Wind environment at a roof-mounted wind turbine on a peaked roof building

Knowledge of the wind resource above peaked roofs is necessary to determine whether installing small wind turbines on low-rise peaked roof buildings is feasible. The wind characteristics at a representative peaked roof barn in southern Ontario, Canada were investigated using a boundary layer wind tunnel and computational fluid dynamics. Field measurements at the barn were collected using sonic anemometers and compared with the simulation results. Wind speed amplification was confined to a region immediately above the roof and was relatively low for wind energy purposes. The presence of nearby trees or buildings adversely impacted wind speed amplification. Considering only wind-related factors, the placing of micro-wind turbines on roof peaks may be warranted. However, if sufficient space is available, it is recommended to place small turbines on a tower rather than on the peaked roof of a low-rise building.     

建筑外形对建筑顶面风力机微观选址的影响

针对城市建筑环境内的风能应用问题,利用CFD方法对4种不同建筑外形的高层建筑物顶面风场湍流特征进行研究,开展建筑外形对风向变化的敏感性分析,并分析4种建筑物顶面风速、湍流强度随高度的变化规律,确定4种建筑外形的建筑顶面风力机的合理安装位置和高度,结果表明：具有圆润曲线外形的2类建筑更利于风力机的安装,风力机的安装高度可更低;建筑物的长轴和短轴越接近,顶面越有利于风力机的安装;4种建筑外形的建筑顶面安装风力机时仅考虑U/U₀≥1的有利安装高度H_u即可保证风力机的有效输出功率和运行安全;无盛行风向情况下,4种建筑物的中心区域更有利于风力机安装,风力机的安装高度最低,圆柱体、椭圆体、正方体及长方体建筑物顶面中心区域风力机最低安装高度分别为1.05H、1.09H、1.11H及1.14H。     

建筑密度对建筑物群内风能利用的影响

针对建筑物群内风能应用问题,采用CFD方法,对建筑密度分别为26%、20%、18%、16%、14%的5种建筑物群周围风速和湍流强度特征开展研究,分析建筑密度对建筑物群内风力机合理安装位置的影响方式。结果表明：在低于1.5H高度范围内,建筑物群的建筑密度越大,同一安装高度上适合于安装风力机的区域就越大,即越有利于建筑物群内风能的应用;在高度高于1.5H后,建筑密度对建筑物群内风力机安装位置的影响消失;无论建筑密度大小,在低于1.2H的高度范围内,建筑物群内不适合安装风力机;在高度高于1.45H后,可优先考虑将风力机安装于建筑物群内中间一排建筑物顶面,在建筑物顶面可优先将风力机安装于拐角位置;5种建筑密度的建筑物群内只考虑风速要求即可确定风力机的合理安装位置。     

Air flow patterns around domed roof buildings

The wind has been regarded as an important criterion in designing building environments since the early days of human settlement. In hot, arid regions of Iran building is designed such as to have adequate protection against the wind storms and also they had provided summer time air conditioning by means of passive cooling. Most of such buildings are domed roof structures. In order to observe wind environment over such structures, simple models of these vaulted roof buildings are made and tested in a two dimensional low speed wind tunnel for low velocity smoke flows.Flow pattern over and around the models are visualized and photographed. From the observations, several features such as flow separation, turbulence, wind shadow area and flow circulation have been identified for different building configurations.     

Wind tunnel study on wind and turbulence intensity profiles in wind turbine wake

In recent years, there has been a rapid development of the wind farms in Japan. It becomes very important to investigate the wind turbine arrangement in wind farm, in order that the wake of one wind turbine does not to interfere with the flow in other wind turbines. In such a case, in order to achieve the highest possible efficiency from the wind, and to install as many as possible wind turbines within a limited area, it becomes a necessity to study the mutual interference of the wake developed by wind turbines. However, there is no report related to the effect of the turbulence intensity of the external flow on the wake behind a wind turbine generated in the wind tunnel. In this paper, the measurement results of the averaged wind profile and turbulence intensity profile in the wake in the wind tunnel are shown when the turbulence intensity of the external wind was changed. The wind tunnel experiment is performed with 500mm-diameter two-bladed horizontal axis wind turbine and the wind velocity in wake is measured by an I-type hot wire probe. As a result, it is clarified that high turbulence intensities enable to the entrainment of the main flow and the wake and to recover quickly the velocity in the wake.     

Implications of the UK field trial of building mounted horizontal axis micro-wind turbines

Building mounted micro-wind turbines and photovoltaics have the potential to provide widely applicable carbon free electricity generation at the building level. Photovoltaic systems are well understood and it is easy to predict performance using software tools or widely accepted yield estimates. Micro-wind, however, is far more complex and in comparison poorly understood. This paper presents the key findings of the building mounted (<2 kWp) turbine component of the UK micro-wind trial undertaken by the Energy Saving Trust in 2008/09. The monitored performance of 39 horizontal axis turbines in urban, suburban and rural locations is discussed alongside the accuracy of predictive wind speed tools for the sites. The performance of urban and suburban micro-wind sites in the trial was poor with annual generation of less than 75 kWh/m² swept area, the majority of which were less than 25 kWh/m². Good rural sites had an annual generation of between 100 and 280 kWh/m², far less than the nominal 360 kWh/m² (10% load factor for a typical turbine) that is often assumed. In the light of these findings, the potential impact of the UK’s latest policy instrument, the 2010 micro-generation tariffs, is considered for both micro-wind and photovoltaics.     

Techno-economic assessment of the application of small-scale wind turbines

In the present work, a tool is proposed for the evaluation of micro-wind turbine performances in the Australian cities. The power curves provided by the manufacturers were combined with the two-parameter Weibull function representative of the local wind conditions. Calculations demonstrate that when taking into account the only wind speed magnitude, the horizontal axis micro-wind turbines have greater capacity factors than the vertical axis micro-wind turbines for similar wind conditions. Turbine capacities were then mapped over a wide range of wind conditions assuming the wind distribution characterised by Weibull parameters. By considering the market price of electricity, the income of a power source can be evaluated. The critical wind-condition boundary needed to provide a beneficial active micro-wind turbine was determined for one horizontal axis micro-wind turbine using a hypothetical income map.     

The wake structure in a 2D grid installation of the horizontal axis micro wind turbines

In order to develop applications for micro-wind turbines, an experimental analysis of the flow field around integrated micro-wind turbines was performed. The wake flows of a single turbine and 5×5 array unit were measured by using hot-wire and ultrasonic anemometers and particle image velocimetry (PIV). The present array of turbines follows a fundamental lattice layout; however, it has the flexibility to optimize its layout according to the environmental conditions. hot-wire and ultrasonic anemometers and PIV measurements were used for stand-alone turbines and their integrated systems. Comparisons between the mean velocity field and turbulent intensity were described for stand-alone full-scale and 1/10-scale wind turbine models. Thereafter, a typical array of the 1/10-scale model was assumed and its wake flow was investigated in a wind tunnel. The velocity profile and turbulence behind the array were measured and studied at different streamwise locations. The scale effect and model similarities were discussed. The experimental results show that a zone exists with constant and linear wake deficit ratios in the downstream regions.     

Thermal behavior of curved roof buildings exposed to solar radiation and wind flow for various orientations

In this study air flow, solar radiation and heat transfer from a two dimensional curved roof with north-south and east-west faced are determined and results are compared with flat roof for the same size and orientation. Comparison are performed for their corresponding roof surface temperature, and heat flow for several roof rim angles and also for various wind flow velocities, as well as for different wind directions. Turbulence is modeled by RNG k–ε method and solar radiation distribution over the roof is determined based on an appropriate model applicable to hot arid regions of Iran. Solar radiation is calculated based on the summation of beam and diffuse radiation and ground reflected radiation. For certain inside roof temperature, over all heat transfer to the building is determined with day time for various wind flows and arc shapes and results are compared with flat roof. It was found that various wind flow condition over the vaulted roof makes substantial difference on the convection heat transfer coefficient and finally on the rate of heat transfer to the building with respect to flat roof. Based on heat transfer simulation, roof temperature, heat transfer convection coefficient and heat flow though the vault for different roof arrangement and flat roofs have been determined and advantages of specific vault orientation and wind direction are specified.     

Wind tunnel simulation of terrain effects on wind farm siting

The turbulent flow above a generic terrain model has been investigated in a wind tunnel. The rather large terrain model was split up into a set of smaller modules, which could be combined in various ways, and consisted of features inspired by the mountainous terrain along the Norwegian coastline. Several vertical velocity profiles were measured above each terrain module, using two‐component laser Doppler anemometry. The mean velocity, turbulence intensity and power spectrum in the simulated incoming atmospheric boundary layer were similar to wind in coastal areas. The flow above hills with sharp and rounded crests and various inclination angles, followed by a plateau, was compared. Results for a straight slope with a sharp crest was compared with a rounded hill with a similar slope, revealing large discrepancies. Flow above rounded hills was seen to be very advantageous for wind turbines, with increased mean velocities and reduced turbulence intensity, compared with the incoming flow. Separated flow occurred in the case with a sharp crest, resulting in highly increased levels of turbulence. Results from cases with two mountains of different heights are also reported. When the flow separated downstream of the first mountain, the flow above the second was affected to a varying extent, depending on the height of the upstream mountain compared to the one downstream. Copyright © 2008 John Wiley & Sons, Ltd.     

Near and far field flow disturbances induced by model hydrokinetic turbine: ADV and ADP comparison

Wake flows downstream of hydrokinetic turbines are characterized by hub and tip vortices, a velocity deficit and an increase in turbulence intensity. Velocity and turbulence recovery in the wakes of individual turbines constrains the density of turbines in an array and limits the amount of energy that can be produced by a turbine farm. However, few hydrokinetic turbine flow recovery studies have been conducted, especially on the far-field flow characteristics. Nor have studies evaluated the accuracy of acoustic Doppler profiler measurements in the wakes of turbines. The present study examines vertical profiles of mean velocity and turbulence, as well as longitudinal profiles of velocity deficit and turbulence levels measured at the symmetry plane of a model three-blade axial flow turbine in a large open channel flow. Mean velocity and turbulence statistics are measured using an acoustic Doppler velocimeter (ADV) and a pulse coherent acoustic Doppler profiler (ADP). ADV and corrected-ADP derived values of mean velocity, turbulence intensity and root-mean-square velocity constitute a well-documented data set that can be used to validate numerical models simulating the effects of hydrokinetic turbine arrays. We found that 80% of the flow recovery occurred about ten diameters downstream from the rotor plane, which suggests that practical values for longitudinal spacing of turbines should be between ten and fifteen diameters. Significant errors observed in mean velocity and turbulence statistics derived from ADP measurements in the near wake region raise concerns on the use of these instruments for such measurements in lab and field studies. Although the cause of some of the errors requires further investigation, we show that errors in turbulence intensity can be successfully corrected with supplemental ADV measurements.     

Tailoring incoming shear and turbulence profiles for lab‐scale wind turbines

An active grid is used to generate a variety of turbulent shear profiles in a wind tunnel. The vertical bars are set to flap through varying angles across the test section producing a variation in the perceived solidity, resulting in a mean shear. The horizontal bars are used in a fully random operational mode to set the background turbulence level. It is demonstrated that mean velocity profiles with approximately the same shear can be produced with different turbulence intensities and local turbulent Reynolds numbers based on the Taylor microscale, λ. Conversely, it is also demonstrated that flows can be produced with similar turbulence intensity profiles but different mean shear. It is confirmed that the length scales and dynamics, the latter being assessed through the velocity spectra and probability density functions, do not vary significantly across the investigation domain. Such flows are of particular relevance for studies investigating the effect of in‐flow conditions on obstacles where these studies wish to decouple the effects of turbulence intensity and mean shear, a feat previously unattainable in experimental facilities. Given that the power output of wind turbines is known to be a function of both mean shear and turbulence intensity, the experimental methodology presented herein is invaluable to the wind turbine model testing community who, at present, cannot exert such control authority over the in‐flow conditions.     

长方体建筑长宽比对屋顶风力机微观选址的影响

采用CFD方法,对长宽比分别为0.25、0.30、0.40、0.50、0.60、0.70、0.80、0.90的8种长方体单体高层建筑物顶面的风流场特征进行模拟计算,确定长宽比对长方体建筑物顶面风力机微观选址的影响.结果表明:长宽比越大,建筑顶面低风速区域越大,且风速随高度增长得越慢,而迎风面拐点风速恢复最快;同时由于建...     

Pole-mounted horizontal axis micro-wind turbines: UK field trial findings and market size assessment

This paper discusses the key findings of the pole-mounted turbine (2.5–6 kWp) component of the UK micro-wind trial. The real world performance of horizontal axis turbines is compared with yield estimates based on site wind speed prediction. The distribution of UK agricultural farms is overlaid with wind resource mapping to estimate the number of potential agricultural farm sites for micro-wind. The yield performance of turbines during the monitoring period was observed to be very close to that predicted by NOABL–MCS wind speed estimates. Based on an installation criterion of a maximum 12 year payback time, with a 6% discount rate and micro-generation feed in tariffs available, there are ∼87,000 farm sites for micro-wind in the UK. If 10% of these farms were to install micro-wind turbines (to a capacity of 48 kWp per farm) this would correspond to a capacity of 418 MWp, with an annual generation yield of 1025 GWh, comparable to that of a large, on shore wind farm in the UK. It should be noted that the feed in tariff considered in this paper is that available in the UK in 2011, which, at 26.7 p/kWh (∼30 Euro cents/kWh) represents a significant subsidy.     

Performance and safety of rooftop wind turbines: Use of CFD to gain insight into inflow conditions

The installation of small and medium-size wind turbines on the rooftops of high buildings has been often suggested by architects and project developers as a potential solution for achieving sustainable energy in building design. In such locations, however, because of the presence of buildings and other adjacent obstructions, wind is normally turbulent, unstable and weak, in terms of direction and speed. The use of wind turbines in the built environment poses challenges to overcome, including energy yield reduction due to lower mean wind speeds in urban areas, and environmental impacts because of their close vicinity to people and property.There is a need to understand the inflow wind conditions for a small wind turbine in the built-environment. A resource assessment of the potential wind turbine site in the built environment can determine the wind characteristics including zones of wind acceleration, recirculation, blocking and channelling. This knowledge is crucial for input into the design process of a small wind turbine to accurately predict blade fatigue loads and ensure that it operates safely, and performs optimally in the environment.Computational Fluid Dynamics (CFD) is a useful method to model wind flows in order to perform a resource assessment for the application of small wind turbines in a manner that requires less time and investment than a measurement campaign. This paper presents the results of research using a CFD code to model wind flows over the roof of a building and assesses the possibility of combining a CFD package with wind atlas software to form a wind energy resource assessment tool for the application of small wind turbines on the roof of a building. Experimentation with the model shows that the results are particularly sensitive to building height and shape, roof shape, wind direction, and turbine installation height and location. The results will be used to help develop a recommended practice of wind resource assessment in the built environment, in an international collaborative effort via the International Energy Agency Task 27.     

Application of a LES technique to characterize the wake deflection of a wind turbine in yaw

When a wind turbine works in yaw, the wake intensity and the power production of the turbine become slightly smaller and a deflection of the wake is induced. Therefore, a good understanding of this effect would allow an active control of the yaw angle of upstream turbines to steer the wake away from downstream machines, reducing its effect on them. In wind farms where interaction between turbines is significant, it is of interest to maximize the power output from the wind farm as a whole and to reduce fatigue loads on downstream turbines due to the increase of turbulence intensity in wakes. A large eddy simulation model with particular wind boundary conditions has been used recently to simulate and characterize the turbulence generated by the presence of a wind turbine and its evolution downstream the machine. The simplified turbine is placed within an environment in which relevant flow properties like wind speed profile, turbulence intensity and the anisotropy of turbulence are found to be similar to the ones of the neutral atmosphere. In this work, the model is used to characterize the wake deflection for a range of yaw angles and thrust coefficients of the turbine. The results are compared with experimental data obtained by other authors with a particle image velocimetry technique from wind tunnel experiments. Also, a comparison with simple analytical correlations is carried out. Copyright © 2009 John Wiley & Sons, Ltd.     

Investigation on the feasibility and enhancement methods of wind power utilization in high-rise buildings of Hong Kong

This paper reports the investigation results of wind power application in buildings. It is found that the concentration effect of buildings and the heights of buildings could enhance wind power utilization by increasing the wind speed by 1.5–2× and wind power density by 3–8× under the given simulation conditions. The wind aerodynamics and wind flows over the buildings are investigated based on local meteorological data and local high-rise building characteristics. This paper concludes that wind power utilization in high-rise buildings in Hong Kong is feasible theoretically, and some effective enhancement methods are proposed based on the simulation results, such as making full use of the heights of buildings and the concentration effect of buildings, and choosing optimal shape of building roof. However, to receive the highest potential wind energy resource and avoid turbulent areas, the tool of Computational Fluid Dynamics (CFD) has to be used to model the annual wind flows over buildings to help analyze, locate, and design wind turbines in and around buildings.     

Low‐cost mounting arrangements for building‐integrated wind turbines

Micro‐generation is being widely promoted as a way for householders in the UK and elsewhere to take part in ‘the Green Revolution’. Building‐integrated wind turbines (BIWTs) provide a way to do this, enabling people to reduce their contribution to the problems of both climate change and decreasing fossil fuel availability. Although energy yields from BIWTs for many householders have been shown to be low, there are still situations where such turbines can make a useful contribution to electricity generation, e.g. in windier areas and for isolated detached buildings. The standards for the installation of BIWTs are still being developed including those for the safe mounting of turbines on domestic buildings. This paper investigates the current trend for mounting small wind turbines on the walls of domestic premises and compares this with an approach which uses roof timbers. It identifies the main characteristics of building construction which affect the integrity of such installations. European and British standards have been used to calculate wind and gravitational loads. Finite element models are used to derive working stresses and, hence, some basic principles of good design. The likely costs of wall and roof mounting are then compared. Installation and health and safety issues are also examined briefly. Copyright © 2010 John Wiley & Sons, Ltd.     

Modelling and measurements of power losses and turbulence intensity in wind turbine wakes at Middelgrunden offshore wind farm

Understanding of power losses and turbulence increase due to wind turbine wake interactions in large offshore wind farms is crucial to optimizing wind farm design. Power losses and turbulence increase due to wakes are quantified based on observations from Middelgrunden and state‐of‐the‐art models. Observed power losses due solely to wakes are approximately 10% on average. These are relatively high for a single line of wind turbines due in part to the close spacing of the wind farm. The wind farm model Wind Analysis and Application Program (WAsP) is shown to capture wake losses despite operating beyond its specifications for turbine spacing. The paper describes two methods of estimating turbulence intensity: one based on the mean and standard deviation (SD) of wind speed from the nacelle anemometer, the other from mean power output and its SD. Observations from the nacelle anemometer indicate turbulence intensity which is around 9% higher in absolute terms than those derived from the power measurements. For comparison, turbulence intensity is also derived from wind speed and SD from a meteorological mast at the same site prior to wind farm construction. Despite differences in the measurement height and period, overall agreement is better between the turbulence intensity derived from power measurements and the meteorological mast than with those derived from data from the nacelle anemometers. The turbulence in wind farm model indicates turbulence increase of the order 20% in absolute terms for flow directly along the row which is in good agreement with the observations. Copyright © 2007 John Wiley & Sons, Ltd.     

Green and cool roofs’ urban heat island mitigation potential in European climates for office buildings under free floating conditions

Heat island which is the most documented phenomenon of climatic change is related to the increase of urban temperatures compared to the suburban. Among the various urban heat island mitigation techniques, green and cool roofs are the most promising since they simultaneously contribute to buildings’ energy efficiency. The aim of the present paper is to study the mitigation potential of green and cool roofs by performing a comparative analysis under diverse boundary conditions defining their climatic, optical, thermal and hydrological conditions. The impact of cool roof’s thermal mass, insulation level and solar reflectance as well as the effect of green roofs’ irrigation rate and vegetation are examined. The parametric study is based on detailed simulation techniques coupled with a comparative presentation of the released integrated sensible heat for both technologies versus a conventional roof under various climatic conditions.