Performance Enhancement in Horizontal Axis Wind Turbine Using Blade With Split

A lift-driven vertical axis wind turbine (VAWT) generates peak power when it is rotating at high tip-speed ratios (TSR), at which time the blades encounter angles of attack (AOA) over a small range from zero to 30 degrees. However, its ability to self-start is dependent upon its performance at low TSRs, at which time the blades encounter a range of AOAs from zero to 180 degrees. A novel vented aerofoil is presented with the intention of improving the performance of a lift-driven VAWT at low TSRs without hampering the performance of the wind turbine at high TSRs. Computational fluid dynamics (CFD) simulation is used to predict the aerodynamic characteristics of a new vented aerofoil based on the well documented NACA0012 profile. Simulations are performed using the SST turbulence model. The results obtained show a reduction in the coefficient of tangential force (the force that generates torque on the wind turbine) at low AOAs (less than 90 degrees) of no more than 30%, while at high AOAs (more than 90 degrees) an improvement in the tangential force of over 100% is observed. Using a simple momentum based performance prediction model, these results suggest that this would lead to an increase in torque generation by a theoretical three-bladed VAWT of up to 20% at low TSRs and a minor reduction in coefficient of performance of up to 9% at TSR of 2 and closer to 1% at higher TSRs.

This study aims to improve the adaptability of vertical axis wind turbines (VAWTs) in urban wind fields and reduce the cost of 3-D optimization. It focuses on 2-D to 3-D optimization of J-shaped blades. The goal is to enhance kinetic energy capture during startup and improve moment and power performance under different tip speed ratios (TSR). Based on the improved blade-element momentum (BEM) theory, 2-D J-shaped blades were developed for multi-environment applications. They were extended into 3-D forms using variable-section lofting. The 2-D optimization used the Northern Goshawk Optimization–Radial Basis Function (NGO-RBF) neural network and the Multi-Objective Harris Hawks Optimization (MOHHO) algorithm. The objectives were average power coefficient (CPA ) and maximum moment coefficient (Cm-max ). Maximum thickness (δ) and length ratio (lo /c) were design variables. For 3-D blades, moment (M) and average power (Pb ) were used as performance indicators. Integrating the 2-D designs produced symmetric tapered and diffused tips and asymmetric tapered blades. CFD simulations revealed aerodynamic behavior under various TSRs. Optimized 2-D blades enhanced drag effects at low TSR and reduced drag loss while improving lift at high TSR. Among the 3-D designs, the symmetric tapered tip performed best. It increased minimum moment by 7.9% at low TSR and average power by 24.2% at high TSR. The 3-D flow results also confirm the conclusions from the 2-D optimization mechanisms. This study provides an effective design method for VAWT blades in urban parallel wind conditions. It also lays the foundation for future research on adaptive intelligent blades for complex wind environments.

The simplest aerodynamic model of horizontal-axis wind turbines is the blade element momentum theory, which assumes that the blades behave as airfoils, but a correct two-dimensional representation is an infinite cascade of lifting bodies. This study analyzes the conventional and impulse forms of the forces on cascades of airfoils at spacings and pitch angles typical of wind turbine applications. OpenFOAM software was used to simulate steady, incompressible flow at a Reynolds number of 6×106 through cascades of NACA 0012 airfoils. The force equations agree well (less than 1% error) with the forces determined directly from OpenFOAM for four spacing ratios. We concentrate on the “wake vorticity” term, which is ignored in blade element momentum analysis. At a pitch angle of 90°, this term balances the viscous drag when the angle of attack is zero. At zero pitch, which models the outer region of a wind turbine blade at a high tip speed ratio, the term can account for 27% of the axial thrust when the angle of attack is about 4°. The normal force equation, like the angular momentum equation for wind turbines, has no viscous term, which forces the body drag to contribute to the circulation in the wake. It is shown that the airfoil assumption is conservative in that cascade elements have higher lift-to-drag ratios than airfoils at the same angle of attack. An associated result is that separation occurs at higher angles of attack on a cascade element compared to an airfoil.

This report presents the study of fluid-structure interaction (FSI) of a 5kW horizontal axis wind turbines (HAWT). The motivation of the current investigation is to address the FSI problem of HAWT in the whole flow field analysis with hybrid turbulence model method which is used to find out the flow and structure characteristics of wind turbine in operation under different tip speed ratio (TSR). The target of current work is to obtain detailed information of flow field and structure response simultaneously using the finite element analysis (FEA) method as well as providing new reliable and accurate approach which could be quickly and economically employed in wind turbine FSI problem analysis. In this report, a detailed and comprehensive literature review related to FSI problem of wind turbine has been presented and includes aerodynamic, wind turbine theory, computational fluid dynamic, fluid structure interaction and aeroelastic stability. A 5kW wind turbine is designed according to blade element momentum (BEM) method and a 3D wind turbine modeling is generated in SolidWorks. The 3D wind turbine modeling is imported into FEA software by coupling with the structure domain and the fluid domain to achieve FSI analysis and different TSR are applied to the computational fluid dynamics (CFD) analysis to gain the wind turbine operation information under different scenarios. The simulation results are investigated to reveal the influence of the FSI phenomenon on wind turbine performance. Analysis of wind turbine aeroelastic stability is conducted to prevent blade from experiencing flutter problem. A multiple objective optimization method for wind turbine design including aerodynamic and structure has been developed to improve wind turbine performance. The flow field analysis results showed that the strength of vortex at the tip and root of the blade increases as wind speed increases when wind turbine is under operating conditions. With wind speed of 10m⁄s, most of the flow over the lower surface remains attached and the wind turbine blade was running under design condition. As the wind speed increases, flow separation emerges. When the wind speed increasing to 16m⁄s, the wind turbine blade works under deep stall condition. The wind turbine power coefficient drops continuously from 0.46 to 0.22, as wind speed increased from 10m⁄s to 20m⁄s. The comparison of wind turbine power and torque coefficient under different CFD configuration shows the advantage of whole flow field analysis with hybrid turbulence model. The structural analysis results demonstrated that most of the pressure force acts at a region around the 1⁄4 chord from the leading edge of the blade under operating condition and the displacement of blade tip in flap direction is bigger than those in the other directions. Modal analysis results deduced that flap is the main type of vibration in first and second modes. The difference of natural frequency between single blade and blade rotor becomes larger as vibration order increases. The dynamic performances of wind turbine blade and rotor are totally different and show the whole flow field method is better than the one third domain method. The wind turbine aeroelastic analysis results illustrate that the bending frequency and torsion frequency are around 23.94Hz and 38.46Hz respectively under TSR=4 and both move towards 30Hz under TSR=1.7. When TSR is smaller than 1.7 the wind turbine will experience flutter due to the overlapping of bending and torsion frequencies. The multiple objectives optimization method takes the aerodynamic performance and blade loading into account to gain better design parameter. The comparison between initial and optimal design shows that optimized method could provide higher electrical energy around 1073kW•h per annum under rated wind speed. The optimized wind turbine critical flutter wind speed will increase from initial design speed of 35m/s to 38m/s under optimization design.

Low-center-of-gravity wind turbines (LCGWTs) characterized by tapered blades whose chord length c increases nonlinearly from the top (where c = 0.11 m) to the bottom (where c = 0.17 m) of each blade. Further, turbines featuring these blades do not need any arms, or even a center pole in the rotor. Two experimental LCGWTs (diameter: 0.4 m; height: 0.25 m) with symmetrical blades (NACA 0018) and cambered blades were built. A dead band, which is a band of tip speed ratio (TSR) where the rotor has negative torque at TSR lower than that where the maximum power-coefficient condition is achieved, was observed when symmetrical blades were subjected to low wind speed. In contrast, no dead band was observed for the cambered blades. Under high wind speeds and over a wide range of TSR values, performance of the LCGWTs was better with cambered blades than with symmetrical blades. Computational fluid dynamics (CFD) analysis of 2-dimensional rotors whose blade sections corresponded to the blade sections at the equatorial planes of both types of LCGWTs showed the same tendency. Performance predictions by the blade element momentum (BEM) method using aerodynamic data on the NACA 0018 blades showed some agreement with the CFD analysis. For the cambered blade rotor, Wilson and Walker’s empirical correction of the thrust coefficient, a correction that is typically used in simulations of horizontal axis wind turbines, brought the BEM prediction closer to the CFD prediction than Glauert’s correction did. However, the agreement between the BEM prediction with Wilson and Walker’s correction and the CFD prediction of the cambered blade rotor was thought to be just a coincidence due to large difference on the torque variations between BEM and CFD. At least, the Wilson and Walker’s correction predicts larger torque than the Glauert’s correction at high TSR region. : Wind power, Vertical axis wind turbine, Low center of gravity, Cambered blade, Flow curvature, Conformal mapping, Blade element momentum method, Computational fluid dynamics

Aerodynamic Optimization and AI-Based Performance Prediction of Vertical Axis Wind Turbines

The challenge of wind turbine blade design is to balance the conflict between high capacity and heavy system loads introduced by the large scale rotor. To solve this problem, we present a multi-objective optimization method to maximize the Annual energy production (AEP) and minimize the blade mass. The well-known Blade element momentum (BEM) theory is employed to predict the aerodynamic performance and AEP of the blade. The blade is simplified as a thin Bernoulli beam. The cross section is modelled as a combination of composite layer, shear webs and spar caps typically. The strain of every cross section has been considered as a constraint to minimize the spar cap thickness for minimizing the blade mass. An improved genetic algorithm (NSGA-II) is applied to obtain the Pareto front set. Several solutions of the Pareto set are selected to compare with the reference blade (NREL 5MW blade). Performance of the rotors on design condition is simulated by STAR-CCM+ to verify the results of BEM theory. Optimal results show that the present blade, which is fully superior to the reference blade, can be selected from the Pareto set. The optimization design method can provide a superior blade with an increase by 2.48% of AEP and a reduction by 5.52% of the blade mass. It indicates the present optimization method is effective. Results of numerical simulations show that the spanwise flow would be increased obviously in tip region of the reference blade. The reason is that chord length variation in blade tip affects the flow and causes minor stall. The abrupt change of chord distribution in blade tip should be avoided to reduce the spanwise flow in initial blade design.

Blade element momentum theory is extensively used to design and characterize the performance of the wind turbine. Aerodynamic characteristics of the airfoils used in the blades of the wind turbine are one of the crucial parameters of the blade element momentum theory. The aerodynamic characteristics of airfoil can be significantly influenced by Reynolds number besides the angle of attack. Thus, an inadequate consideration of Reynolds number for aerodynamic characteristics of the airfoil can result in discrepancy in the optimum design and performance evaluation capability of the blade element momentum theory. Several mathematical models, and semi-empirical relations to parameterize the aerodynamic characteristics of two-dimensional airfoil in blade element momentum theory, inherently lack dependency on Reynolds number and thus open a scope for uncertainty. An artificial neural network-based model is proposed to predict the lift and drag coefficient of airfoil as a function of not only the angle of attack but also the Reynolds number. A series of six in-house developed airfoils for small wind turbine have been considered for the present study. The computational fluid dynamic results of the airfoil with a range of Reynolds number (100,000–2,000,000) and angle of attack (0°–20°) were utilized to develop the model. A high coefficient of determination and low root-mean-square error of the developed models for a test dataset suggests the robust capabilities and effective topology of the artificial neural network-based model to predict the lift and drag coefficient of the airfoils with respect to a given angle of attack and Reynolds number. The developed model can then be used to replace the traditional analytical or semi-empirical model for mathematical representation of airfoil in the blade element momentum theory and thus reduce the uncertainty on account of inadequate consideration of Reynolds number for aerodynamic characteristics of airfoil in design and performance evaluation.

During recent years, wind energy has moved from an emerging technology to a nearly competitive technology. This fact, coupled with an increasing global focus on environmental concern and a political desire of a certain level of diversification in the energy supply, ensures wind energy an important role in the future electricity market. For this challenge to be met in a cost-efficient way, a substantial part of new wind turbine installations is foreseen to be erected in big onshore or offshore wind farms. This fact makes the production, loading and reliability of turbines operating under such conditions of particular interest.

The impact of trailing edge flap on the aerodynamic performance of small-scale horizontal axis wind turbine

Blade Element Momentum (BEM) theory is a well understood and proven method for modeling blade loads and determining steady state performance characteristics of wind turbines. Recently this theory has successfully been applied to horizontal axis marine current turbines when incorporating modifications that lead to better predictions of turbine performance for a range of operating conditions. Relatively little work exists in the implementation of BEM theory in a marine environment with surface gravity waves. A better understanding of the effects of waves on tidal turbines is necessary to predict fatigue loading that can eventually lead to blade failure. This paper presents a BEM numerical model that incorporates the unsteady velocities due to the presence of waves and assesses the effects of waves on tidal turbine performance. Numerical results of the coefficient of power (CP) and coefficient of thrust (CT) match closely with experimental results for the mean Cp and CT for a range of tip speed ratios. The model is also able to predict the instantaneous amplitudes of the performance characteristics as compared with the measured values except for the BEM calculated thrust which shows a 20% reduction in amplitude.

Steadily increasing energy consumption, fluctuating fuel costs and concerns about global climate changes have led to the research and evaluation of alternative renewable energies. The wind turbine is among the promising alternative energy sources that have recently gained more attention. In this work, the challenging aspects involved in modelling the rotor aerodynamics and wakes behind the wind turbines are studied. This report covers the literature review of Blade Element Momentum (BEM) analysis of wind turbine, limitations of BEM method, effect of stall delay, wake aerodynamics, atmospheric boundary layer and its effects on the wake characteristics and previous different wake models of near and far wake regions. In this project, experimental data of NREL Phase VI Turbine (sequence S) were used to corroborate the results. There are two main objectives of this project. The first objective is to improve the BEM analysis to account for the three-dimensional (3D) effects due to the rotation of the turbine. The second objective is to contemplate on the effects of atmospheric boundary layers (ABL) on the wake characteristics. These two objectives act as a roadway to enhance coupled BEM-CFD analysis, which is left for the future works. In the coupled BEM-CFD analysis, the BEM method will be applied to calculate the aerodynamic forces of the aerofoil sections along the blade span. The main drawback of BEM analysis is the use of the twodimensional (2D) aerofoil characteristics (CL and CD ) which considers only the axial flow but not radial or spanwise flow along the blade span. This leads to a considerable difference in the lift coefficients between the rotating and non rotating blades, especially at inboard sections of the blade. This 3D phenomenon is called stall delay. The current study includes different proposals for the extrapolation of 2D aerofoil characteristics of the S809 aerofoil and their implementation in BEM analysis and comparison of power predicted with experimental results. Also, four existing stall delay correction models in BEM analysis are examined. In general, these models result in over-prediction of power, especially at high inflow wind speeds. An improved inverse BEM method is developed to compute 3D aerofoil characteristics at different radial locations along the blade span. In addition to five radial locations as described in the NREL/NASA Ames test analysis, 13 additional radial locations are considered for a better understanding of stall delay. A new BEM model with the local radius effect of aerofoil characteristics (other than as a function of Reynolds number and angle of attack only) is proposed. Implementation of the new model showed a good agreement with aerofoil characteristics distribution along the blade span with the 3D aerofoil characteristics computed from the CFD analyses using inverse BEM method. MATLAB code was developed for both BEM and Inverse BEM analyses. Most important in the wind farm analysis is the effect of the atmospheric boundary layer since the turbulence properties of the atmosphere affect the wake characteristics. In this work, the NREL Phase VI Turbine is virtually placed in different atmospheric boundary layers from open sea to city/forest. Simulations are performed with direct rotor modelling using sliding mesh analysis. Since the experiment results of wake characteristics of the NREL Phase VI Turbine was not available, different empirical models are used for comparison. It was evident that wake recovers at a faster rate as the roughness length of the ground increases. Also, it was noted that the turbulence intensity of the wake varies both laterally and vertically, but existing analytical wake turbulence intensity models provide only an averaged value The possible methods for indirect rotor modelling like Actuator Disk, Actuator Line and Actuator Surface methods that lead to coupled BEM-CFD analysis of wind turbine is discussed in brief. The possible ways of improving these indirect rotor models by using the aerodynamic forces computed from improved BEM analysis and wake aerodynamics are provided for the future works.

Research of asymmetric airfoil on aerodynamic characteristics of vertical axis wind turbines

The powerful wind generators, producing electric power on the order of megawatts are with significant sizes. The studies on the interaction between the air flow and the wind turbine usually assume that the airflow is homogeneous in vertical direction, which admission is made also in the classical Blade element momentum (BEM) theory. While for the smaller wind turbines this assumption does not lead to significant inaccuracies, for powerful generators the vertical gradient of the wind velocity causes a dependence of the aerodynamic interaction from the angle of rotation of the turbine and leads to a significant cyclic variability of the forces and moments acting on the blades, even for a constant flow velocity without a longitudinal turbulence. The study of this problem is important for the blades loads and fatigue calculations as well as for the synthesis of an individual pitch control of the turbine blades. In this publication adaptation of the BEM theory is proposed in order to account of the influence of the vertical gradient of the wind velocity.

Abstract. Load calculations play a key role in determining the design loads of different wind turbine components. To obtain the aerodynamic loads for these calculations, the industry relies heavily on the Blade Element Momentum (BEM) theory. BEM methods use several engineering correction models to capture the aerodynamic phenomena present in Design Load Cases (DLCs) with turbulent wind. Because of this, BEM methods can overestimate aerodynamic loads under challenging conditions when compared to higher-order aerodynamic methods – such as the Lifting-Line Free Vortex Wake (LLFVW) method – leading to unnecessarily high design loads and component costs. In this paper, we give a quantitative answer to the question of load overestimation of a particular BEM implementation by comparing the results of aeroelastic load calculations done with the BEM-based OpenFAST code and the QBlade code, which uses a particular implementation of the LLFVW method. We compare extreme and fatigue load predictions from both codes using sixty-six 10 min load simulations of the Danish Technical University (DTU) 10 MW Reference Wind Turbine according to the IEC 61400-1 power production DLC group. Results from both codes show differences in fatigue and extreme load estimations for the considered sensors of the turbine. LLFVW simulations predict 9 % lower lifetime damage equivalent loads (DELs) for the out-of-plane blade root and the tower base fore–aft bending moments compared to BEM simulations. The results also show that lifetime DELs for the yaw-bearing tilt and yaw moments are 3 % and 4 % lower when calculated with the LLFVW code. An ultimate state analysis shows that extreme loads of the blade root out-of-plane bending moment predicted by the LLFVW simulations are 3 % lower than the moments predicted by BEM simulations. For the maximum tower base fore–aft bending moment, the LLFVW simulations predict an increase of 2 %. Further analysis reveals that there are two main contributors to these load differences. The first is the different way both codes treat the effect of the nonuniform wind field on the local blade aerodynamics. The second is the higher average aerodynamic torque in the LLFVW simulations. It influences the transition between operating modes of the controller and changes the aeroelastic behavior of the turbine, thus affecting the loads.