1,720,963 research outputs found
On the Turbulent Mixing in Horizontal Axis Wind Turbine Wakes
No full textThe wake flow of a horizontal axis wind turbine is characterised by lower wind speed and higher turbulence than the free-stream conditions. When clustered in large wind farms, wind turbines regularly operate inside the wake of one or more upstream machines. This is a major cause of energy production loss and a source of higher fatigue loads on the rotor’s blades. In order to minimise the wake effects, a smart optimisation of the wind-turbine layout is essential and reliable method for modelling the wake behaviour is fundamental. The scientific community has broadly recognised the high level of uncertainty, which still affects the state-of-the-art numerical wake models and, in turn, leads to miscalculation of the wake effect. In order to develop more advanced models it is valuable to follow a back-to-basic approach and to investigate the physics of the transition from near-wake flow to far-wake flow. The near wake is characterised by the presence of organised structures as the tip- and root-vortex helices, which are trailed at the two extremities of each blade. In the far wake, the influence of the blade flow is no longer visible: this is the region where most of the turbulence mixing happens and the wake undergoes a re-energising process. Given the different physics governing the two regions, including in a single model a set of assumptions able to encompass both flow characteristics and to account for the influence of the near-wake features on the far-wake development is still problematic. This research explores two aspects of the wake problem, adopting an experimental, numerical and theoretical approach. In the first place, the physics of the transition from near to far wake is explored. In particular, the main aim is to study how the near-wake turbulent flow structures affect the re-energising process of the far wake, by understanding the relationship between the near-wake vortex system and the resulting turbulence structures in the wake. In second instance, the actuator disc approach, which is at the basis of most rotor as well as wake models, is studied for shedding more light onto its limitations and potentials. Stereo particle image velocimetry (SPIV) is adopted for mapping the three-component velocity field in a meridian plane encompassing a large portion of the near, transition and far wake of a two-blade wind turbine model. Measurements are carried out in the presence of an artificially-triggered tip-vortex pairing instability, the so-called leapfrogging instability, which determines the tip-vortex breakdown and the onset of a more efficient wake mixing. The analysis of the data revealed a major influence of the vortex instability on both the time-average velocity field and on the turbulence field. In particular, it was shown that the wake begins its re-energising process after the tip vortices have completed a 90 degree rotation around each other during the pairing mechanism. A second step in this analysis is the application of the triple decomposition of the flow in the shear layer at the border of the wake. With this approach, the role of the periodic and random flow motions in the turbulent mixing and wake re-energising process can be studied separately. Two components of the mean-flow kinetic-energy transport are quantified for one single phase of the rotor rotation: the mean-flow kinetic-energy flux and the turbulence production. The analysis shows that only the random flow fluctuations are yielding considerable entrainment of kinetic energy, while the near-wake vortex structure seem to act as a shield preventing the wake mixing. The study continues with the analysis of the wake of the wind turbine model compared with the one of an actuator disc. The latter is reproduced experimentally by means of a porous disc manufactured with metal mesh, having the same diameter and drag coefficient of the turbine model. Differences between the two wakes (velocity deficit, turbulence levels, mean-flow kinetic-energy transport, etc.) are quantified with SPIV measurements. The study shows that the actuator disc is in fact able to reproduce the time-average velocity field also in the very near wake with good accuracy, contrary to what is found in previous literature. Proper orthogonal decomposition (POD) of the flow field is adopted as an alternative method for separating periodic and random flow motions without the need of acquiring phase-locked measurements. This also allows estimating the mean-flow kinetic-energy flux and the turbulence production in the time-average field, rather than in one single rotation phase. The analysis confirms that major contribution to the momentum entrainment in the wind turbine wake is provided by the random flow fluctuations, while the periodic fluctuations have a zero or even negative contribution. In the actuator disc wake the kinetic energy transport is only positive and of a larger magnitude compared to the one in the wind turbine wake. The analysis of the turbulence production shows a distinct region characterised by large negative values in correspondence of the tip-vortex instability. This phenomenon constitutes a clear example of the failure of the gradient transport model in the time-mean field, which normally does not account for the possibility of reverse energy transfer from coherent structures to the mean flow. Five state-of-the-art computational fluid dynamics (CFD) codes are validated against the experimental data in a benchmark workshop organised among several academic and industrial organisations. Four large eddy simulation (LES) codes and one vortex models are used for reproducing the near wake of the porous disk. The comparison shows that, despite the lack of viscosity and turbulence models, the vortex model is capable of reproducing the wake expansion and the centreline velocity with very high accuracy. Also all tested LES models are able to predict the velocity deficit in the very near wake well, contrary to what was expected from previous literature. However the resolved velocity fluctuations in the LES are below the experimentally measured values.AWEPAerospace Engineerin
3D flows near a HAWT rotor: A dissection of blade and wake contributions
No full textInvestigating the flow physics in the vicinity of the wind turbine blade is a challenging endeavour. In the past, focus was placed on the understanding of near wake flows arising from wake vorticity and the rotor loading. In this work, a different approach is taken by considering the flow field in the blade vicinity as a consequence of the separate effects of bound and wake vorticity. This enables new insight regarding the role of the blade as having a direct influence on the three-dimensional flow. The approach is applied for the reference axial flow condition and hence for the yawed flow condition where the issue of flow three-dimensionality takes a new level of complexity. Three research hypotheses are investigated in this work: 1. Radial flow components especially close to the wind turbine blade are not negligible. This contradicts the classical momentum approach which treats the flow as two-dimensional. The situation for yawed flow is even more important since wake vorticity not only exhibits an expansion but also a skewness. A fundamental understanding of the behaviour of the radial flow component is hence of paramount importance. 2. The three-dimensional flow field close to a Horizontal Axis Wind Turbine (HAWT) rotor is due to the effects of body and wake vorticity. The blade tip shape plays a fundamental role on the behaviour of the flow field near the blade. 3. The tip vorticity for axial and yawed flow results in a different tip flow behaviour. The hypotheses are linked by a common goal; to establish new insight into three-dimensional flows in the proximity of the rotor in yawed flow, using axial flow as a baseline investigation. Both numerical and experimental approaches have been used to investigate these hypotheses. A 3D unsteady potential flow panel model is used for the numerical computations. The model permits to decompose flow due to diff erent vorticity components. Stereo Particle Image Velocimetry (SPIV) is used for the experimental measurements. This enables measurement of all velocity components in a 2D plane and can then be used to construct a 3D volume of data. Flow data from three different rotors is used: SPIV measurements from the Model Experiments in Controlled Conditions (MEXICO) rotor in the German-Dutch DNW wind tunnel and experiments performed in the Open Jet Facility of TU Delft on two different 2m diameter rotors. The thesis is structured into six parts as follows: Part I - Literature review to support and contextualize the research Part II - Analysis of the hypotheses on ow three-dimensionality Part III - Decomposition of velocities in the rotor proximity Part IV - Origins and dynamics of vorticity Part V - Conclusions Part VI - Appendices The results presented in this thesis challenge the current understanding of flow three-dimensionality in the rotor plane particularly for the yawed flow case. The blade's role as a vorticity generator as well as its active role in disturbing the flow due to its vorticity distribution are both supported. The creation of a HAWT tip vortex over the blade thickness is studied leading to important implications about the induced flow field at the tip. The details of flow three-dimensionality due to the complex behaviour of the tip vortex upon release are presented and the implications of this discussed. The outcome of this research bridges the gap between existing knowledge of the flow on the rotor scale to future lines of research which will be directed to the study of boundary layer flows of rotating blades. By extensively analyzing the rotor blade scale outer flow (outside of the boundary layer) this research gives impetus to a necessary revision of tip corrections in the application to the industry standard BEM design codes which to this day rely on models which are not based on the detailed knowledge of rotor blade flow which this research provides.DUWINDAerospace Engineerin
Cost-sensitivity Analyses for Gearbox Condition Monitoring Systems Offshore
No full textTo compete more successfully with other sources of energy, a decrease in the costs of offshore wind energy needs to be achieved. Operation and maintenance costs represent a large share of these costs. In order to reduce these costs new developments and strategies are considered for operation and maintenance of wind turbine components. Condition monitoring systems (CMSs) could be a vital tool to decrease these costs, especially for expensive components such as a gearbox. This thesis focusses on the gearbox as it is the component, with one of the highest downtime per failure and failure costs. A literature study reveals that the replacement of a gearbox offshore might lead to months of downtime and costs might sum up to one million euro for a 6 MW wind turbine. In order to prevent such high costs, CMSs are assessed in this thesis. CMSs comprise of sensors providing data, which reflects the health status of the component. This data is subsequently analysed by a data-mining technique, capable of detecting trends and anomalies to predict upcoming failures. If the system is sufficiently accurate, a large failure can be prevented, which leads to significant savings in the lifetime gearbox maintenance costs A cost-benefit study is performed to determine the required performance of a CMS in order to be profitably implemented in an offshore wind turbine. The CMS performance is described by two parameters: one reflecting the ability of the system to prevent large gearbox failures, and a second parameter describing its ability to prevent waiting downtime, caused by weather window waiting time, spare part logistics and vessel mobilisation. Based on Monte Carlo simulations, the gearbox maintenance costs are quantified over the wind turbine lifetime. Subsequently a sensitivity analysis is performed. Results show that differences in gearbox failure rate and wind farm distance significantly affect the maintenance costs. Hence to break-even different performance requirements of the CMS are necessary. The model reveals that a system, capable of preventing large failures and/or preventing waiting downtime, can reduce the lifetime maintenance costs to a great extent. The actual performance of a CMS, needed to break-even, can be considered low, as the condition monitoring costs are in no proportion to the total gearbox maintenance costs and thus the potential revenue of the CMS.Aerospace EngineeringSustainable Energy Technolog
The conceptual design of a safety system: For the 5MW Deepwind wind turbine
No full textThis research work proposes the initial design considerations of the safety system of the Deepwind offshore floating vertical axis wind turbine. Deepwind is a wind turbine model and prototype development project under the umbrella of the 7th European Framework Programme. The safety system is one of the aspects of this project and it is dealt within this Thesis work. Safety is one of the most important features that modern wind turbines should include. Statistics and industrial experience have indicated to the regulation organizations (e.g. IEC,DNV) to add safety systems in the complex electromechanical system of a wind turbine. The most crucial safety feature is the over-speeding control. Usually the controller functions alleviate this problem but an additional safety level is more than necessary to avoid irreparable incidents. This safety feature has been developed within the scope of this Thesis. Therefore, the major task is the definition and design of the safety system functions. This task was conducted in the context of the Conceptual Design method. Several possibilities were investigated. This search led to systems using the aerodynamic and hydrodynamic principles of operation. Many aspects were taken into account concerning the functionality and compatibility of these safety systems. These aspects were addressed from literature review and generation of engineering models in MATLAB. Finally, through multi-criteria analysis, which is one of the tools of Conceptual Design, all the systems were compared and a solution was formulated with the initial design configurations for further development. The proposal of this Master Thesis is to sink the wind turbine system inside the sea, by adding seawater into the spar buoy in an effective time response. Consequently, the blades of the wind turbine hit the seawater and thus create enough drag forces to reduce effectively the rotational speed. The particular wind turbine characteristics make this solution the most promising, as presented through the whole process. This report proposes the initial design characteristics but also suggests further steps on the design process of the safety system.Aerospace EngineeringSustainable Energy Technolog
Scenarios for offshore wind development in the Netherlands: An agent-based modelling approach
No full textThe aim of this study is to develop a method to identify the barriers to and opportunities in the development of large-scale offshore wind energy in the Netherlands, taking into account the uncertainties of the future and consequences of decisions, from technological, economical, social, political and environmental perspectives. The research question is stated as: can an agent-based model be used to develop realistic implementation paths towards 6000 MW installed offshore wind power in the Dutch EEZ that show the consequences for the stakeholders? The focus topics for the model are the permit procedures, financial support, layout and timing of an offshore grid, the availability of resources, and innovation, especially of wind turbines. The results show that the agent-based model can indeed simulate different implementation paths that can be used for policy and decision support as a communication tool to show different possible futures and the limiting factors for the implementation in these futures. The methodology given in this study provides a step plan to develop such an agent-based model in analysis, design, implementation and validation phases. The main disadvantages of using agent-based modelling are: the extensive (detailed) data gathering, a long development time dependent on the implementation process and available standards, the required 'mass' and development time before simulations can be made that can be validated, and the limitations in modelling complex actor behaviour. The main advantages of using agent-based modelling are: the model can combine technological and socio-institutional aspects, the model can combine qualitative and quantitative data, the agent-based `as-is' modelling makes design easier, the model is easily extendable and a computer model is transparent.Wind EnergyAerospace Engineerin
Designing a Hybrid Energy System for Arzanah Island Abu Dhabi: Using Synthetic Wind and Solar Resource Data
No full textAerospace EngineeringSustainable Energ
Horizontal Axis Wind Turbine (HAWT) wake stability investigations: Insights Through a Vortex-Ring Modelling Approach
No full textAs wind energy technology continues to take the helm of renewable energy deployed throughout the world, and wind farms become a more common sight on the horizon, increased emphasis is placed on wind farm aerodynamics. Obtaining insight into this complex flow problem requires a deeper understanding of the nature of individual wind turbine wakes, and subsequently wake interactions. The topic of wake stability and wake meandering, has received particular attention in recent years. Recent work by Larsen et al. [1] investigated this wake meandering phenomenon based on the hypothesis that the wake behaves as a passive tracer, governed by large-scale lateral and horizontal turbulent components. In contrast, Medici and Alfredsson [2] propose that a meandering mechanism similar to bluff body vortex shedding is responsible for the wake oscillations of their two-bladed model. Thus conflicting views with respect to the triggering mechanisms of wake instability exist and are addressed in this thesis. The approach taken was to use a simple inviscid vortex ring (VR) modelling method to represent the developing rotor wake. This allows for a straight forward investigation and comparison of the impact of uniform, yawed and sheared flow conditions on the development of the wake, with the additional possibility of including ground effect. The phenomenon of vortex filament interaction or leapfrogging, could play a role in the observation of unsteady phenomena and is therefore also addressed. Such a study is hence performed in light of recent conflicting views on the causes of wake meandering. The main conclusion from this study is that the presence of the ground and external perturbations, most notably changes in the wake pitch and the rotor thrust coefficient, can significantly affect the steady development of the wake. The phenomenon of vortex filament leapfrogging, whilst displaying interesting periodic behaviour, does not correlate with periodic wake behaviour reported in Medici et al. [2]. However, in the absence of unsteady inflow, it is shown that the wake of a Horizontal Axis Wind Turbine (HAWT) is certainly prone to displaying unstable, dynamic behaviour caused by these additional factors.Aerospace EngineeringAerodynamics, Wind Energy & Propulsio
Engineering models in wind energy aerodynamics: Development, implementation and analysis using dedicated aerodynamic measurements
No full textThe subject of aerodynamics is of major importance for the successful deployment of wind energy. As a matter of fact there are two aerodynamic areas in the wind energy technology: Rotor aerodynamics and wind farm aerodynamics. The first subject considers the flow around the rotor and the second subject considers the (wake) flow within a wind farm. For both areas calculational models have been developed which are implemented i rotor design and wind farm design codes respectively. Accurate rotor design codes enable a reliable design of wind turbines and an optimization towards a higher energy production and lower loads, i.e. towards a lower cost of energy. They are also required to avoid design errors and hence to reduce investment risks of wind turbine manufacturers. Accurate wind farm design codes are needed to predict the production losses and the load increase on turbines in a farm due to wake effects. They also support the optimization of wind farms (e.g. through farm control) by which the energy losses and the load increase from wake effects (and consequently the costs/kWh) are minimized. For both areas the complexity of models range from engineering methods to very advanced Computational Fluid Dynamics (CFD) methods. The term engineering method is meant to indicate a model which casts a complicated flow phenomenon into a transparent form. This generally goes together with an economic computer usage. In this respect it is very important to realize that wind energy design calculations are inherently very time consuming by which advanced CFD models are still beyond the routine possibilities of industry. As such engineering methods form the only alternative for that purpose. The main aim of the present thesis is then to describe several developments of the last 25 years which have led to the present generation of aerodynamic engineering models. It will be shown that much progress has been made both on the field of rotor aerodynamics as well as on the field of wind farm aerodynamics and that this progress was highly supported by the fact that dedicated aerodynamic measurement data have become available. The progress is illustrated by the engineering models which are developed and validated by ECN in several large (inter)national cooperation projects in which these measurements played an important role. The author of this thesis was heavily involved in these projects and often acted as coordinator. Since these projects were performed in close cooperation with other institutes (which used different types of models), the activities of the author can be placed in a wider context. The first part of the thesis is devoted to rotor aerodynamics. Basically the subject of rotor aerodynamics can be subdivided in two parts: The first part deals with the global flow field around a wind turbine. This type of modelling is called induction aerodynamics, since its main goal is to determine the induced velocities at the blade. The second part deals with the loads on a wind turbine blade as a response to this flow situation and is called blade aerodynamics. Current engineering models for rotor aerodynamics topic are built around the Blade Element Momentum (BEM) theory. The Blade Element Momentum theory in itself is very basic, e.g. it is derived for 2-dimensional, stationary, homogenous and non-yawed conditions. For this reason several engineering models have been developed which overcome these simplifications and which act as add-on's to the basic BEM theory. These engineering add-on's have been developed for the field of blade aerodynamics and for the field of induction aerodynamics. In this thesis a comparison is made between current engineering models and the engineering models from 25 years ago. The engineering methods from 25 years ago were not much more than the very basic BEM theory with a Prandtl tip loss correction and a turbulent wake correction. Moreover a tower shadow model based on a dipole model and a 'geometric' correction for cone and tilt angle were included, while yaw was modelled with the advancing and retreating blade effect only. Since then the models for airfoil aerodynamics have been improved by adding unsteady and three-dimensional effects. These unsteady effects can be divided in viscous dynamic stall effects and non-viscous effects at low angles of attack. The three-dimensional effects occur at the inner part of the blade where stall is delayed and at the outer part where the tip decreases the loads. In terms of induction aerodynamics, models have been added for dynamic inflow, the azimuthal variation of the induced velocity at yaw and a model for root losses. The progress in the rotor aerodynamic engineering models from ECN is mainly described along results of four subsequent IEA Tasks: IEA Task 14 and 18, IEA Task 20 and IEA Task 29(Mexnext). An IEA Task (sometimes called an IEA Annex) is a cooperative project carried out under auspices of the International Energy Agency IEA. The goal of IEA Tasks 14 and 18 was to create a database of detailed aerodynamic measurements which all have been taken on turbines under atmospheric conditions. The goal of IEA Task 20 was to analyze the measurements which have been taken by the National Renewable Energy Laboratory NREL on a 10 meter diameter wind turbine which was placed in the very large NASA-Ames wind tunnel. Finally IEA Task 29(Mexnext) analyzed the measurements which have been taken in the EU Project Mexico on a wind turbine rotor with a diameter of 4.5 meters placed in the Large Low Speed Facility (LLF) of the German Dutch Wind Tunnel (DNW). In all of these experimental programs pressure distributions were measured at different locations along the rotor blades. Moreover the Mexico experiment mapped the flow field upstream, in and downstream of the rotor plane. The detailed aerodynamic measurements from the IEA Tasks were found to be very useful in the development, improvement and validation of these engineering models because they made it possible to extract aerodynamic phenomena which are hidden in the very global information from conventional measurement programs. It is concluded that only detailed aerodynamic measurements may be used for validation of aerodynamic design models: A validation on basis of global turbine(blade) loads does not give a decisive answer on the accuracy of aerodynamic models due to the fact that 'compensating errors' may occur. Moreover it will be shown that the measurements revealed several shortcomings in aerodynamic engineering methods which partly could be 'repaired', sometimes with the help of more refined models. Several recommendations are made on rotor aerodynamics. This includes some specific further improvements which are still possible to the current state of engineering models. Amongst other things, models for the annulus averaged induction at yaw, tip loss effects and time constants at dynamic inflow can be improved further. These improvements can be established by calibrating engineering methods to results from more advanced aerodynamic models (e.g. CFD or free vortex wake methods). The background for this recommendation lies in the fact that the validation of these advanced aerodynamic models with the detailed aerodynamic measurements from the IEA Tasks showed a clear added value from such methods on these fields. Moreover it is concluded that three-dimensional and unsteady effects on the drag deserve more attention. However the most important recommendation is related to the observation of an unbalance in the aerodynamic wind energy society: Much effort is spent on the development of aerodynamic models (often of little mutual differences) but the amount of experimental validation material is (too) limited. Therefore it is recommended to intensify the activities on rotor aerodynamic measurements in both the wind tunnel and the field. Special attention should be paid to the measurement of those phenomena which, until now, are still largely concealed (e.g. boundary layer phenomena) or unclear (e.g. the relation between blade loads and underlying flow field which is found puzzling in the Mexico experiment). The present thesis also describes the progress which has been made on the field of wind farm aerodynamics. Opposite to the situation for rotor aerodynamics, where the BEM model can be appointed as the main model, the variety of models for wind farm aerodynamics is much larger. This is partly due to the fact that a wind farm aerodynamic model should cover much more aspects: It should model both the aerodynamic behavior of the rotor (which generates the wake) as well as the turbulent wake downstream of this rotor. The fact that calculational time is such an extreme constraint adds to the diversity: As a consequence CFD modelling of wind farm aerodynamics often only refers to the modelling of the wake and not to the modelling of the rotor. It also makes that wind farm and rotor aerodynamics are sometimes considered to be fully separate subjects. This is seen as an undesired development since the aerodynamics of the wake is largely determined by the aerodynamics of the rotor standing in front of the wake. In this thesis the main characteristics of the wake flow behind a wind turbine are described together with a survey of wind farm aerodynamic models. Most of the attention is focussed on an intermediate between the very basic models and the CFD codes, i.e. the parabolized wake models. These models are relatively economic in computer usage (by which they are still considered to be engineering models) where they model the so-called far wake in a physically accurate way. The disadvantage lies in the fact that they generally need an empirical treatment of the near wake. This again goes together with a very simple modelling of the rotor. The progress in wind farm aerodynamic models is then illustrated with ECN's wind farm design code Farmflow (based on the former Wakefarm wake model) which combines a parabolized k-epsilon turbulence model for the far wake with results from a physical free vortex wake method for the near wake. The measurements on wind farm aerodynamics used in this thesis mainly come from the ECN Wind Turbine Test Site Wieringermeer, EWTW. This research farm consists of five wind turbines in a line set up with a rated power of 2.5 MW and a rotor diameter and hub height of 80 meter. The turbines are extensively instrumented, where a meteorological mast is available to measure the free stream or the wake conditions. A major advantage of these measurements lies in the research environment by which data have been recorded over a very long period of high quality. The EWTW measurements revealed various new wake aerodynamic phenomena and they offered validation material for the improvement and validation of the Farmflow code. The observations on the EWTW farm are compared with those on large off-shore wind farms, the measurements of which were supplied within the EU project Upwind. In the EWTW line set-up the largest power loss due to wake effects (and hence the lowest overall power) appears at the second turbine in the farm. The turbines deeper in the farm have a slightly higher power. This is opposite to the situation in large off-shore wind farms where the power keeps decreasing for turbines deeper into the farm. This can be explained by lateral wake effects and the size of those large (array) wind farms. The power behavior in both the EWTW as well as in the large array wind farms was predicted well with Farmflow. Several conclusions on wind farm aerodynamics are drawn. The most important conclusion is that as for the situation on rotor aerodynamics, much progress has been achieved over the past decades. This is illustrated with the developments from Wakefarm to Farmflow. In the beginning of the 1990's only single wakes were considered. These were modelled with a very simple approach: The wind turbine was represented by an actuator disc with a near wake model based on momentum theory (and later empiricism). The far wake was modelled with a turbulence model tuned for non wind energy applications. Since then the near wake models has been refined and multiple wake effects are taken into account in both axial and lateral direction. Furthermore the turbulence model has been calibrated for wind turbine wake situations. For the development of wind farm engineering models in general it is very important that some CFD models entered the (research) scene in which the rotor is modelled with more advanced methods than the actuator disc approach (e.g. with actuator lines). Such advanced models can now be used for calibration of more simple models. Several subjects for wind farm aerodynamics have been identified which still need more attention. As such it is recommended to intensify research on these fields. This holds amongst other things for the validation and improvement of multiple wake models and near wake models in multiple wake situations. Also the interaction of wind farms with the outer atmosphere deserves more attention. Moreover there is a need to refine the turbulence models for wind farm aerodynamics. Another main question to be answered is the importance of rotor aerodynamics for wake aerodynamics. More specifically it should be determined whether it is justified to model the rotor as an actuator disc. The answer to this question can be found by comparing results from CFD codes, which models both the rotor and the wake in a detailed way, with results from a similar code in which the rotor is replaced by an actuator disc. As for the situation on rotor aerodynamics it is again concluded that progress on the field of wind farm aerodynamics is hampered by a shortage of high quality validation material. For this reason it is recommended to intensify the measurement activities for wind farm aerodynamics. In this thesis minimum requirements for such measurement programs are given. Measurements anyhow need to be done on full scale wind farms, preferably in combination with wind tunnel measurements. The first type of measurements yield representative information but generally lack a sufficient degree of detail for a complete interpretation of the wind farm aerodynamic problem. Furthermore field measurements are difficult to interpret due to the stochastic turbulent environment in the free atmosphere. The second type of measurements can yield very detailed and easy interpretable information but the scale of the model turbines is far too small. An interesting intermediate is then the so-called ECN scaled wind farm. This farm consists of 10 wind turbines with a rotor diameter of 7.6 m and a rated power of 10 kW. The farm is heavily instrumented where the size is sufficiently large to make the results at least to some extent, representative for full scale situations. The combination of full scale measurements, scaled farm measurements and wind tunnel measurements then forms the most complete experimental base for wind farm aerodynamics even though each type of measurements has its own drawbacks.Wind EnergyAerospace Engineerin
Performance optimization of wind turbine rotors for wind farm operation
No full textDue to fierce competition on the electricity market, many wind energy related research projects are currently aimed at cost reduction. In order to establish cost reduction, more and more wind turbines are being clustered in (offshore) wind farms. Major drawback of wind farms however, is the energy production loss due to aerodynamic wake interaction between wind turbines. Optimizing wind turbine rotor designs on wind farm scale instead of focussing on wind turbine level, can increase the energy production of a wind farm. In the current project, optimization problems have been investigated that manipulate wind turbine thrust and power curves. The thrust and power curves were manipulated by Matlab's optimization toolbox, using built-in optimization algorithms. The optimization algorithm aims to reduce wake losses, which results in the maximum annual energy production of a wind farm. ECN's wind farm-wake simulation tool “FarmFlow-fast” was used to calculate the annual energy production of a wind farm, for each evaluated set of optimization parameter values. The current project involved two main objectives. First, the FarmFlow-fast model was validated for optimization purposes. This was done by comparing simulation data with power measurement data from the Horns Rev and Lillgrund wind farms. Secondly, the potential of wind turbine performance curve optimization was investigated. This was done by investigating multiple scenarios, in which the increase in annual energy production of a wind farm was calculated as function of the wind turbine thrust and power curves. It is found that FarmFlow-fast is capable to be used for optimization purposes. The most suitable optimization algorithm, concerning time consumption and objective function value decrement, was the “pattern search” algorithm. Unique optimum performance curves were found, depending on the wind farm layout, accounted wind speed and direction ranges and the parametrization and optimization method. Although the potential increase in annual energy production was rather small, the thrust reduction on wind turbine rotors around rated wind speed might as well be an important outcome of the optimization runs. The thrust reduction can result in cheaper wind turbine designs, which might even be a more promising result in terms of cost of energy reduction than the increase in annual energy production. The results of this project contribute to choosing new design parameters for future wind turbine rotor designs, in order to minimize the costs of energy.Wind EnergyAerodynamics & Wind EnergyAerospace Engineerin
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