Sound Propagation From Wind Turbines - A comparison of measurements and calculations at the forest site Ryningsnäs.

Sammanfattning: Today wind power developers are exploring the possibilities of establishing wind turbines in forest areas, therefore improved sound prediction models for complex terrains are valuable. The Swedish model used today is only applicable for hard flat ground without any vegetation, and is not adjustable to any meteorological parameters except the wind speed. The companies DELTA, DONG Energy and EMD International have in cooperation elaborated a prototype model, which implements the Nord2000 model in the software WindPRO. Vattenfall AB has participated in the reference group for the project. Originally, Nord2000 has been mainly applied for traffic noise calculations in the programme SoundPLAN. The objective of this master’s thesis is to compare results obtained from calculations with Nord2000 in WindPRO 2.7.388 Beta version with calculations made with Nord2000 in SoundPLAN, the Swedish model and measurements at the forest site Ryningsnäs. The main findings of the thesis are: ÅF-Ingemansson suggests a revision of the IEC 61400-11 standard for the emission measurement to prevent misinterpretations. The wind speed at hub height is sought instead of the recalculation from hub height to the reference height of 10 m. At present the one-third octave band data are not fully operating with the Nord2000 in WindPRO 2.7.388 Beta version, better results are instead obtained with octave band data. The prototype shows potential to bring adequate sound level results to the original version of Nord2000, but some further validations and parameter improvements need to be done before the final commercial version is released. A minor overestimation occurs at the downwind point EM when comparing the calculations with Nord2000 in WindPRO 2.7.388 Beta version with measurements; underestimations occur instead at the other points. DELTA assumes an overestimation of the sound pressure level calculated with Nord2000 in the upwind case compared with measurements, due to the fact that the shadow zone is not taken in regard, but in this study an underestimation instead occurs at point IM3/UW. The differences between calculations and the measurements may be a result of reverberation in the forest, and/or the positive temperature gradient that appears within the forest, causing the sound waves to turn downward to the ground. These hypotheses need to be further evaluated. A more detailed sound propagation model in comparison to the Swedish model could be very useful both for optimization of electric power and annoyance limitations. The Swedish model only gives a worst-case scenario. It is important for the user to understand the sensitivity of the parameters in Nord2000, to perform reasonable calculations. The Swedish National Environmental Protection Agency (Naturvårdsverket) needs to give clear directives of the model when the commercial version of Nord2000 in WindPRO is released