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The importance of Wind Measurement

In order to design a profitable wind farm, the local wind conditions are measured to perform a detailed wind site assessment. Once the wind farm is set up and running, its best performance has to be ensured. Thus wind measurement does not stop with connecting the wind farm to the grid. The profitability of the wind turbines has to be monitored and the wind farm needs to be controlled, e.g., by implementing SCADA systems.

The construction of a profitable wind farm requires the most accurate and reliable wind measurement technology. The choice of appropriate measurement equipment and its correct installation are crucial. Measuring equipment must perform as precisely as possible to ensure the quality of data essential for producing accurate wind site assessments.

A small discrepancy of even 3% in the evaluation of wind speed data drastically multiplies during assessment calculations and results in a loss of seven digit economic figures. Compared to the costs for the construction of a new wind farm, the costs for a high standard measuring system are minimal.

Feasibility studies and wind site assessment are the basis for the financial decision to build a wind farm. Energy yield forecasts ensure that the wind farm will be profitable. To create those studies, a wind measurement campaign has to be performed and the site for the wind farm has to be analysed in detail. Measurement campaigns usually last for at least twelve months, during which wind measurement data is continuously gathered at regular intervals without interruption. Afterwards the measurement data is processed and evaluated and then compared to long-term meteorological data, e.g., data from adjacent weather stations, to assess whether the site will be suitable for the erection of a wind farm.

ammonit wind measurement mast

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Wind measurement

Learn how you can benefit from accurate wind measurement in your wind energy project - before the wind farm is set up and while operation. We summarized the most important facts in this section. You can also read our brochures for further details:
Download: Information brochure on wind measurement (English, PDF)
Download: Ammonit Wind Resource Assessment Systems (English, PDF)
Download: Ammonit Wind Resource Assessment Systems for Cold Climate Regions (English, PDF)
Download: Ammonit Wind Farm Monitoring Systems (SCADA, English, PDF)


Wind Measurement for Site Assessment

The taller the wind turbine is, the higher is the wind performance and the better is the return on investment of the wind farm.

The installation of a complete, state-of-the-art measuring system at one or several significant locations on a potential site gives the best assessment of the site’s wind conditions and helps to determine its suitability for a wind farm. Measuring the wind speed (velocity) with anemometers is critical for the evaluation of a site’s wind energy potential. During wind measurement campaigns wind statistics are gathered according to the wind atlas method and are cleared of any topographical influences. The preparation of wind statistics usually lasts for at least 12 months. During the campaign, wind measurement data is collected continuously at regular intervals. Weather conditions vary from season to season and from one year to the next. An accurate wind site assessment requires the comparison of new detailed wind measurement data with long-term meteorological data, e.g., data from nearby weather stations. Both must be analysed together to determine whether the site is suitable for the erection of a wind farm. The quality of weather station data does not have to be to the same standard as the wind measurement data, but it is important that both sites are comparable and that the external data is checked for reliability, e.g., the weather station has not moved during the relevant period.

ammonit site assessment

In addition to the assessment of wind speed, the assessment of wind direction and its allocation is required. This parameter is measured with wind vanes. Assessing wind direction and allocation helps to avoid sheltering effects within the wind farm. An analysis of the surrounding topography and ground consistency should also be made and taken into account for the assessment calculations. Although less crucial, values such as air density, air pressure and humidity should be considered as well.

A measuring system consists of several components. The specific components of a system will vary depending on climatic and regional conditions, and the anticipated size of the wind farm. Because the height of the measuring tower depends on the turbine hub height (the level where the rotor blades are to be located), the anticipated turbine hub height is critical. The complexity of the surrounding terrain is also very important in the selection of measuring system components. Masts and wind turbines are becoming increasingly taller; the average mast height is currently approx. 100m. However, in the meantime masts with 200m are in operation. As a general rule: the higher the wind turbine is from the ground, the better is the wind performance; the higher the wind performance, the better is the return on investment of the wind farm.

The mast should be set up, free of obstruction, on a carefully selected position on the site. It should be fully stocked with high-quality measuring components, consisting of a data logger, several sensors, a communication system, a power supply and several additional accessories as required. The main mast can be supported by additional masts, to be located in intervals of 5 - 10km, to assess the overall wind conditions of the region. However, under most circumstances, one mast fully equipped with a self-contained, high-standard measuring system will be sufficient to assess the wind potential, even in remote areas and under extreme climatic conditions.

In addition to a met mast, remote sensing devices like LiDAR and SoDAR can be used to increase the number of available measurement data in the region. If LiDAR or SoDAR devices should be used for wind site assessment, consider the requirements described in the latest IEC 61400-12-1 standard (March 2017) and the German TR6 guideline.

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Global Wind Atlas

The Global Wind Atlas is part of an international collaboration with the Danish DTU Wind Energy as project leader. The online platform provides wind data, which can be used to pre-scan a region for wind resources. The platform does not replace a proper wind resource assessment campaign at the planed site for a wind farm.

Different data sources have been used to develop the Global Wind Atlas. A list is provided on the Global Wind Atlas website. You can display the wind data in a number of plots or download the data for further analysis.

Global Wind Atlas


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Components of our wind measurement systems

The better the quality of the components of a measuring system are, the more accurate are the results of the wind site assessment. An accurate wind site assessment ensures that a wind park will be profitable.

A measurement system has to meet the local requirements of climate and landscape. In general, the measurement system includes the following components:

In addition to the above mentioned components further measurement devices can be installed, although they are not required for wind energy yield calculations, e.g., for precipitation and global horizontal irradiation measurement. The measurement system has to work independent from the grid, has to be weather-resistant and must meet the requirements for flight safety. Even in very remote areas the measurement system has to reliably transfer measurement data to the wind consultant.

There are primarily two types of masts in use:

  • Lattice masts

  • Telescope masts

A steel cabinet containing data logger, communication system, components for the power supply and any additional system components are mounted near the bottom of the tower typically at a height of approx. 6m to make access for thieves or vandals difficult, while allowing maintenance and service access.


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Impact of Sensors on the AEP (Annual energy Production)

In order to obtain reliable and accurate measurement data, the installation of high quality sensors is crucial. Refer to the table below to learn more about the impact of measurement errors on the annual energy production.

  Generated error by 1% measurement error

Δ1% wind speed measurement error → Δ3% energy
(Up to Δ6% energy, if anemometers are used to calculate the wind profile.)

Wind vane

No direct linear relation, but will lead to poor wind farm design.

Barometric pressure sensor

Δ1% air pressure measurement error → Δ1% energy

Temperature sensor

1° measurement error → 0.35% energy

Air humidity sensor

(at 40°C) 1% measurement error → 0.05% energy


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Standards and Guidelines

In order to summarize the high quality requirements in wind measurement, international standards have been developed. The most important standards and guidelines are:

IEC 61400-12-1
The IEC 61400-12-1 is the most important standard for wind measurement. This standard specifies the correct set-up of a met mast including installation of sensors. It describes the quality of sensors installed on the met mast with regard to accuracy and reliability. Additionally, it defines a set of criteria with regard to data quantity and quality.The IEC standard has been updated recently to meet new requirements for wind energy assessment. The new IEC published in March 2017 includes large parts about site assessment. Ultrasonic anemometers and remote sensing devices are accepted for site assessment under certain conditions. Additionally, data integrity, quality and filtering are in the focus of the standard (see also MEASNET).

MEASNET is an international network of measurement institutes, which developed the guideline "Evaluation of site-specific wind conditions". This guideline describes the process of site assessment including data collection, evaluation and interpretation. The MEASNET guideline refers to IEC 61400-12-1 and focuses on data quality, plausibility and integrity.
Download: MEASNET Site Assessment Guideline V2 (April 2016, English, PDF)

TR6 (Rev.9)
The technical guideline TR6, published by the German "Fördergesellschaft Windenergie und andere Erneuerbare Energien", describes processes to determine the wind potential and the energy yield at sites for wind turbines („Bestimmung des Windpotentials und Energieertrags an Standorten für Windkraftanlagen“). The guideline focuses on wind measurement considering wind met masts as well as SoDAR and LiDAR devices. Under certain conditions, the guideline accepts wind speed and direction measurement exclusively with remote sensing systems. The TR6 guideline also defines criteria for data quality. Thus measurement data has to be checked for integrity and plausibility on a regular basis. This guideline is mainly applied in Germany.

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Setting up a Measurement system

The measuring system should always be installed by professionals - no matter whether it is a met mast or a remote sensing system.

The correct set up of the met mast and its booms as well as position, type and quality of the measurement instruments has been defined in internationally accepted standards and guidelines: IEC 61400-12-1, MEASNET and the German TR6 guideline.

The evaluation of wind speed is crucial for wind site assessments. At least four anemometers are required to determine a wind profile that enables wind speed to be evaluated. The anemometers should be mounted onto the mast at different heights. The “top” anemometer should be mounted at the very top of the mast, and ideally should be at the same level as the hub height of the planned turbines. It is essential that unobstructed wind can reach anemometers from every direction.

Servicing high-level anemometers is not always possible, as masts can be very high; a second, backup anemometer should be installed slightly below the top anemometer. At least two additional anemometers should be mounted at the middle and the lower level on the mast. On taller masts, several anemometers are installed at regular height intervals to give the most accurate wind measurement data.

Wind vanes are installed to determine the wind direction. A wind vane is best positioned just below the top anemometer. It is recommended installing 2 or 3 additional wind vanes into the system. For very wide masts (e.g. a converted radio mast) sensors should be installed in pairs (1 on each side of the mast) to ensure that at least one sensor delivers the correct wind data. Humidity-temperature sensors should be mounted with a weather and radiation shield at a height of 10m.

The air pressure sensor should be installed inside the data logger’s steel cabinet, provided that the cabinet is permeable to air. Data loggers are the core of every wind measuring station and should be safely installed in a weatherproof and robust CE certified steel cabinet. Several potential components could be added in the steel cabinet, such as GSM/GPRS communication module, barometric pressure sensor, battery and surge protection. The steel cabinet offers protection against damage from weather, condensation and vandalism. Finally the obstacle lights should be mounted at the top of the tower. The regulations on installation of obstacle lights vary largely from country to country.

ammonit system set up

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Evaluating wind measurement data

There are several methods of recording wind measurement data and often several methods are applied at the same time.

Rayleigh Method
Evaluates the Rayleigh cumulative probability distribution function. This is a direct variational method, in which the minimum of a functional defined on a normed linear space is approximated by a linear combination of elements from that space. This method yields solutions when an analytical form for the true solution may be intractable. It is applied to calculate likely fluctuations at the site when only the average wind speed of the region is known. The Rayleigh distribution uses a fixed general formula and reflects regional features only to a certain extent.

Weibull Method
The knowledge of two parameters is required for this method: the scale factor A and the form parameter C. This method describes wind circumstances more exactly than the Raleigh method, as the shape of actual distribution is taken into account. At a value of parameter C = 2, the formula is identical with the Rayleigh distribution.

Classifying measurement data
This procedure is used to structure and minimise the measurement data to an absolute minimum (only the absolute necessities). There are two methods of classification:

  • Evaluation of wind speed on two levels

  • Scale is divided into segments of constant width. The frequency is recorded and counted at intervals of one or ten minutes to assess, if the values are within the class limits. Over a period of time a frequency distribution is created to assess the relative distribution, via a division of class values through total number of measurements. The measured data has to be corrected in two steps in order to comply with the requirements and standards (long-term validity at the hub height).

Transformation to hub height
Since wind measurements are usually generated at a lower level than the eventual hub height of the potential wind turbine, a transformation of the data is necessary. This value is usually determined with the roughness length of the site possible for each direction sector (e.g. ground contour). Schedules for roughness length, giving a description of the approximate values of the surroundings, can be applied. However, a much more reliable and precise method is to collect the wind measurement at two different heights.

Correlation of long-term data
The measurement data should be collected over a period of at least 1 year to ensure that seasonal fluctuations are taken into account. The data of a single year must then be compared with long-term data. Wind speeds can differ largely – up to 20% – from the long-term average. Long-term correlation data can often be obtained from nearby professional weather stations, airports or from existing wind parks. The measuring standard of this external data may be of a lower standard than that collected from the measuring station. The most important thing is to ensure reliable continuity of the external data (e.g. data from a weather station that has relocated at some point is not acceptable) and that the site being used for correlation data is comparable.

Wind rose
The unobstructed evaluation of the wind direction is essential to enable the ideal positioning of wind turbines. Information on distribution of wind speeds and the frequency of varying wind directions, as well as the evaluation of the roughness length of the site is required. To visualise this, one can draw a wind rose diagram based on meteorological observations of wind speed and direction. A circle is divided in 12 - 36 sectors. The radius of the 12 outermost, wide wedges gives the relative frequency of each of the 12 wind directions, i.e. how much percent of the time is the wind blowing from that direction.

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