The aerodynamic effects of blade pitch angle on small horizontal axis wind turbines

Innovative design method and experimental investigation of a small-scale and very low tip-speed ratio wind turbine

Increasing the solidity in vertical axis wind turbines (VAWT) leads to the decreased coefficient of performance (COP) despite the improved start-up performance. To overcome this problem, the pitch regulation system is proposed in this paper for increasing the solidity. In most of the previous investigations, the effect of pitch angle was tested on low-solidity VAWT at uniform flow conditions and low turbulence intensity in wind tunnel test sections, which are different from the real conditions. In this investigation, the influence of pitch angle on the aerodynamic performance of a small Darrieus-type straight-bladed high-solidity VAWT equipped with a pitch regulation system is investigated numerically and experimentally under realistic condition. The proposed numerical procedure is validated through experimental test results. The COP is measured and calculated at different tip speed ratios and two pitch angles of 0 and 5°. The results reveal 25% enhancement in maximum COP with the increase of pitch angle up to 5°. Moreover, according to the numerical results, higher accuracy can be obtained at lower tip speed ratios for both pitch angles. Then, the numerical method is employed to calculate the power (performance) and torque coefficients as a function of Azimuth position as well as the flow field in rotor affected zone and lateral distance. It is found that increasing the pitch angle at a constant tip speed ratio is followed by accelerated vorticity generation, occurrence of maximum COP at lower tip speed ratio and smoother velocity profile in lateral distances of the rotor.

In the pursuit of renewable energy, wind power plays a key role in reducing fossil fuel dependency and mitigating climate change. Vertical Axis Wind Turbines (VAWTs) have gained interest due to their adaptability in various environments. Unlike Horizontal Axis Wind Turbines (HAWTs), VAWTs capture wind from all directions, making them ideal for urban and offshore locations. However, their power efficiency, represented by Power Coefficient (CP), tends to be lower due to complex aerodynamics. Studies emphasize the need for continued pitch angle optimization research to improve VAWT performance [1]. 2D and 3D CFD simulations were applied to analyze aerodynamic flow in VAWTs [2]. The study in [2] served as the benchmark for this research, utilizing ANSYS Fluent to model flow patterns. Although the k-ε model is not ideal for strong pressure gradients, it provides faster convergence [3]. The effect of pitch angle (β) on VAWT efficiency was explored in [1], showing a 6.6% CP increase with a -2° pitch angle at a tip speed ratio (λ) of 4 for NACA 0015 airfoils. Their work highlights the need for further optimization research to make VAWTs competitive with HAWTs. These findings were backed by [4] and [5]. A five-parameter model was developed to explore optimal pitch control and created a Variable Blade Pitch Automatic Optimization Platform (VBPAOP) integrating genetic algorithms with CFD simulations [4]. The effectiveness of individual blade pitching and its impact on performance were experimentally demonstrated in [5]. In this research, 2D Unsteady Reynolds Averaged Navier Stokes (URANS) CFD simulations were performed and validated using the results from [2]. The study's novelty lies in the following:

In this paper, the numerical and experimental analysis of the effect of leading-edge protuberances on the performance of small horizontal axis wind turbines (SHAWT) at low Reynolds number was carried out. The wind turbine blades were designed using the blade element momentum theory (BEMT) with wake rotation. The E216 profile was chosen over other airfoils because, in low Reynolds number flow conditions, it gives a high lift-to-drag ratio. The tubercle shapes employed for the study are slot, triangular, and sinusoidal, and their effects on the performance of wind turbine were compared with baseline turbine as well as among themselves. The flow behavior and the influence of pitch angle on the performance of baseline wind turbine were investigated. The numerical simulations were conducted in ANSYS FLUENT R2021, and the experiments carried out in a low-speed wind tunnel were used to validate the results. The numerical equations were solved using a three-dimensional Reynolds-averaged Navier-Stokes equation with a shear stress turbulence (SST) <i>k-ω</i> turbulence model. The output power, torque, and coefficient of power (<i>C<sub>P</sub></i>) values for the baseline turbine increased up to 25° pitch angle and afterwards, a decline was seen. The optimum tip-speed ratio (TSR) was also investigated and found to be 2.67. The pitch angle 25° provides the greatest improvement among all pitch angles examined for the same blade profile. Hence, for the study of different-shaped tubercles (triangular, sinusoidal, and rectangular slot) pitch angle of 25° was considered. Sinusoidal tubercles show a greater lift-to-drag (<i>C<sub>L</sub></i>/<i>C<sub>D</sub></i>) ratio than baseline wind turbines, although there is no substantial difference in <i>C<sub>P</sub></i>. Furthermore, the <i>C<sub>L</sub></i>/<i>C<sub>D</sub></i> for triangular and slotted tubercles is more significant than that of the baseline wind turbine, as is the <i>C<sub>P</sub></i>. When all three tubercles are compared, the slot has the highest <i>C<sub>P</sub></i>, while the sinusoidal wind turbine has the highest <i>C<sub>L</sub></i>/<i>C<sub>D</sub></i>.

Optimizing the aerodynamic design of small-scale horizontal-axis wind turbine (HAWT) blades is essential for enhancing annual energy production, performance, and efficiency. This study employs Computational Fluid Dynamics (CFD) with the ANSYS FLUENT solver to investigate the aerodynamic behavior of a newly designed multi-bladed HAWT, specifically introducing toroidal blades. The primary objectives include understanding the aerodynamics of two and three-bladed toroidal rotors, improving HAWT performance by increasing the power coefficient (Cp) and identifying the optimal looping ratio (C) for elliptic toroidal blade shape. The research involves a detailed comparison between toroidal and conventional turbine blades, utilizing Blade Element Momentum (BEM) theory and a NACA 4412 airfoil for blade sections. The geometry characteristics, such as chord length and twist, are carefully calculated along the blade. A 3D geometric domain is created and appropriately meshed for the rotor. Analysis is conducted on a model representing the rotor at a constant wind speed of 7 m/s and various Tip Speed Ratios (TSR). A parametric study for the toroidal blade focuses on the shape ratio (C), determining the optimum ratio as 0.25. The results reveal a notable 14% increase in the power coefficient (Cp) for the three-bladed toroidal blade, calculated based on the optimum TSR for both basic and toroidal blades. The findings suggest that toroidal blades can effectively enhance small HAWT performance, especially at lower TSR. Overall, this research highlights the potential of toroidal blades to improve the power coefficients of small-scale HAWTs.

In this study, an unsteady aerodynamic simulation is performed to realize the influences of platform surge motion on the aerodynamic performance of a high capacity offshore floating wind turbine. A dynamic model with pitch angle control system is utilized to propose a more realistic model of wind turbine and also achieve the rated condition of the rotor. The transient effect of platform surge motion on power coefficient, thrust coefficient and blade pitch angle also is investigated. The 5 MW NREL wind turbine is selected for the simulations. The unsteady aerodynamic model contains unsteady blade element momentum method, dynamic stall and dynamic inflow models. The in-home aerodynamic code and the control system model are implemented in MATLAB/SIMULINK software. It is revealed that reduction in mean power coefficient at tip speed ratios less that 7 is expected by amount of 12-15 % at surge amplitude of 2m and frequency of 0.1 Hz. For high tip speed ratios, the trend is reverse with respect to fixed-platform case. The mean thrust coefficient is also reduced for many tip speed ratios with maximum loss of 32 %. The mean blade control pitch angle is increased due to the surge motion. Since the influence of changing amplitude and frequency of disturbances depends on the tip speed ratio, therefore the special bound of this parameter is being proposed.

The use of vertical-axis turbines in marine-current applications for electric energy generation is still in early developments, and one of the key factors for assessing the applicability of such technology is the power coefficient. To contribute towards the highly competitive market of renewable energy conversions, the turbine system requires good outcomes in terms of energy yield. In this scenario, one of the main challenges regarding the design process to improve the blade performance is to find the best trade-off between the maximization of the power output and the minimization of the structural loadings.In the current work, the influence of blade pitch angles on the hydrodynamics of a vertical-axis five-blade water turbine has been studied. The pitch angles from −5° to +5° were investigated using Computational Fluid Dynamics (CFD). The simulations were validated against experimental data for the power coefficient collected in a river. Overall, a good agreement was found in terms of computed power between simulations and experiments for a wide range of tip speed ratios. The CFD model was proven to be suitable for exploratory analyses and an optimized design was found, providing a 2.3% higher power coefficient by adopting a pitch angle of +2° compared to the zero-referenced pitch angle. Besides validating with the experiment, the CFD simulations were compared with the results of a vortex model. The effect of different pitch angles on the performance prediction and on the blade and turbine loadings was also discussed. It is becoming vital to develop an understanding of the complex interaction of vertical-axis turbines, especially in tidal-current areas where there is a lack of detailed experimental data.

Offshore wind energy is a renewable energy source that is developing fast. It is considered to be the most promising energy source in the next decade. Besides, the expanding trend for this technology requires the consideration of diversified seabeds. In deep seabeds, floating offshore wind technology (FOWT) is needed. For this latter technology, such as for conventional WT, we need to consider aspects related to performance, aerodynamic force, and forces during operation. In this paper, a two-bladed downwind wind turbine model is utilized to conduct experiments. The collective pitch and cyclic pitch angle are adjusted using swashplated equipment. The fluid forces and moments acting on the rotor surface are measured by a six-component balancing system. By changing the pitch angle of the wind turbine blades, attempts are made to manage the fluid forces generated on the rotor surface. Under varied uniform wind velocities of 7, 8, 9, and 10 m/s, the effect of collective pitch control and cyclic pitch control on the power coefficient and thrust coefficient of FOWT is then discussed. Furthermore, at a wind speed of 10 m/s, both the power coefficient and loads are investigated as the pitch angle and yaw angle change. Experimental results indicate that the combined moment magnitude can be controlled by changing the pitch-angle amplitude. The power coefficient is adjusted by the cyclic pitch-angle controller when the pitch-angle phase changes. In addition, the thrust coefficient fluctuated when the pitch angle changed in the oblique inflow wind condition.

A simple unsteady blade element analysis is used to account for the effect of the trailing wake on the induced velocity of a wind turbine rotor undergoing fast changes in pitch angle. At sufficiently high tip speed ratio, the equation describing the thrust of the element reduces to a first order, nonlinear Riccti's equation which is solved in a closed form for a ramp change in pitch followed by a constant pitch. Finite tip speed ratio results in a first order, nonlinear Abel's equation. The unsteady aerodynamic forces on the NREL VI wind turbine are analyzed at different pitch rates and tip speed ratio, and it is found that the overshoot in the forces increases as the tip speed ratio and/or the pitch angle increase. The analytical solution of the Riccati's equation and numerical solution of Abel's equation gave very similar results at high tip speed ratio but the solutions differ as the tip speed ratio reduces, partly because the Abel's equation was found to magnify the error of assuming linear lift at low tip speed ratio. The unsteady tangential induction factor is expressed in the form of first order differential equation with the time constant estimated using Jowkowsky's vortex model and it was found that it is negligible for large tip speed ratio operation.

For vertical axis wind turbines (VAWTs), the increase of the incoming wind speed higher than the rated value will make the tip speed ratio (TSR) lower and lower, resulting in the blade fatigue load becoming more and more severe and the power coefficient weakening gradually. This paper explores whether varying the pitch with the TSR decrease is necessary for improving the power coefficient and reducing the fatigue load. Specifically, the pitch angle effect on the power coefficient and fatigue load of a VAWT at different TSRs was studied by the computational fluid dynamics method. The results show that the optimal pitch angle in terms of the power coefficient varies with the TSR, which means that varying the pitch with the TSR decrease can improve the power coefficient. Meanwhile, the principle to guide the pitch variation is to avoid flow separation in the downwind zone and minimize the angles of attack (AoAs) in the upwind zone. At the lowest TSR of 1.7 in the present work, varying the pitch from the optimal one in terms of the power coefficient reduced the blade normal force amplitude significantly, which is mainly attributed to avoiding the vortex–blade encounter and minimizing the AoAs in the downwind zone. The vortex–blade encounter at the lowest TSR is an important phenomenon related to the variation of the blade torque and blade normal force and will weaken and disappear with the pitch angle increase.

ABSTRACTThe performance of a 0.9 m diameter model wind turbine using the National Renewable Energy Laboratory S826 airfoil profile has been investigated both experimentally and numerically. The geometry was laid out using blade element momentum (BEM) theory, and a detailed description of the geometry is given here. The design was tested experimentally and gave a peak power coefficient of CP = 0.448 at the design tip speed ratio of λ = 6. After the model tests had been undertaken, numerical calculations were performed by means of fully three‐dimensional computational fluid dynamics (CFD) simulations using a k −ω turbulence model. It was found that the BEM correctly predicts the shape of the power and thrust coefficient curves, with the efficiency coefficient virtually identical to the measurements at the design conditions. At higher tip speed ratios, the performance is over‐predicted. The estimated thrust was, however, consistently too low by a shift of the order of ΔCT ∼ 0.1 in the normal operating tip speed range. The high‐resolution CFD predictions (using about 3.5 ×106 grid points) reproduced the model thrust coefficient almost perfectly, and the predicted power coefficients were also very close to the measurements, although the agreement with the measurements at high tip speed ratios were only marginally better than those from the BEM method. At the design tip speed ratio, the CFD over‐predicted the power coefficient by merely 2%. The good agreement between the measured and computed performance at model scale assures that accurate predictions of turbine performance at full‐scale conditions are also possible with high‐resolution CFD. Copyright © 2011 John Wiley & Sons, Ltd.

The paper analyzes the effect of airfoil thickness, camber and blade pitch angle on the performance of the three-bladed Darrieus wind turbines. The research results show that the increase of airfoil thickness, camber and pitch angle of blade, can improve power coefficient when the wind turbine tip speed ratio between zero and four. The increase of thickness and camber of the airfoil leads to running tip speed ratio range of wind turbine get narrowed, and reduces the power coefficient when wind turbine runs in high tip speed ratio range. When the pitch angle of blade is 1˚, power coefficient reaches the maximum value. Negative pitch angle has a bad impact on power coefficient and even creates negative power coefficients.

The aim of this research work is to modulate the pitch angle of both types of wind turbines based on fuzzy logic control (FLC), as changes in the pitch angle have various functions in horizontal and vertical axis wind turbines. For HAWT, pitch angle control is applied to shield the electrical components of the turbine when the wind speed exceeds the rated speed without shutting down the turbine. FLC is used to control the angular velocity using two inputs and one output with three membership functions for both inputs and output. In VAWT, pitch angle control is applied to boost the performance of the turbine and its self-starting torque. FLC utilizes two inputs and one output with five membership functions for both inputs and output. For both turbine types, FLC produces a control signal that drives the actuator to achieve the desired pitch angle. The dynamics of HAWT and VAWT are simulated by the MATLAB/Simulink to demonstrate the influence of pitch controls on their dynamics. For HAWT, the FLC control has successfully maintained the angular speed of the rotor. The values of tip speed ratio and coefficient of performance are reduced in order to maintain the rotor angular velocity at its rated value. On the other hand, the results showed that the torque produced by the VAWT individual blade has improved with the pitch angle control. In addition, using FLC to control the pitch angle gives enhanced output and higher Cp at low tip speed ratios. Gain schedule PI controller is also used in both HAWT and VAWT for comparative study.

This thesis presents numerical simulations and experiments on the effect of the variable pitch angle of blade rotor on the performance of small vertical-axis wind turbines (Darrieus and Orthopter wind turbine). The power, torque, and flow around blade rotor of VAWTS were analyzed by two-dimensional unsteady computational fluid dynamics simulations using ANSYS Fluent 13.0 program. The Darrieus rotor was composed using three NACA0018 airfoil straight blades. Three different rotor blade configurations were employed which i.e., a fixed-pitch blade with a pitch angle, variable blade pitch angle. The Orthopter rotor blades rotate movement around its own axis a half relative to the main rotor in one revolution. The configurations of different aspect ratio and number of blade (of the Orthopter rotor blade were employed. The effects of the variable-pitch angle, tip speed ratio, and three turbulent models, i.e., the RNG k-e, Realizable k-e, and SST k-ω turbulent models, on the performance of Darrieus wind turbine were investigated. The effect of number of blades, tip speed ratio, and aspect ratio of the Orthopter wind turbine with flat-plate blades rotor were also investigated by numerical simulation using RNG k-e turbulent model. The result predictions of numerical were validated by open circuit wind tunnel experimental data. The numerical simulations results of both a VAWT with variable pitch blade and the orthopter wind turbine had good qualitative agreement with the experiments results. The predictions of performance of using the RNG k-e turbulence model were very close with experimental data. The VAWT with variable-pitch blades has better performance than a VAWT with fixed-pitch blades and by RNG k-e and SST k-ω turbulent models, its can suppress the occurrence of a vortex on its blades at a low tip speed ratio. The performance of the Orthopter is influenced by aspect ratio, number of blade, and tip speed ratio. The highest performance of Orthopter wind turbine at AR=1. The high tip speed ratio lead to reducing torque generation especially at downstream area. The simulations show effects of number of blade on the performance. The high number of blade reduces torque generation of one blade. However, the overall of performance was increased.

Constant speed horizontal axis wind turbines operate at power coefficients different than its maximum due to operation at different tip speed ratios (TSRs). Whereas rotational speed and the site wind speed distribution determine the TSR range of the operation, the pitch angle is significant for the aerodynamic performance of the wind turbine. In this paper, a blade element momentum approach is used to study the power production effects of pitch angle and rotational speed of NREL Phase VI wind turbine at different site wind distributions (characterized by Weibull distributions). It is shown that the best combination of rotational speed and pitch angle always occurs at the same pitch angle, but for different rotational speeds (other than the best one) the best pitch angle may vary. The best rotational speed is found to be dependent on both form and scale factors of Weibull wind speed distribution. Improvements in turbine pitch angle and rotational speed are found to increase generated energy up to four times, but the increasing factor is highly dependent on wind speed distribution: For the studied turbine, high-wind-speeds sites can benefit more from the adjustment of these parameters than low-speed ones.