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Differences between GWCs for two hub heights

Lucas Cleto Paiva

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This may not be a very reasonable question, but I have a doubt concerning the difference between the generalised wind climate calculated from two hub heights with the same time series. I did it in my project for measured data at 60 m and 100 m height for a wind turbine with a hub height of 70 m, and I was expecting them to provide the same GWC, which did not occured. That led the wind farm capacity factor of each one to have a difference of almost 4%.

I would like to know if there is any obvious explanation for this, and if there is anything I can do to compensate for that discrepancy when calculating my wind farm AEP.

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Hi Lucas,
I am not sure if I understand your question correctly, but it seems like you are applying the same time series twice to get a GWC? You will need to use the 'true' height where your time series was measured and insert this in the WAsP program. I.e. if you have a measured time series from a height of 60 m, you will get different results when you use this time series with an input height of 100 m. You will need the GWC that was generated using the real height of 60 m, otherwise WAsP will think the wind speed of 60 m was valid at 100, giving you much lower values in the GWC.
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Hi Rogier, thanks for your answer.

In fact, I have two time series, each one corresponding to a different measurement height (60 m and 100 m) in the same met mast, that is to say, I have two anemometers. So, in order to get a comparison between the GWC associated to each one of those anemometers, I used WAsP to calculate the AEP for the same wind farm twice, with the GWC generated. The analysis showed that there are remarkable differences whenever I change the GWC, even if they correspond to the same met mast, but at different heights.

My question is: is that difference due to any vertical extrapolation model limitation?

If it's not clear yet, please let me know. Thank you once again!
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Ok, so in that case the reason why you won't get the same values is indeed due the the modelling. The most common causes are the roughness length of the terrain (if it is too high you will get a too high wind resource). Also the default stability used in WAsP could be off (mostly the offset heat flux in profile model). If these things are wrong you will not be able to reproduce the wind climate from one height to another, so it is worth spending some time investigating what could changed.
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