Description: Wind Resources Version: 8.0.0 Updated: 17.07.14

Wind Resources

Accumulation of results - the wind resource map

Before running the Wind Resource module at least one climatology must exist and all sectors defined in that climatology must exist in the wind database. The wind resource map is established by weighting the wind database against the climatology. If several climatology objects are available the wind resource map will be based on them all, by interpolation of the inverse distance to the climatology objects.

The Wind Resource module contains a tool for area classification. Finding high speed connected areas where the areas are grouped according the wind speed and size. The possible power production in the area is also estimated.

Properties
1. Wind resource map
Heights
The heights above ground level for which the results should be generated. Only heights below the "Height of reduced wind database" specified in the Wind Field module are valid. Multiple heights can be given as a semicolon separated list: e.g. 50;60;70.
Sector interpolation
The wind direction for a simulation in the wind database refers to the wind direction at the inlet. Due to terrain effects the wind direction changes in the inner of the model. In order to weight the wind database against a climatology, an interpolation is required to reproduce the sectors of the climatology at the climatology position, see example in figure 1. A sector interpolation for all sectors in the climatology is performed when sector interpolation is True. The default value is True (-).
Figure 1. Example of sector interpolation, where interpolation of incoming wind from North-northwest and North give wind from North at the climatology location.
Wake effects
Wake effects can be calculated by analytical and CFD based methods. Analytical methods are attractive as they are simpler and less computational demanding than CFD based methods. The three wake models described below are all analytical models. They are all single wake models calculating the normalized velocity deficit; δV =(U-V)/U, see the definition sketch in figure 2. All models are rotational axisymmetric along the x-axis, which imply that reduced wakes will be calculated off hub height.
Figure 2. Definition sketch wake effects.
The velocity deficit is calculated based on the wind database established in the module Wind Fields. As such it is a post-processing treatment, and the mutual interaction between the wakes, the interaction between the wakes and the terrain can not be captured correctly. Alternatively, models of each turbine, could be established in the module Terrain and thus be included in the wind field simulations.
Model 1
Model 1 is based on momentum deficit theory and is often referred to as the "Jensen model" [1]. This model gives a simple linear expansion of the wake, determined by the wake decay factor, k. The wake decay factor increases with increasing level of ambient turbulence, a typical range is from 0.04 to 0.075.
δV = (1 - SQRT(1 - CT)) / (1 + (2kx/D))2
where:
CT = thrust coefficient (-)
k = A/ln(h/z0)
A = 0.5
h = hub height (m)
z0 = roughness height (m)
Model 2
Model 2 is derived from the turbulent boundary layer equations and a similarity assumption, and is often referred to as the "Larsen model" [2].
δV = (1/9)(CT Arx-2)1/3 {r3/2 (3C12CT Ar x)-1/2 - (35/2π)3/10 (3C12)-1/5 }2
where:
CT = thrust coefficient (-)
Ar = πD2/4
D = rotor diameter
C1 = (D/2)5/2 (CT Ar x0)-5/6
x0 = 9.5D/(2R95/D)3 - 1
R95 = 0.5(Rnb + min(h,Rnb))
Rnb = max(1.08D,1.08D + 21.7D(Ia-0.05))
Ia = ambient turbulent intensity at hub height
Model 3
This model introduces a turbulent depending rate of wake expansion [3].
δV = CT1/2/32 (1.666/k1)2 (x/D)-p EXP(-r2/b2)
where:
CT = thrust coefficient (-)
b = k1(CT1/4/0.833) D(1-(p/2))xp/2
D = rotor diameter
p = k2(Ia + Iw)
Iw = k3 (CT/max(Ia,0.03)) (1 - EXP(-4(x/10D)2))
Ia = ambient turbulent intensity at hub height
k1 = 0.27
k2 = 6.00
k3 = 0.004
Roughness height
The roughness height at the turbine position is an input parameter to some wake models, this value can be read from the .gws file or given a constant value. The default option is to read from the .gws file.
Ambient turbulent intensity
The ambient turbulent intensity at the turbine position is an input parameter to some wake models, this value can be read from the wind database or given a constant value. The default option is to read from the wind database file
Number of sub-sectors
Each sector of the climatology is divided in sub-sectors, to distribute the wake effect over the sector. The default value is 30 (-).
Influence range
The influence range, given in rotor diameters, determines where the wake calculation should be performed. The minimum value is used to disregard wake effects in the near-field that might not be represented correctly by some wake models. The maximum value is given for computational reasons, to avoid calculations in the far-field where the wake effects could be neglected. The default range is (1;10) (Rotor diameter).
Multiple wakes
When more than one turbine influence the velocity at the considered location the velocity deficits calculated by the analytical single wake models are combined to obtain an equivalent wake deficit. Several wake combination, multiple wakes, models have been proposed in literature, WindSim uses the linear superposition of the wake deficts (1) or the square root of the sum of the squares (2).
(1) linear superposition
δv = Σδvi
(2) sum of squares of velocity deficits
δv = SQRT(Σδv2i)
Air density correction
Activating Export to WAsP format the wind power density calculation is activated and density calculation is done in every grid point. Choose the desired density correction. The default value is "no correction".
No correction
The density at all points equals the density in the power curve file.
Fixed value
The density at all points equals the given value.
Individual 1
The density is calculated at each point from the U.S. Standard Atmosphere taking into account elevation change: rho = 1.225 - (1.194 * 10-4) * z ; where z is the height above sea level. This will give a good long-term average value of air density in moderately complex areas.
Individual 2
The density is calculated at each point from an isothermal atmosphere approximation to account for elevation changes and mean site temperature: rho = (Po / RT) exp(-g*z/RT). Where Po is the standard sea level atmospheric pressure (1013.25 hPa), g is the gravitational constant (9.8 m/s2), T is the temperature in Kelvin and z the height above sea level.
Individual 3
The density is calculated at each point from the standard atmosphere with a temperature lapse rate of -0.65K/100m and a pressure of 1013.25 hPa at sea level. A reference temperature at a reference height needs to be specified. As reference temperature the mean annual measured temperature can be used.
2. Legend
Legend minimum and maximum values
Specification of the legend interval, if both minimum and maximum are set to zero the full range will be given. Default value is zero (-).
3. Export
Export to ASCII format
Exporting the wind resource map to an ASCII file. A link to the file will be provided in the report. The default value is False (-).
Export to WAsP format
Exporting the wind resource map to the WAsP .rsf and .wrg formats. A link to the files will be provided in the report, likewise plots of the Weibull scale and shape parameters. The default value is False (-).
Export all
Exporting the WAsP files weighted against all climatologies. This procedure is quite time and memory demanding and should only be activated if it is needed. The default value is True (-).
Type
The WAsP files can be exported for different areas: the complete grid, the refinement area if there is a refinement, and a user defined area. For the user defined area the range in global coordinates and the horizontal resolution can be specified.
4. Cross-checking
The cross-checking functionality estimates the errors of the numerical model in predicting measurements. Whenever multiple measurements exist, represented as climatologies with the given time history format .tws, a cross-checking can be performed. All possible pair combinations are cross-checked automatically, defining one climatology as the reference and the other climatologies as targets. In a projects with three climatologies then all six combinations given in figure 3 will be cross-checked.
Figure 3. Cross-checking combinations in a project with three climatologies.
The cross-checking is based on concurrent time series. It calculates the ratios of the wind speeds (speed-ups) and ratios of the standard deviations for all concurrent time records of one reference and one target. These ratios are calculated in two different ways; filtered and unfiltered. The filtered way includes wind speeds inside the operational range of a typical wind turbine, the default range is from 3 to 25 (m/s). Whereas the unfiltered way includes all wind speeds except calm conditions when the wind speed is zero.
Hence two sets of errors are estimated based on filtered and unfiltered data. The errors obtained by filtering should be closer to the errors obtained from the numerical model for the operational range of wind speeds for a wind turbine. Results presented in the report tables are based on filtered data. In the underlying files linked in the report there is also unfiltered results.
References
[1] Katic, I., Højstrup, J., Jensen, N.O. "A Simple Model for Cluster Efficiency." EWEC Proceedings, 7-9 October 1986, Rome, Italy.
[2] Larsen, C. G. "A Simple Wake Calculation Procedure." Risø-M-2760, 1988.
(http://www.risoe.dk/rispubl/VEA/veapdf/ris-m-2760.pdf)
[3] Ishihara, T., Yamaguchi, A., Fujino, Y. "Development of a New Wake Model Based on a Wind Tunnel Experiment." Global Wind Power 2004.
(http://windeng.t.u-tokyo.ac.jp/posters/2004_gwp_poster.pdf)