En Ammonit Windinfo Knowhow

Measuring wind

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Measuring wind

Know-how for successful

wind measurements

Measuring wind


Content:

1) Wind measurements - what for?

2) How can a successful wind measurement campaign be ensured?

3) How often should wind measurements be taken and when?

4) What makes a good wind measurement system?

5) Measurement system - the components

6) How to install a measurement system?

7) Setting up the mast / tower

8) Avoidable mistakes

9) Data recording

10) Notes

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Know-how for successful wind measurements

Measuring wind


Meteorological measurements, including wind measurement, are required for many different applications, such as weather forecasting, agricultural meteorology, industrial monitoring or for other research areas.

All these disciplines have specific conditions that require a customised measuring technology. Wind measure-ments are essential in the field of wind energy generation and the development of wind parks. The highest stan-dard of wind measurement and the most accurate measuring technology is required, for assessing potential wind park sites and for monitoring and maintaining existing wind parks.

Prior to setting up a wind park, wind is measured by professional wind consultants for a wind site assessment. These measuring 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 park. Once a wind park is in operation, wind measurements are required for monitoring performance, such as monitoring turbine functioning and for collecting data to evaluate profitability (for example with SCADA systems).

The construction of a profitable wind park 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. To evaluate whether a wind park will be profitable, investors and banks require wind site assessments based on the most accurate measurement data. Compared to the costs for the construction of a new wind park, the costs for a high standard measuring system are minimal.

Several international standards for wind measurement are in place. The regulations IEC 61400-12-1:2005 and TR6 are the most significant, describing the optimal installation of masts, traverses and sensors. Conforming to these standards, along with taking the advice of professional wind consultants and manufacturers of measuring systems, will ensure that wind park investments will pay off.

Our brochure offers detailed information and advice on how to perform accurate wind measurements. We hope that our information will be helpful and that it will provide a good overview on what is crucial to ensure a successful measurement campaign.

1) Wind measurements - what for? • High quality measuring equipment ensures a successful wind measurement campaign • Inaccurate wind measurements result in losses of seven digit economic figures

Photo: GTZ, Chile (www.gtz.de)

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Measuring wind


The installation of a complete, state-of-the-art measuring system at one or (less often) several significant loca-tions on a potential site will give the best assessment of the site’s wind conditions and help to determine its suit-ability for a wind park.

Measuring the wind speed (velocity) with anemometers is critical for the evaluation of a site’s wind energy poten-tial. 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 compari-son of new detailed wind measurement data with long-term meteorological data (e.g. data from nearby weather stations). Both must be analyzed together to determine whether the site is suitable for the erection of a wind park. 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).

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 park. 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 park. 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 cur-rently approx. 80m, however, masts with a height of 140m are being developed. As a general rule: the higher the wind turbine is from the ground, the better are the wind performance results; the higher are the wind performance results, the better is the financial return of the wind park.

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 data transfer 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.

2) How can a successful wind measurement campaign be ensured? • The taller the wind turbine is, the higher are the wind performance results and the better is the financial return of the wind park

Photo: Ecosem, Spain (www.ecosem.es)

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Measuring wind


Prior to setting up a wind park, wind measurements for wind site assessments are carried out by professional wind consultants to the highest possible standards. Mea-surement campaigns usually last for at least 12 months, in which wind measurement data is continuously gathered at regular intervals.

The measurement data is then processed, evaluated and compared with long-term meteorological data (e.g. data from adjacent weather stations) to assess whether the site will be suitable for the development of a wind park. Wind measurements are also required to monitor performance once a wind park is in operation, for example with SCADA systems.

3) How often should wind measurements be taken and when? • Measuring wind for wind site assessments, prior to setting up a wind park • Measuring wind for wind park monitoring, once the wind park is in operation

Images: typical mast position - wind park monitoring (SCADA)

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Measuring wind



• 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.

The choice of an appropriate, state-of-the-art measurement system and the correct installation of the equipment are crucial. A good measuring system must be robust, reliable and self-contained to perform well in remote regions and extreme weather conditions. The measurement data must be accessible and it needs to be transferred consistently and reliably to the wind consultant’s PC. The measuring equipment should be selected according to regional and climatic requirements.

The basic components of every system are: a data logger, several anemometers, wind vane(s), a humidity-temperature sensor, a barometric pressure sensor, power sup-ply (usually solar panels). Additional components can be added where required, such as a GSM/GPRS data transfer system, a precipitation sensor, a pyranometer and an obstacle light.

There are primarily two types of masts in use: lattice masts and telescope masts. Both types could be applied in combination. The typical standard height is at present between 80 to 100m, with the latest masts being as tall as 140m. Masts are a specialist construction and equipped with sensors, usually mounted prior to erection of the mast. A steel cabinet containing the data logger, data transfer system, compo-nents 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.

Image: typical measuring system (SCADA)

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Measuring wind


ANEMOMETERS

OBSTACLE LIGHTS

PRECIPITATION SENSORS

BAROMETRIC PRESSURE SENSORS

HYGRO-THERMAL SENSORS

PYRANOMETERS

POWER SUPPLY

WIND VANES

ULTRASONICS

COMMUNICATION SYSTEMS

DATA LOGGERS

SOFTWARE

STEEL CABINETS

MAST-CONSTRUCTION

AMMONITORONLINE-PLATFORM


Image: potential components of a measuring system

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AmmonitORONLINE PLATFORM

Measuring wind



Image: components of typical measuring system

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A typical measuring system consists of • 1 Data logger

• 1 Steel cabinet with solar panel and GSM/GPRS communication system

• 3 Anemometers (Thies First Class Advanced)

• 2 Wind vanes (Thies First Class or Thies Compact)

• 1 Barometric pressure sensor

• 1 Humidity-temperature sensor Optional • Precipitation sensor

• Global radiation sensor / pyranometer

• Obstacle light

approx. 80 m to 100 m

approx. 73.5

24 m

9 m

6 m

Measuring wind



5.1 Data Logger

Ammonit data loggers offer a wide range of possibilities and are the core of every wind measuring station. They are robust, reliable, low in energy consumption, and have a nonvolatile memory of 4 MB with a capacity for 2,000,000 recorded values.

The self-contained measuring system runs even in extreme weather conditions and in remote areas, being reliably supplied with power by a 10, 20 or 50 W solar panel. Ammonit data loggers are safely housed in CE certifi ed steel cabinets for protection against damage from weather, condensation and vandalism.

In addition to the data logger and the basic components, several optional components may be included within the steel cabinet, such as a GSM/GPRS communication mo-dule, an air pressure sensor, a battery and surge protection. The data transfer be-tween PC and logger can be carried out via GSM/GPRS, satellite or direct interface. We offer free service software to simplify your communication procedures.

The three Series 32 data logger types differ only in number of input channels.

Meteo-32X

Meteo-32

Wicom-32

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Schaltschrank

Luftdruckmesser

Antenne

Vorhängeschloss

Schaltplan

Montageplatte

Laderegler

Akku

Erdung

GSM Modem

Kabelanschlussoptional Über-spannungsklemme

Datenlogger

Kabeldurchführung

Barometer

Steel CabinetData Logger

Antenna

Grounding

Padlock

Wiring Diagram

Cable Gland

Battery

Cable Clampand optional Surge

Protection clamp

Charge Controller

Assembly Plate

Image: typical steel cabinet system

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Measuring wind


C4 + C5


5.2.0 Logger Accessories

5.2.1 Steel Cabinets

Ammonit offers robust, weatherproof steel data logger cabinets in various sizes. The cabinet will protect your data logger against weather and condensation damage, theft and vandalism. Our CE certified steel cabinets are easy to install and maintain. Op-tional components, such as a GSM/GPRS communication module, barometric pres-sure sensor, battery and surge protection, can also be accommodated within the cabinet. The size of cabinet depends on the size of battery and the number of selected components; we currently offer 2 sizes. Cabinets are typically mounted at a height of approximately 6m and padlocked for protection from vandalism and theft.

5.2.2 Communication-Systems: GSM/GPRS or Satellite

You can communicate with your data loggers from your PC/Laptop in 3 ways:

1) GSM/GPRS System (email or text messages) 2) Satellite, if no GSM network is available 3) Direct interface at your measuring station

Your measuring data can be accessed at time periods of your choice, e.g. daily, sev-eral times a week, or monthly. Ammonit has developed several software programs that simplify communication with your data, such as automatic data mailing and ac-cessibility and archiving procedures. These programs are available on our website for your free download.

5.2.3 Power Supply

Ammonit measuring systems are fully self-contained; a connection to the local power network is not required.

The entire measuring system can be reliably powered with a 10 or 20W solar panel. In some cases, larger solar panels may be required, such as with the application of a larger number of sensors when a satellite communication system is applied or in cold climates requiring sensor heating.

5.2.4 Surge Protection

Surge protection prevents lightning and surges from disrupting DC and AC low vol-tage supplies to the measuring system equipment. The device, a special cable clamp, protects data and signal lines and power cables. The surge protection cable clamp is installed within the steel cabinet instead of a regular unprotected cable clamp. We offer several types of surge protection devices, differing in their number of pins and their voltage levels. Although the installation of surge protection is not mandatory, we highly recommended it. Unprotected lightning can lead to a total breakdown of the measuring system. Surge protection is especially recommended at freestanding measuring stations or when aircraft obstacle lights are employed.

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10W, 20W, 50W

GSM/GPRS-Modem

Measuring wind



5.3.0 Sensors

5.3.1 Anemometers

Anemometers measure the horizontal wind speed (velocity), a crucial parameter for any wind site assessment. Cup anemometers are the standard type of anemome-ter. Ammonit anemometers are robust and resistant to turbulence and skew winds caused by masts and traverses. Each anemometer should be individually calibrated and given a certified calibration report, according to international standards (e.g. MEASNET). The number of anemometers applied at one mast can vary from a mini-mum of 2 sensors up to 6 sensors (using Ammonit data logger Meteo-32X). Most anemometers can be equipped with electronically regulated heating.

5.3.2 Ultrasonic Anemometers

Ultrasonic anemometers measure the horizontal components of wind speed and direction as well as the acoustic virtual temperature. However, because of their high power consumption, a connection to mains power supply is required and their application within a self-contained solar-powered measurement system is not fea-sible. Ultrasonics offer excellent performance ratings at well-powered measuring stations. At present they are not accredited according to the internationally valid MEASNET regulations. Ultrasonic anemometers are typically applied for wind park monitoring, at wind turbines and on offshore projects. Most can be fitted with elec-tronically regulated heating.

5.3.3 Wind Vanes

Wind vanes determine wind direction. A detailed understanding of the wind direction on a site enables the best possible positioning of wind turbines. Ammonit wind vanes offer an excellent and fine resolution of 1º, despite their very low energy consumption. It is essential that a wind vane covers a 360º radius without a north gap. Cheaper wind vanes often have a considerable north gap, as well as a limited life span, due to lower quality internal electromechanical fittings. Both analog or potentiometric wind vanes are available from Ammonit and most can be equipped with electronically regulated heating. To avoid obstruction, wind vanes should be located at the top of the tower and at least 1.5m below the top anemometer.

5.3.4 Propeller Anemometers

Propeller anemometers are applied if a site is suitable according to IEC 1, 2 or 3 clas-sifications. They can also be helpful for wind park monitoring by showing how the turbines react to airflow. A propeller anemometer utilizes a fast-response helicoid propeller and high-quality tach-generator transducer to produce a DC voltage that is linearly proportional to air velocity. Airflow from any direction may be measured, but the propeller responds only to the component of the airflow that is parallel to its axis of rotation. Off-axis response closely approximates a cosine curve with appropriate polarity; with perpendicular air flow, the propeller does not rotate. The output signal of propeller anemometers is suitable for a wide range of signal translators and data logging devices.

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Measuring wind



5.3.0 Sensors

5.3.5 Humidity-Temperature Sensors

Thermal sensors measure the air temperature, while humidity sensors measure the air humidity; they are often applied in combination to reduce costs. The calculation of air humidity does not directly influence a wind site assessment, but knowing this pa-rameter helps assessing the potential for ice build-up at the measuring location. The temperature sensor should always be mounted at a height of at least 10m to ensure sufficient distance from heat radiating from the earth.

5.3.6 Barometric Pressure Sensors

Barometric pressure sensors (barometers) measure the air pressure. Air pressure and air temperature should be evaluated for an accurate wind site assessment, how-ever, because this data could be obtained from nearby weather stations, air pressure sensors are not considered an essential part of a measuring system. Even so, baro-metric pressure sensors are worth considering. Taking into account the duration of measuring campaigns (at least 12 months), savings made on the expense of supply, analysis and incorporation of external data could be substantial. Generally we would recommend the inclusion of an air pressure sensor within the weatherproof steel cabinet for all measuring systems.

5.3.7 Precipitation Sensors

Precipitation sensors are designed to measure the height, quantity and the intensity of precipitation striking the earth’s surface. Precipitation is best measured with a tipping bucket device. Precipitation, collected over a surface of 200cm2, is conducted through an inflow sieve into a tipping bucket. When the bucket has collected 2 sqm = 0.1mm of precipitation, it tips over. This measurement principle is based on the Guide to Meteorological Instruments No 8, WMO. Precipitation sensors are typically applied for meteorological assessments. This robust device can be equipped with heating when appropriate for the climatic region.

5.3.8 Pyranometers

Pyranometers (global radiation sensors) measure global radiation. They are also used as reference instruments due to their measurement precision. The utilisation of this sensor is crucial for measurements with solar-powered applications. It is advi-sable to use only classified and calibrated pyranometers to ensure accurate results. Adjustable feet and a level allow easy horizontal adjustment.

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Measuring wind



5.4.0 Additional Components

5.4.1 Obstacle Lights

Met masts increasingly reach heights that require the application of obstacle lights to protect air traffic. The national regulations as to when and how obstacle lights are to be installed differ largely from country to country.

Ammonit has designed a solar-powered warning light system that is controlled and monitored by our data logger. Obstacle lights are usually maintenance-free for up to 5 years. We would strongly recommend the application of surge protection systems when obstacle lights are installed.

5.4.2 Connection Cables

Appropriate high-quality cables are essential to connect the sensors with the data logger. Corrupted measurement data can occur when connection cables are too long or when the cable resistant is compensated in the supply. Furthermore it is crucial that cables are carefully installed within the lattice mast to avoid cable damage.

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Measuring wind



5.5.0 Software

Ammonit develops two different types of software: 5.5.1 Firmware for the data loggers This software is essential to run the data loggers and will be delivered with every new data logger. Our firmware is regularly updated and can be down loaded from our website (support and software section).

5.5.2 Communication services programs (free of charge) We provide additional software programs to simplify communication between measuring stations and PC/laptop. All software programs are available for free on our website (support and software section) Our software programs offer a variety of functions - Firmware: Configuration of measuring channels, status monitoring of firmware version

Long-term data storage and intercorrelation of wind direction values, e.g. north gap

Setting up of SMR-recorder and RS-485 interface options

Generation of wind statistics for energy forecasts and warnings, plausibility checks of measurement values

Configuration of GSM/GPRS or satellite connection.

- AmmonitMail: Automated collection and archiving of measurement data

- AmmonitOR: Ammonit Online Report - online platform to access, manage, monitor and visualise wind measurement data online, around the clock. To learn more visit our website on: http://www.ammonit.com/en/ammonitor-online-platform

AmmonitConnect

AmmonitMail

AmmonitOR

AmmonitCALLaLOG

AmmonitALWIN

AmmonitConnect

AmmonitMail

AmmonitOR

AmmonitCALLaLOG

AmmonitALWIN

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AmmonitOR

AmmonitAlwin

AmmonitOR: visualise your wind data in diagrams or schedules of your choice.

Measuring wind


The evaluation of wind speed is crucial for wind site assessments. At least two 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 is often installed slightly below the top anemometer. At least one additional anemometer should be mounted at a 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 posi-tioned just below the top anemometer – at a minimum distance of 1.5m. Depending on the tower height, 2 or 3 additional wind vanes can be integrated into the system on lower levels. 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 at a height of 10m and protected with a weather radiation shield. 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. Ammonit data loggers are safely housed 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 vandal-ism. The steel cabinet and the solar panel should be mounted on the tower at a height of approx. 5.5 m, protecting it from thieves and vandalism, while allowing for mainte-nance and servicing access.

Finally the obstacle lights should be mounted at the top of the tower. The regula-tions on installation of obstacle lights vary largely from country to country. With the increasing height of masts & towers, the application of obstacle lights is becoming more and more the rule.

6) How to install a measurement system? • The measuring system should always be installed by professionals

Photo: Windhunter (www.windhunter.com)

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The most important rules are: 7.1.0 Installation

7.1.1 Most components will be mounted before the tower is erected and still lying on the ground. It is essential that the mast is absolutely vertical after the installation. After erection the orientation of the lower tower part should be checked from all sides for any bends. A suitable measuring tool (e.g. an inclinometer) will help. Small deviations can be detected with a final eye-check. It is advisable to mount electronic tilt sensors to the top end of the tower. They are not only helpful for the installation process, but help monitoring the inclination of the tower at all times. With the application of remote data transmission, the station operator will send out warning signals of any impending dangers.

7.1.2 All wind sensors must be fitted absolutely vertically. Even small deviations lead to skew winds resulting in wrong measurement data. Typically sensors are fitted to the tower before it is erected.

7.1.3 Traverses keep the sensors as far away as possible from shaded or turbulent areas. Traverses must be kept free of any vibration. Vibration can not only influence the measurement data, but also lead to bearing damage of the transmitter.

7.1.4 The length of a traverse should be at least 7 times the tower diameter of a cylindrical tower. With a lattice tower structure, a traverse length of approx. 5 times the tower width (approx. 1 - 1.5m) will be sufficient.

7.2.0 Sensors

7.2.1 The top anemometer should be placed centrally at the top of the tower. It must be exposed to wind coming from all directions without any obstruction. The top piece (minimum 0.5m) of the pillar should have a diameter simi- lar to that of the shaft of the transmitter. This should relate to the set-up used during calibration of the wind sensor in the wind channel. A thin lightning rod should be located right next to the top anemometer.

7.2.2 Any other anemometers should be located at lower height on the tower. Anemometers should always be located in regular distances from each other to allow for accurate interpolation. Lower level anemometers should be fitted onto a vertical pipe attached to a traverse, ensuring that the anemometer remains 30 - 60cm above the traverse. A traverse can interfere with measurement if positioned too close to the anemometer. The fitting should lie at a 45˚angle to the main wind direction.

7) Setting up the mast / tower • The correct setting up of the mast is crucial for a successful wind measurement

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Measuring wind


7) Setting up the mast / tower • The correct setting up of the mast is crucial for a successful wind measurement

7.2.3 The lightning rod (approx. 2cm diameter) must be kept at a distance of at least 50cm from the top anemometer, and must be vibration free. The lightning rod should reach slightly beyond the anemometer, by an imagined 60˚ angle from it.

7.2.4 The wind vane should be located as high as possible on the tower, but always 1.5m below the top anemometer to avoid obstruction. The wind vane’s traverse should be fitted as described above. A compass, a good map and a small scale are required in order to locate a prominent fixed point on the horizon during the installation process. A good angle measurement tool will also be helpful.

7.3.0 Cable transfer, steel cabinet and additional components

7.3.1 The best location for all connection cables is within the tower to avoid damage. The weight of any free hanging cables over a length of 50m should be secured. If it is not possible to locate a cable within the tower, it should be fixed onto the tower and traverses at intervals of 1m. It is essential to avoid dangling cables and cables coming into contact with sharp edges. Even a little stress on a cable can affect the transfer of measurement data.

7.3.2 The following general rule can be applied for all other components of a measuring station (steel cabinet, data logger, other sensors, solar power supply and data transfer systems): Locate the components as high as possible to prevent theft and vandalism, but still within reach for access and maintenance. A height of approx. 6m makes access for thieves more difficult, but still relatively practical for legitimate access. Solar panels and GSM an- tennas are especially at risk; a smaller 10W solar panel is less attractive to thieves than a larger 150W solar panel.

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• Wrong choice of anemometers

• Uncalibrated anemometers

• Wrong length of traverses

• Shelter effect /shading

• Wrong measurement of heights

8.1 Wrong choice of anemometers

Anemometers vary largely. Choosing the right anemometer for your specific conditions is crucial. Sensors with small cups and edgy bodies can be susceptible for skew winds and turbulences caused by tower and traverses.

8.2 Uncalibrated anemometers

The measurement accuracy of uncalibrated anemometers will not be sufficient, even if they are of a high quality standard. International wind measure- standards (e.g. MEASNET) are only met with calibrated anemometers providing a certified calibration report.

8.3 Wrong length of traverses

Towers and traverses cause shading and turbulences that influence the wind measurement. Wind sensors should therefore always be installed at a distance of 30 - 60cm from the traverse; mounting the sensor onto a small vertical pipe is sufficient to create this distance. The length of traverses should be approx. 1 - 1.5m. Vibration should be avoided at any times.

8.4 Shelter effect /shading

Wind must reach the sensors without obstruction from any direction; data of obstructed anemometers is not usable. Incorrect installation of wind vanes and anemometers can mutually affect each other. Obstructions such as buildings, trees and hills also cause shading and turbulence that affect the measuring system. A shaded anemometer has a catastrophic impact on the accuracy of the wind measurement data.

8.5 Wrong measurement of heights

It is essential to locate the top anemometer as close as possible on the level of the hub height of the potential turbine(s). Extrapolating the wind speed at hub height during calculation causes uncertainties. This uncertainty in the calculation results accelerates with inaccurate measurement. (The calculation of the level profile of a site is difficult; the applied logarithmic formula is only a convergence.)

8) Avoidable mistakes

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Measuring wind


9.1 Rayleigh Method

Evaluates the Rayleigh cumulative probability distribution function. This is a direct variational method, in which the minimum of a functional defi ned 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 fl uctuations at the site when only the average wind speed of the region is known. The Rayleigh distribution uses a fi xed general formula and refl ects regional fea- tures only to a certain extent.

9.2 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.

9.3 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 classifi cation:

A) Evaluation of wind speed on two levels

B) 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).

9) Data recording

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

• Rayleigh Method

• Weibull Method

• Classifying measurement data

• Transformation to hub height

• Correlation of long-term data

• Wind rose

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9.4 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.

9.5 Correlation of long-term data

The measurement data should be collected over a period of at least 1 year to ensure that seasonal fl uctuations 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.

9.6 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.

9) Data recording 30 32 360 10 28 347 101 101 101 2952 2954 2952 927 927 927 400 520 290 50 61 61 63 83 39 60 82 35 95 95 93 2956 2956 926 926 926 446 670 230 94 61 61 126 126 126 97 97 97 96 96 96 9 355 358 14 338 97 99 97 2952 2952 2951 927 927 927 630 790 450 81 61 61 126 127 126 97 97 97 96 96 96 11 19 358 1 15 348 101 101 101 2954 2955 2954 927 927 342 500 200 60 61 61 61 130 160 128 97 97 97 96 96 96 18 26 12 14 19 5 101 100 2955 2955 295 927 927 927 452560 330 63 61 62 61 131 132 130 97 97 97 96 96 96 30 32 360 10 28 347 101 101 101 2952 2954 2952 927 927 927 400 520 290 50 61 61 63 83 39 60 82 35 95 95 93 2956 2956 926 926 926 446 670 230 94 61 61 126 126 126 97 97 97 96 96 96 9 355 358 14 338 97 99 97 2952 2952 2951 927 927 927 630 790 450 81 61 61 126 127 126 97 97 97 96 96 96 11 19 358 30 32 360 10 28 347 101 101 101 2952 2954 2952 927 927 927 400 520 290 50 61 61 63 83 39 60 82 35 95 95 93 2956 2956 926 926 926 446 670 230 94 61 61 126 126 126 97 97 97 96

10 28 347 101 101 10 28 347 101 101 101 2952 2954 2952 101 2952 2954 2952 927 927 927 400 520 927 927 927 400 520 290 50 61 61 63 83 290 50 61 61 63 83

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Measuring wind


Ammonit Measurement GmbH Wrangelstrasse 100 D-10997 Berlin +49 30 6003188-0 +49 30 6003188-10 [email protected]. ammonit.com

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Dear Reader,

The information as shown in our wind measurement brochure summarizes a number of important aspects relating to wind measurement for wind power generation. The information is based upon years of practical work and it has been discussed in advance with experts in this field. Nevertheless, this study can deviate in some respects from current national and international guidelines and other publications, in particular where these are contradictory or seem to be inadequate.

It is therefore essential to take in advice of professional wind consultants once a wind measurement system is to be put in place.

Disclaimer: Ammonit disclaims any and all promises, presentations and warranties with respect to the informa-tion furnished to the customer. Ammonit assumes no liability for errors or omissions in this text, or for damages arising from the use of results or conclusions generated by it. This publication may not be reproduced or copied or transferred by electronic or mechanical media without prior written approval by Ammonit.

We hope that our information was helpful to you. We will greatly appreciate your feedback and suggestions.

Best regards

Your Ammonit Team

10) Notes

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En Ammonit Windinfo Knowhow

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