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Anemometers Assignment

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Anemometers Assignment

  1. 1. D.I.T. Kevin St. Anemometry 1 | P a g e Department of Electrical Services Engineering Aneomometry Course Code / Year DT 018 Module Electrical services engineering and energy management. Lecturer Mr. Derek Kearney. Student Name Wayne Doyle, Mark Stewart, Eamon Carroll Paul Derwin Student Number D06112499,D05110728,D06111916, D07114349. SubmissionDate 29th November2011
  2. 2. D.I.T. Kevin St. Anemometry 2 | P a g e Word Count 9494 Declaration I hereby certify that the material, which is submitted in this assignment/project, is entirely my own work and has not been submitted for any academic assessment other than as part fulfilment of the assessment procedures for the program Bachelor of Science in Electrical Services and Energy Management (BSc (Hons)) (DT 018). Signature of student:…………….……… Date:…………………………
  3. 3. D.I.T. Kevin St. Anemometry 3 | P a g e
  4. 4. D.I.T. Kevin St. Anemometry 4 | P a g e Table of Contents: Page No. Declaration ......................................................................................................................2 Table of Contents:..........................................................................................................4 Table of Figures:.............................................................................................................6 1.0 Introduction..........................................................................................................7 1.1 Wind. ....................................................................................................................7 2.0 Anemometry........................................................................................................9 3.0 Rotational anemometers .................................................................................10 3.1 Cup anemometer .............................................................................................10 3.2 Propeller anemometer.....................................................................................13 4.0 Phase shift anemometers ...............................................................................14 4.1 LIDAR (Light Detection and Ranging) ..........................................................14 4.2 Sonic anemometers.........................................................................................17 5.0 Pressure Type Anemometers.........................................................................19 5.1 Pressure plate...................................................................................................19 5.2 Pressure Tube Anemometer (Pitot tubes) ...................................................20 6.0 International standards....................................................................................22 6.1 Cup & Propeller Anemometer Test Procedures..........................................22 6.1.1 IEC 61400-12-1.....................................................................................22 6.1.2 ASTM D5096-02...................................................................................24 6.2 Phase shift anemometers...............................................................................25 6.2.1 Sonic Anemometer Test Standards...................................................26 6.2.2 ASTM D 6011-96..................................................................................26
  5. 5. D.I.T. Kevin St. Anemometry 5 | P a g e 6.2.3 ISO 16622..............................................................................................28 6.2.4 IEC standard 61400 .............................................................................30 6.3 Environmental conditions and inspections...................................................31 6.4 Standards Conclusion .....................................................................................31 7.0 Comparison of the performance ....................................................................32 7.1 Over speeding: .................................................................................................32 7.1.1 Dynamic filtering in Turbulent Winds .................................................33 7.1.2 Mechanical operation:..........................................................................33 7.2 Phase shift anemometers–Ultra Sonic anemometers, SODAR and LIDAR anemometers. .............................................................................................34 7.2.1 2.1Ultra sonic anemometers...............................................................34 7.2.2 SODAR and LIDAR..............................................................................35 7.3 Thermoelectric anemometers ........................................................................36 7.3.1 Hot wire anemometers.........................................................................36 8.0 The development and effect of turbulence in relation to wind measurement ....................................................................................................38 8.1 The Importance of Accurate Wind Measurement .......................................38 8.1.1 What is Turbulence ..............................................................................38 8.2 Turbulence Effect on Wind Measurement Devices ....................................40 8.2.1 Cup Anemometer..................................................................................40 8.2.2 Propeller Type.......................................................................................41 8.2.3 Sonic Anemometer...............................................................................42 8.2.4 SODAR...................................................................................................43 8.2.5 LIDAR Anemometer .............................................................................43 8.2.6 Cup vs LIDAR........................................................................................44 9.0 References ........................................................................................................46
  6. 6. D.I.T. Kevin St. Anemometry 6 | P a g e Table of Figures: Page No. Fig 1 (Anon., 2011) ........................................................................................................7 Fig 2 (Gripe, 2004) .......................................................................................................10 Fig 3 (Dines, 1911 ) .....................................................................................................11 Fig 4 (Anon., 2010) ......................................................................................................13 Fig 5 (wright, 2006) ......................................................................................................14 Fig 6 (technology, 2011) .............................................................................................16 Fig 7 (Johnson, 2001)..................................................................................................18 Fig 8 IEC performance requirements........................................................................23 Fig 9 (Brazier, 1914) ....................................................................................................23 Fig 10 ASTM D 5096-02 Calibration Test Speed Protocol, ascending and descending speeds (Coquilla, 2009).........................................................................25 Fig 11 wind tunnel requirements for ASTM D 6011-96 sonic sensor testing (Adam Havner, 2008) ..................................................................................................27 Fig 12 Otech Eng. Wind Tunnel Laboratories (ASTM Inernational, 1996) .........29 Fig 13(Webb, 2007) ....................................................................................................38 Fig 14(Horst, 2007) .....................................................................................................39 Fig 15(Horst, 2007) .....................................................................................................39
  7. 7. D.I.T. Kevin St. Anemometry 7 | P a g e 1.0 Introduction Measurement of wind speed is very important to people such as pilots, sailors, and farmers and engineers. Accurate information about wind speed is important in determining the best sites for wind turbines. Wind speeds must also be measured by those concerned about dispersion of airborne pollutants. Wind speeds are measured in a wide variety of ways, ranging from simple go-no go tests to the most sophisticated electronic systems. The variability of the wind makes accurate measurements difficult, so rather expensive equipment is often required. 1.1 Wind. What is wind? Wind is created by different in pressure. When pressure differential exists the air is accelerated from a higher to a lower pressure. So when the sun strikes the earth it heats the soil near the surface. In turn the soil warms up the surrounding air .With warm air being less dense than cool air,just like a helium balloon it rises. Cool air then rushes in to take its place and is then heated its self and continues the cycle. The cycle is continues just like a crank shaft in a car and as long as the solar engine is driving . (Gripe, 2004) Due to it rotating sphere the error will be deflected by the Cariolis Effect except exactly on the equator. Globally, there are two major driving factors of large-scale winds the atmospheric circulation, and the differential heat between the equator and a pulse and the rotation of the plants. The wind rises from the moves north and south in the higher layers of the atmosphere. At around 30° in both hemispheres the cariolis force prevents the air from moving much further. At this latitude there is a high pressure area, as the air begins sinking down again. As the wind rises from the equator there will be a low-pressure area close to ground level attracting wins from the north and Fig 1(Anon., 2011)
  8. 8. D.I.T. Kevin St. Anemometry 8 | P a g e south. Then at the north and South Pole there will be a high pressure due to the cooling of the air at the poles.
  9. 9. D.I.T. Kevin St. Anemometry 9 | P a g e 2.0 Anemometry An anemometer is device fundamental used for measuring wind speed and is a common weather station instrument .The term its self is derived for the Greek word anemo which means wind. The first known descriptions seam to show that an inventor by the name of Leon Battista gives a description of an anemometer in the year 1450. (W.E. Knowles Middleton, 1968) Anemometers can be broken down mainly into two categories those that measure wind velocity an those that measure wind pressure these are close linked (W.E. Knowles Middleton, 1968) As stated above Anemometers can be segregated into four categories according to their operational and physical characteristics. 1. Rotational anemometers – cup anemometers, and propeller anemometers. 2. Phase shift anemometers – ultra sonic anemometers, and laser Doppler anemometers. 3. Pressure type anemometers – pressure tube anemometers, pressure plate anemometers, and sphere anemometers. 4. Thermoelectric anemometers – hot wire anemometers, and hot plate anemometers. (Kearney, 2011) This document will also review international standards from the International Organization for Standardization (IOS), International Electro technical Commission (IEC) and American Society for Testing Materials (ASTM). whom govern anemometry calibration and wind measurement. The purpose of these standards is to provide the methodology that will ensure accuracy and reproducibility in the measurement and analysis of anemometers. They are intended to aid manufacturers, installers and purchasers of anemometers.
  10. 10. D.I.T. Kevin St. Anemometry 10 | P a g e 3.0 Rotational anemometers 3.1 Cup anemometer Cup anemometers are one off the most common types of anemometer used today worldwide. It is a very simple design and operation. The cup anemometer was invited the Irish astronomer Thomas T. R. Robinson of Armagh Observatory in 1846. Miditon (1969) identified that the first anemometers had four hemispherical cups each mounted on a horizontal arms which are then mounted on equal angles to each other. The unit was then mounted on a vertical access.” The wind would then flow past any the cups in any of the horizontal access inducing a radial motion which was proportional to the wind velocity therefore allowing for counting the cup turns over a set time which allow him to then produce an average wind velocity over a rage of wind velocities (Spilhaus, 1953) (Dines, 1911 ).Around the end of the 1920s much research and experimenting was carried out in relation the number of cups and also the length of the arms. When the incident was designed the anemometer wrongly claimed that no matter how big the cups or how long the arms were cups would always move warned that the speed of the wind. This is apparently confirmed by some early independent experiments but it turned out to be very further from the truth Brazier and Paterson found that a three arm cup rotor is optimal with respect to sensitivity and suppression of the unevenness in the rotation (“wobbling”).A modern cup is shown below in figure 3. (Brazier, 1914) Paterson and Braziar also demonstrated the literate is better on a shorter diameter. (Brazier, 1914) (Kristensen, 2005) Patrson also determined that optimum Fig 2(Gripe, 2004)
  11. 11. D.I.T. Kevin St. Anemometry 11 | P a g e number of cups is three and can be proven in his equation 𝑓 = 𝑢 𝑟𝑠 where r=radious of the cup and U= angular velocity and S= the cup rotor. The wind would then flow past any one of the cups in of the horizontal access inducing a radial motion which was proportional to the wind velocity therefore allowing for counting the cup turns over a set time which allow him to then produce an average wind velocity over a rage of wind velocities (Spilhaus, 1953).A modern cup is shown in figure 3. The cup anemometers are probably the most common type anemometer currently being used today; it can also be fitted with a vain to determine the mean of the horizontal wind velocity components. It has proven over time that such combinations sturdy, reliable and robust Instrumentation package. They are simple to operate and can be used in a wide range of applications in weather stations at airports wind- farms, solar farms and can also be used on large structures such as high-rise buildings and bridges which are under construction or the equipment being used to construct them i.e. tower cranes. The behaviour of a couple anemometers in turbulent wind can cause systematic errors in the measurement of the wind speed. One of the main errors which have been identified is a so-called over speeding. This is gives an increased wind speed measurement or velocity than the actual wind speeds. This is due to the anemometer responding quickly to an increased wind speed and responding much slower to decrease in winds in the wind speed, this allows the cup and anemometer to continue rotating at a faster velocity than the actual wind speed it is measuring. “Based on an equation of the dynamic response of cup anemometer and the contrast observations between cup anemometer and sonic anemometer, it is shown that the error caused by the over speed of cup anemometer is between 1% and 3 % and data processing error (DP error) Fig 3(Dines, 1911 )
  12. 12. D.I.T. Kevin St. Anemometry 12 | P a g e associated with cup anemometer is the most important one because of distinct calculated methods (vector mean and scalar mean). After the reduction for DP error, the rational wind speed values could be obtained”. (Kristensen, 2005) The following equation is used to demonstrate over speeding as a percentage E = I². (1.8d – 1.4) Where: E is the percentage of over speeding error I is the turbulence intensity d is the distance constant for the anemometer(Energy, 2008) In one form or another cup anemometers are often used with a wind vane for determining the mean of the horizontal wind velocity component. Such combinations are sturdy and reliable instrument packages. They are easy to operate and are used at weather stations, airports, wind farms, and sites where large structures, such as bridges, are under construction. Cup anemometers are relatively in expensive to manufacture and are suitable for many wind measurement applications. The three types of cup anemometer has now become the standard for this type of anemometer. The cup anemometer is a mechanical device or instrument with a long proven reliable track record which is easy to find certification for the use in contractual applications. Researchers, Educational Institutes, Meteorologists and many of the general public are familiar with cup anemometers. Although they are one of the oldest ways of measuring wind speed you will find that most people will have no problem accepting easily understood mechanical designs. Cup anemometer designs vary considerably according to the particular application. Several Cup Anemometer Manufacturers offer different models which include low cost portable wind speed measurement, homeowner anemometers , industrial wind speed checking and high specification models for scientific research and weather analysing High specification cup anemometers will normally be designed to withstand changing conditions and more solid construction materials and overall design will not only increase the lifetime of the device but
  13. 13. D.I.T. Kevin St. Anemometry 13 | P a g e also render it more valuable in terms of accuracy and repeatability in conditions that are subject to change. (Tong, 2010) 3.2 Propelleranemometer Propeller anemometer and sometimes also now as wind mill anemometers or aero vanes usual consist of four bladed propeller. The blades are constructed of extremely light weight Martials such as carbon fiber, aluminum or thermo setting plastics. The device functions predominantly in on the lift force. With airflow parallel to its axis the propeller blades experience a lift force which turns the propeller at a velocity or speed proportional to the winds speed or velocity. In this case the wind turns a propeller instead of cups. Small propeller anemometers are used to check the velocity and direction of the turbulence. A propeller anemometer is used where the direction of the wind changes, as it does out-of-doors. The propeller blades indicate the correct wind velocity only when they face the flow of the wind. The vane, which also shows the direction of the wind, keeps the propeller properly aligned. Propeller types generally have a driven shaft with an ac or dc generator integrated to the device. The types of propeller used for wind applications have a fast response time and produce a linear output for the changing speeds. These devices can be as simple as a magnet on the rotating shaft passing a coil inducing a voltage which can be represented as a wind speed depending how many times the magnet passes the coil over a set time. Figure 1 Fig 4(Anon., 2010)
  14. 14. D.I.T. Kevin St. Anemometry 14 | P a g e 4.0 Phase shift anemometers 4.1 LIDAR (Light Detection and Ranging) LIDAR (Light Detection and Ranging) was first demonstrated in the United States of America in the late 1960 early 1970.The widespread use and development of the technology has been hampered over the years due to the complexity and the high cost associated with LIDAR systems. “However the recent year’s developments of new LIDAR systems based on fibre optic components which have been proven in the telecommunication’s industry seem to show a large cost reduction and improvement in the overall design and also an improved compactness”.(wright, 2006) LIDAR is a remote sensing technology analogous radar .The range (distance) to an object is determined by measuring the time delay between transmission and reflection of a signal. LIDAR is used for remote sensing which will give “temporal and spatial resolution” (wright, 2006).Doppler LIDAR works by the emitting light energy and backscattered from the microscopic particulates or aerosols being transmitted by the wind such as dust pollen and water molecules. This difference in the frequency being emitted and the frequency being received back is called the Doppler shift which in turn is proportional to the velocity of the particles. The need for atmospheric LIDAR to meet eye safety standards limits both the intensity of the light energy and its highest frequency (shortest wavelength) that can be focused and radiated. When these criteria are combined, LIDARs used for atmospheric measurements use midrange infrared light generated by lasers operating with wavelengths in the 1.5 to 2 μm range like High resolution Fig 5(wright, 2006)
  15. 15. D.I.T. Kevin St. Anemometry 15 | P a g e Doppler LIDAR (HRDL).Modern equipment is highly sensitive, they are fitted with electronic amplifiers which have allowed LIDAR technics like HRDL to measure wind fields to a distance of several kilometres even when the air is clean and it contains a very low number of particles that result in backscatter (wright, 2006). Generally LIDAR is used to measure atmospheric wind in two general forms continues wave (CW) or pulsed. CW LIDAR continuously emits its light energy through its optics which focus the beam in a predetermined radios ahead of the instrument. Within the focus of radios, the backscatter energy is collected and the velocity of the wind is measured from using the Doppler shift. (N.D. Kelley, 2007) CW wind-finding LIDARs typically perform a conical scan sequence in which one or more 360º scans are performed at a fixed elevation angle and focus position. (N.D. Kelley, 2007) CW LIDARS only produce measurements of the mean horizontal wind vector at a few heights. But as is highlighted in by the national renewable energy laboratory it has proven to be very accurate when some optimal smoothing is applied. Pulse LIDAR emits regular spaced emissions of highly regulated light energy which is spaced apart at regular emissions of highly collimated light energy for a specific and predetermined time (pulse length) similar to Doppler SODAR. The return signals then isolated to a period of time which corresponds to a specific segment of radial distance along a beam called “the range gate” (N.D. Kelley, 2007) The backscattered signals which are digital extracted from the gate are then processed to “derive the line of sight (LOS) or radial velocities along the path of the LIDAR(N.D. Kelley, 2007) Pulsed LIDAR typically operates with a pulse repetition or sampling frequency ranging from 200 to 1000 pulses per second. The resulting vertical wind profile derived from a pulsed LIDAR using a conical scan incorporates a much greater vertical resolution than an equivalent CW profile. (N.D. Kelley, 2007)Sound propagation in the atmosphere has been studied for at least 200 years, but it has only been in the last 50 years that acoustic scattering has been used as a means to study the structure of the lower atmosphere. In the United States during World War II, acoustic backscatter in the atmosphere was used to examine low-level temperature inversions as they affected propagation in microwave communication links. Beginning in the late 1960's and early 1970's, scientists at
  16. 16. D.I.T. Kevin St. Anemometry 16 | P a g e the U.S. National Oceanic and Atmospheric Administration demonstrated the practical feasibility of using acoustic sounders to measure winds in the atmosphere by means of the Doppler shift.(technology, 2011) During the 1980s there was a similar development of Doppler SODAR systems by other U.S companies. The Xondar SODAR system which was capable of making wind profile and turbulence measurement. Another company was developing their pulsed Doppler SODAR system, this system was an Invisible Tower system.(technology, 2011)Like its cousin technologies, radar (radio detection and ranging) and LIDAR (light detection and ranging), SODAR uses waves for measurement; and, as the name suggests, it uses sound waves. Over the past few decades SODAR has been typically used to research atmospheric conditions which would include insect migration patterns and pollution. Commercial SODAR in today's wind industry are ground-based systems that send up focused sonic beams in rapid succession, producing an audible chirp. Wind turbulence sends a portion of the sound back towards the ground as an echo. By precisely measuring the frequency and time delay of the echo (aka the Doppler shift), the SODAR device using trigonometry measures the wind speed and direction at any height up to around 200 meters. This is why the SODAR are also called Doppler SODAR.(secondwind, 2011)Commercial SODAR systems and other remote sensing systems were developed and commercialized for the wind industry to help reduce to overall inaccuracy in measuring wind velocity below the turbines hub height. SODAR are now used as a resource assessment by wind farm operators either in conjunction with a microwave towers or as a standalone SODAR is important to wind meteorologists because it is ideally suited to measuring the lower bound below the sweep of radar and above the ground. This is where wind matters most to wind turbines as shown below in figure 6.SODAR have gained commercial acceptance in wind development applications because they measure the wind at higher Fig 6(technology, 2011)
  17. 17. D.I.T. Kevin St. Anemometry 17 | P a g e heights than is practical using meteorological towers.Sonic Detection and ranging SODAR systems are classified as remote sensing as they do not have to place a sensor at the location where the measurement is required They are used to remotely measure the vertical turbulence structure turbulence structure and the profile of the lower layer of the atmosphere. SODAR systems operate in a similar manner to radar (radio detection and ranging) systems except that sound waves rather than radio waves are used for detecting. (secondwind, 2011) Because of their ability to be easily transported, SODAR systems are also well suited to more specialized wind development application such as wind prospecting (also known as scouting or site finding). Wind shear measurement, because of their ability to measure wind at many different heights, SODAR is particularly valuable to when looking for information in relation the amount and the type wind shear and veer. Because of the extreme difference in the velocity of the wind and the constant changing of direction has been identified as a substantial factor in the increase in wear and tear of wind turbines. The information gained to understand the wind shear on a particular site helps to identify the suitably of the site for a wind turbine.(secondwind, 2011) 4.2 Sonic anemometers Some articles show that sonic anemometers were developed first in the 1970s others state that it was developed by a Dr Andreas Pflitsch in 1994. And in a report written by the department of metrology Indian the sonic anemometer was first developed in the 1950s with some confusion over its origins but there is no confusion over the usefulness of this instalment its self. But whichever year it was developed there is no doubt in the benefits of the device. Wind velocity is measured based on the time it takes for sonic pulses to travel between a pair of transducers.Using the
  18. 18. D.I.T. Kevin St. Anemometry 18 | P a g e combined measurements from several transducer pairs mount on the end of arms mounted perpendicular to each other, we can then yield a measurement of air flow in 1, 2 or 3 dimensions. The technic used are also based on the principles of Doppler shift. The transducers emit acoustic signals which travel up and down through the air and because the speed of the sound moving through still air is different from that through still air. A typical path length between the transducers is 10 - 20cms and this gives us the special resolution. A temporal resolution of 20 Hz or better can be achieved which makes this technology well suited for the purposes of turbulence measurements.(Mathew, 2009) One of the great advantages of ultrasonic anemometers is their lack of moving parts which makes them appropriate for usage in Automatic Weather Stations and in harsh environments. The lack of moving parts makes them appropriate for use in automated weather stations in the wind turbine industry and renewable energy industry. Some limitations of sonic anemometers are when they are exposed snowy conditions the anemometer does not work. Fig 7(Johnson, 2001)
  19. 19. D.I.T. Kevin St. Anemometry 19 | P a g e 5.0 Pressure Type Anemometers 5.1 Pressureplate The pressure plate or normal plate anemometer consists of a swinging plate which is held at the end of a horizontal arm. The arm is attached to a shaft which it is free to rotate around. A wind vane ensures that the pressure plate is always facing perpendicular to the wind flow. The wind blows against the plate and the distance that the arm travels is directly calibrated to the force of the wind. It operates on the principle that the force of moving air on a plate held normal to the wind is Fw = cAρ u2/2 Where Ais the area of the plate ρis the density of air uis the wind speed cis a constant depending on the size and shape of the plate but not greatly different from unity. This force produced is then used to drive a recording device directly or as input to a mechanical to electrical transducer. Research of gust studies is the main application of this type of anemometer because of its very short response time. Gusts with a duration of as little as 10ms can be examined with this type of anemometer. A pressure plate anemometer is however not very sensitive to low winds and is likely only to give an estimate of wind speed and is also not very responsive to the turbulence dynamic of wind. A pressure plate anemometer is useful for spraying agricultural chemical or for estimating wind chills. (Johnson, 2001)
  20. 20. D.I.T. Kevin St. Anemometry 20 | P a g e 5.2 PressureTube Anemometer(Pitot tubes) Pitot tubes are commonly used to measure air flow in ventilation systems because they can be easily installed into ventilation systems through small ports in the duct work. Accuracy can be as high as +/- 2% if great care is taken when taking Pitot tube readings. The pressure tube anemometer measure the height of a liquid in a u tube which has one bent into a horizontal direction to face the wind and the other end would usually be open and facing up so the wind can pass over, this will create a vacuum due to the Bernoulli effect. The height of the liquid is usually measured with a float connected to a mechanical linkage to a recording device. A disadvantage of the pressure tube anemometer is that the moving parts can wear out. The liquid manometer can also be fragile. To get an accurate reading the Pitot tube must be faced into the wind stream. External weather conditions can have an effect on the performance of the pressure tube anemometer.
  21. 21. D.I.T. Kevin St. Anemometry 21 | P a g e
  22. 22. D.I.T. Kevin St. Anemometry 22 | P a g e 6.0 International standards 6.1 Cup & PropellerAnemometer TestProcedures 6.1.1 IEC 61400-12-1 The International Electro-technical Commission (IEC) is a global standards organization that consists of committees that review operational practices in electrical and electronic industry and research. The IEC also publishes international standards in collaboration with the International Organization for Standardization (ISO) (Coquilla, 2009). Cup and Propeller Anemometer test procedures are carried out to determine the transfer of the instruments voltage output to wind speed. According to the IEC, these procedures should be conducted in a uniform, horizontal and steady state flow of low turbulence levels these conditions are best achieved in a purpose built wind tunnel.  The anemometer shall be mounted on a stand similar to that used in the field in the wind tunnel.  Prior to every calibration the set up shall be verified by means of a comparative calibration of a reference anemometer.  The anemometer shall be run for five minutes prior to calibration to avoid the effect of large temperature variations on the mechanical friction of the anemometer bearings.  Calibration shall be carried out in wind speeds ranging between 4 and 16m/s at 1m/s interval, the sequence of speeds are as follows, 4, 6, 8, 10, 12, 14, 16, 15, 13, 11, 9, 7, and 5m/s. This rising and falling speed sequence is to identify any hysteresis during the test.  For data collection at the desired speed, the sampling frequency shall be at least 1 Hz at a sampling interval of 30 seconds (Coquilla, 2009).
  23. 23. D.I.T. Kevin St. Anemometry 23 | P a g e below lists the IEC performance requirements for a wind tunnel calibration facility. Wind characteristic tunnel Description Minimum requirement Blockage ratio Anemometer ratio plus mount frontal area to the total wind tunnel test section area Not to exceed 0.1 for open test sections, 0.05 for closed sections Flow uniformity Percent difference in wind speed within the test section volume Less than 0.2% in the longitudinal, transversal, and vertical directions Horizontal wind gradient Dynamic pressure differential at the area covered by the rotating cup anemometer Must be less than 0.2% Turbulence intensity Ratio between the wind speed standard deviation to the main speed Must be less than 2% Fig 8IEC performance requirements . It is noted by the IEC that it is important that the anemometer calibration test is carried out with a steady uniform horizontal flow across the cup or propeller anemometer, with no vertical or cross flow conditions. The wind tunnel section should be turbulence free. According to the IEC, cup anemometers are to be calibrated to a pilot-static tube system, which is also calibrated to an appropriate test speed (Coquilla, 2009).The IEC standard states the reference wind speed from a Pitot- static tube system is calculated using the differential pressure Fig 9(Brazier, 1914)
  24. 24. D.I.T. Kevin St. Anemometry 24 | P a g e measured at the inlet of the Pitot-static tube and also the measurements of the local conditions inside the wind tunnel test section. The IEC requires equipment used to measure pilot tubes to be traceable back to a standards authority. 6.1.2 ASTM D5096-02 ASTM D 5096-02 is a Standard Test Method for Determining the Performance of a Cup and Propeller Anemometer Test Procedures, which were published in 1990(Smith, 2009). The ASTM (American Society for Testing Materials) is a non-profit voluntary standards organization, which develops and publishes standards. The purpose of ASTM D5096-02 is to provide a method for calibration and performance characteristics for the cup and propeller anemometers. Some of the characteristics include starting threshold and off- axis response. Wind tunnel requirements for ASTM 5096-02 cup and propeller anemometer testing include some of the following criteria.  Anemometer front area must be less than 5% of the test section cross- section area of the wind tunnel testing equipment.  Turbulence must be less than 1% in the test area.  Wind speed-reading must maintain an accuracy of 0.1m/s.  Air density profile in test area must be less than 3% difference.  Flow uniformity in the test area must be constant to within 1% (Coquilla, 2009) 6.1.2.1Steps from ASTM calibration procedure 1. Install the anemometer at an angle of attack maintained within 0.50 2. Acquire wind tunnel speed and anemometer rotation rate along with test section environmental conditions. Each test will run between 30 and100 seconds. At each test speed collect data. 3. Calibration is carried out at two times the threshold (U0) of the anemometer and up to 0.5 times the max application speed (Umax). Incremental test speeds are defined in table 2 that shows ascending and descending speeds on the next page. They are 20 different test speeds.
  25. 25. D.I.T. Kevin St. Anemometry 25 | P a g e 4. Calculate and report wind speed residuals by determining a wind speed value for a range of anemometer rotation rates using linear transfer functions and then subtracting the predicted value from the measured wind speed. 5. Construction of new data beyond the range of measurement is not required therefore if the max test speed is 50% of Umax, then the transfer function only applies for that test speed. Ascending speeds Descending speeds 2 times U0 3 times U0 4 times U0 5 times U0 6 times U0 0.1times Umax 0.2 times Umax 0.3 times Umax 0.4 times Umax 0.5 times Umax 0.5 times Umax 0.4 times Umax 0.3 times Umax 0.2 times Umax 0.1 times Umax 6 times U0 5 times U0 4 times U0 3 times U0 2 times U0 Fig 10ASTM D 5096-02 Calibration Test Speed Protocol, ascending and descending speeds (Coquilla, 2009) 6.2 Phase shift anemometers Standards that are available for the performance testing of sonic anemometers are ASTM D 6011-96 and ISO 16622, which involves a test program that evaluates the three-dimensional characteristics of sonic anemometers. For sonic sensors used in research applications, where it may be necessary to map complex flows at high resolution, such detailed test procedures for the sonic instrument may be necessary. However, some of the most common industry
  26. 26. D.I.T. Kevin St. Anemometry 26 | P a g e applications for sonic anemometers generally only require a certain level of uncertainty in two dimensional wind speed and direction measurements such as on weather stations, airports, oceanic buoys, nuclear power plants, wind plants, and many others. Thus, a more practical test protocol, extracted from published test standards, may be applicable for verifying sonic anemometer measurement 6.2.1 Sonic Anemometer Test Standards Two standards that define procedures for sonic anemometer testing are ASTM D 6011-96 and ISO 16622. These standards describe the initial calibration for a sonic anemometer in a zero-wind chamber, which involves the measurement of the path length and transient times between the transmitter and receiver. ASTM D 6011-96 is a sonic anemometer test standard, which was released in 1996 and is intended to assist wind instrument manufacturers in the development and design of sonic anemometers. Test facilities also use it as a guide for evaluating the performance of sonic anemometers used in specific applications. 6.2.2 ASTM D 6011-96 ASTM D 6011-96 procedures include measurements of the following  Acoustic path length  System delay  System delay mismatch  Thermal stability range  Velocity resolution  Shadow correction,  Velocity calibration range  The acceptance angle The first five tests are done in a zero-wind chamber, which defines the initial calibration transfer characteristics of the instrument. The following three tests, shadow correction, velocity calibration range, and acceptance angle, define the
  27. 27. D.I.T. Kevin St. Anemometry 27 | P a g e necessary correction to the initial calibration transfer characteristics these are determined in a wind tunnel Wind tunnel characteristic Minimum requirement Blockage Anemometer front area is less than 5% of the test section C.S.A. Wind speed capability Must be able to reach speeds up to 50% of the application range and must maintain speed within +/- 0.2m/s. wind tunnel speeds from 1.0 to 10m/s be maintained at +/- 0.1m/s. Flow uniformity Flow profile in the test section must be constant to within 1% Turbulence Must be less than 1% in the test section Air density uniformity Density profile in test section must be less than 3% difference Wind speed reading Maintain a relative accuracy of 0.1m/s to its traceable source Fig 11wind tunnel requirements for ASTM D 6011-96 sonic sensor testing (Adam Havner, 2008)
  28. 28. D.I.T. Kevin St. Anemometry 28 | P a g e 6.2.3 ISO 16622 ISO 16622, an international standard for used for anemometer performance testing was released in September 2002 and is similar to ASTM D 6011-96, the following steps define ISO 16622,  “Zero wind chamber tests: the offset of the measured wind speed is determined over the operational temperature range.  Wind tunnel test: the deviation of the measured from the true velocity (vector) is determined over the operational range of flow speed and direction.  Pressure chamber test: the operational range of air density is determined. Although the measuring principle does not depend on air density, a minimum density is required to transmit detectable sound.  Field test: addresses the response to potentially adverse environmental conditions, which are difficult to simulate in the laboratory” (ISO 16622 International Standard, 2002). The ISO 16622 requirements for wind tunnels are similar to ASTM D 6011-96 but the wind tunnels shall be capable of producing wind speeds that cover the full range of the anemometer to be calibrated. The wind speed must be kept within +/- 0.2m/s. For angle testing the rotating fixture must have a 1° angular resolution. The wind tunnel test procedures are as follows  Senor positioned at zero angle of attack, low wind speed is selected. To test for orientation +/- 360 degrees around its vertical axis at 5-degree increments and step has an increase in wind speed (e.g. 18%, 32%, 56% and 100% of the max test speed).  With the sensor positioned vertically at zero angle of attack and rotated at the worst-case orientation. Tests are carried out at different percentages of the test speed.  The steps above are repeated with the sensor tilted 15 degrees into the wind and 15 degrees away from the wind (Coquilla, 2009).
  29. 29. D.I.T. Kevin St. Anemometry 29 | P a g e Fig 12 Otech Eng. Wind Tunnel Laboratories (ASTM Inernational, 1996)
  30. 30. D.I.T. Kevin St. Anemometry 30 | P a g e 6.2.4 IEC standard 61400 Technical requirements for LIDAR and SODAR anemometers have being revised under the IEC standard 61400-12-1. The purpose of these standards is to ensure traceability, repeatability of SODAR/LIDAR measurements and uncertainties of measurements. The main requests of these standards are an accuracy test and sensitivity test/classification (Albers, 2010). Accuracy Test & Calibration Unlike mechanical anemometers such as cup and propeller types, LIDAR and SODAR anemometers cannot be tested in wind tunnels due to the large size of the wind scanning volume. The only way to calibrate the instruments is to compare each individual LIDAR and SODAR to calibrated reference sensors in open field tests. According to IEC 61400-12-1the comparisons of the LIDAR and SODAR anemometers against a reference sensor must capture the wind speed range between 4 and 16m/s similar to wind tunnel test speeds. If the difference of the measured wind speed exceeds the reference sensor measurement, the relation of the LIDAR/SODAR wind speed measurement and the reference sensor measurement should be applied for a calibration of LIDAR/SODAR. This calibration is then linked to the following; statistical uncertainly of the comparison and uncertainty of the reference sensor. The test and calibration cannot be more accurate than the cup anemometer reference sensor (Albers, 2010). 6.2.4.1Sensitivity Test/ Classification Environmental conditions such as wind shear, turbulence intensity effects the operation and accuracy of the LIDAR/SODAR anemometers, therefore uncertainty of the LIDAR/SODAR wind measurements due to sensitivity to environmental conditions needs to be assessed. According to IEC 61400-12-1, the outline for the sensitivity test/classification is the same as the accuracy test/calibration. The sensitivity of the LIDAR/SODAR anemometers is based on set environmental variables. The percentage change
  31. 31. D.I.T. Kevin St. Anemometry 31 | P a g e of the LIDAR/SODAR and cup anemometer per 10-minute period is considered as one environmental variable at a time. 6.3 Environmentalconditions and inspections A wind vane/ sonic anemometer should be capable of use under the following conditions. a) Operating temperature range:  Equipment installed outdoors − 25 °C to + 55 °C  Equipment installed indoors − 15 °C to + 55 °C. b) Operating time: continuous. c) Operating power range: The operating level can be set from 100V to 240V for AC voltage, or from 12V to 24V for DC voltage. The tolerance level for the set figures shall satisfy the requirements of IEC 60945. 6.4 Standards Conclusion In this report, the four published standards that define procedures for calibrating anemometers are IEC 61400-12-1, ASTM D5096-02, ASTM D6011-96 and ISO 16622. According to Coquilla the only publication related to the wind industry is IEC 61400-12-1. Most test facilities are able to perform test procedures from the standards listed above. The test procedures explains and defines necessary steps manufacturers need to carry out to calibrate anemometers, this ensures that all anemometers are traceable back to international standards organization. LIDAR/SODAR and sonic anemometers use the same standards, ISO 16622 and IEC 61400-12-1 for calibrating.
  32. 32. D.I.T. Kevin St. Anemometry 32 | P a g e 7.0 Comparison of the performance The cup anemometer is the preferred wind speed measurement instrument for site assessments or proposed wind farm performance evaluations. This anemometer is an inexpensive and rugged sensor appropriate for turbulence measurements for wind-energy measurement applications. Both propeller and cup anemometers are the most suitable for a wide variety of environments including marine applications. The propeller anemometer differs in that they require an orienting vane to keep the propeller facing into the wind thus provides both wind speed and direction information in a single unit. The design of the rotational anemometer is such that the rate of rotation is linearly proportional to the wind speed. This means the anemometer responds primarily to that portion of the wind vector that is parallel to the rotor axis. Aspects of rotational anemometry which may cause concern when specifying are:  Over Speeding  In-ability to sense vertical wind components  Mechanical operation  Dynamic filtering in turbulent winds. (Energy, 2008) 7.1 Over speeding: Over speeding occurs during fluctuating wind speeds. Instruments have time constant characteristics, i.e. the response to change of an input parameter. Generally the time to react to change is not linearly proportional to the magnitude of that change in parameter however this is not the case with cup anemometers, with fluctuating wind speed the mean indication of a cup anemometer will be higher than the actual average wind speed, the results showing that a cup anemometer will respond more quickly to positive changes in wind speed and not as quick to negative changes.
  33. 33. D.I.T. Kevin St. Anemometry 33 | P a g e 7.1.1 Dynamic filtering in Turbulent Winds As stated above cup anemometers cannot follow the wind speed fluctuations exactly, wind fluctuations of higher magnitude reduce the ability of the anemometer to provide an accurate reading and indication of the changes in magnitude of the wind vector. However by reducing the arms that support the cups shall improve the performance and ability to operate in turbulent conditions. Sensitivity to the wind vector depends upon the ratio of torque to rotational inertia. As a consequence to dynamic filtering in turbulent winds the spectral power indicated by a cup anemometer will be lower than what is the actual power of the wind resulting in an incorrect evaluation of the true turbulence intensity. The inability to react to the turbulent dynamic is a main disadvantage of a rotational anemometer. This will result in directional overshoots that can place the rotor of the main wind axis resulting in a lower wind speed reading that the actual wind speed. This is more common in low wind speeds with variance flow conditions and the wind direction is consistently changing rapidly. False wind speed measurements can also occur due to off axis flows in a vertical direction. This is particularly true for propeller – vane anemometers installed on sloping terrain or near abrupt topographical features which can have a constant vertical wind component or frequent large vertical velocities. The dynamic response of a helicoid propeller to wind speed changes decreases when the angle between the flow and axis increases, for example when the angle with respect to the rotor axis reaches 85 degrees, its distant constant has tripled. 7.1.2 Mechanical operation: In the absence of mechanical friction the performance and steady state calibration results should be perfectly linear but friction is always present to some extent. Rotational Anemometers like their counterparts can fail totally, however more seriously can partially fail. Due to their mechanical construction they are more susceptible to breakdown i.e. bearing seizure. Preventative
  34. 34. D.I.T. Kevin St. Anemometry 34 | P a g e maintenance is necessary for the anemometer and/ or introducing forms of redundancy to the measurement system. Such preventative maintenance and calibration should be performed on a regular basis to maintain the low-wind speed performance and to detect increased drag from bearing wear. (Energy, 2008) Environmental concerns must be taken into consideration. Low temperatures can cause particular problems for cup anemometers. The accumulation of ice/snow in the cups will cause a change in aerodynamic behaviour, and appropriate measures need to be taken to detect when such conditions occur. Lightning strikes must be considered to ensure that a strike does not damage the test equipment, a lightning finial/spike should be mounted at the top of the mast, complete with a down conductor installed to carry the strike to Earth and surge arrestors installed should they not be incorporated in the data logging equipment. Another particular disadvantage of the rotational anemometer is relative to the necessity of the erection of a mast. It may prove difficult to attain planning permission if the mast was to impose on the environmental landscape. 7.2 Phase shift anemometers–UltraSonic anemometers, SODAR and LIDAR anemometers. 7.2.1 2.1Ultra sonic anemometers Ultra sonic anemometers main operational attribute is that they are non- mechanical, this in turn omits many of the problems associated with cup and propeller anemometers explained previously as the lack of moving parts reduces continued calibration and maintenance needs, although they are similar in that, both rotational and Ultrasonic anemometers are both linearly proportional to the wind speed. Accurate measurement of the three-dimensional wind vector can be obtained using the 3-axis sonic anemometer, and provide both wind speed and direction from a single unit as with the bi-vane propeller, where cup anemometers
  35. 35. D.I.T. Kevin St. Anemometry 35 | P a g e cannot. Sonic anemometers do it in a time resolution much higher than both a cup anemometer and propeller anemometer. Although hot wire anemometers can have sampling rates in the order of 10 kHz, their fragility in harsh environmental conditions and the necessity to keep the wire free of dust deposits can pose a great problem in their operation. Due to their complex construction the cost of these anemometers can be significant. Other notable problems associated with sonic anemometers include false readings at the sensor can be obtained should the complex structure cause flow distortions by impeding the wind vector. Acoustic blockages of the transducers can also be caused by rain drops, ice, snow and other debris on the transducers, and therefore not suited for marine applications as with the cup anemometer. 7.2.2 SODAR and LIDAR SODAR is classed as a remote sensing system as it does not use a sensor to directly measure the wind speed (Manwell, et al., 2003) SODAR and LIDAR both are remote sensing device systems that are ground level based and capable of measuring wind speeds at a range of heights .A clear advantage is not requiring a large mast to mount the measurement devices on thus having less visual impact on the surrounding environment. SODAR is also primarily only able to give information on mean wind speed and does not provide information on wind gusts. (Anon., 2011) . SODAR and Ultra sonic anemometers both operate using acoustic principles however differ greatly in their operating measurement range. As discussed earlier Sonic anemometers study wind structure by emitting frequency between closely spaced transmitters and receivers, whereas SODAR instruments look at larger scale structures using a combined transmitter/receiver and being a remote sensing tool, SODAR does not disturb the flow in the way that a met mast does. Defining wind profiles and evaluating higher elevation wind speeds using SODAR, proves to be much more cost effective than their counterparts using mast mounted meteorological instruments therefore SODAR is of clear relevance to MW scale turbines. SODAR and LIDAR are ideal for short term
  36. 36. D.I.T. Kevin St. Anemometry 36 | P a g e data collection as they are portable, however considerations should be taken should they be required for longer terms as the instruments could be subject to vandalism or theft. Although portable, SODAR requires a steady surface ruling them out of off-shore. SODAR can also measure the three dimension wind vector should more than one antenna orientated in a different direction be utilised. Vertical wind profiling can be derived using the Doppler effect of both SODAR and LIDAR anemometry by measuring the propagation time. The typical uncertainty in measurement of wind speed using SODAR is about 2 to 4%. Recent developments in SODAR have indicated improvement in accuracy in correlation with wind speed and direction of up to 80 m/s with a 2% difference for mean wind speed between SODAR and cup anemometry. (Lang & McKeogh, 2011)SODAR systems are not yet in widespread use for wind energy applications. They are not cost effective for smaller turbines and the choice of suppliers is limited. 7.3 Thermoelectric anemometers 7.3.1 Hot wire anemometers A hot-wire anemometer as stated earlier consists of a tungsten-wire element heated by an electrical current. The hot wire anemometers works on the principle of temperature coefficient of resistance. There are two types of circuitry utilised in the construction of the heated sensing element constant- temperature or constant-power. The constant-temperature sensor maintains a constant temperature differential between a heated sensor and a reference sensor; the amount of power required to maintain the differential is measured as an indication of the mass flow rate of the wind vector. Constant-temperature anemometers are popular because of their high-frequency response, low electrical noise level, immunity from sensor burnout when airflow suddenly drops, compatibility with hot-film sensors, and their applicability to liquid or gas flows but have also serious limitations. The hot wire anemometers are susceptible to atmospheric contamination, are expensive, require large power inputs and do not respond to low wind speeds resulting in false readings.
  37. 37. D.I.T. Kevin St. Anemometry 37 | P a g e Atmospheric contamination such moisture will cause the thermometer to be inaccurate and will ultimately lead to corrosion. They are less popular because their zero-flow reading is not stable, temperature and velocity response is slow, and temperature compensation is limited. The importance to correct flow temperature changes during calibration, as well as changes which occur between calibration and measuringsituations to standard conditions is a cause for concern. The main cause of error in thermal anemometry is not accounting for flow temperature changes. To reduce this affect calibration by measuring both velocity and temperature effects of the thermal anemometer or by taking measurements should be carried out in the same temperature controlled apparatus that the calibration of the anemometer was completed. (Finaishand, 1994).
  38. 38. D.I.T. Kevin St. Anemometry 38 | P a g e 8.0 The development and effect of turbulence in relation to wind measurement In this section will discussed the development and effect of turbulence in relation to wind measurement and discuss the how anemometers perform in these conditions. 8.1 The Importanceof Accurate Wind Measurement When assessing a site for a potential location for the introduction of wind energy it is important to understand the necessity of collecting accurate information. In turbulent environments it can be difficult to collect accurate data due to problems such as volatile wind conditions. This type of problem can cause errors in initial calculations of data collection for a proposed wind farm which could lead to a project becoming a finical disaster. 8.1.1 What is Turbulence Turbulence can be caused by surface ‘roughness’, this describes the nature of the land in which the wind flows over (Katerina Syngellakis, 2007). It is a major problem as it can cause wind turbine operation to perform irregularly and ultimately cause the wind turbine to break down due to wear and tear caused by the turbulent winds. Below table 1 gives an indication where hot spots for turbulence may arise. Fig 13(Webb, 2007) In relation to wind energy/measurement the problem of turbulence can be found in built up areas such as urban cities. In this section it is hope to explain the role of the built environment in the creation of turbulence. For example Figure 1 and 2 show a graphic of wind flowing against a building; the arrows represent the
  39. 39. D.I.T. Kevin St. Anemometry 39 | P a g e current state of the wind flow at the point of interaction with the building. This is hoped to highlight the danger of placing wind energy technology such as wind turbines in turbulent environments without taking all the necessary elements/data into account. The graphics in figure 1 and 2 show how this turbulence occurs when wind flows around buildings and other obstacles found in urban environments. These obstructions in the path of the wind flow cause the wind flow to become distorted, this creates turbulence. These flow disturbances can be seen in Figures 1 and 2. In Figure 1, a view of the side of building can be seen; this shows how the wind flows over the top of the building. The size of the arrows in the graphics demonstrates the intensity of the wind. The shorter arrows and the arrow in the blue colour show how weak the wind flow is while the longer arrows represent the wind at a high velocity. The section with yellow arrows shows the area where there is the strongest wind, the direction of this wind is mostly constant. It can be concluded from the two graphics that if the turbine is best placed in the middle of the roof, this area is the location with the least amount of turbulence and where the wind is also at its strongest(Horst, 2007). Fig 14(Horst, 2007)Fig 15(Horst, 2007)
  40. 40. D.I.T. Kevin St. Anemometry 40 | P a g e 8.2 Turbulence Effecton Wind MeasurementDevices Traditional large scale wind turbines are designed to be located in open areas such as rural areas or off shore where the wind characteristics can be measured accurately. Measuring and collecting wind data for wind turbines located in turbulent areas such as an urban environment can be a problem. In non-turbulent areas conventional methods of collecting wind data experiences very little fluctuations in wind speed or direction. This is not the case when it comes to turbulent environments, in these environments conventional methods of collecting data experiences sudden changes in wind speed and direction frequently. This leads to the technologies which are very successful in their traditional roles such as a cup anemometer in an open rural area becoming inaccurate and failing to provide the correct data when applied to a turbulent area. The following section will focus on individual types of anemometer and explain the effects of turbulence on these technologies and discuss their performance in turbulence environment. 8.2.1 Cup Anemometer A cup anemometer encounters certain difficulties when expose to turbulent areas. There are two so called difficulties that affect the devices capability to collect accurate data.  Over speeding  Vertical component of turbulent wind The first problem discussed will be the concept of over speeding and its possible effects on poor data collection. Over speeding occurs in turbulent winds where the cup anemometers mean speed result is higher than the true speed of the wind (B Maribo Pedersen, 2003). This is created by the fundamental nature of how the cup anemometer works. For this device to work at all it must respond quicker to an increase in wind speed rather than response to a decrease in wind speed. These results in a distortion of the true wind speed as the device spend more time measuring above the mean wind speed then the time spent below the mean wind speed. This distortion occurs with the presence
  41. 41. D.I.T. Kevin St. Anemometry 41 | P a g e of turbulence and will mean that the reading produced from the cup anemometer will be larger than the true wind speed (Kristensen, 2005). The second difficulty with the cup anemometer is the response to the vertical element of the wind encountered in turbulent areas. It is said that cup anemometers struggle with the three dimensional nature of the wind flow in a complex terrain. Cup anemometers are designed to measure the horizontal wind flow whereas an additional factor is introduced in turbulent environments, this being the vertical component of the wind (Walker, 2004). These difficulties have been thought to be the main factors in the problems cup anemometers have in turbulent areas. Although this may be true, articles featuring experiments performed have disagreed with these being the main problems and believe a more fundamental problem is the cause for the larger readings. One document states that over speeding only accounts for 0.2% of the errors encountered with over speeding (B Maribo Pedersen, 2003). While another document puts forward that the design of the cup anemometer is partly the cause of the higher readings. It details that the shape of the cup can be the deciding factor on how much an affect the vertical component with have on the overall reading, stating that a design of a certain type of cup exposed to turbulent winds can create drag and affect the overall operation of the device. (A. Albers, 2000). 8.2.2 Propeller Type The propeller anemometer is a rotational anemometer similar to the cup anemometer. From research it was found that not many projects trust to incorporate propeller anemometers solely for wind measurement in turbulent environments. This is due to them providing limited information on turbulence or the vertical component of the wind. In turbulent locations propeller type devices tend to register a lower wind speed; the reason for this is the device has difficulty maintaining its alignment with the instantaneous wind direction (D.C Anderson, 2008). Like the cup anemometer the propeller anemometer does not react accurately to sudden changes in wind speed or direction, this type of environment being commonplace in turbulent/urban areas. This will have a
  42. 42. D.I.T. Kevin St. Anemometry 42 | P a g e crippling effect and result in essential data not being recorded by the device. Another interesting point was found in one such article where it stated that a propeller anemometer cannot measure the vertical component of wind (M.C.H. Hui, 2009). In turbulent/urban locations the presence of buildings adds a strong vertical component to the wind (D.C Anderson, 2008). This can in addition to the reasons shown above explain why a propeller type anemometer may not be suited to turbulent applications unless incorporated with an additional device. This statement is backed up by a journal article where it states a propeller device “dynamic behaviour in turbulent environments may be less than satisfactory” (Morris et al., Not Stated). Also in this article a cup, sonic and propeller anemometer are compared in a turbulent environment with the result being the propeller device been labelled inferior to the two other devices. The reason stated being the wind speed discrepancy caused by under speeding (Morris et al., Not Stated). It may also be mentioned that the propeller type device has problems structurally when exposed to turbulent environments. 8.2.3 Sonic Anemometer A sonic anemometer is seen as one of the best instrument for the measurement of turbulence (Bowen, 2008). Advantages of the sonic anemometer include no moving parts and the fact that it can detect the vertical component of wind successfully over an extremely quick period of time (D.C Anderson, 2008). Another advantage is its capability to capture wind and turbulence data in very low wind. The sonic anemometer compared to cup anemometers in low wind speed (2ms) shows that cup devices measure higher wind speed more than sonic 93% of the time. This once again is a problem associated with turbulent conditions and over speeding (Bowen, 2008). This has caused a lot of organisations to replace traditional device such as cup anemometers with sonic anemometers. As discussed above this type of anemometer has an advantage over cup or propeller devices in that it can measures all three components of wind (U, V, and W), W being the vertical component which means the sonic anemometer does not produce non-linearity’s errors which is common with the more mechanical type devices (B Maribo Pedersen, 2003). One problem with
  43. 43. D.I.T. Kevin St. Anemometry 43 | P a g e this device is how it is affected by the external environment, for example the presence of rain can affect performance. 8.2.4 SODAR When a SODAR anemometer is utilized to perform in a turbulent area the result usually ends up with the device underestimating data. This is due to the devices inability to detect small scale fluctuations. A SODAR anemometer has a low sampling rate and when used in turbulent areas it is exposed to large sampling rates. This causes the underestimation in the devices produced wind data (R THOMAS, 1991). Another disadvantage of SODAR is that when placed in urban areas they tend to pick up noise disturbance causes by passing cars etc. This distortion can lead to the device underperforming in these conditions. 8.2.5 LIDAR Anemometer LIDAR anemometers are challenging cup anemometers in some areas of wind measurement as hub heights increase. Remote sensors are becoming popular in this area. This type of situation would be located at wind farms where there is a flat terrain. The performance of LIDAR in more complex terrain where there is unpredictable wind flow is in question. LIDAR performance in flat terrain is good where it only shows a small percentage of error (Ferhat Bingol, 2009). Its has been found though when LIDAR is assigned to collecting wind data in more turbulent areas the error level can go as high as ten per cent. This error occurs as result of the workings of the LIDAR anemometer, LIDAR use the “assumption of horizontally homogeneous flow” (Courtney, 2011) to calculate horizontal wind speed. This works well in non-turbulent flat areas. The error of up to ten per cent occurs in the unpredictable wind flow which is caused by turbulence; this turbulence interrupts the element of horizontal wind flow which the LIDAR usually uses to create good accurate results in settled flat terrain (Ferhat Bingol, 2009). A possible solution to this problem would be to incorporate many LIDAR anemometers and aim each one at one point. This will
  44. 44. D.I.T. Kevin St. Anemometry 44 | P a g e result in the assumption of homogeneity of the flow not being required. Also a downside of the technology is its inability to measure the horizontal component of wind as accurately as cup or sonic devices (Courtney, 2011) 8.2.6 Cup vs LIDAR From research it was found that the leading devices in this field are cup and LIDAR anemometers. It is universally recognised that the cup anemometer is leading the way but only by a short distance from the emerging LIDAR technology. It was thought the difference between the two was random noise on the side of the LIDAR device which caused errors in result. Though one document argues that evidence from experiments undertaking describe the difference more to do with the two different probing methods. The cup employs point measurements while the LIDAR using volume measurements (Courtney, 2008). This problem stands from the fact that when LIDAR measures in volume it takes into account the vertical and horizontal extents with non-linear weighting, the difference between the vertical and horizontal measurements cannot be interpreted (Michael Courtney, 2008). This problem along with the trouble measuring the horizontal component means the cup anemometer is the most suited device for use in turbulent environments. It should be noted that all the devices included in this section are evolving and improving at a rapid rate.
  45. 45. D.I.T. Kevin St. Anemometry 45 | P a g e
  46. 46. D.I.T. Kevin St. Anemometry 46 | P a g e 9.0 References A. Albers, H. K. D. W., 2000. Outdoor Comparison of Cup Anemometers. Messung von Wind und Leistung, 1(1), pp. 107-111. Adam Havner, R. V. C. a. J. O., 2008. Verification Testing of Wind Speed Measurements from 2D Sonic Anemometers. Albers, A., 2010. How to gain acceptance for LIDAR measurements. [Online] Available at: http://www.windguard.de/filedmin/media/pdfs/UEber_Uns/DEWK_2010/paper_ WindGuard_LIDAR_Acceptance_DEWEK10.pdf [Accessed 14 november 2011]. Anon.,2010.Camblescientific.[Online] Availableat:http://www.campbellsci.com.au/05103-l [Accessed novmber2 7 november 2011]. Anon.,2011.AboutSODAR.Online] Available:http://www.SODAR.com/about_SODAR.htm [Accessed 2 October 2011]. Anon.,2011:waterencyclopedia.[Online] Available_http://www.waterencyclopedia.com/Ce-Cr/Climate-and-the- Ocean.html [Accessed november 2011]. ASTM Inernational, 1996. ASTM International Standards Worldwide. [Online] Available_at:_http://www.astm.org/index.shtml [Accessed sat 19 november 2011]. B Maribo Pedersen, T. F. P. H. K. N. v. d. B. N. K. J. Å. D., 2003. WIND SPEED MEASUREMENT AND USE OF CUP ANEMOMETRY. Research and Development on Wind Energy Conversion Systems, pp. 1-45.
  47. 47. D.I.T. Kevin St. Anemometry 47 | P a g e Bowen, B. M., 2008. Improved wind and turbulence measurements using a low cost 3-D sonic anemometer at a low wind site. The Open Atmopsphere Journal , 2(131 - 138), pp. 1-9. Brazier, C. E., 1914. Recherches expérimentales sur les moulenets anémometrique, in. s.l.:s.n. Coleman, H. a. W. S., 1999.Otech.Engineering.[Online] Available,at:http://otechwind.com/services [Accessed 25 october 2011]. Coquilla, R. V., 2009. Review of anemometer calibration standards. Courtney,M.,2008.http://www.upwind.eu.[Online] Availableat:http://www.upwind.eu/media/618/D6.5.2.pdf [Accessed 19th November 2011]. Courtney, M., 2011.Integrated Wind Turbine Design. Project UpWind, Feburary , 1(1), pp. 1-33. D.C Anderson, J. W., 2008. Rooftop wind resource assessment using a Three - Dimmensional Ultrasonic Anemometer. May, 1(1), pp. 1-7. Dines, W. H., 1911 . Anemomete. s.l.: Encyclopædia Britannica. Energy,I.W.,2008.Irish.Wind.Energy.Association..[Online] Available at:http://www.iwea.com/index.cfm/page/faqs?rfaqId=2. Ferhat Bingol, J. M. D. F., 2009. LIDAR performance in complex terrain. March, 1(1), pp. 1-9. Finaishand, H., 1994. ASHRAE Research Project RP-698, s.l.: s.n. Gripe, P., 2004. Wind Power. In: Wind power . LOndon: Jamses and Jamses Science publisher, pp. 1-26. IEC, 2005. IEC 61400-12-1, windturbines, Part 12-1: Power performance measurements of electricity producing wind turbines, first edition. [Online] Availableat:http://www.IECstandards/windturbines/part12-
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