Wake Losses Research Articles

Wind Farm Micro-Siting Optimization and Benefits Analysis

This paper puts forward a creative turbine micro-siting scheme breaking the 3D-5D rule, and calculates wake losses, power production and its economic benefits in traditional wind farm micro-siting scheme and creative wind farm micro-siting scheme by evaluating software WEPAS and MWVE. Results show that the micro-siting scheme breaking 3D-5D is more feasible and effective. It not only ensures the safe and stable operation of the wind turbines and technical parameters, but also the power production and the initial investment have been optimized.

Compressible Direct Numerical Simulation of Low-Pressure Turbines—Part I: Methodology

Modern low pressure turbines (LPT) feature high pressure ratios and moderate Mach and Reynolds numbers, increasing the possibility of laminar boundary-layer separation on the blades. Upstream disturbances including background turbulence and incoming wakes have a profound effect on the behavior of separation bubbles and the type/location of laminar-turbulent transition and therefore need to be considered in LPT design. Unsteady Reynolds-averaged Navier–Stokes (URANS) are often found inadequate to resolve the complex wake dynamics and impact of these environmental parameters on the boundary layers and may not drive the design to the best aerodynamic efficiency. LES can partly improve the accuracy, but has difficulties in predicting boundary layer transition and capturing the delay of laminar separation with varying inlet turbulence levels. Direct numerical simulation (DNS) is able to overcome these limitations but has to date been considered too computationally expensive. Here, a novel compressible DNS code is presented and validated, promising to make DNS practical for LPT studies. Also, the sensitivity of wake loss coefficient with respect to freestream turbulence levels below 1% is discussed.

Steady Vortex-Generator Jet Flow Control on a Highly Loaded Transonic Low-Pressure Turbine Cascade: Effects of Compressibility and Roughness

A new high-speed linear cascade has been developed for low-pressure turbine (LPT) studies at The Ohio State University. A compressible LPT profile is tested in the facility and its baseline performance at different operating conditions is assessed by means of isentropic Mach number distribution and wake total pressure losses. Active flow control is implemented through a spanwise row of vortex-generator jets (VGJs) located at 60% chord on the suction surface. The purpose of the study is to document the effectiveness of VGJ flow control in high-speed compressible flow. The effect on shock-induced separation is assessed by Mach number distribution, wake loss surveys and shadowgraph. Pressure sensitive paint (PSP) is applied to understand the three dimensional flow and shock pattern developing from the interaction of the skewed jets and the main flow. Data show that with increasing blowing ratio, the losses are first decreased due to separation reduction, but losses connected to compressibility effects become stronger due to increased passage shock strength and jet orifice choking; therefore, the optimum blowing ratio is a tradeoff between these counteracting effects. The effect of added surface roughness on the uncontrolled flow and on flow control behavior is also investigated. At lower Mach number, turbulent separation develops on the rough surface and a different flow control performance is observed. Steady VGJs appear to have control authority even on a turbulent separation but higher blowing ratios are required compared to incompressible flow experiments reported elsewhere. Overall, the results show a high sensitivity of steady VGJs control performance and optimum blowing ratio to compressibility and surface roughness.

Wind farm layout optimization using imperialist competitive algorithm

In this work, the wind farm layout optimization problem is dealt with using a new approach. The aim of wind farm layout optimization is to maximize the output power of a wind farm considering the wake losses. Layout optimization minimizes the wake losses regarding the location of the turbines. Three different wind scenarios with different wind direction angles, wind direction blowing probabilities, and Weibull distribution parameters are assumed. Since, the problem is nonlinear and constrained, imperialist competitive algorithm is used as a modern and powerful algorithm for continuous optimization problems. The optimization outcomes indicate that imperialist competitive algorithm yields promising results.

The Potential of Wind Energy in Malaysian Renewable Energy Policy: Case Study in Kudat, Sabah

Recently, a study of wind energy potential at selected areas in Malaysia was completed by Universiti Malaysia Terengganu (UMT). One of selected sites is at Kudat, Sabah. The wind atlas analysis and application Program (WAsP) was used to investigate the wind energy potential by developing the regional wind diagram in the considered site at Kudat. Different types of data have been collected about climate, topography, obstacles and surface roughness of the land. The observed wind climates (OWC) of the wind mast had been created. The wind atlas and the resource grid have been calculated and analyzed, especially in the high wind resource areas and using criteria based on proximity, similarity and ruggedness index (RIX). The 22kW rated power wind turbine was selected due to it's best capacity factor. Then, the annual energy production of wind farms were evaluated by taking the turbine wake losses into account. The obtained results showed that the optimal place for single units of wind energy production is located in the farm zone 2 with a potential varying from 37.5–43.1 MWh per year. In economic aspect, the predicted FiT rates in between RM0.30 and RM1.30 were tested by using net present value (NPV) and payback period (PBP) approach. Finally, the conducive FiT rates for wind projects with secured investment were identified, and the rates were in between RM0.46 and RM0.80 per kWh.

Optimal output power of not properly designed wind farms, considering wake effects

The goal of this paper is to present an algorithm for increasing output power in wind farms with high wake losses. In some wind directions, wake effects cannot be neglected because the distance between turbines may be less than 5–8 times of rotor diameter. The purpose of presented algorithm is to determine coefficient of performance (CP), thrust coefficient (CT), pitch angle (β) and rotational speed (ω) of each turbine by using wake effects equations so that wake losses in the wind farm becomes minimum which result in increasing output power of wind farm. PSO algorithm is used for optimization. Finally, a sample wind farm consisting of 16 turbines is used as a case study and the results show that there is a noticeable increase in the amount of wind farm output power rather than the time which no control and optimization is used. The conclusion is that the presented algorithm is suitable for wind farms with high wake losses and little distance between their turbines and is very effective for increasing their output power.

Offshore winds mapped from satellite remote sensing

Around 2000 wind turbines in 58 offshore wind farms produce wind energy in the Northern European seas and many new wind farms are foreseen. The wind resource assessment is costly to observe using traditional meteorological masts and therefore atmospheric modeling is state of the art. However, to reduce the uncertainty on the model results on the offshore wind resource, it is necessary to compare model results with observations. Observations from ground‐based wind lidar and satellite remote sensing are the two main technologies that can provide new types of offshore wind data at relatively low cost. The advantages of microwave satellite remote sensing are (1) horizontal spatial coverage, (2) long data archives, and (3) high spatial detail both in the coastal zone and of far‐field wind farm wake. Passive microwave ocean wind speed data are available since 1987 with up to six observations per day with near‐global coverage. The data are particularly useful for investigation of long‐term wind conditions. Scatterometer ocean surface wind vectors provide a continuous series since 1999 with twice‐daily near‐global coverage. Both types of data have grid cells around 25 km. In contrast, synthetic aperture radar (SAR) wind maps can be retrieved at 1‐km grid resolution. SAR‐based wind maps have been used for wind resource assessment far offshore and in the coastal zones with good results when compared to e.g., meteorological data and mesoscale model results. High‐resolution SAR data show very long far‐field wind farm wakes. Thus wind farm wake loss is foreseen in wind farm clusters. WIREs Energy Environ 2014, 3:594–603. doi: 10.1002/wene.123This article is categorized under: Wind Power > Science and Materials

Optimization of Exogenous and Endogenous Variables for a Three Column Wind Farm Using CMAES

Uneven terrains in mountain regions, where wind mills are to be erected cause concerns on the matrix of location, variation in wind direction, wake effects and due to location which may take a toll on efficiency, frequent changes in wind velocity, limitation of the hub height are a fear of the exogenous variables that influence the operation of wind farm. An attempt is made in this work to analyze the effect of those parameters on the efficiency of wind farm. Energy efficiency and exergy efficiency for a three column wind farm are determined and compared. The mathematical model developed considers wake deficit loss, transmission losses and resource losses the loss due to change in the wind direction, overall efficiency factor and locational specifications. A new objective function is derived for the wind farm with multidirectional wind flow and it is solved by Covariant Matrix Adaptation Evolutionary Strategy algorithm. This algorithm is used to maximize the wind farm exergetic efficiency. Location specification is the main variable to optimize and the other dimensionless variables remain same. Exergy efficiency is improved when compared to the reference layouts. The results projected will help the wind farm promoters to optimally utilize the resources to get maximum output.

Wakes in very large wind farms and the effect of neighbouring wind farms

We present the first analysis of wake losses in some of the largest offshore wind farms built to date. In addition, we give an example of the external wake losses that can be imposed by a neighbouring wind farm. Both situations lend insights to the wake phenomena in large offshore wind farm clusters. A simple wake model is compared to the data to assess the need for a more detailed physical description of large wind farm wakes.

A comparison of actuator disc and BEM models in CFD simulations for the prediction of offshore wake losses

In this paper computational fluid dynamics (CFD) simulations are performed using ANSYS CFX to compare wake interaction results obtained from two rotor modelling methodologies: the standard actuator disc and the blade element momentum model (BEM). The unsteady simulations embed Coriolis forces and neutral stability conditions in the surface layer and stable conditions in the free stream. The BEM method is implemented in the CFD code through a pre-processing set of files that employs look-up tables. The control system for the wind turbines is considered through look-up tables that are constructed based on operational wind farm data. Simulations using the actuator disc and BEM methodologies have been performed using a number of different turbulence models in order to compare the wind turbine wake structure results. The use of URANS and LES numerical methods, coupled with the two different methodologies of representing the turbine, enables an assessment to be made of the details required for varying degrees of accuracy in computing the wake structures. The findings stress the importance of including the rotation of the wake and the non-uniform load on the rotor in LES simulations to account for more accurate turbulence intensity levels in the near wake.

Modeling the Influence of Land-Shape on the Energy Production Potential of a Wind Farm Site

During wind farm planning, the farm layout or turbine arrangement is generally optimized to minimize the wake losses, and thereby maximize the energy production. However, the scope of layout design itself depends on the specified farm land-shape, where the latter is conventionally not considered a part of the wind farm decision-making process. Instead, a presumed land-shape is generally used during the layout design process, likely leading to sub-optimal wind farm planning. In this paper, we develop a novel framework to explore how the farm land-shape influences the output potential of a site, under a given wind resource variation. Farm land-shapes are defined in terms of their aspect ratio and directional orientation, assuming a rectangular configuration. Simultaneous optimizations of the turbine selection and placement are performed to maximize the energy production capacity, for a set of sample land-shapes with fixed land area. The maximum farm capacity factor or farm output potential is then represented as a function of the land aspect ratio and land orientation, using quadratic and Kriging response surfaces. This framework is applied to design a 25 MW wind farm at a North Dakota site that experiences multiple dominant wind directions. An appreciable 5% difference in capacity factor is observed between the best and the worst sample farm land-shapes at this wind site. It is observed that among the 50 sample land-shapes, higher energy production is accomplished by the farm lands that have aspect ratios significantly greater than one, and are oriented lengthwise roughly along the dominant wind direction axis. Subsequent optimization of the land-shape using the Kriging response surface further corroborates this observation.

Micro-siting Wind Resource Assessment and Near Shore Wind Farm Analysis in Pakpanang District, Nakhon Si Thammarat Province, Thailand

This paper presents the micro-siting wind resource assessment and near shore wind farm analysis in Pakpanang district, Nakhon Si Thammarat province, southern Thailand. One year of observed wind speed and wind direction data from a 120 m met tower installed near the shoreline are analyzed in order to investigate the monthly mean wind speeds and dominant wind directions at the site. The observed wind data is used to develop a 20 m resolution microscale map of the study area in order to analyze several configurations of near shore wind farms in the range of 10 to 90 MW. To this end, annual energy productions (AEP), wake effects, and theoretical capacity factors (TCF) are estimated using WAsP. Results show that the monthly mean wind speeds at 120 m above ground level are in the range of 3.3 to 5.3 m/s. The dominant wind at the site are from the NEE and SSW directions. In terms of the near shore wind farm analysis, the maximum net AEP is approximately 170 GWh/year (90 MW wind farm scenario), while the corresponding wind farm's wake loss is 3.65%, which corresponds to a 21.3% wind farm TCF.

Investigation of the Shear Flow Effect on Secondary Flow and Losses in a Low Speed Axial Flow Compressor Cascade

This paper explores the effect of a prescribed intense shear flow on the flow structures in an axial flow compressor cascade. An approximate shear flow is generated in the test section of an open circuit cascade wind tunnel by using a planar grid of parallel rods with varying solidity. To study only the effect of shear, a compressor cascade based on NACA65 series with a relatively low camber was chosen. The cascade was analysed experimentally as well as computationally (using ANSYS Fluent). The computational results of the cascade were compared with the available measured data. The results agreed well with the experimental data. Detailed analysis of the numerical results was then carried out to explore the complex flow features caused by the shear flow in a cascade. With uniform flow, the secondary flow was found to be negligible from experiments as well as from the computations. Therefore, in an attempt to amplify secondary flow, a shear flow generator was placed upstream of the cascade. Experiments showed quite contrasting results with shear flow, as compared to the uniform flow, in terms of the wake loss. Numerical analysis revealed the formation of vortices in the wake of the cascade due to secondary flows caused by the incoming shear flow and other interesting flow features.

First Verification Test and Wake Measurement Results Using a SHIP-LIDAR System

Measuring wind offshore in deep water depths will be a future challenge. Where the sea bed foundation installation for fixed meteorological masts is impossible, floating systems tend to be a sophisticated solution. In addition to the use of moored lidar- buoy systems, ship-lidar systems are an alternative solution for a number of different applications. In this paper we describe general aspects of motion influences on lidar measurements as well as two motion-correction methods for motion-influenced lidar measurements. The implementation of the ship-lidar system and different scanning modes will be presented. First measurements were carried out as part of the EERA-DTOC project. Hence a verification of one of the two correction algorithms as well as first results from wake measurements behind the Alpha Ventus offshore wind farm will be shown. This comprises distinct wind speed wake losses and an increasing turbulence intensity in a distance of approximately 2km behind the wind farm.

Simulation of Wake Interactions in Wind Farms Using an Immersed Wind Turbine Model

The accurate modeling of the wind turbine wakes in complex terrain is required to accurately predict wake losses. In order to facilitate the routine use of computational fluid dynamics in the optimized micrositing of wind turbines within wind farms, an immersed wind turbine model is developed. This model is formulated to require grid resolutions that are comparable to that in microscale wind simulations. The model in connection with the k-ω turbulence model is embedded in a Reynolds-averaged Navier–Stokes solver. The predictions of the model are compared to available wind tunnel experiments and to measurements at the full-scale Sexbierum wind farm. The good agreement between the predictions and measurements demonstrates that the novel immersed turbine model is suited for the optimized micrositing of wind turbines in complex terrain.

Exploitation of Subharmonics for Separated Shear Layer Control on a High-Lift Low-Pressure Turbine Using Acoustic Forcing

The mechanism of separation control by sound excitation is investigated on the aft-loaded low-pressure turbine (LPT) blade profile, the L1A, which experiences a large boundary layer separation at low Reynolds numbers. Previous work by the authors has shown that on a laminar separation bubble such as that experienced by the front-loaded L2F profile, sound excitation control has its best performance at the most unstable frequency of the shear layer due to the exploitation of the linear instability mechanism. The different loading distribution on the L1A increases the distance of the separated shear layer from the wall and the exploitation of the same linear mechanism is no longer effective in these conditions. However, significant control authority is found in the range of the first subharmonic of the natural unstable frequency. The amplitude of forced excitation required for significant wake loss reduction is higher than that needed when exploiting linear instability, but unlike the latter case, no threshold amplitude is found. The fluid-dynamics mechanisms under these conditions are investigated by particle image velocimetry (PIV) measurements. Phase-locked PIV data gives insight into the growth and development of structures as they are shed from the shear layer and merge to lock into the excited frequency. Unlike near-wall laminar separation sound control, it is found that when such large separated shear layers occur, sound excitation at subharmonics of the fundamental frequency is still effective with high-Tu levels.

A Global Approach to Turbomachinery Flow Control: Passage Vortex Control

A flow control scheme was implemented in a low-pressure turbine cascade that simultaneously mitigated profile and endwall losses using midspan vortex generator jets (VGJs) and endwall suction. The combined system had an approximate zero-net mass flux. During the design, a theoretical model was used that effectively predicted the trajectory of the passage vortex using inviscid results obtained from two-dimensional computational fluid dynamics (CFD). The model was used in the design of two flow control approaches: the removal and redirection approaches. The emphasis of the removal approach was the direct application of flow control along the passage vortex (PV) trajectory. The redirection approach attempted to alter the trajectory of the PV with the judicious placement of suction holes. A potential flow model was created to aid in the design of the redirection approach. The model results were validated using flow visualization and particle image velocimetry (PIV) in a linear turbine cascade. Detailed total pressure loss wake surveys were measured while matching the suction and VGJ mass flow rates for the removal and redirection approaches at ReCx = 25,000 and blowing ratio, B, of 2. When compared with the no control results, the addition of VGJs and endwall suction reduced the wake losses by 69% (removal) and 68% (redirection). The majority of the total pressure loss reduction resulted from the spanwise VGJs, while the suction schemes provided modest additional reductions (<2%). At ReCx = 50,000, the endwall control effectiveness was assessed for a range of suction rates without midspan VGJs. Area-averaged total pressure loss reductions of up to 28% were measured in the wake at ReCx = 50,000, B = 0, with applied endwall suction (compared to no suction at ReCx = 50,000), at which point the loss core of the PV was almost completely eliminated.

The Effect of Acoustic Excitation on Boundary Layer Separation of a Highly Loaded LPT Blade

An experimental investigation of the effect of acoustic excitation on the boundary layer development of a highly loaded low-pressure turbine blade at low-Reynolds number is investigated. The aim of this work is to study the effect of excitation at select frequencies on separation which could give indications about active flow control exploitation. The front-loaded L2F blade is tested in a low-speed linear cascade. The uncontrolled flow presents a separation bubble on the suction surface at Reynolds numbers below 40,000. For these conditions, the instability of the shear layer is documented using hot-wire anemometry. A loudspeaker upstream of the cascade is directed towards the passage inlet section. A parametric study on the effect of amplitude and frequency is carried out. The effect of the excitation frequency is observed to delay separation for a range of frequencies. However, the control authority of sound is found to be most effective at the fundamental frequency of the shear layer. The amplitude of perturbation is significant in the outcome of control until a threshold value is reached. PIV measurements allow a deeper understanding of the mechanisms leading to the reduction of separation. Data has been acquired with a low inlet turbulence level (<1%) in order to provide a cleaner environment which magnifies the effects of the excitation frequency, and with an increased turbulence intensity level of 3% which is representative of more typical engine values. Integrated wake loss values are also presented to evaluate the effect on blade performance.

Wakes Calculation in a Offshore Wind Farm

This paper is focusing on the wake modeling in large offshore wind farms. Over the sea the ambient turbulence is much lower than onshore, wakes persist for long distances, mixing in a complex pattern. Hence an accurate evaluation of the wakes become crucial in the estimation both of the production and loads. Regarding loads the main effect of the turbines wakes is an increase of turbulence compared to the ambient one. A careful assessment of the wakes is therefore required when analyzing offshore wind farms. The approaches in estimating the wake losses go from simple theoretical or empirical laws to full rotor aerodynamic calculations; in between there is a range of intermediate calculations. Two approaches will be presented: the use of relative simple equations (also called analytical models) which are the standards in the wind resource assessment and a more accurate, but computationally more demanding, the actuator disc technique.

Meteorological Controls on Wind Turbine Wakes

The primary control on the magnitude of the power losses induced by wind turbine wakes in large wind farms is the hub-height wind speed via its link to the turbine thrust coefficient. Hence, at low to moderate wind speeds (between cut-in and rated turbine wind speeds) when the thrust coefficient is high, wake losses are proportionally larger and decrease to be virtually undetectable at wind speeds above rated wind speeds. Wind direction is also critical. Not only does it determine the effective spacing between turbines but also the wind speed distribution is primarily determined by synoptic forcing and typically has a predominant direction from which wind speeds tend to be higher (from southwest for much of the central United States and northern Europe). Two other interlinked variables, turbulence intensity (TI), and atmospheric stability also dictate wake losses. Quantifying, understanding, modeling, and predicting this complex and interdependent system is therefore critical to understanding and modeling wind farm power losses due to wakes, and to optimizing wind farm layout. This paper quantifies the impact of these variables on the power loss due to wakes using data from the large offshore wind farms located at Horns Rev and Nysted in Denmark.