Wind resources are best along coastlines and on hills, but usable wind resources can be found in most other areas as well. As a power source wind energy is less predictable than solar energy, but it is also typically available for more hours in a given day. Wind resources are influenced by the ground surface and obstacles at altitudes up to 100 metres. The wind energy is thus much more site specific than solar energy. In hilly terrain, for example, two places are likely to have the exact same solar resource. But it is quite possible that  wind resource can be different at both places because of site condition and different  exposure to the prevailing wind direction. In this regard, wind turbines planning must be considered more carefully than solar technology. Wind energy follows seasonal patterns that provide the best performance in the winter months and the lowest performance in the summer months. This is just the opposite of solar energy. For a Denmark conditions a PV plant has a production per month varying between 18% in January and 100% in July. The wind power plant produces 55% in July and 100% in January. For this reason small wind and solar systems work well together in hybrid systems. These hybrid systems provide a more consistent year-round output than either wind-only or PV-only systems.
It is important to know that the amount of wind power generated is proportional to the density of air, area swept by the rotor blades of the wind turbine, and to the cube of the wind speed.

AIR DENSITY
Blades of the wind generator rotate because air mass is moving them. The more air can move the blades, the faster the blades will rotate, and the more electricity the wind generator will produce. From the physics comes out that the kinetic energy of a moving body (e.g. air) is proportional to its mass (or weight) so the energy in the wind depends on the density of the air. Density refers to the amount of molecules in unit volume of air. At normal atmospheric pressure and at 15° Celsius air weighs some 1,225 kg per cubic metre, but the density decreases slightly with increasing humidity. Air is more dense in winter than in the summer. Therefore, a wind generator will produce more power in winter than in summer at the same wind speed. At high altitudes, (in mountains) the air pressure is lower, and the air is less dense. It is obvious that the density of air is variable that we can’t do anything about.

ROTOR AREA
The rotor of the wind turbine “captures” the power in the mass of the air that are passing through. It is clear that the larger area covered by a rotor means, the more electricity it can produce. The rotor area determines how much energy a wind turbine is able to use from the wind. Since the rotor area increases with the square of the rotor diameter, a turbine which is twice as large will receive four times as much energy. But increasing rotor area is not as simple as putting bigger blades on a wind generator. At first glance, this appears to be a very easy way to increase the amount of energy that a wind generator can capture. But by increasing the swept area we have also increased all of the stresses on the wind system at any given wind speed. In order to compensate for this change and let the wind system survive, it is important  to make all of the mechanical components stronger. Obviously this approach is going to get very expensive.

WIND SPEED
The wind speed is most important factor influencing the amount of energy a wind turbine can convert to electricity. Increasing wind velocity increases the amount of air mass passing the rotor, so increasing wind speed will also have an effect on the power output of the wind system. The energy content of the wind varies with the cube (the third power) of the average wind speed. Thus, if wind speed doubles, the kinetic power gained by the rotor increases eight times. From the following table you can estimate the power of the wind for standard conditions (dry air, density 1.225 kg/m3, at sea level pressure). The formula for the power in Watts per m2  = 0.5 * 1.225 * v3, where v is the wind speed in m/s (according to Danish Wind Turbine Manufacturers Association).

Nature provide us with a different wind conditions and wind speed is continuously changing. Wind turbines are specially build to make use of wind which range in speed between 3 to 30 m/s. Higher wind speed can damage the turbine so large turbines are equipped with the brakes. Smaller turbines can make use of wind speeds lower than 3 m/s. Wind speed scale:

Wind speed m/s	Type of wind
0-1,8	Calm
1,8-5,8	Light
5,8-8,5	Moderate
8,5-11	Fresh
11-17	Strong
17-25	Gale
25-43	Strong gale
more than 43	Hurricane

ROUGHNESS CLASS OF THE TERRAIN
Earth surface with its vegetation and buildings is the main factor reducing the wind speed. This is sometimes described as roughness of the terrain. As you move away from the earth’s surface, roughness decreases and the laminar flow of air increases. Expressed another way, increased height means greater wind speeds. High above ground level, at a height of about 1 kilometre, the wind is hardly influenced by the surface of the earth at all. In the lower layers of the atmosphere, however, wind speeds are affected by the friction against the surface of the earth. For the wind power utilisation it means the higher the roughness of the earth’s surface, the more the wind will be slowed down. Wind speed is slowed down considerably by forests and large cities, while plains like water surfaces or airports will only slow the wind down a little. Buildings, forests and other obstacles are not only reducing the wind speed but they often create turbulence in their neighbourhood. The lowest influence on the wind speed have the water surfaces. When people in the wind industry evaluate wind conditions in a landscape they describe it by roughness class. Higher roughness class means more obstacles in terrain and larger wind speed reduction. Sea surface is described as roughness class 0.

Roughness Class and Landscape Type:
0 = Water surface
0.5 = Completely open terrain with a smooth surface, e.g. runways in airports, mowed grass, etc.
1 = Open agricultural area without fences and hedgerows and very scattered buildings. Only softly rounded hills
1.5 = Agricultural land with some houses and 8 metre tall sheltering hedgerows with a distance of approx. 1250 metres
2 = Agricultural land with some houses and 8 metre tall sheltering hedgerows with a distance of approx. 500 metres
2.5  = Agricultural land with many houses, shrubs and plants, or 8 metre tall sheltering hedgerows with a distance of approx. 250 metres
3 = Villages, small towns, agricultural land with many or tall sheltering hedgerows, forests and very rough and uneven terrain
3.5 = Larger cities with tall buildings
4 = Very large cities with tall buildings and skyscrapers

In the industry also the term wind shear is used. It describe the fact that the wind profile is twisted towards a lower speed as we move closer to ground level. Wind shear may also be important when designing wind turbines. Here large rotor diameter and only a few meter higher tower could mean that the wind is blowing with higher speed when the tip of the blade is in its uppermost position, and wit much lower speed when the tip is in the bottom position.

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