A study of the performance benefits of closely-spaced lateral wind farm configurations

Scaled wind turbine experiments were conducted in order to evaluate the beneficial effect of closely-spaced lateral wind turbine configurations on the performance of a wind farm. Two outer wind turbines were spaced apart with a particular gap distance and the longitudinal setback of a central rotor was varied at each gap width. The turbine placement resulted in tip-to-tip separation distances that ranged from 1 diameter (D) to 0.25D. Additionally, the performance of a wind farm layout in rough and smooth boundary layers, designed to mimic onshore and offshore conditions, respectively, was evaluated. It was observed that a narrow gap between several laterally-aligned rotors creates an in-field blockage effect that results in beneficial flow acceleration through the gap. This increase in speed increases the power output of the central turbine when its longitudinal setback is between 0D and 2.5D. A cumulative increase in power output of 17% was observed when 3 rotors were aligned in a lateral plane with a blade tip separation of 0.5D or 0.25D, compared to the same number of rotors in isolation. While the benefits of closely-spaced wind turbines were observed in both of the tested boundary layers, the performance benefits with a smooth boundary layer were smaller than with a rough boundary layer. These results may lead to new wind farm design methodologies for certain topology- and wind distribution-specific sites and suggest that wind turbines can be closely-spaced in the lateral direction in order to obtain substantial increases in power.     

实度对低风速风电机组风轮气动性能影响研究

为提高低风速地区的风能利用率,研究风轮实度对低风速风电机组气动性能的影响。考虑影响风轮实度因素（叶片数量、弦长及安装角）,设计2组不同弦长叶片与可调安装角轮毂。安装角改变时不仅会引起实度变化,还会使叶尖速比发生改变。通过车载试验验证安装角不同时对风轮气动性能的影响主要与叶尖速比相关。根据不同风轮表面压力分布数值模拟结果得出：相同风速下,弦长由叶根到叶尖逐渐增大的叶片更易启动。相同条件下,试验机组输出功率与数值模拟机组输出功率最大相差5.37%,说明数值模拟结果可信。随着风轮实度的增加,风速5 m/s时,其风能利用系数呈增大趋势,风速8 m/s时,其风能利用系数呈减小趋势,两趋势相交时实度为25.38%,得出该实度下风轮气动性能较优,即可得到适合低风速地区的风轮实度。     

Simulation of wind turbine wake interaction using the vorticity transport model

The aerodynamic interactions that can occur within a wind farm can result in the constituent turbines generating a lower power output than would be possible if each of the turbines were operated in isolation. Tightening of the constraints on the siting of wind farms is likely to increase the scale of the problem in the future. The aerodynamic performance of turbine rotors and the mechanisms that couple the fluid dynamics of multiple rotors can be most readily understood by simplifying the problem and considering the interaction between only two rotors. The aerodynamic interaction between two rotors in both co‐axial and offset configurations has been simulated using the Vorticity Transport Model. The aerodynamic interaction is a function of the tip speed ratio, and both the streamwise and crosswind separation between the rotors. The simulations show that the momentum deficit at a turbine operating within the wake developed by the rotor of a second turbine is governed by the development of instabilities within the wake of the upwind rotor, and the ensuing structure of the wake as it impinges on the downwind rotor. If the wind farm configuration or wind conditions are such that a turbine rotor is subject to partial impingement by the wake produced by an upstream turbine, then significant unsteadiness in the aerodynamic loading on the rotor blades of the downwind turbine can result, and this unsteadiness can have considerable implications for the fatigue life of the blade structure and rotor hub. Copyright © 2009 John Wiley & Sons, Ltd.     

Wind turbine rotor imbalance detection using nacelle and blade measurements

Wind power is the world's fastest growing renewable energy source, but operations and maintenance costs are still a major obstacle toward reliability and widescale adoption of wind power, accounting for a large part of the cost of energy for offshore installations. Structural health monitoring systems have been proposed for implementing condition‐based maintenance. The wind energy industry currently uses condition monitoring systems that are mostly adapted from roating machinery in other power generation industries. However, these systems have had limited effectiveness on wind turbines because of their atypical operating conditions, which are characterized by low and variable rotational speed, rapidly varying torque, extremely large rotors and stochastic loading from the wind. Although existing systems primarily take measurements from the nacelle, valuable information can be extracted from the structural dynamic response of the rotor blades to mitigate potentially damaging loading conditions. One such condition is rotor imbalance, which not only reduces the aerodynamic efficiency of the turbine and therefore its power output but can also lead to very large increases in loading on the drivetrain, blades and tower. The National Renewable Energy Laboratory's fast software was used to model both mass and aerodynamic imbalance in a 5 MW offshore wind turbine. It is shown that a combination of blade and nacelle measurements, most of which can be obtained from standard instrumentation already found on utility‐scale wind turbines, can be formulated into an algorithm used to detect and locate imbalance. The method described herein allows for imbalance detection that is potentially more sensitive than existing on‐line systems, while taking advantage of sensors that are already in place on many utility‐scale wind turbines. Copyright © 2014 John Wiley & Sons, Ltd.     

New high profitable wind turbines

To generate more quantities of electric energy from wind it is necessary to use a new type of wind turbine built in the regulable mantle's nozzle. This wind turbine type replaces the free air stream from wind by a programmed, i.e. regulated, and partially concentrated stream of air. The nozzle shell is designed as an aerodynamically shaped ring with wings with its lower pressure side pointed towards the centre so that the lift force on each part of the wing is directed radially towards the centre. This induces centrifugal reaction force in the airflow that causes the stream field to expand strongly downstream of the rotor and includes a greater number of streamlines in the active stream in front of the rotor (upstream). Thus the nozzle forces a higher mass flow rate of air through the turbine. The higher mass flow and higher velocity reduction behind the rotor result in a higher energy output from the wind turbine in the nozzle. In this way the wind turbine efficiency is multiplied. New turbines induce more power from weaker and medium winds and their lasting time, because of the relation P=f(v³) (i.e. the power corresponds to wind velocity raised to third power). Wind turbine nozzle produces three times more energy than conventional wind turbine. Short economic analysis for conditions of the island of Lastovo indicates that profit gained by new turbines is up to five times higher than by conventional turbines. The new wind turbine nozzle should generate interest and demand on an international market, even for regions with weaker winds.     

Studies of some high solidity symmetrical and unsymmetrical blade H-Darrieus rotors with respect to starting characteristics,dynamic performances and flow physics in low wind streams

Vertical axis wind turbines can be successfully installed in low wind speed conditions but its detailed starting characteristics in terms of starting torque, starting time and dynamic performances have not been investigated thoroughly which is important for increasing the energy yield of such turbines. Amongst their designs, H-Darrieus rotor, in spite of having good power coefficient, possesses poor self-starting features as symmetrical blade profiles are used most of the times. Instead of using symmetrical blades if unsymmetrical or cambered blades are used with high solidity, then starting performance of H-Darrieus rotor along with its power coefficients can be improved. Though this performance improvement measures are correlated with improvement in the starting characteristics, a detailed work in this direction would be useful and for this reason the present work has been carried out. Three types of blade designs have been considered; two unsymmetrical blades namely S815 and EN0005 and one conventional symmetrical NACA 0018 blade, and experiments are conducted using a centrifugal blower test rig for three-bladed H-Darrieus rotors using these three considered blades at low wind streams (4 m/s, 6 m/s and 8 m/s). Considering reality, the effects of flow non-uniformity and turbulence intensity on the rotor performance at optimum condition as well as flow physics have also been studied. It has been found that unsymmetrical S815 blade rotor has higher dynamic torque and higher power coefficient than unsymmetrical EN0005 and symmetrical NACA 0018 blade H-Darrieus rotors.     

Wind electric power in the world and perspectives of its development in India

The global market for wind power is expanding faster than any other source of renewable energy. From just 4,800 MW in 1995 raise to fifteen-fold to reach 73,904 MW at the end of 2006. Top five wind electric power generating countries at the end of 2006 were Germany, Spain, United States of America (USA), India and Denmark. Since 1980s, when the first commercial wind turbine was deployed, their capacity, efficiency and visual design have all improved a lot. A modern wind turbine annually produces 180 times more electricity at less than half the cost per unit (kWh) than its equivalent twenty years ago. The largest turbines being manufactured now are of rated power of 5 MW capacity and a rotor diameter of 126 m. Modern turbines are modular and quick to install, whilst wind farms vary in size from a few MW to several hundred MW. Keeping these factors in view, an attempt has been made in this paper to present current advances in wind turbine generator technology. Wind energy scenario in the world in general and in India in particular have been presented. Further the cost components of wind turbine electric generation system have been included.     

Innovative improvement of a drag wind turbine performance

Drag type wind turbines have strong potential in small and medium power applications due to their simple design. However, a major disadvantage of this design is the noticeable low conversion efficiency. Therefore, more research is required to improve the efficiency of this design. The present work introduces a novel design of a three-rotor Savonius turbine with rotors arranged in a triangular pattern. The performance of the new design is assessed by computational modeling of the flow around the three rotors. The 2D computational model is firstly applied to investigate the performance of a single rotor design to validate the model by comparison with experimental measurements. The model introduced an acceptable accuracy compared to the experimental measurements. The performance of the new design is then investigated using the same model. The results indicated that the new design performance has higher power coefficient compared with single rotor design. The peak power coefficient of the three rotor turbine is 44% higher than that of the single rotor design (relative increase). The improved performance is attributed to the favorable interaction between the rotors which accelerates the flow approaching the downstream rotors and generates higher turning moment in the direction of rotation of each rotor.     

风力机扩散放大器流场的数值仿真

应用CFD软件对二维风力机扩散放大器模型进行数值仿真，计算最小截面处流速增加比率，分析扩散器内压力场和速度场，找出流动特性，比较不同型号及相同型号不同尺寸的扩散器之间的特性曲线，分析原因，对扩散器的研究起到理论指导作用。     

Effect of guide vanes on the performance of a self-rectifying air turbine with constant and variable chord rotors

A Wells turbine is a self-rectifying air flow turbine capable of converting pneumatic power of the periodically reversing air stream in Oscillating Water Column into mechanical energy. The Wells turbine has inherent disadvantages; lower efficiency, poorer starting characteristics, higher axial force and low tangential force in comparison with conventional turbines. Guide vanes before and after the rotor suggest a means to improve the tangential force, hence its efficiency. Experimental investigations are carried out on a Wells turbine with the constant chord and variable chord blade rotors fitted with inlet and outlet guide vanes to understand the aerodynamics. Experiments were also conducted for the above said rotors with various stagger angles to validate the design stagger angle. In addition, the starting and running characteristics of the rotors have been studied and compared with the case without guide vanes. Studies were done at various flow coefficients covering the entire range of flow coefficients over which the turbine is operable. The efficiency, starting characteristics of the turbines with guide vanes have improved when compared with the respective turbines without guide vanes.     

A hybrid actuator disc – Full rotor CFD methodology for modelling the effects of wind turbine wake interactions on performance

The performance of individual wind turbines is crucial for maximum energy yield, however, their performance is often reduced when turbines are placed together in an array. The wake produced by the rotors interacts with downstream turbines, resulting in a reduction in power output. In this paper, we demonstrate a new and faster modelling technique which combines actuator disc theory, modelled using wind tunnel validated Computational Fluid Dynamics (CFD), and integrated into full rotor CFD simulations. This novel hybrid of techniques results in the ability to analyse performance when simulating various array layouts more rapidly and accurately than using either method on its own.It is shown that there is a significant power reduction from a downstream turbine that is subjected to the wake of an upstream turbine, and that this is due to both a reduction in power in the wind and also due to changes in the aerodynamics. Analysis of static pressure along the blade showed that as a result of wake interactions, a large reduction in the suction peak along the leading edge reduced the lift generated by the rotor and so reduced the torque production and the ability for the blade to extract energy from the wind.     

Noise simulations of flap devices for wind turbine rotors

The rotor of a large diameter wind turbine experiences more substantial and more dynamic loads due to the fluctuating and heterogeneous wind field. The project SmartBlades 2.0 investigated rotor blade design concepts that alleviate aerodynamic loading using active and passive mechanisms. The present work evaluates the acoustics of the two load alleviating concepts separately, an inboard slat and an outboard flap, using the Fast Random Particle Mesh/Fast Multipole Code for Acoustic Shielding (FRPM/FMCAS) numerical prediction toolchain developed at DLR with input from the averaged flow field from RANS. The numerical tools produce a comparable flap side-edge noise spectrum with that of the measurement conducted in the Acoustic Wind Tunnel Braunschweig (AWB). The validated FRPM/FMCAS was then used to analyze the self-noise from a slat at the inboard section of a rotor blade with a 44.45 m radius and compared with that from the outboard trailing edge. Furthermore, the rotational effect of the rotor was included in the post-processing to emulate the noise observed at ground level. The findings show an increase in the slat's overall sound pressure level and a maximum radiation upwind of the wind turbine for the case with the largest wind speed that represents the off-design condition. In operational conditions, the slat adds at most 2 dB to the overall sound pressure level. The toolchain evaluates wind turbine noise with conventional or unconventional blade design, and the problem can be scaled up for a full-scale analysis. As such, the tools presented can be used to design low-noise wind turbines efficiently.     

Wave energy harvesting using a novel turbine rotor geometry

Wells turbines provide a practical solution for wave energy harvesting. The low aerodynamic efficiency of Wells turbines tangibly reduces their output power. Both the turbine efficiency and output power depend on the turbine solidity. The turbine solidity decreases from rotor hub to rotor tip for the commonly used rotors with constant chord‐length blades. The present work introduces a novel Wells turbine rotor geometry. This geometry was obtained by numerically optimizing the rotor's radial solidity distribution. The turbine performance with different rotor geometries was numerically simulated by solving the three‐dimensional Reynolds‐averaged Navier–Stocks equation under incompressible and steady state flow conditions. Simple and multi‐objective optimization were implemented in order to obtain the optimum rotor geometry. The present work showed that an improved turbine performance can be achieved by optimizing the turbine radial solidity distribution. Two different optimized rotor geometries were obtained and presented. The first rotor geometry improved the turbine efficiency by up to 4.7% by reducing its pressure drop. The second rotor geometries enhanced the turbine output power by up to 10.8%. Copyright © 2016 John Wiley & Sons, Ltd.     

Optimization of turbine design in wind farms with multiple hub heights,using exact analytic gradients and structural constraints

Wind farms are generally designed with turbines of all the same hub height. If wind farms were designed with turbines of different hub heights, wake interference between turbines could be reduced, lowering the cost of energy (COE). This paper demonstrates a method to optimize onshore wind farms with two different hub heights using exact, analytic gradients. Gradient‐based optimization with exact gradients scales well with large problems and is preferable in this application over gradient‐free methods. Our model consisted of the following: a version of the FLOw Redirection and Induction in Steady‐State wake model that accommodated three‐dimensional wakes and calculated annual energy production, a wind farm cost model, and a tower structural model, which provided constraints during optimization. Structural constraints were important to keep tower heights realistic and account for additional mass required from taller towers and higher wind speeds. We optimized several wind farms with tower height, diameter, and shell thickness as coupled design variables. Our results indicate that wind farms with small rotors, low wind shear, and closely spaced turbines can benefit from having two different hub heights. A nine‐by‐nine grid wind farm with 70‐meter rotor diameters and a wind shear exponent of 0.08 realized a 4.9% reduction in COE by using two different tower sizes. If the turbine spacing was reduced to 3 diameters, the reduction in COE decreased further to 11.2%. Allowing for more than two different turbine heights is only slightly more beneficial than two heights and is likely not worth the added complexity.     

Urban wind energy conversion: The potential of ducted turbines

The prospects for urban wind power are discussed. A roof-mounted ducted wind turbine, which uses pressure differentials created by wind flow around a building, is proposed as an alternative to more conventional approaches. Outcomes from tests at model and prototype scale are described, and a simple mathematical model is presented. Predictions from the latter suggest that a ducted turbine can produce very high specific power outputs, going some way to offsetting its directional sensitivity. Further predictions using climate files are made to assess annual energy output and seasonal variations, with a conventional small wind turbine and a photovoltaic panel as comparators. It is concluded that ducted turbines have significant potential for retro-fitting to existing buildings, and have clear advantages where visual impact and safety are matters of concern.     

Sensitivity of southern California wind energy to turbine characteristics

Using output from a high‐resolution meteorological simulation, we evaluate the sensitivity of southern California wind energy generation to variations in key characteristics of current wind turbines. These characteristics include hub height, rotor diameter and rated power, and depend on turbine make and model. They shape the turbine's power curve and thus have large implications for the energy generation capacity of wind farms. For each characteristic, we find complex and substantial geographical variations in the sensitivity of energy generation. However, the sensitivity associated with each characteristic can be predicted by a single corresponding climate statistic, greatly simplifying understanding of the relationship between climate and turbine optimization for energy production. In the case of the sensitivity to rotor diameter, the change in energy output per unit change in rotor diameter at any location is directly proportional to the weighted average wind speed between the cut‐in speed and the rated speed. The sensitivity to rated power variations is likewise captured by the percent of the wind speed distribution between the turbines rated and cut‐out speeds. Finally, the sensitivity to hub height is proportional to lower atmospheric wind shear. Using a wind turbine component cost model, we also evaluate energy output increase per dollar investment in each turbine characteristic. We find that rotor diameter increases typically provide a much larger wind energy boost per dollar invested, although there are some zones where investment in the other two characteristics is competitive. Our study underscores the need for joint analysis of regional climate, turbine engineering and economic modeling to optimize wind energy production. Copyright © 2012 John Wiley & Sons, Ltd.     

Grounding and the influence of the total pressure losses in ducted wind turbines

Ducted wind turbines are known to exceed the Betz limit for the power that may be extracted from free currents per unit cross‐sectional area of their rotors. The present study points to another advantage: the fact that the turbine rotor (and the associated subassemblies) need not be lifted up on the air and rotated to align with the wind. They can function just as effectively by installing them on the ground and sucking air inside it. The analysis evaluates the efficiency of a ducted wind turbine plant, exhausting into the wake of a bluff body, under both ideal and realistic component performance. The results indicate that there exists a strong need to develop diffusers with a very low loss coefficient (e.g. by employing suction or transpiration techniques). Under such conditions and by employing the above exhaust assist techniques, the ducted wind turbine concept may deliver efficiencies more than an order of magnitude larger than those of the conventional plants. Copyright © 2009 John Wiley & Sons, Ltd.     

新型双风轮风力机气动特性的三维流场数值模拟

基于Simplic算法,采用SST κ-ω湍流模型,利用Fluent6．3数值模拟软件对新型的小型双风轮风力机的气动特性进行了三维流场研究,并与同规格单风轮风力机的三维流场进行了比较．结果表明：与单风轮风力机相比,随着后风轮叶片数目的增加,新型双风轮风力机的湍流强度变大,风力机运行的稳定性在一定程度上有所降低;当后风轮的叶片数目合理时,后风轮对前风轮的影响较小,且可以有效地捕捉到前风轮的漏风,使得新型双风轮风力机的风轮在获得较大迎风面积的同时可以保持较高的转速,进而能够高效地实现风能的两级利用,明显提高发电功率和增大风能利用系数．     

A numerical investigation into the effect of diffusers on the performance of hydro kinetic turbines using a validated momentum source turbine model

Kinetic hydropower involves the use of hydro turbines, submerged within existing currents for power generation. They are applicable to ocean and tidal currents, rivers, and human-made channels. This versatility gives them advantages over conventional hydropower, however they suffer from low power densities. This numerical study investigates the use of diffusers to enhance the performance and viability of kinetic hydro turbines. To simplify the problem, the turbine is modeled as a momentum source region, a strategy that is first validated against Betz theory. The diffuser configuration produces 3.1 times more power than the turbine with no diffuser. A scaling analysis also shows a turbine with a diffuser outperforms a larger size turbine with no diffuser.     

Characterization of aerodynamic performance of wind-lens turbine using high-fidelity CFD simulations

Wind-lens turbines (WLTs) exhibit the prospect of a higher output power and more suitability for urban areas in comparison to bare wind turbines. The wind-lens typically comprises a diffuser shroud coupled with a flange appended to the exit periphery of the shroud. Wind-lenses can boost the velocity of the incoming wind through the turbine rotor owing to the creation of a low-pressure zone downstream the flanged diffuser. In this paper, the aerodynamic performance of the wind-lens is computationally assessed using high-fidelity transient CFD simulations for shrouds with different profiles, aiming to assess the effect of change of some design parameters such as length, area ratio and flange height of the diffuser shroud on the power augmentation. The power coefficient (C_p) is calculated by solving the URANS equations with the aid of the SST k–ω model. Furthermore, comparisons with experimental data for validation are accomplished to prove that the proposed methodology could be able to precisely predict the aerodynamic behavior of the wind-lens turbine. The results affirm that wind-lens with cycloidal profile yield an augmentation of about 58% increase in power coefficient compared to bare wind turbine of the same rotor swept-area. It is also emphasized that diffusers (cycloid type) of small length could achieve a twice increase in power coefficient while maintaining large flange heights.