Stall-regulated Wind Turbine Research Articles

UNICO: an open-source controller optimized for stall-regulated wind turbines

Stall regulation turbines still represent the preferred solution for small wind turbines. In stall-controlled rotors the controller plays a key role but, differently from pitch-based ones, no open-source controller was available to date. The study presents the UNICO (UNIfi research COntroller) controller, which has been specifically developed for variable speed stall-regulated turbines. The controller has been developed in MATLAB® Simulink® and a dynamic link library (.dll) has been generated, which can be coupled with common simulation codes such as OpenFAST and QBlade using a Bladed-style interface. UNICO includes features that are specifically tailored to variable-speed stall-regulated turbines. For below-rated conditions, the controller employs either the commonly used k-ω2 law or a tracking of the optimal tip speed ratio. For above-rated conditions, a PI controller is used to track a user-imposed reference speed. The reference speed is set to decrease linearly with wind speed, providing a safety margin for turbine operation at higher wind speeds. UNICO has been tested on a 50-kW stall-regulated reference turbine. Preliminary results show how the proposed controller can achieve better overall performance in comparison to the simplified control laws implemented in state-of-the-art codes. Additionally, the rotor speed can be controlled in above-rated conditions, providing an increased run away safety margin.

Power generation enhancement in a horizontal axis wind turbine blade using split blades

The positive effects of a split on the aerodynamic performance of fixed wings are well known. However, for horizontal axis wind turbines (HAWTs), changes in the local angle of attack (AoA) and relative velocities along the blade create complex three-dimensional flow over blades, making the analysis of the split location very challenging. In this paper, the effects of the radial position of a split are investigated using CFD simulations of five different split versions of the NREL Phase VI blade and the original blade. The CFD models are solved as transient simulations at different tip speed ratios (TSRs). Results demonstrate that the power generated by all of the split blades at any TSR<3.5 is higher than the No-split blade. Also, when the split is located at an outer radius, the split blade generates more power. Besides, it was observed that for this stall-regulated wind turbine, at high winds and especially at a stall condition, a blade in which the split extends from the blade root to the blade tip generates considerably higher power than the No-split blade. Finally, to visualize the physical phenomenon behind each case, flow patterns and streamlines of different cases are compared and discussed.

Assessing the Sensitivity of Stall-Regulated Wind Turbine Power to Blade Design Using High-Fidelity Computational Fluid Dynamics

Abstract This study provides a novel contribution toward the establishment of a new high-fidelity simulation-based design methodology for stall-regulated horizontal axis wind turbines. The aerodynamic design of these machines is complex, due to the difficulty of reliably predicting stall onset and poststall characteristics. Low-fidelity design methods, widely used in industry, are computationally efficient, but are often affected by significant uncertainty. Conversely, Navier–Stokes computational fluid dynamics (CFD) can reduce such uncertainty, resulting in lower development costs by reducing the need of field testing of designs not fit for purpose. Here, the compressible CFD research code COSA is used to assess the performance of two alternative designs of a 13-m stall-regulated rotor over a wide range of operating conditions. Validation of the numerical methodology is based on thorough comparisons of novel simulations and measured data of the National Renewable Energy Laboratory (NREL) phase VI turbine rotor, and one of the two industrial rotor designs. An excellent agreement is found in all cases. All simulations of the two industrial rotors are time-dependent, to capture the unsteadiness associated with stall which occurs at most wind speeds. The two designs are cross-compared, with emphasis on the different stall patterns resulting from particular design choices. The key novelty of this work is the CFD-based assessment of the correlation among turbine power, blade aerodynamics, and blade design variables (airfoil geometry, blade planform, and twist) over most operational wind speeds.

Design, electromechanical simulation, and control of a variable speed stall-regulated PMSG-based wind turbine

ABSTRACTWind turbines are complicated systems including different aerodynamic, mechanical, electrical, and control aspects. A few research works have considered the detailed models which include all aspects of the wind energy systems. These models can be used to cope with the disadvantages of the simple models that neglect some important features and electromechanical interactions. However, in these studies, the existing aerodynamic designs were used which limits the studies to available designs. In the present study, the site-specific blades of a variable-speed stall-regulated wind turbine are firstly designed then employed in simulation with detailed models. Since the stall-regulated turbines do not require active aerodynamic devices to control the captured power in high wind speeds, they are appropriate alternatives to be used in small-scale turbines. Hence, this type of turbine is studied in the present paper and the details of blade design and electromechanical simulation are discussed. The controllers are designed for maximum power point tracking in low and moderate wind speeds and limiting the captured power using the stall phenomenon in high wind speeds. The performance of the turbine with the designed blades is investigated in different wind conditions including simple hub-height and turbulent wind profiles, generated by TurbSim software.

Platform for design, simulation, and experimental evaluation of small wind turbines

Wind turbines are complicated systems with different aerodynamic and electromechanical aspects. An integrated platform which includes design, simulation, and experimental evaluation of wind energy conversion systems is very helpful to design, develop, and examine the performance of different wind turbine sub-systems. The previous studies exclude such a platform and this study tries to fill this gap. In this study, the blades of a 5.5 kW fixed-speed stall-regulated wind turbine are initially designed then employed in simulation and emulation. The same as the simulation setup, the utilised emulator uses AeroDyn and FAST software tools to model the aerodynamic and mechanical aspects of the turbine in a laboratory environment. The emulator is capable of reproducing the static and dynamic behaviour of the turbine in a laboratory similar to the real turbines. For simulation, the electrical parts are implemented in MATLAB/Simulink, whereas the real electrical parts are used for the emulator. The performance of the turbine with the designed blades is investigated in simulation and emulation considering a simple hub-height and turbulent wind profile, generated by TurbSim software tool based on the IEC standards. Moreover, the start-up process of the wind turbine is evaluated using the wind turbine emulator and the results are discussed.

A stall-regulated wind turbine design to reduce fatigue

Variable-speed stall-regulated (VS-SR) wind turbines can be designed to produce power as efficiently as variable-speed pitch-controlled (VS-PC) systems. However, amongst the main drawbacks of VS-SR systems high transient power and low predictability have been the primary factors in favour of adopting VS-PC system for multi-MW wind turbines. Cyclic and stochastic loads leading to fatigue failure is one of the prime considerations for large wind turbines. In contrast to the current trend of research, which is focused on load alleviation by integrating active flow controllers, this paper highlights the potential benefits of VS-SR wind turbines in reducing fatigue loads. Adopting the NREL 5 MW wind turbine as the baseline, blades are redesigned for stall-regulation. It is shown that a well-designed VS-SR wind turbine experiences significantly less fatigue loads compared to VS-PC systems. It also results in low power transients near and above rated wind speed. Taking into account added complexity, mass and maintenance costs of wind turbines utilising active flow controllers and in view of the recent progresses that have been made regarding the aeroelastic stability of stalled blades, VS-SR systems seem to have a role to play in the design of future wind turbines.

Performance assessment of five MCP models proposed for the estimation of long-term wind turbine power outputs at a target site using three machine learning techniques

Various models based on measure-correlate-predict (MCP) methods have been used to estimate the long-term wind turbine power output (WTPO) at target sites for which only short-term meteorological data are available. The MCP models used to date share the postulate that the influence of air density variation is of little importance, assume the standard value of 1.225 kg m−3 and only consider wind turbines (WTs) with blade pitch control.A performance assessment is undertaken in this paper of the models used to date and of newly proposed models. Our models incorporate air density in the MCP model as an additional covariable in long-term WTPO estimation and consider both WTs with blade pitch control and stall-regulated WTs. The advantages of including this covariable are assessed using different functional forms and different machine learning algorithms for their implementation (Artificial Neural Network, Support Vector Machine for regression and Random Forest).The models and the regression techniques used in them were applied to the mean hourly wind speeds and directions and air densities recorded in 2014 at ten weather stations in the Canary Archipelago (Spain). Several conclusions were drawn from the results, including most notably: (a) to clearly show the notable effect of air density variability when estimating WTPOs, it is important to consider the functional ways in which the features air density and wind speed and direction intervene, (b) of the five MCP models under comparison, the one that separately estimates wind speeds and air densities to later predict the WTPOs always provided the best mean absolute error, mean absolute relative error and coefficient of determination metrics, independently of the target station and type of WT under consideration, (c) the models which used Support Vector Machines (SVMs) for regression or random forests (RFs) always provided better metrics than those that used artificial neural networks, with the differences being statistically significant (5% significance) for most of the cases assessed, (d) no statistically significant differences were found between the SVM- and RF-based models.

Design of advanced airfoil for stall-regulated wind turbines

Abstract. Nowadays, all the modern megawatt-class wind turbines make use of pitch control to optimise the rotor performance and control the turbine. However, for kilowatt-range machines, stall-regulated solutions are still attractive and largely used for their simplicity and robustness. In the design phase, the aerodynamics plays a crucial role, especially concerning the selection/design of the necessary airfoils. This is because the airfoil performance is supposed to guarantee high wind turbine performance but also the necessary machine control capabilities. In the present work, the design of a new airfoil dedicated to stall machines is discussed. The design strategy makes use of a numerical optimisation scheme, where a gradient-based algorithm is coupled with the RFOIL code and an original Bezier-curves-based parameterisation to describe the airfoil shape. The performances of the new airfoil are compared in free- and fixed-transition conditions. In addition, the performance of the rotor is analysed, comparing the impact of the new geometry with alternative candidates. The results show that the new airfoil offers better performance and control than existing candidates do.

Design of advanced airfoil for stall-regulated wind turbines

Nowadays, all the modern MW-class wind turbines make use of pitch control to optimize the rotor performance and control the turbine. However, for kW-range machines, stall-regulated solutions are still attractive and largely used for their simplicity and robustness. On the design phase, the aerodynamics plays a crucial role, especially concerning the selection/design of the necessary airfoils. This is because the airfoil performance should guarantee high wind turbine performance, but also the needed machine control capabilities. In the present work, the design of a new airfoil dedicated for stall machines is discussed. The design strategy makes use of numerical optimization scheme where a gradient-based algorithm is coupled with XFOIL code and an original Bezier-curves-based parameterization to describe the airfoil shape. The performances of the new airfoil are compared in free and fixed transition conditions. In addition, the performance of the rotor is analysed comparing the impact of the new geometry with alternative candidates. The results show that the new airfoil offers better performance and control than existing candidates do.

The application of passive air jet vortex-generators to stall suppression on wind turbine blades

An experimental study was performed to assess the feasibility of passive air jet vortex-generators to the performance enhancement of a domestic scale wind turbine. It has been demonstrated that these simple devices, properly designed and implemented, can provide worthwhile performance benefits for domestic wind turbines of the type investigated in this study. In particular, this study shows that they can increase the maximum output power coefficient, reduce the cut-in wind speed and improve power output at lower wind speeds while reducing the sensitivity to wind speed unsteadiness. A theoretical performance analysis of a 500 kW stall-regulated wind turbine, based on blade element momentum theory, indicates that passive air jet vortex-generators would be capable of recovering some of the power loss because of blade stall, thereby allowing attainment of rated power output at slightly lower average wind speeds. Copyright © 2016 John Wiley & Sons, Ltd.

Novel Image Analysis Method for Blade Aerodynamic Performance on Operational Turbine

Tuft flow visualisation has been used to study aerodynamic stall on wind turbine blades for several decades. In recent years, advances in the processing power of personal computers have made digital image processing vastly more accessible. In this paper, therefore, a novel method is presented to digitally analyse tuft flow visualisation on the blade of an operating wind turbine. Examination of the outboard 40% of the blade of a 10 m diameter wind turbine revealed stalled flow in wind speeds from 5m/s to 20m/s. The region of stall at those speeds increased from 5% to 40% of the coverage area of the tufts. This increase in the amount of stalled flow is expected for stall-regulated wind turbines. Overall, the results are very promising and demonstrate potential for a wide range of aerodynamics applications including real-time blade stall determination and classical wind tunnel aerodynamics studies.

Rapid optimization of stall-regulated wind turbine blades using a frequency-domain method: Part 2, cost function selection and results

A fast and effective frequency-domain optimization method was developed for stall-regulated blades. It was found that when using linearized dynamics, typical cost functions employing damage-equivalent root bending moments are not suitable for stall-regulated wind turbines: when the cost function is minimized, the edgewise damping can be low, and the flapwise damping can approach zero during an extreme operating gust. A new cost function is proposed that leads to nicely balanced stall behavior and damping over the entire operating windspeed range. The method was used to design the blades of two multi-MW, stall-regulated, offshore wind turbines, comparable with the NREL 5 MW and NTNU 10 MW pitch-regulated turbines. It is shown that the optimal stall-regulated blade has a unique aerodynamic profile that gives high flapwise and edgewise damping and a uniform mean power output above the rated windspeed. The blades are described in sufficient detail that they can be used in further aeroelastic analyses, to compare large stall-regulated and pitch-regulated turbines. Copyright © 2014 John Wiley & Sons, Ltd.

The Design of Stall-Regulated Wind Turbine Blade for a Maximum Annual Energy Output and Minimum Cost of Energy Based on a Specific Wind Statistic

The design of a stall-regulated wind turbine to achieve a maximum annual energy output is still a formidable task for engineers. The design could be carried out using an average wind speed together with a standard statistical distribution such as a Weibull with k = 2.0. In this study a more elaborated design will be attempted by also considering the statistical bias as a design criterion. The wind data used in this study were collected from three areas of the Lamtakong weather station in Nakhonratchasima Provice, the Khaokoh weather station in Phetchaboon and the Sirindhorn dam weather station in Ubonratchathani, Thailand. The objective is to design a best aerodynamic configurations for the blade (chord, twist and pitch) using the same airfoil as that of NREL Phase VI wind turbine. Such design is carried out at a design wind speed point. Wind turbine blades were optimized for both maximum annual energy production and minimum cost of energy using a method that take into account aerodynamic and structural considerations. The work will be carried out by the program “SuWiTStat” which was developed by the authors and based on BEM Theory (Blade Element Momentum). Another side issue is the credibility of the Weibull statistic in representing the real wind measurement. This study uses a regression analysis to determine this issue.

Dynamic Optimization of Drivetrain Gear Ratio to Maximize Wind Turbine Power Generation—Part 1: System Model and Control Framework

Wind is considered to be one of the most promising resources in the renewable energy portfolio. Still, to make wind energy conversion more economically viable, it is necessary to extract greater power from the wind while minimizing the cost associated with the technology. This is particularly important for small wind turbines, which have the highest cost per kilowatt of energy produced. One solution would be a variable ratio gearbox (VRG) that can be integrated into the simple and low-cost fixed-speed induction generator. Through discrete variable rotor speed operation, the VRG-enabled system affects the wind speed ratio, the power coefficient, and ultimately the power produced. To maximize electrical production, mechanical braking is applied during the normal operation of the wind turbine. A strategy is used to select gear ratios (GRs) that produce torque slightly above the maximum amount the generator can accept while simultaneously applying the mechanical brake, so that full load production may be realized over greater ranges of the wind speed. To characterize the performance of the system, a 100 kW, fixed speed, stall-regulated wind turbine, has been developed for this study. The VRG-enabled wind turbine control system is presented in two papers. Part 1 focuses on the turbine simulation model, which includes the rotor, VRG-enabled drivetrain, disk brake, and electric generator. A technique for estimating the performance of a disk brake, in the wind turbine context, is also presented. Part 2 of the research will present a dynamic optimization algorithm that is used to establish the control protocol for competing performance objectives.

Performance analysis of indirect vector control algorithm for small-sized wind turbine induction generator

A power limitation on a small-sized of a stallregulated wind turbine is presented. The turbine is designedwith a rated power of 25kWatt and operating in variablespeed. The turbine is designed to use a 30kWatt SquirrelcageInduction Generator (SCIG) to generate electricity. Acontrol algorithm using Indirect Vector Control (IVC) hasbeen designed, and the performance has been analysed. Agood result is achieved where the power can be limited atthe rated value during high wind velocity. The comparisontransient response when the different controller's gain istuned is also demonstrated. This proposed design is to beused in the stall regulated wind turbine, with active speedcontrol.

CFD Investigation on the aerodynamic characteristics of a small-sized wind turbine of NREL PHASE VI operating with a stall-regulated method

The objective of this investigation is to clearly understand the aerodynamic characteristics of a small-sized wind turbine of NREL Phase VI, operating with a stall-regulated method using CFD code. Based on this, it is possible to provide turbine designers with the aerodynamic design data to increase efficiency and improve performance in the design phase of future small-sized wind turbine blades. Moreover, a comparison was made between experimental datasets, in order to verify the reliability and validity of the analysis results. The first height in the normal direction from the surface of a rotor blade is about 0.2 mm, and the average value of y+ is about 7 at 7 m/s. The domain is chosen to consist of only two hexahedral mesh regions, namely the interior region, including the wind turbine blade, and the external region excluding the rectangle. The total cell number of the numerical grid is about Ng = 3.0 × 106. Five different inflow velocities, in the range Vin = 7.0−25.1 m/s, are used for the rotor blade calculations. The calculated power coefficient is about 0.35 at a TSR of 5.41, corresponding to 7 m/s, and showed considerably good agreement with the experimental measurements, to within 0.08%. It was observed that the 3-D stall begins to generate near the blade root at a wind speed of 7 m/s. Therefore, root design approaches considering the appropriate selection of the angle of attack and the thickness are very important in order to generate the stall on the blade root. Through a clear understanding of aerodynamic characteristics of a small-sized NREL Phase VI wind turbine, it is expected that this useful aerodynamic data will be made available to designers as guidance in designing stall-regulated wind turbine blades in the development phase of small-sized wind turbine systems in the future.

A simple frequency-domain method for stress analysis of stall-regulated wind turbines

A simple method, based in the frequency domain, was developed for calculating the dynamic response of a stall-regulated wind turbine. Emphasis is placed on two aspects of the method, which are necessary in order to obtain a reasonable linearization of behavior when the blades are stalled. First, the tangential (in-plane) component of turbulence is included, in addition to the axial component. Second, the linearized relationship between lift coefficient and angle-of-attack is adjusted to account for the effects of dynamic stall: separate linearizations are used for excitation and damping of vibration. A thorough comparison is made between linear and non-linear dynamic-stall methods, with the conclusion that the accuracy of the linear method depends upon the frequency and amplitude of oscillation. The linear dynamic-stall method is accurate at blade vibrational frequencies, but it can be inaccurate at frequencies in the vicinity of 1P or below, when the angle-of-attack oscillates with an amplitude of 3° or more. Load spectra of a Nordtank 500 turbine, calculated using the frequency-domain method, are compared with measurements. The frequency-domain method provides estimates of load spectra and aerodynamic damping (stability) that are useful for preliminary design and optimization, but the method lacks sufficient accuracy and generality to be used for certification. Copyright © 2011 John Wiley & Sons, Ltd.

Wind energy conversion with a variable-ratio gearbox: design and analysis

Variable-speed wind turbines are able to adapt to low wind speeds and therefore have greater efficiency than fixed-speed turbines during partial-load operation. Unfortunately, the high cost and low reliability of the electronics that enable variation in speed have discouraged this mode of operation for distributed wind turbines. Alternatively, a Variable-Ratio Gearbox (VRG) can be integrated into the fixed-speed wind turbine to facilitate operation with a discrete set of variable speeds that boost efficiency. The VRG concept is based upon mature technology taken from the automotive industry and is characterized by low cost and high reliability. In this paper, a model-based design methodology is introduced to study the performance gain of integrating a VRG into a fixed-speed stall-regulated wind turbine system. The results demonstrate how this device can improve the efficiency of the fixed-speed turbine in the partial-load region and the potential to use the VRG to limit power in the full-load region where pitch control is often used.

Optimization model for rotor blades of horizontal axis wind turbines

This paper presents an optimization model for rotor blades of horizontal axis wind turbines. The model refers to the wind speed distribution function on the specific wind site, with an objective to satisfy the maximum annual energy output. To speed up the search process and guarantee a global optimal result, the extended compact genetic algorithm (ECGA) is used to carry out the search process. Compared with the simple genetic algorithm, ECGA runs much faster and can get more accurate results with a much smaller population size and fewer function evaluations. Using the developed optimization program, blades of a 1.3 MW stall-regulated wind turbine are designed. Compared with the existing blades, the designed blades have obviously better aerodynamic performance.

Effect of dust on the performance of wind turbines

An experimental investigation on the effect of blade surface roughness, due to dust accumulation, on the performance of wind turbines was performed. The development of the energy generating costs of wind turbines directly depends on the wind turbine output, which depends upon the characteristics of the turbine blades and their surface roughness. An important operating requirement that relates to a wind turbines airfoils are its ability to perform when the smoothness of its surface has been degraded by the dust. The effect of surface roughness of rotor blades due to accumulated dust on the blade surface of stall-regulated, horizontal axis 300 kW wind turbine was investigated. The mechanism of dust built up and accumulation on the blade surface of wind turbine was investigated, and the effect of operation period of wind turbine on the blade surface roughness intensity was investigated experimentally. Also, the quantity of dust accumulated on the blade leading edge; and the effect of changing dust area on blade surface were studied. Standard roughness in Hurghada site was chosen and put in various leading edge areas. The roughness area on blades was changed from 5 to 20% from the chord line towards the leading edge. The effect of dust on the performance of pitch-regulated 100 kW horizontal axis wind turbine was investigated. These results from pitch-regulated wind turbine were compared with 100 kW stall-regulated wind turbine.