Pitch Vertical Axis Wind Turbines Research Articles

Sistem for variable pitch vertical axis wind turbine

Sistem for variable pitch vertical axis wind turbine

Toward improving the performance of a variable pitch vertical axis wind turbine (VP-VAWT), Part 1: Sensitivity analysis using Taguchi-CFD approach

Vertical axis wind turbines (VAWTs) are feasible to be improved due to lower efficiency compared to the horizontal axis type and suffer from fatigue issues as a result of cyclic aerodynamic loading. The performance of VAWTs can definitely be enhanced by utilizing a variable pitch angle mechanism and specific-efficient airfoils, especially at low tip speed ratios (TSRs). With this background, the scope of the present investigation is 2-fold. In the first part, Taguchi and computational fluid dynamics (CFD) methods will be used to perform a thorough sensitivity analysis. Then, in the second part, a multi-objective optimization problem will be defined based on the results of part-1 to address the mentioned drawbacks. This part proposes a novel procedure to accomplish sensitivity analysis. A MATLAB code was used to automatically manage suitable simulations for implementing the proposed innovative statistical-aerodynamic analysis. Five objective functions are offered, including power indices, structural and fatigue demands. Design variables consist of 12 parameters related to the pitch angle profile and the blade section geometry. The sensitivity analysis was implemented in three main stages: executing various simulations under distinct conditions based on orthogonal tables, assessing the hypothesis of using different pitch angle profiles for each blade instead of the same pitch profile for all blades, and describing sensitivities using signal-to-noise ratio (S/N) analysis. The same pitch profile idea was proved appropriate based on the results. The third stage result would reflect the order of importance of design variables regarding each objective in detail. Finally, the best-simulated cases are reported and different aerodynamic aspects will be explored. By using the proposed modifications, both upwind and downwind power performance could be enhanced. However, the fatigue problem could be reduced by lowering the maximum local moment coefficient. The integrated systematic approach used in this paper could be considered a unified framework for sensitivity analysis of variable pitch wind turbines.

Toward improving the performance of a variable pitch vertical axis wind turbine (VP-VAWT), Part 2: Multi-objective optimization using NSGA-II with CFD in the loop

In this study, a multi-objective optimization is performed to improve the performance of a Darrieus vertical axis wind turbine (VAWT) at low tip speed ratios considering the aerodynamic and structural objectives. In this regard, firstly, a multi-objective optimization platform is developed. Using the results of part-1 of the current research, design of experiment (DOE) analysis, more effective design variables and their limited range of variation were extracted. These quantities were selected from pitch profile and blade section geometry variables, which will form a minimal optimization problem including the essential physics with reduced computational cost. The optimization is executed using a homemade NSGA-II code, validated by a benchmark model. It is embedded within the main MATLAB code, which relates and controls the flow field solution and optimization loop. Average moment coefficient in the downwind phase, the maximum moment coefficient in the upwind phase and the bending stiffness were selected as objective functions. These parameters would reflect power, structural fatigue and strength, respectively. The Pareto front charts are provided, which depict the relationship between objectives. It will then be followed by trend analysis and provide trade-off capability. Different aerodynamic aspects of the optimized VAWT are also compared with the reference VAWT, demonstrating the effects of optimum pitch profile and blade section. The trend analysis shows a compromise between structural and aerodynamic objectives, while the two indices of maximum moment and stiffness are not opposing. Moreover, a practical method for effective angle of attack identification is employed. Based on this estimation, one may deduce that the angle of attack in the upwind phase is more influenced by pitch variation than in the downwind. Interestingly, thrust coefficient and thrust angle variation range reduction would be achieved as a result of the proposed optimization, which is beneficial for the whole system's stability. According to the results, the developed framework is capable of performing other multi-disciplinary optimization challenges.

Estimation of blade loads for a variable pitch Vertical Axis Wind Turbine with strain gage measurements

AbstractThe blade pitch of a Vertical Axis Wind Turbine can have a profound impact on the aerodynamic loading experienced by the turbine. This in turn impacts the structural loads and the performance of the machine. In order to characterize the effects of changing pitch, studies are conducted with fixed pitch offsets from a neutral pitch position in the open jet wind tunnel of TU Delft. Measurements with strain gages bonded to the turbine struts are used to estimate the normal loading of the blades. The measured behavior gives insights into the sensitivity of the turbine loading to the blade pitch angle. Aerodynamic phenomena associated with VAWTs are evident in the data including dynamic stall and blade vortex interaction. Shifting of turbine blade pitch is shown to alter the azimuthally varying normal loading, causing changes in magnitude and direction of rotor thrust. Frequency responses of the turbine and platform mounting structure are presented for rotating and fixed reference frames, respectfully. The effects of stall due to high pitch offsets is shown to excite higher per rev frequencies in both the rotor normal measurements and platform accelerations. The data sets are made available for validation of numerical models.

Estimation of blade loads for a variable pitch vertical axis wind turbine from particle image velocimetry

AbstractThis paper presents the flow fields and aerodynamic loading of a two bladed H‐type vertical axis wind turbine with active variable pitch for load and circulation control. Particle Image Velocimetry is used to capture flow fields at six azimuthal positions of the blades during operation, three upwind and three downwind. Flow phenomena such as dynamic stall and tower shadow are captured in the flow fields. The phase‐averaged velocity fields and their time and spatial derivatives are used to calculate the normal and tangential loading at each position for each pitching configuration using the Noca formulation of the flux equations. The results show the effect of load shifting from the upwind to downwind region of the actuator using pitch and the effects of dynamic stall on the blades. The results also provide an unique database for model validation.

Experimental and Numerical Evaluation of Performance of a Variable Pitch Vertical-Axis Wind Turbine

Abstract The research work depicts the study of the comparison of a 1 kW fixed pitch vertical-axis wind turbine (VAWT) and a variable pitch VAWT via analytical, numerical, and experimental results. Being an emerging technology, wind turbine is becoming a source of attraction for the researchers. The VAWT in comparison to the horizontal-axis wind turbine (HAWT) has shown numerous benefits. The fundamental purpose of this study is to maximize the output power and output torque of the wind turbine. For achieving an improved output, a novel and unique mechanism, termed as pitching mechanism, is employed that follows the variable pitch concept. The mathematical modeling was performed for the straight blade variable pitch VAWT and for the fixed rotor. The four-bar mechanism was developed to execute the variable pitch mechanism and implemented in the form of the computer-aided design, model. A scaled down three-dimensional (3D) model of the rotor was manufactured using the 3D printing technique. The aerodynamic forces such as lift and drag were measured upon the rotor as per the testing on the rotor in the wind tunnel. Computational fluid dynamics (CFD) simulations were run for the fixed pitch and the variable pitch rotor. The transient analysis was performed for the azimuthal angle ranging from 0 to 360 deg and for a pitch angle varying from +25 to −25 deg in ansys software. The comparative study was undertaken, keeping in view the analytical, simulation, and experimental results. A worthy agreement was observed among analytical, software, and experimental results, and a promising increase in power and torque was observed due to the introduction of the variable pitch mechanism. The power produced by the variable pitch design showed a significant increase in the power production compared to the fixed pitch design. The numerical and experimental values of cp for the variable pitch design were quite comparable.

Numerical analysis of the effect of the mutual interaction between closely separated variable pitch vertical axis wind turbines

Recent researches have proven that mutual interaction between vertical axis wind turbines (VAWTs) results in enhancement in the average power coefficient. Efficient VAWT clusters have been created to provide wind farms having higher power densities compared to conventional horizontal vertical axis wind turbine farms. The created clusters adopted fixed pitch VAWTs in studying the mutual interaction in close vicinity. This paper extends the investigation of the mutual effect between variable pitch VAWTs in closely oriented turbine clusters. A numerical study is performed using commercial Fluent ANSYS code in order to study the effect of gap distances, phase shifts, and oblique angles for co-rotating and counter-rotating arrangements of two variable pitch VAWTs. The results showed improvement in the performance of two turbine clusters up to 26% compared to isolated turbines. Three turbine clusters are also tested numerically based on the results of the two turbine clusters. The created three turbine cluster represents a basic unit to construct a more efficient wind farm. The results of the developed three turbine cluster showed an increase in the average power coefficient by 38% higher than that of isolated turbines.

CFD analysis of the angle of attack for a vertical axis wind turbine blade

The Angle of Attack (AOA) of the Vertical Axis Wind Turbines (VAWTs) blades has a dominant role in the generation of the aerodynamic forces and the power generation of the turbine. However, there is a significant uncertainty in determining the blade AOAs during operation due to the very complex flow structures and this limits the turbine design optimization. The paper proposes a fast and accurate method for the calculation of the constantly changing AOA based on the velocity flow field data at two reference points upstream the turbine blades. The new method could be used to calculate and store the AOA data during the CFD simulations without the need for extensive post-processing for efficient turbine aerodynamic analysis and optimisation. Several single reference-points and pair of reference-points criteria are used to select the most appropriate locations of the two reference points to calculate the AOA and It is found that using the flow data from the two reference points at the locations 0.5 aerofoil chord length upstream and 1 chord away from each side of the aerofoil can give most accurate estimation across a range of tested AOAs. Based on the proposed AOA estimation method, the performance of a fixed pitch and the sinusoidal variable pitch VAWT configurations are analysed and compared with each other. The analysis illustrates how the sinusoidal variable pitch configuration could enhance the overall performance of the turbine by maintaining more favourable AOAs, and lift and drag distributions.

Effect of solidity on the performance of variable-pitch vertical axis wind turbine

The Darrieus vertical axis wind turbine (VAWT) has been the subject of a large number of recent studies to improve self-starting capability and aerodynamic performance. This study presents a computational fluid dynamics simulation of the vertical axis wind turbine with fixed and variable pitch at different solidities. In order to introduce the best efficiency value of solidity for a variable-pitch vertical axis wind turbine, a computational modeling of two-dimensional transient flow around a VAWT at solidities between 0.2 and 0.8 and turbine with two, three and four blades is conducted. The numerical simulation models the flow field around a turbine rotor by solving the strong nonlinearity of URANS and SST k-ω turbulence model, utilizing the semi-empirical numerical model. To simulate the turbine in a variable pitch method, the user-defined function (UDF) code and the moving mesh method are used. The results show that variable pitch blades with high solidities are preferred when the initial self-starting torque is required. Moreover, the variable pitch blades are capable to produce more torque at high solidities. Therefore, the vortex interaction between upwind vortices and blades in downwind stream is decreased. The results verify that the power reduction of fixed pitch VAWT at high solidities could be solved by utilizing the variable pitch technique.

Predicted and measured performance of a vertical axis wind turbine with passive variable pitch compared to fixed pitch

The performance of a 5-m diameter Darrieus vertical axis wind turbine was predicted using both a double multiple streamtube model and a two-dimensional unsteady Reynolds-averaged Navier–Stokes computational fluid dynamics simulation with constant rotational speed for a series of operational points. The actual performance was measured in both fixed and variable pitch modes. The aims were (1) to compare starting torque and peak efficiency in fixed and variable pitch modes and (2) to test an overspeed control mechanism. Starting torque was approximately three times higher in variable pitch mode and the maximum efficiency on some runs was significantly higher. The overspeed control mechanism functioned consistently as designed. Thus, variable pitch was shown to overcome two major disadvantages of normal fixed pitch vertical axis wind turbines, self-starting and overspeed control. Discrepancies between the predicted and measured results showed the importance of accurately assessing parasitic drag losses and the need for three-dimensional simulation to give reliable performance predictions.

A new approach for optimization of Vertical Axis Wind Turbines

A new approach has been developed for the optimization of Vertical Axis Wind Turbines (VAWTs). The approach is derived from double multiple streamtube theory (DMST) and adopts the concept of “Representative streamtube” wherein the entire aerodynamic property of the VAWT is assumed to be represented by a single streamtube occupying a specific azimuthal location. Five input parameters namely; power, wind velocity, aspect ratio, air viscosity and air density and six output parameters that are minimally required for the construction of a straight bladed, constant pitch VAWT are considered in the study. The discrete data of lift and drag coefficients pertaining to specific values of Reynolds number and angle of attack are arranged in the order of decreasing lift to drag ratio and starting with the first set of coefficients, a check is made if it is eligible to become Representative streamtube for the particular problem. The check is done through a tri-directional “Demand Factor” test that seeks the compatibility of Reynolds number and angle of attack values of the aerofoil data under consideration. The first set which gains the eligibility to become “Representative streamtube” marks the solution for the particular problem.

Geometrical Parameters Influencing the Aerodynamic Efficiency of a Small-Scale Self-Pitch High-Solidity VAWT

In this paper, four key design parameters with a strong influence on the performance of a high-solidity variable pitch vertical axis wind turbine (VAWT) operating at low tip-speed-ratio (TSR) are addressed. To this aim, a numerical approach, based on a finite-volume discretization of two-dimensional (2D) unsteady Reynolds-averaged Navier–Stokes (URANS) equations, on a multiple sliding mesh, is proposed and validated against experimental data. The self-pitch VAWT design is based on a straight-blade Darrieus wind turbine with blades that are allowed to pitch around a feathering axis, which is also parallel to the axis of rotation. The pitch angle amplitude and periodic variation are dynamically controlled by a four-bar linkage system. We only consider the efficiency at low and intermediate TSR; therefore, the pitch amplitude is chosen to be a sinusoidal function with a considerable amplitude. The results of this parametric analysis will contribute to define the guidelines for building a full-size prototype of a small-scale wind turbine of increased efficiency.

Modeling, Simulation, Hardware Implementation of a Novel Variable Pitch Control for H-Type Vertical Axis Wind Turbine

Abstract It is well known that the fixed pitch vertical axis wind turbine (FP-VAWT) has some disadvantages such as the low start-up torque and inefficient output efficiency. In this paper, the variable pitch vertical axis wind turbine (VP-VAWT) is analyzed to improve the output characteristics of FP-VAWT by discussing the force of the six blade H type vertical axis wind turbine (VAWT) under the stationary and rotating conditions using built the H-type VAWT model. First, the force of single blade at variable pitch and fixed pitch is analyzed, respectively. Then, the resultant force of six blades at different pitch is gained. Finally, a variable pitch control method based on a six blade H type VP-VAWT is proposed, moreover, the technical analysis and simulation results validate that the variable pitch method can improve the start-up torque of VAWT, and increase the utilization efficiency of wind energy, and reduce the blade oscillation, as comparable with that of FP-VAWT.

Small Vertical Axis Wind Turbines: aerodynamics and starting behavior

In urban areas the wind is very turbulent and unstable with fast changes in direction and velocity. In these environments, the use of small vertical axis wind turbines (VAWT) becomes increasingly attractive due to several advantages over horizontal axis wind turbines (HAWT). However, such designs have received much less attention than the more common propeller-type designs and the understanding of same aspects of their operation remains, to this day, incomplete. This is particularly true of their starting characteristics. Indeed, same authors heuristically maintain that they cannot start without external assistance. This paper reviews the cause of the inability of the low solidity fixed pitch vertical axis wind turbines to self-start, and investigates the way of overcoming this draw back.

Double Multiple Streamtube Model and Numerical Analysis of Vertical Axis Wind Turbine

The present paper contributes to the modeling of unsteady flow analysis of vertical axis wind turbine (VAWT). Double multiple streamtube (DSMT) model was applied for the performance prediction of straight bladed fixed pitch VAWT using NACA0018 airfoil at low wind speed. A moving mesh technique was used to investigate two-dimensional unsteady flow around the same VAWT model with NACA0018 airfoil modified to be flexible at 150 from the main blade axis of the turbine at the trailing edge located about 70 % of the blade chord length using fluent solving Reynolds average Navier-strokes equation. The results obtained from DMST model and the simulation results were then compared. The result shows that the CFD simulation with airfoil modified has shown better performance at low tip speed ratios for the modeled turbine.

Features to aid or enable self starting of fixed pitch low solidity vertical axis wind turbines

This paper reviews the cause of the inability of low solidity fixed pitch vertical axis wind turbines to self start, and investigates a number of ways of overcoming this problem. One approach is only applicable to variants of the Musgrove VGVAWT and involves changes of blade geometry. Another approach is to utilize blades having aerofoil sections that strongly exhibit the laminar separation bubble phenomenon below the stall and that have low immediate post-stall drag. Evidence is given that VAWT's can be made to consistently self start.