Interactive flow field around two Savonius turbines

The use of a Savonius type of vertical axis wind turbine is expanding in urban environments as a result of its ability to withstand turbulence as well as its relatively quiet operation. In the past, single turbine performance has been investigated primarily for determining the optimum blade configuration. In contrast, combining multiple Savonius turbines in the horizontal plane produces extra power in particular configurations. This results from the interaction between the two flow fields around individual turbines. To understand quantitatively the interaction mechanism, we measured the flow field around two Savonius turbines in close configurations using particle image velocimetry. The phase-averaged flow fields with respect to the rotation angle of the turbines revealed two types of power-improvement interactions. One comes from the Magnus effect that bends the main stream behind the turbine to provide additional rotation of the downstream turbine. The other is obtained from the periodic coupling of local flow between the two turbines, which is associated with vortex shedding and cyclic pressure fluctuations. Use of this knowledge will assist the design of packaged installations of multiple Savonius turbines.     

A review of materials degradation in utility scale wind turbines

Abstract

This paper presents a comprehensive review of the state of knowledge of materials used in the major wind turbine components of both land based and offshore wind turbines. The paper is divided into the following seven major sections: utility scale wind turbine design overview; current state of wind turbine technology; review of degradation of materials used in wind turbines; a summary of materials degradation service experience; condition monitoring overview; review of materials based research and development for wind turbines; a summary of missing knowledge and future materials challenges. The review points out that the most important degradation mechanism is fatigue which limits the life, reliability and performance of current wind turbines. As even larger machines are built in the future, with pressures to cut weight and cost, continued materials research and development, as summarised in this paper, is warranted. This critical assessment and review of materials based degradation should be of interest to a wide range of technical energy specialists including those from manufacturers, research and development centres, end users (i.e. electric power generation companies) and financiers and insurers.     

Savonius风力机力矩特性的数值计算与风洞试验研究

利用数值计算和风洞试验相结合的方法对Savonius风力机的力矩特性进行了研究。首先在数值计算中选用了2种湍流模型,即单方程Splart-Allmaras湍流模型和双方程k-ε湍流模型,计算了Savonius风力机在不同攻角下的静力矩以及在不同转速下的输出力矩和输出功率,得到了风力机周围的流场,并对流场进行了分析;利用风洞试验对数值计算结果进行了验证,对比分析了不同湍流模型对Savonius风力机力矩特性和风轮周围流场的影响。     

A review of resonance response in large, horizontal-axis wind turbines

Field operation of the Mod-0 and Mod-1 wind turbines has provided valuable information concerning resonance response in large, two-bladed, horizontal-axis wind turbines. Operational experience has shown that 1/rev excitation exists in the drive train, high aerodynamic damping prevents resonance response of the blade flatwise modes, and teetering the hub substantially reduces the chordwise blade response to odd harmonic excitation. These results can be used by the designer as a guide to system frequency placement. In addition, it has been found that present analytical techniques can accurately predict wind turbine natural frequencies.     

Influence of wake dynamics on the performance and aeroelasticity of wind turbines

Wind turbines are currently a rapidly expanding form of renewable energy. However, there are numerous technological challenges that must be overcome before wind energy provides a significant amount of power in the United States. One of the primary challenges in wind turbine design and analysis is accurately accounting for the aerodynamic environment. This study is focused on a comprehensive verification and validation of the NREL FAST code, which is enhanced to include a free vortex wake model. The verification and validation is carried out through a comparison of blade lift distribution, wind turbine power and force and moment coefficients using a combination of CFD and experimental data. The results are also compared against Blade Element Momentum theory, and results from a 2001 double-blind NREL study on the prediction capabilities of wind turbine modeling tools. Results indicate that the enhanced aeroelastic code generally provides improved predictions. However, in several notable cases the predictions are only marginally improved, or even worse, than those generated using Blade Element Momentum theory aerodynamics. It is concluded that modeling of the aerodynamic environment remains incomplete, even after inclusion of wake effects. One important aspect identified is modeling of the unsteady aerodynamic lift characteristics of the rotor. Finally, the aeroelastic response in the combined presence of wake effects and inflow turbulence is examined. Significant differences are observed in loads, power, and structural response between results computed using the free wake model or simpler models, such as Blade Element Momentum theory.     

Characterization of aerodynamic performance of ducted wind turbines: A numerical study

The complex aerodynamic interactions between the rotor and the duct has to be accounted for the design of ducted wind turbines (DWTs). A numerical study to investigate the characteristics of flow around the DWT using a simplified duct–actuator disc (AD) model is carried out. Inviscid and viscous flow calculations are performed to understand the effects of the duct shape and variable AD loadings on the aerodynamic performance coefficients. The analysis shows that the overall aerodynamic performance of the DWT can be increased by increasing the duct cross‐sectional camber. Finally, flow fields using viscous calculations are examined to interpret the effects of inner duct wall flow separation on the overall DWT performance.     

A torque matched aerodynamic performance analysis method for the horizontal axis wind turbines

An analysis method is developed to test the operational performance of a horizontal axis wind turbines. The rotor is constrained to the torque–speed characteristic of the coupled generator. Therefore, the operational conditions are realized by matching the torque generated by the turbine over a selected range of incoming wind velocity to that needed to rotate the generator. The backbone of the analysis method is a combination of Schmitz' and blade element momentum (BEM) theories. The torque matching is achieved by gradient‐based optimization method, which finds correct wind speed at a given rotational speed of the rotor. The combination of Schmitz and BEM serves to exclude the BEM iterations for the calculation of interference factors. Instead, the relative angle is found iteratively along the span. The profile and tip losses, which are empirical, are included in the analysis. Hence, the torque at a given wind speed and rotational speed can be calculated by integrating semi‐analytical equations along the blade span. The torque calculation method is computationally cheap and therefore allows many iterations needed during torque matching. The developed analysis method is verified experimentally by testing the output power and rotational speed of an existing wind turbine model in the wind tunnel. The generator's torque rotational speed characteristic is found by a separate experimental set‐up. Comparison of experiments with the results of the analysis method shows a good agreement. Copyright © 2013 John Wiley & Sons, Ltd.     

Optimization of Savonius turbines using an obstacle shielding the returning blade

Due to the worldwide energy crisis, research and development activities in the field of renewable energy have been considerably increased in many countries. In Germany, wind energy is becoming particularly important. Although considerable progress has already been achieved, the available technical design is not yet adequate to develop reliable wind energy converters for conditions corresponding to low wind speeds and urban areas. The Savonius turbine appears to be particularly promising for such conditions, but suffers from a poor efficiency. The present study considers a considerably improved design in order to increase the output power of a Savonius turbine with either two or three blades. In addition, the improved design leads to a better self-starting capability. To achieve these objectives, the position of an obstacle shielding the returning blade of the Savonius turbine and possibly leading to a better flow orientation toward the advancing blade is optimized. This automatic optimization is carried out by coupling an in-house optimization library (OPAL) with an industrial flow simulation code (ANSYS-Fluent). The optimization process takes into account the output power coefficient as target function, considers the position and the angle of the shield as optimization parameters, and relies on Evolutionary Algorithms. A considerable improvement of the performance of Savonius turbines can be obtained in this manner, in particular a relative increase of the power output coefficient by more than 27%. It is furthermore demonstrated that the optimized configuration involving a two-blade rotor is better than the three-blade design.     

Structural load mitigation control for wind turbines: A new performance measure

Structural loads of wind turbines are becoming critical because of the growing size of wind turbines in combination with the required dynamic output demands. Wind turbine tower and blades are therefore affected by structural loads. To mitigate the loads while maintaining other desired conditions such as the optimization of power generated or the regulation of rotor speed, advanced control schemes have been developed during the last decade. However, conflict and trade‐off between structural load reduction capacity of the controllers and other goals arise; when trying to reduce the structural loads, the power production or regulation performance may be also reduced. Suitable measures are needed when designing controllers to evaluate the control performance with respect to the conflicting control goals. Existing measures for structural loads only consider the loads without referring to the relationship between loads and other control performance aspects. In this contribution, the conflicts are clearly defined and expressed to evaluate the effectiveness of control methods by introducing novel measures. New measures considering structural loads, power production, and regulation to prove the control performance and to formulate criteria for controller design are proposed. The proposed measures allow graphical illustration and numerical criteria describing conflicting control goals and the relationship between goals. Two control approaches for wind turbines, PI and observer‐based state feedback, are defined and used to illustrate and to compare the newly introduced measures. The results are obtained by simulation using Fatigue, Aerodynamics, Structures, and Turbulence (FAST) tool, developed by the National Renewable Energy Laboratory (NREL), USA.     

A simulation model for wind turbines

The equation of motion of a wind turbine may be written as $\text{J(}\text{dΩ}\text{d}\text{t}\text{) = g(V,Ω) ? T}$, where J is moment of inertia, T load torque, V wind speed and Ω angular velocity. It is proposed that the generated torque $\text{g(V,Ω)}$ may be described by an homogeneous second-degree function in V and Ω. This function is defined and some relevant properties are explored.     

单桩海上风机灌浆连接的抗震性能研究

在地震作用下,灌浆连接段的稳定性直接关系到风机结构的安全。为了探讨灌浆连接的抗震性能,文章以5 MW单桩风机为研究对象,采用OpenSees和ANSYS软件分别建立风机结构及灌浆连接有限元模型;以最不利的地震动为输入,同时考虑风和波浪耦合作用分析灌浆连接的受力及反应。结果表明,地震动使风机灌浆连接处产生较大荷载作用,容易使灌浆层出现高拉应力,导致灌浆材料拉裂,致使灌浆连接失效;地震动强度较小时,风和波浪荷载对灌浆连接受力影响较大,随着地震动强度增大,风和波浪的影响逐渐减弱。风机灌浆连接的抗震设计,应重点分析灌浆层的震动破坏,并充分考虑风和波浪的荷载效应。     

A review and design study of blade testing systems for utility-scale wind turbines

Since the blades are one of the most critical components of a wind turbine, representative samples must be experimentally tested in order to ensure that the actual performance of the blades is consistent with their specifications. In particular, it must be demonstrated that the blade can withstand both the ultimate loads and the fatigue loads to which the blade is expected to be subjected during its design service life. In general, there are basically two types of blade testing: static testing and fatigue (or dynamic) testing. This paper includes a summary review of different utility-scale wind turbine blade testing methods and the initial design study of a novel concept for tri-axial testing of large wind turbine blades. This new design is based on a blade testing method that excites the blade in flap-wise and edgewise direction simultaneously. The flap motion of the blade is caused by a dual-axis blade resonance excitation system (BREX). Edgewise motion is delivered by the use of two inclined hydraulic actuators and linear guide rail system is used to move the inclined actuators in the flap-wise direction along the blade motion. The hydraulic system and linear guide rail requirements are analyzed and an initial cost estimate of the proposed system is presented. Recommendations for future work on this proposed system are given in the final section of this work.     

The use of a curtain design to increase the performance level of a Savonius wind rotors

In this study, a curtain design has been arranged so as to improve the low performance levels of the Savonius wind rotors. Designed to prevent the negative torque on the convex blade of the rotor, this curtain has been placed in front of the rotor, and performance experiments have been carried out when the rotor is with and without curtain. It has been determined from here that a significant increase can be achieved in the rotor performance by means of the curtain design. Experiments of the curtain design have been conducted in three different dimensions when the Savonius wind rotor is static, and the highest values have been obtained with the curtain 1. Therefore, the curtain designs and curtain angles in which the highest values obtained have been analyzed numerically with Fluent 6.0 program and the results obtained experimentally have been supported with numerical analysis. Moreover, performance experiments have been made for the curtain 1 with which the best performance values have been obtained when the rotor is in its dynamic position, and the results obtained have been given in figures.     

Condition monitoring and fault detection of wind turbines and related algorithms: A review

Renewable energy sources like wind energy are copiously available without any limitation. Wind turbines are used to tap the potential of wind energy, which is available in millions of MW. Reliability of wind turbine is critical to extract this maximum amount of energy from the wind. We reviewed different techniques, methodologies and algorithms developed to monitor the performance of wind turbine as well as for an early fault detection to keep away the wind turbines from catastrophic conditions due to sudden breakdowns. To keep the wind turbine in operation, implementation of condition monitoring system (CMS) and fault detection system (FDS) is paramount and for this purpose ample knowledge of these two types of systems is mandatory. So, an attempt has been made in this direction to review maximum approaches related to CMS and FDS in this piece of writing.     

Monetary‐based availability: A novel approach to assess the performance of wind turbines

With the background of numerous wind turbines phasing out of fixed feed‐in tariffs in the following years and an increasing share of negative electricity market prices on the spot market, the availability definitions of “time‐based” and “production‐based” availability are possibly no longer suitable for assessing the overall performance of a wind turbine. This paper introduces a novel definition: the “monetary‐based” availability. The differences between the established definitions and the “monetary‐based” availability are highlighted by comparing the measures on an empirical data set. Furthermore, results on the impact of scheduling planned downtimes towards a monetary‐based optimum show that revenues can be increased. By shifting only a small share of the annual downtime to an optimum to maximize the revenue from electricity, a strong increase in additional earnings and thereby an increasment of the monetary‐based availability can be achieved.     

Fluidic flow control applied for improved performance of Darrieus wind turbines

The focus of the present research is performance enhancement of a vertical axis Darrieus‐type wind turbine using flow control techniques. The academic and industrial interest in vertical‐axis wind turbines (VAWTs) is increasing because of its suitability to urban areas, characterized by high turbulence and low wind speeds. The paper describes experimental work performed on a GOE222 asymmetrical airfoil intended to be used in a straight‐bladed Darrieus VAWT. Airfoil characteristics were measured in a wide range of incidence angles and Reynolds numbers, relevant for the operation of a small to medium size wind turbine. A variety of passive flow control (passive porosity and surface roughness) and active flow control techniques (boundary layer suction, pulsed suction) were tested in order to evaluate their effects on the airfoil performance. The measured effects of flow control on the 2D airfoil are integrated into a modified version of a double‐multiple streamtube model in order to predict the effects on the performance and efficiency of the turbine. It was found that the improvement of 2D airfoil characteristics can be translated into improvement of total turbine performance. By the use of active flow control, it was possible to increase the VAWT maximum mechanical output. When active flow control is properly activated taking into account the azimuth and Reynolds number conditioning, the effects could be greatly increased while consuming less energy, increasing the net efficiency of the entire system. Copyright © 2015 John Wiley & Sons, Ltd.     

Upgrading conventional wind turbines

When approaching a conventional wind turbine, the air flow is slowed down and widened. This effect causes a loss in the efficiency of the turbine. By creating a field of low pressure behind the turbine, this effect and the corresponding loss in efficiency can be avoided. In order to maintain this low pressure field, the air passing near, but not through the turbine needs to do work.Based on these considerations we have made a model of a wind turbine with a wing profiled ring around it. We present various fluidodynamical calculations in order to study the resulting increase in power and in order to estimate what the geometrical size of such an apparatus would need to be and whether it could be of advantage compared to conventional devices from an economic point of view.     

Unsteady Aerodynamics of a Savonius wind rotor: a new computational approach for the simulation of energy performance

When compared with of other wind turbine the Savonius wind rotor offers lower performance in terms of power coefficient, on the other hand it offers a number of advantages as it is extremely simple to built, it is self-starting and it has no need to be oriented in the wind direction. Although it is well suited to be integrated in urban environment as mini or micro wind turbine it is inappropriate when high power is requested. For this reason several studies have been carried-out in recent years in order to improve its aerodynamic performance. The aim of this research is to gain an insight into the complex flow field developing around a Savonius wind rotor and to evaluate its performance. A mathematical model of the interaction between the flow field and the rotor blades was developed and validated by comparing its results with data obtained at Environmental Wind Tunnel (EWT) laboratory of the “Polytechnic University of Marche”.     

Capacity factor of wind turbines

The power generated by a wind turbine depends on both the design characteristics of the turbine and the properties of the wind resource. These parameters determine the capacity factor (ratio of average power output to rated power of the turbine). Since detailed information on the wind-speed frequency is often lacking, the shape factor (k) of the Weibull distribution is taken to be equal to 2, which may lead to significant errors. It is our aim to estimate capacity factors for shape factors between 1.5 and 3 and for sites with average wind speeds ranging from 4 to 8 m/s. The results have been analyzed to obtain a general form for the capacity factor. Our method may be useful to turbine manufacturers in designing electronic control systems.