Abstract
In this study, large-eddy simulations are performed to investigate the flow inside and around large finite-size wind farms in conventionally-neutral atmospheric boundary layers. Special emphasis is placed on characterizing the different farm-induced flow regions, including the induction, entrance and development, fully-developed, exit and farm wake regions. The wind farms extend 20 km in the streamwise direction and comprise 36 wind turbine rows arranged in aligned and staggered configurations. Results show that, under weak free-atmosphere stratification ( Γ = 1 K/km), the flow inside and above both wind farms, and thus the turbine power, do not reach the fully-developed regime even though the farm length is two orders of magnitude larger than the boundary layer height. In that case, the wind farm induction region, affected by flow blockage, extends upwind about 0.8 km and leads to a power reduction of 1.3% and 3% at the first row of turbines for the aligned and staggered layouts, respectively. The wind farm wake leads to velocity deficits at hub height of around 3.5% at a downwind distance of 10 km for both farm layouts. Under stronger stratification ( Γ = 5 K/km), the vertical deflection of the subcritical flow induced by the wind farm at its entrance and exit regions triggers standing gravity waves whose effects propagate upwind. They, in turn, induce a large decelerating induction region upwind of the farm leading edge, and an accelerating exit region upwind of the trailing edge, both extending about 7 km. As a result, the turbine power output in the entrance region decreases more than 35% with respect to the weakly stratified case. It increases downwind as the flow adjusts, reaching the fully-developed regime only for the staggered layout at a distance of about 8.5 km from the farm edge. The flow acceleration in the exit region leads to an increase of the turbine power with downwind distance in that region, and a relatively fast (compared with the weakly stratified case) recovery of the farm wake, which attains its inflow hub height speed at a downwind distance of 5 km.
Highlights
Among all of the available renewable energy forms, wind energy is the fastest-growing electricity source on the globe
The identified flow regions are: (i) The induction region, immediately upwind of the wind farm, where flow deceleration is induced by the wind farm due to the cumulated wind turbine blockage effect
The deceleration results in an upward mass flux from the wind farm top, which slows down the flow above the farm and causes an internal boundary layer (IBL) growth
Summary
Among all of the available renewable energy forms, wind energy is the fastest-growing electricity source on the globe. Various studies [1,2,3,4,5,6,7] have investigated the interaction between the ABL and very large wind farms (i.e., infinite wind farms) Inside such wind farms, the ABL flow reaches its asymptotic fully-developed regime, in which the power extraction of the wind turbines is balanced by the vertical turbulent transport of kinetic energy entrained from the flow above [2,4,5,6,7,8,9]. Few studies have focused on the whole flow field inside and around large finite size wind farms, including the induction and wake regions, and its adjustment to the fully-developed regime. The rest of the manuscript is structured as follows: Section 2 introduces the LES framework and numerical setup, Section 3 describes the suite of simulations performed, Section 4 presents the results obtained from the numerical simulations and Section 5 concludes and discusses the results of the study
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