Description: Energy Version: 8.0.0 Updated: 17.07.14

Energy

The annual energy production, AEP

The annual energy production, AEP is calculated for all visible Turbine objects. If several climatology objects are available the AEP based on each climatology is calculated separately. Any discrepancies between the AEP's based on different climatologies are easily accessible.

A climatology is given by its frequency distribution and presented graphically in the wind rose. Additionally a climatology is given by its Weibull distribution. The AEP is calculated for both representations.

Properties
1. Calculations
Air density correction
Specification of the air density at the turbine positions. A power curve is given for a specific air density. If the air densities given in the power curves differ from the air density given here, a correction will be applied to the power curves used for the energy calculation (see method for density correction). The default value is "no correction".
No correction
The density at the turbine position equals the density specified in the power curve. The given power curve is not corrected.
Fixed value, Individual 1, Individual 2 & Individual 3
See description page of Wind Resources
Method for density correction
Two different correction methods can be applied to the power curve depending on the power control system of the WECS (EN 61400-12).
Pitch-regulated WECS
In the case of pitch-regulated wind turbines the power output of the WECS is calculated by entering in the original power curve with a corrected wind speed. The corrected wind speed is obtained by the wind speed times the fraction (air density AEP)/(air density power curve) at the power 1/3.
Stall-regulated WECS
In the case of stall-regulated wind turbines the fraction (air density AEP)/(air density power curve) is used in the AEP calculations as a multiplication factor of the reference power curves.
Sector interpolation
See description page of Wind Resources
Wake effects
See description page of Wind Resources
Height of reference production
By default the production is calculated at the measurement mast position in the measurement height and in 80 m above ground. The 80 m value can be changed and additional heights can be added in this line (i.e. "80;100").
2. Export
Export vertical profiles
An ASCII file with vertical profiles at all visible Turbine positions is produced. The profiles are extracted from the wind database from ground level up to the "Height of reduced wind database" specified in the Wind Field module. A link to the file will be provided in the report.
Figure 1. Definition sketch of vertical profiles, e.g. Speed_2D. Variable values are given in cell centres.
The vertical profile file contains the following variables:
UCRT - wind speed scalar in East-West direction (m/s)
VCRT - wind speed scalar in North-South direction (m/s)
WCRT - wind speed scalar in vertical direction (m/s)
Speed_2D - wind speed scalar in horizontal plane, SQRT(UCRT2+VCRT2) (m/s)
Inflow - angle with respect to the horizontal, ATAN(WCRT/Speed_2D) (deg)
Shear - derivate of Speed_2D with respect to vertical direction, ΔSpeed_2D/ΔZ (1/s), where ΔSpeed_2D = Speed_2Dk+1 - Speed_2Dk-1, ΔZ = Zk+1 - Zk-1
Shear_low - derivate of Speed_2D with respect to vertical direction, ΔSpeed_2D/ΔZ (1/s), where ΔSpeed_2D = Speed_2Dk - Speed_2Dk-1, ΔZ = Zk - Zk-1
Shear_high - derivate of Speed_2D with respect to vertical direction, ΔSpeed_2D/ΔZ (1/s), where ΔSpeed_2D = Speed_2Dk+1 - Speed_2Dk, ΔZ = Zk+1 - Zk
KE - turbulent kinetic energy (m2/s2)
TI - turbulent intensity assuming isotropic KE, 100*SQRT((4/3)*KE)/SQRT(UCRT2+VCRT2) (%)
alpha - is the wind shear power exponent of a power law tangent to the calculated wind speed profile, Shear/(Speed_2D/za.g.l.) (-)
The default value is False (-).
Export rotor profiles
An ASCII file with rotor profiles at all visible Turbine positions is produced. Data is extracted from the wind database in the corners of a square centred at the hub. A link to the file will be provided in the report.
Figure 2. Definition sketch of rotor profiles.
The rotor profile file contains the same variables as the vertical profiles, see above. The variables are given at the HUB and in the corners of a square centred at the hub, the extension of each side is equal to the rotor diameter given in the module Objects. The following name syntax is used for corners; Upstream (U), Downstream (D), Right (R), Left (L), Top (T) and Bottom (B). The upstream plane of the cube is perpendicular to the incoming wind direction. The position in the vertical direction is referred to: above sea level (asl), above ground level (agl) and referred to HUB (rth).
The default value is False (-).
Export power history
The visible climatologies of type time history (.tws) are transferred to the hubs of each visible turbine; the relative .tws files are placed in the object folder of the project. ASCII files containing the electrical power output of each WECS for each record of the time history files are furthermore created and saved into the report folder. A link to the power history files will be provided in the report.
Export weighted power history
All time histories transferred to the hub height of each visible turbine used to calculate power output (“powerhistory_clim…_wes_....dat”) are loaded and weighted again inverse distance. A weighted time series is obtained out of it and ASCII files containing the electrical power output are created. A link to the power history files will be provided in the report files called “powerhistory_clim_all_to_wecs_....dat/.csv”.
Note: the time series of the layout need to be consistent each other, same length, same time step and same time period.
Export turbine assessment
Important sectorwise parameters are written at the turbine positions: basic information are the annual energy production (without and with wakes losses), the Weibull parameters and the wakes losses. In case the IEC classification is executed additional parameters are written as the representative turbulence intensity (Irep), the effective turbulence intensity (Ieff), the wind shear exponent and the inflow angle.
3. IEC Classification
In this option of the Energy module the suitable IEC class for each turbine of the current layout is calculated, as described in the standards IEC 61400-1 [2,3,4]. The IEC classification of the turbines is performed for both the 2nd edition [2] and 3rd edition [3] of the standards, accounting for the amendment 1 (2010) [4] of the 3rd edition.
Each .tws is transferred to each hub position, and then the parameters Vref, Iref (mean value and standard deviation) are computed for the transferred .tws. The reference velocity, an extreme wind (10 min average) with a recurrence period of 50 years, is computed by a Gumbel fitting the annual peaks of transferred wind speeds. Annual peaks are fitted with the method of the maximum likelihood. At least two years of measurements are therefore required to perform a Gumbel fitting, with uncertainties reducing with longer time histories. Measurement periods of more than ten years are recommended.
In case the time stepping of the reference time series (.tws) is not 10 minutes there will be need to change the Gust factor accordingly. For the 2nd edition the IEC class is determined also according to the site annual average wind speed Vave. For some sites the restriction over the Vave is more severe than the one on the extreme wind speeds. The 3rd edition substituted the verification over the average wind speed with a more detailed verification of the site probability density function (pdf) over the turbine design pdf, which is a Rayleigh distribution. The check of the site pdf against the design pdf is not present in WindSim as output of the IEC classification, though it can be carried out analyzing the site through the WAT export or the turbines_assessment files. The Iref is given as expected value of the turbulence intensity for the 15 m/s bin, its standard deviation is also computed for the same samples. In the case of classification of wind turbines according to the second edition of the standards [2] a characteristic turbulent intensity is required (84th percentile, mean plus the standard deviation for a normal distribution). For the 3rd edition, as amended in the 2010, the turbulence class is instead determined by veryfing equation (35) of [4] in the range Wind speeds range Ieff. All values of Ieff are reported in the energy_IEC_classification.log file which is linked in the report of the Energy module.
In the case of classification according to the third edition, it is verified that the standard deviation of the longitudinal component from the normal turbulence model σ1 of the wind velocity at hub height is greater or equal to the estimated 90th percentile of the effective turbulence standard deviation (accounting for both ambient and wake turbulence):
σ1(Vhub) ≥ Ieff(Vhub) Vhub
where:
σ1 = standard deviation of the longitudinal component of the wind velocity;
Vhub = wind speed at the hub height;
Ieff = effective turbulence intensity.
The verification is performed between for Vhub ranging from 60% of the rated wind speed Vr and the cut-out wind speed Vout. In the case rated and cut-out wind velocity are not specified in the power curve (.pws) file, the verification will be carried out automatically for Vhub varying in the range 7-25 m/s if a user defined wind speed range is not given.
The effective turbulence Ieff is defined in the IEC standards as weighted average of turbulence intensity to the power of m, being m the Wöhler (SN-curve) exponent. For the IEC classification m is by default equal to 10, number valid to verify the glass fiber of the blades, the most fragile component of the wind turbine. The effective turbulence is therefore that turbulence intensity that would produce a fatigue damage after the same number of cycles of a failure caused by the actual turbulence wind rose.
Ieff = [∫ p(θ|Vhub) Im(θ|Vhub)dθ]1/m
where:
m = Wöhler exponent;
θ = wind direction;
p = frequency of occurrence;
I = representative value (90th percentile) of the turbulent intensity of the wind;
Vhub = wind velocity at the hub height.
The turbulence intensity I accounts for the presence of the neighboring turbines with the version of the Frandsen model as proposed in the Annex D of the last standards amendment [4].
The integral defining Ieff is therefore discretized in a set of wind directional sectors. Using the subscript s for sector-wise properties, NS number of directional sectors:
Ieff[∑ pS(Vhub) ISm(Vhub)]1/m
For the general sector s Is is the representative value of the turbulence intensity, accounting for wake induced turbulence by Ns neighboring (di < 10) turbines in the sector s.
Is = σeff_ss,Vhub)/Vhub
σeff_ss,Vhub) = [(1-Ns pws) σr_sms,Vhub) + pws ∑σT_sms,Vhub)]1/m
Ns = neighboring turbines within sector s;
pws = probability to be under wake for a given sector;
σr_s = representative value of ambient turbulence intensity for sector s;
σT_s = representative value of turbulence intensity in wake condition.
The turbulence intensity under wake condition is modeled with the following expression:
σT_s(d_i ) = √((Vhub2)/(1.5+(0.8 di)/√(CT))2 + σr_s2s,Vhub) )
where:
CT = thrust coefficient of i-th wind turbine generating the wake;
di = distance (in rotor diameters) to the i-th wind turbine.
15 m/s bin width
Width of the velocity bin centered at 15 m/s used to compute mean and standard deviation of turbulent intensity.
Gust factor
In the case the .tws file does not contain records averaged over 10 minutes there will be the need to adjust the Gust factor. In case of a .tws with stepping longer than 10 minutes, e.g. a hour, the Gust factor is the averaged ratio of the maximum 10 minutes peaks within the recording period and the mean wind speed in the same time; so a number bigger than unity. The Gust factor will multiply each yearly peak at the turbine location before fitting with a Gumbel distribution.
WAT Export
Enabling this option allows to export a set of files to the report\WAT folder that can be later imported in Windfarm Assessment Tool (WAT), a free software by Risø and DTU Wind Energy.
A basic WAT file (.txt) will be exported, together with turbulence files (.txt), power characteristics (.wtg) and a terrain file (.grd). Turbulence files are obtained by the transferred .tws at each hub position. Power characteristics in .wtg format are converted from the .pws files loaded in the Objects module while the digital terrain model (DTM) in .grd format is converted from the .gws file loaded in the current WindSim project.
Wöhler coefficient
The Wöhler coefficient is the exponent m appearing in the definition of the effective turbulence Ieff. It is specific for the material used to build the component that will be verified to fatigue loads. In the case of a complex structure as a wind turbine it has be chosen the maximum coefficient between all components. A coefficient of 10, slightly conservative, is meant for the glass-fibers, generally present in the composite materials of the blades.
Ieff filter
A filter is set, given by the minimum number of samples needed to account for a sector in the Ieff calculation
Wind speeds range Ieff
According to the IEC standards [3,4] the amended equation (35) in [4] has to be verified ranging from 60% of the rated wind speed (Vr) and the cut-out wind speed (Vout) of the turbine. If the properties of the turbine are unknown [4] recommends to verify in the range of wind speeds 0.2 Vref-0.4 Vref. When the Wind speeds range Ieff is set to its default value "According to IEC standards" the Ieff will be verified in the range 0.6 Vr-Vout; though, if turbine properties are unknown, the check will be performed between 7 and 25 m/s. Alternative to the default value is to set a user defined wind speed range.
References
[1] Risø and DTU Wind Energy.
Windfarm Assessment Tool (WAT)
software official web site and installation package (MSI) download
http://www.wasp.dk/Products/WAT.aspx
[2] International Electrotechnical Commission (IEC)
IEC 61400-1 ed2.0 (1999)
Wind turbine generator systems - Part 1: Safety requirements
[3] International Electrotechnical Commission (IEC)
IEC 61400-1 ed3.0 (2005)
Wind turbine generator systems - Part 1: Safety requirements
[4] International Electrotechnical Commission (IEC)
IEC 61400-1-am1 ed3.0 (2010)
Wind turbine generator systems - Part 1: Safety requirements