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Vertical Axis Wind Turbines (VAWTs) are suitable for use in populated areas. The main parameter of the design of this type of the wind turbine is the cross sectional profile of the turbine blade. Additionally, the turbine performance and the generated aerodynamic noise of the VAWTs are critical issues that could limit the spreading of these turbines especially in the residential zones. In the last years, some publications investigated the J-shape Darrieus turbine and praised the performance of this design. Therefore, the present investigation introduces a criticism of the J-shape Darrieus Vertical Axis Wind Turbine (J-SD-VAWT) with three-bladed rotor. An accurate comparison has been introduced for three different turbines with different airfoils sections (NACA 0015, NACA 0021 and S1046). The comparison included the standard airfoil design and the J-shape of the same airfoils from two points of views of turbine performance and noise generation. Aero-acoustics and performance of the proposed design was thoroughly investigated using the Computational Fluid Dynamics (CFD). Both the Unsteady Reynolds-averaged Navier-Stokes (URANS) equations and the impermeable Ffowcs Williams and Hawkings (FW-H) equation were simultaneously solved. The effects of J-shape cut ratio, speed ratio, and turbine solidity were investigated. For all configurations, the results indicated that J-shape reduced the performance and did not reduce the turbine noise. • The present study considers J-shape Darrieus wind turbine. • Effect of different J-shape airfoils on the turbine performance is investigated. • J-shape airfoils noise is evaluated and compared with standard turbine. • J-shape design is not recommended for Darrieus wind turbine. [ABSTRACT FROM AUTHOR]

Abstract This paper investigates the effects of lateral wind gusts on the aerodynamic performance of vertical axis wind turbines. A synthetic approach coupling computational fluid dynamics (CFD) simulations and the double multiple streamtube method is utilized to calculate vertical axis wind turbine performance. For simulating the wind gust response, the resolved gust approach (RGA) models the gust transport throughout the wind field. The effects of wind gusts on the effective wind velocity, angle of attack, torque and power performance are evaluated. The presence of gusts slightly changes the effective wind velocity and angle of attack. Lateral gusts increase the angles of attack in the upwind half period causing premature flow separations in the flowfield, while suppress flow separation by decreasing the angles of attack in the downwind period. The mean torque and power of the three-bladed turbines are enhanced by the lateral sharp-edge gust of 2 m/s by 5% in comparison to the steady wind. Reducing the number of blades to one and doubling the gust velocity make the performance further enhances by 14.7% and 34%, respectively. Finally, the effects of several gust shapes are investigated and discussed. Highlights • Wind turbine power output under gusts is calculated. • The process of gust passing wind turbines is simulated. • The method to calculate three-dimensional turbine performance is of low cost. • The effects of gusts on wind turbine performance are analyzed comprehensively. [ABSTRACT FROM AUTHOR]

Abstract The wake of upstream wind turbines is known to affect the operation of downstream turbines and the efficiency of the wind farm. In this study, a systematic experimentation on performance and wake spatial evolution was carried out using a wind turbine model varying tip speed ratio, pitch and yaw angles. The change of pitch angle was observed to induce a greater effect on the wake velocity as compared to the tip speed ratio. This is interpreted in terms of "force viewpoint", which describes more quantitatively the relationship between the turbine performance and the wake, as compared to the "power viewpoint", based on the sole energy conversion. The turbine yaw angle is observed to cause not only a decrease in power and thrust, but also an offset and an asymmetry in the wake. The offset, quantified using the spatial distribution of the velocity minima, is modeled analytically. Comparisons of model estimations with the experimental measurements show that the proposed model can acceptably predict the wake offset of a yawed turbine. The observed dependencies of the mean velocity deficit and wake turbulence on power, thrust, and yaw angle, may suggest new derating strategies for wind farm optimization. Highlights • A small-scale turbine model is tested under different operating conditions. • Different tip speed ratios, pitch and yaw angles are explored. • Velocity deficit, thrust and power coefficients are estimated. • A model to predict the wake offset for yawed turbines is proposed and validated. [ABSTRACT FROM AUTHOR]

Abstract Compared with large HAWT, the power coefficient and economical performance of the micro HAWT are inferior. However, due to flexible deployment, rapid installation, easy-to-use and low cost, it is still necessary to carry out research on micro HAWT to improve their performance. As a structure proven to be able to increase the power coefficient of HAWT effectively, the aerodynamic characteristics of the micro counter-rotating HAWT (CRWT) is studied in detail by experiments and LBM-LES simulation in this paper. Firstly, the results show the micro CRWT can enhance the power coefficient by two schemes. One is that the equal diameter counter rotating rotors with certain pitch angle can achieve a maximum power coefficient in a wider range of speed. The other is to use the vice rotor to enhance the power output of the main rotor, whether the vice rotor is in front rotor or rear, the rotation of the vice rotor can significantly improve power coefficient of the main rotor, so as to enhance the power coefficient of the total system. Then the aerodynamic mechanism of the Vice-Main and Main-Vice rotor schemes is further discussed with LBM-LES result in detail. Highlights • Performance and aerodynamics of Micro CRWT are investigated. • The power coefficient of upstream rotor can be effected by the downstream rotor. • The upstream rotor can improve the blade’s lift-drag ratio of downstream rotor. • The improper assumptions of the previous CRWT theoretical model are pointed out. [ABSTRACT FROM AUTHOR]

A numerical wake model based on the vortex filament method is proposed to predict the velocity deficit in the wake of a horizontal axis wind turbine (HAWT). By solving the evolution of the vortex system behind the wind turbine, the model calculates the distribution of the downstream velocity indirectly with very low computational cost. Instead of the usual scheme of the vortex method, the more efficient and mature blade element momentum (BEM) theory is used for the blade aerodynamics and to initialize the calculation of the vortex system evolution. The model is tested by a published wind tunnel experiment of a miniature wind turbine. The numerical results agree well with the experimental data. It is found that the core growth of the vortex filaments due to turbulence mainly dominate the velocity deficit along the downstream distance in the wake. In addition, the generalization of the model to full-scale wind turbines is discussed and within the framework of the present model, a reasonable conclusion can be obtained: wakes of wind turbines with different scales are similar. [ABSTRACT FROM AUTHOR]

This paper presents the development of a comprehensive open-source numerical model to study the dynamic response and load analysis of floating offshore wind turbines. The model accounts for the wind inflow, rotor aerodynamics, multibody structural model of the system, wave and current kinematics, hydrodynamics, and mooring-line dynamics. This coupled simulation tool can be used for analysis, optimization and preliminary design to determine the technical and economic feasibility. Several verification and validation cases are performed to show the correctness of the numerical simulations. The results show that the proposed approach provides an accurate estimate of the wind turbine dynamics and loads. The simulation tool is then applied in the analysis of a 5 MW wind turbine aimed to characterize the dynamic response and to identify potential loads and instabilities resulting from the dynamic couplings between the turbine and the external conditions. This open-source fully coupled aero-hydro-elastic model provides a modular framework to enable investigating a variety of wind turbine configurations, support systems, and mooring lines. Therefore, it is expected that researchers and design engineers worldwide use the model to study, investigate and analyze different aspects of floating offshore wind turbine design, which results is the promotion and advancement of science and technology for floating offshore wind turbines. [ABSTRACT FROM AUTHOR]

The relatively low power coefficient restricts the wide application of vertical axis wind turbines (VAWTs). An effective solution to this problem is to design specific airfoil profiles which directly influence the capture ratio of wind power. The main aim of the present study is to develop an automatic airfoil profile optimization system to improve the power performance of a VAWT. A three-bladed high-solidity VAWT is adopted as the research object with its chord length, blade span and rotor diameter being 0.2 m, 0.8 m and 0.8 m, respectively. The optimization is conducted at a moderate tip speed ratio (TSR) with a value of 1.0 and the method of coupled CFD simulations with genetic algorithms is employed. The following points make this paper different from previous studies: (a) introducing Multi-Island Genetic Algorithm to optimize airfoils for VAWTs; (b) investigating the airfoil as part of the VAWT rather than as a single isolated body with 3D simulations. The results show that the power coefficient of the VAWT equipped with the optimized blades sensibly improves at all TSRs from 0.4 to 1.5 and the maximum growth rate of it occurs at TSR = 0.9 with a value of 26.82%. The integrated optimization system used in this paper provides an effective way to generate suitable airfoil profiles for given VAWTs with the goal to achieve higher power efficiency. [ABSTRACT FROM AUTHOR]

In this study, the effect of deploying microtabs on performance improvement of a horizontal axis wind turbine blade is numerically investigated in three-dimensions. The NREL Phase VI, a stall-regulated upwind wind turbine, is used as the baseline case. Different cases are considered to investigate the effects of spanwise location as well as the height variation of tabs along the blade span, on the flow over the rotor blade. In all cases, the tab is located at 95% chord of the airfoil section on the lower surface of the blade. Results reveal that locating microtabs at the outboard part of the blade has a greater impact on the rotor performance than the inboard part. However, both cases improve the output power in comparison with the baseline blade. Furthermore, due to the variations in Reynolds number and local angle of attack along the blade span, the influence of microtab height profile on output power is explored in details. In the best case, 17% of the wasted wind energy in the baseline blade is saved for the sub-rated wind speeds. [ABSTRACT FROM AUTHOR]

Due to the time-varying wind direction, yaw operation of wind turbines is a common state. Under yaw, the aerodynamic behavior of wind turbines is complicated, and also causes complex energy capture performance. To clarify some vague knowledge, a detailed investigation of yaw effect based on SCADA data is carried out. Firstly, a yaw coefficient definition and its specific calculation method are presented. Furthermore, to analyze the energy capture mechanism, the loss factor of energy capture (power coefficient) is subdivided into aerodynamic loss factor and inertia loss factor, which means both the effects of aerodynamic characteristic and mechanical inertia on energy capture are considered. According to this understanding, power coefficient is expanded as a function of four factors. Then, a single-valued processing method is employed to investigate the relationship between wind speed and output power. Subsequently, relationship between yaw coefficient and output power is investigated. Comparative investigations are carried out by two different methods, that is, least square fitting (LSF) method and kernel density estimation (KDE) method. Also, characteristics of power coefficient and rotor torque under yaw are investigated. Effect laws of yaw coefficient on wind turbine power, power coefficient, and rotor torque are obtained. Finally, the relationship between the internal control mechanism and external output of wind turbines is discussed. [ABSTRACT FROM AUTHOR]

Modelling wind profile is crucial to the adequacy assessment of wind-integrated generation system. This paper characterizes the wind profile as a wind vector that consists of wind speed and direction. A wind vector simulation model is proposed to produce long-term wind vector samples, whilst maintain the probability distribution of actual wind vector as well as its gusty characteristics. Such model is incorporated into sequential simulation process of wind-integrated generation system, while the wake effect is considered with Jensen model. Then, adequacy assessment procedure considering wake effect is developed. Collected wind vector data from four distinctive sites in North Dakota in US are used to verify the proposed model. The wind-integrated IEEE Reliability Test System (IEEE-RTS) is used to demonstrate the application of the proposed model and the procedure to adequacy assessment. The impacts of wake effect, peak load, and wind turbine type on system adequacy are investigated in detail. [ABSTRACT FROM AUTHOR]

Due to the growing importance of wind power as a clean and renewable energy source, the use of small-scale wind turbines in urban environments has increased lately. The blade pitch control is an effective method to improve the aerodynamic response of a wind turbine, usually applied to large-scale wind turbines. This study presents the effects of varied blade pitch on the aerodynamic performance of a small-size wind turbine. The blades were sketched out according to the Blade Element Moment (BEM) theory, applying the aerodynamic profile NREL-S809 and designed for a tip speed ratio of eight. To analyze the influence of the blade pitch angle on the energy conversion, a comparative study was carried out varying the pitch angle to five different values. Using the analysis of variance (ANOVA), it was possible to demonstrate that blade pitch control could be an effective method also for small-sized wind turbines. A performance chart from the results of blade pitch experiments shows that the power coefficient varies significantly when the angle changes. As conclusion, it is highlighted that an enhanced behavior could be attained by the use of a pitch angle controller resulting in a better recovery of the energy available in the wind. [ABSTRACT FROM AUTHOR]

This paper investigates the power performance and wake characteristics of a straight-bladed Vertical Axis Wind Turbine (VAWT) with field experiments. In order to observe the aerodynamic forces characteristics, the power performances measured by a torque meter are compared between wind tunnel and field experiments. The values of reference wind and wake velocities are measured through three cup-type anemometers and ultrasonic anemometers. As a result, it is clarified that field experiment result of power coefficient shows higher value than that of wind tunnel experiment at low tip speed ratio in the case of high turbulence intensity. Furthermore, the turbulence intensity has an inhibitory effect on power performance at optimal and high tip speed ratios. The maximum velocity deficit is in the position of −0.2 < y/D < 0. Meanwhile, the recovery process in the velocity deficit region is asymmetric. The predictions of wake velocity distributions in different inflow velocities are also investigated with Gaussian function model. It is estimated that wake velocities recover near the positions of x/D = 3.3 and 4.5 in the cases of U ref = 5.0 and 9.0 m/s. The results have a guiding significance for comprehending the development process of straight-bladed VAWTs. [ABSTRACT FROM AUTHOR]

To enhance the performance of horizontal axis wind turbines, it is proposed to place a cylindrical disc in front of the rotor in order to lead the incoming flow from the inner part to the outer part of the rotor blades. This is expected to increase the power output, as the kinetic energy is mainly captured at the outer part of the blades, where the relative wind speed is high. To assess the impact of this novel design idea, a hybrid numerical technique, based on solving the Reynolds-averaged Navier-Stokes equations, is utilized to determine the aerodynamic performance. The in-house developed EllipSys3D code, which is employed as basic numerical solver, is combined with an actuator disc representation of the wind turbine rotor and an immersed boundary technique for representing the upstream cylindrical disc. The impact of the disc on the rotor performance is assessed by systematically changing the size of the circular disc and its axial distance to the rotor. Based on a numerical study of a Megawatt size commercial wind turbine, it is found that up to 1.5% additional energy can be captured by placing a circular disc with a suitable diameter upstream of the rotor plane. [ABSTRACT FROM AUTHOR]

Wind energy plays a vital role in the development sustainable energy due to its vast availability, commercially ready technology, low cost, and great contribution to CO 2 reduction. Wind turbines account for most of the cost involved in both onshore and offshore wind projects. Therefore, wind turbine aerodynamics is the backbone in the wind energy area as it is directly related to its performance. We first review and discuss available engineering models and the Reynolds-Averaged Navier Stokes (RANS) methods for wind turbine aerodynamics. The widely used momentum method is restricted significantly by the availability of reliable airfoil data and its empiricism. Potential flow methods are limited by its exclusion of viscous effect. RANS methods can produce reasonable integrated quantities, but fail to capture complex flow features such as separation and vortex shedding. The hybrid RANS-LES method (HRLM), which is a technique to bridge the gap between less accurate RANS and more computational costly LES method, is a remedy to turbine aerodynamics in complex flow conditions. We then present existing HRLMs and review their applications to wind turbine aerodynamics. They have obvious advantages over the RANS models in the prediction of flow unsteadiness. Finally, we recommend the best practice guidelines for the HRLM to facilitate and promote the implementation of the HRLMs for an improved understanding of flow physics around wind turbine blades. The enhanced knowledge of complex flow characteristics will further benefit subsequent aeroelastic and aeroacoustic analysis. Therefore, the HRLM is a promising tool for the research and development of wind turbine technology. [ABSTRACT FROM AUTHOR]

India is rapidly growing in the economy and population leading to a continuous increase in electricity demand. The wind is a better energy source because of its negligible greenhouse gas emissions, cost competitiveness, price stability, and energy security. This paper presents Geospatial Information System (GIS) based methodology to characterise the offshore wind power potential in the western and the eastern coast of India with technical, economical, and marine ecosystem consideration. Spatial distribution of levelized production cost (LPC) and cost-supply curves was developed under different ocean conditions (human impact on marine ecosystem). The influence of project lifetime, discount rate, and assumptions about investment cost, annual energy production, and availability is presented using sensitivity analysis. Results show that total potential available along the shallow waters (50 m water depth) of Indian coast is approximately equal to 12.8% and 42% respectively of the existing renewable power and wind power capacity. In terms of wind resources and economic costs, the shallow waters of Gujarat and Tamil Nadu states are the best suitable regions for offshore wind power development. [ABSTRACT FROM AUTHOR]

The aerodynamics of the blade sections constitute the core problem in the design of new-generation wind turbines. Aerodynamic theories for blade design suffer from the unavailability of aerodynamic coefficients for the airfoils involved in the blade. The aim of this work was therefore to develop an efficient and accurate tool for computing the flow parameters, reducing the need for complex and costly wind tunnel tests. Our approach is based on Computational Fluid Dynamics (CFD) and consists in the adoption of a laminar-to-turbulent transition model. For the flow regimes involved in wind energy applications, the fully turbulent or laminar flow model fails completely in the prediction of aerodynamic performance. We consequently propose a Reynolds Averaged Navier Stokes (RANS)-based approach capable of modeling transitional flows with a limited computational cost. This is a crucial aspect because standard RANS models assume a fully turbulent regime. Our proposed approach couples the well-known γ − Re θ , t ˜ technique with the Spalart-Allmaras turbulence model. The effectiveness, efficiency and robustness of the computational method introduced here were tested by computing the flow over several wind turbine airfoils. [ABSTRACT FROM AUTHOR]

Recently, Savonius vertical axis wind turbines due to their capabilities and positive properties have gained a significant attention. The objective of this study is to design and model a Savonius-style vertical axis wind turbine with direct discharge flow capability in order to ventilate buildings. For this purpose, a modeling procedure is defined and validated using available experimental results in literature. In addition, two design modifications, variations in cross-section with respect to the height of rotor and conical shaft in the middle of wind rotor are proposed. The variable cut plane changes the pressure in inner region of rotor and enhances the discharge flow rate. However, this increases the negative torque acting on returning blade thus reducing the power coefficient. The inlet flow to Savonius wind rotor goes along the surface of conical shaft and is diverted to lower pressure in order to improve the discharge flow rate. Results indicate that the twist on Savonius wind rotor reduces the negative torque and improves its performance. According to the results, a twisted Savonius wind turbine with conical shaft is associated with 18% increase in power coefficient and 31% increase in discharge flowrate compared to simple Savonius wind turbine. Also, wind turbine with variable cut plane has a 12% decrease in power coefficient and 5% increase in discharge flow rate compared to simple Savonius wind turbine. Therefore, it can be inferred that twisted wind turbine with conical shaft indicated a proper aerodynamic performance. [ABSTRACT FROM AUTHOR]

A crucial problem of designing thick airfoils is balancing structural and aerodynamic requirements. This paper documented a new idea to deal with the thick airfoil's design. Firstly, the relative thickness of the original airfoil was increased to enhance its structural property. Then the overall aerodynamic performance was improved by the optimization design method. Specifically, this paper put forward a mathematical model of the overall optimization employing airfoil's performance evaluation indicators which represent modern rotor blades' aerodynamic requirements of “high efficiency, low extreme load, wide range of operating angle of attack and stability with varying operating conditions”. Based on this model, an integrated optimization platform for thick airfoils' overall design was established. Through an optimization experiment, a new 35-percent relative thickness airfoil was obtained. The new airfoil was predicted with high design lift coefficient, acceptable maximum lift to drag ratio, moderate stall parameter, and desirable stability parameters. These characteristics contribute to a high overall performance which could be competent with commonly used thick DU airfoils. Lift characteristics of the new airfoil have been validated by tests. These results confirmed the proposed method has effectively balanced airfoil's complicated requirements and successfully improved the new airfoil's overall performance. [ABSTRACT FROM AUTHOR]

Abstract: A new concept in wind power harnessing is described which significantly outperforms traditional wind turbines of the same diameter and aerodynamic characteristics under the same wind conditions and it delivers significantly higher output, at reduced cost. Its first innovative feature is the elimination of tower-mounted turbines. These large, mechanically complex turbines, and the enormous towers used to hoist them into the sky, are the hallmark of today's wind power industry. They are also expensive, unwieldy, inefficient, and hazardous to people and wildlife. The second innovative feature of INVELOX is that it captures wind flow through an omnidirectional intake and thereby there is no need for a passive or active yaw control. Third, it accelerates the flow within a shrouded Venturi section which is subsequently expanded and released into the ambient environment through a diffuser. In addition, INVELOX provides solutions to all the major problems that have so far undermined the wind industry, such as low turbine reliability, intermittency issues and adverse environmental and radar impact. Simulating the performance of this wind delivery system is quite challenging because of the complexity of the wind delivery system and its interaction with wind at the front end and with a turbine at the back end. The objectives of the present work are to model and understand the flow field inside the INVELOX where the actual wind turbine is located as well the external flow field which not only provides the intake flow but also has to match the exhaust flow of the system. The present computations involved cases with different incoming wind directions and changes in the intake geometry. The results show that it is possible to capture, accelerate and concentrate the wind. Increased wind velocities result in significant improvement in the power output. These results led to the design of a demonstration facility which has provided actual data which verified the significantly increased power expectations. [Copyright &y& Elsevier]

Abstract: An innovative design for the Troposkien concept is introduced by means of an advanced chord distribution, computed using the WOMBAT (Weatherly Optimization Method for Blades of Air Turbines) algorithm for the performance optimization of vertical axis wind turbines. Five rotor blade architectures, characterized by a constant value of the thickness-to-chord ratio and a varying chord length along the blade span, are evaluated. The optimization process is conducted with respect to the power coefficient for a target wind speed of 9 m/s, obtaining a consistent improvement of rotor performance with respect to the baseline blade configuration. [Copyright &y& Elsevier]

Abstract: Wind energy represents a very important source of energy for many countries nowadays. Wind energy provides an efficient and an effective solution to reduce fusel fuel consumption as well as pollutant emissions. VAWTs (vertical axis wind turbines) were originally considered as very promising, before being subrogated by the present, horizontal axis turbines. There is now a resurgence of interests for VAWTs, in particular Darrieus turbines. VAWTs (vertical axis wind turbines) like the Darrieus turbine appear to be particularly promising for the conditions of low wind speed, but suffer from a low efficiency compared to horizontal axis turbines. Additionally, VAWTs are not always self-starting, which is a major drawback. The present paper introduces the main problem of the self-stating capability of Darrieus turbine and investigates some techniques to improve this drawback. The effect of the turbine solidity and the usage of hybrid system between drag and lift types have been investigated in this paper numerically using CFD (Computational Fluid Dynamics) technique and experimentally. A considerable improvement of the H-rotor Darrieus turbine self-starting capability can be obtained by these techniques. [Copyright &y& Elsevier]