Noise is generation can be seen from two aspects; the aerodynamic forces of the wind on the turbine blades, and the mechanical operations of the turbine. Modern gearboxes are now very quiet and therefore the dominant noise sources are located on the blade. Noise produced by turbines can be broken down into separate categories.
Tonal – Noise at discrete frequencies. It is caused by turbine components such as meshing gears, non-aerodynamic instabilities interacting with a rotor blade surface, or unstable flows over holes or slits or a blunt trailing edge (Rogers & Manwell, 2002).
Broadband – Continuous distribution of sound pressure with frequencies above 10 Hz. Broadband noise is caused by interaction of blades with atmospheric turbulence.
Low Frequency Noise – Noise in the range of 20 Hz to 100 Hz.
Infrasound – Noise below the 20Hz range.
Impulsive – Short acoustic impulses or thumping sounds that vary in amplitude with time. Impulsive noise is caused by interaction of blades with disturbed air flow around the tower of a downwind machine.
Source: Renewable Energy Research Laboratory
Fig 1: Sound pressure level
Amplitude modulation (AM) of aerodynamic noise from wind turbines is a phenomenon that occurs when broadband noise is modulated (slowly changing the amplitude with time). It arises when the blades of the rotor pass through different zones or directions of wind. The “swishing” or “thumping” sounds that can be heard near wind farms is a product of amplitude modulation. This is considered to be the most annoying aspect of wind turbine noise for residents that live near these farms (Davidsen, 2009). The causes of amplitude modulation are not fully understood and it cannot be fully predicted at the current state of art (Larsson & Ohlund, 2014). Experts concluded that the sounds from wind turbines are not unique and there is no evidence to believe that the sounds from wind turbines could plausibly have direct adverse health consequences.
Wind turbine operation creates varying levels of noise dependent on many factors, which has potential to be of concern for human health (McMurty, 2010) (McMurtry & Krogh, 2014). The topic of noise being harmful to humans is controversial with studies showing results for both sides and no definitive answer to date. The effects of noise on people can be classified into three general categories.
· Subjective effects including annoyance, nuisance and dissatisfaction.
· Interference with activities such as sleep, speech, and learning.
· Physiological effects such as anxiety, tinnitus, or hearing loss.
Modal Analysis is used to find out modal parameters so as to study vibration patterns and parameters for dynamic properties. Includes several subsystems it can be simple or a complex model. The linear systems of excitation can be classified as sum of contributions of modes of the system. There are some limitations that operational modal analysis can overcome experimental modal analysis. In classical modal analysis are analyzed using measurements of both input force and output force.
Impulse Response Function (IRF) and Frequency Response Function (FRF) are mainly used in the data for modal parameter extraction can be determined by artificial excitation. Input force is very difficult to measure in the large structures like wind turbines. Inputs like wind are complicate to characterize instead, a localized excitation can be sufficient to excite heavy structure, these excitations are impossible to separate from the environmental excitation.
2. Analysis of Noise & Vibration:
The major aspect of the noise caused by the wind turbine is due to the design aspect of the tower and blades attached to rotor shaft. Due to the rigidly attached blades to the rotor shaft the noise will occur. Some designs have blades that can be pitched and other designs change the rotor speed as the wind speed changes. Generally, turbine motors will be upwind or downwind of the tower. Based on the designs of the blades the noise varies as per the different ways of operation. In general, lower rotational speeds and pitch control result in lower noise generation, as the upwind rotors are opposed to downwind rotors.
Noise generated from the aerodynamic design is very sensitive as it occurs from the tip of the blade. To reduce the aerodynamic noise from the blades many of the modern designs of turbines are introduced in such a way the rotation of the blades will not exceed 65m/s. Generally, we can observe the speed wind turbines rotate at a low speed in low wind increase in higher speed till the limiting speed is reached. This results in much quieter operation in low winds than a comparable constant speed wind turbine. Small wind turbines (with ratings less than 30 kW) are also often variable speed wind turbines. These smaller wind turbine designs do not always limit the rotor tip speed in high winds to about 65 m/s. This can result in greater noise generation than would be expected, compared to larger machines. This is also perhaps due to the lower investment in noise reduction technologies in these designs. Some smaller wind turbines regulate power in high winds by turning out of the wind. This type of operation may affect the nature of the sound generation from the wind turbine. Noise Propagation in order to predict the sound pressure level at a distance from a known power level, one must determine how the sound waves propagate.
In general, as noise propagates without obstruction from a point source, the sound pressure level decreases. The initial energy in the noise is distributed over a larger and larger area as the distance from the source increases. Thus, assuming spherical propagation, the same energy that is distributed over a square meter at one meter from a source is distributed over 10,000 m2 at a distance of 100 meters away from the source and with spherical propagation; the sound pressure level is reduced by 6 dB per doubling of distance. This model of spherical propagation should be modified in the presence of reflective surfaces and effects. Details of sound propagation in general are discussed in Beranek and Vers (1992).
The development of an accurate noise propagation model generally must include the following factors: Source characteristics (e.g., directivity, height, etc.), distance of the source from the observer, air absorption, which depends on frequency, ground effects (i.e., reflection and absorption of sound on the ground, dependent on source height, terrain cover, ground properties, frequency, etc.), blocking of sound by obstructions and uneven terrain, weather effects (i.e., wind speed, change of wind speed or temperature with height). The prevailing wind direction can cause considerable differences in sound pressure levels between upwind and downwind positions.
3. Solutions for Noise and Vibrations from wind Turbine:
There are diverse ways in reduction of noise from the wind turbines based on the characteristics of noise generation. The main way is to design wind turbine with acoustic behavior. Majorly, researchers must design wind turbine in such a way that it should not produce much noise on the other hand it shouldn’t reduce the power generation.
3.1. Aerodynamic noise reduction:
Aerodynamic noise reduction can be done through adaptive approaches and blade modification methods.
There are many adaptive ways to reduce the noise from the wind turbine. Increase in rotational speed of the turbines will produce more noise. So, in order to decrease the noise, the rotational speed of the blades is to be decrease but it affects the power input of the wind turbine. In this case the turbines should be rotated to certain wind velocities which can reduce the sound when the heavy winds occur. The other approach is to change the pitch angle of the blades which also effects in the noise control. If we increase the pitch angle of the blade results in reduction of angle of attack, if the angle of attack increases the turbulent boundary on the airfoil grows which leads to increase in noise by wind turbine. But the major drawback is due to decrease of angle of attack the power generation will be decreased.
Blade Modification Methods:
Many studies have been focused mainly on the trailing edges which produce more noise. One such method is through usage of acoustically optimized airfoils, the project conducted on in 2003-2007 in Europe in replacing of silent airfoils with the existing airfoils called the SIROCCO (Silent rotors by acoustic optimization) in the outer parts of the base line blades which are exposed to maximum velocities. Some of the turbines which are tested by SIROCCO project showed a good result in minimizing the noise levels. In the present mounted wind turbines have a controller to change the pitch angle to reduce the angle of attack based on the wind velocity and optimize the power generation. Similar controllers are used to change blade between different modes by retracting at higher frequency and serration in lower frequency to reduce the noise reduction.
Fig 2: Components and Total Sound Power Level of a Wind Turbine
3.2. Mechanical Noise Reduction:
Mechanical noise reduction can be done majorly on the rotating components. Vibration controllers are used to eliminate unwanted vibrations. Different control laws can be chosen to reduce the unwanted vibrations. Inherently, absorbers work very effectively to reduce the vibrations. This includes closing holes of nacelles which decrease sound transmitted to air, insulation and isolating materials. Aside the loss of power and maintenance cost are occurred due to fault gear box. In order to that researches have to develop fault diagnostic system for gearboxes. Several fuzzy classical techniques are introduced for fault detection.
Noise and vibration caused by wind turbines are from various sources. Even though the turbines have gotten quieter with time due to technology, they are still of environmental concern. Government need to address the issues of the public living in the close proximity of the wind farm. At times, wind turbines run with partial load so that during that time, living being and wind farms can co-exist peacefully. However, this leads to production of less energy which is a loss.
Installing damping systems produce counter-vibrations which in turn can reduce the noise to an extent. But solutions like these are not cost-effective, further research is needed in this area. Along with following the standard and regulations with respect to noise and vibration, the manufacturers of the small wind turbines need to make comprehensive sound power level measurements based on these standards.
Anthony L Rogers, James F Manwell, “Wind Turbine Noise Issues”, 2004
2. Beranek, Leo L; Istvan L., “Noise and Vibration Control Engineering: Principles and Applications”, 1992.
Conny Larsson and Olof Ohlund, “Amplitude modulation of sound from wind turbines under various meteorological conditions”, June 2014
Davidsen B, “Low Frequency Noise Emission from Wind Farms, Lloyd’s Register ODS, 2009
5. J.F.Manwell, et.al., “Wind Energy Explained: Theory, Design and Application”, Jun 2002.
Robert Y McMurtry1,2 and Carmen ME Krogh, “Diagnostic criteria for adverse health effects in the environs of wind turbines”, 2014