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Asymmetric airfoils are commonly used in horizontal axis wind turbines, while symmetrical airfoils are currently the focal point of the researches for most vertical axis wind turbines' airfoils studies. The purpose of this paper is to research the influence of asymmetric airfoils on the aerodynamic performance of vertical axis wind turbines. The influence of asymmetric airfoils on the aerodynamic characteristics of vertical axis wind turbines is investigated by numerical simulation method. The symmetric airfoils are chosen as NACA0021, while the asymmetric airfoils are chosen as DU97-W-300. Single-blade, 3-blade, different tip speed ratios, and three wind speeds (7, 8, 9 m/s) are set as the parameters. The wind turbine with symmetric airfoils performed better aerodynamically at high tip speed ratios, whereas the wind turbine with asymmetric airfoils performs well at low tip speed ratios. The wind turbine with asymmetric airfoils has outstanding start ability at low wind speeds. [ABSTRACT FROM AUTHOR]

This study explored the aerodynamics of a new multi-stage co-axial vertical-axis wind turbine based on bio-inspiration from natural swimming habit of fish. The turbine was formed from a conventional straight-bladed vertical axis turbine (VAWT) with an additional small inner rotor, also of three blades. The azimuthal and radial locations of the inner rotor were varied. Using numerical simulations, performance of the proposed new design was evaluated over a range of tip-speed ratios. The preliminary results identified a 600% increase in power output for multi-stage VAWTs at tip-speed ratios TSR < 3 , and a substantial drop in power coefficient at TSR > 3. 0. The wake dynamics analyses revealed that the increase was due to interactions between the blades of one rotor and the other. This reduced the unsteady separation from the outer rotor, which produced most of the power. A detailed parametric study was also completed, which showed the implications of geometric and kinematic details on the performance of the proposed multistage VAWT. [ABSTRACT FROM AUTHOR]

In this work, static and dynamic analyses are carried out on VAWT blade using analytical and numerical methods to figure out reasons behind the failure in the blade for off-shore applications arising due to turbulence of wind. The design flaws and different operating conditions play major role in failure of blades. Fatigue life cycles, natural frequencies of blade in different modes and turbine harmonic frequency have been calculated analytically using Goodman's and vibration analysis theories respectively. In analysis, life cycles, natural frequencies and mode shapes for VAWT blade are studied. Analytical and numerical results have been compared, life cycles and frequencies are determined. The numerical results showed good agreement with theoretical concepts. [ABSTRACT FROM AUTHOR]

Location and geometry optimisations of Straight Plate Upstream Deflector (SPUD) for performance enhancement of a three-straightbladed Vertical Axis Wind Turbine (VAWT) are carried out at three regimes of tip speed ratios (TSRs). Results analysis reveals that SPUD location has significant effect on the power coefficient (Cp) increment of VAWT. Placing two SPUDs both upward and downward in upstream of VAWT can achieve the highest Cp improvement at all regimes of TSRs. Overall, placing SPUD in upstream of VAWT can improve the Cp value at all regimes of TSRs, but the percentage of improvement can vary. The largest Cp improvement of 126.7% is achieved at low regime of TSRs, while at the medium and high regimes of TSRs, Cp improvements decrease to 52.7% and 52.5%, respectively. Increasing the width of SPUD can increase the average VAWT Cp improvement by 2.83% at all regimes of TSRs. [ABSTRACT FROM AUTHOR]

This study presents experimental results of power performance characterizations for of 12 different Savonius turbine blades. There are three different models and the numbers of each blade types have been selected as 2, 3, 4, and 5. Three different blade types are drawn in a CAD program and all blade models are produced from 3D printer. The experiments are carried out in the subsonic wind tunnel laboratory of the Faculty of Aeronautics and Astronautics at Erciyes University. The wind tunnel has 15 kW motor power to produce wind flow. Test zone inlet and outlet section is 0.57 m × 0.57 m and 0.59 m × 0.59 m respectively. Shaft to shaft type torque meter is mounted between turbine and electric motor. Measurements have been made at wind speeds between 1 and 30 m/s. The performance values of each turbine have been measured. While the 3-bladed turbine showed the highest power coefficient values in the whole study, the maximum value was obtained from the modified helical turbine type. [ABSTRACT FROM AUTHOR]

One of the important challenges for Vertical Axis Wind Turbine (VAWT) is to fully understand its dynamic characteristics in different operating conditions. Meanwhile, it is necessary to seek a fast and accurate method to evaluate the dynamic characteristic of VAWT. In this study, we improve the LB model by considering the operating principle of VAWT to study the dynamic characteristics of the dedicated and commonly used VAWT airfoils in different operating conditions. The results show that the improved LB model is suitable for simulating the dynamic characteristic of VAWT with a thick airfoil. Although the asymmetric airfoil shows the higher lift coefficient, their dynamic characteristic appears huge fluctuation as the increases of tip speed ratio. Moreover, at a low tip speed ratio, the advantages of the asymmetric airfoil are not obvious. While the dynamic characteristic of the symmetric airfoil is relatively stable with the variation of tip speed ratio. [ABSTRACT FROM AUTHOR]

This paper addresses the torque ripple problem of vertical axis wind turbines (VAWT) by applying an oscillating bump which was previously optimised with a numerical model. Since the element primarily affects the drag of an airfoil, a low-speed wind-tunnel experiment using the wake survey method was conducted in order to highlight the effect on the drag of a symmetrical airfoil. Measurements were taken along the centreline of the foil to compare with numerically simulated results on a 2D foil. This study provides an experimental data set for an active flow control device. When the stationary results were compared against numerical simulation they showed an overall good agreement. The moving bump increased the drag coefficient for higher frequencies of oscillation and induced less drag compared to the steady maximum distortion. Uncertainties in the experiments were primarily caused by fluctuations in the test section and the data reduction error. Future work should include the measurement of lift in order to determine the influence of the element on the torque. [ABSTRACT FROM AUTHOR]

Vertical Axis Wind Turbine (VAWT) can be a promising solution for electricity production in remote ice prone territories of high north, where good wind resources are available, but icing is a challenge that can affect its optimum operation. A lot of research has been made to study the icing effects on the conventional horizontal axis wind turbines, but the literature about vertical axis wind turbines operating in icing conditions is still scarce, despite the importance of this topic. This paper presents a review study about existing knowledge of VAWT operation in icing condition. Focus has been made in better understanding of ice accretion physics along VAWT blades and methods to detect and mitigate icing effects. [ABSTRACT FROM AUTHOR]

The vertical axis wind turbine (VAWT) configuration has many advantages for an offshore wind turbine Installation. In this paper, the three dimensional (3D) computational fluid dynamics analysis of a large-scale 5 MW VAWT is conducted. At the optimum tip-speed ratio (TSR), the VAWT maximum inline force was 75% larger than the maximum lateral force. It was found the dynamic stall effects cause the VAWT flow field to become increasingly asymmetrical at the mid-span plane, when the TSR is reduced. The attachment of end plates to the blade tips, resulted in a performance improvement during the upwind phase with the average blade torque coefficient in this range being increased by 4.71%. Conversely, during the blade downwind phase a reduction in performance was found due to the increase in drag from the end plates and the average blade torque coefficient in this phase was reduced by 23.1%. [ABSTRACT FROM AUTHOR]

The design, construction, and development of micro-turbines using 3D printing has been an important advance in the wind energy field to explore the possibility of offering viable alternative for the electric power generation, reducing the pollution caused by fossil fuels. In this work, a five blade vertical axis wind turbine prototype was tested. The components were designed using CAD and the turbine was manufactured by additive technology in a 3D printer. The material employed for 3D printing was commercial polylactic acid. The mechanical properties of the material used were obtained by tensile tests under the ASTM D-638 standard. On the other hand, a static structural simulation was performed by finite element method. Maximum tensile stress safety factor, maximum principal stresses, and fatigue analyses of the main turbine components were computed. The turbine performance as function of rotational velocity and relative wind velocity was analyzed implementing an experimental set-up. [ABSTRACT FROM AUTHOR]

Experimental investigations of wind turbine blades having NACA airfoils 0021 and 4412 with and without tubercles on the leading edge have been performed in a wind tunnel. It was found that the lift coefficient of the airfoil 0021 with tubercles was higher at Re = 1.2×105 and 1.69×105 in post critical region (at higher angle of attach) than airfoils without tubercles but this difference relatively diminished at higher Reynolds numbers and beyond indicating that there is no effect on the lift coefficients of airfoils with tubercles at higher Reynolds numbers whereas drag coefficient remains unchanged. It is noted that at Re = 1.69×105, the lift coefficient of airfoil without tubercles drops from 0.96 to 0.42 as the angle of attack increases from 15° to 20° which is about 56% and the corresponding values of lift coefficient for airfoil with tubercles are 0.86 and 0.7 at respective angles with18% drop. [ABSTRACT FROM AUTHOR]

The present paper investigates the aerodynamic and aeroacoustic characteristics of the H-rotor Darrieus vertical axis wind turbine (VAWT) combined with very promising energy conversion and steering technology; a fixed guide-vanes. The main scope of the current work is to enhance the aerodynamic performance and assess the noise production accomplished with such enhancement. The studies are carried out in two phases; the first phase is a parametric 2D CFD simulation employing the unsteady Reynolds-averaged Navier-Stokes (URANS) approach to optimize the design parameters of the guide-vanes. The second phase is a 3D CFD simulation of the full turbine using a higher-order numerical scheme and a hybrid RANS/LES (DDES) method. The guide-vanes show a superior power augmentation, about 42% increase in the power coefficient at λ = 2.75, with a slightly noisy operation and completely change the signal directivity. A remarkable difference in power coefficient is observed between 2D and 3D models at the high-speed ratios stems from the 3D effect. As a result, a 3D simulation of the capped Darrieus turbine is carried out, and then a noise assessment of such configuration is assessed. The results show a 20% increase in power coefficient by using the cap, without significant change in the noise signal. [ABSTRACT FROM AUTHOR]

Vision 2030 of the Kingdom of Saudi Arabia envisions a shift towards renewable sources of energy. In accordance with this vision, we have developed an off-grid application using a vertical axis wind turbine (VAWT) to power a street lamp in a highway. VAWT are more suited for this application as they are easy to install than the horizontal axis wind turbine, can function with low-speed blades under varying wind speeds, and can prevail in the harsh weather condition prevailing in the Kingdom. MATLAB is utilized to design the circuit and to analyze the state of charge (SOC) of the battery under no load, varying load, and full load. Arduino based weather station and automatic control of street lamp are implemented to conserve energy. The model was tested in the highway and the test indicates the proposed model is able to harness power from wind to power the street lamp. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

An experimental study is presented on the performance of a vertical axis wind turbine with variable blade geometry of the design developed by Austin Farrah. This is experimentally compared with the performance of a correspondingly sized Bach-type Savonius turbine using the same electrical generator and measurement instrumentation in a wind tunnel. Experiments were performed for Reynolds numbers, based on blade chord, in the range 5 × 103 to 1 × 105, and for blade settings between −40° and +40o. The study shows that for the tip speed ratios that have been investigated, the Farrah vertical axis wind turbine design can only marginally outperform a corresponding two-bladed Bach-type Savonius turbine and then only when its blades are set to 40° pitch angle. The presence of a small inner cylinder, which rotates with the turbine, does not enhance its performance due to the fact that it is immersed in an extensive column of relatively static air. [ABSTRACT FROM AUTHOR]

Mast structure is one of the most important parts of a vertical-axis wind turbine which supports generator and rotor and represents one-third of the overall costs in the production of a standard wind turbine (approximately 30%). All this may cause significant economic and physical losses when it is damaged or collapsed. The purpose of this research is to investigate numerically the static strength and structural dynamic responses of 10-kW vertical-axis wind turbine masts subjected to the aerodynamic and gravity loadings (according to the IEC 61400-2:2006 and EN 1991-1-4:2005 standards) using the SolidWorks finite element software. Mast structures with four different heights (12, 14, 16, and 18 m) and three various outer diameters (0.6, 0.7, and 0.8 m), in each height configuration, were evaluated. These analyses were performed to identify the stiffness, resistance, reliability, and natural frequency stiffness requirements within the mast structures, in order to save manufacturing cost. Based on static analysis, no structural failure is predicted for all masts during wind turbine operation according to maximum von Mises stresses at the bottom of the mast and maximum total deflections on the top of the mast. In addition, the dynamic parameters of these 12 models of masts have been studied to obtain the natural frequencies and corresponding mode shapes. Finally, the recommendations to avoid resonance and design strategy for each mast model are discussed. [ABSTRACT FROM AUTHOR]

A simple model for accounting for tower mass when estimating the first-mode eigenfrequency of a semi-guy-wired tower has been derived. This extends previous work where an analytical model of the semi-guy-wired tower of a 200-kW vertical-axis wind turbine was developed. The model was primarily used to estimate the eigenfrequencies as a result of adding guy wires to a free-standing tower (thus creating a semi-guy-wired setup). However, a weakness with the model was that the tower mass was accounted for in a rough way that essentially ignored the guy wires, which gave a larger-than-necessary error. In this work, an effective top mass, that takes into account the tower mass and the constraints from the guy wires, is derived to achieve a higher accuracy when estimating the first-mode eigenfrequency. This, together with the earlier models, gives a more complete method to estimate the eigenfrequencies for a semi-guy-wired wind turbine. [ABSTRACT FROM AUTHOR]

Vertical-axis wind turbines have been confined to small-scale generation in urban environments where their omnidirectional capability offers them an advantage over the more ubiquitous horizontal-axis wind turbine. With a drive towards renewable energy, more opportunities exist for the implementation of wind turbines in a multitude of environments. Based on its inherent operational drawbacks, the vertical-axis wind turbine has not undergone extensive investigation. Recently, there has been a resurgence of interest in the technology. This article addresses the torque ripple, a variation in torque produced by the turbine, present during operation. The variation in torque generated by a vertical-axis wind turbine increases the likelihood of failure due to fatigue. Current treatment is symptomatic and addresses the result of the torque fluctuation and not the cause. A novel blade design, capable of altering the lift and drag response through shape alteration, is presented as a solution. The blade design and operation is achieved through genetic algorithm optimization and computational fluid dynamic simulations. Comparisons with previous work show the novel blade presented here surpasses the reduction seen with pitching solutions. A 25% reduction in torque ripple was demonstrated for a 17% reduction in performance coefficient using the surface distortion approach. This surpasses the foil pitching approach which achieved a 15% torque ripple reduction for the same loss in performance coefficient. [ABSTRACT FROM AUTHOR]

This article analyses the effect of best blade pitching positions on the power coefficient of vertical axis wind turbine at different tip speed ratios. Analysis of the power coefficient of the vertical axis wind turbine is carried out for optimized six blade pitching curves. The first three pitching curves are designed for the tip speed ratios below 1.5 (0 < λ < 1.5) and other three are for the tip speed ratios in the range of 0 < λ < 3. The double multiple stream tube model is used for the present analysis. The results are compared between the optimized and sinusoidal pitching curves. It is concluded that the best optimized pitch position blade method improves the power coefficient than the typical sinusoidal blade pitching in the range of tip speed ratios 0 < λ < 3. [ABSTRACT FROM AUTHOR]

Vertical axis wind turbines have a place in the small scale renewable energy market. They are not currently implemented on a commercial scale but have found a niche space in urban areas. Here, the turbulent wind conditions and limited space are more easily tapped into with a vertical axis wind turbine. However, the challenges facing these types of turbines have hampered deployment. One of these issues is the fluctuating torque experienced during operation, which can lead to over-designed power trains. Genetic- and gradient-based optimization is applied to an analytical model of a vertical axis wind turbine, in order to reduce the torque fluctuation while attempting to maintain a high power coefficient. The reduction in torque ripple is achieved through a sinusoidal pitching motion of the blades. The torque ripple can be reduced by 10% with a similar reduction in power coefficient. [ABSTRACT FROM AUTHOR]

This work presents an innovative technique to enhance the performance of the Savonius wind turbine. The new technique is based on introducing an upstream deflector and downstream baffle. The shape and location of both devices are optimized using a genetic algorithm. The performance of the turbine with the optimized devices is compared with the single Savonius turbine performance. The study employs the finite volume solver (ANSYS-FLUENT) to solve unsteady Reynolds Averaged Navier–Stokes equations and turbulence model equations. The optimized configuration results in much higher power coefficient than the Savonius turbine. The average peak power coefficient using both deflector and baffle is 0.47 compared to 0.24 of the Savonius turbine. The peak power coefficient of the turbine corresponds to a speed ratio close to unity. This improved performance is attributed to the favorable aerodynamic interaction between the turbine and the downstream baffle which accelerates the flow around the rotor and generates larger turning torque. The baffle generates a jet effect on the advancing bucket and accelerates the flow behind the bucket creating a large zone of negative pressure and thereby increases the driving torque. Furthermore, the upstream deflector (also called shield or curtain) produces a shield for the returning bucket of the turbine which diminishes the adverse effect associated with the returning bucket on the aerodynamic torque of the turbine. This remarkable improvement of turbine performance will encourage the future application of the Savonius wind turbine in small power applications of wind energy. [ABSTRACT FROM AUTHOR]

The aerodynamic performance of horizontal-axis wind turbines is strongly dependent on many parameters, among which the airfoil type and the blade geometry (mainly defined by the chord and the twist distributions) are considered the most critical ones. In this article, an approach giving the appropriate airfoil for a small wind turbine design was conducted by performing an aerodynamic improvement of the blade's airfoil. First, a preliminary design of the rotor blades of a small wind turbine (11 kW) was conducted using the small wind turbine rotor design code. This preliminary approach was done for different airfoils, and it resulted in a maximum power coefficient of 0.40. Then, the aerodynamic efficiency of the wind turbine was improved by modifying the geometry of the airfoils. This technique targets the optimization of the lift-to-drag ratio (Cl/Cd) of the airfoil within a range of angles of attack. Also, a non-uniform rational B-spline approximation of the airfoil was adopted in order to reduce the number of the design variables of the optimization. This methodology determined the best airfoil for the design of a small wind turbine, and it gave an improved power coefficient of 0.42. [ABSTRACT FROM AUTHOR]

Improving the H-Darrieus rotor is often followed by the investigation of the influence of the turbine's parameter design, notably, the aspect ratio, the solidity (σ), the tip speed ratio, and the airfoil profile shape. In this work, we are interested in both the aerodynamic flows around a straight cambered blade profile and the rotor turbine wake separation of a Darrieus vertical axis wind turbine. The aim of this study is to better understand the evolution of the instantaneous torque and the generated-separated blade vortex during full rotation. Indeed, a three-dimensional computational fluid dynamics model of a vertical axis wind turbine with a straight cambered blade profile NACA4312 operating over a large range of tip speed ratio is considered. The flows are governed by Reynolds-averaged Navier–Stokes equations and the turbulence is modeled with shear stress transport formulations k - ω. This research revealed a high correlation between the evolution of the torque coefficient and the generated-separated blades vortex. In particular, a good correlation between the maximum tip vortices size and the torque coefficient peak is demonstrated. [ABSTRACT FROM AUTHOR]

This article aims to present a two-dimensional parametric analysis of a modified Savonius wind turbine using computational fluid dynamics. The effects of three independent parameters of the rotor, namely, shape factor, overlap ratio, and tip speed ratio on turbine performance were studied and then optimized for maximum coefficient of performance using response surface methodology. The rotor performance was analyzed over specific domains of the parameters under study, and three-variable Box-Behnken design was used for design of experiment. The specific parametric combinations as per design of experiment were simulated using ANSYS Fluent®, and the response variable, coefficient of performance (Cp), was calculated. The sliding mesh model was utilized, and the flow was simulated using Shear Stress Transport (SST) k - ω model. The model was validated using past experimental results and found to predict parametric effects accurately. Minitab® and ReliaSoft DOE++® were used to develop regression equation and find the optimum combination of parameters for coefficient of performance over the specified parametric domains using response surface methodology. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The vertical axis wind turbine generators are mainly used for small size wind turbine generators, and are independent on wind direction. The rotor of the vertical axis wind turbine is required to start rotating at a low wind speed and to smoothly rotate with small torque loss so that the energy loss from the energy of wind is minimized and the power generation efficiency is increased. But when the rotor is designed according to the wind conditions and the safety factors in the international safety standard for small wind turbines, IEC61400-2, commercially-available bearings are not suitable for the rotor, because both of the static and dynamic load ratings are larger than those demanded for the vertical axis wind turbine. That is, the frictional force and the rotating torque resistance of the commercially-available bearings are too large for the rotor to meet the above mentioned requirements.In this study, the bearings having the most suitable load ratings and the very low frictional force for the rotor were newly designed conforming to the wind conditions and the safety factors in the above mentioned IEC61400-2. And the low torque shaft unit for the rotor using these newly designed bearings was also designed and experimentally produced. This low torque shaft unit was equipped in the real vertical axis wind turbine and a demonstration experiment was conducted. The result of the experiment showed that the low torque shaft unit started rotating at low wind speed and the power generation efficiency was improved. It is verified that the low torque shaft unit satisfies the above mentioned requirements well. [ABSTRACT FROM AUTHOR]

The extreme response of a multi-column tension-leg platform (TLP) floating wind turbine (the WindStar TLP system) is analyzed here under operating conditions. Coupled dynamic response simulations are performed using FAST, a fully coupled aero-hydro-servo-elastic time-domain simulation tool. A land-based system (a U.S. National Renewable Energy Laboratory offshore 5 MW baseline wind turbine installed on land) is quantitatively compared with the WindStar TLP system. Extreme loads with different return periods are obtained by statistical load extrapolation. Peaks are extracted using peak-over-threshold analysis with time separation between peaks. The WindStar TLP system shows almost similar mean generator power output to the land-based system under operating conditions owing to its small dynamic motion responses to environmental loads. It shows greater characteristic load (blade 1 out-of-plane moment at the blade root and tower base fore-aft bending moment) under operating conditions than does the land-based system, with the characteristic load ratio reaching approximately 1.2. These results will help identify the extreme response differences between the two systems, and thus aid the development of relevant modifications to the initial WindStar TLP design. [ABSTRACT FROM AUTHOR]

Despite having been used for a long time, tubular lattice towers with three or four legs have not been systematically analysed for use with small wind turbines. We present a design procedure based on modelling the towers as either tripods or quadrapods to allow analytic approximations to the tower stresses. Following the IEC standard for small wind turbine design, the critical load occurs at the 50-year extreme wind speed acting on a stationary turbine and tower. To avoid buckling in the downwind leg, three separate methods of estimating the critical buckling resistance are shown to give very similar results. The analytic models also allow the tower-top deflection to be simply approximated. We use an arbitrary limit on deflection as 5% of the tower height, to ensure linear, static behaviour for extreme wind loads. Two example tower designs are considered: an 18 m tower for a 5 kW turbine and a 12 m tower for a 500 W turbine. [ABSTRACT FROM AUTHOR]

In order to improve the performance of small horizontal axis wind turbines at low wind speed, this study designs a novel airfoil section with an optimum transition ramp through multipoint inverse design method. A viscous analysis code is used to close the design loop. Further, Shear stress transport-transition model in ANSYS-Fluent is employed with modified constants to analyze the flow and aerodynamic performance of the airfoil at Reynolds numbers 60 000, 100 000, 200 000, 300 000 and 500 000. Next we consider the designing of a 2m rotor from the new airfoil section using an evolutionary algorithm for optimization. The power coefficient and self starting of a small 2m wind turbine is improved significantly with the chosen generator resistive torque of 0.5Nm, the new airfoil section and the optimization technique for finding the optimal values of the parameters. [ABSTRACT FROM AUTHOR]

We present a novel Vertical Axis Wind Turbine (VAWT) that uses flat delta-wings instead of aerodynamically shaped blades with airfoil cross-sections. This choice lowers the manufacturing costs significantly. Furthermore, delta-wings have a good aerodynamic efficiency at low Reynolds-numbers compared to conventional blades. This is in particular an advantage, if delta-wings are used in small wind turbines. We have performed wind tunnel measurements at low tip-speed ratio and showed that the power output of the delta-wing turbine is higher than that of a conventional straight-bladed VAWT. [ABSTRACT FROM AUTHOR]

Blade root fatigue stress, primarily resulting from wind shear and turbulence, is a critical factor in wind turbine design. Blade mounted aerodynamic control devices have been shown to have the potential to reduce this. However, limited research exists into suitable devices. The blade designed in this work addresses this by employing an actuated compliant mechanism, contained within a flexible matrix composite structure. The resulting mechanism design achieves a sectional change in lift coefficient of ΔCL + 0.4 to -0.15. The performance of the blade is analysed with a quasi-steady time marching BEM model. A reduction of 21.59% in the standard deviation of the flap-wise bending moment comparable to previous load control investigations has been achieved. [ABSTRACT FROM AUTHOR]

Eigen frequencies of a vertical axis wind turbine tower made out of laminated wood which are both bolted to the ground and supported by guy wires are studied and compared. Using beam theory, an analytical model taking the guy wires into account for calculating the first mode eigen frequency of the tower has been derived. The analytical model is then evaluated by comparing with FEM-simulations and measurements performed on the actual tower. The model is found to be reasonably accurate keeping in mind that the estimated masses and second moments of area are somewhat rough. Furthermore the model can be used to give an indication of the magnitude of change in eigen frequency when modifying a tower or guy wire property. [ABSTRACT FROM AUTHOR]

Banki wind turbine is a kind of vertical axis wind turbine (VAWT) with high solidity, and has been proved to have a relatively high starting torque and maximum power coefficient under low wind conditions. This study introduces an arc-shaped windshield to further improve the performance of Banki wind turbines. The windshield is placed in front of the turbine to deflect wind and reduce the negative torque on the blades in the upwind direction. Computational investigations are carried out to study the influence of the angle of the arc-shaped windshield on the performance of the turbine using the commercial computational fluid dynamics (CFD) code Fluent 13.0. The results demonstrated that the maximum power coefficient of the Banki wind turbine was increased by 66.9% with the optimal windshield angle of 60°. The maximum power coefficient was found to be 0.2784 at a tip speed ratio of 0.5 in the presence of the windshield. [ABSTRACT FROM AUTHOR]

Turbulence evolution in the near wake of a VAWT blade at a constant Reynolds number of 80,000 and an angle of attack of 14 degrees has been studied experimentally under two different levels of free-stream turbulence, 0.5 and 4.6%. Mean velocity, turbulence intensity, turbulence rms velocity and turbulence strain rate profiles have been examined at several streamwise locations in the wake using hot-wire anemometry. External free-stream turbulence has been shown to notably affect the aerodynamic performance of the blade as well as turbulence characteristics of the wake flow. Aerodynamic performance of the blade has been improved under higher free-stream turbulence in the stall region, shown by increase in the lift-to-drag ratio. Free-stream turbulence also decreases the mean velocity deficit and turbulence intensity consistently in the entire wake region, while its effect on the turbulence rms velocity and turbulence strain rate is not monotonic. Substantial asymmetry has been observed in wake profiles for both Tu = 0.5 and 4.6% cases at very near wake regions. The asymmetry decreased going downstream, becoming essentially symmetric past 1.5 chord lengths downstream of the blade trailing edge. At very near wake regions, significant flow characteristic changes occurred between the wake center, wake lower, and upper parts, these changes became more subtle moving downstream. [ABSTRACT FROM AUTHOR]

The following work represents the most recent advances in design and testing of vertical axis wind turbines (VAWT) rotors. VAWTs have received much attention as of late due to proposed advantages in small scale and off grid wind power generation. Thus, many recent works have surfaced involving analysis, design and optimization of VAWT rotors in order to more efficiently convert wind energy to electricity or other readily usable means. This paper is a collection of most of the recent literature works involving VAWT rotor design and testing, the majority of which published after 2005. We discuss research in the designing of various lift based rotors as well as some drag based rotors, hybrids, and various others. The recent work in this area suggests VAWT capacity could dramatically increase in the near future, and play a vital role in obtaining cleaner, more sustainable energy when global energy demand is increasing at an unprecedented rate.HIGHLIGHTS• A review of various works involving rotor design and testing of both lift and drag Vertical Axis Wind Turbines (VAWTs) is presented;• Benefits of vertical axis wind turbines in small scale and off grid wind power generation is summarized;• Much of the recent work, published after 2005, has been directed towards analyzing, designing, and optimizing rotor shapes.The body of this recent work suggests that research on VAWT rotor design continues to flourish and could make VAWTs a viable competitor to more traditional Horizontal Axis Wind Turbines (HAWTs) worldwide. [ABSTRACT FROM AUTHOR]

A numerical simulation has been performed to assist in evaluating vertical-axis Savonius-style wind turbines. The simulation was fully three-dimensional and unsteady. Through the simulation, it was possible to assess the performance of a wind turbine that possessed novel features such as wing venting and blade caps. Results from the simulation were compared with experimental data and it was found that they were in general agreement. The simulations provide continuous information regarding air-flow patterns and velocity distributions during the rotation. Representative patterns of flow are shown. [ABSTRACT FROM AUTHOR]

The dynamics of a straight-bladed vertical axis wind turbine is investigated with respect to oscillations due to the elasticity of struts and shaft connecting to the hub. In particular, for the three-bladed turbine, a concept is proposed for dimensioning the turbine to maximize the size of the resonance free rpm range for operation. The effect of aerodynamic damping on the struts is also considered. The damping of these types of oscillations for a typical turbine is found to be good. [ABSTRACT FROM AUTHOR]

This work introduces a modified double-multiple streamtube model for Vertical Axis Wind Turbines (VAWT) that takes into account some parameters that were neglected till now. For example, the model considers the directions of the flow, rather than assuming that the flow is parallel to the free-wind. Also a formulation is established to calculate the effect of inclination of the blades in any direction. The modifications are facilitated by considering the momentum conservation in a direction perpendicular to the force (which was abandoned in the traditional models). The model results are compared to other models and to experimental results with a fair agreement. [ABSTRACT FROM AUTHOR]

This study reports an investigation on the influence of the virtual camber effect on the performance prediction of an H-Darrieus VAWT using a theoretical approach based on the Momentum Models.In detail, a simplified model is proposed which highlights a limit point below which the virtual camber effect must be neglected to correctly evaluate the performance of the airfoils. This correction provides a more accurate description of the unstable region of the power curve, for which very few in-depth studies are available. An accurate description of the left side of the power curve is, however, of particular importance for a correct prediction of the self-starting capabilities of the rotor, which are presently considered as a decisive requirement for small-power machines. On this basis, the effects of the new virtual camber estimation on the transient study of an H-Darrieus rotor are also presented here. This has been carried out by means of a specifically developed numerical code.All the theoretical conjectures and the numerical models have been validated with the experimental data of two wind-tunnel campaigns on two different models of H-Darrieus turbine. [ABSTRACT FROM AUTHOR]

This paper presents a computational study of a rooftop size vertical axis wind turbine with straight blades (H-type turbine). The computational model solves for the two-dimensional and three-dimensional unsteady flow fields around the turbine based on the unsteady Reynolds averaged Navier-Stokes equations and a sliding mesh technique to connect the farfield fixed mesh to the near field rotating mesh around the rotor. Interesting flow features such as dynamic stall around the blades and the interaction of the blade wakes with the following blades are illuminated. Comparison of the 2D and 3D simulations highlight strong three dimensional effects, including the blade tip losses and the effects of the blade supporting shaft and arms. These effects significantly degrade the performance of the VAWT under investigation, pointing a way for improving VAWT designs. [ABSTRACT FROM AUTHOR]

The paper demonstrates the application of inverse airfoil design method to improve performance of a low-speed straight-bladed Darrieus-type VAWT. The study shows that an appropriate tailoring of the airfoil surface using the inverse airfoil design technique can help improve performance by eliminating undesirable flow field characteristics at very low Re, such as early transition due to presence of separation bubbles. The increase aerodynamic efficiency then translates into an improved aerodynamic performance of VAWTs specifically at very low chord Reynolds numbers. The study employs an interactive inverse airfoil design method (PROFOIL) that allows specification of velocity and boundary-layer characteristics over different segments of the airfoil subject to constraints on the geometry (closure) and the flow field (far field boundary). Additional constraints to satisfy some desirable features, such as pitching moment coefficient, thickness, camber, etc., along with a merit of performance of the VAWT, such as the required power output for a given tip-speed ratio, are specified as part of the inverse problem. Performance analyses of the airfoil and the VAWT are carried out with the aid of state-of-the-art analyses codes, XFOIL and CARDAAV, respectively. XFOIL is a panel method with a coupled boundary-layer scheme and is used to obtain the aerodynamic characteristics of resulting airfoil shapes. The final airfoil geometry is obtained through a multi-dimensional Newton iteration. A design example is presented to demonstrate the merits of the technique in improving performance of small VAWTs at low speeds. The main findings of the study suggests that the strength of the method lies in the inverse design methodology whereas its weaknesses is in reliably predicting aerodynamic characteristics of airfoils at low Reynolds numbers and high angles of attack. This weakness can, however, be overcome by assessing relative performance of the VAWT with the assumption that the changes in airfoil characteristics be kept small. The results indicate that a 10-15% increase in the relative performance of the VAWT can be achieved with this method [ABSTRACT FROM AUTHOR]

The performance of a prospective candidate vertical axis wind turbine airfoil S1223 has been investigated at Reynolds numbers of 55,000, 75,000, and 100,000. The airfoil was tested at angles of attack from -5 to 25 deg in a quasi-isotropic turbulent flow generated using orificed plates. The independent effects of the turbulence intensity were examined with a constant integral length scale & Lambda;/c = 0.14 (where c is the chord length). By increasing the turbulence intensity independently from 4.1 to 9.5%, the airfoil exhibits strong stall characteristics at low turbulence, whereas the stall behaviour becomes less obvious at high turbulence. The independent roles of integral length scale were examined by varying integral length Λ/c from 0.14 to 0.23. The less turbulent free-stream with smaller length scales appears to delay the stall of the airfoil based on the observation of the lift and drag curve, implying that the smaller integral length under low turbulence intensity (4.1%) delays the boundary layer separation on the suction surface. At a higher turbulence of intensity 9.5%, the effect of varying Λ/c from 0.08 to 0.15 only leads to subtle changes in the lift and drag. [ABSTRACT FROM AUTHOR]

In order to operate a modern wind turbine an effective start procedure is required. This article presents an electrical start for a VAWT which is shown to have advantages over an aerodynamic start. An electrical starter for a 12 kW VAWT has been designed and built. The starter is based on a six-step modulated inverter that is feedback-controlled by the rotor position. The inverter is connected to an auxiliary winding in the generator to separate the starter electronics from the main system. The system has been simulated and experimentally verified in the wind turbine setup. [ABSTRACT FROM AUTHOR]

Horizontal and vertical axis type wind turbines using sails (instead of rigid blades) are experimentally investigated. Cretan type wind turbines are horizontal axis sail type wind turbines. The simple design, low cost and ease of construction of these turbines make them an attractive option for areas having low wind velocities. Coefficient of power and coefficient of torque for Cretan type rotor with 5 blades for a solidity ratio of 0.85 is studied. The type of the cloth used is polyester with porosity ranging between 75 to 300 μm. Experiments are conducted for Reynolds number of 1 ×105 and 1.2 × 105 and tip speed ratios ranging between 0 and 0.1. Influence of sail angle (5°, 15°, 25° and 30°) on the performance of the horizontal axis sail type wind turbines is investigated. Sail angle of 25° gives the maximum coefficient of power of 0.31 at a tip speed ratio of 0.71. Sail Savonius wind turbines are investigated on the vertical wind turbine domain. The enhanced performance of Cretan type wind rotors using sails laid the foundation for studying the behavior of Savonius wind turbines with flexible blades. The sail Savonius rotor used in the study has a rotor diameter of 180 mm with an overlap ratio of 0.15. The type of cloth used is polyester with the porosity ranging between 75 to 300 μm. The performance characteristic curves are measured for Reynolds number of 9 × 104 and 1 × 105. The peak value of coefficient of power is around 0.05 at a tip speed ratio of 0.43 for a Reynolds number of 1 × 105. [ABSTRACT FROM AUTHOR]

A high solidity, small scale, 2.5m diameter by 3m high Vertical Axis Wind Turbine (VAWT) consisting of three NACA 0015 profile blades, each with a span of 3m and a chord length of 0.4m, was tested in an open-air wind tunnel facility to investigate the effects of preset toe-in and toe-out turbine blade pitch. The effect of blade mount-point offset was also investigated. The results from these tests are presented for a range of tip speed ratios, and compared with an extensive base data set obtained for a nominal wind speed of 10m/s. Results show measured performance decreases of up to 47% for toe-in, and increases of up to 29% for toeout blade pitch angles, relative to the zero preset pitch case. Also, blade mount-point offset tests indicate decreases in performance as the mount location is moved from mid-chord towards the leading edge, as a result of an inherent toe-in condition. Observations indicate that these performance decreases may be minimized by compensating for the blade mount offset with a toe-out preset pitch. The trends of the preset blade pitch tests agree with those found in literature for much lower solidity turbines. [ABSTRACT FROM AUTHOR]

The influence of load level on the performance of a stand-alone, straight-wing, vertical axis wind turbine system in terms of its power generation capability and output fluctuation during operation is numerically investigated using a dynamic simulation model. The system is mainly operated at a constant tip-speed ratio, i.e., at a maximum power coefficient point, by load control in proportion to the square of rotational speed. Thus, the steady-state performance and dynamic behavior of the system under four load levels are analyzed. Then, the dynamic system performance under different load levels is discussed. This work shows that, under high mean wind speed conditions, a low load level is undesirable because it would cause an increase in the output fluctuation. However, under low mean wind speed conditions, a low load level could be effective in improving the power generation capability, with only a slight increase in the output fluctuation. [ABSTRACT FROM AUTHOR]

The power output characteristics of a novel vertical axis wind turbine system were investigated via computational fluid dynamics. The system was tested at seven wind velocities. Results showed that the efficiency of the new system increases with increasing wind velocities up to approximately 30 m/s, after which the efficiency remains constant at 0.45. Moreover it was found that system reacts more quickly to changes in wind velocity once it is in motion and reaches a steady state. A new equation was developed to predict the power generation as a function of time and wind velocity. The use of the equation in predicting power performance is discussed in the paper. [ABSTRACT FROM AUTHOR]

The two-dimensional characteristics of airfoil NACA 0018 have been measured for Reynolds numbers between 0.15×106 and 1.0×106 to establish the lift, drag and moment curves that serve as input to performance calculations of vertical axis wind turbines. At the lower surface laminar separation occurs at low to medium angles of attack, which is of significant influence on the characteristics and the radiated noise. For the situation with a lower surface laminar separation bubble, span wise wake rake traverse measurements showed an irregular three-dimensional pattern. Noise reduction could be achieved with zigzag tape at the 70% to 80% lower surface chord station. Significant post-stall hysteresis loops occurred showing a high loss in lift. [ABSTRACT FROM AUTHOR]

The fixed-pitch straight-bladed vertical axis wind turbine (SB-VAWT) is one of the simplest types of wind turbine. One of the main challenges of wide spread application of the smaller-capacity SB-VAWT is to design and develop it in a cost-effective manner. The overall cost of the SB-VAWT will mainly depend on judicious choice of multiple design parameters. An attempt has been made in this paper to identify and critically analyze the main design parameters related to smaller-capacity fixed-pitch SB-VAWT. It has been demonstrated in this paper that proper selections of these parameters are vital for a cost-effective smaller-capacity SB-VAWT which can be considered as a candidate for urban and off-grid rural applications. [ABSTRACT FROM AUTHOR]

Selection of airfoil is one of the most critical factors in achieving better aerodynamic performance and in determining the optimum dimensions of a fixed-pitch straight-bladed Darrieus type vertical axis wind turbine (SB-VAWT), along with solidity, radial arm parasitic drag, aspect ratio etc. Airfoil related design changes also have the potential for increasing the cost effectiveness of SB-VAWTs which is one of the simplest types of wind turbine and they are prospective candidate for diversified urban and rural applications. After identifying the main aerodynamic challenges of this type of wind turbine, the authors short listed the salient aerodynamic and geometric characteristics of a desirable airfoil for a selfstarting and better performing SB-VAWT. Based on these findings, detailed performance analyses were done with several prospective airfoils. Finally a special-purpose airfoil has been designed for smaller capacity SB-VAWT. [ABSTRACT FROM AUTHOR]