Downwind Rotor Research Articles

Changes in design driving load cases: Operating an upwind turbine with a downwind rotor configuration

AbstractThis work considers the design driving load cases from a full design load basis analysis on an upwind turbine changed into a downwind configuration. The upwind turbine is a commercial class IIIA 2.1‐MW turbine, manufactured by Suzlon. The downwind turbine shows an increase in the normalized tower clearance by 6%, compared with the upwind concept. Removing the blade prebend increases the normalized minimum tower clearance by 17% in the downwind configuration compared with the upwind configuration. The extreme loads on the longitudinal tower bottom bending moment are seen to generally increase by 17% because of the overhanging gravity moment of the rotor‐nacelle assembly. The extreme blade root bending moments are reduced by 10% flapwise, because of the coning of the rotor in downwind direction. The fatigue loads suffer from the tower shadow, leading to an overall increase of the fatigue loads in the blades with up to 5% in flapwise direction in the downwind configuration. Because of blade deflection and coning direction, the downwind configuration shows a 0.75% lower annual energy production. Removing the prebend increases the annual energy production loss to 1.66%.

Numerical investigation on the performance of a small counter-rotating wind turbine

The article presents results of the investigation on the performance of a small counter-rotating wind turbine. Wind turbine has been simulated using Computational Fluid Dynamics methods. Actuator Line Model has been successfully used to represent rotors in computational domain. Parametric study has been carried out, taking into account changes in the tip speed ratio of the rotors while maintaining a constant distance between upwind and downwind rotor. Study results revealed noticeable increase in power coefficient for optimal configuration. Dynamic interaction between rotors has been investigated exposing no significant interference in both torque and power.

Self-aligning behaviour of a passively yawing floating offshore wind turbine

ABSTRACTFloating offshore wind turbines are a promising concept for expanding offshore wind energy. In comparison with fix-founded offshore wind turbines, the overall costs are less dependent on water depths, which leads to a variety of potential locations and markets worldwide. Furthermore, floating platforms allow for new structural designs with the potential to save material and installation costs. In this paper, a self-aligning platform equipped with a 6 MW turbine is presented. The platform is moored on a single point and uses a turret buoy to be able to rotate freely around its anchor point. A downwind rotor and an airfoil-shaped tower induce self-aligning turning moments to passively follow changes of the wind direction. The first order boundary element method panMARE is used to simulate the motion behaviour considering aerodynamic, hydrodynamic and mooring loads. The self-aligning capability is demonstrated under partial turbine load for steady and dynamic conditions with waves and current.

Performance of SB-VAWT using a modified double multiple streamtube model

The present study introduces a new relation to evaluate the equilibrium induced air velocity in the mid plane between the upstream and downstream rotor zones to give a good improvement in prediction of the maximum power coefficient. The relation is derived as a function of both interference factor in upwind and downwind rotor zones determined using classical DMST model. The root-finding algorithm is used to obtain the interference factor both in upwind and downwind zones of the turbine at each streamtube. The comparison between the numerical results and the experimental data of 5-m, 17-m, and 34-m Darrieus VAWTs shows that the use of the introduced relation gives a sensible improvement in the prediction of maximum value for turbine power coefficient. A theoretical study is presented based on the use of well documented symmetrical NACA0012 turbine blade profile. The effect of wind speed and rotor solidity on maximum power coefficient and corresponding optimal speed ratio on the turbine rotor is evaluated in order to determine an improved aerodynamic configuration for a given application.

Combined Blade-Element Momentum—Lifting Line Model for Variable Loads on Downwind Turbine Towers

Downwind rotors are a promising concept for multi-megawatt scale large wind turbines due to their advantages in safety and cost reduction. However, they have risks from impulsive loads when one of the blades passes across the tower wake, where the wind speed is lower and locally turbulent. Although the tower shadow effects on the tower loads have been discussed in former studies, there is currently no appropriate model for the blade-element and momentum theory so far. This study formulates the tower shadow effects on the tower load variation induced by blades using the lifting line theory, which does not require any empirical parameters. The method is verified via computational fluid dynamics for a 2 MW(megawatt), 3-bladed downwind turbine. The amplitude and the phase of the variation are shown to be accurate in outboard sections, where the rotor-tower clearance is large (>3.0 times of the tower diameter) and the ratio of the blade chord length is small (<0.5 times of the tower diameter), in both of rated and cut-out conditions.

Comparison between two-dimensional and three-dimensional computational fluid dynamics techniques for two straight-bladed vertical-axis wind turbines in inline arrangement

This study is dedicated to drawing a comparison between two- and three-dimensional approach capabilities for the simulation of two similar rotors placed in three inline (or tandem) arrangements. This arrangement is generally recognized as the worst-case scenario for the downwind rotor considering the vortices and disorders produced by the upwind rotor. The rotor in question with the diameter of 2.5 m is made up of three NACA0015 blades with the chord length and span size equal to 0.4 and 3 m, respectively. Based on the authors’ previous works, the [Formula: see text] shear stress transport model was selected for this comparative study. According to the results, there is an appreciable deviation in the aerodynamic performance of the upwind rotor predicted by the two-dimensional and three-dimensional simulation techniques. There is no tangible difference between the two-dimensional and three-dimensional results in terms of the averaged power output for the downwind rotor. However, the study of flow field employing different means like vortex structures, axial velocity, and even torque variation indicates that the two-dimensional approach is unable to achieve realistic and reliable output data. The introduced “pillar effect” regarding the dimensional limitations of the two-dimensional approach, which affects the vorticity shape and its dissipation, is plausible evidence for this discrepancy.

An experimental study on the aerodynamic performances and wake characteristics of an innovative dual-rotor wind turbine

A novel dual-rotor wind turbine (DRWT) concept with an auxiliary upwind rotor and a bigger downwind rotor was introduced for improved turbine performance. An experimental study was performed in an Atmospheric Boundary Layer (ABL) wind tunnel with scaled turbine models to investigate the aerodynamic performances and wake characteristics of DRWTs in either co-rotating or counter-rotating configuration (i.e., CR-DRWT or CO-DRWT), in comparison to those of a conventional single-rotor wind turbine (SRWT). In additional to measuring the aerodynamic forces and power outputs, the wake flow characteristics behind DRWTs and SRWT were also quantified by using a high-resolution Particle Image Velocimetry (PIV) system. In comparison to those of SRWT, while the aerodynamic force acting on the CR-DRWT was found to increase 13.3%, the power coefficient of the CR-DRWT design was improved by 7.2%. Faster wake recovery behind the DRWT designs was confirmed from both the measured power outputs of the same turbine model sited in the turbine wakes and the measured flow velocity at various downstream locations. The wake flow measurements were correlated with the aerodynamic force and power output data to elucidate underlying physics for the higher efficiency of DRWT designs in either isolation or wind farm settings.

Pre-aligned downwind rotor for a 13.2 MW wind turbine

Abstract While there is continued demand for larger wind turbines with cheaper cost of energy, conventional upwind blade design may be reaching a size limit stemming from blade failure at extreme wind conditions. For turbines rated above 10 MW, decreasing the structural loads to more manageable levels may be achieved by better aligning the forces along the rotor blade to reduce ultimate bending moments and fatigue loadings. To achieve such moment reduction, a downwind configuration with a coning angle prescribed to allow load alignment for critical conditions could be used. Such a configuration also increases the rated clearance distance between the rotor and the tower since wind loads push the blades further from the tower. This concept is explored herein for a 13.2 MW rated downwind design compared to a conventional upwind three-bladed rotor design that uses the Sandia SNL100-02 blades. The simulation results show in Class IIB winds that the two-bladed downwind pre-aligned rotor with 15° coning, decreases the blade damage equivalent loads by 19.0%, and decreases rotor blade mass by 27.4%, compared with the unmodified conventional upwind three-bladed rotor. However, further design optimization is needed to ensure annual energy production is preserved with this concept.

Wind farm hub height optimization

The objective of this study is to assess the effect of hub height optimization on the Annual Energy Production (AEP) of a wind farm. The only optimization variable is the hub height of each wind turbine and all other characteristics of the wind farm, including base location, rotor diameter (D), and total number of wind turbines, remain unchanged. The first case study consisted of two wind turbines aligned with the wind direction, with the hub height of the upstream turbine fixed and that of the downstream turbine varying. Two competing effects were identified. On one hand, lowering the hub height of the downwind turbine exposes a larger fraction of the downwind rotor to the undisturbed wind and therefore increases its power production. On the other hand, turbines with lower hub heights experience lower wind speeds due to shear. The balance between these two effects is complicated by surface roughness and spacing between turbines. The maximum benefits are found offshore, with tight axial spacing (⩽5D). To put into perspective the effectiveness of using multiple hub heights in a realistic setting, the second case compared two 20-turbine wind farms with the same horizontal layout, turbine type, and wind direction, but one with all turbines installed at the same hub height (80m) and the second with alternating rows of tall (100m) and short (57.5m) wind turbines. The vertically staggered configuration was found to produce approximately 5.4% more power. Lastly, a real 48-turbine wind farm (Lillgrund in Sweden) was optimized under all 360 wind directions and wind speeds ranging from the cut-in to the cut-out using a greedy search algorithm. The optimal configuration presented a non-intuitive and balanced mix of 26 tall and 22 short turbines. Although the entire range from a minimum to a maximum predefined hub height was searched to determine the optimal hub height for each wind turbine, the optimization algorithm ended up with an optimal layout in which the hub height of each turbine was equal to either the minimum hub height or the maximum hub height and no value in between was assigned to any of the turbines. The AEP of the optimized wind farm with multiple hub heights was approximately 2% (10GWh) higher than that of the wind farm with a single hub height. The PARK model was used in this study to calculate wake losses and to perform the optimization. To validate its results, Large-Eddy Simulations (LES) of the flow around the turbines, represented as actuator lines, were conducted at fine spatial and temporal resolutions. The LES results confirmed that optimizing hub heights is beneficial and indicated that the net gains were actually higher (by up to a factor of two) than those calculated with PARK.

ダウンウィンド型風力発電システム開発のためのCAE活用

ダウンウィンド型風力発電システム開発のためのCAE活用

Variations and relations of meteorological parameters between upwind and downwind small-scale wind turbine rotor area

Renewable energy sources are becoming increasingly important due to climate change and the energy crisis. Wind energy projects and applications in particular have been growing recently. Meteorological parameters such as wind speed, ambient temperature, relative humidity, and air pressure directly affect wind turbine electricity generation. Understanding the relationships among these parameters is necessary to determine wind energy potential and to predict electricity generation. The current work is based on data obtained from a 1.5 kW wind turbine constructed in Istanbul Technical University's meteorological park in Turkey. A one minute time interval was used in the data analysis. In this study, upwind/downwind meteorological variables of the wind turbine rotor area were investigated and analyzed in detail. It was observed that the turbine used in this study performs better under low wind speed conditions than under high wind speed conditions. The difference in relative humidity between the upwind and downwind rotor area reached 4%. It is also important to note that, depending on wind speed, temperature differences between the upwind and downwind rotor area reached 5%. Equations for upwind/downwind variables were obtained for each meteorological parameter.

Design Considerations of Rotor Cone Angle for Downwind Wind Turbines

Due to their potential to use light, flexible blades, downwind turbines are well suited for offshore floating platforms, for which there is a need to substantially lower the cost of wind-generated electricity. However, downwind rotors must operate in the presence of the tower's wakes with which are associated strong changes in flow incidence, and thus high fatigue loads. In order to guide the development of design rules for multimegawatt downwind turbines, a comprehensive experimental study has been conducted to better understand the characteristics of the unsteady rotor torque on downwind turbines. High-frequency measurements of the unsteady rotor torque on a model turbine that can be configured with rotors of different cone angles and operated either downwind or upwind in well-controlled flow conditions are conducted. The measurements show that in the case of the downwind turbine, the blade's passage through the tower's wake accounts for 56% to 61% of the variance of the rotor torque; the proportion of this unsteadiness is independent of the cone angle. For nonoptimum tip speed ratios (TSRs), the increase in unsteadiness is consistently less for downwind configurations than for upwind configurations. For the 5 deg-cone downwind configuration, the increase in rotor torque unsteadiness is 13–18% of the increase observed for the 5-deg-cone upwind configuration for nonoptimum TSRs. Thus from a design perspective, downwind rotor configurations offer above or below rated wind speed, a smaller increase in unsteadiness of the rotor torque compared to upwind turbine configurations. These characteristics differ from upwind turbines, on which broadband vortex shedding from the blade is the primary source of the unsteadiness, which may be reduced by increasing the rotor-tower clearance. It is suggested that given the strong periodic character of the blade's passage through the tower's wake, the turbine control system may be designed to reduce fatigue loads and there is a broader design space on downwind turbines that can be exploited for peak load mitigation by moderately adjusting the blade's stiffness.

Downwind wind turbine yaw stability and performance

In this experimental work, the yaw stability and performance of three different downwind rotors are measured and compared to those of the corresponding upwind rotors. A modular scale-model wind turbine that allows for either upwind or downwind operation, with a range of rotor cone and yaw angles, is used. The measurements, at near full-scale Reynolds numbers, show all downwind rotor configurations have yaw stability. On the otherhand, upwind turbines are shown to be either unstable or to have significantly reduced yaw stability compared to the corresponding downwind rotor configurations. Downwind configurations with zero, 5° and 10° cone have higher shaft power and rotor thrust than the corresponding upwind configurations. For zero yaw, the 5° and 10° cone angle, the downwind configurations each yield 5% more power, and have only 3% higher thrust, than the upwind configurations, indicating a potential benefit in performance compared to upwind configurations. Overall, it is concluded that coned downwind configurations provide for easier yaw control and yield more power.

A morphing downwind-aligned rotor concept based on a 13-MW wind turbine

To alleviate the mass-scaling issues associated with conventional upwind rotors of extreme-scale wind turbines (>/=10 MW), a morphing downwind-aligned rotor (MoDaR) concept is proposed herein. The concept employs a downwind rotor with blades whose elements are stiff (no intentional flexibility) but with hub-joints that can be unlocked to allow for moment-free downwind alignment. Aligning the combination of gravitational, centrifugal and thrust forces along the blade path reduces downwind cantilever loads, resulting in primarily tensile loading. For control simplicity, the blade curvature can be fixed with a single morphing degree of freedom using a near-hub joint for coning angle: 22 degrees at rated conditions. The conventional baseline was set as the 13.2-MW Sandia 100-m all glass blade in a three-bladed upwind configuration. To quantify potential mass savings, a downwind load-aligning, two-bladed rotor was designed. Because of the reduced number of blades, the MoDaR concept had a favorable 33% mass reduction. The blade reduction and coning led to a reduction in rated power, but morphing increased energy capture at lower speeds such that both the MoDaR and conventional rotors have the same average power: 5.4 MW. A finite element analysis showed that quasi-steady structural stresses could be reduced, over a range ofmore » operating wind speeds and azimuthal angles, despite the increases in loading per blade. However, the concept feasibility requires additional investigation of the mass, cost and complexity of the morphing hinge, the impact of unsteady aeroelastic influence because of turbulence and off-design conditions, along with system-level Levelized Cost of Energy analysis.« less

A proposal and a theoretical analysis of a novel concept of a tilted-axis wind turbine

A novel wind turbine is proposed with a downwind rotor, tilted upward. An external cable that is attached to the blades' center of force supports each rotor blade, and the tower is supported by guy wires at the top. The nacelle is eliminated and the main axle is replaced by power take off from an annular ribbon, rotating with the rotor; thus most large bending moments are eliminated and replaced by tensile and compressive loads. Further, the proposed wind turbine does not use a gearbox or a direct drive generator. The proposed system is structurally analyzed and compared with a conventional wind turbine. The comparison shows that manufacturing the proposed system may be 2.5 times less expensive than manufacturing a conventional wind turbine of the same power on land. The cost advantage promises to be even bigger offshore.

Advanced Concept Offshore Wind Turbine Development

Development of novel offshore wind turbine designs and technologies are necessary to reduce the cost of offshore wind energy since offshore wind turbines need to withstand ice and waves in addition to wind, a markedly different environment from their onshore counterparts. This paper focuses on major design challenges of offshore wind turbines and offers an advanced concept wind turbine that can significantly reduce the cost of offshore wind energy as an alternative to the current popular designs. The design consists of a two-blade, downwind rotor configuration fitted to a fixed bottom or floating foundation. Preliminary results indicate that cost savings of nearly 25% are possible compared with the conventional upwind wind turbine designs.

Rotor Design and Performance Study of a Vertical-Axis Wind Turbine Based on DMS

Specifically address the design of a 5KW H-type vertical-axis wind turbine (H-VAWT) with NACA 0018 airfoil considering the factors that affect wind turbine power. The double-multiple streamtube (DMS) theoretical model is analyzed and summarized and calculated by Matlab. The 5KW H-type vertical axis wind turbines aerodynamic performance is calculated by the model written in Matlab. The curve of the power coefficient as a function of the tip-speed ratio and the curve of the normal force coefficient and the tangential farce as a function of the blade position is given by Matlab. From the curves we can see that upwind rotor aerodynamic load is larger, downwind rotor aerodynamic load is smaller and there is a serious flow retarding effect in the rotor downwind area.

A calibration method for downwind wake models accounting for the unsteady behaviour of the wind turbine tower shadow behind monopile and truss towers

Traditionally wind turbines are built in the upwind configuration, but the alternative of a downwind rotor has distinct advantages. A main issue with such a configuration is the tower shadow effect. The presence of the tower generates a complex wind field, consisting of an averaged velocity deficit, unsteady fluctuations from vortex shedding processes and turbulence. Since this tower shadow is commonly implemented using parametric steady wake models, the dynamic behaviour of the wake is not directly accounted for. The present paper introduces a general method for calibrating the parametric steady wake models and an effective turbulence intensity (accounting both for the velocity profile and the unsteady effects) from computational fluid dynamic simulations of the unsteady structural shadow.To demonstrate its potential the method is used in a blade fatigue comparison study. A 15 percent more flexible and lighter blade for the downwind mounted rotors showed a decrease in blade fatigue loads of three, four, and two percent compared to the conventional upwind mounted rotor on a monopile tower for the monopile tower and truss towers at 0 and 22.5 degrees, respectively. This example shows that the truss tower at 0 degree angle seems to result in the lowest blade fatigue loads.The proposed calibration method is completely general and can be used to obtain efficient, effective engineering models for the aerodynamic flow behind complex, multi-membered structures.

Computational Analysis of 30 Kw Contra Rotor Wind Turbine

The aim of this study is to optimize and analyze the characteristics of the upwind primary rotor and the downwind secondary rotor of the contra rotor wind turbine to increase the aerodynamic performance by using computational fluid dynamic (CFD). The main objective is to provide maximum energy that can be extracted by the primary and secondary rotor from the renewable resource of wind to increase the thrust power. For this purpose, two kinds of rotor configurations which are 3-bladed single and 3-bladed contra-rotating rotor were compared by using CFD. The results of the primary and secondary rotor are validated with measurements of the 30 KW Contra rotor wind turbine available on literature and plotted for power output. The optimum axial distance between the two rotors are investigated through CFD for increased performance. In addition, the increased thrust and torque for each rotor configuration are compared for aerodynamic feasibility.

Stochastic dynamic response analysis of a tension leg spar‐type offshore wind turbine

ABSTRACTThis paper presents a stochastic dynamic response analysis of a tension leg spar‐type wind turbine subjected to wind and wave actions. The dynamic motions, structural responses, power production and tension leg responses are analyzed. The model is implemented using the HAWC2 code. Several issues such as negative damping, rotor configuration (upwind or downwind rotor) and tower shadow effects are discussed to study the power performance and structural integrity of the system. The operational and survival load cases considering the stochastic wave and wind loading are analyzed to investigate the functionality of the tension leg spar‐type wind turbine. Amelioration of the negative damping applied for this concept reduces the structural dynamic responses, which are important for fatigue life. It is found that the responses induced by wave and wind actions at the wave frequencies are not affected much by the aerodynamic excitation or damping forces. Because of the nonlinear effects of the tension leg, all of the motion responses are strongly coupled. The global responses of upwind and downwind versions of the turbine are found to be close because the tower shadow has a limited effect on the global responses. However, the structural dynamic responses of the blades are more affected by the tower shadow. In this study, the extrapolation methods are applied to efficiently estimate the maximum responses. The maximum response is found to occur in the survival cases as a result of the wave actions and the increased aerodynamic drag forces on the tower. The results show that the maximum responses corresponding to the up‐crossing rate of 0.0001 (corresponding to the maximum response within a 3 hour period) can be expressed by the mean plus 3 to 5 standard deviations. Copyright © 2012 John Wiley & Sons, Ltd.