Noise from wind turbines under non-standard conditions

Transcription

1 Noise from wind turbines under non-standard conditions Lars S. Søndergaard a) DELTA Acoustics Agro Food Park 13, 8200 Aarhus N, Denmark Noise prediction for wind turbines and wind farms are often based on noise data achieved from measurements on single wind turbines assuming standard conditions with regard to both wind inflow to the turbine and wind shear. But wind turbines in wind farms do influence each other through the turbulent wake downwind of the wind turbines and also different wind shear situations is seen to result in different noise emission characteristics. In a full scale measurement setup the influence of turbulent inflow with respect to noise emission has been investigated. Correlation of the pressure variation on the surface of a fully instrumented wind turbine blade with far field acoustic noise measurements for the wind turbine in operation was made using an acoustic parabola measurement system. Another measurement campaign was carried out to investigate the influence of different wind shear conditions on sound power determination for the wind turbine and on the characteristics of the noise both in the near field and the far field of the turbine. The results from the two campaigns are presented and contribute to the understanding of the influence from both turbulent inflow and wind shear on noise emission. 1 INTRODUCTION Noise from wind turbines is often considered as a major factor when planning new wind farms to ensure that complaints from neighbours to the wind farms can be avoided. Noise prediction for wind turbines and wind farms are often based on noise data achieved from measurements on free standing wind turbines assuming standard conditions with regard to both wind inflow to the turbine and wind shear. This paper highlights some of the methods and main results from the two projects, Madsen 1 and Madsen 2 : a) lss@delta.dk

2 2 NOISE FROM WIND TURBINES IN WAKE (WAKE PROJECT) In a wind farm where the turbines influence each other through the turbulent wake downwind of the wind turbines source descriptions assuming standard conditions are expected to be inadequate. This motivates a full scale measurement setup where investigation of the influence of wake effects on noise emission from the wind turbines has been investigated. This has been executed as a supplement to the research project Experimental Rotor- and Airfoil Aerodynamics on MW Wind Turbines, led by Risø DTU 4. In the main project an extensive experimental work has been carried out to provide full scale results for validation of advanced numerical models used for prediction of noise radiation from wind turbine blades under different inflow conditions. The concept in the WAKE project Noise from wind turbines in wake, led by DELTA was to correlate the pressure variation on the surface of a fully instrumented wind turbine blade with far field acoustic noise measurements for the wind turbine in operation. For this purpose an acoustic parabola measurement system was designed to be able to measure in the far field the noise emitted from the wind turbine blade under different degrees of wake. 2.1 Methods for WAKE Project Determination of noise radiation patterns for wind turbines based on far field measurements work has earlier been presented using acoustic arrays 5. The experimental approach presented in the wake project however is rather simple and is based on a parabolic microphone measurement system (PMMS). In Fig. 1 left the measurement system is shown at the test site. The parabolic microphone measurement system is based on the properties of a parabolic reflector. The parabolic reflector amplifies the sound coming at normal incidence thus suppressing the sound coming from the sides. This makes the PMMS useful where directivity for focused measurements is needed. An improvement of the PMMS is made by using a cardioid microphone placed in the focus point of the parabolic reflector pointing towards the dish. The cardioid microphone suppresses the direct sound on the PMMS and ensures that mainly the normal incidence reflections on the dish will be measured thereby minimizing interference patterns. The registered acoustic signal from the PMMS has to be synchronized with blade positions and turbine operation. The directivity of a parabolic reflector is dependent on its physical size. Generally a parabolic reflector has a small aperture angle at high frequencies while it becomes larger with decreasing frequency. The designed PMMS uses a dish having a diameter of 2.4 m and the directivity has been tested and is shown in Fig. 2. The work is based on measurements carried out on a M80 2MW wind turbine, where the relationship between the far field noise levels and the surface pressure and inflow angles measured by sensors on an instrumented wind turbine blade was investigated, see Fig. 1 right. 2.2 Results for WAKE project Based on the measurement results obtained with surface pressure sensors and results from the far field measurements using the PMMS it is found that the variance of surface pressure at the trailing edge (TE) agrees with the theory with regard to variation of pressure spectra with varying inflow angle (Angle-of-Attack, AoA, see Fig. 4 right) to the blade. Low frequency TE surface pressure increases with increased AoA and high frequency surface pressure decreases with increased AoA, see Fig. 3. It also seems that the TE surface pressure remains almost unaltered during wake operation

3 Results from the surface transducers at the leading edge (LE) and the inflow angles determined from the pitot tube indicates that the inflow at LE is more turbulent in wake for the same AoA and with a low frequency characteristic, thereby giving rise to more low frequency noise generated during wake operation. The trailing edge surface pressure hardly varies with wake or power but is dependent of the inflow angle, AoA. The far field measurements supports that on one hand there will be produced relative more low frequency noise due to a turbulent inflow to the blade and on the other hand there will be produced less noise in the broader frequency range/high frequency range due to a lower inflow angle caused by the wind deficit in the wake. The net effect of wake on the total noise level is unresolved. The noise observed from a position on the ground is related to directional effects of the noise radiated from the wind turbine blade. Theoretical results confirmed by PMMS measurements concludes that for an observer position close to the ground directly downwind of the wind turbine the major part of the noise is received from the part of the rotor plane with downwards movement of the blades. This is also shown in Fig. 5. Due to a limited number of recordings for situations with wake and without wake at comparable situations regarding wind speed and blade configuration it has not been possible to get a precise estimation of the effect on the noise generation for a given wake degree based on the far field noise measurements. This means that the implementation of a more complex wake model in the program WindPRO has not been possible. 3 NOISE FROM WIND TURBINES DURING NIGHT (NOISE AT NIGHT PROJECT) For noise propagation from different noise sources like wind turbines it is well known that the atmospheric stability influences both wind speed and air temperature variation with height above terrain. This motivates another work - Noise from wind turbines during night - which purpose is to investigate to which extent the meteorological conditions influences noise from wind turbines in the surroundings with special regard to the difference between day and night more precisely to answer the following questions: Do meteorological conditions at night cause a change in the wind speed gradient between wind turbine hub height and 10 m above terrain causing a higher noise emission from the wind turbine, than assumed in the Danish noise regulation for wind turbines? Do meteorological conditions occur during night that causes a different noise emission from a wind turbine than measured during day time? Do meteorological conditions occur during night that causes that sound propagation is attenuated less than assumed in the Danish noise regulations for wind turbines? 3.1 Methods for Noise at Night In a stable atmosphere, which typically occurs during night and especially in clear weather, the wind speed observed in hub height of a wind turbine, are higher than observed in a neutral atmosphere at same wind speed for 10 m height. In a unstable atmosphere, which typically occurs during daytime and especially in clear weather, the wind speed in hub height of a wind

4 turbine equivalently will be lower than observed in a neutral atmosphere with the same wind speed in 10 m height. Since the noise transmission from the wind turbine depends of the wind speed around the blades of the turbine, it will therefore be experienced, that the transmitted noise (sound power level) at a constant wind speed at 10 m height will vary depending on the stability of the atmosphere. The biggest difference between wind speed in hub height and in 10 m height will occur during night. Analysis of meteorological data has been performed for four chosen synoptic Danish weather stations (Karup, Skrydstrup, Thyborøn and Copenhagen Airport) supplemented with extensive meteorological data from Høvsøre (Danish test site for large wind turbines). Two of the stations are located far from the coastline and the rest are located close to the coastline. All locations are spread geographically over Denmark. On the basis of these data, general statistics about wind conditions has been made, and wind speeds for the height 10 m and 90 m above terrain are compared. Change of wind speed by height for the 6 Klug-Manier stability classes 9 is shown in Fig. 6. From the above found weather statistics calculations of the terrain effect (A-weighted sound pressure level compared to free field) can be done by using the calculation model Nord and 8. Complementary to the analysis above noise measurements on wind turbines both single turbines and turbines in a park has been performed both during day and night time. 3.2 Results for Noise at Night The difference between wind speed at hub height and in 10 m height is seen to be larger at night than during day as also described in litterature 6. Generally wind speed at 10 m height is lower at night than during day, whereas the wind speeds at hub height generally do not differ significantly. In fig. 7 the cumulative frequency for day and night for wind speeds shows that the difference between night and day is small for wind speeds measured in 90 m height but large for wind speeds measured in 10 m height at Høvsøre. This is supplemented by fig. 8 showing the distribution for both night and day for the different Klug-Manier stability classes. Based on above, the noise emission from wind turbines is not expected to be higher at night than during the day. But if the sound power level of a tall wind turbine is determined on the basis of the measured wind speed at 10 m height, sound power level measurements performed during the night overestimate the noise transmission from the wind turbine significantly compared to the reference conditions from the Danish regulations 3. Seasonal variation is seen only to influence the conditions at 10 m height, whereas the influence at wind speed at hub height will be insignificant. No indication of influential differences is seen between coast near locations compared to inland locations besides the differences to be explained by higher average wind speed. Lower wind speed at 10 m height is expected to cause a lower background noise level at the residents close to wind turbines. It is possible that the lower masking from background noise level could lead to higher nuisance under these conditions. However the probability is low for these special conditions to occur at 8 m/s at hub height (2 % - 6 % for the examined weather stations) The influence on the sound propagation during downwind conditions due to variation of meteorological differences from day to night is without any real influence (±0.3 db) at distances from minimum distances for erection of wind turbines in Denmark to at least 2000 m. This is shown in fig. 9 where the general terrain effect prescribed by Danish regulation should be compared to the relevant meteo-classes.

5 The measurement method described in the Danish regulations 3 for determination of the sound power levels are tested both for neutral and stable metrological conditions both theoretically and by measurements, and there was no indication of an influence by the meteorological variations. The difference between the two sound power levels on 1/1 octave band level is shown in fig. 10. The deviations for the highest and lowest frequencies occur in the frequency area where the noise from the turbine is not significant and is influenced by background noise for the two measurement periods. Examples of pulsating noise with a variation of up to 5-7 db of the instantaneous noise level are seen from measurements carried out at night at distances m, see fig. 11. The pulsating character is also seen for comparable measurements at neutral weather conditions at day time with the same amount of variation although not for as steady periods as during the night. No examples were found, where the operation of more wind turbines at the same time increased the pulsation variation. In Denmark the sound power level is measured close to the wind turbine and the sound power level is then used to calculate the noise level to be expected at the closest neighbors to the turbine. For a case the noise level at a neighbor situated 537 m to the wind turbine was measured. The difference between the measured noise level and the expected noise level based on the guideline in the Danish regulation 3 was less than 1 db. This case contain prominent pulsation in the noise, which indicates that the average noise level, even with pulsation, does not increase compared to the calculated values. 4 ACKNOWLEDGEMENTS The noise at night project is funded by the Danish Environmental Protection Agency. Supplementary work was done by Risø-DTU. The wake project is publicly funded by the Danish Energy Agency under journal nr and co funded by DONG Energy, Statkraft Development, StatoilHydro and Vattenfall A/S. The project partners are DELTA (Project manager), Risø-DTU, EMD International and DONG Energy. 5 DISCUSSION AND CONCLUSIONS Different aspects for noise from wind turbines under nonstandard conditions has been investigated and presented to give a better understanding of the influence from both turbulent inflow and wind shear on noise emission. 6 REFERENCES 1. K. D. Madsen, EFP07-II, Noise emission from wind turbines in wake, DELTA, (2011) 2. K. D. Madsen, Noise from wind turbines during night time, Environmental Project no. 1415, Danish Environmental Protection Agency/DELTA, (2012) 3. Miljøstyrelsen (Danish Environmental Protection Agency), Bekendtgørelse om støj fra vindmøller nr. 1518, December 2006.

6 4. H. A. Madsen et al, The DAN-AERO MW Experiments - Final Report, Risø-R-1726, Risø DTU, September S. Oerlemens, B. M. López, Localisation and quantification of noise sources on a wind turbine. Proceedings from Wind Turbine Noise, Berlin G. P. van den Berg, Effects of wind profile at night on wind turbine sound, J. Sound Vib. 277(4-5), , (2004) 7. B. Søndergaard and B. Plovsing, PSO-07 project. Noise and energy optimization of wind farms. Validation of the Nord2000 propagation model for use on wind turbine noise, DELTA Report AV 1236/09, (2009) 8. Miljøstyrelsen (Danish Environmental Protection Agency), Bekendtgørelse om kortlægning af ekstern støj og udarbejdelse af støjhandlingsplaner, nr. 717, 13. juni TA-Luft, Erste Allgemeine Verwaltungsvorschrift zum BundesImmissionsschutzgesetz Technische Anleitung zur Reinhalturng der Luft, 1986

7 Magnitude [db] Fig. 1 Left: Parabolic measurement system (PMMS). Right top: Instrumented wing. Right bottom: Microphone section Directivity of Parabolic Measurement System - Normalized to On-axis Response Frequency [Hz] - 1/3 octave On-axis Fig. 2 Directivity of the PMMS

8 Fig. 3 Theoretical spectra for blade surface pressure at trailing edge (TE) as a function of inflow angle of attack (AoA). T up T down OP OT OM c AoA Fig. 4 Left: Sketch showing the focus areas OT, OP and OM for the PMMS and the time windows used for analysis of each blade passage T down and T up. Right: Illustration of inflow angle. The inflow angle is denoted AoA (Angle of Attack)

9 Height above terrain [m] , L eq (db) re 20 upa Up- and downwards movement of blade 1 NM (16/ ) and OM (16/ ) - pitch minus 1 - wind speed 6 m/s Frequency, Hz Blade 1 Up Blade 1 Down Fig. 5 Spectra obtained with focus on NM and OM for blade 1 on 16 July; pitch angle minus 1. Wind speed approx. 6 m/s at hub height. N for downward movement and O for upward Wind speed [m/s] I II III,1 III,2 IV V Fig. 6 Change of wind speed by height for each of the 6 stabilityclasses at a wind speed in 10 m height at 6 m/s. Klug-Manier stability classes: I: Very stable II: Moderately stable III/1: Neutral III/2: Neutral, slighter unstable IV Moderately unstable V: Very unstable

10 Fig. 7 Cumulative frequency for day and night for wind speeds measured in 10 and 90 m height at Høvsøre. Fig. 8 Frequency of the Klug-Manier stability classes at the synoptic weather stations FSN Karup and Thyborøn at day and night.

11 Difference in sound power level [db] Terrain effect [db] Horizontal propagation distance [m] Ref. meteorology Meteo-class 20 Meteo-class 24 Danish Regulation Fig. 9 Terrain effect in db as a function of distance from 100 m to 2000 m in steps of 100 m for propagation setup as provided by the Danish regulation and for the night time with meteorological setup represented by meteo-classes 20 and neutral - stable st dev neutral st dev stable Frequency, 1/1 octave [Hz] Fig. 10 Differences in 1/1-octave sound power level values determined at 8 m/s for the same wind turbine for either neutral and stable weather conditions

12 Fig. 11 Example of the variation in wind turbine noise measured for 20 seconds at a distance of 500 m