Presentations

Abstract

The interaction between wind farms and the surrounding atmosphere is extremely complex and brings significant challenges when it comes to site atmospheric characterisation and turbine interaction (wakes & blockage) modelling. This can be seen in recent publications covering the global blockage effect [1], gravity waves [2], far-field farm-to-farm wakes [3][4][5] and complex coastal transitions [6]. The scale of offshore wind farm development is ever increasing, thereby increasing the risk associated with accurate energy yield predictions. Offshore substations (OSS) have historically been excluded from energy yield predictions primarily due to their small size and overall aerodynamic footprint. However, as export capacity and the need for cost reduction, larger OSS structures with heights of up to 80m are being proposed and pose an aerodynamic a risk to yield and turbine loading. As advancements in the high-fidelity solutions such as Computational Fluid Dynamics (CFD) are made and cost decreases, opportunities are emerging to assess this risk and potential impacts to energy yield predictions. RWE has undertaken bespoke CFD modelling of an OSS with a height of 77m to assess its aerodynamic impact on a hypothetical but realistic future 2GW scale wind farm in a far-offshore North Sea wind regime. The atmosphere has been modelled using RWE’s standard CFD modelling approach assuming a fully-developed marine boundary layer [6]. The OSS has been represented using a simplified but realistic geometry for the main super-structure as a bluff body with a minimum grid resolution of 0.5m, while a momentum sink approach has been used to represent the support structure drag effect. As a large bluff body structure, an OSS represents a unique modelling challenge with downstream wake characterised by sharp pressure gradients and large coherent vortex structure differing substantially from that of a wind turbine. Nevertheless, modelling the OSS as a bluff body requires a locally fine and correspondingly expensive computational grid. Alternative computationally cheaper options are explored including representing the OSS as an overall momentum sink block approach and as a small turbine. It will be seen that neither of these can replicate the sharp flow gradients and therefore complex downstream flow patterns exhibited in the case of a mesh-conformal bluff body. An assessment has been performed demonstrating a negative impact on predicted energy yield through a combination of reduced wind resource and possible load mitigation strategies triggered by adverse wind conditions. Delegates will learn details of how the CFD model has been set including boundary conditions, meshing and modelling approaches. They will also learn how different OSS modelling approaches differ and why challenges exist with respect to current industry standard approaches. Delegates will see how bespoke CFD can leveraged to generate understanding of complex wind conditions and what this might mean for the design of future large-scale offshore wind farms.