Wind turbine noise at neighbor dwellings, comparing calculations and measurements

Wind turbine noise at neighbor dwellings, comparing calculations and measurements Rune EGEDAL 1 ; Morten Bording HANSEN 2 ; Lars Sommer SØNDERGAARD 3 1, 2, 3 DELTA a part of FORCE Technology, Aarhus Denmark ABSTRACT Danish noise regulation for wind farms (currently BEK 1736) is based on calculated noise levels at residents from sound power level measurements at a wind speed of 6 and 8 m/s (10 m height). Doubt has been raised on whether the above-mentioned method gives the same results as actual measurements where residents live. The Danish Environmental Protection Agency (Danish EPA) has therefore initiated several projects with noise measurements at residents. DELTA has conducted one of the measurement campaigns where synchronized measurements were carried out both close to the turbines (IEC distance), at an intermediate position and at the residents both indoor and outdoor. The project consists of two types of measurements, the first being highly surveyed short-term measurements of 2-3 hours and the second being long term continuous measurements. The two different approaches ensure that different ranges of wind direction and wind speed were included in the measurements and hence describe the noise around the turbines and not necessarily only in downwind direction. The short-term measurements ensure that part of the measurements is conducted in a close to controlled environment. The results of the measurements are compared with the calculated values. The calculations are carried out using the prescribed method in BEK 1736 and a more advanced model, in this project the Nord2000 model. The goal of the comparison is to clarify the following: I) Are there systematic differences between calculation and measurements both indoors and outdoors? II) What is the uncertainty using measurements contra calculation? III) Does other wind speeds than 6 and 8 m/s and other wind directions, cause the wind turbines to emit more pronounced tones or low frequent noise? Keywords: Wind turbines and wind farms, Measurements, Sound propagation, Nord2000 model I-INCE Classification of Subjects Numbers: 23, 24 1. INTRODUCTION This paper compares noise measurements made at neighbor dwellings near a wind farm with calculations made for the same neighbor dwellings. Two different calculation methods are used, the first being the method described in the Danish noise regulation for wind farms BEK1736 (1), and the second being the more advanced Nord2000 model (2). In this paper, several parameters are investigated beside the comparison of measurements and calculations. For the long-term measurements, the noise emitted by the wind turbine is investigated for differences in regard to wind direction and wind speed. For the short-term measurements, indoor low frequency levels at the neighbor dwellings are compared to calculations conducted with the Nord2000 model. The project was initiated by the Danish Environmental Protection Agency, EPA, because there has been criticism of the current guidelines for measuring and calculating wind turbine noise. Complaints from neighbors have also been raised because they have measured higher indoor values than calculated according to BEK1736. 1 rue@force.dk 2 mbh@force.dk 3 lss@force.dk 2477

2. THE WIND TURBINE SITE 2.1 General The site used in this project was chosen by the Danish EPA who had the initial contact with the wind farm owners and the neighbors. Afterwards DELTA has visited the wind farm and the neighbors, where they have pointed out relevant measurement points indoor and provided their perception of the noise from the wind turbines. Furthermore, DELTA inspected the surroundings of the wind farm and neighbors to plan the measurement campaign. 2.2 Wind Turbines The wind farm consists of three wind turbines of the type Vestas V112 3.075 MW with a hub height of 84 meters and a rotor diameter of 112 meters. The Vestas V112 turbine is one of the most common platforms used in Denmark in relation to larger wind turbines. The turbines are situated in flat cultivated agricultural land. 20 older wind turbines (4 clusters) are identified around the wind farm in distances between approximately 1400 meters and 4500 meters. The noise contribution from the older wind farms have been calculated and evaluated as negligible compared to the noise from the wind farm in question. 2.3 Measurement setup The Danish EPA has as mentioned chosen the site and has chosen and made arrangements with three neighbor dwellings for measurements. The three dwellings are placed in three different directions from the wind farm. In Figure 1 an overview of the wind farm and surroundings is shown. Figure 1 Overview of the wind farm and placement of microphones, wind mast and neighbors. The three neighbors are marked with a circle and a line is drawn from the neighbor and towards the windfarm. The three wind turbines are marked with T1 (WT1), T2 (WT2) and T3 (WT3) and are situated close to the center point of the neighbor dwellings. Approximately along the line between the wind turbine and the corresponding neighbor 3 microphones are placed, they are marked with an x in Figure 1. The two microphones closest to the wind turbine are placed on a board according to the IEC 61400-11 ed. 3 standard (3). The microphones at the neighbors are placed on a tripod at a height of 1.5 meters (1). All outdoor microphones are equipped with both a primary and a secondary wind screen to suppress wind induced noise. The distance to the first two microphones measured from the corresponding wind turbine is 140 meters for the first microphone (P1) and approximately 300 meters for the second microphone (P2). The distance between neighbor microphones (P3) and the wind turbines are as follows: T1 neighbor east, 627 meters; T2 neighbor north, 841 meters; T3 neighbor southwest, 620 meters. The 9 microphone measurements were synchronized in time and have been set up for the entire measurement period. Measurements indoor has also been conducted during periods where the neighbors were not at home. The indoor measurements were only measured for shorter periods. As 2478

many appliances as possible, have been shut down for the measurement period. Indoor measurements have only been conducted for two neighbors respectively north and southwest. Besides measuring the noise at the 9 outdoor microphone positions the wind speed and direction is recorded simultaneously. Two wind masts were placed at two neighbors and one wind mast was placed near wind turbine T3, the height of the wind masts were 10 meters. In order to calculate the true wind speed, data from the wind turbines was logged e.g. produced power, yaw angle, nacelle wind speed etcetera. 2.4 Weather conditions The measurements were conducted from late evening the 3 rd, to afternoon the 6 th of January 2017. In Table 1 the weather data for the measurement days can be seen in a brief overview. Table 1 Weather conditions during the measurements Day Temperature Humidity Cloud cover Air pressure 3 rd Jan 3 to 7 o 70 to 80% 4/8 to 8/8 990 to 1000 hpa 4 th Jan -3 to 5 o 50 to 85% 0/8 to 7/8 990 to 1020 hpa 5 th Jan -10 to -3 o 57 to 92% 0/8 to 6/8 1020 to 1040 hpa 6 th Jan -12 to -5 o 80 to 90% 0/8 to 8/8 1030 to 1040 hpa During the measurements, there was a brief period with rain in the morning the 4 th of January. Wind speed and direction was measured during the entire period at the wind turbines. For some of the time measurements were conducted at two of the neighbor dwellings. In Figure 2 the wind speed and direction for the entire period is shown. Figure 2 Wind speed and direction. The upper plot is deducted from turbine T2 and the lower is measured at the neighbor to the north. The wind speed at 10 meter height is deducted from the measured power from the wind turbine where appropriate (1). The periods where the power produced exceeded 95% of rated power, a kappa factor based on the measurements where the rated power was below 95% of rated power was calculated. The kappa factor was used to correct the wind speed measured by the wind turbine nacelle anemometer. There is an overall good correlation between the measurements from the wind turbine and the measurements from the wind mast at the neighbor to the north. The wind speed range for the entire period covers wind speeds from 2 m/s to 14 m/s and directions going from west (270 o ) over north(360 o ) to south(180 o ). The fluctuating direction seen at the neighbor measurement is due to the low wind speed which affects the wind vane. 2479

3. MEASUREMENTS 3.1 General measurements Measured L A90 noise values for the three measurement positions (P1, P2 and P3) at each wind turbine for the 5 th of January are shown on Figure 3. The positions P1 and P2 are +6dB measurements, (3) compared with P3. The noise levels at P1 and P2 in Figure 3 are not corrected for the +6dB reflection. L A90 represents the continuous background noise in the area, filtering out spurious noise sources such as cars passing. The black line is a binary line indicating whether the turbine is on/off (15/0) and whether valid turbine data is available (missing data). Disruptions in L A90 lines indicate invalid noise data due to overload or maintenance of the measurement system. Overload is mainly caused by wind induced noise in the microphone. Figure 3 Time slice day 3, 5 th of January, wind speeds in the range 2-6 m/s In general, it is clear from Figure 3 that in periods with the wind turbines turned off, the noise at P1 and P2, nearest to the wind turbine, decreases to approximately the same level as the noise at P3, farthest from the wind turbine. It is also visible that the noise level at P3 decreases when turbines are turned off, indicating that the wind turbine noise should be audible at neighbor positions. At WT1 P3 and WT3 P3, the total noise level at times, are above the noise level at P1 and P2. WT1 P3 and WT3 P3 are both locations surrounded by medium to high vegetation, whereas WT2 P3 has low vegetation, but are exposed to spurious noises such as cars passing and wind induced noise. This shows that, even with the statistical filtering of L A90, background noise at neighbor positions makes it difficult to measure wind turbine noise at these positions. To be able to measure the noise from the wind turbine, a significant signal-to-noise ratio is necessary (The Danish regulation specifies a minimum of 6 db between total noise and background noise). In Figure 4, two scatter plots for WT1 P1 & WT1 P3 are shown. The colors on the scatter plot indicates the wind direction, with red being downwind direction towards the receiver-point, and green being upwind away from the receiver-point. 2480

Figure 4 L A90 for different wind speeds and directions At P1 the general tendency for a pitch-regulated wind turbine (4) is seen in the scatter plot. The wind turbine starts at 3 m/s, and the noise increases up until 8 m/s, at which it reaches a steady plateau. The background noise at 4 9 m/s is significantly lower, 15-20 db, than the total noise, indicating that it indeed is wind turbine noise which is measured. At higher wind speeds, e.g.12 m/s, there is an indication that the background noise reaches a level within 3 db of the total noise, at which the summation of the wind turbine noise and background noise shows an increase in total noise level. At P3 the background noise is approximately at the same level as the total noise, indicating that the wind turbine noise is less or equal to the background noise. The noise increases linearly as expected for vegetation noise at increasing wind speeds (5). As mentioned a precondition, for determining the wind turbine noise level alone, is to have a significant signal to noise ratio. From Figure 4 it is seen that the total noise levels measured and the background noise levels are very similar. Hence it is difficult to validate whether the noise measured originates from the wind turbines. Furthermore, it introduces an uncertainty to the measurement because of the unpredictable nature of the background noise, which means it is difficult to conduct reproducible results for shorter measurement periods. 3.2 Direction and wind speed In Figure 5, a polar scatterplot for WT2 P3 is shown to visualize the effect of wind direction on wind turbine noise. The colormap has increased color-resolution at 4-8 m/s, because of the large change in wind turbine noise at these wind speeds. Downwind direction towards the receiver-point is at 180 degrees. The wind speed is visualized with the different colors, i.e. red is low wind 2-4.5 m/s. 2481

Figure 5 Left, polar scatterplot of L A90 10min averages, right, Polar scatterplot L pa,lf 10min averages WT2 The polar plot to the left in Figure 5 gives an indication that for similar wind speeds, around 6-7 m/s, measurements within 170-180 degrees shows a slightly higher noise level between ~32-38 db(a) than measurements at 325-355 degrees with corresponding wind speeds at which noise levels are at ~27-33 db(a). The Polar plot to the right is structured in the same way as the left, but shows the outlier unfiltered low frequency (10-160 Hz) noise level. Because the data is unfiltered, spurious noise sources, such as cars passing, are included in the 10 minute average. The tendency as seen in the left figure repeats for the right figure, but is less clear due to outliers caused by spurious noise sources. It is seen that low frequency noise at lower wind speeds around 5-6 m/s in the downwind direction is at the same noise level, ~30-40 db(a) as the noise level at wind speeds above 7 m/s but in the upwind direction 320-355 degrees. Hence for this scenario, it seems there is a higher low frequency noise level in downwind conditions. 3.3 Tonality Objective tone analysis for each 1 minute period has been performed for all recordings with the method prescribed in BEK1736, similar to the ISO 1996 Annex C method (6). Tone frequencies above 2 khz were found to be unimportant at neighbor distances due to air absorption, and only tone frequencies below 2 khz will be shown here. Pattern recognition on tone frequency / generator speed correlation was used to identify tones which most likely origin from the wind turbines. However, when listening to samples this works for higher frequencies, whereas the method, due to the fact that the lowest critical band is positioned with center frequency at 60 Hz, doesn t work well below 100 Hz. Since it is not possible to listen to all samples conclusions should not be made for frequencies below 100 Hz, since tonality in this area just as well can be planes, tractors and lorries. The tonal audibility, adjusted to the Danish limit resulting in a tone penalty, is shown in Figure 6 as a function of wind speed and tone center frequency for each wind direction. In Figure 6 the colored dots show tonal audibility at neighbor East, adjusted to the Danish limit, for each 1 minute recording as a function of; 1: wind speed calculated at 10 m height, 2: tone center frequency 3: wind direction (wind turbine yaw angle). For each wind direction, a histogram in grey is showing the number of 1 minute periods for each wind speed. Downwind ± 45 for the neighbor in question is at a wind direction of 279 to 9 degrees. A color bar for the tonal audibility in all of the four plots is shown in the bottom right corner, adjusted so a red colored dot shows a 1-minute period which should give a 5 db tone penalty according to the Danish regulation. Looking at downwind (± 45 ) the numbers of 1 minute recordings for the wind speeds 5-9 m/s are very similar, and the tonality seen at the different wind speeds should be comparable. The highest tonality seems to occur at 5-6 m/s, and nearly no tonality is observed for higher wind speeds. For the other wind directions, there is not sufficient data above 6 m/s to conclude whether tonality is lower at high wind speeds. Regarding wind speeds lower than 6 m/s there is almost only data at 5 m/s for downwind. Comparing 5- and 6 m/s it seems that the tonality is similar, maybe a bit lower for 5 m/s 2482

than 6 m/s. Looking at 9-99 degrees (crosswind) and 99-189 degrees (upwind) it seems that the tonal audibility is lower for upwind than for downwind for the wind speeds of 3-5 m/s. Comparing downwind and side-wind (9-99 degrees) at 5 m/s it seems that the level of tonality is higher in side-wind than in downwind, although it should be noticed that there are approximately 50 % more data at the side-wind direction than at the downwind direction. Tone center frequency [Hz] Tone center frequency [Hz] 279-9 degrees (downwind) 2000 400 1500 300 1000 200 500 100 0 2 4 6 8 10 12 0 189-279 degrees (crosswind SW) 2000 400 1500 300 1000 200 500 100 0 2 4 6 8 10 12 0 Wind speed [m/s] 9-99 degrees (crosswind NE) 2000 1500 1000 500 400 300 200 100 0 2 4 6 8 10 12 0 99-189 degrees (upwind) 2000 10 400 1500 5 1000 0 500-5 0-10 2 4 6 8 10 12 0 Wind speed [m/s] 300 200 100 WS histogram [1 min periods] WS histogram [1 min periods] Figure 6 Tonality at neighbor east at different wind directions and conditions. The analysis of the measurements indicates that there might be relevant and audible tones outside of the region where BEK1736 prescribes to analyze for tones (wind speeds of 6 and 8 m/s and downwind). Further sample listening and analysis is needed, and the recordings for the other neighbors will also be investigated in terms of tonality. 4. MEASUREMENTS AND CALCULATIONS COMPARED 4.1 BEK1736 Noise propagation calculations according to the Danish regulation BEK1736 assumes downwind for all wind directions. The following Figures 7-9 compares the measurements and BEK1736 calculations, includes total noise samples for wind directions within 45 degrees (± 22.5). The Danish regulation BEK1736 calculations are based on measured sound power levels for the wind turbines, during the compliance measurements carried out in January 2013. According to BEK1736, the uncertainties on these calculations are ±2 db. Figure 7 L A90 for measurements, 10min average, and calculations. 2483

In Figure 7 it is seen, that for measurements at high wind speeds 7-11 m/s, there is a low correlation between the measurements and the calculations, where for lower wind speeds there is a better correlation. As described earlier, WT1 P3 is a position with medium to high vegetation, which increases the background noise with increasing wind speeds. This seems a probable reason, for the low correlation between calculations and measurements at high wind speeds. Figure 8 L A90 for measurements, 10min average, and calculations. For WT2 P3 there is only downwind data for 4-6 m/s available as shown in Figure 8. For these three wind speeds there is a better correlation between calculations and measurements compared with the high wind comparisons of WT1 P3. The large confidence interval at 4 m/s, is due to the small number of samples, 4, for this wind bin. Figure 9 L A90 for measurements, 10min average, and calculations. In Figure 9, the measured mean values for low wind speeds 3-5 m/s, shows a good correlation between calculations and measurements for WT3 P3. It is also seen that the mean measured noise is lower than the predicted. 2484

Figure 10 Scatterplot showing L A90 for measurements, 1 minute sorted data, and calculations. In Figure 10, the data for WT1 P3 is analyzed in greater depth in order to make a fair comparison of the measurements and the calculated data. As mentioned for Figure 7 there is much vegetation at WT1 P3 and hence a higher background noise level contributing to the total noise level. In Figure 10 it is seen that the measured total noise and the calculated levels does not correlate very well, however taking the high background noise level into account the measured and calculated values are comparable for 7-9 m/s. When listening to the recordings for wind speeds around 6 m/s it is clear that the higher total noise level is due to wind induced noise. 4.2 Nord2000 comparison The more advanced Nord2000 model takes several conditions into account compared to the Danish regulation BEK1736. The main difference is that the Nord2000 model includes the shape of the terrain and several meteorological parameters. The uncertainty is estimated to be ±2 db. In this study, 18 different scenarios which occurred during the measurement period between the 3 rd and the 6 th of January, are calculated and compared with the measured noise levels during a time interval with the parameters set for the calculation. In Table 2 the 18 different scenarios are listed together with the corresponding input data for the Nord2000 model. Table 2 Meteorological parameters for Nord2000 calculations Scenario 1 2 3 4 5 6 7 8 9 Cloud cover [1/8] 6 1 1 3 4 7 6 6 0 Wind speed 10m [m/s] 9.6 10.8 8.4 8.6 8.4 8.8 8.7 7.2 7.0 Wind dir. [deg.] 294 311 329 222 355 349 345 348 346 Temperature [deg.] 5 5 4 4 4 4 4 4-1 Humidity [%] 70 74 70 68 69 69 69 73 55 Scenario 10 11 12 13 14 15 16 17 18 Cloud cover [1/8] 0 1 5 5 0 0 0 0 8 Wind speed 10m [m/s] 6.4 5.3 4.3 3.9 4.0 5.4 6.1 6.3 6.6 Wind dir. [deg.] 355 358 20 14 48 179 171 170 175 Temperature [deg.] -2-3 -5-5 -6-8 -7-4 -3 Humidity [%] 63 93 73 85 56 87 80 68 70 2485

All the scenarios are calculated with all three wind turbines operating. The calculations were conducted for all 9 microphone positions. In Figure 10, 11 and 12 the comparison of the measurements (red) and the calculated noise level by using Nord2000 (blue) is shown for possible combinations. All measured results are shown as 10 minute L A90 levels in order to exclude spurious noise. Figure 10 Comparison at position P1 for turbine WT1 For the positions near the turbines, P1 and P2, it is seen that there is an overall good correlation between the measured values and the calculated values. For scenario 2 the measurements deviates, which might be wind induced noise caused by the fairly high wind speed despite the windscreen. For scenario 16 a similar deviation is seen. This deviation might be due to negative temperatures causing a change in the ground absorption; hence the higher measured noise level. Figure 11 Comparison at position P2 for turbine WT1 2486

Figure 12 Comparison at position P3 for turbine WT1 In Figure 12 the measured and calculated values for position P3 is shown. The deviations seen for the high wind scenarios (1 and 2) are very clear for P3 compared to P2. The deviation is caused by vegetation noise during high wind speeds. For the rest of the measurements a general god correlation is seen. Indoor measurements were conducted on the 4 th of January at the neighbor to the north (7). These measurements were highly surveyed, and afterwards analyzed in 10 second intervals. The measurements have been subjectively sorted on a 1/3-octave level in order to remove outliers which do not inherit from the wind turbines. The noise indoor was measured in 3 positions, in a living room facing the wind turbines. One microphone placed 0.5 meters from a corner (P3 P2 ), and two positions pointed out by the neighbor. One in front of a window section facing the wind turbines (P3 P3 ) and one above the sofa (P3 P4 ). The calculations and measurements are conducted for low frequency as well as for normal frequencies. In Figure 12 the low frequency result is shown. The measured result is compared with the calculated noise level from Nord2000, corrected for the sound insulation of the dwelling. The sound insulation values are taken from the Danish BEK1736 (1). Figure 12 Comparison at position P2 for turbine WT1 Figure 12 shows two low frequency noise levels, the outdoor level measured at position P3 (P3 P1 ) and the low frequency noise level measured indoor at the 3 mentioned positions. For the outdoor 2487

position it is seen that the measured total noise level in general is higher than the calculated Nord2000 levels. It is also seen that the background noise is close to the total noise which affect the measured noise. The calculated level is only the contribution from the wind turbines and is hence lower than the measured level. For the indoor measurements a clear level difference is seen for the total noise and the background noise. In the figure the calculated noise levels are comparable to the noise levels from the corner position (P3 P2 ), which is the worst case measurement. Comparing calculated and measured indoor noise levels shows a good correlation. 5. FINAL REMARKS This project shows that for specific scenarios, there is a good correlation between calculations of wind turbine noise either following BEK1736 or using the Nord2000 model and noise measurements at neighbor dwellings. For scenarios where the calculated results deviate from the measurements, explanation has been sought and is found to be due to high background noise at high wind speeds where vegetation noise primarily becomes dominant. For high wind speeds, there seems to be a better correlation between calculations and measurements of indoor noise levels, in this case indoor low frequency noise levels. No systematic differences between calculations and measurements are observed. Using measurements at the neighbors compared to calculations are difficult because of the low signal to noise ratio at large distance from the wind turbines, which introduces a large uncertainty to whether the noise measured is from the wind turbine or the surroundings. Regarding the noise emitted from the wind turbines in other directions than downwind it is seen that the noise level downwind is higher than the noise level during upwind conditions. For tonality it seems that tonal audibility might be higher in other wind speeds than downwind. The tonal audibility at lower wind speeds seems also to be more prominent than at higher wind speeds. ACKNOWLEDGEMENTS The Project is financed by the Danish EPA, and is a part of several projects currently in Denmark. The authors would like to acknowledge; Carsten Thomsen for his great support in analysis of data, Erik Thysell for contributing to the Nord2000 calculations, Claus Backalarz for his critical eye reading the paper. REFERENCES 1. The Danish EPA, 2015, BEK 1736 Bekendtgørelse om støj fra vindmøller. (Statutory Order on Noise from Wind Turbines; Statutory Order no. 1736.) 2. Plovsing B, Proposal for Nordtest Method: Nord2000 - Prediction of Outdoor Sound Propagation, AV 1106/07, 2007. 3. IEC 61400-11:2012 Ed. 3, Wind turbines - Part 11: Acoustic noise measurement techniques. 4. Søndergaard LS, Nielsen SM, Pedersen TH. Støj fra vindmøller ved andre vindhastigheder end 6 og 8 m/s, TC-100531, DELTA 2014. (Noise from Wind turbines at other wind speeds than 6 and 8 m/s) 5. Søndergaard LS, Egedal R, Hansen, MB. Variation of wind induced non-turbine related noise due to position, shelter, wind direction and season, Proc 7th Wind Turbine Noise 2017, 2-5 May 2017; Rotterdam, Netherlands 2017. 6. ISO 1996-2:2007, Description, measurement and assessment of environmental noise - Part 2: Determination of environmental noise levels - Annex C 7. Orientering fra Miljøstyrelsen, Nr. 9 1997, Lavfrekvent støj, infralyd og vibrationer I eksternt miljø. (Orientation from the Danish Environmental Protection Agency, no. 9 1997, Low frequent noise, infrasound and vibrations in external environment) 2488