Open Field Cup Anemometry

Open Field Cup Anemometry Axel Albers, DEWI Summary Within the EC co-funded project "Identification of Variables for Site Calibration and Power Curve Assessment in Complex Terrain" (SiteParIden, contract: JOR3-CT98-0257) large deviations between cup anemometers of different type under natural conditions have been identified. Deviations in the order of 2 % have been found to be common even in flat terrain conditions although all compared cup anemometers have been calibrated according to the latest standards in the wind tunnel. These large deviations are of major significance for the whole wind energy industry because the result of wind turbine power performance tests as well as site assessments are strongly dependent on the type of cup anemometer in use. The deviations between the cup anemometers have been found to be influenced by site characteristics. The most important flow and turbulence parameters influencing the cup anemometer deviations have been identified. From these results follows a first attempt to correct cup anemometer measurements and to classify cup anemometers as well as an advice for further research on the field of cup anemometry.. Introduction In the SiteParIden project detailed comparisons of cup anemometers of different type under natural conditions have been performed. Surprisingly large deviations between measurements with different types of cup anemometers have been found (Fig. ). As all compared cup anemometers have been calibrated in the same wind tunnel according to the latest standards [] this observation must be linked to the turbulent nature of free air flow (in contrast to the low turbulence present in wind tunnels). The results have been verified at various sites in flat and complex terrain up to heights of 83 m above ground (see reference [2] and [3] for details). The effects on cup anemometers have been found to be site dependent. However, even in flat terrain conditions 83 m above ground significant deviations between the measurements with different types of cup anemometers in the order of typically 2 % have been observed [3]. This finding is of major interest for the wind energy industry because it implies that wind turbine power performance tests are comparable only when they are performed with the same type of cup anemometer and under comparable turbulence characteristics. Furthermore, additional uncertainties for site assessments are present when the cup anemometer in use for the site assessment does not match the type of cup anemometer used for the determination of the power curve of the projected wind turbine type. Since the effect has been discovered the bodies occupied with the standardisation of power performance testing aim to overcome the problem. Basically three different solutions are under discussion [2]: use of a standardised cup anemometer, a classification of cup anemometers, a correction of cup anemometers. Difference: v-x - v-thies [%] 3 2 0 - -2-3 -4-5 Risoe P2445b, 30m Risoe P2445b, 8m Vector A00, 30m Vector A00, 8m Metone 00C-, 30m Mierij, 8m Vaisala WAA5 Friedrichs 4033.00x, 30m Friedrichs 4033.00x, 8m Thies Compact, 8m 0 5 0 5 20 25 v-thies 4.3303.22.000 [m/s] Fig. : Deviations between measurements with various cup anemometers in flat terrain 8 m and 30 m above gorund Fig. 2: Test rig for outdoor cup anemometer comparison. The distance between the cup anemometers is 2.5 m. 53

As has been outlined in [3] until lately it was not clear which flow and turbulence parameters and anemometer properties have to be considered for a proper solution of the problem. Major progress in this respect has been gained recently from a new large data base of high resolution time series by DEWI [4], which is subject of this presentation. 2. Methodology Two cup anemometers of type Thies 4.3303.22.000 and Risø P2445B have been mounted in parallel position on a special test rig on top of a 30 m high meteorological mast in flat terrain (Fig. 2). An ultra sonic anemometer (USA) has been placed in middle of the test rig in order to evaluate turbulence characteristics as well as the vertical flow inclination. The whole test rig is aligned perpendicular to the actual wind direction by a micro processor controlled drive system. Before anemometers of different type were compared, detailed tests with two anemometers of the same type mounted on the test rig have been performed in order to ensure that the two cup anemometer positions are exposed to the same wind conditions. Furthermore, before the outdoor tests, all anemometers including the USA have been calibrated in the same wind tunnel according to the MEASNET procedure []. A large data base of time series sampled with a frequency of 0 Hz (output rate of USA 4 Hz) has been collected over a period of 2440.5 h. The aim of this intense data collection was to study which turbulence and flow parameters are significant to explain the differences between the cup anemometer measurements. From the time series about 00 turbulence and flow parameters have been calculated for each 0 minute period. The ratios between the 0 minute averages of the anemometer measurements have been analysed in detail on their dependence on turbulence and flow parameters. The Risø P2445B and Thies 4.3303. 22.000 anemometer have been selected for this investigation because these anemometers have formerly shown large deviations, both anemometers are widely used in Germany and Denmark respectively and because there is hope that the anemometers may represent two groups of anemometers consisting of the faster anemometers Friedrichs 4033.00X, the Thies 4.3303.22.000 and the Thies compact series and the slower anemometers Vector A00, Risø P2445b and Metone 00C-. 3. Results 3. Comparison between Cup Anemometers and Ultra Sonic Anemometer A key finding is that the Thies 4.3303.22. 000 anemometer clearly overestimates even the magnitude of the full three dimensional wind speed vector (in short vector wind speed v-vector) in highly turbulent air flow as it is present in wakes of wind turbines (Fig. 3). Under the same conditions the Risø P2445 B anemometer still tracks well the horizontal wind speed component (Fig. 4). Furthermore, in unperturbed air flow the wind speed ratio of the Thies anemometer and the USA is more dependent on site specific flow parameters than the Risø anemometer (compare Fig 3 and Fig 4 for the sector 0-350 ). Wind Speed Ratio [-] Wind Speed Ratio [-].2.5..05 0.95 0.9 0 40 80 20 60 200 240 280 320 360.2.5..05 0.95 wake sector wake sector Wind Direction [ ] 0.9 0 40 80 20 60 200 240 280 320 360 Wind Direction [ ] free sector v-thies/u v-thies/v-horizontal v-thies/v-vector Fig. 3: Ratio of measurements with a Thies 4.3303.22.000 cup anemometer and the longitudinal wind speed component (U), the horizontal wind speed component (v-horizontal) and the magnitude of the full 3D-wind speed vector (v-vector) as evaluated with a calibrated ultra sonic anemometer. free sector v-risoe/u v-risoe/v-horizontal v-risoe/v-vector Fig. 4: Ratio of measurements with a Risø P2445B cup anemometer and the longitudinal wind speed component (U), the horizontal wind speed component (v-horizontal) and the magnitude of the full 3Dwind speed vector (v-vector) as evaluated with a calibrated ultra sonic anemometer. 54

3.2 Correlation of Time Series The correlation coefficient between the cup anemometers and the ultra sonic anemometer within 0 minute periods has been investigated. Key findings of this analysis are: The Risø P2445 B anemometer correlates clearly better with the horizontal flow component than with v-vector or the longitudinal wind speed component. The Thies 4.3303.22.000 anemometer has a small tendency to correlate better with v-vector than with the horizontal flow component. It correlates clearly better with v-vector or the horizontal flow component than with the longitudinal wind speed component. 3.3 Influence of Flow and Turbulence Variables on Cup Anemometer Measurements The influence of flow and turbulence parameters on the wind speed ratios of the two cup anemometers and the wind speed ratios of cup anemometers and the longitudinal wind speed component, the horizontal wind speed component and v- vector has been investigated following three different procedures: The correlation coefficient between the wind speed ratios based on 0 minute averages and the flow and turbulence variables has been evaluated. Multi variate regression analysis has been performed following the technique described in [5] with the wind speed ratios based on 0 minute averages as dependent variables and the flow and turbulence variables as independent variables. A multi classification technique as described in [6] has been applied in order to care for the co-linearity of the flow and turbulence parameters. The following results can be drawn in full consistence from all three evaluation techniques: The wind speed ratios with the Thies 4.3303.22.000 anemometer correlate significantly with different turbulence variables. In very turbulent air flow the anemometer measures a much higher wind speed than the vector wind speed. 55

0.025.6 Significance Coefficient [-] 0.02 0.05 0.0 0.005 std(x): standard deviation of flow component x within 0 minutes R(x,y): corellation coefficient between flow component x and y within 0 minutes L(x): integrale turbulence length scale of flow component x tensor(xy): turbulent stress between flow component x and y (Reynolds tensor element) inclination: vertical flow inclination angle averaged over 0 minutes v-thies/v-horizontal, 0-350 (free air flow) v-thies/v-horizontal, 350-0, (wake situation) v-risoe/v-horizontal,0-350 (free air flow) v-risoe/v-horizontal,350-0, (wake situation) Wind Speed Ratio [-].4.2..08.06.04.02 v-risoe/v-horizontal v-thies/v-horizontal 0 0.98-0.005 0.96 std(u)/u std(v)/u std(w)/u R(U,V) R(U,W) R(V,W) L(U) L(V) L(W) tensor(uv) tensor(uw) tensor(vw) inclination v-vector 0.94 0 0.05 0. 0.5 0.2 0.25 0.3 0.35 std(w)/u [-] Fig. 5: Influence of flow and turbulence variables on wind speed ratios of cup anemometer measurements and the horizontal wind speed component. The significance coefficient as gained from multi variate regression analysis describes the relative change of the wind speed ratio when the respective flow or turbulence variable is increased by one standard deviation. Fig. 6: Influence of the vertical turbulence intensity on wind speed ratios of cup anemometer measurements and the horizontal wind speed component. The wind speed ratios of the Risø P2445 B anemometer and the horizontal wind speed component shows only a very low correlation with all considered flow and turbulence parameters. Turbulence effects on this anemometer are much smaller than in case of the Thies 4.3303.22.000 anemometer (Fig. 5) The vertical turbulence intensity is the most significant variable for the explanation between the differences in the cup anemometer measurements. It has the largest contribution to the variation of the wind speed ratios (Fig. 5, Fig. 6). Other significant variables are the flow inclination, the correlation between the lateral wind speed component and the other two flow components and the integral turbulence length scale of the vertical flow component (Fig. 5). The influence of the longitudinal turbulence intensity is much smaller than the influence of the vertical turbulence intensity. The most significant flow and turbulence variables are the same for both types of anemometers (Fig. 5). The air density, air temperature (considered range 5-20 C), air pressure and the wind speed have shown no significant influence on the ratios between the cup anemometers measurements. 4. Correction of Cup Anemometer Measurements A correction of cup anemometers is needed to make wind turbine power curve measurements performed with different cup anemometers comparable and to match existing power curve measurements to the cup anemometer in use for site specific wind speed measurements. The results given in the previous chapter imply that a cup anemometer correction should consider at least the vertical turbulence intensity. The simplest correction would be a linear correction according to the vertical turbulence intensity. Fig. 7 demonstrates how the variation of the wind speed ratios with the wind direction is reduced by the wind speed correction. 5. Classification of Cup Anemometers A classification of cup anemometers should take into account the sensitivity of the cup anemometers to flow and turbulence parameters. As a first attempt the sensitivity of cup anemometers on only the most significant turbulence parameter has been used for the classification of cup anemometers on the bases of field tests. The sensitivity of the cup anemometers on the vertical turbulence intensity is expressed by the slope of the regression through the ratios of the cup anemometer measurements and the horizontal flow component (or v-vector) versus the vertical turbulence intensity. From the slope the (simplified) measurement error can be calculated by a multiplication with the vertical turbulence intensity. Then the maximum allowable vertical turbulence intensity can be calculated for each class when the class index is defined as the maximum measurement error in percent. The resulting classification scheme in Tab. should be considered only as a first attempt to classify cup anemometers by means of open field measurements. Furthermore, it must be pointed out that except of the Thies 56

4.3303.22.000 and the Risø P2445B anemometer the presented slopes of the regressions have been evaluated from measurements with some shortcomings (see [4]). However, the ranking of the anemometers according to the regression slopes coincides very well with the differences in the cup anemometer measurements as observed in flat terrain (compare Tab. and Fig. ). An exception is the Metone 00C- anemometer which has been very close to the Risø P2445B and the Vector A00 anemometer in the outdoor comparison while its regression slope is relatively large. 6. Conclusions Large deviations between measurements with cup anemometers of different type occur even in flat terrain and when the anemometers are calibrated uniformly in the same wind tunnel. The deviations are in the range of some percent. Wind turbine power curve measurements are strongly effected by the choice of the cup anemometer. The observed differences in wind speed measurements are dependent on the site specific flow and turbulence regime. The vertical turbulence intensity seems to be the flow variable with the largest influence on the differences in the cup anemometer measurements. Among other important flow variables seem to be the flow inclination, the correlation between the lateral and the other flow components and the integral turbulence length scale of the vertical flow component. Classifications of cup anemometers and corrections of wind speed measurements should account for the identified important flow and turbulence parameters. The Risø P2445B cup anemometer seems to be nearly insensitive against turbulence effects, at least in flat terrain conditions. The Thies 4.3303.22.000 cup anemometer seems to be seriously influenced by the vertical turbulence intensity. In very turbulent flow the anemometer overestimates even the vector wind speed significantly. A simple correction algorithm for cup anemometer measurements in flat terrain conditions has been proposed. This correction, however, requires a good knowledge about the vertical turbulence intensity. Although shortcomings of the correction are evident, its application can lead to more accurate results of wind speed measurements at least in flat terrain when anemometers with a high sensitivity on turbulence effects are in use. A first attempt to classify cup anemometers according to their sensitivity on the vertical turbulence intensity, as it is determined by outdoor measurements, coincides well with v-risoe/v-thies [-].05.025 0.975 0.95 0.925 0.9 0.875 wake sector not corrected linear correction for std(w)/u from free sector 0.85 0 40 80 20 60 200 240 280 320 360 Direction [ ] free sector Fig. 7: Ratios of measurements with a Thies 4.3303.22.000 and a Risø P4225B cup anemometer for the not corrected Thies anemometer and the Thies anemometer corrected for the effect of the vertical turbulence intensity. The correction has been established from the free sector 0-350. Instrument for Horizontal Wind Speed Instrument for Vector Wind Speed Slope Maximum Vertical Turbulence Intensity Class Slope Maximum Vertical Turbulence Intensity Class Thies 4.3303.22.000 Thies 4.3303.22.000 0,35 0,032 0,225 0,045 0,063 2 0,090 2 0,095 3 0,35 3 0,27 4 0,8 4 0,59 5 0,226 5 Risoe P2445b Risoe P2445b 0,05 0,96-0,039-0,255 0,392 2-0,5 2 0,588 3-0,766 3 0,784 4 -,02 4 0,979 5 -,276 5 Metone 00C- (limited validity) 0,2387 0,042 0,084 2 0,26 3 0,68 4 0,209 5 Results from 8m mast (limited validity) Vector A00 (limited validity) 0,0632 0,58 0,37 2 0,475 3 0,633 4 0,792 5 Mierij (limited validity) 0,743 0,057 0,5 2 0,72 3 0,230 4 0,287 5 Vaisala WAA5 (limited validity) 0,2792 0,036 0,072 2 0,07 3 0,43 4 0,79 5 Friedrichs 4033.00x (limited validity) 0,3699 0,027 0,054 2 0,08 3 0,08 4 0,35 5 Tab. : Classification of cup anemometers according to their sensitivity on the vertical turbulence intensity 57

the observed differences between the cup anemometer measurements. Future research should emphasise on the verification of the findings regarding the turbulence parameter analysis with other types of cup anemometers and at other locations, especially in complex terrain. Simulations of turbulence effects on cup anemometers in wind tunnels require a good control over the here identified important turbulence measures. 7. Acknowledgements The work has been partially supported by the European Commission under contract number JOR3- CT98-0257. 8. References [] MEASNET: Measurement Procedure Cup Anemometer Calibrations, 997 [2] A. Albers, H. Klug, D. Westermann; Outdoor Comparison of Cup Anemometers; Pro-ceedings of DEWEK 2000 [3] A. Albers, H. Klug, D. Westermann; Cup Anemometry in Wind Engineering, Struggle for Improvement, DEWI Magazine no. 8, February 200 [4] A. Albers; Identification of Variables for Site Calibration and Power Curve Assessment in Complex Terrain, JOR3-CT98-0257, Project Task 6, Relative Power Curve Measurements in Flat terrain, Final Report, Report-No. JOR3-CT98-0257-060-DEWI07, June 200 [5] F. Mouzakis, E. Morfiadakis, P Dellaportas; Parameter Identification on Power Performance of Wind Turbines Operating at Complex Terrain, 2nd European & African Conference on Wind Engineering Geneva, Italy, 997 [6] A. Albers; Comments on Techniques for Multi-Parameter Site Calibration, DEWI Report-No.: JOR3- CT98-0257-98-DEWI0, 998 Problematik der Windböen Frank Böttcher, Christoph Renner, Hans-Peter Waldl und Joachim Peinke Fachbereich Physik, Carl-von-Ossietzky Universität Oldenburg. Einleitung Windenergieanlagen (WEA) sind im Betriebszustand unterschiedlichen Belastungen ausgesetzt, die die Lebensdauer der Anlagen, vor allem durch die wechselnden (instationären) Belastungen, herabsetzen. Neben den deterministischen Anteilen (wie z.b. Schwer- und Kreiselkräfte) sind es insbesondere stochastische Lastanteile, die die mechanische Auslegung von Windkraftanlagen sehr komplex werden lassen. [] Die stochastischen mechanischen Belastungen haben ihre Ursache in der schwankenden Windgeschwindigkeit. Sie werden in der Regel in Windturbulenzen und in Windböen untergliedert. Der Einfluß der Windturbulenz wird oftmals durch den Turbulenzgrad S σ = u () wiedergegeben, der das Verhältnis der Standardabweichung des Windgeschwindigkeitsfeldes zu seiner mittleren Geschwindigkeit beschreibt. Die Frage nach der Natur von Windböen ist trotz ihrer scheinbaren Trivialität bis heute nicht allgemein geklärt und eine strenge Definition des Begriffes findet sich in der Literatur nicht [3]. Dabei ist die Kenntnis der maximal zu erwartenen Geschwindigkeit einer Windböe besonders hinsichtlich der Abschätzung von Extrembelastungen von großem Interesse. Ähnlich relevant und auch schwierig ist die Einschätzung der Häufigkeit von Böenereignissen und deren zeitliche Abfolge [2]. Die Unterscheidung von Böen und Windturbulenzen wird in der Regel folgendermaßen formuliert. Während die Windturbulenz u (t) als permanente, fluktuierende Überlagerung der mittleren Geschwindigkeit u(t) aufgefaßt werden kann, stellen Böen nach [3] eine erhebliche Abweichung von der mittleren Windgeschwindigkeit im Bereich von einigen bis einigen zehn Sekunden" dar. Letztere werden demnach im Gegensatz zur Windturbulenz als einzelne und extreme Ereignisse charakterisiert. Um zu untersuchen wie Windböen vom statistischen Standpunkt aus erfaßt werden können, werden wir im folgenden zwei Geschwindigkeitszeitreihen auf ihre statistischen Eigenschaften untersuchen: Einen Freifeld-Windgeschwindigkeits- und einen Labor-Geschwindigkeitsdatensatz. In diesem Artikel 58