Environmental impacts

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Environmental issues associated with wind energy – a review
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Kaoshan Daia, Anthony Bergotb, Chao Lianga, and Wei-Ning Xiangc,d
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a College of Civil Engineering, Tongji University, 1239 Siping Rd., 200092 Shanghai, China,
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Email: [email protected], Tel: +(86)2165985374 (corresponding author)
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b ENSI de Bourges, 88, boulevard Lahitolle, 18020 Bourges cedex, France, Email:
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[email protected]
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a College of Civil Engineering, Tongji University, 1239 Siping Rd., 200092 Shanghai, China,
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Email: [e-mail]
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c Shanghai Key Lab for Ecological Processes and Eco-Restroration, East China Normal
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University, 500 Dongchuan Rd., 200214 Shanghai, China, Email: [e-mail]
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d Department of Geography and Earth Science, University of North Carolina, Charlotte, 9201
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University City Blvd., Charlotte, NC 28223, USA
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Abstract
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Being recognized as one of the most mature renewable energy technologies, wind energy
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developed rapidly in recent years. Although many countries show great interest toward wind
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power, it is not free of environmental impacts. As the wind energy industry continues to grow
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in many parts of the world, especially in some developing countries and ecologically
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vulnerable regions, a comprehensive appreciation of wind energy environmental impacts is
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important. This paper reviews environmental problems brought about from wind farms
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through summarizing existing studies. It also identifies some mitigation measures that are
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available to minimize these adverse environmental impacts. It is intended to provide the wind
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energy planners and developers a state-of-the-art knowledge about environmental issues
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associated with wind energy.
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Keywords: wind energy; environmental impact; mitigation measure
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1. Introduction
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The combustion of fossil fuels is believed to be one of the causes that lead to global
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warming. Renewable energy sources such as wind energy, solar energy and biomass attract
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interests, as alternatives, of many researchers, industrial professionals, and government
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officers. Modern biomass, wind and geothermal energies are becoming more commercially
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competitive and are making relatively fast progress[1]. According to the European Renewable
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Energy Council, approximately half of the global energy supply will come from renewable
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energy in 2040[2]. Predictions also indicate a large increase in renewable energy and energy
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efficiency by 2050 and therefore, a great decrease of carbon dioxide emissions[3].
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As one of the most mature renewable energy technologies, wind power has accelerated
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its growth during the past decade. Wind becomes a preferable choice as energy planners and
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national governments seek to diversify their energy mix, reduce CO2 emissions, build new
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industries with the investment, and provide more employment opportunities. According to the
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latest Global Wind Report, the global total of wind power is 282.5 GW at the end of 2012[4].
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However, wind energy developments are not free of environmental impacts. As wind power
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continues to develop in many parts of the world, especially in some developing countries and
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ecologically vulnerable regions, a poor understanding of its impacts, is a concern[5,6].
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In this paper, we review potential environmental problems brought about from wind
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farms, summarize evidences collected from existing studies, and identify available mitigation
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measures to minimize these adverse environmental impacts. Through this review, it is
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anticipated to provide the planner and developers a clear warning that an inappropriate wind
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farm project design can adversely affect local environment. Suitable mitigation efforts should
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be considered in the design, construction, and operation phases of a wind farm to avoid, if
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possible, concentrations of vulnerable ecological systems.
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2. Wind energy induced environmental problems
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Wind power plants use wind turbines to convert the most common and available
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resource, wind, into electricity or mechanical energy. The output power of a turbine is
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function of the density of the air, the area swept out by turbine blades and the cube of the
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wind speed[7]. Environmental issues related to wind farms mainly include noise, bio-system
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disturbance, visual pollution, changes to local climate, avian safety and electromagnetic
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interference[8].
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2.1 Noise
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Noise is one of the major environmental hindrances of wind power. According to Van
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den Berg[9], at quiet nights, people living as far as 500m away from wind farms react
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strongly to the noise and those living up to 1900m away from wind farm can experience
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annoyance. It is also found that people seem to be more annoyed with wind turbine noise than
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the same dose of transportation noise[10]. In addition, wind turbine visual and aesthetic
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impacts on landscape could increase annoyance to wind turbine noise[10]. However,
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compared to large quantity of data on annoyance from transportation noise, studies on
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correlation between annoyance and wind turbine noise are limited. Interpretation should be
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done with care and further research is in need.
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In terms of frequency, there are two types of noises produced by wind turbines: tonal and
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broadband noise. Tonal noise is defined by discrete frequencies (in the range of 20Hz to
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100Hz) and is caused by non-aerodynamic instabilities, such as unstable flows over holes,
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slits or a blunt trailing edge[11]. Broadband noise is a random, non-periodic, signal with
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frequency superior to 100 Hz. It contains continuous sound level distribution caused by the
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interaction of wind turbine blades with atmospheric turbulence and generated by the airflow
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right along the surface of the airfoil[11]. In terms of noise sources, the sound of wind turbines
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comes from aerodynamic and mechanical noise. Aerodynamic noise comes from blades
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passing through the air. This sound, perpendicular to the blade rotation, varies with turbine
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size, wind speed and blade rotation speed. A strong wind and a big turbine are obviously
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noisier. Since modern turbines can rotate in order to face to wind, noise comes from different
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directions at different time. To make sure that a turbine rotates with a constant speed, the
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blade pitch can automatically adjust with the change of wind, and therefore, produce different
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levels of noises. The aerodynamic sound contains different frequencies and they are
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considered as a broadband noise[12]. Mechanical noise comes from turbine’s internal gears,
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generator and other auxiliary parts[13]. This noise is often considered as a tonal noise because
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it often contains only discrete frequency. These tonal noises are noticeable and irritating,
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especially for small wind turbines with insufficient insulation[12]. Contrary to aerodynamic
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noise, mechanical noise doesn’t increase very fast with dimensions of turbines and it can be
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controlled through proper insulation during manufacturing[14]. The total noise, characterized
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with sound pressure level dBA, is a combination of mechanical and aerodynamic noise.
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Sound pressure levels, as a function of wind speed for different wind turbines reported in
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literatures, are listed in Table 1. The AOC 15/50 wind turbine, with a hub height of 25 m, has
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15-m-length blades, which have a nominal speed of 65 rpm. Microphones were located 32.5
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m from the center of the tower base. The National Renewable Energy Laboratory (NREL)
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used the equation: R = H + D/2, where R is the reference distance, H is the vertical distance
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from the ground to the rotor center and D is the diameter of the rotor, to estimate the distance
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where microphones will be installed[15]. The Bergey Excel wind turbine was mounted on a
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37-m lattice tower. The measured wind was obtained with an anemometer located at hub
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height (37meters) and 22.7 meters upwind from the testing turbine, and microphone were
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located 40.5 m from the center of the tower base[16]. The whisper H40 wind turbine is a
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three-bladed system, mounted on a 10 cm tube tower with a hub height of 9.1m. The same
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method as the NREL was used to determine the place to establish the microphone, which is
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10.2 m from the tower base center[17]. The International Standard IEC 61400-11[18] was
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followed in these three studies, thus providing a uniform method that ensures consistency and
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accuracy for noise measurements.
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The sound pressure level of one single wind turbine at 40 meters away can vary from 50
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to 60 dBA[8]. For a wind farm, the noise level at a certain distance from the wind turbine is
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related to the number of turbines in operation. The equivalent sound pressure in a house
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located 500 meters away from a single wind turbine center normally varies from 25 to 35
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dBA. Under the same condition, the noise level generated by 10 wind turbines ranges from 35
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dBA to 40 dBA[8]. The relationship between wind turbine number and the minimum distance
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needed to reduce the noise level to 40dBA is shown in Table 2[19].
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Besides distance, many other factors contribute to how sound propagates and attenuates,
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including air temperature, humidity, barriers, reflections and ground surface materials. Inside
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a building, wind direction and building material absorption have influences on the attenuation
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of sound coming outside[12]. At night, the sound can be perceived differently: the whooshing
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(amplitude-modulated noise from wind turbines) is perceived to increase in intensity and can
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even become a thumping. The reason is that ambient noise or background noise is lower at
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night due to lower human-made noise and a stable atmosphere[7]. According to Van den
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Berg[9], a person living at 1.5 km from the wind park perceives the sound as an “endless
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train” on quiet night. For some wind farms located on the seaside, background noise such as
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noise of the waves and wind itself can be so loud that the noise of wind turbine cannot be
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differentiated[20]. Therefore, the measured noise pressure level of wind turbine is modified
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by the background noise. The modification factors provided in the Chinese standard[21] are
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listed in Table 3, which are used for those scenarios when the difference between background
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noise level and the measured noise ranges from 3 dBA to 10 dBA.
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Noise is one of the main causes why there is a minimum separation distance between
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wind farms and habitations. This distance is often recommended by government or medical
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institutions and it varies among countries. Recommendations given by different countries are
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summarized in Table 4. Another approach is to limit the noise that can be heard at the closest
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inhabited dwelling. Such restrictions set by different countries or regions are collected in
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Table 5. In Table 5, another scale to measure noise, L90, is provided. It measures a sound level
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that is exceeded 90% of the time and it is considered as the noise someone hears late in the
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evening or at night when there is very little sound[22]. This scale is useful to characterize the
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background noise that masks the noise effects of wind farm [23,24]. Kamperman and
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James[22] also argue that common use of a single A-Weighted (dBA) noise descriptor is not
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adequate to limit wind turbine noise which has significant low frequency sound. To
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supplement current standards, they proposed to limit C-Weighting or dBC within L90+20dB
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and with the maximum of 50 dBC[22].
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Noise can induce sleep disturbance and hearing loss. Exposure to high noise can trigger
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headaches, irritability, fatigue, constricted arteries and a weakened immune system[25]. The
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disturbing noise can also have subjective effects such as annoyance or dissatisfaction[24].
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Shepherd et al.[26] conducted a questionnaire study for people who live within two
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kilometers of wind turbines. Results show that wind turbines affect life quality and cause
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amenity for some residents, who are not willing to accept wind turbines and keep a virulent
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attitude against the wind turbine project. Studies have also shown that sleep deprivation due
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to wind turbine noise can cause health problems but none studies have yet proved that this
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noise per se can cause direct health problems or whether infrasound from wind turbines can
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impact the vestibular system[7]. Due to the paucity of literature and the facts that annoyance
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can be caused by other factors, the clear association between annoyance and wind turbine
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noise still need more rigorous study.
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2.2 Effects on animals
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2.2.1 Bird
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Wind turbines induce the risks of mortality and disturbance for birds. Numerous birds
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die because of a collision with rotating propellers; they also die or suffer lethal injuries by
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colliding with towers, nacelles and other associated structures such as guy cables, power lines
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and meteorological masts in a wind farm[27]. Although many birds die every day because of
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pesticides and collision with other facilities, wind towers kill more treasured species such as
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golden eagles or swans[28]. Researchers used different methods to calculate the number of
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