Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

374 Fluid Mechanics, Thermodynamics of Turbomachinery
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. 1–5MW size) wind turbines the rotor tip speed on land is limited (in the USA the
limit is 70m/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 et al. 2004) on eight small wind turbines with power
ratings from 400W to 100kW in order 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 10.4b) 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.
Addendum
A 5 MW REpower Systems wind turbine has now been installed (Oct 1, 2004) at
Brunsbüttel in Schleswig-Holstein, Germany, according to a report in Renewable
Energy World (Nov–Dec 2004). The 3-bladed rotor has a tip diameter of 126.3 m (blade
length 61.5 m, maximum chord 4.6 m) and a hub height of 120 m.
The various speeds and rotor speed range quoted are:
Rotor speed 6.9–12.1 rev/min
Rated wind speed 13 m/s
Cut-in wind speed 3.5 m/s
Cut-out wind speed 25 m/s (onshore)
30 m/s (offshore)