Operation Of Vertical Axis Wind Turbines Research Articles

Validation of the structural response of a 70kW VAWT

This paper describes a project for predicting and measuring the structural response of a 70 kW vertical axis wind turbine (VAWT), while offering insights for optimizing efficiency and ensuring longevity. We employ the traditional frequency response approach and model the turbine behavior using its stationary modal shapes. Results for three key sensor locations – lower central mast bending, upper blade flapwise bending, and central horizontal arm out-of-plane bending – reveal that the codes accurately predict the critical resonance and capture the dominant harmonic amplitudes. In addition, it is shown how a statistical treatment of the steady state solution can lead to predictions of the fatigue cycle distribution enabling accurate life predictions. The program demonstrates the potential to significantly improve the design and operation of VAWT using simple, steady state, simulation codes for capturing VAWT aerodynamics. The model demonstrates sufficient accuracy while offering computational efficiency compared to more complex models.

Experimental study on rooftop flow field of building based on the operation of vertical-axis wind turbines

Vertical-axis wind turbines (VAWTs) demonstrate good adaptability for harnessing wind energy on the building rooftops. However, knowledge gaps still exist in the understanding of the wind resources on the rooftop, the operation of VAWTs, and its effect on the wind field. To add new knowledge on this subject, the operation of wind turbines modeled by the NACA0018 airfoil on the rooftop of a building is experimentally studied via wind tunnel. The results indicate that flow above the rooftop of building shows an obvious speed reduction due to the blunt body effect of the turbines, and the turbulence intensities are dramatically enhanced. The different tip-speed ratios, wind directions, and installation locations of wind turbine have significant effect on flow field on the roof. Due to the flow characteristics above the rooftop, the power spectral density of the fluctuating wind speed exhibits high energy below the top position of the VAWT on the rooftop. Additionally, the wind profiles and probability distribution of the wake of VAWTs on the rooftop are analyzed and mathematically fitted for quantifying wind field characteristics rooftop turbines on the roof. For the VAWTs investigated in this study, the impacts of the operation of VAWTs on the wind field are non-negligible on the roof of building. The findings of this study will provide valuable insights for the optimal placement of VAWTs and utilization of wind energy on the rooftops.

Nonlinear Structural Vibration of Multi-Megawatt Vertical Axis Wind Turbine Blades-Part 1: Derivation of Motion Equations

Replacing fossil fuels with renewable energy is one of the important means to improve energy scarcity and environmental protection, and is of great significance for achieving sustainable development of human society. Vertical axis wind turbine (VAWT) is one of the key equipment for applying green energy — wind energy. During the operation of VAWT, its blades not only have to withstand periodic aerodynamic loads, but also suffer from centrifugal force effects and the Coriolis force effect caused by coupling with multi-dimensional deformation. The complex stress situation makes the blades susceptible to damage, resulting in structural resonance and instability of aeroelastic stability. This paper applies the Euler beam model and Hamilton principle to establish a nonlinear structural vibration motion equation for large-span vertical axis wind turbine blades with fully coupled four-dimensional deformation (i.e. lateral bending deformation, chord bending deformation, axial torsion deformation, and axial tensile deformation), and retains the nonlinear term of the motion equation to the third-order infinitesimal. Separating the motion equation into equilibrium equation and dynamic equation, it can be seen that the displacement of equilibrium state caused by the centrifugal force of the rigid body exists in the form of a coefficient in the dynamic equation, which will have an impact on the dynamic characteristics. This modeling work can provide a research foundation for exploring the dynamic characteristics and aeroelastic stability of VAWT blades in the future.

Effect of Stator Blades on the Startup Dynamics of a Vertical Axis Wind Turbine

Vertical Axis Wind Turbines (VAWTs) are omni-directional, low-cost, low-efficiency wind power extractors. A conventional drag-based VAWT consists of multiple thin rotor blades with a typical peak Tip Speed Ratio (λ) of < 1. Their lower cut-in speed and maintenance cost make them ideal for power generation in urban environments. Numerous studies have been carried out analysing steady operation of VAWTs and quantifying their performance characteristics, however, minimal attention has been paid to their start-up dynamics. There are a few recent studies in which start-up dynamics of lift-based VAWTs have been analysed but such studies for drag-based VAWTs are severely limited. In this study, start-up dynamics of a conventional multi-blade drag-based VAWT have been numerically investigated using a time-dependant Computational Fluid Dynamics (CFD) solver. In order to enhance the start-up characteristics of the drag-based VAWT, a stator has been integrated in the design assembly. The numerical results obtained in this study indicate that an appropriately designed stator can significantly enhance the start-up of a VAWT by directing the flow towards the rotor blades, leading to higher rotational velocity (ω) and λ. With the addition of a stator, the flow fields downstream the VAWT becomes more uniform.

Formulation, Validation, and Application of a Novel 3D BEM Tool for Vertical Axis Wind Turbines of General Shape and Size

Low order models based on the Blade Element Momentum (BEM) theory exhibit modeling issues in the performance prediction of Vertical Axis Wind Turbines (VAWT) compared to Computational Fluid Dynamics, despite the widespread engineering practice of such methods. The present study shows that the capability of BEM codes applied to VAWTs can be greatly improved by implementing a novel three-dimensional set of high-order corrections and demonstrates this by comparing the BEM predictions against wind-tunnel experiments conducted on three small-scale VAWT models featuring different rotor design (H-shaped and Troposkein), blade profile (NACA0021 and DU-06-W200), and Reynolds number (from 0.8×105 to 2.5×105). Though based on the conventional Double Multiple Stream Tube (DMST) model, the here-presented in-house BEM code incorporates several two-dimensional and three-dimensional corrections including: accurate extended polar data, flow curvature, dynamic stall, a spanwise-distributed formulation of the tip losses, a fully 3D approach in the modeling of rotors featuring general shape (such as but not only, the Troposkein one), and accounting for the passive effects of supporting struts and pole. The detailed comparison with experimental data of the same models, tested in the large-scale wind tunnel of the Politecnico di Milano, suggests the very good predictive capability of the code in terms of power exchange, torque coefficient, and loads, on both time-mean and time-resolved basis. The peculiar formulation of the code allows including in a straightforward way the usual spanwise non-uniformity of the incoming wind and the effects of skew, thus allowing predicting the turbine operation in a realistic open-field in presence of the environmental boundary layer. A systematic study on the operation of VAWTs in multiple environments, such as in coastal regions or off-shore, and highlighting the sensitivity of VAWT performance to blade profile selection, rotor shape and size, wind shear, and rotor tilt concludes the paper.

CFD Investigation of Separation Control on a Vertical Axis Wind Turbine: Steady and Unsteady Suction

High-fidelity two-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) simulations are employed to investigate the influence of boundary layer suction through a slot located near the leading edge of a vertical axis wind turbine operating in dynamic stall. The analysis includes both steady and unsteady suction with different frequencies. The results shows that: (i) when the suction slot is located within the chordwise extent of the laminar separation bubble, dynamic stall can be avoided with minimal suction amplitude; (ii) the most promising suction location is the most upstream suction location studied, at 8.5%c where c is the blade chord length; (iii) the suction only needs to be applied during the azimuthal angles when dynamic stall occurs; (iv) the oscillation frequency of the suction velocity has insignificant influence on the obtained turbine power gain; (v) applying unsteady suction is interesting as it reduces the energy consumption of the suction system, thus the net power gain.

Performance Evaluation of H-Type Darrieus Vertical Axis Wind Turbine with Different Turbine Solidity

Among renewable energy resources, wind energy is one of the best alternative for power generation. Recently, vertical axis wind turbine (VAWT) received renewed interest as small-scale wind energy converter due to its suitability for urban application, where the wind condition is known to be unsteady and turbulence. Amongst various type of VAWTs, H-type Darrieus rotor has become more popular, thanks to its simple construction features, resulting to low manufacturing and installation cost. The aim of this paper is to evaluate numerically the power performance of straight-bladed Darrieus VAWT with different turbine solidity using computational fluid dynamic (CFD) technology. A series of two-dimensional CFD simulations of a three-bladed H-type Darrieus rotor were performed with 3 different solidities, σ (0.3, 0.5 and 0.7) to evaluate their power performance. Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations were used to calculate the instantaneous moment coefficient, Cm and power coefficient, Cp over a range of tip speed ratio, λ (0.5–4.5) with a free stream velocity of 8.0 m/s. The simulation results show that high solidity turbine performed well at low values of λ while turbine with low solidity has a wider operating range of λ and performed better at λ > 3.0 due to less blade-wake interactions between upstream and downstream halves of the turbine and lower blockage effect. The findings lend substantially to our understanding of physics flow around blades and turbine in order to optimize the power performance of small scale straight-bladed Darrieus VAWT operating in unsteady and turbulence wind condition.

Power performance assessment of H-rotor vertical axis wind turbines with different aspect ratios in turbulent flows via experiments

Vertical axis wind turbines (VAWTs) are suitable for applications in urban areas. Their ultimate power production relies on both the structural parameters and wind conditions. In this study, the power performance of H-rotor VAWTs immersed in turbulence of various length scales was assessed via wind tunnel testing. The influence of turbine aspect ratio on the power performance was investigated in the turbulent flows. A range of length scales were produced by wooden grids of different mesh sizes. The power coefficients, CP, of the VAWTs were measured by connecting an external resistance in the rotor circuit. Uncertainty analysis of the experimental data was conducted thereafter. Encouragingly, the rated CP was raised by approximately 100% in the turbulent flows compared to that in the smooth flow. The results evidently proved that turbulence was favorable for the operation of VAWTs. Moreover, the rated CP was inversely proportional to length scale, whereas it increased with aspect ratio, irrespective of flow regimes, the underline reasons for which were addressed. Further, the power-law equations were proposed to mathematically express the length scale and aspect ratio effects. This study contributes to the design and application of VAWTs in built environments.

Large Eddy Simulation of an isolated vertical axis wind turbine

A computational study is presented on the wake features of a Vertical Axis Wind Turbine (VAWT), with a particular focus on the way stall phenomena lead to the formation of large coherent structures. In our earlier work (Int. J. Heat Fluid Flow 61: 75–84, 2016) numerical results were compared with experiments carried out in similar conditions. The operation of VAWTs is characterized by the generation of large vortices, especially at low Tip Speed Ratios (TSRs), corresponding to larger angles of attack. In the present study the use of a Large Eddy Simulation (LES) approach, coupled with an immersed-boundary (IB) formulation, allowed the solution of such coherent structures, which were found to affect substantially the flow within the rotor and downstream of the turbine. Two values of TSR are investigated here, in order to discuss its influence on the wake structure. Lower TSRs are associated to boundary layer separation closer to the leading edge of the blades and larger rollers, during both upwind and downwind stall. Upwind stall affects more substantially wake properties, producing larger structures populating the leeward side of the overall wake.

Leading-edge serrations for performance improvement on a vertical-axis wind turbine at low tip-speed-ratios

The performance of vertical-axis wind turbines (VAWTs) are substantially affected by the phenomenon of dynamic stall which is induced by the variations of angle of attack of rotating blades, especially at low tip-speed-ratios (TSRs). Large and sudden torque fluctuations are observed to take place when the dynamic stall vortices, formed near the blade leading-edge, are transported downstream. At low TSRs (λTSR<4) and relatively low Reynolds number (Re<105), dynamic stall occurs periodically during the rotation of turbine blades. This results in a sharp drop in lift coefficient and therefore rotor torque and power output are essentially reduced. The purpose of the present study is to investigate the concepts for improving the power performance of a conventional H-type VAWT model by implementing sinusoidal serrations on the leading-edge of turbine blades to control the dynamic flow separation at low TSRs. A thorough numerical study has been carried out to obtain the detailed flow fields for analysis and visualization. The power output results show that the improved turbine design with the sinusoidal serration profile of the wave amplitude h=0.025c and the wavelength λs=0.33c not only increases the power generation at low TSRs, but also enhances the capability of wind energy extraction at the optimal TSR in comparison to the baseline model. The flow separation is significantly controlled in the azimuth angle ranges from 75° to 160°, where the positive torque generation is also found to be considerably increased in the improved turbine model. Counter-rotating vortex pairs are generated due to the existence of serrations, which suppress the flow separation, especially in the regions near the peak-serration sections. Additionally, the lift coefficients illustrate a delay of occurrence of dynamic stall and a notable improvement of maximum lift in the improved wind turbine model in comparison to the baseline model. The effects of Reynolds number variation also reveal that the improved model would gain more benefits in power generation at low Reynolds number compared with that at high Reynolds number. The simulated results demonstrate that the leading-edge serration strategy could be an effective solution to control the dynamic stall in the operation of VAWTs.

Simulating the dynamic behavior of a vertical axis wind turbine operating in unsteady conditions

The present work aims at assessing the reliability of a simulation tool capable of computing the unsteady rotational motion and the associated tower oscillations of a variable speed VAWT immersed in a coherent turbulent wind. As a matter of fact, since the dynamic behaviour of a variable speed turbine strongly depends on unsteady wind conditions (wind gusts), a steady state approach can't accurately catch transient correlated issues.The simulation platform proposed here is implemented using a lumped mass approach: the drive train is described by resorting to both the polar inertia and the angular position of rotating parts, also considering their speed and acceleration, while rotor aerodynamic is based on steady experimental curves. The ultimate objective of the presented numerical platform is the simulation of transient phenomena, driven by turbulence, occurring during rotor operation, with the aim of supporting the implementation of efficient and robust control algorithms.

Airfoil Design for Vertical Axis Wind Turbine Operating at Variable Tip Speed Ratios

Airfoil Design for Vertical Axis Wind Turbine Operating at Variable Tip Speed Ratios

Airfoil Design for Vertical Axis Wind Turbine Operating at Variable Tip Speed Ratios

A new airfoil design method for H type vertical axis wind turbine is introduced in the present study. A novel indicator is defined to evaluate vertical axis wind turbine aerodynamic performance at variable tip speed ratios and selected as the airfoil design objective. A mathematic model describing the relationship between airfoil design variables and objective is presented for direct airfoil design on the basis of regression design theory. The aerodynamic performance simulation is conducted by computational fluid dynamics approach validated by a wind tunnel test in the study. Based on the newly developed mathematic model, a new airfoil is designed for a given wind turbine model under constant wind speed of 8 m/s. Meanwhile, the comparison of aerodynamic performance for newly designed airfoil and existing vertical axis wind turbine airfoils is studied. It has been demonstrated that, by the novel indicator, the rotor aerodynamic performance at variable tip speed ratios with the newly designed airfoil is 6.78% higher than the one with NACA0015 which is the airfoil widely used in commercial H type vertical axis wind turbine.

Design Optimisation of Shroud-augmented Dual-rotor Exhaust Air Energy Recovery Wind Turbine Generator Using Hybrid Non-destructive Evaluation Approach

An innovative system for recovering exhaust gas is created to recycle the energy from industry cooling tower waste. Two Vertical Axis Wind Turbines (VAWTs) with enclosure are positioned at a specific position at the outlet of cooling tower to harness the wind energy for electricity generation. From structural dynamics aspect, adding two VAWTs or dual-rotor with enclosure can contribute extra burden to the cooling tower. This has changed the dynamic characteristics of the cooling tower. Therefore, a hybrid non-destructive evaluation approach is proposed to investigate the dynamic behaviour and reliability of this new design. From this approach, it was found that the overall vibration of the cooling tower increased from 3mm/s to 26mm/s after the installation of the VAWTs with enclosure. The VAWTs are not operating at its optimum speed due to energy loss in vibration. The operating speed of VAWTs (i.e. 7.5Hz) was found close to one of the natural frequencies of VAWT (i.e. 7.0Hz). This indicates structural dynamic weakness which causes high vibration as recorded. Structural Dynamic Modification (SDM) is performed on the new design in the attempt to reduce the vibration at VAWTs and improve the reliability of VAWTs with enclosure in order to ensure optimum operation of VAWTs. The SDM is expected to shift the natural frequency away from the operating frequency with around 81% of reduction in vibration level which leads it to safe and unrestricted long-term operation. In addition, reducing vibration at VAWTs also recovers the energy loss of 419 Joules and subsequently maximises the efficiency of wind turbine.

Enhancing vertical axis wind turbine by dynamic stall control using synthetic jets

A novel application of modern active flow control on vertical axis wind turbine (VAWT) dynamic stall is explored in this paper. Operation of VAWT at low blade speed ratios offers numerous noise and safety advantages over higher blade speed ratios but suffer from dynamic stall that degrades their performance. Consequently, an experimental investigation was carried out on VAWT operating at low blade speed ratios that employed synthetic jets as a means of active flow control. Large angles of attack on VAWT rotor blades were simulated by sinusoidal oscillation of a thick NACA 0020 turbine blade about its quarter-chord. The pressure distribution and thrust hysteresis results showed synthetic jet actuation to be effective when dynamic stall effects were present, and clearly demonstrated that they could be utilised to improve low blade speed ratio VAWT performance and thus enhance turbine safety and noise. The findings from this study have significant practical implications for the design and development of safe and efficient wind turbines, both small- and large-scale, and further the cause of wind as a viable renewable energy source.

Improving Safety and Performance of Small-Scale Vertical Axis Wind Turbines

Although horizontal axis wind turbines (HAWT) are considered more efficient in operation than their vertical axis wind turbine (VAWT) counterpart and are more commonly used in wind farms as large wind turbines, the VAWT may offer greater advantages in safety and operation when it comes to their application within the urban environment. Yaw control systems are an essential requirement for the safe operation of HAWT, which are costly and require high levels of maintenance, but are inherently unnecessary for VAWT. At low blade speed ratios, the performance of VAWT degrades owing to strong dynamic stall effects. This necessitates VAWT operation at high blade speed ratios to suppress them. However, the consequent large rotational speeds lead to hazardous operation especially in confined urban areas. Thus to improve the low blade speed performance, a preliminary experimental investigation has been carried out at the Aerodynamics Laboratory of the University of New South Wales on an H-type VAWT blade that employed zero-net mass flux actuation. This technique has traditionally been used for static stall delay and flow separation mitigation on aircraft wings. In the present study, large relative angles of incidence were simulated by sinusoidally oscillating the blade about its quarter-chord, and resulted in the formation of dynamic stall vortices. The application of zero-net mass flux actuation was found to have a beneficial effect on the blade aerodynamic performance by either suppressing dynamic stall or delaying its onset to higher angles of attack. This study, therefore, suggests that reduced oscillatory loads and more robust output power can be achieved with zero-net mass flux actuation on VAWT operating at low blade-speed ratios. Consequently, the findings have positive practical implications for the design of small-scale VAWT for widespread use in the urban environment.

Wind Tunnel Hotwire Measurements, Flow Visualization and Thrust Measurement of a VAWT in Skew

The results of experimental research on the wake and induced flow around a vertical axis wind turbine (VAWT) in skew are presented. The previous research on VAWTs in skew is limited because this operation mode has only recently been found to be significant in the operation of VAWTs in the built environment. These results contain hotwire measurements of the incoming flow and wake of a VAWT in nonskewed and skewed flow. The high sampling rate of the hotwire data allows the effects of blade passing to be identified. Flow visualization of the tip vortices is also presented. Thrust measurements of the rotor were performed to understand the effect of skew on thrust variation and to compare with analytical predictions.