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MicroSiting (Part I). UNESCO Desire – Net Project Roma, 21/02/2007. Roberto Nardi Wind Research Engineer, Siting & Performance Department. Siting & Wind Analysis. What is micro siting and why do it? What kind of data do you need for micro siting?

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(Part I)

UNESCO Desire – Net Project

Roma, 21/02/2007

Roberto Nardi

Wind Research Engineer, Siting & Performance Department

Siting & Wind Analysis

What is micro siting and why do it?

What kind of data do you need for micro siting?

What are the results coming from the micro siting activities?

What is micro siting?

“Micro siting” is a way to optimize the park layout in any given site to obtain the highest production on site.

Calculate the production

Calculate the wake losses due to other turbines

Calculate sound emission from the turbines to the nearest neighbor

Create a visualization of the park

Ensure a 20 year design lifetime

All this is something that is done before the park is erected so you can calculate the feasibility of the project.

Why do it?

This can be said with a few words:

To optimize production and reduce loads

Where does Wind Energy come From?

All renewable energy (except tidal and geothermal power), and even the energy in fossil fuels, ultimately comes from the sun. The sun radiates 100,000,000,000,000 kilowatt hours of energy to the earth per hour. In other words, the earth receives 10 to the 18th power of watts of power.

About 1 to 2 per cent of the energy coming from the sun is converted into wind energy. That is about 50 to 100 times more than the energy converted into biomass by all plants on earth.

For a good micrositing is needed
For a good micrositing is needed:

  • Min. 1 year of wind data measured on site

  • Wind direction measurements

  • The wind speed measurements must be conducted for at least 2 heights → wind shear

  • The measuring height should be as close to hub height as possible

  • Standard deviation measurements → turbulence

  • If possible temperature measurements → air density

  • A digital 3-D contour map covering an area of a radius of 5 – 10 km from the site centre

In order to do a proper wind assessment on site wind measurements are necessary
In order to do a proper wind assessmenton-site wind measurements are necessary!

  • One full year of measurements are needed in order to take all seasonal variations into account.

  • If more than one year of raw data are used the year to year uncertainty is taken into account.

  • If the temperature is measured simultaneity with the wind speed, it is possible to estimate weather or not, a high/low temperature turbine is needed.

  • On site measurements are needed in order to investigate the wind regime on site. Wind shear, turbulence, wind rose, and wind speed are factors that can easily change with the complexity of the landscape.

Wind rose

Anemometer mast

The measurement of wind speeds is usually done using a cup anemometer, such as the one in the picture to the right. The cup anemometer has a vertical axis and three cups which capture the wind. The number of revolutions per minute is registered electronically.

Normally, the anemometer is fitted with a wind vane to detect the wind direction.

Other anemometer types include ultrasonic or laser anemometers, hot wire anemometers.

The advantage of non-mechanical anemometers may be that they are less sensitive to icing.

Nrg logger with gsm modem
NRG Logger with GSM modem

  • standalone wind logger

  • automatic data transmission

  • data on web

  • data in database

  • logger network

10min wind data
10min Wind data

Data measured every 10sec - but only a mean value for 10min is logged on data logger.

Typical wind data from a meteorological station

The wind data must include:

Date, time, 10 min mean wind speed and direction

10 min mean standard deviation

Temperature (if possible)

The following parameters are important to check the loads from the turbines:

Weibull fit, turbulence

Air density

Inflow angles

Typical Wind Data from a Meteorological Station

F(u) = exp.(-(u/A)k)

A = scale parameter

k = shape parameter

Weibull fit

The wind rose
The wind rose

  • The wind rose is needed in order to make the optimal layout.

  • The optimal layout is perpendicular to the mean wind direction.

  • More than one wind vane should be used, in order to check for errors in the direction data.

  • An optimal layout has a closer spacing perpendicular to the main wind direction and bigger spacing along the mean wind direction.

Wake effect
Wake Effect

Since a wind turbine generates electricity from the energy in the wind, the wind leaving the turbine must have a lower energy content than the wind arriving in front of the turbine.

This follows directly from the fact that energy can neither be created nor consumed.

A wind turbine will always cast a wind shade in the downwind direction.

In fact, there will be a wake behind the turbine, i.e. a long trail of wind which is quite turbulent and slowed down, when compared to the wind arriving in front of the turbine. You can actually see the wake trailing behind a wind turbine, if you add smoke to the air passing through the turbine, as was done in the picture on the right.

Wind turbines in parks are usually spaced at least three rotor diameters from one another in order to avoid too much turbulence around the turbines downstream. In the prevailing wind direction turbines are usually spaced even farther apart, as explained on the next page.

Park layout
Park Layout

Ideally, it is suggested to space turbines as far apart as possible in the prevailing wind direction. On the other hand, land use and the cost of connecting wind turbines to the electrical grid would tell us to space them closer together. At last the layout will be a balance between technical and commercial issues.

As a rule of thumb, turbines in wind parks are usually spaced somewhere between 5 and 9 rotor diameters apart in the prevailing wind direction, and between 3 and 5 diameters apart in the direction perpendicular to the prevailing winds.

In this picture have been placed three rows of five turbines each in a fairly typical pattern.

The turbines (the white dots) are placed 7 diameters apart in the prevailing wind direction, and 4 diameters apart in the direction perpendicular to the prevailing winds.

Speed up effects 1
Speed Up Effects (1)

If you take a walk in a narrow mountain pass, you will notice that the air becomes compressed on the windy side of the mountains, and its speed increases considerably between the obstacles to the wind. This is known as a "tunnel effect".

So, even if the general wind speed in open terrain may be, say, 6 meters per second, it can easily reach 9 meters per second in a natural "tunnel".

Placing a wind turbine in such a tunnel should be one clever way of obtaining higher wind speeds than in the surrounding areas.

To obtain a good tunnel effect the tunnel should be "softly" embedded in the landscape. In case the hills are very rough and uneven, there may be lots of turbulence in the area, i.e. the wind will be whirling in a lot of different (and rapidly changing) directions.

If there is much turbulence it may negate the wind speed advantage completely, and the changing winds may inflict a lot of useless tear and wear on the wind turbine.

Speed up effects 2
Speed Up Effects (2)

A common way of siting wind turbines is to place them on hills or ridges overlooking the surrounding landscape. In particular, it should be always an advantage to have as wide a view as possible in the prevailing wind direction in the area.

On hills, normally the wind speeds are higher than in the surrounding area. Once again, this is due to the fact that the wind becomes compressed on the windy side of the hill, and once the air reaches the ridge it can expand again as its soars down into the low pressure area on the lee side of the hill.

You may notice that the wind in the picture on the right starts bending some time before it reaches the hill, because the high pressure area actually extends quite some distance out in front of the hill.

Also, you may notice that the wind becomes very irregular, once it passes through the wind turbine rotor.

As before, if the hill is steep or has an uneven surface, it will appears a significant amounts of turbulence, which may negate the advantage of higher wind speeds.


The principle of wake effects in wind farms:

  • Wind farm turbulence consists of 2 elements:

    • Back ground turbulence;

    • Wake turbulence, turbulence made by other turbines.

  • Close spacing among turbines gives high wake turbulence hence high turbulence.

  • High turbulence reduces the turbine life time dramatically.

k = Wake decay constant

Wake loss should be less then 4-5%

  • Mean goal of micrositing is to reduce loads and optimise production!