State of the Art in the Optimisation of Wind Turbine Performance Using CFD

Wind energy has received increasing attention in recent years due to its sustainability and geographically wide availability. The efficiency of wind energy utilisation highly depends on the performance of wind turbines, which convert the kinetic energy in wind into electrical energy. In order to optimise wind turbine performance and reduce the cost of next-generation wind turbines, it is crucial to have a view of the state of the art in the key aspects on the performance optimisation of wind turbines using Computational Fluid Dynamics (CFD), which has attracted enormous interest in the development of next-generation wind turbines in recent years. This paper presents a comprehensive review of the state-of-the-art progress on optimisation of wind turbine performance using CFD, reviewing the objective functions to judge the performance of wind turbine, CFD approaches applied in the simulation of wind turbines and optimisation algorithms for wind turbine performance. This paper has been written for both researchers new to this research area by summarising underlying theory whilst presenting a comprehensive review on the up-to-date studies, and experts in the field of study by collecting a comprehensive list of related references where the details of computational methods that have been employed lately can be obtained

A Comparison Between CFD-Based Aerodynamic Models and BEM Theory-Based Models Applied in Coupled Simulations of Floating Offshore Wind Turbines

Interest in floating offshore wind as a renewable energy source is growing, as it offers the potential to access deeper waters than those suited to bottom-fixed offshore turbines. A key design challenge for floating offshore wind turbines (FOWTs) is capturing the aerodynamic behaviour using numerical models, which is significantly more complex than for bottom fixed turbines due to the motions of the floating platform that result in unsteady relative wind flow at the rotor. Many of the engineering models available for analysing wind turbine aerodynamics such as the blade element momentum (BEM) method were designed for fixed turbines and require empirical corrections to account for unsteady aerodynamic effects, and may not be suitable for analysing the more complex aerodynamics associated with FOWTs. Higher order modelling approaches including computational fluid dynamics (CFD) may offer improved accuracy as they capture more of the flow physics, however, they can have extremely high associated computational costs. In this thesis, the performance of different aerodynamic models for FOWTs is investigated by studying the motion and load response of a FOWT in a range of load cases covering operational and extreme conditions using a BEM method and two different CFD-based models. Firstly, the BEM method used in the wind turbine engineering tool FAST is compared with an actuator line model (ALM) from the CFD wind turbine code package SOWFA for a range of load cases. Comparisons are made in load cases that have specific challenges for FOWTs and where the BEM method has known limitations, including rotor misalignment with the wind due to yaw, and varying wave conditions. The two modelling approaches are then used to study FOWT behaviour in realistic operational and extreme environmental conditions, and the model results are compared against available field data from full scale FOWT demonstration projects. The impact of using high order large eddy simulation (LES) to generate a turbulent wind field is also compared against a lower order statistical approach. Finally, a high order modelling approach is proposed that couples a geometry-resolved CFD model of a wind turbine blade with a structural model based on 3D finite element analysis (FEA) to enable two way coupled fluid structure interaction simulation. This model provides detailed information on the loading and deformation of blades, and is compared against FAST for studying a large flexible wind turbine blade in the parked and feathered position. This research provides improved understanding of the impact that the choice of aerodynamic and wind models have on the predicted response of a floating offshore wind turbine. ALM predictions are found to diverge from BEM predictions in increasingly large rotor yaw misalignment angles. Turbine loads and platform motions are found to be sensitive to the atmospheric stability condition, with stable conditions having a significant effect, however the use of high fidelity LES modelling of neutral conditions has little effect on turbine response (in either operational or extreme conditions) compared to using more efficient statistical modelling of turbulence using the Kaimal model. The results of the presented comparisons in this work are used to make recommendations on the use of different models in the design process for FOWTs. It is found that FAST is suitable for the majority of load cases, and may provide improved predictions of a FOWT in extreme conditions over an ALM that may underestimate aerodynamic loading on the tower. However, an ALM may provide improved predictions for a yawed turbine. The use of a high fidelity coupled CFD-FEA approach has potential to be a useful tool for analysing the detailed response of highly flexible blades where low fidelity methods are less reliable, though further work is needed to validate the modelling

Study on Actuator Line Modeling of Two NREL 5-MW Wind Turbine Wakes

The wind turbine wakes impact the efficiency and lifespan of the wind farm. Therefore, to improve the wind plant performance, research on wind plant control is essential. The actuator line model (ALM) is proposed to simulate the wind turbine efficiently. This research investigates the National Renewable Energy Laboratory 5 Million Watts (NREL 5-MW) wind turbine wakes with Open Field Operation and Manipulation (OpenFOAM) using ALM. Firstly, a single NREL 5-MW turbine is simulated. The comparison of the power and thrust with Fatigue, Aerodynamics, Structures, and Turbulence (FAST) shows a good agreement below the rated wind speed. The information relating to wind turbine wakes is given in detail. The top working status is proved at the wind speed of 8 m/s and the downstream distance of more than 5 rotor diameters (5D). Secondly, another case with two NREL 5-MW wind turbines aligned is also carried out, in which 7D is validated as the optimum distance between the two turbines. The result also shows that the upstream wind turbine has an obvious influence on the downstream one

Reliability analysis of 15MW horizontal axis wind turbine rotor blades using fluid-structure interaction simulation and adaptive kriging model

Over the course of the last four decades, the rotor diameter of Horizontal Axis Wind Turbines (HAWTs) has undergone a substantial increase, expanding from 15 m (30 kW) to an impressive 240 m (15MW), primarily aimed at enhancing their power generation capacity. This growth in blade swept area, however, gives rise to heightened loads, stresses and deflections, imposing more rigorous demands on the structural robustness of these components. To prevent sudden failure and to plan effective inspection, maintenance, and repair activities, it is vital to estimate the reliability of the rotor blades by considering all the forces (aerodynamic and structural dynamics) acting on them over the turbine’s lifespan. This research proposes a comprehensive methodology that seamlessly combines fluid-structure interaction (FSI) simulation, Kriging model/algorithm and Adaptive Kriging Monte Carlo Simulation (AKMCS) to assess the reliability of the HAWT rotor blades. Firstly, high-fidelity FSI simulations are performed to investigate the dynamic response of the rotor blade under varying wind conditions. Recognizing the computationally intensive nature and time-consuming aspects of FSI simulations, a judicious approach involves harnessing an economical Kriging model as a surrogate. This surrogate model adeptly predicts blade deflection along its length, utilizing training and testing data derived from FSI simulations. Impressively, the Kriging model predicts blade deflection 400 times faster than the FSI simulations, showcasing its enhanced efficiency. The optimized surrogate model is then used to estimate the flap wise blade tip deflection for one million wind speed samples generated using Weibull distribution. Thereafter, to evaluate the reliability of the blades, statistical modeling using methods such as Monte Carlo Simulation (MCS), AKMCS is performed. The results demonstrate the faster convergence of AKMCS requiring only 21 samples, as opposed to 1 million samples for MCS with minimal reduction in the precision of the estimated probability of failure (Pf) and reliability index (β). Demonstrated on the backdrop of an IEA-15MW offshore reference WT rotor blade, the proposed methodology underscores its potential to be seamlessly incorporated into the creation of WT digital twins, due to its near real-time predictive capabilities for Pf and β assessments.Reliability analysis of 15MW horizontal axis wind turbine rotor blades using fluid-structure interaction simulation and adaptive kriging modelacceptedVersio

Vortex-lattice-based nonlinear aeroservoelastic modelling and analysis of large floating wind turbines

Wind turbine blades have significantly increased in length over the last few decades and are being operated in increasingly complex inflows such as the wake of other wind turbines or on floating platforms. This is increasing the unsteady and three-dimensional aerodynamic effects and the nonlinear structural dynamics that are neglected by the industry-standard Blade-Element Momentum and linear structural theories, respectively. In this dissertation, we employ Unsteady Vortex-Lattice Method for the aerodynamics and nonlinear Geometrically-Exact Beam Theory for the structural dynamics computations to describe these phenomena and their role on wind turbine aeroelasticity. We show that, Unsteady Vortex-Lattice Method fails to provide accurate drag estimation that we overcome with a semiempirical correction to include drag from steady-state tabulated data. We also show that, in cases of yaw, Blade-Element Momentum theory predicts accurate root-bending moments and rotor coefficients up to about ten degrees of yaw, for larger yaw angles, it over estimates the loads decay with the yaw angle. Furthermore, the interaction between radial sections of the blade under turbulent inflow is significant but not accounted for by Blade-Element Momentum theory so we study this phenomenon with Unsteady Vortex-Lattice Method and propose a correction to include the interaction between blade sections in Blade-Element Momentum theory that improves the prediction of loads along the span. We also reduce the computational cost of the Unsteady Vortex-Lattice Method by proposing a new wake discretisation scheme of the wake convection equation. We study the change in aerodynamic surface orientation in long flexible blades and conclude that capturing the twist degree of freedom is important for loads and power estimation. Moreover, we describe the influence of the platform pitch and roll motions in the unsteady character of the aerodynamic loads. Finally, we redesign the controller of the blade pitch to account for nonlinear structural dynamics and unsteady aerodynamics showing a reduction in the fluctuations of the main platform motions and energy production around the equilibrium position.Open Acces

[Report of] Specialist Committee V.4: ocean, wind and wave energy utilization

The committee's mandate was :Concern for structural design of ocean energy utilization devices, such as offshore wind turbines, support structures and fixed or floating wave and tidal energy converters. Attention shall be given to the interaction between the load and the structural response and shall include due consideration of the stochastic nature of the waves, current and wind

State of the art in the aeroelasticity of wind turbine blades: Aeroelastic modelling

With the continuous increasing size and flexibility of large wind turbine blades, aeroelasticity has been becoming a significant subject for wind turbine blade design. There have been some examples of commercially developed wind turbines experiencing aeroelastic instability problems in the last decade, which spokes for the necessity of aeroelastic modelling of wind turbine blades. This paper presents the state-of-the-art aeroelastic modelling of wind turbine blades, provides a comprehensive review on the available models for aerodynamic, structural and cross-sectional analysis, discusses the advantages and disadvantages of these models, and outlines the current implementations in this field. This paper is written for both researchers new to this research field by summarising underlying theory whilst presenting a comprehensive review on the latest studies, and experts in this research field by providing a comprehensive list of relevant references in which the details of modelling approaches can be obtained