Investigation of boundary‐layer wind predictions during nocturnal low‐level jet events using the Weather Research and Forecasting model

The accuracy of boundary‐layer wind profiles occurring during nocturnal low‐level jet (LLJ) events, and their sensitivities to variations of user‐specifiable model configuration parameters within the Weather Research and Forecasting model, was investigated. Simulations were compared against data from a wind‐profiling lidar, deployed to the Northern Great Plains during the U.S. Department of Energy‐supported Weather Forecast Improvement Project. Two periods during the autumn of 2011 featuring LLJs of similar magnitudes and durations occurring during several consecutive nights were selected for analysis. Simulated wind speed and direction at 80 and 180 m above the surface, the former a typical wind turbine hub height, bulk vertical gradients between 40 and 120 m, a typical rotor span, and the maximum wind speeds occurring at 80 and 180 m, and their times of occurrence, were compared with the observations. Sensitivities of these parameters to the horizontal and vertical grid spacing, planetary boundary layer and land surface model physics options, and atmospheric forcing dataset, were assessed using ensembles encompassing changes of each of these configuration parameters. Each simulation captured the diurnal cycle of wind speed and stratification, producing LLJs during each overnight period; however, large discrepancies in relation to the observations were frequently observed, with each ensemble producing a wide range of distributions, reflecting highly variable representations of stratification during the weakly stable overnight conditions. Root mean square error and bias values computed over the LLJ cycle (late evening through the following morning) revealed that, while some configurations performed better or worse in different aspects and at different times, none exhibited definitively superior performance. The considerable root mean square error and bias values, even among the ‘best’ performing simulations, underscore the need for improved simulation capabilities for the prediction of near‐surface winds during LLJ conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

Properties of the offshore low level jet and rotor layer wind shear as measured by scanning Doppler Lidar

The atmospheric flow phenomenon known as the Low Level Jet (LLJ) is an important source of wind power production in the Great Plains. However, due to the lack of measurements with the precision and vertical resolution needed, particularly at rotor heights, it is not well‐characterized or understood in offshore regions being considered for wind‐farm development. The present paper describes the properties of LLJs and wind shear through the rotor layer of a hypothetical wind turbine, as measured from a ship‐borne Doppler lidar in the Gulf of Maine in July–August 2004. LLJs, frequently observed below 600 m, were mostly during nighttime and transitional periods, but they were also were seen during some daytime hours. The presence of a LLJ significantly modified wind profiles producing vertical wind speed shear. When the wind shear was strong, the estimates of wind power based upon wind speeds measured at hub‐height could have significant errors. Additionally, the inference of hub‐height winds from near‐surface measurements may introduce further error in the wind power estimate. The lidar dataset was used to investigate the uncertainty of the simplified power‐law relation that is often employed in engineering approaches for the extrapolation of surface winds to higher elevations. The results show diurnal and spatial variations of the shear exponent empirically found from surface and hub‐height measurements. Finally, the discrepancies between wind power estimates using lidar‐measured hub‐height winds and rotor equivalent winds are discussed. Copyright © 2016 John Wiley & Sons, Ltd.     

Large‐eddy simulations of wind‐farm wake characteristics associated with a low‐level jet

In this study, we performed a suite of flow simulations for a 12‐wind‐turbine array with varying inflow conditions and lateral spacings, and compared the impacts of the flow on velocity deficit and wake recovery. We imposed both laminar inflow and turbulent inflows, which contain turbulence for the Ekman layer and a low‐level jet (LLJ) in the stable boundary layer. To solve the flow through the wind turbines and their wakes, we used a large‐eddy simulation technique with an actuator‐line method. We compared the time series for the velocity deficit at the first and rear columns to observe the temporal change in velocity deficit for the entire wind farm. The velocity deficit at the first column for LLJ inflow was similar to that for laminar inflow. However, the magnitude of velocity deficit at the rear columns for the case with LLJ inflow was 11.9% greater because of strong wake recovery, which was enhanced by the vertical flux of kinetic energy associated with the LLJ. To observe the spatial transition and characteristics of wake recovery, we performed statistical analyses of the velocity at different locations for both the laminar and LLJ inflows. These studies indicated that strong wake recovery was present, and a kurtosis analysis showed that the probability density function for the streamwise velocity followed a Gaussian distribution. In a quadrant analysis of the Reynolds stress, we found that the ejection and sweep motions for the LLJ inflow case were greater than those for the laminar inflow case.     

Impact of low‐level wind maxima below hub height on wind turbine sound propagation

An analysis of the effect of low‐level wind maxima (LLWM) below hub height on sound propagating from wind turbines has been performed at a site in northern Sweden. The stably stratified boundary layer, which is typical for cold climates, commonly features LLWM. The simplified concept for the effects of refraction, based on the logarithmic wind profile or other approaches where the wind speed is continuously increasing with height, is often not applicable there. Long‐term meteorological measurements in the vicinity of a wind farm were therefore used to identify LLWM. Sound measurements were conducted simultaneously to the meteorological measurements. LLWM below hub height decrease the sound level close to the surface downwind of the wind farm. This effect increases with increasing strength of the LLWM. The occurrence of LLWM as well as strength and height of the LLWM are dependent on the wind direction.     

Evaluation of the Weather Research and Forecasting model on forecasting low‐level jets: implications for wind energy

Nocturnal low‐level jet (LLJ) events are commonly observed over the Great Plains region of the USA, thus making this region more favorable for wind energy production. At the same time, the presence of LLJs can significantly modify vertical shear and nocturnal turbulence in the vicinities of wind turbine hub height, and therefore has detrimental effects on turbine rotors. Accurate numerical modeling and forecasting of LLJs are thus needed for precise assessment of wind resources, reliable prediction of power generation and robust design of wind turbines. However, mesoscale numerical weather prediction models face a challenge in precisely forecasting the development, magnitude and location of LLJs. This is due to the fact that LLJs are common in nocturnal stable boundary layers, and there is a general consensus in the literature that our contemporary understanding and modeling capability of this boundary‐layer regime is quite poor. In this paper, we investigate the potential of the Weather Research and Forecasting (WRF) model in forecasting LLJ events over West Texas and southern Kansas. Detailed observational data from both cases were used to assess the performance of the WRF model with different model configurations. Our results indicate that the WRF model can capture some of the essential characteristics of observed LLJs, and thus offers the prospect of improving the accuracy of wind resource estimates and short‐term wind energy forecasts. However, the core of the LLJ tended to be higher as well as slower than what was observed, leaving room for improvement in model performance. Copyright © 2008 John Wiley & Sons, Ltd.     

Error features and their possible causes in simulated low‐level winds by WRF at a wind farm

The WRF model is applied to simulate the low‐level wind field for a wind farm located in a typical arid region in northwest China for February 2008. The selected region has complex terrain with sparse vegetation. Overall, the WRF model reproduced the variation features of wind speeds and wind directions. However, the model overestimated the observed low‐level wind speeds, and there were large discrepancies for the low wind velocity (i.e. the errors of simulated winds increase with height and will be larger when the observed wind speeds are lower than 2.5 m/s). The features of the simulated errors and the possible causes in the model were analysed. The simulated low‐level wind in the afternoon is more accurate than that in early morning, which is usually unstable. Copyright © 2013 John Wiley & Sons, Ltd.     

Uncertainties of the 50‐year wind from short time series using generalized extreme value distribution and generalized Pareto distribution

This study examines the various sources to the uncertainties in the application of two widely used extreme value distribution functions, the generalized extreme value distribution (GEVD) and the generalized Pareto distribution (GPD). The study is done through the analysis of measurements from several Danish sites, where the extreme winds are caused by the Atlantic lows. The simple extreme wind mechanism here helps us to focus on the issues mostly related to the use of limited wind measurements. Warnings are flagged and possible solutions are discussed. Thus, this paper can be used as a guideline for applying GEVD and GPD to wind time series of limited length. The data analysis shows that, with reasonable choice of relevant parameters, GEVD and GPD give consistent estimates of the return winds. For GEVD, the base period should be chosen in accordance with the occurrence of the extreme wind events of the same mechanism. For GPD, the choices of the threshold, the definition of independent samples and the shape factor are interrelated. It is demonstrated that the lack of climatological representativity is a major source of uncertainty to the use of both GEVD and GPD; the information of climatological variability is suggested to be extracted from global or mesoscale models. © 2013 The Authors. Wind Energy published by John Wiley & Sons, Ltd.     

Modification of the Baltic Sea wind field by land‐sea interaction

The wind and turbulence fields over a small, high‐latitude sea are investigated. These fields are highly influenced by the proximity to the coast, which is never more than 200 km away. Simulations with the WRF model over the Baltic Sea are compared with a simplified, stationary wind model driven by the synoptic forcing. The difference between the models is therefore representative of the mesoscale influence. The results show that the largest wind‐field modifications compared with a neutral atmosphere occur during spring and summer, with a mean monthly increase of up to approximately 1 ms^?1 at typical hub heights and upper rotor area (120‐170 m height) in the WRF model. The main reason for this is large‐scale low‐level jets caused by the land‐sea temperature differences, likely increasing in strength due to inertial oscillations. These kind of events can be persistent for approximately 12 hours and cover almost the entire basin, causing wind speed and wind shear to increase considerably. The strongest effect is around 2000 to 2300 local time. Sea breezes and coastal low‐level jets are of less importance, but while sea breezes are mostly detected near the coastline, other types of coastal jets can extend large distances off the coast. During autumn and winter, there are fewer low‐level jet occurrences, but the wind profile cannot be explained by the classical theory of the one‐dimensional model. This indicates that the coastal environment is complex and may be affected by advection from land surfaces to a large degree even when unstable conditions dominate.     

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.     

Comparison of the atmospheric stability and wind profiles at two wind farm sites over a long marine fetch in the North Sea

A comparison of the atmospheric stability and wind profiles using data from meteorological masts located near two wind farm sites in the North Sea, Egmond aan Zee (up to 116 m) in the Dutch North Sea and Horns Rev (HR; up to 45 m) in the Danish North Sea, is presented. Only the measurements that represent long marine fetch are considered. It was observed that within a long marine fetch, the conditions in the North Sea are dominated by unstable [41% at Egmond aan Zee Offshore Wind Farm (OWEZ) and 33% at HR] and near‐neutral conditions (49% at OWEZ and 47% at HR), and stable conditions (10% at OWEZ and 20% at HR) occur for a limited period. The logarithmic wind profiles with the surface‐layer stability correction terms and Charnock's roughness model agree with the measurements at both sites in all unstable and near‐neutral conditions. An extended wind profile valid for the entire boundary layer is compared with the measurements. For the tall mast at Egmond aan Zee, it was found that for stable conditions, the scaling of the wind profiles with respect to boundary‐layer height is necessary, and the addition of another length scale parameter is preferred. At the lower mast at HR, the effect was not noticeable. Copyright © 2011 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.     

Analysis of impact of grid‐connected wind power on small signal stability

The power system small signal stability analysis considering wind generation intermittence is studied comprehensively in this paper. The modelling of doubly‐fed induction generator (DFIG) involving the converters with application of stator flux‐oriented vector control strategy is addressed briefly. In order to reveal how the intermittent nature of wind power affects the operating behaviour of an existing power system, a probabilistic small signal stability analysis method based on Monte Carlo simulation technique is proposed to explore and exploit the impact of intermittent grid‐connected wind power on small signal stability. The IEEE New England test system is applied as benchmark to verify the proposed model and approach. Total 3 scenarios are elaborately designed to figure out the potential relationship between the small signal stability indices and the wind generation intermittence. Finally, some preliminary conclusions and comments were drawn based on the numerical simulation results. Copyright © 2010 John Wiley & Sons, Ltd.     

Avoided demand charges and behind‐the‐meter wind: insights from an application at the University of Minnesota

One common ownership structure for community‐scale wind development in the USA is a behind‐the‐meter installation. In addition to allowing the displacement of retail energy, such installations may also affect peak demand, which is frequently an important component of electricity tariffs (via ‘capacity’ or ‘demand’ charges). This paper uses Monte Carlo simulation techniques on original wind and load data for the University of Minnesota at Morris in order to estimate the savings associated with lower peak demand, as a result of the installation of a 1.65‐MW turbine in 2005. Results represent the first (to our knowledge) quantitative effort to estimate this aspect of the economics of wind power projects, and they suggest these previously ignored savings comprise nearly 10% of this project's gross projected revenue stream, even though the local utility's demand charge in this case is only 63% of the industry average. Copyright © 2008 John Wiley & Sons, Ltd.     

Assessment of wind energy potentiality at Kudat and Labuan, Malaysia using Weibull distribution function

The wind resource is a crucial step in planning a wind energy project and detailed knowledge of the wind characteristic at a site is needed to estimate the performance of a wind energy project. In this paper, with the help of 2-parameter Weibull distribution, the assessment of wind energy potentiality at Kudat and Labuan in 2006-2008 was carried out. “WRPLOT” software has been used to show the wind direction and resultant of the wind speed direction. The monthly and yearly highest mean wind speeds were 4.76 m/s at Kudat and 3.39 m/s at Labuan respectively. The annual highest values of the Weibull shape parameter (k) and scale parameter (c) were 1.86 and 3.81 m/s respectively. The maximum wind power density was found to be 67.40 W/m² at Kudat for the year 2008. The maximum wind energy density was found to be 590.40 kWh/m²/year at Kudat in 2008. The highest most probable wind speed and wind speed carrying maximum energy were estimated 2.44 m/s at Labuan in 2007 and 6.02 m/s at Kudat in 2007. The maximum deviation, at wind speed more than 2 m/s, between observed and Weibull frequency distribution was about 5%. The most probable wind directions (blowing from) were 190° and 269° at Kudat and Labuan through the study years. From this study, it is concluded that these sites are unsuitable for the large-scale wind energy generation. However, small-scale wind energy can be generated at the turbine height of 100 m.     

Worldwide wind energy status and the characteristics of wind energy in Iran,case study: the province of Sistan and Baluchestan

In this paper, the recent trend of the worldwide wind energy utilisation is reviewed and the recent activities in using renewable energy sources in Iran are explained. As a case study, the wind characteristics of the province of Sistan and Baluchestan are statistically analysed. The wind characteristics such as the monthly mean wind speed and the wind power density of each station are presented. The monthly variation of the wind direction is presented and also the dominant wind direction is shown in a wind rose diagram. The values of turbulence intensity at different heights are calculated. The results show that the stations of Khash and Nosratabad are more suitable for limited off-grid utility applications. Lootak with the average annual wind power density of 388?W?m^?2 at the height of 40?m and constant wind direction is recommended for large-scale grid-connected wind turbines.     

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.     

A Tomo‐PIV study of the effects of freestream turbulence on stall delay of the blade of a horizontal‐axis wind turbine

Volumetric velocity fields were measured using tomographic particle image velocimetry on a model of the blade of a 5 kW horizontal‐axis wind turbine to study the effects of freestream turbulence levels (FTLs) at 0.4%, 4% and 13% on stall delay phenomenon at two different global tip speed ratios of 3 and 5 with Reynolds number (Re) ? 5000. Static pressures were measured, and results illustrated that FTL has stronger effect on the surface pressures of the static airfoil. Magnitudes of the absolute velocities within the separated flows above the static airfoil's suction surface increase significantly with higher FTL, while the changes of these velocities above the rotating blade's surface are less obvious. Radial flows from rotating blade's root to tip were also observed with strong spanwise velocity component located in the vicinities of the vortices. At the root and middle sections of the rotating blade, the flows with strong radial velocity component, w, become wider with higher FTL near to the rotating blade's leading edge when the angles of attack (AOAs) are large. At large AOAs, the strength and size of the vortices shed from the rotating blade's leading and trailing edges decrease significantly with higher FTL. However, at small AOAs, the size and coherence of the vortices near the rotating blade's trailing edge increase significantly with higher FTL. Surface streamlines of the rotating blade illustrated that at the rotating blade's root region and at large AOAs, the streamlines tend to lean toward the rotating blade's trailing edge at higher FTL. Copyright © 2014 John Wiley & Sons, Ltd.     

Flow characterization in the near‐wake region of a horizontal axis wind turbine

An experimental study is conducted to investigate the flow dynamics within the near‐wake region of a horizontal axis wind turbine using particle image velocimetry (PIV). Measurements were performed in the horizontal plane in a row of four radially distributed measurement windows (tiles), which are then patched together to obtain larger measurement field. The mean and turbulent components of the flow field were measured at various blade phase angles. The mean velocity and turbulence characteristics show high dependency on the blade phase angle in the near‐wake region closer to the blade tip and become phase independent further downstream at a distance of about one rotor diameter. In the near‐wake region, both the mean and turbulent characteristics show a systemic variation with the phase angle in the blade tip region, where the highest levels of turbulence are observed. The streamlines of the instantaneous velocity field at a given phase allowed to track a tip vortex which showed wandering trend. The tip vortices are mostly formed at r/R > 1, which indicates the wake expansion. Results also show the gradual movement of the vortex region in the axial direction, which can be attributed to the dynamics of the helical tip vortices which after being generated from the tip, rotate with respect to the blade and move in the axial direction because of the axial momentum of the flow. The axial velocity deficit was compared with other laboratory and field measurements. The comparison shows qualitative similarity. Copyright © 2015 John Wiley & Sons, Ltd.     

Impact of wall roughness and turbulence level on the performance of a horizontal axis wind turbine with the U‐RANS solver THETA

The presented work investigates the impact of different sheared velocity profiles in the atmospheric boundary layer on the characteristics of a wind turbine by modifying the wall roughness coefficients in the logarithmic velocity profile. Moreover, the rotor and wake characteristics in dependence of the turbulence boundary conditions are investigated. In variant I, the turbulence boundary conditions are defined in accordance to the logarithmic velocity profile with different wall roughness lengths. In variant II, the turbulent kinetic energy and turbulent viscosity remain independent of the velocity profile and represent the free‐stream turbulence level. With an increase of the shear in the velocity profile, the amplitudes in the 3/rev characteristics of rotor thrust and rotor torque, induction factors, and effective angles of attack are increased. In variant I, the overall levels of thrust coefficient are hardly affected by the velocity profiles resulting from different wall roughness length values. The power coefficient is reduced about 1%. Conversely, compared with variant II, a difference of 2% in the power coefficient has been detected. Moreover, the wake recovery process strongly depends on the turbulence boundary condition. Simulations are carried out on an industrial 900‐kW wind turbine with the incompressible U‐RANS solver THETA.     

Collapse analysis of wind turbine tower under the coupled effects of wind and near‐field earthquake

In this paper, the pattern of wind turbine tower collapse as a result of the coupled effects of wind and an intense, near‐field earthquake is investigated. The constitutive relation of the tower cylinder steel is simulated via a nonlinear kinematic hardening model, and the specific value of each parameter in the constitutive model is provided. A precise model of the tower structure coupled with the blade is created using a nonlinear, finite element method. This method is compared with the results from a static pushover test of a small cylindrical tower to validate the finite element modeling method in this research. Two earthquake wave sets are selected as inputs. One contains 20 near‐field velocity pulse‐like ground motion waves with various pulse periods; the other contains 20 ordinary far‐field ground motion waves. A wind turbine tower with a hub height of 60 m is selected as an example for analysis. The dynamic response of this tower as a result of the coupled effects of the two ground motion wave sets and a transient wind load is calculated using nonlinear time‐history analysis. The calculation results shows that the average horizontal displacement of the tower top as a result of the near‐field velocity pulse‐like ground motion is 33% larger than the case with far‐field ground motion. Finally, the seismic collapse vulnerability curve of this wind turbine tower is calculated. The seismic collapse capacity of the tower is evaluated, and the seismic collapse pattern of the tower is analyzed.