Ct (Thrust Coefficient) calculations for Ct > 1 (2004-10-06)

[Old Forum Archivist]

Hi,

In the formula WAsP uses to calculate the effective wind speed deficit in waked loading conditions (found at http://www.wasp.dk/Products/WAsP/WakeEffectModel.html), the term (1-sqrt(1-Ct)), becomes an imaginary number when Ct, the thrust coefficient, becomes higher than 1.0. So since we don't want imaginary numbers, I believe WAsP rounds down to 1 any input Ct values over 1. However, I would note that Ct curves (thrust coefficient per wind speed) for different turbine manufacturers frequently contain Ct values over 1. When WAsP rounds these down to 1, there is some information lost. So my question is: Please explain why it is appropriate for WAsP to round down Ct values over 1 (e.g. does this underestimate the wake losses)?, and my other question is: Can the formula be changed to use values over 1?



David Wahl

GE Wind Energy

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The wake model in WAsP is an approximate model according to which Ct cannot exceed 1. I guess it is fair to say that this may lead to sligth underestimation of the wake effects. On the other hand, Ct-values higher than 1 would only occur at very low wind speed, just above cut-in, where the produced power is anyway very low. Thus the impact on the calculated annual energy production is to my judgement negligible.

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