|
Noise from offshore wind turbines 2 Noise Measurements2.1 Loudspeaker measurementsThe proposed measurement setup with the microphone at a height of 3 to 5 m above the sea surface is expected to correspond to a measurement above a fully reflecting ground surface. Above a certain frequency the result of this is a +3 dB addition to the free field value. To test if this works in practice, a pink noise signal from a calibrated loudspeaker hanging from the nacelle was measured at the new reference position. The source height was 60 m, the microphone height was 3.5 m and the horizontal distance was 125 m. In Figure 2. the measured noise levels are compared to the calculated noise levels using the NORD2000 model and a simple model using spherical spreading + 3dB. The input data for the NORD2000 calculation is shown in Figure 4. Figure 2 Comparison of measured noise and calculated noise Figure 3 Comparison of Excess attenuation (sound pressure levels in excess of spherical spreading) for NORD2000 and + 3 dB Figure 4 Computation parameters applied in Nord 2000 to obtain the results shown in Figure 2 and Figure 3
Generally there is a good agreement between measurement and calculation as the overall A-weighted sound pressure levels are within 1 dB of each other. Above 500 Hz Spherical spreading + 3 dB and NORD2000 are almost identical. Between 100 Hz and 500 Hz NORD2000 fits the general behaviour of the measurements better than Spherical spreading + 3 dB. Figure 3 shows the large variation in the ground effect with deviations from +3 dB of up to 4 dB. Below 100 Hz the measurements are expected to be highly influenced by background noise, giving too high values. The background noise at the frequencies from 25 Hz to 80 Hz is estimated to be around 40 dB to 45 dB. The deviation in ground effect from + 3 dB in this region is up to 10 dB. The difference between NORD2000 and Spherical spreading + 3 dB is so small that for simplicity it is recommended to use Spherical spreading + 3 dB, when calculating the Apparent Sound Power Level from measurements. If however the noise from the wind turbine has a significant content of noise below 100 Hz, it would be more correct to use the NORD2000 correction for the ground effect. 2.1.1 Loudspeaker calibrationThe loudspeaker was calibrated through a measurement setup with a similar geometry as for the noise measurements. The loudspeaker was hanging from a ladder at a height of 4 m to the centre. Noise measurements were made with the microphone on the hard asphalt paved ground at a distance of 8 m. Thus, the directivity and the efficiency of the loudspeaker were included in the calibration. The sound power level of the loud speaker in 1/3-octave bands is shown in Figure 5. Figure 5 Sound power level of the loudspeaker in 1/3-octave bands 2.2 Wind Turbine Noise measurementsThe noise measurements were made at a 2MW wind turbine situated in interior Danish waters. The wind speeds were from 7 m/s to 10 m/s, and the wave height was app. 1 m. The microphone height was 3 m. The results of the measurements are given in Figure 6. Figure 6 Measurement results
The average difference between the total noise measurements and the regression line through the background noise measurements is 4.3 dB. According to [1] the difference shall be 6 dB or more. The background noise was generated by waves hitting the vessel. As the measurements were made at a location, where the wave height was rather small, this can prove to be a general problem with the method. The background noise can be reduced by using a microphone placed at a higher position or at a larger distance from the vessel. The measurement distance was at the outer limit of the range given in [1] i.e. R0 + 20 % = 125 m. If the shortest distance, R0 - 20% = 85 m, had been applied, the noise level from the turbine would be expected to be app. 2 dB higher, and the 6 dB difference criteria would have been met. Increasing the microphone height from 3 m to 5 m would give a smaller impact of the background noise. The background noise was dominated by wave noise, while wind induced noise in this case was negligible. In Figure 7 the background noise and the total noise at a wind speed of 8 m/s is given as 1/3-octave band spectra. Figure 7 Comparison of Total Noise and Background Noise in 1/3-octave band spectra
The results given as the Apparent Sound Power level of the measurements relative to the results from an accredited measurement for a land based wind turbine of the same make and type are given in Table 1. The offshore measurements are corrected for the background noise even though the 6 dB difference was violated. A second order regression is applied, as this was done in the accredited report to make a direct comparison possible.
Table 1. Apparent sound power level for the offshore wind turbine relative to results from an accredited measurement at a similar wind turbine on land. The value at 6 m/s has been extrapolated from a second order regression through the data points. The results of the offshore measurements are 1 dB to 3 dB above the results from the land based wind turbine. This difference is within what could be expected when comparing two different wind turbines of the same type on land. A comparison of the frequency distribution in 1/3-octave bands is shown in Figure 8. Figure 8 Comparison of sound power levels in 1/3-octave bands
2.3 CommentsThe results of the measurements indicate that it is possible to make noise measurements on offshore wind turbines according to the general principles in IEC 61400-11:2002 with a few changes.
where LAeq,c is the background corrected A-weighted sound pressure level R1 is the slant distance in meters from the rotor centre to the microphone and S0 is the reference area, S0 = 1 m2 The background noise from the waves hitting the vessel is the main problem. To reduce the influence of this, it is recommended to use the shortest allowable measurement distance given in the standard, i.e. R0 – 20 %, and use as high a microphone position as possible.
|
