Wind Turbines
S.L. Dixon B. Eng., Ph.D., C.A. Hall Ph.D., in Fluid Mechanics and Thermodynamics of Turbomachinery (Seventh Edition), 2014
Acoustic emissions
Wind turbines undoubtedly generate some noise but, with the improvements in design in recent years, the level of noise emitted by them has dropped remarkably.
Aerodynamic broadband noise is typically the largest contributor to wind turbine noise. The main efforts to reduce this noise have included the use of lower blade tip speeds, lower blade angles of attack, upwind turbine configuration, variable speed operation, and specially modified blade trailing edges and tip shapes. For the new, very large (i.e., 1–5 MW size) wind turbines, the rotor tip speed on land is limited (in the United States the limit is 70 m/s). However, large variable speed wind turbines often rotate at lower tip speeds in low speed winds. As wind speed increases, the rotor speed is allowed to increase until the limit is reached. This mode of operation results in much quieter working at low wind speeds than a comparable constant speed wind turbine.
The study of noise emitted by wind turbines is a large and complex subject. No coverage of the basic theory is given in this chapter. Numerous publications on acoustics are available and one particularly recommended as it covers the study of fundamentals to some extent is the white paper by Rogers and Manwell (2004), prepared by NREL. A wide ranging, deeper approach to turbine noise is given in the NASA/DOE publication “Wind Turbine Acoustics,” by Hubbard and Shepherd (1990).
A particular problem occurs in connection with small wind turbines. These turbines are sold in large numbers in areas remote from electric utilities and are often installed close to people’s homes, often too close. There is an urgent need for reliable data on the levels of noise generated so that homeowners and communities can then reliably anticipate the noise levels from wind turbines prior to installation. The NREL have performed acoustic tests (Migliore, van Dam, & Huskey, 2004) on eight small wind turbines with power ratings from 400 W to 100 kW to develop a database of acoustic power output of new and existing turbines and to set targets for low noise rotors. Test results will be documented as NREL reports, technical papers, seminars, colloquia, and on the Internet. In comparing the results, Migliore et al. reported that, following improvements to the blading, the noise from the Bergey Excel (see Figure 10.4(b)) was reduced to the point that the turbine noise could not be separated from the background noise. As a result, any further testing will need to be done in a much quieter location.
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