Significant difference between averaged wind speeds and averaged wind power density, for wind generation modelling?
In ECMWF data, wind direction and wind speed are collected from observation separately. Then based on a well known transformation the wind vectors, u and v are calculated. 

Their documentation: http://www.ecmwf.int/sites/default/files/elibrary/2015/9208-part-i-observation-processing.pdf

You can focus on this part in their documentation:

" **Wind.**There are four wind variables: wind direction(DDD), wind force(FFF),u and v components. For each of these variables the first thing which is done is to get a local copy of it together with its related parameters from an ODB supplied array (GETSETE).Once a variable is made available locally a check is made to ensure that the vertical coordinate is pressure; if instead of pressure a flight level is supplied it is converted into pressure by assuming a standard ICAO atmosphere(Z2PICAO). If the variable in question is either u or v, then DDD and FFF are converted into u and v wind components. Furthermore, for each of the four variables appropriate observation error statistics are assigned (ERRSTAT, FINOERR). Also, if any flags are set at this stage an appropriate word in the local copy is updated (PPVAFL). Finally, an updated local copy of an observed quantity and its related parameters are returned back into the ODB (AIREPBE)."*

Once wind speed and direction are obtained calculations are made to get the two components of wind u and v. 
e.g. 
    $$u = -FFF sin(DDD \frac{\pi}{180})$$
    $$v = -FFF cos(DDD \frac{\pi}{180})$$

In calculating wind power, we need wind speed and not wind direction. If we are given u,v components of wind, we can calculate wind speed as 
$$ U = \sqrt{u^{2} + v^{2}}$$.
Then, the power density,
   $$ P = \frac{1}{2}\rho A U^{3}$$
To sum up when we calculate wind power, what we need is the speed and not the direction. 

This may not answer your question but I hope it may give some hint about it!