Impact of aerodynamic modelling on seakeeping performance of a floating vertical axis wind turbine

This study focuses on the impact of the aerodynamic model on the dynamic response of a floating vertical axis wind turbine (VAWT). It compares a state‐of‐the‐art quasi‐steady double multiple streamtube (DMS) solver, a prescribed vortex wake (PVW), and a free vortex wake (FVW) solver. The aerodynamic loads acting on a bottom‐fixed VAWT and computed with the three aerodynamic solvers are compared, then the dynamic responses of the floating turbine in irregular waves and turbulent wind with the different aerodynamic solvers are compared. Differences are observed, particularly in the mean motions of the platform. Eventually, the aerodynamic damping computed by the solvers are estimated with aerodynamic simulations on the turbine with imposed surge and pitch motions. The estimated damping can then be correlated with the dynamic response amplitude of the VAWT. Substantial discrepancies are observed between the three solvers at high tip speed ratio, when the rotor is highly loaded. It is shown that the quasi‐steady DMS solver seems to give greater amplitude of motions for the floating VAWT because of strong rotor/wake interaction that are not correctly accounted for.     

新型螺旋扰流片漂浮式风力机半潜平台动态响应研究

为改善半潜式平台漂浮式风力机动态响应,提出将扰流片螺旋布置于半潜式平台浮筒下部的结构设计方法,将附加螺旋扰流片的半潜平台漂浮式风力机与原平台进行对比,并研究扰流片布置螺距对平台性能的影响。结果表明：频域分析中扰流片能够显著降低平台所受横摇波浪激励力矩,对平台垂荡、横摇、纵摇辐射阻尼及附加质量均有一定提升,平台频域幅值响应算子(RAO)峰值明显降低;安装扰流片后,平台固有周期增加,抗共振性能更好;时域分析中,在垂荡、纵摇方向上扰流片布置螺距对平台运动响应影响较小,对横摇运动响应抑制明显,其中0.29D(D为半潜式平台浮筒半径)扰流片平台性能较佳。     

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

This 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.     

Environmental activism and vertical‐axis wind turbine preferences in California

Wind energy is widely recognized as a key element of the worldwide effort to limit greenhouse gas emissions. As compared with the general population, environmental activists have a much higher level of knowledge, interest, and capacity to affect the final outcome of a proposed wind turbine facility. To explore how their opinions on wind energy, particularly on vertical‐axis wind turbines, differ from the general public, we administered the same online experimental survey to a general population sample of adult Californians and to a self‐selected sample of online energy and environmental activists. We find that support for wind energy increases with the degree of environmental activism and engagement. The general public prefers vertical‐axis wind turbines in open spaces, away from one's residence. Location and price sensitivity, however, are weaker among activists. Among activists, attitudes about specific vertical‐axis wind turbine technologies are more crystalized and less susceptible to the information effects except on the topic of minimizing bird deaths.     

A comparative study on dynamic responses of spar‐type floating horizontal and vertical axis wind turbines

Interest in the exploitation of offshore wind resources using floating wind turbines has increased. Commercial development of floating horizontal axis wind turbines (FHAWTs) is emerging because of their commercial success in onshore and near‐shore areas. Floating vertical axis wind turbines (FVAWTs) are also promising because of their low installation and maintenance costs. Therefore, a comparative study on the dynamic responses of FHAWTs and FVAWTs is of great interest. In the present study, a FHAWT employing the 5MW wind turbine developed by the National Renewable Energy Laboratory (NREL) and a FVAWT employing a Darrieus rotor, both mounted on the OC3 spar buoy, were considered. An improved control strategy was introduced for FVAWTs to achieve an approximately constant mean generator power for the above rated wind speeds. Fully coupled time domain simulations were carried out using identical, directional aligned and correlated wind and wave conditions. Because of different aerodynamic load characteristics and control strategies, the FVAWT results in larger mean tower base bending moments and mooring line tensions above the rated wind speed. Because significant two‐per‐revolution aerodynamic loads act on the FVAWT, the generator power, tower base bending moments and delta line tensions show prominent two‐per‐revolution variation. Consequently, the FVAWT suffers from severe fatigue damage at the tower bottom. However, the dynamic performance of the FVAWT could be improved by increasing the number of blades, using helical blades or employing a more advanced control strategy, which requires additional research. Copyright © 2016 John Wiley & Sons, Ltd.     

Aerofoil optimization for vertical‐axis wind turbines

An expression for the aerodynamic optimization of aerofoils for 2D lift driven vertical‐axis wind turbines is derived as a function of lift slope and drag. As lift slope is proportional to aerofoil thickness, the aerodynamic optimum is found in thick aerofoils, which are also structurally advantageous. Using a genetic optimization algorithm, the objective function is used to generate aerofoils whose performance in a vertical‐axis wind turbine is calculated using a potential flow solution of the induction field and 2D polars calculated with XFOIL. The results demonstrate power and structural gains. This approach can lead to reductions in rotor mass due to the thicker and thus stiffer aerofoils, without compromising aerodynamic performance. Copyright © 2014 John Wiley & Sons, Ltd.     

Demonstrating that power and instantaneous loads are decoupled in a vertical‐axis wind turbine

The flow in the meridian plane of a high aspect ratio vertical‐axis wind turbine (VAWT) can be described as two dimensional. The wake that is generated by the VAWT in a two‐dimensional flow consists of shed vorticity and is a result of the temporal variation of bound circulation on the blades, following Kelvin's theorem. The strength and location of the vorticity that is produced by the VAWT in a two‐dimensional flow are thus independent of the average bound circulation on the blade. Two independent computational models—a potential flow panel model and a method that is based on the vorticity–velocity formulation of the Navier–Stokes equations—have been used to show that the VAWT can produce the same power for different azimuthal distributions of the blade aerodynamic loading. It is thus demonstrated that the instantaneous blade aerodynamic loading and the power conversion of a VAWT are decoupled. This observation has, potentially, significant impact on the design of the VAWT and reopens the research on asymmetric blade shapes in order to optimize the performance of this turbine configuration. Copyright © 2013 John Wiley & Sons, Ltd.     

Simulating the aerodynamic performance and wake dynamics of a vertical‐axis wind turbine

The accurate prediction of the aerodynamics and performance of vertical‐axis wind turbines is essential if their design is to be improved but poses a significant challenge to numerical simulation tools. The cyclic motion of the blades induces large variations in the angle of attack of the blades that can manifest as dynamic stall. In addition, predicting the interaction between the blades and the wake developed by the rotor requires a high‐fidelity representation of the vortical structures within the flow field in which the turbine operates. The aerodynamic performance and wake dynamics of a Darrieus‐type vertical‐axis wind turbine consisting of two straight blades is simulated using Brown's Vorticity Transport Model. The predicted variation with azimuth of the normal and tangential force on the turbine blades compares well with experimental measurements. The interaction between the blades and the vortices that are shed and trailed in previous revolutions of the turbine is shown to have a significant effect on the distribution of aerodynamic loading on the blades. Furthermore, it is suggested that the disagreement between experimental and numerical data that has been presented in previous studies arises because the blade–vortex interactions on the rotor were not modelled with sufficient fidelity. Copyright © 2010 John Wiley & Sons, Ltd.     

Modelling the aerodynamics of vertical‐axis wind turbines in unsteady wind conditions

Most numerical and experimental studies of the performance of vertical‐axis wind turbines have been conducted with the rotors in steady, and thus somewhat artificial, wind conditions—with the result that turbine aerodynamics, under varying wind conditions, are still poorly understood. The vorticity transport model has been used to investigate the aerodynamic performance and wake dynamics, both in steady and unsteady wind conditions, of three different vertical‐axis wind turbines: one with a straight‐bladed configuration, another with a curved‐bladed configuration and another with a helically twisted configuration. The turbines with non‐twisted blades are shown to be somewhat less efficient than the turbine with helically twisted blades when the rotors are operated at constant rotational speed in unsteady wind conditions. In steady wind conditions, the power coefficients that are produced by both the straight‐bladed and curved‐bladed turbines vary considerably within one rotor revolution because of the continuously varying angle of attack on the blades and, thus, the inherent unsteadiness in the blade aerodynamic loading. These variations are much larger, and thus far more significant, than those that are induced by the unsteadiness in the wind conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Airfoil optimisation for vertical‐axis wind turbines with variable pitch

To advance the design of a multimegawatt vertical‐axis wind turbine (VAWT), application‐specific airfoils need to be developed. In this research, airfoils are tailored for a VAWT with variable pitch. A genetic algorithm is used to optimise the airfoil shape considering a balance between the aerodynamic and structural performance of airfoils. At rotor scale, the aerodynamic objective aims to create the required optimal loading while minimising losses. The structural objective focusses on maximising the bending stiffness. Three airfoils from the Pareto front are selected and analysed using the actuator cylinder model and a prescribed‐wake vortex code. The optimal pitch schedule is determined, and the loadings and power performance are studied for different tip‐speed ratios and solidities. The comparison of the optimised airfoils with similar airfoils from the first generation shows a significant improvement in performance, and this proves the necessity to properly select the airfoil shape.     

两种常用漂浮式风力机平台动态特性分析

分别针对MIT/NREL TLP和Umaine-Hywind Spar两种海上风力机浮式平台主体的水动力特性进行研究,旨在分析两者的系泊稳定性.基于海洋水动力学和结构动力学理论,建立了平台/缆索系统耦合模型,在水深和外界载荷激励相同的情况下,利用有限元分析ANSYS软件中的水动力学计算模块进行时域、频域响应分析,研究了两种平台在海风、海流和随机波联合作用下的动态响应,并分析了两种平台随波浪频率的响应变化.结果表明:MIT/NREL TLP平台的动态响应较大,而Umaine-Hywind Spar平台动态响应较小;两平台均在低频波浪作用下产生响应峰值.     

Impact of aerodynamic modeling on seakeeping performance of a floating horizontal axis wind turbine

Over the last decade, several coupled simulation tools have been developed in order to design and optimize floating wind turbines (FWTs). In most of these tools, the aerodynamic modeling is based on quasi‐steady aerodynamic models such as the blade element momentum (BEM). It may not be accurate enough for FWTs as the motion of the platform induces highly unsteady phenomena around the rotor. To address this issue, a new design tool has been developed coupling a seakeeping solver with an unsteady aerodynamic solver based on the free vortex wake (FVW) theory. This tool is here compared with the reference code FAST, which is based on the BEM theory in order to characterize the impact of the aerodynamic model on the seakeeping of a floating horizontal axis wind turbine (HAWT). Aerodynamic solvers are compared for the case of the free floating NREL 5MW HAWT supported by the OC3Hywind SPAR. Differences obtained between the models have been analyzed through a study of the aerodynamic loads acting on the same turbine in imposed harmonic surge and pitch motions. This provides a better understanding of the intrinsic differences between the quasi‐steady and unsteady aerodynamic solvers. The study shows that differences can be observed between the three aerodynamic solvers, especially at high tip speed ratio (TSR) for which unsteady aerodynamic phenomena and complex wake dynamics occur. Observed discrepancies in the predictions of the FWT dynamic response can raise issues when designing such a system with a state‐of‐the‐art design tool.     

垂直轴风力机动态流场及其气动性能分析

垂直轴风力机气动性能研究是风力机设计、实验的重要部分,对其运动状态下的流场进行分析是观测垂直轴风力机性能重要环节.基于NACA0012对称翼型,建立二维几何模型并进行模拟计算.采用k-ωSST湍流模型及滑移网格技术,通过CFD软件数值计算得到达里厄型直叶片垂直轴风力机运行时周边流场分布情况.通过比较不同方位角下流场涡量以及升、阻力系数得出:在方位角为105°附近时,翼型下表面产生流动分离,并导致失速;下风区翼型运行的流场由于受到上风区尾流的影响,翼型周围没有产生明显的流动分离.     

Modelling and analysis of floating spar‐type wind turbine drivetrain

This paper studies the drivetrain dynamics of a 750 kW spar‐type floating wind turbine (FWT). The drivetrain studied is a high‐speed generator, one‐stage planetary, two‐stage parallel and three‐point support type. The response analysis is carried out in two steps. First, global aero‐hydro‐elastic‐servo time‐domain analyses are performed using HAWC2. The main shaft loads, which include the axial forces, shear forces and bending moments, are obtained in this integrated wind–wave response analysis. These loads are then used as inputs for the multi‐body drivetrain time‐domain analyses in SIMPACK. The investigations are largely based on comparisons of the main shaft loads and internal drivetrain responses from 1 h simulations. The tooth contact forces, bearing loads and gear deflections are the internal drivetrain response variables studied. The comparisons are based on the mean values, standard deviations and maximum values extrapolated using a 10 ^{? 5} up‐crossing rate. Both operational and parked conditions are considered. The investigation consists of three parts. First, the responses are compared between the FWT and its equivalent land‐based version. Second, the contributions from the main shaft loads (shear forces, axial forces and bending moments) and nacelle motions are investigated individually. Third, an improved four‐point support (4PT) system is studied and compared against the original three‐point support system for the FWT. The results show that there are general increases in the standard deviations of the main shaft loads and internal drivetrain responses in the FWT. In addition, these increases are a result of the increased main shaft loads in the FWT, especially the non‐torque loads. Last, the 4PT system, when applied to a FWT drivetrain, significantly reduces the tooth contact forces and bearing loads in the low‐speed stage, but this result comes at the expense of increased main bearing radial loads. Copyright © 2013 John Wiley & Sons, Ltd.     

基于有限元法的漂浮式风力机Spar平台动态响应分析

以美国可再生能源实验室(NREL)的5 MW漂浮式风力机和Spar平台为参考模型,采用有限元分析法分析了Spar平台的波浪载荷频域特性,包括对振幅响应算子、绕射力和F-K力频域动态响应分析,并对比了三种波浪谱(P-M谱、JONSWAP谱和Wen’s谱)海况下的时域响应.结果表明:Spar平台载荷峰值出现在低频波浪作用下,且响应明显,而随着频率的增大,其值逐渐减小;在P-M谱和Wen’s谱作用下的平台动态响应无明显差异,且两者的振幅和周期大致相同,而在JONSWAP谱作用下的平台动态响应最小,但其往复周期小,频率大,使得系泊系统承受较大的周期性张力.     

Assessment of a comprehensive aeroelastic tool for horizontal‐axis wind turbine rotor analysis

This paper presents the development of a computational aeroelastic tool for the analysis of performance, response and stability of horizontal‐axis wind turbines. A nonlinear beam model for blades structural dynamics is coupled with a state‐space model for unsteady sectional aerodynamic loads, including dynamic stall effects. Several computational fluid dynamics structural dynamics coupling approaches are investigated to take into account rotor wake inflow influence on downwash, all based on a Boundary Element Method for the solution of incompressible, potential, attached flows. Sectional steady aerodynamic coefficients are extended to high angles of attack in order to characterize wind turbine operations in deep stall regimes. The Galerkin method is applied to the resulting aeroelastic differential system. In this context, a novel approach for the spatial integration of additional aerodynamic states, related to wake vorticity and dynamic stall, is introduced and assessed. Steady‐periodic blade responses are evaluated by a harmonic balance approach, whilst a standard eigenproblem is solved for aeroelastic stability analyses. Drawbacks and potentialities of the proposed model are investigated through numerical and experimental comparisons, with particular attention to rotor blades unsteady aerodynamic modelling issues. Copyright © 2016 John Wiley & Sons, Ltd.     

Response analysis and comparison of a spar‐type floating offshore wind turbine and an onshore wind turbine under blade pitch controller faults

This paper analyses the effects of three pitch system faults on two classes of wind turbines, one is an onshore type and the other a floating offshore spar‐type wind turbine. A stuck blade pitch actuator, a fixed value fault and a bias fault in the blade pitch sensor are considered. The effects of these faults are investigated using short‐term extreme response analysis with the HAWC2 simulation tool. The main objectives of the paper are to investigate how the different faults affect the performance of wind turbines and which differences exist in the structural responses between onshore and floating offshore wind turbines. Several load cases are covered in a statistical analysis to show the effects of faults at different wind speeds and fault amplitudes. The severity of individual faults is categorized by the extreme values the faults have on structural loads. A pitch sensor stuck is determined as being the most severe case. Comparison between the effects on floating offshore and onshore wind turbines show that in the onshore case the tower, the yaw bearing and the shaft are subjected to the highest risk, whereas in the offshore case, the shaft is in this position. Copyright © 2014 John Wiley & Sons, Ltd.     

Simulation and wake analysis of a single vertical axis wind turbine

Because of several design advantages and operational characteristics, particularly in offshore farms, vertical axis wind turbines (VAWTs) are being reconsidered as a complementary technology to horizontal axial turbines. However, considerable gaps remain in our understanding of VAWT performance since cross‐flow rotor configurations have been significantly less studied than axial turbines. This study examines the wakes of VAWTs and how their evolution is influenced by turbine design parameters. An actuator line model is implemented in an atmospheric boundary layer large eddy simulation code, with offline coupling to a high‐resolution blade‐scale unsteady Reynolds‐averaged Navier–Stokes model. The large eddy simulation captures the turbine‐to‐farm scale dynamics, while the unsteady Reynolds‐averaged Navier–Stokes captures the blade‐to‐turbine scale flow. The simulation results are found to be in good agreement with three existing experimental datasets. Subsequently, a parametric study of the flow over an isolated VAWT, carried out by varying solidities, height‐to‐diameter aspect ratios and tip speed ratios, is conducted. The analyses of the wake area and velocity and power deficits yield an improved understanding of the downstream evolution of VAWT wakes, which in turn enables a more informed selection of turbine designs for wind farms. Copyright © 2016 John Wiley & Sons, Ltd.     

驱动热泵的垂直轴风力机风轮气动特性数值分析

根据风能热泵系统的工作环境及匹配特性,针对给定供热面积的系统各参数选取方法以及垂直轴风力机风轮设计进行了探讨,在此基础上初步完成了额定功率为300 W的风轮设计,并利用二维数值模拟方法对不同叶尖速比下的性能曲线进行计算,分析得到了额定风速9.00 m/s时,驱动压缩机的最佳风轮转速为350~375r/min。结果表明,300 W垂直轴风力机的输出特性可满足风能热泵机组的工作要求,该数值分析结果可为风能供热技术的应用示范提供理论支撑。