Hayat

doi:10.3390/en11010093

Huiwen Liu, Yaqing Jin + Show 2 more

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https://doi.org/10.3390/en11010093

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Abstract

A wind-tunnel investigation was carried out to characterize the spatial distribution of the integral time scale ( T u ) within, and in the vicinity of, two model wind farms. The turbine arrays were placed over a rough wall and operated under high turbulence. The two layouts consisted of aligned units distinguished only by the streamwise spacing ( Δ x T ) between the devices, set at five and ten rotor diameters d T (or S x = Δ x T / d T = 5 and 10). They shared the same spanwise spacing between turbines of 2.5 d T ; this resulted in arrays of 8 × 3 and 5 × 3 horizontal-axis turbines. Hotwire anemometry was used to characterize the instantaneous velocity at various vertical and transverse locations along the central column of the wind farms. Results show that T u was modulated by the wind farm layout. It was significantly reduced within the wind farms and right above them, where the internal boundary layer develops. The undisturbed levels above the wind farms were recovered only at ≈ d T / 2 above the top tip. This quantity appeared to reach adjusted values starting the fifth row of turbines in the S x = 5 wind farm, and earlier in the S x = 10 counterpart. Within the adjusted zone, the distribution of T u at hub height exhibited a negligible growth in the S x = 5 case; whereas it underwent a mild growth in the S x = 10 wind farm. In addition, the flow impinging the inner turbines exhibited T u / T i n c u < 1 , where T i n c u is the integral time scale of the overall incoming flow. Specifically, T u → β T i n c u at z = z h u b , where β < 1 within standard layouts of wind farms, in particular β ≈ 0.5 and 0.7 for S x = 5 and 10.

Highlights

Installed power capacity from wind energy has been experiencing a monotonic increase worldwide in the last years; it accounted for almost half of all the electricity growth in 2015 [1], with large new installations of 30.75 GW in China [1]
Understanding the distribution and dynamics of such flow structures is of paramount relevance to quantify the unsteady loading and power output fluctuations of wind turbines [2]
A similar roughness configuration was used in wind tunnel experiments by Ohya [33] and Chamorro et al [32] to study stable boundary layers and wind turbine wakes over rough surfaces, respectively

Summary

Introduction

Installed power capacity from wind energy has been experiencing a monotonic increase worldwide in the last years; it accounted for almost half of all the electricity growth in 2015 [1], with large new installations of 30.75 GW in China [1]. Energies 2018, 11, 93 structure on the power output of single turbines and wind farms. Power measurements on a full-scale 2.5 MW wind turbine by Chamorro et al [7] showed that power fluctuations are strongly modulated by the scales of turbulence in a complex way. They suggested that the spectral content of the power fluctuations, Φ P , and that of the incoming flow, Φu , exhibit a relationship that can be characterized by a nonlinear transfer function G ( f ) ∝ f −2 across relevant length scales. Tobin et al [8]

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