#1 Re: WAsP » Power curve and thrust co efficient » 2019-09-11 14:39:08

Alternative power curves could reflects different ambient conditions, like air density, and modes of turbine operation, like reduced tip-speed as a means for noise reduction. The changes are made by the wind turbine controller managing tip speed and blade pitch angle, so yes, you are right, this should both affect power curve and thrust-coefficient curve. However, we normally don’t know the exact strategy of the control system or the detailed rotor aerodynamics, so it is difficult to predict changes in the thrust just from changes in the power.


Some manufactures supply sets of power- and thrust-coefficient curves for different conditions, and then you type both curves into the WAsP turbine editor. However, I have also noticed manufactures who supply the same thrust-coefficient curve for different power curves. I think the argument is that the power curve is the most important one, as it influences the AEP more directly. The usually wake loss is only 5-10%, so a modified thrust-coefficient curve will not have as significant an effect on the AEP as a modified power curve.


Cheers,
Morten

#2 Re: WAsP Engineering » Uncertainty in Extreme Wind Report » 2019-06-04 11:19:23

Hi Windfrosch,


It is a standard deviation of the extreme wind estimate associated with the uncertainty of the fitted statistical model for the extreme-wind distribution. There are actually alternative models, see the post at http://www.wasptechnical.dk/forum/viewtopic.php?id=1060
When running the script you mention WEng will use the model defined by the selected extreme-wind-climate object, either annual maximum (AM), peak over threshold (POT) or spectral correction method (SC).   


Cheers,
Morten

#3 Re: Climate Analyst » +/- range in Oewc and Oewc-POT results » 2019-05-29 00:03:56

Dear Sinisa Knezevic,


1) The extreme wind in WEng may be calculated by three alternative methods called the
annual maximum (AM), peak-over-threshold (POT) and spectral correction (SC) methods.
The theory differ for the three methods, but to generalize we could say that they
first fit a statistical model for extreme events and then used that model
to estimate the level of the extreme event with a fifty year return period.
The uncertainty of the 50-year extreme wind estimate depends on the uncertainty of the fitted model. Read more in the WEng help file section 'WAsP Engineering modelling| Extreme winds', specifically in the subsections called 'Extreme wind estimators' for the AM method, 'Extreme wind POT method' for the POT method and 'Spectral correction' for the SC method.


2) The footnote in IEC 61400-1 Ed.4 expresses the coefficient of variation (COV) by the alpha and beta parameters of the Gumbel distributions. Thus, COV depends on the fitted model and not directly on the uncertainty of the fit. However, the uncertainty is closely related to the Gumbel alpha parameter, so you could say that there is an indirect relation. Unfortunately, WEng no longer reports the fitted alpha and beta parameters, but if you look in the 'WAsP Engineering modelling| Extreme winds| Extreme wind estimators' section of the WEng help file you will find an easy way to calculate them by yearly maxima exactly as in WEng when using the AM method. Alternatively, you can read the 100-year event from the graphs in WEng showing extreme winds as function of return period and then use the formula in the footnote of the IEC standard.


With best regards,
Morten

#4 Re: WAsP » Wind Farm Power Curve for Turbine Site Group » 2019-04-19 17:17:04

Hi Windfrosch,


WAsP does not support this directly, but as a work-around method you could try a modified power curve for the turbines you wish to exclude from the windfarm power curve. This modified power curve must have the usual Ct-coefficient curve but a close-to-zero power curve. Ideally that should be exactly zero power for all wind speeds, but then WAsP would fail to import the file, so better use tiny values instead.


The easiest way to modify the turbine power curve is to
- export to a WTG file,
- open the WTG file in the WAsP turbine editor,
- export to the old POW format
- open the POW file in an ASCII editor
- reduce the power scaling parameter (second parameter, third line), say from 1000 to 0.0001
- save the modified POW file
You now have a model for an incredibly inefficient turbine, producing almost no power but with the usual wake loss at the other turbines.


Now go back to the WAsP project, insert the modified POW file in the subgroup representing turbines you want to exclude from the windfarm power curve and recalculate. As a check you might compare the NetAEP of the turbines of interest to their NetAEP using the original WTG for all turbines. I got small deviations on the last digit, but hope that is OK.


Finally, when you have the special-purpose wind farm power curve, you should revert to the original WTG file for all turbines.


Cheers,
Morten

#5 Re: WAsP » Roughness with many windbreaks » 2018-11-02 18:13:10

Dear Vinh Le Thanh,


I inserted the coordinate into Google Earth, and I agree that the roughness pattern at this site is quite complex with many small tiles and a large variation in surface roughness. From the pictures I also saw that fields are flooded in part of the year, and open water surfaces will reduce the roughness roughness. For WAsP applications you should, however, apply a roughness typical for the entire year as this is more relevant for the yearly energy production. 


It would be a big job to digitize a roughness map for this site. Instead, you could try to press ‘File| Import| from Web-database| GWA Map Warehouse – roughness’ in the WAsP map editor to download a roughness map. The resolution is not ideal, so you may need to add more details for the area close to your windfarm and met mast. But you don’t have to edit the roughness far from the sites of interest, as these details should not affect the WAsP results much.


With best regards,
Morten Nielsen

#6 Re: WAsP » Lib Interpolator » 2018-10-10 14:55:56

The Lib Interpolation applies simplified version of the method described in

Nielsen, M. (1999), A method for spatial interpolation of wind climatologies. Wind Energ., 2: 151-166. doi:10.1002/(SICI)1099-1824(199907/09)2:3<151::AID-WE26>3.0.CO;2-5

where it is referred to as LibInt LT.

#7 Re: WAsP Engineering » Setting up a met tower » 2018-09-28 15:50:46

It depends a little on the job you need to do. Maybe you want to study a particular problem occurring for a specific wind direction or maybe you need to check turbine safety in general?


For the latter you could try an IEC 61400-1 site assessment, which includes a list of wind conditions to check. You start by creating and selecting a group of turbines sites and their hub height. You also need to create an extreme wind climate by the WAsP Climate Analyst and import this in the WEng project. Then you open ‘Tools| Prepare data for WAT’ from the main menu. This tool will calculate the fifty-year extreme winds for all turbine positions plus tables of wind conditions (shear, turbulence, inclination angle, etc.) for a number of wind directions. You can direct these results to an Excel file and use them as they are or import them in WAT to complete the IEC site assessment.


Cheers, Morten

#8 Re: WAsP Engineering » Setting up a met tower » 2018-09-27 12:04:33

The basic wind input to the flow model of WAsP Engineering is a single wind condition. You may insert multiple winds in the project, but usually you have to select only one of these as reference for the calculations. Quite often you also need to specify a height and a site or a group of sites. Some of the ‘tools| scripts’ will iterate over all wind in your project, e.g. ‘Summary site report…’ and ‘Turbulence report…’, or even create a number of winds with different direction and report for these, e.g. ‘Wind speedup and deflection’.


WEng can also operate on extreme wind climates (EWC). This is a little like the mean wind climates in WAsP, i.e. you specify an observed EWC for the reference site and then predict the EWC for other sites.


There are some video demonstrations of basic operations in WEng at https://www.youtube.com/playlist?list=P … FQMI2iinpb


Cheers,
Morten

#9 Re: WAsP » .Wtg files » 2018-09-25 13:20:58

Hi LinkeshD,


You can create a WAsP WTG file by clicking on 'Tools| WAsP Turbine Editor' from the main menu inside WAsP.


I usually start by preparing a performance table inside Excel with three columns (speed, Power, Ct) and then copy this table (without headers) to clipboard. Back in the turbine editor I select the 'Enable Edit' option and adjust the 'Table size' to the number of data points. Then I enter the upper left cell of the empty table and paste the performance table from the clipboard. Next step is to edit the 'description' string, rotor diameter, hub height and air density. If multiple performance tables are available, I add them one by one and finally save the WTG file to disk. 


Starting from WAsP 12 we can interpolate and extrapolate power performance tables to air densities at individual turbine sites. To reduce uncertainty it will be best if the starting point is a file with performance tables for air densities close to the local conditions.


Cheers,
Morten

#10 Re: WAsP » air density correction of power curves » 2018-09-04 12:56:37

A note for third-party readers:


The assumption in my first reply was wrong. The real problem is that WAsP applies an automatic routine to detect whether a turbine is pitch or stall regulated. It simply consider a turbine to be pitch-regulated when the power curve has at least three points equal to the maximum. Unfortunately, this routing fails for measured power curves with a bit of noise in the power-curve data. It should not be a problem with power curves supplied by manufactures.

#11 Re: WAsP Engineering » Flow inclination definition » 2018-09-04 12:42:36

Hi Ebazus,


Normally WAsP Engineering calculates for specific wind conditions, not sectors. However, the tool called 'prepare data for WAT' produces a result table for the same number of sectors as defined in the supplied wind atlas file, which is a result from WAsP. These calculations are made with one reference wind for each sector with a wind direction equal to the center angle of each sector. The reference winds are generalized winds, i.e. for flat terrain with surface roughness z0=0.05m, and they may deflect over real terrain. Thus the flow inclination angle (or tilt) are calculated for a case where the local wind direction is a bit off the sector center angle, especially in complex terrain.


Cheers, Morten

#12 Re: Map Editor » WAsP Turbine Editor » 2018-07-24 10:41:08

Hi Roella,


The Weibull probability distribution describes the mean wind speed at the turbine position. If you have an observed time series of wind speeds at the right position and height above terrain you can fit a Weibull distribution to the data and then estimate the annual energy production by a probability-weighted integral of the power curve of all wind speeds. With a program like WAsP you can observe the wind climate at a reference mast, then use a flow model to predict wind climates at your turbine sites and finally use these turbine-specific wind climates for the AEP estimates. WAsP operates with different Weibull distributions for individual wind sectors as the relative speedup between reference site and sites of interest and the corrections for wake effects of neighbouring turbines depend on wind direction. So the AEP calculation is a more detailed than in the turbine editor. In any case the observed wind data should cover a whole number of years to avoid seasonal bias.


Best regards,
Morten

#13 Re: WAsP Engineering » WEng and WindPRO » 2018-07-16 08:58:16

Hi Victoria,

Good to hear that you were able to solve the problem.

Cheers, Morten

#14 Re: WAsP Engineering » WEng and WindPRO » 2018-07-14 13:00:31

Hi Victoria,


Mark forwarded this question to me, but I am afraid that I cannot help much. All I know is that the flow model of WAsP Engineering operates with a local coordinate system which must be Cartesian with metric coordinates. However, if you try to connect to our servers you also need to describe the map projection and datum for your map projection, e.g. UTM 30 and WGS84, so the system can transform your local coordinates to geographical coordinates.


I am not familiar with WindPro. It might operate with the same map conventions as WEng, but it is also possible that some information needed by WEng is provided automatically without troubling the user. Maybe you have a user guide for WindPro, which can show you the correct way.


Not sure what a 'position type' is, so it is probably a term specific for WindPro.


Cheers,
Morten

#15 Re: WAsP Engineering » Cant import *gwc or *lib files to Assesment Tool » 2018-07-12 14:20:29

Hi,


The ‘Prepare data for WAT tool’ calls some WAsP DLLs to add mean climate predictions (Weibull distributions) to the result table for each turbine site. That is why it asks for a wind atlas file and a map in vector format. The format of the wind atlas file has changed over the years and I suspect that this could be the reason for the problem. I did some experiments and it seems like the WAT launcher tools reads the old LIB format without problems. It also reads a GWC file exported from WAsP 11 correctly but I did not manage to get it to read a GWC file exported from WAsP 12. The GWC format differences are the barometric reference data added in WAsP 12.0 and parameters for the background flow profile added in WAsP 12.1. Since you write about temperature and pressure information, I am guessing that you are in fact using WAsP 12, so your problem is probably related to the modified GWC file format. A work-around solution could be to apply the LIB format instead. If you are worried about the accuracy, then try to compare the Weibull distributions in your WAsP project and the ones which are written in the input file for WAT.


I am not sure about the technical reason for the problem, and I rather not ask the programmer who is on vacation right now. Maybe WEng 4 can only work with the DLLs from WAsP 11 or maybe it gets confused when you, like me, both have WAsP 11 and WAsP 12 installed on the same PC. Anyway, thanks for reporting this. I shall forward the message to the programmers.


Cheers,
Morten

#16 Re: WAsP » air density correction of power curves » 2018-06-14 16:27:50

Hi Stefan,


The extrapolation is done in two steps. First we correct the wind speeds of the reference points of the original power curve. For a decrease in air density, this shifts the power curve to the right, but we take care of not moving the point at the cut-out wind speed.
We then interpolate the shifted power curve and find new values at wind speeds with regular spacing. I have seen one case where this interpolation failed because the interpolation function was forced into unrealistic undulations. The problem was an unconventional additional reference point near cut out inserted by the user. It did not use to disturb WAsP but fooled the air density correction method. If you like me to investigate the problem then please send a WAsP workspace file illustrating the problem to waspsupport@dtu.dk.


Best regards,
Morten

#17 Re: WAsP Engineering » IEC 61400-12-1: Turbulence structure σu/σv/σw » 2018-06-08 15:30:01

Dear Sinisa,


IEC 61400-12-1 on power performance measurements has an informative Annex M discussing how to correct a measured power curve for effects of turbulence. However, it does not depend on ratios of standard deviations of turbulent fluctuations in different directions (sigmaU, sigmaV and sigmaW). I am therefore guessing that your actually ask about the turbulence structure correction parameter CCT, which is specified in IEC 61400-1 on turbine safety. Actually you can calculate this in two alternative ways – by terrain assessment or by a formula based on sigma-ratios. You can make either assessment with our WAT program, which is a postprocessor of results from WAsP and WEng.  WAT calculates CCT by probability-weighted averages of standard deviations of turbulence components in a number of sectors as calculated by WEng. The data for WAT is calculated by a tool called ‘Prepare data for WAT’ inside WEng 4. Learn more in videos no. 2, 3 and 6 at https://www.youtube.com/playlist?list=P … qJ5UXlHvYA


I am not sure, but I think the preparation of data for WAT worked the same way with WEng 3.1 If not, you may have to upgrade or make equivalent assessments manually using the turbulence calculated for a number of wind directions inside WEng. 


With best regards,

Morten Nielsen

#18 Re: WAsP » AEP & dRIX » 2018-05-13 22:11:29

WAsP CFD is not free. However, if you are a student at DTU then please ask your supervisor to contact the WAsP group about your needs.

#19 Re: Fuga » Exception occurred in Fuga-64 'ComboBox9Change' » 2018-02-06 17:34:10

The file format of the WAsP workspace file has changed, because of the new air-density corrections of the power curve in WAsP 12. We will prepare an update to Fuga, which supports the new workspace format. If you are in a hurry you could work with input from WAsP 11.

Regards,
Morten Nielsen

#20 Re: WAsP » AEP & dRIX » 2018-01-24 10:48:39

Hi Seenu,


The ruggedness index (RIX) is a measure of the terrain complexity in the area around a site. Delta-RIX (dRIX) is the difference between the RIX numbers at the site of interest and at the reference mast. Terrain complexity leads to uncertainty in linearized flow models, such as the one used in WAsP. But errors related to generalization of the wind climate at reference mast data and errors related to climate prediction at the site of interest will often compensate each other when dRIX is close to zero, i.e. when the RIX numbers for the two sites are comparable. A positive dRIX value means that the terrain around the site of interest is more complex than near the reference mast and the model is likely to over-predicted the wind speed. Best practice is to install a reference mast at the site of interest, let it measure for 2-3 years, and use the local data for the wind resource estimate. You could also try the WAsP CFD model, which generally gives better predictions in complex terrain than the default flow model in WAsP.


Best regards,
Morten

#21 Re: Map Editor » Constant recalculation of cross points » 2017-12-22 14:34:14

Hi Seenu,


I am not sure what the 'tiresome refresment' problem is, but I suppose it is a technical matter and I think it will be better to write to waspsupport@dtu.dk than to discuss at the WAsP forum. Please describe the problem in sufficient detail in your mail to the help desk and include a test file or similar information so we can investigate the problem.


Cheers, Morten


PS: We are closed between Christmas and New Year.

#22 Re: Map Editor » Constant recalculation of cross points » 2017-12-19 12:19:40

Hi Phil,

The programmer is not here today, but to save time I suggest that you already now send a mail to our help desk at waspsupport@dtu.dk

Please include the map you are working with in a state where we can see the problem. If you are not able to save the map before the problem occurs, then please send some screen dumps, or maybe a video, so we can reproduce the problem here.

Cheers,
Morten

#23 Re: Map Editor » Constant recalculation of cross points » 2017-12-18 11:35:22

Hi Phil,
I just asked our Map Editor programmer, who reminded me that roughness consistency checks are optional and may be deactivated under 'Tools' in the main menu on the main Map Editor window.
Cheers,
Morten

#25 Re: WAsP » Sector-wise Weibull parameters calculation method » 2017-10-30 16:38:58

The main principle is first to compute the two types of statistics mentioned above based on the observations and then to select the A and k parameters which give the same statistics when calculated by the Weibull probability model. This is done for every wind sector. The situations with calm weather need special treatment, since we cannot trust the wind direction sensor when the wind speed is close to zero. Calm data are therefore distributed proportionally over the wind sectors using the frequency of occurrence determined by winds above the calm threshold.

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