Performance of a Propeller-Vane Compared to Two Cup Anemometers

Performance of a Propeller-Vane Compared to Two Cup Anemometers Mark A. Taylor, Senior Meteorologist (mtaylor@awstruewind.com) Erik Hale, Meteorologist (ehale@awstruewind.com) Michael C. Brower, Chief Technical Officer (mbrower@awstruewind.com) AWS Truewind, LLC 3 New Karner Road Albany, New York 05 Tel: (5) 3-00 Fax: (5) 3-005 Abstract A key goal of wind resource assessment is to ensure that wind speed measurements conform to known standards. Morris et al. (99) document that an RM Young Wind Monitor prop-vane sensor can record wind speeds that are as much as % lower than those measured by NRG Maximum #0 cup anemometer in turbulent settings. Their explanation is that this underspeeding is most likely the result of the prop-vane failing to point perfectly upwind with sudden direction shifts. When not pointing perfectly upwind, the propvane does not conform to its specification; the response is approximately a cosine function of the direction deviation. The results of several case studies are summarized and they provide further understanding of how the response of propeller-vane (prop-vane) anemometers differs from that of the more widely used NRG Maximum #0 and Risoe cup anemometers. We discuss the physical reasons for those differences and propose a method for converting wind speed observations from one type to the other. Introduction A key goal of wind resource assessment is to ensure that wind speed measurements conform to known standards. This study provides insights into how the response of R.M. Young 0503 propeller-vane (propvane) anemometers differs from those of the more widely used NRG Maximum #0 and Risoe P5A cup anemometers, and proposes a method for converting wind speed observations from one type to the other. According to Morris et al. (99), in non-turbulent flow from a wind tunnel, the prop-vanes and cup anemometers in their analyses recorded virtually the same speed, as is to be expected if they have been calibrated correctly. However, in their field studies, the prop-vane anemometers consistently recorded lower wind speeds. The study concluded that the prop-vane s inability to remain perfectly aligned with the horizontal wind vector under turbulent conditions was responsible. This study attempts to confirm and extend this finding using observed wind speed data from five pairs of R.M. Young prop-vane and NRG Maximum #0 cup anemometers, mounted on three masts, and three pairs of R.M. Young prop-vanes and Risoe P5A cup anemometers, mounted at different heights on the same mast. Anemometer Characteristics Cup and propeller anemometers observe the wind speed by employing an apparatus that detects the wind flow by rotating about an axis at a rate proportional to the wind speed. A propeller anemometer rotates about a horizontal axis and ideally responds only to the component of the wind velocity vector that is parallel to that

axis. To accurately measure the wind speed from a different direction, the propeller (impelled by an attached vane) must rotate until it faces fully into the wind. Conversely, a cup anemometer rotates on a vertical axis enabling it to measure the horizontal wind from any direction. In this study, the following three instrument models were employed to measure wind speed: the R.M. Young 0503 Wind Monitor, a prop-vane sensor, and the NRG Maximum #0 and Risoe P5A cup sensors. According to specifications published by each respective manufacturer, the anemometer starting thresholds range from less than 0. m/s (Risoe P5A) to.0 m/s (R.M. Young 0503). Another measure of anemometer sensitivity, the distance constant, indicates the length of a passing air column needed for the sensor to recover 3% of an abrupt change in speed. The Risoe model has the shortest distance constant (. + 0.0 m), while the NRG and R.M. Young anemometers have longer constants (3.0 m and.7 m, respectively). A shorter distance constant indicates a more rapid response to fluctuations in speed. Table summarizes the characteristics of each anemometer type. Table. Wind Measurement Instrument Specifications R.M. Young 0503 NRG Maximum #0 Risoe P5A Distance Constant.7 m (3% Recovery) 3.0 m (3% Recovery). + 0.0 m Default Slope / Offset 0.09 m / 0 m/s 5 m / 0.35 m/s 0.0 m / m/s Starting Threshold.0 m/s m/s < 0. m/s Monitoring Site Characteristics We chose four monitoring masts configured with side-by-side cup and R.M. Young propeller-vane anemometers. Two of these masts are located in a high-turbulence environment, while the remaining masts are located in an area characterized by moderate turbulence. Information about the sites, including the monitoring heights, anemometer types, and observed average turbulence intensities at 5 m/s are provided in Table. Table. Monitoring Site Characteristics Monitoring Mast Period of Record Monitoring Heights (m) Cup Anemometer Model Site 00 Jun 00 Sep 007 0, NRG Maximum #0 Site 00 Jan 007 Sep 007 0, 0, 0 Risoe P5A Site 003 Feb Mar 00 53, 3 NRG Maximum #0 Site 00 Feb Mar 00 53, 3 NRG Maximum #0 Mean TI at 5 m/s 9 (0 m), 0.0 ( m) 0. (0 m), 0 (0 m), 0. (0 m) 0. (53 m), 0.3 (3 m) 0. (53 m), 0.3 (3 m)

Data Analysis Methods The data consisted of 0-minute average wind speed, direction, and temperature readings and their standard deviations, and were received in various formats from the tower owners. All of the prop-vane anemometers were uncalibrated; the raw binary data for these sensors were converted to physical values according to the manufacturer s specified slope with no offset. The NRG Maximum #0 cup anemometers were calibrated, but the manufacturer s standard slope and offset were used to convert the raw data. The Risoe P5A cup anemometers were all calibrated, and the raw binary data were converted according to the calibration information provided with the sensors. At Site 00, the prop-vane instrument had been in operation for roughly months before the cup anemometers were installed; furthermore, the original -m cup anemometer failed shortly after its installation, calling into question its overall reliability. A new calibrated anemometer was installed almost months later. At Site 00, all instruments were installed when the tower was erected. However, the Risoe anemometer at 0 m did not become operational until months later. At Sites 003 and 00, instruments were installed on three separate occasions. The masts were initially installed with single prop-vanes at 53 m and 3 m; an additional set of prop-vanes was installed 9 months later at the same heights; the NRG Maximum #0 anemometers were installed an additional months later. For tubular monitoring towers, the impact of the tower on the wind flow is approximately symmetric on either side of the wind vector. Therefore, to minimize differential tower effects, we analyzed wind speed observations only from direction sectors that were within 0 degrees of the midpoint between each sensor pair on the upwind side. It has been documented that off-horizontal flow impacts anemometers differently. However, due to the absence of vertical anemometers on three of these masts, this phenomenon was not assessed. An experienced meteorologist examined the data for completeness and reasonableness and determined the functionality of each instrument. Each sensor pair showed satisfactory operation except for the 53-m level at Site 00, which was disqualified from the study. For all turbulence-related calculations, we only used R.M. Young-reported wind speeds between m/s and m/s, the range typically used to calibrate the NRG Maximum #0 anemometers. Finally, to avoid the potential for undetected icing of the anemometers to contaminate the data, we selected only observations between the months of May and September at Sites 00 and 00. Icing was not expected to impact Sites 003 and 00. Theory The turbulent energy in the wind flow manifests itself through variations in both the speed and direction. Longitudinal turbulence causes a change in speed, whereas lateral or transverse turbulence, which is usually of a comparable magnitude, causes a change in direction. The turbulence intensity (TI) is computed as the quotient of the standard deviation of the observed speed and the mean speed for each 0-minute period. The standard deviation of direction (DSD) provides a comparable measure of transverse turbulence. Both cup and propeller anemometers experience a lag in response to longitudinal speed changes, as indicated by their distance constants. This lag is often implicated as a cause of anemometer overspeeding. The NRG and R.M. Young anemometers have similar distance constants and therefore should experience similar degrees of such overspeeding under the same conditions. The Risoe anemometer should be somewhat less susceptible to this problem. However, in any case the impact is relatively small under most conditions. Separate analyses were run using the instrument-specific calibration slopes and offsets at Site 00. 3

The R.M. Young anemometer, however, is uniquely susceptible to underspeeding caused by changes in wind direction. Such changes force the wind vane to constantly realign itself with the horizontal velocity vector, introducing a directional lag. Since the component of the scalar speed v that is parallel to the propeller axis is given by vcos(θ), where θ is the angle between the wind vector and the axis, it is reasonable to suppose that the degree of underspeeding over an extended period is proportional to the cosine of the standard deviation of direction. To correct for this effect and recover the free-stream speed, we propose dividing the observed wind speed by cos(dsd). Results and Discussion Dependence on Speed Plots of the cup anemometer wind speeds as a function of those observed concurrently by the R.M. Young anemometers reveal very tight correlations between the two readings. Figure contains two examples of these plots. In the first plot, it is notable that the scatter at very low speeds tends to drop below the regression line; in the other graph, the scatter at the same speeds drifts above the regression line. Site 00 0-m Speeds Site 00 0-m Speeds NRG Maximum #0 Speed (m/s) 0 0 0 y =.057x + 0.035 R² = 9 0 5 0 5 0 Risoe Speed (m/s) 0 0 0 y =.005x + R² = 99 0 5 0 5 0 Figure. Scatterplots of Simultaneous 0-m Wind Speeds Observed at Site 00 To show the different responses of the sensors in more detail, Figures and 3 plot the wind speed ratios between sensor pairs as a function of the wind speeds observed by the R.M. Young anemometer. Underspeeding by the R.M. Young is indicated by a ratio less than one. For anemometers with a similar response, the plots should show a tight scatter near a ratio of one at most speeds, with random scatter at speeds near the instruments cut-in. The two series of plots show decidedly different signatures, however. For the R.M. Young / NRG anemometer pairs at Site 00, the scatter in the speed ratio as a function of speed is tightly clustered at a ratio near one at speeds greater than about m/s; at lower speeds, random scatter centered near the same ratio was observed. There is, however, a hint that the scatter at low speeds may be biased toward speed ratios greater than one. The R.M. Young / Risoe anemometer pairs at Site 00 demonstrate mostly tight scatter at all speeds. However, the center point of the scatter varies with speed. At speeds greater than about m/s, the average speed ratio is nearly constant at a value near ; below m/s, the mean ratio drops slowly with declining speed until it reaches about m/s, where it drops rapidly.

0-m Anemometer Pair -m Anemometer Pair RM Young / NRG Speed Ratio.5..3.. 0. 0. 0 5 0 5 0 RM Young /NRG Speed Ratio.5..3.. 0. 0. 0 5 0 5 0 Figure. Plots of R.M. Young / NRG Wind Speed Ratio as a Function of Wind Speed at Site 00 0-m Anemometer Pair RM Young / Risoe Speed Ratio.5..3.. 0. 0. 0 5 0 5 0 0-m Anemometer Pair 0-m Anemometer Pair RM Young / Risoe Speed Ratio.5..3.. 0. 0. 0 5 0 5 RM Young / Risoe Speed Ratio.5..3.. 0. 0. 0 0 Figure 3. Plots of R.M. Young / Risoe Wind Speed Ratio as a Function of Wind Speed 5

We hypothesize that the different patterns indicate that the problem of overspeeding due to longitudinal turbulence becomes worse at low speeds for sensors with relatively large distance constants. The NRG and R.M. Young sensors, both of which have large distance constants, provide similar speed readings at low speeds, despite some scatter, but the R.M. Young sensor reads significantly higher than the Risoe anemometer at speeds below 5 m/s. The different patterns may also reflect the lower cut-in speed of the Risoe anemometer, since the wind speed will more often drop below the cut-in for the NRG and R.M. Young models. Neither hypothesis could be confirmed, however. Dependence on Transverse Turbulence Turbulent energy in the horizontal wind flow manifests itself through fluctuations both along the axis of the flow (turbulence intensity - TI) and transverse to the flow (directional standard deviation - DSD). Figure provides examples of the strong linear relationship between the two. (It should be noted that turbulent energy also is present in the vertical component of the flow but is not analyzed here due to lack of vertical speed and turbulence measurements.) Site 00 -m Site 00 0 m DSD 0 0 y =.3x +.73 R² = 9 0 0. 0. 0.3 0. TI DSD 0 0 0 y = 7.57x + 39 R² = 97 0 0. 0. 0.3 0. TI Figure. Plots of DSD as a Function of TI We assessed the relationship between the turbulence transverse to the horizontal wind flow and the variation between the anemometer types. Figure 5 contains scatterplots of the wind speed differences between the anemometers as a function of cos(dsd) for the NRG and Risoe sensor pairs. For both cup models, the amount of underspeeding by the RM Young anemometer increases with the transverse turbulence, and at high turbulence levels can amount to several percent, a substantial effect for wind energy. However, for the Risoe anemometer pairs, the underspeeding becomes noticeable only for cos(dsd) values greater than about (DSD <.5 o, or TI ~0.-). For the NRG pairs, the impact of turbulence is evident over the full range. We next multiplied the wind speeds reported by the R.M. Young anemometers by /cos(dsd) and plotted the adjusted speeds as a function of the cos(dsd). Figure contains separate plots for the NRG and Risoe anemometer pairs. The NRG anemometer pairs appear to confirm that transverse turbulence causes propvane underspeeding, as the slopes are not statistically different from zero for all but the -m level at Site 00. While this suggests that the DSD correction is useful, the variation of the intercepts implies that other factors are impacting the anemometer relationships.

For the adjusted Risoe anemometer pairs, the portion of the graph where the observed speed difference was dependent on the cos(dsd) has become flat, but the slope is now reversed at low turbulence. We believe this pattern may be because the Risoe anemometer itself underestimates the scalar wind speed when the wind vector has a vertical component. Since horizontal turbulence is generally accompanied by vertical turbulence, which causes the wind vector to stray off-horizontal, the effects of transverse turbulence on the Risoe and RM Young anemometers may be comparable at low to moderate turbulence levels and therefore cancel each other. NRG Anemometer Pairs Risoe Anemometer Pairs Speed Difference (%) % 0% % % 3% % 70 75 0 5 90 95.000 Cos (DSD) Site 00 m Site 00 0 m Site 003 53 m Site 003 3 m Site 00 3 m Speed Difference (%) 0% % % 3% % 5% % 7% Site 00 0 m Site 00 0 m Site 00 0 m 5 7 9.00 Cos (DSD) Figure 5. Plots of Observed Speed Difference as a Function of cos(dsd) NRG Anemometer Pairs Risoe Anemometer Pairs % 0% Adjusted Speed Difference (%) 0% % % 3% % 70 75 0 5 90 95.000 Cos (DSD) Site 00 m Site 00 0 m Site 003 53 m Site 003 3 m Site 00 3 m Adjusted Speed Difference (%) % % 3% % 5% % 7% Site 00 0 m Site 00 0 m Site 00 0 m 5 7 9.00 Cos (DSD) Figure. Plots of Adjusted Speed Difference as a Function of cos(dsd) Summary and Conclusions In this analysis, we have compared the wind speed measurements from several R.M. Young 0503 prop-vane instruments with those of NRG Maximum #0 and Risoe P5A cup anemometers. Consistent with previously published results, the R.M. Young instruments recorded lower wind speeds with respect to the two types of cup anemometers. In general, our results suggest that the precise method for adjusting each R.M. Young anemometer to a cup anemometer standard should be determined on-site when side-by-side data are available. However, when employed for resource assessment, a single R.M. Young prop-vane is often the sole 7

instrument installed at a monitoring level. This situation necessitates the derivation of a turbulence-based adjustment that can be computed from the speed and direction data provided by a single instrument. We have assumed that the standard deviation of the wind direction for each 0-minute observation is a good measure of the impact of off-axis winds not observed by the R.M. Young anemometer. Based on the cosineresponse of a propeller anemometer, we multiplied the reported speeds by /cos(dsd) in order to estimate the free-stream horizontal wind speed observed by each corresponding cup anemometer. For the NRG / R.M. Young pairs, the result was to eliminate most evidence of underspeeding. Conversely, for the Risoe / R.M. Young pairs, our results suggest that the same adjustment is only applicable when the cos(dsd) was less than about (DSD >.5 o ); under lower turbulence conditions, no relationship is implied and this adjustment is, therefore, not recommended. This research has addressed the underspeeding by prop-vane anemometers associated with turbulence. It is stressed that other factors, such as differing anemometer distance constants, calibration, degradation, tower effects, and off-horizontal flow must also be analyzed in order to fully understand differences between anemometers used for wind resource assessment. These topics are the subject of continuing research at AWS Truewind. References Hunter, R.S. 990. The Accuracy of Cup Anemometer Calibration With Particular Regard to Testing Wind Turbines. Wind Engineering, ()3 3 Morris, V.R., et al. Comparison of Anemometers for Turbulence Characterization, Windpower 9, Seattle, Washington Pedersen, T.F.; Schmidt Paulsen, U., Classification of operational characteristics of commercial cupanemometers. In: Wind energy for the next millennium. Proceedings. 999 European wind energy conference (EWEC '99), Nice (FR), -5 Mar 999. Petersen, E.L.; Hjuler Jensen, P.; Rave, K.; Helm, P.; Ehmann, H. (eds.), (James and James Science Publishers, London, 999) p. -5