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Energy Systems and Technologies for the Coming Century ...

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ise to increased wind speeds. As wind power density is a function of wind speed to <strong>the</strong>power 3 [Eq. 1], <strong>the</strong> impact of <strong>the</strong> resolved terrain features is significant.For <strong>the</strong> 50 x 50 km area <strong>the</strong> mean wind power density is estimated to be around 320Wm -2 <strong>for</strong> <strong>the</strong> resolutions of 10, 5, <strong>and</strong> 2.5 km. For <strong>the</strong> 100 m resolution <strong>the</strong> mean powerdensity is around 505 Wm -2 , i.e. an increase of 50% compared to <strong>the</strong> lower resolutionestimates.The comparison becomes more striking when <strong>the</strong> distribution of <strong>the</strong> wind power densityis considered. Consider this: we split <strong>the</strong> 50 x 50 km into two areas, <strong>the</strong> first area where<strong>the</strong> wind power density is below <strong>the</strong> median value <strong>and</strong> <strong>the</strong> second area where <strong>the</strong> windpower density is above <strong>the</strong> median value. Next we calculate <strong>the</strong> mean wind powerdensity in <strong>the</strong> second higher wind area, we get <strong>for</strong> <strong>the</strong> 10, 5, 2.5 km resolution estimates380 Wm -2 , where as <strong>for</strong> <strong>the</strong> 100 m resolution we get 640 Wm -2 , an increase of nearly70%. The impact gets stronger as we look at <strong>the</strong> even windier areas. As wind turbineswill be deployed at <strong>the</strong> favourable sites, it is important to be able to capture <strong>the</strong>distribution of wind power density due to terrain features, <strong>and</strong> this is only possible byconsideration of high resolution effects.Even <strong>for</strong> ra<strong>the</strong>r simple terrain, such as in Denmark, <strong>the</strong> effect of resolution is important.A similar 50 x 50 km area showed an increase of wind power density of around 25% <strong>for</strong><strong>the</strong> windiest 5-percentile (windiest 1/20th of <strong>the</strong> area).Figure 3: Map showing <strong>the</strong> orography (elevation) of <strong>the</strong> 640 x 400 km Columbia Gorgetest region located in north-western USA. The elevation ranges from 0 m as <strong>the</strong> sea toover 2000 m in <strong>the</strong> mountains.Figure 4: Map showing <strong>the</strong> variations in <strong>the</strong> surface roughness length <strong>for</strong> <strong>the</strong> 650 x 400km Columbia Gorge test region.Risø International <strong>Energy</strong> Conference 2011 Proceedings Page 219

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