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Due to growth of energy consumption and depletion of fossil fuels sources, power generation of renewable energy sources such as wind energy has become one of the main interests of researchers. Among different types of wind turbines used for extracting electric power from wind flow, vertical axis wind turbines can be implemented in urban areas and in proximity of energy consumers because of their independence of wind direction, low sensitivity to wind turbulence and lower noise production. In this paper a straight-bladed vertical axis wind turbine has been simulated 3 dimensionally by use of a commercial CFD code. The numerical results have been validated against available experimental data. To improve the performance of the turbine, the effects of blade mount point offset and preset pitch have been investigated. The results show that appropriate blade offset and preset pitch for this case study leads to a 60 and 65 percent increase in the maximum performance coefficient respectively.

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In this paper, three-dimensional boundary layer flows on wind turbine blades as well as separation event have been studied. At first, boundary layer and three-dimensional momentum integral equations were obtained for incompressible flow considering rotation effects. Next, the effects of pitch angle and the angle between the flow direction and rotation vector on the Coriolis terms were applied using geometry factor definition and Blade Element Momentum (BEM) theory. Then, the integral parameters and effective geometry factors on separation positions and stall structure were investigated for a rotating blade. The obtained results show that rotational ratio, aspect ratio and radial position are three basic parameters for separation occurrence and separation and stall can be delayed via controlling them. Moreover, the results show that the area near the root is strongly influenced by rotational effects. In addition, it is concluded that the centrifugal pumping due to rotation decreases the boundary layer thickness and delays separation especially in the near root region and increases the blade aerodynamic coefficients.

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The ratio of lift to drag coefficient in wind turbine blades is within the most important parameters affecting the power coefficient of wind turbines. Due to the performance of Magnus wind turbines in low speed air flow; such turbines are attractive for research centers. In the present work, a new geometry for the blades of Magnus wind turbines is defined. The defined geometry is based on the geometry of a Treadmill with a difference that the diameter of its leading circle is greater than that of its trailing one. In the present work, the body is supposed to a low speed air flow while a tangential velocity is applied to the airfoil surfaces and then, its effect on the lift and drag coefficient is studied by numerical method. The effect of generated tangential velocity on the surfaces is investigated for different air flow speed and attack angles and then, its results are compared with that for stationary surfaces. The results show that generating tangential velocity along the surfaces caucuses the lift and drag coefficients and, their ratio to be varied, greatly. By the tangential movement of the surfaces, the maximum ratio of lift to drag coefficient occurs in zero attack angle which is equal to 109. Moreover, maximum magnitude of lift to drag coefficient for attack angles 5, 10, and 15 degrees are 81, 64, and 57; respectively.

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In the recent years, wind energy had a faster growth compared with the other renewable energies. The interaction between fluid and structure becomes more important as the wind turbine size and its power production capacity increases. In the present research, the effect of wind speed and blade materials on static deformation of a small size horizontal axis wind turbine blade has investigated. The shaft torque and root flap bending moment values obtained from simulation are in a good agreement with experimental data. Results demonstrated that the deformation of the blade increases as the wind speed grows although the increase rate has declined in the mean wind speed range because of the occurrence of separation phenomenon on the blade surface. The effect of blade components materials on blade deformation was investigated and the least deformed configurations were introduced. The thickness of the designated blade components has been investigated by means of the maximum strain theory. The final thickness of the skin, spur and root was estimated by 2.1 mm, 2.8 mm and 10 mm respectively which are 30% less than the primary one.

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Darrieus turbine is a type of vertical axis wind turbines that unlike it's simple structure, behavior analysis is a hard computational task. Because of the complex flows around the machines, aerodynamic optimization problem that still remains an open question. In this paper, a numerical algorithm based on the Double Multiple Stream tube model is used to calculate the effect of the parameters that influence the efficiency of the Darrieus turbine. This method is a semi-empirical method using lift and drag coefficients obtained from experimental data. The comparison between the results of the present study with the experimental measurements shows that although the developed algorithm gives acceptable results, but, for higher rotational speeds gets than nominal rotational velocity, the model accuracy gets lower. The aim of this paper is to find optimal conditions, parametrically analyze the effect of blade thickness, solidity, Reynolds number, pitch angle and aspect ratio on turbine efficiency and start. The results show that increasing thickness, Reynolds number and solidity cause an increase in the turbine self-start capability. On the other hand, increasing the solidity of the turbine will reduce working range, and increasing the aspect ratio will increase efficiency especially at the nominal rotational velocity. The results also show that the designed turbine having variable solidity, can have the benefits of both low and high solidity turbines simultaneously. But manufacturing variable thickness blades doesn't have proper justification. Limited increase in pitch angle can also have positive effect on efficiency.

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In this study, the self-starting of a Darrieus vertical axis wind turbines (VAWT) is enhanced using a J-Shaped airfoil profile. The paper investigated the performance of VAWT with the J-shaped blades. Since the J-shaped blades utilize the lift and drag forces simultaneously, the turbine performance at low tip speed ratios (TSRs) enhances. Thus, it is expected that using these blades improves the starting torque and output power. The main goal in this study is to find an optimum J-shaped profile acquiring the best performance of wind turbine. For this purpose, a 3kW J-Shaped straight-bladed Darrieus type VAWT is investigated numerically using OpenFOAM computational fluid dynamic package. It employs the finite volume method to solve the Navier-Stokes equations. The J-Shaped profile is designed by means of eliminating a fraction of pressure side of Du 06-W-200 airfoil. The results indicate that the performance of turbine is optimized for J-shaped profile which eliminates the pressure side of airfoil from the maximum thickness toward the trailing edge. Moreover, employing this J-Shaped profile, the wind turbine performance is intensified TSRs and self-starting of turbine is improved.

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The Modal analysis is one of the applicable methods used to identify the dynamic characteristics of structures. Inspection of structures to avoid resonance conditions can be achieved by extracting vibration modes using modal analysis. Since every point of the vibrating structure has its own characteristics such as the displacement, speed and acceleration, therefore the measurement of these parameters in a specific time interval can be used to extract modal parameters. In this study, stereo vision as a non-contact measuring system is used to obtain the displacement of several points of the blade of a 2.5kW wind turbine with a length of 3m under the operational modal condition. At first, the camera calibration process is performed and then the three-dimensional data of the turbine blade are extracted from images recorded during the test. Consequently, modal parameters of the blade are calculated by analyzing the data. Finally, modal parameters obtained by three different methods including the stereo vision system, the finite element analysis and the testing accelerometer are compared. The results show that visually obtained data are sufficiently accurate to find the natural frequency of the first mode of the blade. The first natural frequency mode extracted by the stereo vision System shows a difference of 10.36% and 2.67% compared to the those obtained by finite element method and the accelerometer respectively.

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In this paper, in order to improve the control performance and increase the efficiency of Vestas 660 kW wind turbine, a research based on theory and practice, using real data is done, and a state feedback controller is designed. The actual data obtained from Binalood wind power plant, show that these turbines have a low efficiency. This is due to the poor performance of the classical controller in tracking maximum power in the partial load area, and the significant error in measurement of wind speed. In this research, to solve these problems, a state feedback controller is designed which improves the turbine performance. In this controller, in order to control the generator torque, feedback of generator speed and aerodynamic torque are taken. Also, using the rotor speed and aerodynamic torque, the wind speed is estimated much more accurately than it is measured by an anemometer. Since, an accurate model of the system is needed for controller design and simulation, wind turbine modeling is done in different subsystems, and its parameters are identified using real data. Simulations performed in MATLAB, indicate the improvement of the system performance with the designed state feedback controller, compared to the classical controller in the actual wind turbine

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In this paper, equations of motion for a horizontal axis wind turbine with movable base are extracted and natural frequencies and vibration of the system is studied. The wind turbine tower is assumed rigid while its blades are modeled as flexible beams. In-plane bending and twisting are considered as two degrees of freedom for blades motion.The shaft connected the tower to blades is assumed rigid and its rotational velocity is considered.In this paper, specifically, a 5-megawattfloating horizontal axis wind turbine, which it’s basehas three angular velocities in different directions,is studied.Due to the complex shape and variation of the properties along the length, the turbine blade properties such as mass and geometric parameters are extracted by curve fitting in MATLAB.The equations of motion and boundary conditions are derived by Hamilton's principle and then are transformed to ordinary differential equations by Galerkin method. By setting the governing equations to standard form (space state), eigenvalues and frequencies are calculated. The numerical results are compared with published results and good agreement is observed.Then the effect of various parameters on turbine blades frequencies and time responses are demonstrated. Results show that the tower base angular velocity and blades rotational speed have considerable effects on turbine blades time response and vibration frequencies.

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Wind turbines are subject to various unsteady aerodynamic effects. This includes the wind gust and the change of wind direction. In this work, the aeroelastic behavior of a reference horizontal axis wind turbine has been investigated under different wind gusts and yaw conditions. Unsteady blade element momentum (UBEM) theory and Euler-Bernoulli beam assumption were used to rotor power estimations. To take into account the time delay in aerodynamic loads due to a sudden change in inflow conditions, a dynamic wake model was implemented. The ONERA dynamic stall model was coupled into the BEM theory to improve the aerodynamic loads prediction in the unsteady inflow and yaw conditions. To verify this method, the results in the case of steady-state are compared with the NREL 5 MW wind turbine and in the unsteady case are compared with the Tjaereborg test turbine. The results indicate that sudden change in wind speed causes sharp fluctuations in terms of elastic torsion of the blade and other parameters such as rotor power. Increasing in wind gradient can leads to increasing time delay to a new equilibrium. The increase in yaw angle can be contributed to the rotor power and the periodic loads reduction. The method presented here may facilitate improvements in the controller design for wind turbines.

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This paper presents a fast and efficient aerodynamic optimization method for megawatt class wind turbines. For this purpose WP_Baseline 1.5 MW wind turbine is used as a test case. Modified particle swarm optimization (PSO) algorithm is used in this study. PSO parameteric studies are conducted, to increase both efficiency and speed of optimization cycle. Since in aerodynamic optimization, it is very desirable to limit the number of the variables, in this study geometric 'class function/shape function' transformation technique (CST) is used for blade geometry parameterization and the appropriate order of shape function polynomial is proposed for S818, S825 and S826 airfoils. Improved Blade Element Momentum (IBEM) theory is implemented for wind turbine power output estimation, validated with experimental and Computational Fluid Dynamic (CFD) data of AOC wind turbine. The aerodynamic data needed for IBEM is provided by XFoil software. XFoil output data for pressure coefficient and wall shear stress which are validated against experimental and CFD data, are applied as the aerodynamic input data for IBEM method.
The twist, the chord and 3 types of airfoil for all sections of the turbine blade are optimized using IBEM method. Optimization is performed with realistic constraints to produce feasible geometry. The performance of the final optimized geometry is simulated via 3D steady incompressible Navier–Stokes equations coupled with Transition SST Model CFD simulation to predict the performance improvement. The results show about 4 percent power enhancement for WP_Baseline wind turbine.

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The attenuation of mechanical load is one of the most effective approaches in wind turbine components cost reduction, and improving the control system reduces mechanical loads with minimum effort. In modern wind turbines, electrically-excited synchronous generators are mostly applied in direct-drive structure. In current research, generator field voltage along with the blade pitch angle is employed for tower load reduction in a novel multivariable-adaptive control structure. The controller is designed based on the extracted model with aerodynamic, vibratory and electrical interactions. The centralized multivariable structure is chosen to simultaneously reduce rotor speed fluctuations and tower vibrations. Since the nonlinear wind turbine model is complex, the controller is designed via optimization process. The nonlinear aerodynamic behavior of blades influences the closed-loop performance in different operating condition; therefore controller is adapted to the condition by employing gain-scheduling method. The effects of signal noise, digital control and higher-order dynamics of electrical system might defect the closed-loop stability. The designed controller is implemented on a wind turbine simulator which includes the before-mentioned effects. By comparing the performance of the multivariable adaptive controller with a two input-one output multivariable controller, it is proven that the mechanical loads acting on tower have been greatly decreased.

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The wind is one of developing sources of renewable energy in recent years. Wind power often is unusable at peak times. Therefore, a storage system or backup power is always necessary. In this study, a hybrid system was applied for a wind turbine to provide the reliable power. The hybrid system consisted of four main components: a wind turbine, electrolyzer, hydrogen storage and fuel cell. The extra electricity produced in fewer demand hours by wind turbine was conducted to a hydrogen and oxygen generator system. Hydrogen were stored in a tank. Then, hydrogen was introduced to a fuel cell unit in order to produce electricity at peak times (when the electricity produced by wind power was less than demand). The hydrogen production rate by alkaline electrolysis as well as the electricity production by PEM fuel cell was investigated. The maximum hydrogen produced by the system per hour average was 304 ml and the power produced by the fuel cell was 1008 mW. After the construction of the prototype, a case study for this system was done in Kouhin area.

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In this study Large Eddy Simulation method has been employed in order to investigate the effects of blade rotation direction of downstream turbine in two co-rotating and counter-rotating configurations. The acquired results are in good agreement with presented experimental data in literatures. Counter-rotating configuration is used in order to investigate the effect of blade rotation on the efficiency of downstream wind turbine. The results show that the efficiency of downstream wind turbine is increased about 4 percent without any change in wind farm layout and type of wind turbines. The upstream wind turbine absorbed a portion of wind energy. Hence stream wise velocity is decreased and lateral velocities are increased in downstream direction. The flow behind the upstream turbine is rotated in same direction with downstream turbine in a counter-rotating configuration. This is why the efficiency of downstream turbine is increased in a counter-rotating configuration. The results of the present study show that streamwise velocity profile is almost identical in both configurations, while, lateral velocities are changed considerably. In other words, a better efficiency of wind farm could be due to the lateral velocities. Hence, the efficiency of wind farm could be increased by decreasing the distance between two consecutive wind turbines in a counter-rotating configuration.

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In this paper, boundary layer control technique is investigated on the NREL-5MW offshore baseline wind turbine blade with numerical simulation of linear DBD plasma actuator in a three-dimensional manner. This wind turbine uses pitch control system to adjust its generated power above its rated speed; but below that the controller is not in function. In the current study, operating condition is set such that the control system is off. Plasma actuator consists of two electrode and dielectric material. One of these electrodes is connected with the air and the other one is encapsulated with the dielectric material. When the necessary high-level AC voltage is applied to electrodes, electric field forms around the actuator and an induced wall jet forms with the ionization of the air around the actuator. Electrostatic model is applied to simulate the effects of plasma actuator and the resulted body force is inserted into flow momentum equations. In the present study, three different control cases are studied. Results show that in all cases, using this actuator leads to improvement of the velocity profile in controlled section, which influences on pressure distribution and results in rotor torque increment. Finally, increasing in torque leads to grows in produced power of the wind turbine. The most increment in output power occurs, when the actuator installed near the root of the blade in the spanwise direction and before low-speed region in the chordwise direction.

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When a Horizontal axis wind turbine works under yaw condition, each blade element can be considered as an oscillating pitch airfoil while the free stream velocity oscillates horizontally. The unsteady free stream velocity, which is usually ignored, oscillates with the same frequency as the airfoil oscillations and has a great impact on the periodic forces produced by the airfoil oscillation. In order to study the effects of unsteady free stream
velocity on the aerodynamic loads, a 2D NACA0012 oscillating airfoil at Reynolds number of 135000 has been simulated. In this simulation, reduced frequency, reduced amplitude and the phase difference between the free stream velocity oscillation and the airfoil angle of attack oscillation are 0.1≤k≤0.25 ، 0.2≤λ≤0.8 و ϕ=0 ,π, respectively. Results show that free stream oscillations affect the aerodynamic loads, vortex strengths
and dynamic stall characteristics. The lift force can be increased by more than 7 times than that of static case and 3 times compared to the load from steady free stream velocity. Depending on 𝜙 value, the dynamic stall angle of attack can be advanced 1 degree or delayed by more than 7 degrees by increase of reduced amplitude. Also, increase of k always causes delay in leading edge vortex formation and consequently delay in dynamic stall occurrence.

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In this paper, an optimal Fractional-order Proportional-Integral-Derivative (FOPID) controller is proposed to control an offshore 5MW wind turbine’s pitch angle in above rated speed. The proposed pitch controller regulates the generator angular speed and consequently the generator power to its nominal value without any knowledge of the model. In order to find the parameters of the controller, a hybrid cost function is proposed, which consists of sum of absolute error signal and absolute rate of control signal in three different wind speeds. The wind speeds are chosen in the beginning, middle and at the end of the interval, thus, the optimized controller is able to show an acceptable performance in whole range of wind speeds, without any demand to nonlinear and complex controllers. To this end, the proposed cost function is minimized using three optimization algorithms: Differential Evolution (DE), Firefly algorithm and Particle Swarm Optimization (PSO). In order to evaluate the robustness of proposed FOPID, numerous wind profiles with different speeds and fluctuations are applied and the results are compared with the optimal integer order PID controller. The comparison demonstrates that the proposed FOPID has more effective performance and robustness than optimal integer order PID.

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One of the disadvantages of drag driven vertical axis wind turbines, is low aerodynamic performance of the turbine which is mainly due to adverse torque of the returning blade. A recently introduced design suggests using opening/closing blades for the rotor to eliminate the negative torque of the returning blade. In this study, the aerodynamic performance of the newly proposed turbine has been investigated experimentally and numerically. The experimental measurements are performed in a subsonic open-jet type wind tunnel facility. However, the numerical simulations are performed using the Ansys-Fluent commercial software, using the Multiple Reference Frame model (MRF). The effects of the number of blades (3, 4 and 6-bladed), end plates and turbulence intensity on the torque and power coefficients are examined in details, in several Reynolds numbers. Results show that the new rotor has no negative torque in one complete revolution and the 3-bladed rotor has the best aerodynamic performance, in a manner that, it reaches a maximum power coefficient of 0.21 at TSR=0.5. Although increasing the number of blades decreases the output torque oscillations, it also decreases the average power coefficient of the rotor. Results also show that, Reynolds number does not have significant effects on the average power coefficients of the rotors, in the studied range of Reynolds numbers, 7.7×104 ≤ Re ≤ 1.2×105.

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In researches on ducted wind turbines, in order to consider the effects of the duct, the solution process is dependent on parameters which arise from experimental tests or computational fluid dynamics. In the present study, our goal is to present a method for considering the effects of the duct and hub on the wind turbine enclosed in a duct without needing to costly experimental tests or time-consuming numerical simulations. For this purpose, the potential flow method which requires only lift and drag coefficients as input parameters is used. The surface vorticity method and the lifting line theory based on the Biot-Savart law are implemented as a numerical method to analyze the performance of the ducted horizontal axis wind turbine. The proposed method is programmed in the MATLAB software. The validation is carried out with experimental result of the DONQI horizontal axis wind turbine. The results are in good agreement with experimental data in the literature. The output power of the ducted wind turbine is compared to the same bare wind turbine to show the effect of the duct on the performance of the wind turbine. The power curve is illustrated that the ducted wind turbine produces more power than an unducted wind turbine in the same condition.

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Optimization of the arrangement of turbines with the aim of producing the maximum power in a wind farm is inherently part of continuous and nonlinear problems. In the present study, for the linearization of the Wake constraint and the connection between turbine power and single Wake and discrete models. Also, the criterion of placing a turbine in another turbine has been applied indirectly and linearly. The proposed mathematical model compares to continuous nonlinear mathematical models, while maintaining the advantage of achieving exact optimum, has a lower runtime and higher stability. Comparison of the results of the present study with the results of previous studies suggests that metaheuristics algorithms may not be obtained in absolute optimal answer. In addition to the power output, environmental issues can also affect the arrangement of turbines. As an example, the maximum noise level is applied in the present model. In order to calculate the intensity of sound, Euclidean distance based on the spread of the hemisphere and the effects of atmospheric absorption has been used. According to the results, it can be said that under the conditions under consideration, the noise level can cause a significant reduction in the output power of the wind farm. Therefore, in selecting the field, attention should be paid to the distance to residential areas. In addition, the effect of cell count on the accuracy of the results was investigated. The results show that there is no clear relationship between optimal power and number of cells.