Methods Of Measuring Of Acoustic Emissions For Wind Energy Systems

Abstract

Wind Power is one of the prominent power resources that generate power with natural resources. Power generation using Wind Energy Systems such as Wind Turbines have been developed and have gained unimaginable growth. WES with rated power close to 12MW are being built in the current industries. Various industries utilize the generated power for their respective purposes. There are many different types of WES but the major hurdle for the WES is the space where they can be built without affecting the landscape of the region and the noise control. This high level of Noise is due to the gigantic sizes of the rotor blades and mass of each blade which is equivalent to multiple tons. This paper assesses the methods that are used to measure the sound levels of the Wind systems complying to IEC (International Electrotechnical Commission) standards. According to International Wind Turbine noise regulations, sound pressure levels of 35 dBA during the day and 30 dBA at night located inside residences are allowed. There are multiple methods that have been used to measure the acoustics for the wind energy systems and were instrumental in getting high efficiencies of the wind energy systems without causing any alarming inconvenience to the natural habitat near them.

Introduction

Noise from wind turbines may be characterized as aerodynamic or mechanical in origin. Aerodynamic noise components are either narrow-band (containing discrete harmonics) or broadband (random) and are related closely to the geometry of the rotor, its blades, and their aerodynamic flow environments. Mechanical noise components from operating bearings, gears, and accessories for many of the WES configurations are of lesser importance. The spectrum generally contains broadband random noise of aerodynamic origin, although discrete components identified as mechanical noise from the gearbox are also evident. Acoustic “noise” is defined as any unwanted sound. Concerns about noise depend on the level of intensity, frequency, frequency distribution, and patterns of the noise source, background sound levels. The effects of noise on people are subjective effects including annoyance, nuisance, dissatisfaction, and interference with activities such as speech, sleep, and learning. In this paper, Acoustic Emission and its effects to the Wind Turbines are reviewed.

Factors Сontributing to Цind Turbine Noises

Significant factors are relevant to the potential environmental impact of wind turbine noise. All acoustic technology is built on an understanding of three primary elements: Noise sources, propagation paths, and receivers as shown in Figure 1. The noise produced by wind turbines ranges in frequency from low values that are sometimes inaudible to higher values in the normal audible range. Although increased distance is beneficial in reducing noise levels, the wind can enhance noise propagation in certain directions and impede it in others. A unique feature of wind turbine noise is that it can result from essentially continuous periods of daytime and nighttime operation. This contrasts with the more common aircraft and road traffic noises that vary markedly as a function of time of day. Perception thresholds for humans are defined for both narrow-band and broadband spectra from systematic tests in the laboratory and from observations in the field. Also summarized are structural vibrations and interior sound pressure levels, which could result from the low frequency noise excitation of buildings.

Acoustic Emission (AE) is commonly defined as transient elastic waves generated during rapid release of strain energy, caused by a deformation of damage within, or on the surface of a material. AE is a technique capable of detecting dynamic changes and effectively in detecting the initiation and progression of defects particularly in metallic structures, where the attenuation of signal is low. The mechanical event can be produced by different sources such as cracks, plastic deformation, rubbing, cavitation, leakage, etc. Particularly, AE in rotating machinery is produced by the interaction at the interface of two different surfaces in relative motion, i. e. asperity contact. AE is a well-established NDT technique that allows defect initiation and evolution to be monitored in a component. In rotating machinery, these displacements are caused by the high frequency noise generated by the interaction of the interface of two different components rotating with respect to each other. AE is also capable of detecting dynamic changes and, therefore, capable of monitoring the evolution of damage effectively and efficiently in various materials including metals, composites, fiberglass, plastics, ceramics, wood and concrete. Thus, it is well suited for deployment in wind turbines.

Acoustic Emissions

There are four types of sound that can be generated by wind turbine operation: tonal, broadband, low frequency, and impulsive:

1. Tonal: Tonal sound is defined as sound at discrete frequencies. It is caused by 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.
2. Broadband: This is sound characterized by a continuous distribution of sound pressure with frequencies greater than 100 Hz. It is often caused by the interaction of wind turbine blades with atmospheric turbulence, and also described as a characteristic 'swishing' or 'whooshing' sound.
3. Low frequency: Sound with frequencies in the range of 20 to 100 Hz is mostly associated with downwind rotors (turbines with the rotor on the downwind side of the tower). It is caused when the turbine blade encounters localized flow deficiencies due to the flow around a tower.
4. Impulsive: This sound is described by short acoustic impulses or thumping sounds that vary in amplitude with time. It is caused by the interaction of wind turbine blades with disturbed air flow around the tower of a downwind machine.

Various sound generation mechanisms are discussed by Wagner, et al. (1996) in the Table shown below. This table depicts the mechanisms with which different noises generate and their importance.

The internationally accepted standard to ensure consistent and comparable measurements of utility-scale wind turbine sound power levels is the International Electrotechnical Commission IEC 61400-11 standard. All utility-scale wind turbines available today in the US comply with IEC 61400-11. It defines the quality, type and calibration of instrumentation to be used for sound and wind speed measurements, locations and types of measurements to be made. The standard requires measurements of broad-band sound, sound levels in one-third octave bands and tonality. These measurements are all used to determine the sound power level of the wind turbine at the nacelle, and the existence of any specific dominant sound frequencies. Measurements are to be made when the wind speeds at a height of 10 m (30 ft) are 6, 7, 8, 9 and 10 m/s (13-22 mph). Manufacturers of IEC-compliant wind turbines can provide sound power level measurements at these wind speeds as measured by certified testing agencies. Measurements of noise directivity, infrasound (< 20 Hz), low-frequency noise (20-100 Hz) and impulsivity (a measure of the magnitude of thumping sounds) are optional.

As it is observed, Sound Pressure Levels of the Wind Turbines have gradually reduced as the methods to measure the sound levels have been discovered and new techniques to reduce the sound levels have been implemented for lesser Acoustic Emissions without a loss in energy generation. The development of an accurate sound 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) and shape of land.

Methods to measure Acoustic sound levels: Equivalent continuous A-weighted sound pressure level The equivalent continuous A-weighted sound pressure level shall be determined at the five measuring positions and during the following conditions:

1. Wind turbine in operation

• Wind speed as close to cut-in as possible.
• Wind speed as high as possible.

2. Wind turbine parked

• Wind speed as close as possible to the wind speed during the Al measurement.

The difference in wind speed during the conditions given by Al and A2 shall be at least 3 m/s. Each measurement is recommended to be at least 2 minutes in duration where practicable and during periods of steady wind. Remarks on subjective impression of noise (audible discrete tones, impulsive character, spectral content, temporal characteristics, etc. ) should be noted. Measurements shall be carried out simultaneously at the reference position and at least at one of the other points. Measurements at the reference position In addition to the measurement of the equivalent continuous A-weighted sound pressure level, the following quantities shall be determined at the reference position:

• The equivalent continuous sound pressure level in third-octave bands with centre frequencies from 50 Hz up to 5000 Hz.
• The A-weighted percentiles.
• In the case of audible pure tones, a narrow band spectrum covering the appropriate part of the frequency spectrum.

It is recommended that the measurement time period in the first case is at least 15 sec for each frequency band and second case is at least 2 minutes. These measurements shall be carried out at a wind speed as near cut-in + 2 m. /s (measured at hub height) as possible.

Methods to reduce Acoustic Emissions

Turbines can be designed or retrofitted to minimize mechanical sound. This can include special finishing of gear teeth, using low-speed cooling fans and mounting components in the nacelle instead of at ground level, adding baffles and acoustic insulation to the nacelle, using vibration isolators and soft mounts for major components, and designing the turbine to prevent sounds from being transmitted into the overall structure. Efforts to reduce aerodynamic sounds have included the use of lower tip speed ratios, lower blade angles of attack, upwind rotor designs, variable speed operation and the use of specially modified blade trailing edges.

Conclusion

Acoustic Emissions in wind turbines and recent advances to prevent it have been critically reviewed and compared. To successfully reduce or prevent the emissions generated, the sources of noise must be identified. Two major sources of noise are present during operation: mechanical and aerodynamic. Mechanical noise generally originates from the many different components within the wind turbine, such as the generator, hydraulic systems and the gearbox. Different mechanical noise prevention strategies such as vibration suppression, vibration isolation and fault detection techniques are utilized for this type of noise. Aerodynamic noise is the dominant source of noise from wind turbines. It occurs at high velocities when turbulent boundary layers develop over the airfoil and they pass over the trailing edge or at lower velocities as a result of laminar boundary layers leading to vortex shedding at the trailing edge.

Other factors presented include flow separation and blunt trailing edge flow leading to vortex shedding. Strategies for reducing aerodynamic noise include adaptive approaches and wind turbine blade modification methods. Advanced research is being done even by the current industries and is going to be developed gradually in the future to reduce Acoustic Emissions and improve the efficiency of the wind turbines.

References:

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