Presentations

Abstract

This study investigates the interaction between two neighbouring offshore wind farms operating in a Conventionally Neutral Boundary Layer (CNBL) where the atmospheric boundary layer (ABL) is neutrally stable, capped by a stable inversion layer and a less stable free atmosphere above. We compare the performance of the two wind farms in a CNBL to that in a truly neutral boundary layer (TNBL) with neutral conditions throughout the ABL and above. When the wind approaches a wind farm in a TNBL, the combined induction of the turbines creates a zone with increased pressure in front of the wind farm, redirecting the flow laterally and vertically. In a CNBL, the vertical component of the redirected wind interacts with a stable inversion layer aloft, creating an enhanced high-pressure area at the start of the wind farm, a phenomenon commonly known as global blockage. At the end of the wind farm, the wind is directed down towards the low-pressure region in the wind farm wake, lowering the inversion layer. This process reduces the wind speed at the start of the wind farm and accelerates it towards the end.   We look at the interaction of two neighbouring wind farms at varying distances in both a TNBL and CNBL using the relatively fast Multi-Scale Coupled model which uses a simplified mesoscale model to determine a background flow field coupled with an engineering wake model. Each wind farm consists of a regular aligned array of five turbines in the spanwise and ten turbines in the streamwise direction with a five rotor diameter (5D) spacing in both dimensions. The turbines are based on the 5 MW NREL reference and simulations are run at a wind speed of 9 ms−1 at hub height. For the CNBL simulations, we choose a Froude number (Fr) of 1 for the inversion layer and a value of Fr=0.10 for the free atmosphere.   Key findings (also summarised in Table 1): * For infinitely spaced wind farms in a CNBL, lower power output is seen compared to a TNBL due to stronger global blockage effects at the start of each wind farm despite a small speed-up towards the end of the farm. * For small wind farm separations (up to ∼75D)), the speed-up effect at the downstream end of the first wind farm enhances the power output of the second wind farm, whilst the blockage effect of the second wind farm negatively impacts the first wind farm. * The combined output of the two wind farms at small wind farm spacings is higher than when the farms are infinitely spaced though lower than in a TNBL.   Table 1: Wind farm power output in a CNBL for an infinite wind farm spacing and a spacing of 25D, relative to a TNBL.                                                                                                                                                                                                 Wind farm TNBL CNBL infinitely spaced CNBL 25D spacing 1 100% 98.1% 96.7% 2 100% 98.1% 101.8% Total 1 and 2 100% 98.1% 99.1%   This study is being expanded for a range of practical CNBL conditions validated by the use of large-eddy simulations.