L. Folkerts’s scientific contributions

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Publications (16)


TABLE 1 . Details of the sodar experiments used in the model comparison. Here U is the wind speed, Dir shows the mean and range of directions, L is the Monin-Obukhov length, D is the distance in rotor diameters, I 0 is the ambient turbulence, and C T is the turbine thrust coefficient.
FIG. 2. Free-stream wind speed profiles for the three free-stream wind speed cases and six wake experiments.  
FIG. 3. Modeled wake velocity profiles for the three different free-stream cases.  
FIG. 4. Observed frequency distribution of wind direction (upper frame, solid bars) and time evolution of wind direction (lower graph, solid line) over each experiment. The thick solid line in the upper graph shows the mean measured velocity deficit, and the lines with symbols show the velocity deficit for each model simulation corrected for wake meandering.
FIG. 6. Measured and modeled wind speed profile. Experiment number 3, case 2, free-stream winds.  

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Comparison of Wake Model Simulations with Offshore Wind Turbine Wake Profiles Measured by Sodar
  • Article
  • Full-text available

July 2006

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1,278 Reads

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319 Citations

Journal of Atmospheric and Oceanic Technology

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L. Folkerts

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This paper gives an evaluation of most of the commonly used models for predicting wind speed decrease (wake) downstream of a wind turbine. The evaluation is based on six experiments where free-stream and wake wind speed profiles were measured using a ship-mounted sodar at a small offshore wind farm. The experiments were conducted at varying distances between 1.7 and 7.4 rotor diameters downstream of the wind turbine. Evaluation of the models compares the predicted and observed velocity deficits at hub height. A new method of evaluation based on determining the cumulative momentum deficit over the profiles is described. Despite the apparent simplicity of the experiments, the models give a wide range of predictions. Overall, it is not possible to establish any of the models as having individually superior performance with respect to the measurements.

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ENDOW (Efficient Development of Offshore Wind Farms): Modelling wake and boundary layer interactions

July 2004

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96 Reads

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71 Citations

While experience gained through the offshore wind energy projects currently operating is valuable, a major uncertainty in estimating power production lies in the prediction of the dynamic links between the atmosphere and wind turbines in offshore regimes. The objective of the ENDOW project was to evaluate, enhance and interface wake and boundary layer models for utilization offshore. The project resulted in a significant advance in the state of the art in both wake and marine boundary layer models, leading to improved prediction of wind speed and turbulence profiles within large offshore wind farms. Use of new databases from existing offshore wind farms and detailed wake profiles collected using sodar provided a unique opportunity to undertake the first comprehensive evaluation of wake models in the offshore environment. The results of wake model performance in different wind speed, stability and roughness conditions relative to observations provided criteria for their improvement. Mesoscale model simulations were used to evaluate the impact of thermal flows, roughness and topography on offshore wind speeds. The model hierarchy developed under ENDOW forms the basis of design tools for use by wind energy developers and turbine manufacturers to optimize power output from offshore wind farms through minimized wake effects and optimal grid connections. The design tools are being built onto existing regional-scale models and wind farm design software which was developed with EU funding and is in use currently by wind energy developers. Copyright © 2004 John Wiley & Sons, Ltd.



TABLE 1. Characteristics of the Aeroenvironment 4000 minisodar operated during the Vindeby experiment. 
FIG. 8. Relative velocity deficit profiles for each of the 13 experiments grouped by distance of the measurements to the turbine (expressed as number of rotor diameters). Numbers shown refer to the experiment designations given in Table 3.  
FIG. 10. Relative velocity deficit and transport time calculated for two different roughnesses [representing onshore (0.05 m) and offshore (0.0002 m) and from the sodar data]. Numbers shown refer to the experiment designations given in Table 3.  
Offshore wind turbine wakes measured by SODAR

April 2003

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382 Reads

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110 Citations

Journal of Atmospheric and Oceanic Technology

A ship-mounted sodar was used to measure wind turbine wakes in an offshore wind farm in Denmark. The wake magnitude and vertical extent were determined by measuring the wind speed profile behind an operating turbine, then shutting down the turbine and measuring the freestream wind profile. These measurements were compared with meteorological measurements on two offshore and one coastal mast at the same site. The main purposes of the experiment were to evaluate the utility of sodar for determining wind speed profiles offshore and to provide the first offshore wake measurements with varying distance from a wind turbine. Over the course of a week, 36 experiments were conducted in total. After quality control of the data (mainly to exclude rain periods), 13 turbine-on, turbine-off pairs were analyzed to provide the velocity deficit at hub height as a function of the distance from the turbine. The results are presented in the context of wake measurements at other coastal locations. The velocity deficit is predicted with an empirical model derived from onshore measurements based on transport time dependent on surface roughness. The measurements are closer to those predicted using an onshore rather than an offshore roughness despite the relatively low turbulence experienced during the experiments.






Sodar Wind Velocity Measurements of Offshore Turbine Wakes

September 2001

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31 Reads

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11 Citations

Wind Engineering

In A pril 2001 the first of fshore wak e m easurem ents with a SODA R (sonic detection and ranging) were conducted at V indeby. Wak e effects were m easured at distances from the turbine varying from 1.4 to 7.1 rotor diameters. The corresponding calculated relative velocity deficits cover a range from 12% up to 56%. The recorded wak e profiles provide excellent reference cases for the developm ent and evaluation of of fshore wak e models.


Citations (13)


... The calibration of wake model parameters is notoriously difficult and susceptible to error [42,43]. We use a standard choice of k for the Park model of 0.05 for offshore conditions and 0.075 for onshore conditions [44]. However, analysis of wind farm boundary layers suggests ...

Reference:

FLOWERS AEP: An Analytical Model for Wind Farm Layout Optimization
Comparison of wake model simulations with offshore wind turbine wake profiles measured by sodar
  • Citing Article
  • January 2005

Journal of Atmospheric and Oceanic Technology

... As part of the project, the Coastal Discontinuity Model (CDM) was also developed; it uses air and sea temperature, together with the geostrophic wind speed to calculate the stability parameter z/L to correct the wind speed profile and determine the internal boundary layer height giving a prediction of the wind speed at each grid location (Watson et al. 2000). (iv) Efficient Development of Offshore Wind Farms (ENDOW) (Barthelmie et al. 2004) included a component of mesoscale modelling of wind resources in the Baltic Sea and a comparison with predictions from WAsP based on the same dataset as the model (Bergstroem 2001). ...

ENDOW: Efficient Development of Offshore Windfarms: modelling wake and boundary-layer interactions
  • Citing Conference Paper
  • April 2003

... Higher wind speeds offshore result in higher capacity factors (the ratio of actual to maximum power possible production) than for neighbouring onshore areas unless wind speeds at the onshore locations are significantly enhanced by orography. For example, comparison of data from Danish sites suggested capacity factors were twice as high (54%) at an offshore site 10 km from shore than an inland site (27%) (Pryor and Barthelmie 2002a). ...

ENDOW: Efficient Development of Offshore Windfarms.
  • Citing Conference Paper
  • April 2002

... A key issue in designing wind farms is the placement of wind turbines. Typically, the spacing between the turbines is kept at between 4 and 8 rotor diameters [1] to avoid major losses due to wake-effects. However, due to increasing penetration of wind energy, wind farms are required to operate numerous wind turbines placed close to each other which may result in significant losses due to wake. ...

Efficient Development of Offshore Windfarms (ENDOW)
  • Citing Technical Report
  • January 2003

... While previous offshore wind farm wake benchmark comparisons have been carried out during the past decade in the ENDOW project [19] and UpWind project [18], the new and refined models available for the industry combined with the better understanding and refined data of the wind farm SCADA make it relevant now to initiate a new benchmark based on the Horns Rev wind farm within the EERA-DTOC project. ...

ENDOW: Efficient Development of Offshore Windfarms: Modelling Wake and Boundary-Layer Interactions

... Six models of varying complexity were evaluated against experimental data from the Vindeby and Bockstigen wind farms. The simplest approaches were the analytical model developed in the Uppsala University (MIUU) and the semi-analytical model of RISØ [2]. The former one was based on the Taylor hypothesis using the transport time for the wake development [3]. ...

Efficient Development of Offshore Windfarms: a new project for investigating wake and boundary-layer interactions
  • Citing Conference Paper
  • January 2001

... Shallower coastal waters allowed an earlier expansion of offshore wind energy in Europe, where the first installations were in 1993 and installed capacity now exceeds 30 GW, including more than 100 MW of floating offshore wind turbine installed capacity [3,4]. The rated capacity of offshore wind turbines increased over 20 times from the 0.5 MW turbines installed in 1993 at Vindeby [5] to 13 MW (and larger) turbines planned for deployment off the US East Coast. Offshore wind turbine hub-heights increased from 37 m in 1991 to 100 m in 2021 and rotor diameters from 38 m to over 156 m [6]. ...

Offshore wind turbine wakes measured by SODAR

Journal of Atmospheric and Oceanic Technology

... Although real-life measurements show a distorted wake velocity profile due to the atmospheric boundary layer (ABL), 11,43,44 the second assumption in this study will be to negate ABL influences, wake meandering and ground surface interactions thereby enforcing axial symmetric wake shapes. † This assumption further aids to isolate the effects of changing array layout in power variation. ...

Sodar Wind Velocity Measurements of Offshore Turbine Wakes
  • Citing Article
  • September 2001

Wind Engineering

... The configurations of the four-unit arrays of Savonius rotors are selected based on wake models reported in the literature [49][50][51][52][53] as well as our previous CFD-based wake studies performed on single Savonius rotors. 37 As discussed in the introduction, the four-unit Savonius rotor array, as shown in Fig. 2, is chosen as a step forward in the research community's efforts on the computational fluid dynamics analysis of rotor arrays (see the comparison reported in Table III for 2-unit rotor arrays). ...

Comparison of Wake Model Simulations with Offshore Wind Turbine Wake Profiles Measured by Sodar

Journal of Atmospheric and Oceanic Technology