On the Wind Turbine Wake and Forest Terrain Interaction

Shyuan Cheng,Leonardo P. Chamorro,Fanghan Lu,Mahmoud Elgendi

doi:10.3390/en14217204

Abstract

Future wind power developments may be located in complex topographic and harsh environments; forests are one type of complex terrain that offers untapped potential for wind energy. A detailed analysis of the unsteady interaction between wind turbines and the distinct boundary layers from those terrains is necessary to ensure optimized design, operation, and life span of wind turbines and wind farms. Here, laboratory experiments were carried to explore the interaction between the wake of a horizontal-axis model wind turbine and the boundary layer flow over forest-like canopies and the modulation of forest density in the turbulent exchange. The case of the turbine in a canonical boundary layer is included for selected comparison. The experiments were performed in a wind tunnel fully covered with tree models of height H/zhub≈0.36, where zhub is the turbine hub height, which were placed in a staggered pattern sharing streamwise and transverse spacing of Δx/dc=1.3 and 2.7, where dc is the mean crown diameter of the trees. Particle image velocimetry is used to characterize the incoming flow and three fields of view in the turbine wake within x/dT∈(2,7) and covering the vertical extent of the wake. The results show a significant modulation of the forest-like canopies on the wake statistics relative to a case without forest canopies. Forest density did not induce dominant effects on the bulk features of the wake; however, a faster flow recovery, particularly in the intermediate wake, occurred with the case with less dense forest. Decomposition of the kinematic shear stress using a hyperbolic hole in the quadrant analysis reveals a substantial effect sufficiently away from the canopy top with sweep-dominated events that differentiate from ejection-dominated observed in canonical boundary layers. The comparatively high background turbulence induced by the forest reduced the modulation of the rotor in the wake; the quadrant fraction distribution in the intermediate wake exhibited similar features of the associated incoming flow.

Highlights

Wind energy has become a competitive contributor in the energy portfolio, and, as a consequence, it has experienced monotonic growth
This section discusses the impact of the forest-like canopy and its density in the wind turbine wake statistics and turbulent transport above the forest canopy in the near and intermediate wake regions
The experiments with a model wind turbine operating within two forest terrains with different densities showed similarities and distinct departures with the case of a turbine in a canonical turbulent boundary layer

Summary

Introduction

Wind energy has become a competitive contributor in the energy portfolio, and, as a consequence, it has experienced monotonic growth. Future developments are expected to occur in complex topographic and harsh environments due to reduced advantageous sites. Characterization of wind turbines operating in difficult terrains requires significant attention [1]. Particular scenarios include wind turbines operating in forest terrains. There, the interaction of multiple wakes with the canopy can result in turbulent exchange that may modulate the local ecological equilibrium and climate and the performance of large arrays of turbines. Characterization of coherent motions in boundary layer flows developed over forestry, and vegetative canopies, have been of high relevance over past decades due to their impact on mass, momentum, and energy exchange in the mixing process [2,3,4].

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Conclusion