Optimal Tip Speed Ratio Research Articles

Optimization of a vertical axis wind turbine for application in an urban/suburban area

The goal of this study is to investigate the effect of various design parameters on the performance of a Vertical Axis Wind Turbine (VAWT) subjected to realistic unsteady wind conditions. Thirteen turbine design configurations are examined to determine if an optimal VAWT has applications in an urban/suburban environment. The four design parameters of interest include the height-to-diameter aspect ratio (0.83≤H/D≤1.34), blade airfoil shape (NACA 0012, 0015, 0018), turbine solidity (12≤S≤25%), and turbine moment of inertia. The height and diameter of the turbine varied between 1.89 and 2.54 m, depending on the aspect ratio. The turbine moment of inertia was calculated using a computer-aided design drawing of the turbine, along with the realistic material properties of blades, shafts, and supports. The energy generated by each VAWT design configuration is simulated using a full year of actual wind speed data collected in 2009 at 9 different locations around Oklahoma City spanning an area of approximately 500 km2. The wind data were acquired from the top of traffic light posts at a height of about 9 m above the ground. In all cases, an active control strategy is used that allows the turbine to continuously adjust its rotational speed in response to the fluctuating wind. The results suggest that, for the case of operation in unsteady winds, the optimal power coefficient (Cp) versus tip speed ratio curve is not necessarily the one exhibiting the highest peak Cp value but rather the broadest shape. Of the thirteen configurations examined, the optimal wind turbine design capable of harvesting the most energy from the gusty winds was found to have an aspect ratio of H/D=1.2, a solidity of S=12%, and a blade shape using the NACA 0015 airfoil. This design also displayed the lowest moment of inertia. However, when the effects of mass were removed, this design still performed the best. The site-to-site variation in terms of energy captured relative to the available energy in the gusty winds was only about 5% on average and increased slightly with turbine moment of inertia. Four of the suburban sites studied were deemed to be economically viable locations for a small-scale VAWT. The results further indicate that, at one of these sites, the levelized cost of energy associated with the top performing turbine designs examined in the study was about 10% less than the national electricity price, meaning that wind energy provides a cheaper alternative to fossil fuel at this location. It is surmised that VAWTs could economically harvest wind energy in the urban center as well if the turbines were located higher than 9 m, such as on the rooftops of commercial/residential buildings.

Performance optimization of a wind turbine column for different incoming wind turbulence

Abstract Optimization of the performance for a wind turbine column is performed by coupling a RANS solver for prediction of wind turbine wakes and dynamic programming. Downstream evolution of wind turbine wakes is simulated with low computational cost comparable to that of wake engineering models, but with improved accuracy and capability to simulate different incoming wind turbulence. Dynamic programming is used to estimate optimal tip speed ratio ( TSR ) and streamwise spacing of the turbines by using a mixed-objective performance index consisting of total power production from the entire turbine array with the penalty of the average turbulence intensity impacting over the rotor discs. The penalty coefficient, representing the economic impact of fatigue loads as ratio of wind energy revenue, is varied in order to mimic different economic periods. The results suggest that a general strategy for wind farm optimization should consist in coupling design performed through spacing optimization and using a relatively low penalty coefficient for the fatigue loads, while wind turbine operations are optimized by varying TSR .

An influence of the different incoming wake-like flows on the rotor vibrations

The aim of the current investigation is the rotor vibrations generated by the disturbances caused the different types of incoming wake-like flows. Those wakes arriving at the tested rotor were created by two ways: a passive wake generator (immobile disk) and an upstream rotating rotor as an active wake generator. The influence of both wakes on the tested rotor was studied in a water flume. A model of the tested three-bladed rotor designed using Glauert’s optimum theory at an optimal tip speed ratio λ = 5 was placed in both “passive” and “active” wakes to recognize dissimilarities on the vibrations of the tested rotor. The distance from the wake generators to the tested rotor was varied from 4 to 8 rotor diameters. Also, the shift between the rotor axis and axis of the incoming wakes was changed to 0, 0.5 and 1 rotor diameters. The flow condition before rotor was measured with high temporal accuracy using LDA. The turbulent intensity of the incoming wake flows changed from 3 to 16% due to the types of the wake generators. Power and thrust characteristics and their pulsations of the tested rotor were measured by strain gauges. The dependences of power coefficients from tip speed ratios and positions of the wake generators were documented. The present study showed a strong influence of the initial flow from the two different wake generators on the rotor vibrations.

Near wake region of an industrial scale wind turbine: comparing LES-ALM with LES-SMI simulations using data mining (POD)

Accurate prediction of power generation capability needs proper assessment of blade loading and wake behavior. In this regard, the Sliding Mesh Interface (SMI) approach and the Actuator Line Model (ALM) are two diverse computational fluid dynamics (CFD) based approaches of simulating the turbine behavior, each having its own merits and demerits. The SMI technique simulates the unsteady flow by explicitly modeling the blades and their rotation using a dynamic mesh, while in Actuator Line Model, the blades are not modeled explicitly but each blade is resolved as a rotating line (made of N actuator segments), over which the forces are computed. The current work focuses on simulating an industrial scale reference turbine and in differentiating the near wake dynamics predicted by these two approaches using Large Eddy Simulation (LES) and Proper Orthogonal Decomposition (POD) technique (a data mining tool). Initially, the ALM is compared with FAST model for the prediction of variation of power coefficient with the Tip Speed Ratio (TSR). The ALM is able to capture the varying trend and it predicts a similar optimum tip speed ratio as the FAST model. At this optimum TSR condition, the ALM is compared with the SMI method for a study limited to the near wake region. Comparisons between SMI and ALM shows that : (a) The SMI is predicting more complex 3D nature of the flow, and (b) the POD shows that ALM captures the shear regions of wake but it does not capture the vast compendium of length and time scales of eddies as SMI does. However, despite these limitations, the ALM has been able to capture the qualitative trend in wake deficit and the power coefficient variation with tip speed.

Numerical simulation of hydrodynamic performance of blade position-variable hydraulic turbine

This paper presents a study of the movement and the hydrodynamic performance of a new tide-powered hydraulic turbine through numerical simulations. By means of the moving mesh method, the open-closed sequences of the blades and the movement of the rotors are obtained and the angular velocity and the average energy utilization coefficient under different tip speed ratios are also obtained. Moreover, the optimum tip speed ratio is identified by integrating the output power and the energy utilization coefficient of the hydraulic turbine with different tip speed ratios, providing data support for the prototype design of the hydraulic turbine.

Comparison of low-order aerodynamic models and RANS CFD for full scale 3D vertical axis wind turbines

A Double Multiple Streamtube model, a free-wake vortex model (both widely used for vertical axis wind turbine design) and RANS CFD simulations are used in this work to predict the performance of the 17 m Vertical Axis Wind Turbine, field tested by Sandia National Laboratories. The three-dimensional, full scale calculations are compared with the experiments in terms of power coefficient, power and instantaneous turbine torque to assess the validity of each model. Additionally, the two aerodynamic models and RANS CFD are compared to each other in terms of thrust and lateral force. The two models and CFD agree well with the experiments at the turbine optimal tip speed ratio. However, away from the optimal tip speed ratio, the streamtube model significantly deviates from the experimental data and from the other numerical models. RANS CFD gives a good agreement with the experiments, slightly underestimating the power coefficient at every tip speed ratio tested. The vortex model proves to be a useful tool with a better accuracy than the streamtube model and a much lower computational cost compared to RANS CFD.

Wind Turbine Power Curve Design for Optimal Power Generation in Wind Farms Considering Wake Effect

In modern wind farms, maximum power point tracking (MPPT) is widely implemented. Using the MPPT method, each individual wind turbine is controlled by its pitch angle and tip speed ratio to generate the maximum active power. In a wind farm, the upstream wind turbine may cause power loss to its downstream wind turbines due to the wake effect. According to the wake model, downstream power loss is also determined by the pitch angle and tip speed ratio of the upstream wind turbine. By optimizing the pitch angle and tip speed ratio of each wind turbine, the total active power of the wind farm can be increased. In this paper, the optimal pitch angle and tip speed ratio are selected for each wind turbine by the exhausted search. Considering the estimation error of the wake model, a solution to implement the optimized pitch angle and tip speed ratio is proposed, which is to generate the optimal control curves for each individual wind turbine off-line. In typical wind farms with regular layout, based on the detailed analysis of the influence of pitch angle and tip speed ratio on the total active power of the wind farm by the exhausted search, the optimization is simplified with the reduced computation complexity. By using the optimized control curves, the annual energy production (AEP) is increased by 1.03% compared to using the MPPT method in a case-study of a typical eighty-turbine wind farm.

Study on approach of performance improvement of VAWT employing double multiple stream tubes model

On the circular blade path of a Darrieus-type vertical axis wind turbine (VAWT), there are two azimuth positions where torques are small or even negative. These two azimuths have the lowest aerodynamic performance and greatly weaken the overall performance of the VAWT. An approach is proposed to rebuild the flow field of the turbine blades by applying two additional airflows at the two azimuths to enhance the local performance. The aim is to improve the overall power coefficient of the VAWT. A Φ-type VAWT with a diameter of 2 m and a height of 2 m is studied to evaluate the effectiveness of the proposed approach. The effect of additional airflow on the aerodynamic performance of the blade and the turbine is researched using the double multiple streamtube model. The simulation results suggest that the additional airflows are effective in promoting the performance of the rotor blades and the overall rotor; the increase in maximum power coefficient achieved was 6% at an optimum tip speed ratio. Some rules of the effect on the performances of the rotor are obtained in this paper.

Wind estimation with a non-standard extended Kalman filter and its application on maximum power extraction for variable speed wind turbines

To maximize power extraction at below-rated wind speeds, variable-speed wind turbines must be controlled by tracking the optimal TSR (tip speed ratio) and pitch angle, which depend on the wind speed measured by nacelle anemometers or provided by an EWS (effective wind speed) estimator. However, the measured values are imprecise and existing estimators cannot provide qualified estimates. This paper addresses this problem by presenting a novel solution with a non-standard extended Kalman filter. To avoid using imprecise wind speed measurements or other costly measurement devices, the proposed solution employs a virtual measurement that is calculated from related estimated states. In addition, the solution presents an internal EWS model by considering the tower shadow effect, so the obtained model is more general than the statistical model that is difficult to obtain in practice. Compared with existing estimators, the proposed estimator provides more precise estimated results and is suitable for control application. Its application is investigated on the MPE (maximum power extraction) of a variable speed wind turbine, for which an industrial baseline controller is optimized by enhancing the optimal TSR tracking and pitch adjustment. The proposed solutions are validated using both simulation and field testing results. Comparing the proposed estimation solution to two existing methods demonstrates that the former gives the best estimate results. Moreover, its application for the MPE increases annual energy production by approximately 0.8% in comparison with the baseline controller, which is a considerable energy production increment.

Effect of rotor aspect ratio and solidity on a straight-bladed vertical axis wind turbine in three-dimensional analysis by the panel method

Due to the complated flow field and aerodynamic forces characteristics, the performance and safety standard of straight-bladed VAWT have not been full developed. The objective of this study is to investigate the effect of rotor aspect ratio and solidity on the power performance in three-dimensional analysis by panel method. The panel method is based on the assumption of an incompressible and potential flow coupled with a free vortex wake. First of all, the fluctuations of power coefficient and the circulation amount distribution of the bound vortex are discussed at the fixed solidity of σ = 0.064 during rotation. Then, the fluctuations of power coefficient and the circulation amount ratio are also investigated in the spanwise direction of the blade. It can be observed from the results that the peak of power coefficient increases with the increase of the ratio of the diameter and blade span length H/D at the fixed solidity. However, the optimum tip speed ratio was expected to be increased with the increase of H/D. Moreover, in the case of the fixed rotor aspect ratio of H/c = 6, the power coefficient depends on the rotor aspect ratio, rather than the ratio of the diameter and blade span length. Compared with the H/D = 1.2, the circulation amount ratio of H/D = 0.9 indicates a large negative value in the blade center position.

Large Eddy Simulations on Vertical Axis Hydrokinetic Turbines and flow phenomena analysis

Large Eddy Simulations have been performed on a Vertical Axis Hydrokinetic Turbine (VAHT) at various tip speed ratios. The turbine power coefficient and the flow through the turbine show good agreement with experimental data. To better understand the evolution of the VAHT power coefficient through the tip speed ratios the contribution of the VAHT main regions to the global power coefficient has been evaluated. At the optimal tip speed ratio (λ = 2) blade tip vortex and blade/arm connection drag generate losses and decrease the efficiency of the regions around the blade tip and blade/arm connection. The region around the blade tip is the most degraded. When the tip speed ratio decreases to λ = 1, deep dynamic stall with the presence of a Leading Edge Vortex is observed at early angular positions and leads to the power coefficient drop. The power coefficient drop around the blade tip and the blade/arm connection happens at higher angular position than on the middle part of the blade. For a tip speed ratio higher than optimal, the region around the blade/arm connection shows the highest decrease in efficiency. Despite its small height compared to the blade this region is responsible for about 36% of the VAHT power coefficient decrease at λ = 2.5.

Turbulence influence on optimum tip speed ratio for a 200 kW vertical axis wind turbine

The influence of turbulence intensity (TI) on the tip speed ratio for maximum power coefficient, here called λCp_max, is studied for a 200 kW VAWT H-rotor using logged data from a 14 month period with the H-rotor operating in wind speeds up to 9 m/s. The TI - λCp_max relation is examined by dividing 10 min mean values in different turbulence intensity ranges and producing multiple CP(λ) curves. A clear positive relation between TI and λCp_max is shown and is further strengthened as possible secondary effects are examined and deemed non-essential. The established relation makes it possible to tune the control strategy to enhance the total efficiency of the turbine.

Dynamic vibrations in wind energy systems: Application to vertical axis wind turbine

Dynamic analysis of Darrieus turbine bevel spur gear subjected to transient aerodynamic loads is carried out in the present study. The aerodynamic torque is obtained by solving the two dimensional unsteady incompressible Navies Stocks equation with the k–ω shear stress transport turbulence model. The results are presented for several values of tip speed ratio. The two-dimensional Computational Fluid Dynamics model is validated with experimental results. The optimum tip speed ratio is achieved, giving the best overall performance.In this study, we developed a lamped mass dynamic model with 14 degrees of freedom. This model is excited by external and internal issues sources. The main factors of these excitations are the periodic fluctuations of the gear meshes' stiffness and the unsteady aerodynamic torque oscillations. The vibration responses are obtained in time and frequency domains. The originality of our work is the correlation between the complexity of the aerodynamic phenomenon and the non-stationary dynamics vibration of the mechanical gearing system.The effect of the rotational speed on the dynamic behavior of the Darrieus turbine is also discussed. The present study shows that the variation of rotor rotational speed directly affects the torque production. However, there is a small change in the dynamic vibration of the studied gearing system.

An offshore hydraulic wind turbine generator with variable-diameter rotor: Design, modeling and experiment

Offshore wind power is more abundant and stronger than the onshore, and more and more research enthusiasms have been raised in recent years. However, there are still many issues in the utilization of the offshore wind power such as the cost of installations and maintenance and the ability to resist extreme weather conditions. In this article, an offshore hydraulic wind turbine generator with variable-diameter rotor is presented. The diameter of the rotor can be regulated according to the wind speeds to achieve the maximum power coefficient. The hydraulic energy working as the transmission medium can improve the output power quality. The high-speed gearbox is removed, and the generator is installed on the platform, which facilitates the installations and maintenance. Here, the power conversion principle of the wind turbine generator was introduced first. Then, the dynamics and performance of the wind turbine generator was obtained. The relationship between the diameter of the rotor and the wind speed was established according to the dynamics and the optimum tip-speed ratio. Relying on the specific parameters, the dynamic response was calculated in Simulink. The results show that the instantaneous output of the wind turbine generator is relatively stable. Based on the power recovery method, the test platform was built, and the efficiency of the energy conversion device was tested. The experimental results demonstrate that the efficiency of the energy conversion device can be 88%. Finally, the total efficiency of the offshore hydraulic wind turbine generator was predicted to be 33.7%.

Active yaw control in a horizontal axis wind system without requiring wind direction measurement

This study presents an improved active yaw control technique for a horizontal-axis wind turbine that is driven by a full power converter system with maximum power point tracking control. The turbine is aligned to the wind direction through the error between the rotor speed determined by the optimal tip speed ratio and the real rotor speed commanded by the maximum power point tracking control. Thus, the implementation of the yaw control is not based on the measurement of the wind direction and therefore, it is not influenced by inaccuracies that may be caused by the vortex flow downstream of the blades. The proposed yaw control method is intended for large turbines, since larger increase in the generated electric energy can be attained. However, since it is cost-effective, it is affordable to be applied in low power wind systems. Selective simulation and experimental results are presented in order to demonstrate the effectiveness and the resulting improvements of the proposed control scheme. The simulation analysis is conducted in a large wind turbine with permanent magnet synchronous generator and the experimental verification in a low power real wind system with squirrel cage induction generator. However, it can also be applied to a doubly fed induction generator.

Design for the turbine of solar chimney power plant system with vertical collector

Based on the design theory of the wind turbine, a turbine applied to solar chimney power plant with a vertical collector is designed, and the performance of the wind turbine is conducted by wind tunnel test. The results point out that the experimental performance is consistent with the designed performance. Then the number of blades and tip speed ratio are conducted, as well as we obtained the optimum blade number and tip speed ratio of turbine for the system.

Tip Speed Ratio Based MPPT Algorithm and Improved Field Oriented Control for Extracting Optimal Real power and Independent Reactive Power Control for Grid Connected Doubly Fed Induction Generator

<p>Doubly Fed Induction Generator (DFIG) needs to get adopted to change in wind speeds with sudden change in reactive power or grid terminal voltage as it is required for maintaining synchronism and stability as per modern grid rules. This paper proposes a controller for DFIG converters and optimal tip speed ratio based maximum power point tracking (MPPT) for turbine to maintain equilibrium in rotor speed, generator torque, and stator and rotor voltages and also to meet desired reference real power during the turbulences like sudden change in reactive power or voltage with concurrently changing wind speed. The performance of DFIG is compared when there is change in wind speed only, changes in reactive power and variation in grid voltage along with variation in wind speed.</p>

Tip Speed Ratio Based MPPT Algorithm and Improved Field Oriented Control for Extracting Optimal Real power and Independent Reactive Power Control for Grid Connected Doubly Fed Induction Generator

<p>Doubly Fed Induction Generator (DFIG) needs to get adopted to change in wind speeds with sudden change in reactive power or grid terminal voltage as it is required for maintaining synchronism and stability as per modern grid rules. This paper proposes a controller for DFIG converters and optimal tip speed ratio based maximum power point tracking (MPPT) for turbine to maintain equilibrium in rotor speed, generator torque, and stator and rotor voltages and also to meet desired reference real power during the turbulences like sudden change in reactive power or voltage with concurrently changing wind speed. The performance of DFIG is compared when there is change in wind speed only, changes in reactive power and variation in grid voltage along with variation in wind speed.</p>

A Multi-Point Method Considering the Maximum Power Point Tracking Dynamic Process for Aerodynamic Optimization of Variable-Speed Wind Turbine Blades

Due to the dynamic process of maximum power point tracking (MPPT) caused by turbulence and large rotor inertia, variable-speed wind turbines (VSWTs) cannot maintain the optimal tip speed ratio (TSR) from cut-in wind speed up to the rated speed. Therefore, in order to increase the total captured wind energy, the existing aerodynamic design for VSWT blades, which only focuses on performance improvement at a single TSR, needs to be improved to a multi-point design. In this paper, based on a closed-loop system of VSWTs, including turbulent wind, rotor, drive train and MPPT controller, the distribution of operational TSR and its description based on inflow wind energy are investigated. Moreover, a multi-point method considering the MPPT dynamic process for the aerodynamic optimization of VSWT blades is proposed. In the proposed method, the distribution of operational TSR is obtained through a dynamic simulation of the closed-loop system under a specific turbulent wind, and accordingly the multiple design TSRs and the corresponding weighting coefficients in the objective function are determined. Finally, using the blade of a National Renewable Energy Laboratory (NREL) 1.5 MW wind turbine as the baseline, the proposed method is compared with the conventional single-point optimization method using the commercial software Bladed. Simulation results verify the effectiveness of the proposed method.

A new measure of the degree of controllability for linear system with external disturbance and its application to wind turbines

This paper is concerned with the measure of degree of controllability (DOC) for linear system with external disturbance. A new measure of DOC, in which the initial condition is regarded as a random vector, is proposed in this paper by solving the fixed-time expected minimum-energy transfer control problem. Since this new measure is dependent on the statistical information of initial condition rather than its estimated value, it is more suitable to apply the proposed measure in the design and optimization of the structural parameters of controlled plants. Furthermore, the simulations on the NREL (National Renewable Energy Laboratory) CART3 wind turbine demonstrate that the relation of the proposed measure to turbine parameters (including rotor inertia and optimum tip speed ratio) coincides with that of the MPPT efficiency to turbine parameters. This indicates that the proposed measure is applicable to guide the design and optimization of the structural parameters of wind turbines. Meanwhile, a mass-spring-damper system is also simulated to validate the proposed measure.