Reference Turbine Research Articles

Spinner anemometer: wind speed and Spinner Transfer Function seasonal robustness in an offshore wind farm

This work presents the first, long-term, demonstration case of the use of spinner anemometers for power curve measurements in an offshore wind farm. The stability of the spinner anemometer wind speed and its STF during a 22-month measurement period was quantified, using a nacelle lidar as reference. Power curves and annual energy productions (AEPs) were obtained using this long-term data set, to assess the spinner anemometer wind speed stability. Next, several STFs were obtained at a reference turbine in different periods (four two-months periods) to assess the STF seasonality. Finally, the AEPs of three neighbouring turbines were analysed using the STFs generated at the reference turbine.

Multi-Sectional Lift Actuation for Wind Turbine Load Alleviation

The development of large rotors can be limited by the ability to manage dynamic loads. Active Flow Control (AFC) actuators have been proposed to reduce these dynamic loads. This paper aims to study and quantify the advantages of using multiple AFC actuators compared to using a single AFC actuator for each blade. For this study, AFC actuators are idealized and modeled as instantaneous changes to the local lift coefficient at the corresponding blade sections. We refer to this idealized actuators as sectional lift actuators (SLA). A comparison between a single SLA and two SLAs per blade is made using feedforward and feedback control. Blade loads in above-rated wind conditions are used for the comparative analysis, which is conducted using the DTU-10MW reference turbine model.

Modelling, Parametrization and Observer Design of a 20 MW Reference Wind Turbine for Control Purposes

In the present contribution, the model development of a wind turbine with focus on an advanced control system design is reported. The aim is to produce a model that captures essential dynamics over the simplest cases but maintaining an acceptable level of complexity. The problem of parameter reduction from high-resolution models, as used in the simulation of reference turbines, to models with low number of parameters, as normally used for control purposes, is also studied. The obtained parametric model is validated by developing an observer, which can be used for state-space control as well as effective wind speed estimation. As numerical example, a 20 MW reference wind turbine is used. Very large wind turbines are very flexible and nonlinear, therefore very simple rigid models do not lead to satisfactory results. The proposed observer is adjusted during operation in order to overcome this problem.

Emulating aerodynamic forces and moments for hybrid testing of floating wind turbine models

The use of devices with multiple propellers to simultaneously emulate several aerodynamic loads during hybrid testing of floating wind turbines is the emerging state of the art. In this study a validation methodology and a metric are defined for the standardization of the calibration process for multiple propeller hybrid actuators. A statistical validation between the numerical simulations and experimental results is applied and Power Spectral Density is used to calculate the validation metrics. In this paper, the proposed validation method is applied to a novel design for an actuator, which consists of a custom designed frame with six aerial drone propellers. The actuator is named Multi-Propeller Device (MPD). As a test case for the proposed validation method, the MPD is used in this study to emulate the aerodynamic loads of the NREL 5 MW reference turbine at 1:37 scale. The numerical input is generated with the aero-hydro-elastic solver FAST. The aerodynamic loads and effects investigated are rotor thrust and torque, and gyroscopic moment. The recommended validation metric is the Fraction of Measurements within a user defined Tolerance (FMT), which is 1 for a flawlessly operating device. The MPD performs well at emulating rotor thrust and torque loads, with FMT = 0.97 and 0.98 respectively. However, the MPD underperforms at emulating more complex wind loads, such as gyroscopic moment with FMT = 0.27. The poor results for gyroscopic moment are attributed to the generation of significant amounts of high-frequency vibration when propeller pairs of the MPD are operating intermittently at high rotational speeds.

CFD Simulation of Two Tandem Floating Offshore Wind Turbines in Surge Motion

High-fidelity unsteady Reynolds-averaged Navier-Stokes (URANS) CFD simulation is employed to investigate the variations in the power performance of two tandem in-line floating offshore horizontal axis wind turbines for the scenario in which the upstream rotor is oscillating in surge motion and the downstream rotor is positioned in a distance of 3D (D: turbine diameter) and is stationary. The rotors are the NREL-5MW reference turbine. The platform surge period and wave amplitude are 9 s and 1.02 m, respectively. The results are presented for 100 full surge periods. It is found that the surge motion of the upstream rotor results in: (i) sinusoidal fluctuations in the power and thrust coefficients (CP and C T) of the upstream rotor with a standard deviation (std) of 9.7% and 5.5%, respectively; (ii) such fluctuations in CP and CT are less regular with a std of 4.2% and 2.8% for the downstream rotor, respectively. A low-frequency oscillating mode with a period nearly 10 times the surge period is also observed for the downstream rotor. The mean Cp and Ct of the downstream rotor are 28.9% and 38.5% of the upstream one.

Optimization of Francis Turbines for Variable Speed Operation Using Surrogate Modeling Approach

Abstract Previous studies suggested variable speed operation (VSO) of Francis turbines as a measure to improve the efficiency at off-design operating conditions. This is, however, strongly dependent on the hydraulic design and, for an existing turbine, improvements can be expected only with a proper redesign of the hydraulic surfaces. Therefore, an optimization algorithm is proposed and applied to the runner of a low specific speed Francis turbine, with an optimization strategy specifically constructed to improve the variable speed performance. In the constrained design space of the reference turbine, the geometry of the replacement runner is parametrically defined using 15 parameters. Box–Behnken method was used to populate the design space with 421 unique samples, needed to train fully quadratic response surface models of three characteristic efficiencies defined by the proposed objective function. Computational fluid dynamics (CFD) was used to calculate the responses for each sample. The parametric study showed that the anticipated variation of the shape of the hill chart, needed to improve the variable speed performance of the turbine, is limited within a narrow range. The presented method is general and can be applied to any specific speed in the Francis turbine range, for both synchronous speed and variable speed optimization tasks.

Uncertainty Quantification of the Effects of Blade Damage on the Actual Energy Production of Modern Wind Turbines

Wind turbine blade deterioration issues have come to the attention of researchers and manufacturers due to the relevant impact they can have on the actual annual energy production (AEP). Research has shown how after prolonged exposure to hail, rain, insects or other abrasive particles, the outer surface of wind turbine blades deteriorates. This leads to increased surface roughness and material loss. The trailing edge (TE) of the blade is also often damaged during assembly and transportation according to industry veterans. This study aims at investigating the loss of AEP and efficiency of modern multi-MW wind turbines due to such issues using uncertainty quantification. Such an approach is justified by the stochastic and widely different environmental conditions in which wind turbines are installed. These cause uncertainties regarding the blade’s conditions. To this end, the test case selected for the study is the DTU 10 MW reference wind turbine (RWT), a modern reference turbine with a rated power of 10 MW. Blade damage is modelled through shape modification of the turbine’s airfoils. This is done with a purposely developed numerical tool. Lift and drag coefficients for the damaged airfoils are calculated using computational fluid dynamics. The resulting lift and drag coefficients are used in an aero-servo-elastic model of the wind turbine using NREL’s code OpenFAST. An arbitrary polynomial chaos expansion method is used to estimate the probability distributions of AEP and power output of the model when blade damage is present. Average AEP losses of around 1% are predicted mainly due to leading-edge blade damage. Results show that the proposed method is able to account for the uncertainties and to give more meaningful information with respect to the simulation of a single test case.

A study of grease lubricants under wind turbine pitch bearing conditions

Pitch bearings in wind turbines are affected by reciprocating motion due to pitch control. The combination of oscillating operation, high loads, and mixed lubrication often leads to wear. Grease lubricants in wind turbine pitch bearings should be designed to avoid such wear. Due to different available grease lubricants, the anti-wear properties are investigated under downscaled wind turbine pitch bearing conditions. The downscaling is accomplished by load simulation for a pitch bearing and analysis of the pitch movements for a 7.5 MW reference turbine. The operational conditions of the four point contact ball bearing with 5 m outer diameter are scaled to angular contact ball bearings of the size 7208. The investigations are concluded with contact model experiments. Six Industrial grease lubricants for wind turbine pitch bearings are tested that follow very different compositions. The investigations reveal, that none of the tested grease lubricants was able to reduce wear for all tested conditions. After 250000 cycles certain conditions lead to severe wear. Therefore, the pitch controller should avoid such conditions if possible. Nevertheless, grease lubricants with low base oil viscosities and high bleeding rates are best on average for the tested conditions. Furthermore, the results of the bearing experiments are comparable to the model experiments.

The effects of blade structural model fidelity on wind turbine load analysis and computation time

Abstract. Aero-servo-elastic analyses are required to determine the wind turbine loading for a wide range of load cases as specified in certification standards. The floating reference frame (FRF) formulation can be used to model the structural response of long and flexible wind turbine blades. Increasing the number of bodies in the FRF formulation of the blade increases both the fidelity of the structural model and the size of the problem. However, the turbine load analysis is a coupled aero-servo-elastic analysis, and computation cost not only depends on the size of the structural model, but also depends on the aerodynamic solver and the number of iterations between the solvers. This study presents an investigation of the performance of the different fidelity levels as measured by the computational cost and the turbine response (e.g., blade loads, tip clearance, tower-top accelerations). The analysis is based on aeroelastic simulations for normal operation in turbulent inflow load cases as defined in a design standard. Two 10 MW reference turbines are used. The results show that the turbine response quickly approaches the results of the highest-fidelity model as the number of bodies increases. The increase in computational costs to account for more bodies can almost entirely be compensated for by changing the type of the matrix solver from dense to sparse.

Representation of wind turbine blade responses in power production load cases by linear mode shapes

AbstractThe wind turbine industry is designing large MW size turbines with very long blades, which exhibit large deflections during their operational life. These large deflections decrease the accuracy of linear models such as linear finite element and modal‐based models, in which the structure is represented by linear mode shapes. The aim of this study is to investigate the competence of the mode shapes to represent the large blade responses in normal operation load cases. For this purpose, blade deflections are projected onto the linear modal space, swept by mode shape vectors. The projection shows the contribution of each mode and the projection error. The blade deflections are calculated by a nonlinear aero‐servo‐elastic solver for power production fatigue load cases with normal turbulence. The mode shapes are calculated at the steady‐state deflected blade position computed at different wind speeds. Three reference turbine blades are used in the study to evaluate the effects of various blade design parameters such as length, stiffness, mass, and prebend. The results show that although the linear mode shapes can represent the flapwise and edgewise deflections accurately, axial and torsional deflections cannot be captured with good accuracy. The geometric nonlinear effects are more apparent in the latter directions. The results indicate that the blade deflections occur beyond the linear assumptions.

The actuator line method for wind turbine modelling applied in a variational multiscale framework

The actuator line method is implemented into a residual-based variational multiscale (VMS) fluid modelling framework to simulate airflow around a wind turbine. In the actuator line method, the turbine blades are represented as rotating lines of body force applied to the flow field. The NREL 5MW turbine is simulated first to validate the model for a large scale reference turbine and investigate the effect of model parameters such as the width of the body force projection and the mesh resolution. The wind turbine designed by Norwegian University of Science and Technology and the NREL Phase VI wind turbine are simulated next and results are compared with the corresponding wind tunnel experiments. Velocity deficit and turbulence kinetic energy in the wake region between three and five down-stream diameters show good agreement with experimental data. We conclude with a view towards using this framework in an integrated simulation of a wind farm in complex terrain by simulating a single turbine in an area of complex terrain.

Application of the New IEC International Design Standard for Offshore Wind Turbines to a Reference Site in the Massachusetts Offshore Wind Energy Area

The IEC 61400-3-1 standard for a fixed-bottom offshore wind turbine was applied through simulation of specified Design Load Cases. A metocean data set was assembled based on probability distributions of wind and wave data from a representative site in the Massachusetts Offshore Wind Energy Area. A reference turbine was attached to a monopile substructure created for simulation. FAST simulation outputs from feasible ultimate and fatigue loading design load cases are analyzed and discussed. The requirements of the standard are analyzed for the site-specific certification case. Results show importance of control system failure safety and of mitigation during electrical network loss. Hurricane loading is also discussed.

Large eddy simulations of offshore wind turbine wakes for two floating platform types

The growing prospect for large farms of floating offshore wind turbines requires a better understanding of wake effects for floating turbines. In this work, large eddy simulations with an actuator line model are used to study the wake of the NREL 5 MW reference turbine mounted on the OC3-UMaine spar and OC4-DeepCwind semi-submersible platforms. The simulations are carried out in the Simulator fOr Wind Farm Applications (SOWFA) coupled with OpenFAST for the platform and turbine motion. The wake location, deficit, and turbulence levels are compared for the two floating platforms and equivalent fixed-turbine cases. The effects of neutral versus stable atmospheric conditions are also compared. Most notably, floating-turbine wakes are deflected upwards compared to fixed-turbine wakes, because of mean platform pitch. The spar wake deflects upwards more than the semi-submersible, while the stable atmosphere increases this vertical deflection compared to the neutral. The time-varying rotor motions do not significantly affect the mid-to-far wake, though the stable atmosphere shows larger fixed-floating differences in horizontal wake fluctuations.

Studies on High Reliable Turbine Stage Development for Rocket Engine

To improve the launch capability of rocket, rocket engine turbines are required to have high efficiency and high reliability. In reality it is quite difficult to achieve these two goals at the same time. In quest for highly efficient and reliable rocket turbine, the present authors have performed time-accurate calculation using Transient Blade Row (TBR) method of the commercial software ANSYS CFX to investigate unsteady flow behaviors in the two types of turbine stages (so called reference turbine and optimized turbine). This study then attempts to conduct detailed structural analyses of the 1st and 2nd stage turbine blades under the influence of unsteady fluid forces caused by rotor-stator interaction phenomena. Frequency response analysis is performed using the unsteady fluid forces calculated by ANSYS CFX, and the structural analyses are executed by use of MSC Nastran. In the analyses, two materials (Inconel 718 and Ti-6Al-4V) are adopted as blade material.

Influence of stall fences on the performance of Wells turbine

The Wells turbine is one of the candidates for use in Oscillating Water Column (OWC) type wave energy conversion devices. The narrow operating range of a Wells turbine limits its application to a small range of ocean environments. At a high angle of attack, the turbine performance drops suddenly due to a phenomenon known as stall. The present study introduces stall fences in the Wells turbine blade to postpone stall and widen the operating range. The stall fences are defined by the length, height, and thickness in percentage of blade chord length. For the present study, dimensions of the stall fences are determined by using a surrogate-based optimization technique. The modified and reference turbine are numerically studied by solving the Reynolds-Averaged-Navier-Stokes equations in the commercial CFD software ANSYS CFX 16.1. The comparison of fenced turbine with reference turbine shows 16.6% improvement in operating range at the cost of peak torque developed by the turbine. The peak-to-average power ratio in the stall-free range is reduced by 16.7% when stall fences are used. The change in internal flow due to the presence of stall fences is analyzed in detail in the present work.

Investigation of wind turbine wake superposition models using Reynolds‐averaged Navier‐Stokes simulations

AbstractIt is well accepted that the wakes created by upstream turbines significantly impact on the power production and fatigue loading of downstream turbines and that this phenomenon affects wind farm performance. Improving the understanding of wake effects and overall efficiency is critical for the optimisation of layout and operation of increasingly large wind farms. In the present work, the NREL 5‐MW reference turbine was simulated using blade element embedded Reynolds‐averaged Navier‐Stokes computations in sheared onset flow at three spatial configurations of two turbines at and above rated flow speed to evaluate the effects of wakes on turbine performance and subsequent wake development. Wake recovery downstream of the rearward turbine was enhanced due to the increased turbulence intensity in the wake, although in cases where the downstream turbine was laterally offset from the upstream turbine this resulted in relatively slower recovery. Three widely used wake superposition models were evaluated and compared with the simulated flow‐field data. It was found that when the freestream hub‐height flow speed was at the rated flow speed, the best performing wake superposition model varied depending according to the turbine array layout. However, above rated flow speed where the wake recovery distance is reduced, it was found that linear superposition of single turbine velocity deficits was the best performing model for all three spatial layouts studied.

Surrogate model uncertainty in wind turbine reliability assessment

Lowering the cost of wind energy entails the optimization of wind turbine material consumption without compromising structural safety. Traditionally, wind turbines are designed by the partial safety factor method, which is calibrated by probabilistic models and presented in the IEC 61400-1 design standard. This approach significantly reduces the amount of aero-elastic simulations required to assess the fatigue limit state of wind turbines, but it may lead to inconsistent reliability levels across wind farm projects. To avoid this, wind turbines may be designed by probabilistic methods using surrogate models to approximate fatigue load effects. In this approach, it is important to quantify and model all relevant uncertainties including that of the surrogate model itself. Here we quantify this uncertainty according to Eurocode 1990 for polynomial chaos expansion (PCE) and Kriging using wind data from 99 real sites and the 5 MW reference turbine designed by NREL. We investigate a wide range of simulation efforts to train the surrogate models. Our results show that Kriging yields a higher accuracy per invested simulation compared to PCE. This improved understanding of utilizing PCE and Kriging in fatigue reliability assessment may significantly benefit decision support in probabilistic design of wind turbines.

Effect of load reduction mechanisms on loads and blade bearing movements of wind turbines

AbstractThe power control of wind turbines is usually realized via a change in the pitch angle of the rotor blades. Pitching facilitates the exact control of the turbines and the reliable deceleration of the rotor when required. Pitch movements can moreover be used for load control. One of these methods is called individual pitch control (IPC). IPC controls the blades individually and brings about a significant reduction in the fatigue loads and extreme loads placed on the structural components, while at the same time reducing the yield of the turbine only slightly. The lower loads reduce material costs, and thus, the cost of energy (CoE) is reduced, despite the slight reduction in yield. The method is nevertheless not used everywhere since the additional movement cycles put the rotor blade bearings in particular under stress. Special attention must be paid to small amplitude oscillating movements, which carry a high risk of inducing surface damage in the rolling contacts of the blade bearings. This paper uses a cycle analysis of the IWT7.5‐164 reference turbine to illustrate the differences in the movement patterns of wind turbine blade bearings with and without IPC. Moreover, model calculations with single contacts are used to show which of the movement patterns carries a risk of inducing surface damage. The use of IPC leads to the expected load reduction at the blade root. In current literature, IPC is usually assumed to have a negative influence on the life expectancy of blade bearings, but the findings of this study contradict this. The summed blade bearing movement is increased, although the number of very small pitch angles occurring is reduced. This reduction reduces the risk of wear in the blade bearings.

Digital diagnostic complex for power turbine units equipment

The study is devoted to the problem formulation for creation a concept of digital complex for power turbine equipment diagnostics using a reference turbine model formed by a set of computational models of thermal and gas-dynamic processes, by diagnostic expert systems (probabilistic type and decision tree type), by functional standard characteristics and statistical models. As part of the study it is proposed to develop diagnostic methods and algorithms for characteristic malfunctions causes for various subsystems of power equipment, which will allow them to be detected at an early stage and to prevent unplanned (emergency) equipment shutdown.

The need for dynamic inflow models for vertical axis wind turbines

This paper investigates the need for dynamic inflow models for vertical axis wind turbines (VAWTs). The approach is two-fold. First, dynamic inflow is realised by dynamic thrust on an actuator disk in OpenFOAM. The induction phase shift and amplitude showed a significant dependency on the streamwise location. Second, a reference turbine in surging motion is studied using an actuator line OpenFOAM model as reference and an actuator cylinder model (with and without dynamic inflow model). The Larsen and Madsen dynamic inflow model is able to capture the overall behaviour in dynamic inflow conditions, however, it may be improved in the most upwind and downwind location. This study indicates that the modelling of VAWTs in dynamic inflow conditions may be enhanced by improving the dynamic inflow models.