Loads Control Aerodynamic in Offshore Wind Turbines

Due to the increase of rotor size in horizontal axis wind turbine (HAWT) during the past 25 years in order to achieve higher power output, all wind turbine components and blades in particular, have to withstand higher structural loads. This upscaling problem could be solved by applying technologies capable of reducing aerodynamic loads the rotor has to withstand, either with passive or active control solutions. These control devices and techniques can reduce the fatigue load upon the blades up to 40% and therefore less maintenance is needed, resulting in an important money savings for the wind farm manager. This project consists in a study of load control techniques for offshore wind turbines from an aerodynamic and aeroelastic point of view, with the aim to assess a cost effective, robust and reliable solution which could operate maintenance free in quite hostile environments. The first part of this study involves 2D and 3D aerodynamic and aeroelastic simulations to validate the computational model with experimental data and to analyze the interaction between the fluid and the structure. The second part of this study is an assessment of the unsteady aerodynamic loads produced by a wind gust over the blades and to verify how a trailing edge flap would influence the aerodynamic control parameters for the selected wind turbine blade

Considerations for the Design Optimization of Floating Offshore Wind Turbine Blades

Floating offshore wind turbines are an immature technology with relatively high costs and risk associated with deployment. Of the few floating wind turbine prototypes and demonstration projects deployed in real metocean conditions, all have used standard turbines design for onshore or offshore fixed bottom conditions. This neglects the unique unsteady aerodynamics brought on by floating support structure motion. While the floating platform has been designed and optimized for a given rotor, the global system is suboptimal due to the rotor operating in conditions outside of which it was design for. If the potential offered by floating wind turbines is to be realized, offering access to deep water near-shore, costs need to continue to be reduced. This dissertation is the first known design study that considers the optimization of wind turbine rotors specifically for floating conditions. Two design optimization methodologies are presented using different analysis fidelity levels. A relatively computationally efficient, state-state blade element moment optimization of floating wind turbine blades is presented that will be useful for future systems level optimization studies. A higher fidelity methodology is then presented, using time-domain aeroelastic simulations to fully capture the unsteady aerodynamics and dynamic couplings between the rotor and platform motion throughout the optimization process. The principal finding of these studies is that low induction rotors are a promising technology pathway for future FOWT systems, reducing the severity of cyclical loading due to platform motion

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

MODEL UPDATING AND STRUCTURAL HEALTH MONITORING OF HORIZONTAL AXIS WIND TURBINES VIA ADVANCED SPINNING FINITE ELEMENTS AND STOCHASTIC SUBSPACE IDENTIFICATION METHODS

Wind energy has been one of the most growing sectors of the nation’s renewable energy portfolio for the past decade, and the same tendency is being projected for the upcoming years given the aggressive governmental policies for the reduction of fossil fuel dependency. Great technological expectation and outstanding commercial penetration has shown the so called Horizontal Axis Wind Turbines (HAWT) technologies. Given its great acceptance, size evolution of wind turbines over time has increased exponentially. However, safety and economical concerns have emerged as a result of the newly design tendencies for massive scale wind turbine structures presenting high slenderness ratios and complex shapes, typically located in remote areas (e.g. offshore wind farms). In this regard, safety operation requires not only having first-hand information regarding actual structural dynamic conditions under aerodynamic action, but also a deep understanding of the environmental factors in which these multibody rotating structures operate. Given the cyclo-stochastic patterns of the wind loading exerting pressure on a HAWT, a probabilistic framework is appropriate to characterize the risk of failure in terms of resistance and serviceability conditions, at any given time. Furthermore, sources of uncertainty such as material imperfections, buffeting and flutter, aeroelastic damping, gyroscopic effects, turbulence, among others, have pleaded for the use of a more sophisticated mathematical framework that could properly handle all these sources of indetermination. The attainable modeling complexity that arises as a result of these characterizations demands a data-driven experimental validation methodology to calibrate and corroborate the model. For this aim, System Identification (SI) techniques offer a spectrum of well-established numerical methods appropriated for stationary, deterministic, and data-driven numerical schemes, capable of predicting actual dynamic states (eigenrealizations) of traditional time-invariant dynamic systems. As a consequence, it is proposed a modified data-driven SI metric based on the so called Subspace Realization Theory, now adapted for stochastic non-stationary and timevarying systems, as is the case of HAWT’s complex aerodynamics. Simultaneously, this investigation explores the characterization of the turbine loading and response envelopes for critical failure modes of the structural components the wind turbine is made of. In the long run, both aerodynamic framework (theoretical model) and system identification (experimental model) will be merged in a numerical engine formulated as a search algorithm for model updating, also known as Adaptive Simulated Annealing (ASA) process. This iterative engine is based on a set of function minimizations computed by a metric called Modal Assurance Criterion (MAC). In summary, the Thesis is composed of four major parts: (1) development of an analytical aerodynamic framework that predicts interacted wind-structure stochastic loads on wind turbine components; (2) development of a novel tapered-swept-corved Spinning Finite Element (SFE) that includes dampedgyroscopic effects and axial-flexural-torsional coupling; (3) a novel data-driven structural health monitoring (SHM) algorithm via stochastic subspace identification methods; and (4) a numerical search (optimization) engine based on ASA and MAC capable of updating the SFE aerodynamic model

CFD Simulation of a Floating Wind Turbine in OpenFOAM: an FSI approach based on the actuator line and relaxation zone methods

Floating offshore wind turbines (FOWTs) have the potential to harness wind resources in deepwater, which is so far prohibitive for conventional approaches. This, however, comes at a cost: the platform’s extra degrees of freedom (DoFs) introduce complex aerodynamic and hydrodynamic behaviours. Therefore, FOWTs must be accurately modeled to reduce load uncertainties that ultimately prejudice their economic viability. This project implements a framework for the coupled, high-fidelity simulation of FOWTs in OpenFOAM. The tool is built upon two existing libraries: turbinesFoam [1] —for rotor modeling based on the actuator line method— and waves2Foam [2] —for wave-field generation and absorption based on the relaxation zone method. The multi-phase simulation uses the interFoam solver in combination with a morphing mesh technique and rigid-body model to represent the platform. The mooring restraints are computed with a quasi-steady, catenary model from waves2Foam. The turbinesFoam library, targeted at bottomfixed turbines, is modified so that it can accommodate arbitrary motions along the rigid-body DoFs. The platform-turbine FSI coupling follows a serial sub-iterating strategy based on the PIMPLE scheme. The simulation framework is built in a sequential style. First, the propagation of second-order waves in an empty tank is studied, followed by the decay oscillation of floating buoys from the experiments by Ito and Palm et al. [3, 4]. Then, the modified version of turbinesFoam is tested for the conditions from the OC6 Phase III campaign —a series of wind-tunnel tests carried out at Politecnico di Milano that analyzed the performance of a scaled 10-MW turbine under prescribed motions in pitch and surge [5]. Lastly, the coupled simulation of a 2-DoF (surge and pitch) semi-submersible FOWT under combined wind-wave conditions is achieved. The presented framework proved capable of modeling the aerodynamic performance of turbines under prescribed motion and produced plausible results for a semi-submersible FOWT under combined wind and wave conditions. Once carefully validated, this tool will have the potential to serve as a reliable technique for the advanced modeling of FOWTs

Experimenal study of the aerodynamics of a horizontal axis wind turbine

One of the challenges of the wind energy community today is to improve the existing background on the aerodynamic phenomena of a Horizontal Axis Wind Turbine (HAWT), the prediction of the wind speed distribution on the rotor plane, and the estimation of the design loads. This dissertation aims at contributing to the fulfillment of these objectives. In this way, this study assessed the feasibility of measuring the loads exerted on a HAWT blade by means of Stereoscopic Particle Image Velocimetry (SPIV), which is a non intrusive technique that provides with the whole 3D velocity field in a plane. Thus, with this PIV-Loads method, the velocity and pressure fields, as well as the resultant aerodynamic forces around a section of the blade, would be available simultaneously, without the need of modifying the model or disturbing the flow. In order to achieve this goal progressively, the PIV-Loads method, based in a Momentum Equation contour-based approach, was firstly validated using DNS data, both for a laminar unsteady flow case, as for a velocity averaged turbulent flow. Secondly, the method was tested in the wind tunnel with a bidimensional problem, measuring forces in a stationary flat plate, for different angles of attack (with laminar and turbulent flow conditions). The force estimation results were compared with those provided by a high sensitive balance. Finally, the PIV-Loads method was applied to a HAWT model working both in axial and yaw flow conditions, measuring forces on a rotating blade for steady and unsteady cases. Final load calculations were compared with those resulting from a numerical simulation based in the Panel method approach. Bringing the project to completion, the near vortex wake of a HAWT was characterized by means of Time Resolved PIV. Regarding the PIV-Loads methodology, load predictions are more reliable if the integration path does not cross a shear layer or a boundary layer. In addition, it is neither recommended to neglect the third velocity component when measuring forces on a rotating HAWT blade, nor to eliminate the velocity fluctuation terms when dealing with turbulent flows. All implemented codes and experimental results were validated or compared with numerical or experimental alternative data showing good consistency. The conclusion is that the PIV-Loads method provides with precise results if the available velocity data is sufficiently accurate. However, any PIV errors such as lack of resolution, velocity gradients inside the interrogation window or laser reflections, may lead to uncertainties in the load measurements. Any future improvement in this sense will certainly lead to better results

Power Electronics Applications in Renewable Energy Systems

The renewable generation system is currently experiencing rapid growth in various power grids. The stability and dynamic response issues of power grids are receiving attention due to the increase in power electronics-based renewable energy. The main focus of this Special Issue is to provide solutions for power system planning and operation. Power electronics-based devices can offer new ancillary services to several industrial sectors. In order to fully include the capability of power conversion systems in the network integration of renewable generators, several studies should be carried out, including detailed studies of switching circuits, and comprehensive operating strategies for numerous devices, consisting of large-scale renewable generation clusters

Modeling and simulation of hydrokinetic composite turbine system

The utilization of kinetic energy from the river is promising as an attractive alternative to other available renewable energy resources. Hydrokinetic turbine systems are advantageous over traditional dam based hydropower systems due to zero-head and mobility. The objective of this study is to design and analyze hydrokinetic composite turbine system in operation. Fatigue study and structural optimization of composite turbine blades were conducted. System level performance of the composite hydrokinetic turbine was evaluated. A fully-coupled blade element momentum-finite element method algorithm has been developed to compute the stress response of the turbine blade subjected to hydrodynamic and buoyancy loadings during operation. Loadings on the blade were validated with commercial software simulation results. Reliability-based fatigue life of the designed composite blade was investigated. A particle swarm based structural optimization model was developed to optimize the weight and structural performance of laminated composite hydrokinetic turbine blades. The online iterative optimization process couples the three-dimensional comprehensive finite element model of the blade with real-time particle swarm optimization (PSO). The composite blade after optimization possesses much less weight and better load-carrying capability. Finally, the model developed has been extended to design and evaluate the performance of a three-blade horizontal axis hydrokinetic composite turbine system. Flow behavior around the blade and power/power efficiency of the system was characterized by simulation. Laboratory water tunnel testing was performed and simulation results were validated by experimental findings. The work performed provides a valuable procedure for the design and analysis of hydrokinetic composite turbine systems --Abstract, page iv

Development of horizontal axis hydrokinetic turbine using experimental and numerical approaches

“Hydrokinetic energy conversion systems (HECSs) are emerging as viable solutions for harnessing the kinetic energy in river streams and tidal currents due to their low operating head and flexible mobility. This study is focused on the experimental and numerical aspects of developing an axial HECS for applications with low head ranges and limited operational space. In Part I, blade element momentum (BEM) and neural network (NN) models were developed and coupled to overcome the BEM’s inherent convergence issues which hinder the blade design process. The NNs were also used as a multivariate interpolation tool to estimate the blade hydrodynamic characteristics required by the BEM model. The BEM-NN model was able to operate without convergence problems and provide accurate results even at high tip speed ratios. In Part II, an experimental setup was developed and tested in a water tunnel. The effects of flow velocity, pitch angle, number of blades, number of rotors, and duct reducer were investigated. The performance was improved as rotors were added to the system. However, as rotors added, their contribution was less. Significant performance improvement was observed after incorporating a duct reducer. In Part III, a computational fluid dynamics (CFD) simulation was conducted to derive the optimum design criteria for the multi-turbine system. Solidity, blockage, and their interactive effects were studied. The system configuration was altered, then its performance and flow characteristics were investigated. The experimental setup was upgraded to allow for blockage correction. Particle image velocimetry was used to investigate the wake velocity profiles and validate the CFD model. The flow characteristics and their effects on the turbines performance were analyzed”--Abstract, page iv