H-Darrieus Vertical Axis Wind Turbine Research Articles

The effect of geometry variants on the performance of VAWT-rotor with incline-straight blades

This study was conducted theoretically with the aim of investigating the effect of the straight blade angle change on the aerodynamic performance of the vertical axis wind rotor (VAWR). The work was done within the blade angle range of 25°–85° with an angular gradient of 20°. A special name has been given to the VAWR concerned in this paper, where it was called Straight Slant Blades (SSBs). The so-called multi-stream tube (MST) model was modified to be suitable for SSBs. MST is an aerodynamic model, which was previously used to work on what is known as Darrieus and H-Darrieus vertical axis wind turbines. A computer program is designed and developed using Microsoft Visual Basic to be consistent with the objectives of this research. As for verifying the effectiveness of this program, its theoretical results were compared with previous laboratory results from other research studies as it proved its effectiveness in achieving satisfactory results commensurate with the objective of this study. This study is applied to four groups of different engineering designs for the SSB rotor, where each group is concerned with one of the geometric variables confined to the rotor height 2H, the rotor radius R, the number of rotor blades N, and the airfoil C. The results obtained are displayed as rotor power coefficient curves as a function of blade tip speed ratio. As it resulted in the ideal range for the angle of the blade between 45° and 65°, according to the rotor in this range, the value of maximum power coefficient CpMAX is at its maximum and the range of tip speed ratios is the highest in addition to the diminution of the problem of self-movement.

An experimental investigation into performance characteristics of H-shaped and Savonius-type VAWT rotors

The effects of flow regime and rotor configuration strongly influence the power performance of vertical axis wind turbines (VAWTs). Yet, there exits few experimental investigations from literature that have been carried to explain this effect. This could be attributable to huge wind tunneling cost requirement and an array of design skills required including time constrains. The balance between cost and power performance has limited development of VAWTs towards utility scale type. In this paper therefore, we seek to investigate in detail the performance characteristics of vertical axis wind rotors in a wind tunnel environment. The H-Darrieus and Savonius-type VAWT configurations have been utilized to carry out the investigation. A systematic analysis of effect of blade number, rotor configuration, and flow regime including comparison of the two configurations was presented to predict the power performance using a rotary sensor. Results revealed a significant impact of flow field regime on power performance of the VAWT models, with the Savonius-type rotor attaining lower power coefficient (CP) than the Darrieus-type rotor across all the tested wind speeds. In addition, increase in number of blades for Darrieus-type rotor revealed narrower negative CP band that decreased with increase in wind speeds. In both configurations, the CP decreased at low tip speed ratios while an increase in CP was revealed at high tip speed ratios. The understanding gained from the results could be useful to the VAWT research community, and the findings can be extended to the design of wind turbines for mini-grid and utility scale applications.

Effect of blade attachments on the performance of an asymmetric blade H-Darrieus turbine at low wind speed

ABSTRACT Wind energy is a significant source of renewable energy and vertical axis wind turbine (VAWT) is one of the advanced wind energy harvesters. Maximum extraction of energy from wind harvesters is a major goal. Researchers are faced with the challenge of enhancing the performance of VAWTs in low wind speed conditions. To this end, researchers have contributed substantially by doing research on the blade design parameters thereby generating innovative and improved VAWT designs. However, there is hardly any work on the performance augmentation of H-Darrieus VAWT with blade attachments on asymmetric blades, which is the main objective of this work. Further, this work investigates the combined effect of turbulence intensity and flow dissimilarity on turbine performance for low wind speed condition. In this paper, blade attachment of length 20 cm (equal to 1/2.5 times blade span) is attached to asymmetric NACA 63–415 blade turbine having fixed blade pitch angle (+5°). NACA 63–415 blade profile is one of the prominent asymmetric series of NACA blades; hence it is considered in this work. The performances of seven different configurations of the turbine with blade attachments have been investigated experimentally at 5.0 m/s and 6.0 m/s using wind tunnel testing. The performance of the VAWT design is systematically improved by changing the number and position of these attachments on the turbine blades. The result shows that both side attachments on turbine blade on its top position have maximum power coefficient (Cp) of 0.13 at tip speed ratio (TSR) 1.8 at wind speed 6.0 m/s among all the turbine configurations tested. There is an improvement of power coefficient by 4.34% when attachments are placed at top of the turbine blade compared to turbine blade without attachment at wind speed 6.0 m/s. Further, maximum power coefficient is increased by 3.28% when turbine is operated from 5.0 m/s to 6.0 m/s for both side attachments at top of the turbine blade. Moreover, due to the combined effect of turbulence intensity and flow dissimilarity, the power coefficient of the turbine configuration having both side attachments has also improved a little. In this case, an improvement of 0.6% in power coefficient of turbine having both side attachments on top of the blade is achieved. As the turbine is a prototype having smaller size hence the effect of turbulence intensity and flow dissimilarity is less, in case of actual turbine of bigger size the effect of turbulence intensity and flow dissimilarity will be more and hence the percentage enhancement of power coefficient will also be more. Hence, it can be concluded that a turbine blade with higher degree of thickness (by using blade attachments) at top portion will increase the performance of an asymmetric blade H-Darrieus VAWT.

A Computational Fluid Dynamics Study of an Integrating Savonius-Darrieus Vertical Axis Wind Turbine

To combine the advantages of performance of both Savonius and Darrieus wind turbines, this paper aims to study computationally a novel design configuration of a Vertical Axis Wind Turbine (VAWT). The new design would help cover a broad range of performance parameters between both types of wind turbines as well as overcome drawbacks of the Savonius type at higher rotational speed. The proposed design is to integrate both of Savonius and straight blade Darrieus wind turbines in a single VAWT design configuration. In this design, the turbine rotor consists of a number of similar NACA0015 airfoil blades set in a particular arrangement. By regulating individual angles of rotor blades, the turbine would work as either Savonius or H-Darrieus VAWT. The turbine could also be set to work at different configurations between those for the Savonius and H-Darrieus ones. The computational results revealed that the performance of the proposed integrating wind turbine would be controlled and optimized to cover the operating range of both of Savonius and Darrieus wind turbines. Besides, the performance of Savonius VAWT would be modified when applying this design. When the proposed design is practically implemented, characteristics of both of Savonius and H-Darrieus based wind turbines would be found in a single design.

Low wind speed aerodynamics of asymmetric blade H-Darrieus wind turbine-its desired blade pitch for performance improvement in the built environment

An asymmetric blade vertical-axis wind turbine (VAWT) is one of the emerging technologies for harvesting power in the built environment, which has low wind speed. Although asymmetric blade improves VAWT’s performance, the effect of blade pitch angle on its design is hardly ascertained. In this paper, unsteady 2D Reynolds-averaged Navier–Stokes CFD simulations are carried out to investigate the effect of blade pitch angle on the aerodynamic performance of a NACA 63-415 asymmetric blade H-Darrieus VAWT at a low wind speed of 6.0 m/s. Its detailed flow physics at different operating and pitch angle conditions is investigated, and important performance insights are obtained to elucidate its desired blade pitch for performance improvement. The present study shows that positive pitch angle (+ 5°) improves the turbine performance in upwind position, whereas negative pitch angle (− 5°) augments the turbine performance in downwind position as well as causes less wake effect than positive pitch angle. Further, optimal pitch angle (+ 5°) is found out at which the maximum power coefficient of 0.271 is obtained for an operating tip speed ratio 2.4. The present study delineates how desired blade pitch improves the performance of asymmetric blade VAWT for sustainable power generation in the built environment.

Effect of Tip Speed Ratio on the Performance of H-Darrieus Wind Turbine with NACA 0018 Airfoil

The effect of Tip Speed Ratio (TSR) to the performance of an H-Darrieus Vertical Axis Wind Turbine was studied using numerical method. The blade of the turbine was formed by using NACA 0018 mode. Three-dimensional governing equations are developed. The turbulent flow was modelled using k – epsilon equation. The commercial code Computational Fluid Dynamic is employed to solve the problem. The focus in here is to explore effect of the TSR. The tip speed ratio is varied from 1.65 to 1.8. As a results, the contour velocity was generated and discussed. The average velocity of the wind leaving from the turbine is employed to estimate the power coefficient. The power coefficient for tip speed ratio 1.65 and 1.8 are discussed. The conclusion here is that tip speed ratio and the power coefficient strongly affected each other. Increasing the tip speed ratio will increase the coefficient power.

Unsteady aerodynamic performance of Dual-Row H-Darrieus vertical axis wind turbine

H-rotor Vertical Axis Wind Turbine (VAWT) is one of the most efficient energy suppliers which have been investigated in many recent types of research. The aim of this work is to study the aerodynamic performance of a doubled-row H-Darrieus VAWT. First, an ordinary three-bladed VAWT with NACA4415 profile is simulated by means of 3D computational fluid dynamics (CFD) and results are compared to a recently published research work based on Blade Element Momentum (BEM) theory. Afterward, a doubled-row H-Darrieus VAWT is simulated and analyzed in two different geometric configurations. In the first configuration, a second row with the same blade characteristics of the first row is added aligned with the first row and with 0.2 m distance toward it. In the second one, again with the same blade characteristics, the secondary blade is added with 0.2 m distance toward first row, but with 60 degrees angular offset. Renormalization-Group (RNG) k-ɛ turbulence model besides wall function is applied in all unsteady simulations. As comparative tools, based on other studies using the same coefficients, momentum coefficient ( ) and power coefficient are calculated in all simulations to investigate which case operates more efficiently. It is observed that adding a second row to an ordinary H-Darrieus VAWT will improve these coefficients up to 314% which is a considerable leap in power production ability of the VAWT. Also, different turbulence models, geometries (with a central shaft and without central shaft) and solution methods were also analyzed and the effect of each one was computed and compared with other cases.

Design and aerodynamic performance of new Floating H-Darrieus Vertical Axis Wind Turbines

In the last decade, several countries started work on the development of Floating Vertical Axis Wind Turbines, which have significant advantages over Floating Horizontal Axis Wind Turbines. In the present work, we expose a brief history of the use of this type of wind turbines, and we contribute to understanding the existing technologies and we also propose a new design of Floating H-Darrieus Vertical Axis Wind Turbine with three stage rotors. This design solves the problem of starting a large turbine and facilitates maintenance by adopting three mechanisms Bearing Swivel Rollers at each stage. We use the Double Multiple StreamTube method for aerodynamic simulations. The numerical results of the aerodynamic performance analysis show that variable radius rotors maximize the power generated.

Effect of Chord Length on the Performance of H-Darrieus Wind Turbine with NACA 4415 Airfoil

The effect of chord length to the performance of an H-Darrieus Vertical Axis Wind Turbine (VAWT) was investigated numerically. The turbine blade made of NACA 4155 airfoil. A set of governing equations has been developed. In order to incorporate the turbulence effect, k – epsilon model was adopted. A Computational Fluid Dynamic code has been used to solve the problem. The non-dimensional chord length of 0.2, 0.3, 0.4, and 0.5 are investigated. The contour velocity in the computational domain was plotted and discussed. By using the average velocity of the wind leaving the turbine, power coefficient is estimated. The results show that the power coefficient for non-dimensional chord length of 0.2, 0.3, 0.4, and 0.5 is 0.51, 0.57, 0.59, and 0.59, respectively. The conclusion here is that non-dimensional chord length strongly affects the power. Increasing the chord length will increase the coefficient power. However, there exists an optimum chord length it is 0.4. Thus, it is suggested to use the non-dimensional chord length of 0.4.

Improved double-multiple-streamtube approach for H-Darrieus vertical axis wind turbine computations

Accurate predictions on vertical axis wind turbine performance are important for the design process but also challenging because the flow is highly unsteady involving a strong variation of the angle of incidence. In the present work, a double-multiple-streamtube (DMS) approach is improved by analytical corrections. First, two-dimensional and three-dimensional computational fluid dynamics (CFD) simulations are performed for wind turbines at various tip speed ratios. The CFD results are used as a database to investigate the velocity field of the turbines and to derive reasonable physical basis for the modelling approach. Second, new analytical models are introduced to correct the angle of incidence seen by the airfoil section together with correction models for the streamtube expansion and dynamic stall effects. DMS simulations are then carried out and compared with experimental data for turbines with different rotor solidity and various operating ranges. The prediction accuracy is significantly improved by the new model not only for the power coefficient but also for the azimuthal variations of the blade loads.

Comparison of low wind speed aerodynamics of unsymmetrical blade H-Darrieus rotors-blade camber and curvature signatures for performance improvement

Unsymmetrical blade Vertical axis wind turbine (VAWT) can be nicely adapted in the built environment because of its improved performance, self-starting ability, simple construction, easy maintenance and capability to adapt to wind direction changes. For proliferation of such VAWT installations, the challenge is to improve its efficiency in low wind speed condition (usually less than 10 m/s) of the built environment, which depends on its blade profile design. In an unsymmetrical blade, blade thickness-to-chord ratio, percentage camber, blade curvature around aerodynamic moment center are the controlling parameters for effective blade-fluid interactions for performance improvement, thus requiring a detailed investigation of the aerodynamics of unsymmetrical blade VAWT. The objective of this paper is to compare low wind speed aerodynamics of two different unsymmetrical airfoil blade profiles to obtain design information of blade camber and blade curvature around aerodynamic moment center for performance improvement of H-Darrieus VAWT, which is hardly available in the existing literature. Detailed blade-fluid interactions are analysed by Ansys Computational Fluid Dynamics (CFD) software for different low wind speeds between 4 m/s and 8 m/s. It is found that more rounded curvature of the suction side surface of advancing S815 blade around aerodynamic moment center and thicker blade profile (maximum thickness is 26.2% at 25.7% chord) has a distinct influence for performance improvement of the rotor in the power stroke. In the returning stroke, the VAWT is benefited from higher camber of EN0005 blade (maximum camber percentage of 10% at 55% chord) and lesser blade curvature of the blade suction side. Next, vorticity structures of each blade profile have also been monitored at different azimuthal positions and necessary performance insights with regards the development of static/deep stall vortices and other boundary layer features have been drawn. Further, blade polars, power performance, and blade loading have also been obtained and compared with published results. Overall better designs of unsymmetrical blade profiles have also been recommended. The present study leads to the understanding of the design blade camber and curvature signatures in both power and returning strokes for possible performance improvement of small-sized VAWT in low wind speed condition.

Effects of instantaneous tangential velocity on the aerodynamic performance of an H-Darrieus wind turbine

Recently, small scale H-Darrieus vertical axis wind turbines (VAWTs) have got great attention of researchers and wind turbine manufacturers due to their application at roof-tops of residential buildings. They are generally preferred over horizontal axis wind turbines (HAWTs) in terms of lower levels of noise production, operation at low wind speeds and lower manufacturing costs. But the main problem with such wind turbines is their low efficiency, which is mainly due to the complex flow around the turbine blades. It is of great importance to first fully understand the nature of complex aerodynamics associated with such wind turbines. Present study analyzes the instantaneous tangential velocity (ITV) and its impact on the power performance of H-Darrieus wind turbine at five different tip speed ratios (TSRs) i.e. 1–5. Computational models based on large eddy simulations (LES) have been developed using commercial software SC/Tetra V12. Throughout the numerical simulation, it is found that ITV of single blade has a negative value (reverse rotation) for TSRs 1–3 when the azimuth position ranges from 80° to 140°, whereas it has positive values during the remainder of the rotation for the mentioned TSRs. Furthermore, the magnitude of the negative ITV for a single blade was found to increase from TSR 1 to 3; the maximum value of the negative peak is equal to approximately −17.2 m/s while at the azimuth position of 110° for blade number 1 at TSR 3. On the other hand, no negative value of ITV is found for any of the three blades during a complete rotation at relatively higher values of TSRs i.e. 4 and 5. The possible physical reasons behind this behavior have been explained through aerodynamic forces and 2D color contours of important parameters, such as vorticity and static pressure. Finally, a discussion has been included in the final section of this paper explaining how negative ITV affects the power performance of H-Darrieus wind turbines. The current CFD model has been compared and validated with experimental data.

Performance and midspan wake measurements on a H-Darrieus in controlled conditions

Performance and wake measurements on a H-Darrieus vertical axis wind turbine are discussed on the basis of an extensive experimental campaign performed at the large scale, high speed wind tunnel of the Politecnico di Milano (IT). The paper proposes a combined mechanical and fluid-dynamic investigation by virtue of an advanced measurement system that allows to reconstruct both phase-resolved thrust and torque acting on the machine and the phase-resolved multi-dimensional velocity field in the wake. The aerodynamic torque is presented as a function of the azimuthal angle of revolution, while the velocity field in the wake is fully characterized by means of a point to point detailed measurement on the midspan plane, where the influence of both blade support struts and tip vortices is negligible.Particular care is taken in the description of the experimental setup as well as in the presentation of the measurements. The here proposed achievements can be considered as a benchmark for the validation of several classes of computational tools.

Combined experimental and numerical study on the near wake of a Darrieus VAWT under turbulent flows

Small Darrieus Vertical-Axis Wind Turbines (VAWTs) are presently seen as a relevant research topic for the wind energy community, since they are thought to perform better than horizontal-axis rotors in the complex and highly-turbulent flow typical of the urban environment. Indeed, a preliminary wind tunnel test campaign on a H-Darrieus VAWT showed a significant increase of the performance for high turbulence levels. The present study analyses in detail the near wake of the turbine in the same turbulent conditions, enabling a better insight on the reasons of this power increase, and on the means to take advantage of it. Near-wake measurements are also benchmarked with a CFD simulation of the entire wake, in order to match the experimental wake measurements with the detachment of flow structures observed in CFD simulations. By doing so, a deeper and useful insight on the reasons why VAWTs perform better in turbulent environments is gained.

Performance investigations of modified asymmetric blade H-Darrieus VAWT rotors

Wind is an important renewable energy resource, which can be extracted for power generation by horizontal and vertical axis wind turbines (VAWTs). VAWTs can be installed in low wind speed regimes for small-scale power generation. However, for better performance in such a scenario, an improved rotor blade configuration is essential. This paper investigates the performances of an asymmetric blade H-Darrieus rotor with a blade span constant relative thickness (RT) and a variable thickness (VT) at different low wind speeds between 4 and 6 m/s. Both static and steady-state performances have been evaluated. Different blade span thickness to chord ratios (t/c) of 30% and 45% (constant RT case) are considered by increasing the blade thickness of the considered base profile NACA 63-415 while keeping the chord length same (5.0 cm). After determining the optimal blade RT, a new blade span variable thickness (VT) H-Darrieus rotor has been designed with a thicker portion (t/c =37.5%) on the top end and a thinner portion (t/c = 22.5%) on the bottom end of the blade, i.e., the thickness of the blade profile throughout the blade span is not the same. The thickness gradually decreases from top to bottom of the blades. The effects of all the thickness modifications have been studied with respect to the performance improvement of the H-Darrieus rotor. It has been shown that constant blade span RT (t/c = 45%) can largely improve the starting characteristics of the asymmetric blade H-Darrieus rotor, whereas blade span VT design can result in better power coefficients of the same in low wind speed conditions. Furthermore, an attempt has also been made to study the fluctuations of steady state torque at the selected wind speeds for understanding the stability and smoothness of operation. Moreover, the fluctuations of power and torque coefficients due to steady turbulence intensity and flow non-uniformity over a constant wind speed have also been demonstrated to connect with real-life applications of the H-Darrieus rotor.

Development of the Dual Vertical Axis Wind Turbine Using Computational Fluid Dynamics

Small vertical axis wind turbines (VAWTs) are good candidates to extract energy from wind in urban areas because they are easy to install, service, and do not generate much noise; however, the efficiency of small turbines is low. Here-in a new turbine, with high efficiency, is proposed. The novel design is based on the classical H-Darrieus VAWT. VAWTs produce the highest power when the blade chord is perpendicular to the incoming wind direction. The basic idea behind the proposed turbine is to extend that said region of maximum power by having the blades continue straight instead of following a circular path. This motion can be performed if the blades turn along two axes; hence, it was named dual vertical axis wind turbine (D-VAWT). The analysis of this new turbine is done through the use of computational fluid dynamics (CFD) with two-dimensional (2D) and three-dimensional (3D) simulations. While 2D is used to validate the methodology, 3D is used to get an accurate estimate of the turbine performance. The analysis of a single blade is performed and the turbine shows that a power coefficient of 0.4 can be achieved, reaching performance levels high enough to compete with the most efficient VAWTs. The D-VAWT is still far from full optimization, but the analysis presented here shows the hidden potential and serves as proof of concept.

Wind Tunnel testing of small Vertical-Axis Wind Turbines in Turbulent Flows

This study presents an innovative wind tunnel approach to evaluate the efficiency of Vertical-Axis Wind Turbines (VAWT) in turbulent flows, to study their integration in urban environments.The first part of the research is devoted to obtaining highly-turbulent wind profiles in the wind tunnel, with the use of different configurations of square grids. A careful study and validation of this technique is done, in order to obtain uniform wind conditions with the adequate values of turbulence intensity and length scales to model the urban flows.The set-up is used to test a H-Darrieus VAWT under values of turbulence over 5%, in comparison with the operation of the turbine under free stream. The preliminary results show that high levels of turbulence do have a significant effect in turbine performance, causing a drop of power for high rotational speeds, increased vibrations in the structure and more difficult control of the rotor. More tests are advised to validate these observations, as well as to expand the study over higher values of turbulence intensity, in order to present a more detailed study with empirical conclusions.

Study on start-up characteristics of H-Darrieus vertical axis wind turbines comprising NACA 4-digit series blade airfoils

Installation of H-type vertical axis wind turbines is in many cases limited by the inherent start-up issues associated with this type of turbine. This could be crucial in environments with low wind speed. The aim of this study is to provide an appropriate CFD modeling setup for investigation of the start-up behavior associated with this class of turbines. For this purpose, a series of transient CFD simulations were carried out using ANSYS Fluent. In contrast with the conventional approach, whereby a constant angular velocity is specified for the rotor, in the present work, the turbine was left free to accelerate based on the torque experienced over time. Careful considerations were made regarding turbulence modeling and grid generation, which are key to ensuring accuracy in this investigation. The result of this simulation, in the form of an accelerating time series, demonstrates good agreement with the published experimental data, and the method yields a high level of accuracy, proving its usefulness for similar problems. In another attempt, the validated CFD setup was utilized to evaluate the effects of several geometric attributes of the turbine rotor on the starting characteristics. Symmetric and cambered airfoils of different thicknesses with a wide range of pitch angles were examined. The optimum start-up characteristics were observed with the use of a medium-thickness cambered airfoil, NACA2418, put to use with an outward pitch angle of 1.5°; this configuration decreased the start-up time while retaining the turbine's peak performance.

Iterative Approach in Design and Development of Vertical Axis Wind Turbines

Universal step-by-step method of design and development of multi-tier H-Darrieus Vertical-Axis Wind Turbine is described. The method is based on the systemic scientific approach of component-wise optimizing of the Turbine including the design of air foils, rotor outline configuration, alternator calculation and the development of power takeoff algorithm.

Aerodynamic characteristics of two-bladed H-Darrieus at various solidities and rotating speeds

Three-dimensional unsteady numerical analysis has been performed in order to analyze the aerodynamic characteristics of an H-Darrieus vertical axis wind turbine with two straight blades. The reliability of the numerical models has been demonstrated through good agreement between the calculated and measured efficiency of an H-Darrieus. Flow characteristics are closely investigated according to tip speed ratios and solidities. A comparison of aerodynamic characteristics at various operational conditions, including the maximum power point is performed. The direction of the free stream approaching the blade is considerably bent following the interaction between blade-to-blade and blade-to-free stream. Even though, the peak value of a torque increases as solidity increases, the blockage and interaction also increase, and thus, increasing the solidity alone does not improve the performance of the H-Darrieus. On the other hand, decreasing the solidity can reduce the effect of blockage and interaction, but the self-starting features via the negative torque at the low tip speed ratio becomes lost. Therefore, a theoretical model such as the DMST (double multiple stream tube) is not suitable for predicting the performance of H-Darrieus with a high solidity. The blockage by blades in the upwind revolution and the interaction between blades significantly change the magnitude of an incidence velocity, and the angle of attack. Thus, the tip speed ratio of the operation point (i.e. the highest power coefficient point) is found to be lower than it is expected.