Wind obstacles/Wind shade

Wind obstacles
Wind turbines behind trees         This movie comes from a coastal wind site with the wind coming from the right side of the picture. It shows an interesting phenomenon: eventhought it would be expected the wind turbine to the right, which is facing the wind directly, to be the one to start first when the wind starts blowing, in reality this wind turbine will not start at the low wind speeds which are sufficient to drive the other two wind turbines. The reason is the small wood in front of the wind turbines which shelters the rightmost turbine in particular. In this case, the annual production of these wind turbines is probably reduced by some 15% on average and even more in case of the rightmost turbine.
        Note that the turbines are located some five rotor diameters apart and the wood is located at a similar distance from the first wind turbine. The reason why the turbines look like they are standing very close together, is that the movie was shot from about a mile away with the equivalent of a 1200 mm lens for a 35 mm camera.

        Obstacles to the wind such as buildings, trees, rock formations etc. can decrease wind speeds significantly, and they often create turbulence in their neighbourhood.

        In the next two figures the side and top view of the wind flow around an obstacle is shown. As it can be seen from this drawing of typical wind flows around an obstacle, the turbulent zone may extend to some three time the height of the obstacle. The turbulence is more pronounced behind the obstacle than in front of it. Therefore, it is best to avoid major obstacles close to wind turbines, particularly if they are upwind in the prevailing wind direction, i.e. in front of the turbine.

      Side view of wind flow around an obstacle       Top view of wind flow around an obstacle

Shelter behind obstacles
         Obstacles will decrease the wind speed downstream from the obstacle. The decrease in wind speed depends on the porosity, defined as the open area divided by the total area of the object facing the wind of the obstacle, i.e. how "open" the obstacle is. A building is obviously solid and has no porosity, whereas a fairly open tree in winter with no leaves may let more than half of the wind through. In summer, however, the foliage may be very dense, so as to make the porosity less than, say one third.
        The slowdown effect on the wind from an obstacle increases with the height and length of the obstacle. The effect is obviously more pronounced close to the obstacle and close to the ground. When manufacturers or developers calculate the energy production for wind turbines, they always take obstacles into account if they are close to the turbine, say less than 1 km away in one of the more important wind directions.

Wind shade
Wind shade behind an obstacle (% of wind speeds)         This graph gives an estimate of how wind speeds decrease behind a blunt obstacle, i.e. an obstacle which is not nicely streamlined. In this case the obstacle is a seven story office building, 20 m tall and 60 m wide placed at a distance of 300 m from a wind turbine with a 50 m hub height.
        The wind shade can be shown as different shades of grey. The blue numbers indicate the wind speed in per cent of the wind speed without the obstacle. At the top of the yellow wind turbine tower the wind speed has decreased by some 3%, thus to 97% of the speed without the obstacle. Note that this means a loss of wind energy of some 10%.