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| Anemometers for Wind Turbine Testing and Wind Farm Site Assessment |
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Vector Instruments has over 35 years experience in the manufacture and development of wind sensors and has a proven record of accuracy and reliability which is vital for the prediction of return on investment for the wind power generation industry.
To obtain the best results when evaluating prospective wind farm sites (or when performing wind turbine testing) requires cup anemometers which have undergone extensive performance testing and which are optimised to obtain the best results in real-world situations.
Many cup anemometers designed for general meteorological applications are not sufficiently accurate, even though they may be quoted as "1% accurate". Their quoted accuracy only applies to "ideal" wind tunnel conditions and they do not acheive that level of performance in "real world" turbulent conditions.
Industry testing has shown that sonic anemometers are difficult to calibrate and can fail to give an accurate measure of mean wind speed as required for these applications. Similarly, the dynamic response of propellor anemometers in the turbulent conditions experienced on many prospective wind farm sites means that they also fail to give sufficiently accurate measurements of mean wind conditions.
By comparison, the A100 Series cup anemometers ARE accurate enough for the most demanding applications.
Because wind farms today are generally large financial investments, it is essential to make every effort to predict the likely generating capacity for a site as accurately as possible in order to best calculate the likely viability of a proposed wind farm on that site. The available power from the wind is proportional to the CUBE of the mean wind speed, so it follows that any errors in the measurement of mean wind conditions will be magnified and produce much larger errors in any estimates/predictions of a site"s power generating potential.
All wind turbine manufacturers test their products to provide curves of power output versus windspeed. Over the years they have discovered that their results can vary wildly unless they use cup anemometers which have specific closely defined performance characteristics. A number of publicly funded research projects have been carried out (CLASSCUP, ACCUWIND etc) which have ultimately led to the development and refinement of the international standard IEC 61400-12-1 which specifies these cup anemometer performance characteristics as well as details of mast/tower design requirements and anemometer mounting requirements. It is logical, therefore, that the same requirements for high quality anemometer performance, accuracy and mounting apply to the gathering of wind information for proposed sites for those turbines, as these measurement (and any errors) will be combined with those same power performance curves to predict likely generating potential.
IEC 61400-12-1, CLASSCUP and ACCUWIND describe "classification figures" for anemometer performance. These figures are intended to be a measure of the likely overall accuracy of the measurements of mean wind conditions obtained using a specific type of anemometer, with small class figures indicating better results. The "perfect" cup anemometer would have a classification of 0.0, however the best cup anemometers available at present have class figures typically between 0.5 and 5.0 depending on the range of site/environmental conditions, including type of terrain, turbulence and temperature. Determining the classification figure for an anemometer is a difficult task involving lengthy measurements of the cup anemometer properties in both the field and wind tunnels followed by extensive calculations.
For the commonly used "Horizontal Definition" of wind speed, a good "cosine response" to off-axis winds is essential to obtaining a good classification figure. The Vector Instruments A100 series cup anemometer is one of a very few designs suitable for use under this definition. In some of the hilly sites favoured for many wind farms, it is often the case that turbines are likely to be sited at or near the top of a hill/ridge so many of these sites will have significant off-axis winds/turbulence. Most cup anemometers from other manufacturers have either a flat response to off-axis winds, or one which "peaks" in the event of winds with an up/down component, resulting in a poor classification figure, as a consequence they will produce inferior measurements in these situations compared to the A100 Series.
Many prospective wind farm sites are likely to be subject to "Class B conditions" where there is likely to be significant turbulence (hilly country, or areas with trees/hedges/buildings etc. which can interfere with the airflow). Only purpose-designed (and none of the lower-cost cup anemometers) perform well enough in these conditions and achieve acceptably low class B figures. The recent independent ACCUWIND report includes an assessment of five cup anemometers. The results of that report show that the A100 series anemometers are amongst a group of three which perform similarly, and significantly better than the others in the more arduous "Class B" conditions likely to be encountered on many prospective wind farm sites. Of that "group of three anemometers", the A100 series anemometer has built up a good track record in wind assessment, and is the only type which has been used in significant numbers around the world over many years.
The A100, also known as the "Porton Anemometer", was first developed for atmospheric diffusion studies in the 1960s for accurate measurement in turbulent air near the ground. The basic geometry remains unchanged, while developments over the years have resulted in a reliable and versatile range of instruments. The A100 series cup anemometer achieves "Class 1.0" performance under certain special conditions ("Class S" conditions, see the independent certification document which is available detailing this), however the A100 series also performs well in the standard "Class A" and "Class B" conditions. ACCUWIND indicates that the A100 series can be classified as 1.8 for "Class A" conditions, and 4.5 for "Class B" conditions which is amongst the best (i.e. "Class 1.8A" and "Class 4.5B")
Reliability and consistency of the A100 series calibrations are addressed in a number of ways:
Every A100 series anemometer (cup rotor) is tested as standard practice in our own wind tunnel, with regular checks for consistency against accurate calibrations performed at other reputable wind tunnels.
The A100 series anemometers (and their cup rotors) have been shown by independent research to retain their calibration and produce consistent measurements for extended periods.
Individual MEASNET calibrations are routinely provided from various wind tunnels in the MEASNET group as required for IEC61400-12-1.
Obtaining an individual MEASNET calibration for a cup anemometer is now generally considered in the industry to be essential if the anemometer is to be used for wind farm site measurements (or for turbine power performance testing). Over a period of several years, the MEASNET group of independent wind tunnels and test-houses have developed a test procedure and "round-robin" checking procedure to ensure that cup anemometer calibrations can be performed accurately, producing consistent results across all of the wind tunnels within the MEASNET group. The MEASNET calibrations are directly traceable to basic international standards, rather than being "traceable to another wind tunnel" or ?standard anemometer?. The MEASNET procedure is in line with the calibration requirements of international standard IEC61400-12-1 mentioned earlier, and IEC 17025.
Using high quality "purpose designed" cup anemometers may initially appear costly, but in fact does not really add much to the overall cost of installing a met mast on a site compared to one using inferior anemometers. Using high-performance cup anemometers does, however, significantly improve the confidence the wind farm developer can have in the mean wind conditions data and any estimates of generating capacity they may calculate from that data. The cost of the high-performance wind sensors themselves typically represents a relatively small percentage of the total cost of a wind assessment project when the costs of masts, dataloggers, installation, maintenance and data analysis are taken into account. Hence we believe the A100 Series of cup anemometers to be the best choice for these applications.
For these applications, it is recommended that the /PC3 Anti-Surge circuit is specified for the A100L2 and A100LK/A100LM anemometers. While it cannot protect against a direct lightning strike, it greatly reduces the incidence of failure due to nearby lightning strikes or surges from sources such as static discharge or induced voltages from power cables. Should icing be a problem, the newly available HE-4 internal anti-icing heater option can be used while retaining full anemometer accuracy and performance.
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