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Title-Wind as alternative energy source
  Turbine Siting


  Roughness and Wind Shear
  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. In the wind industry one distinguishes between the roughness of the terrain, the influence from obstacles , and the influence from the terrain contours, which is also called the orography of the area. We shall be dealing with orography, when we investigate so called speed up effects, i.e. tunnel effects and hill effects , later.

In general, the more pronounced the roughness of the earth's surface, the more the wind will be slowed down.

Forests and large cities obviously slow the wind down considerably, while concrete runways in airports will only slow the wind down a little. Water surfaces are even smoother than concrete runways, and will have even less influence on the wind, while long grass and shrubs and bushes will slow the wind down considerably.

    Roughness Classes and Roughness Lengths
Roughness class 0.5 imageRoughness class 0.5 imageSheep are a wind turbine's best friend. In this picture from Akaroa Spit, New Zealand, the sheep keep the roughness of the landscape down through their grazing. Photograph Soren Krohn © 1998 DWIA

In the wind industry, people usually refer to roughness classes or roughness lengths, when they evaluate wind conditions in a landscape. A high roughness class of 3 to 4 refers to landscapes with many trees and buildings, while a sea surface is in roughness class 0.

Concrete runways in airports are in roughness class 0.5. The same applies to the flat, open landscape to the left which has been grazed by sheep.

The term roughness length is really the distance above ground level where the wind speed theoretically should be zero.

    Wind Shear

Wind Shear Graph
This graph shows you how wind speeds vary in roughness class 2 (agricultural land with some houses and sheltering hedgerows with some 500 m intervals), if we assume that the wind is blowing at 10 m/s at a height of 100 metres.

The fact that the wind profile is twisted towards a lower speed as we move closer to ground level, is usually called wind shear. Wind shear may also be important when designing wind turbines. If you consider a wind turbine with a hub height of 40 metres and a rotor diameter of 40 metres, you will notice that the wind is blowing at 9.3 m/s when the tip of the blade is in its uppermost position, and only 7.7 m/s when the tip is in the bottom position. This means that the forces acting on the rotor blade when it is in its top position are far larger than when it is in its bottom position.

    Wind Shear and Escarpments
Escarpment with four wind turbines
Aerial photograph Soren Krohn
© 1999 DWIA
    Do not Include the Altitude of Your Terrain in Wind Shear Calculations

The aerial photograph above shows a good site for wind turbines along a shoreline with the turbines standing on a cliff which is about 10 m (30 ft.) tall. It is a common mistake to believe that in this case one may add the height of the cliff to the height of the wind turbine tower to obtain the effective height of the wind turbine, when one is doing wind speed calculations, at least when the wind is coming from the sea.

This is patently wrong. The cliff in the front of the picture will create turbulence , and brake the wind even before it reaches the cliff. It is therefore not a good idea to move the turbines closer to the cliff. That would most likely lower energy output, and cause a lower lifetime for the turbines, due to more tear and wear from the turbulence.

If we had the choice, we would much rather have a nicely rounded hill in the direction facing the sea, rather than the escarpment you see in the picture. In case of a rounded hill, we might even experience a speed up effect.

    The Roughness Rose

Roughness roseIf we have measured the wind speed exactly at hub height over a long period at the exact spot where a wind turbine will be standing we can make very exact predictions of energy production. Usually, however, we have to recalculate wind measurements made somewhere else in the area. In practice, that can be done with great accuracy, except in cases with very complex terrain (i.e. very hilly, uneven terrain).

Just like we use a wind rose to map the amount of wind energy coming from different directions, we use a roughness rose to describe the roughness of the terrain in different directions from a prospective wind turbine site.

Normally, the compass is divided into 12 sectors of 30 degrees each, like in the picture to the left, but other divisions are possible. In any case, they should match our wind rose, of course.

    Averaging Roughness in Each Sector

In most cases, however, the roughness will not fall neatly into any of the roughness classes, so we'll have to do a bit of averaging. We have to be very concerned with the roughness in the prevailing wind directions. In those directions we look at a map to measure Øhow far away we have unchanged roughness.

Photograph Soren Krohn,© 1999 DWIA

    Accounting for Roughness Changes Within Each Sector
    Vestlig sektor Let us imagine that we have a sea or lake surface in the western sector (i.e. roughness class 0) some 400 m from the turbine site, and 2 kilometres away we have a forested island. If west is an important wind direction, we will definitely have to account for the change in roughness class from 1 to 0 to 3.
    Accounting for Wind Obstacles
    It is extremely important to account for local wind obstacles in the prevailing wind direction near the turbine (closer than 700 m or so), if one wants to make accurate predictions about energy output.
    Wind Speed Variability
    Short Term Variability of the Wind

Graph showing short term wind fluctuationThe wind speed is always fluctuating, and thus the energy content of the wind is always changing.

Exactly how large the variation is depends both on the weather and on local surface conditions and obstacles.

Energy output from a wind turbine will vary as the wind varies, although the most rapid variations will to some extent be compensated for by the inertia of the wind turbine rotor.

    Diurnal (Night and Day) Variations of the Wind

Diurnal wind variation In most locations around the globe it is more windy during the daytime than at night. The graph to the left shows how the wind speed at Beldringe, Denmark varies by 3 hour intervals round the clock. (Information from the European Wind Atlas).

This variation is largely due to the fact that temperature differences e.g. between the sea surface and the land surface tend to be larger during the day than at night. The wind is also more turbulent and tends to change direction more frequently during the day than at night. From the point of view of wind turbine owners, it is an advantage that most of the wind energy is produced during the daytime, since electricity consumption is higher than at night.

Many power companies pay more for the electricity produced during the peak load hours of the day (when there is a shortage of cheap generating capacity)

    Seasonal Variations of the Wind

Wind Energy Index, DenmarkWind Matches Seasonal Electricity Consumption Patterns In temperate zones summer winds are generally weak compared to winter winds. Electricity consumption is generally higher in winter than in summer in these regions.

In the cooler areas of the globe, electrical heating is therefore ideal in combination with wind energy, because the cooling of houses varies with the wind speed much like the electricity production of wind turbines vary with wind speeds.

In electricity systems that are not based on hydropower and wind there may be good reasons to avoid electrical heating, however:

Conventional power plant wastes a lot of heat, and thus fuel (at least 60%), i.e. for every unit of useful heat consumed by a household, the power station will waste 1.5 units of heat (and fuel).

    Annual Variation in Wind Energy
    Annual variation in wind energy in Denmark Just like harvest yields vary from year to year in agriculture, you will find that wind patters may vary from year to year. Typically, the variations are less than the changes in agricultural production.

Turbulence pictureYou have probably experienced how hailstorms or thunderstorms in particular, are associated with frequent gusts of wind which both change speed and direction.

In areas with a very uneven terrain surface, and behind obstacles such as buildings there is similarly created a lot of turbulence, with very irregular wind flows, often in whirls or vortexes in the neighbourhood.

Turbulence decreases the possibility of using the energy in the wind effectively for a wind turbine. It also imposes more tear and wear on the wind turbine. Towers for wind turbines are usually made tall enough to avoid turbulence from the wind close to ground level.

    Wind Obstacles

This movie was shot at a coastal wind site with the wind coming from the right side of the picture. It shows an interesting phenomenon: Wind turbines behind trees

We would really expect the wind turbine to the right (which is facing the wind directly) to be the one to start first when the wind starts blowing. But you can see, that the wind turbine to the right 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 per cent on average, and even more in case of the rightmost turbine.

(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).

Side view of wind flow around obstacle Side view of wind flow around an obstacle. Note the pronounced turbulent airflow downstream

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

As you can see 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.

Top view of wind flow around obstacleTop 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 of the obstacle, i.e. how "open" the obstacle is. (Porosity is defined as the open area divided by the total area of the object facing the wind).

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 kilometre away in one of the more important wind directions.

    Wind Shade

Wind Speed behind obstacle (in %)This graph gives you an estimate of how wind speeds decrease behind a blunt obstacle, i.e. an obstacle which is not nicely streamlined. In this case we use a seven story office building, 20 metres tall and 60 metres wide placed at a distance of 300 m from a wind turbine with a 50 m hub height. You can quite literally see the wind shade 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 per cent to 97 per cent of the speed without the obstacle. You should note that this means a loss of wind energy of some 10 per cent, i.e. 1.03 3 - 1, as you may see in the graph up

Wind energy behind obstacle (in %)



    Selecting a Wind Turbine Site
    Wind Conditions

Bent Tree PictureBent Tree PicturePhotograph Soren Krohn © 1997 DWIA

Looking at nature itself is usually an excellent guide to finding a suitable wind turbine site.

If there are trees and shrubs in the area, you may get a good clue about the prevailing wind direction , as you do in the picture to the left. If you move along a rugged coastline, you may also notice that centuries of erosion have worked in one particular direction.

Meteorology data, ideally in terms of a wind rose calculated over 30 years is probably your best guide, but these data are rarely collected directly at your site, and here are many reasons to be careful about the use of meteorology data, as we explain in the next section.

If there are already wind turbines in the area, their production results are an excellent guide to local wind conditions. In countries like Denmark and Germany where you often find a large number of turbines scattered around the countryside, manufacturers can offer guaranteed production results on the basis of wind calculations made on the site.

  Grid Connection

Obviously, large wind turbines have to be connected to the electrical grid.

For smaller projects, it is therefore essential to be reasonably close to a 10-30 kilovolt power line if the costs of extending the electrical grid are not to be prohibitively high. (It matters a lot who has to pay for the power line extension, of course).

The generators in large, modern wind turbines generally produce electricity at 690 volts. A transformer located next to the turbine, or inside the turbine tower, converts the electricity to high voltage (usually 10-30 kilovolts).

    Grid Reinforcement
    The electrical grid near the wind turbine(s) should be able to receive the electricity coming from the turbine. If there are already many turbines connected to the grid, the grid may need reinforcement, i.e. a larger cable, perhaps connected closer to a higher voltage transformer station.
    Soil Conditions
    Both the feasibility of building foundations of the turbines, and road construction to reach the site with heavy trucks must be taken into account with any wind turbine project
    Pitfalls in Using Meteorology Data

Meteorologists already collect wind data for weather forecasts and aviation, and that information is often used to assess the general wind conditions for wind energy in an area.

Precision measurement of wind speeds, and thus wind energy is not nearly as important for weather forecasting as it is for wind energy planning, however.

Wind speeds are heavily influenced by the surface roughness of the surrounding area, of nearby obstacles (such as trees, lighthouses or other buildings), and by the contours of the local terrain.

Unless you make calculations which compensate for the local conditions under which the meteorology measurements were made, it is difficult to estimate wind conditions at a nearby site. In most cases using meteorology data directly will underestimate the true wind energy potential in an area.