Environmental and Structural Safety Issues Related to Wind Energy
Kaoshan Dai1, Kewei Gao1 and Zhenhua Huang2, 1Tongji University, Shanghai, China, 2University of North Texas, Denton, TX, United States Email: 1[email protected]
Abstract
Wind energy is one of the most mature renewable energy technologies and has been developing rapidly in recent years. Many countries have shown interest in utilizing wind energy, but its impact on the environmental is a concern. The primary environmental impacts include avian safety issues, biosystem disturbances, noise, visual pollutions, electromagnetic interferences, and local climate changes. This chapter provides a review and summary of the environmental impacts caused by the wind energy industry. Available mitigation technologies to minimize these adverse environmental impacts are also discussed. In addition to environmental impacts, structural safety is becoming an issue as many structural failures of wind turbines have been reported. A summary of studies related to these structural safety problems are included in this chapter; the focus here is on the structural performance of wind turbine towers under wind and seismic loads together with health monitoring and vibration control technologies. The intention of this chapter is to provide state-of-the-art knowledge about the aforementioned issues associated with wind energy development as well as strategies for wind energy planners and developers to mitigate the negative effects.
Keywords
Wind energy; environmental impact; mitigation strategy; wind turbine tower structure; structural safety
22.1 Introduction
Wind power is one of the most mature renewable energy technologies, and industry has been experiencing accelerated growth during recent decades. However, wind energy developments are not free of environmental impacts. A poor understanding of these environmental impacts is a serious concern for the current wind energy industry, especially in developing countries and ecologically vulnerable regions. In addition, structural safety issues have gradually appeared, such as the structural reliability issues of wind turbine towers. In this chapter, the authors review the potential environmental impacts and structural safety issues associated with wind farm developments and identify potential methodologies to mitigate these adverse effects.
22.2 Wind-Energy-Induced Environmental Issues and Countermeasures
Wind power plants use wind turbines to convert wind energy into electrical or mechanical energies. The output power of a wind turbine is a function of air density, area swept by turbine blades, and wind speed [1]. The primary environmental issues related to wind turbines include avian safety, biosystem disturbance, noise, visual pollution, electromagnetic interference, and local climate changes [2,3]. These issues could be grouped into ecological effects, impacts on humans, and climate-related issues [4].
22.2.1 Effects on Animals and Mitigation Strategies
22.2.1.1 Birds and Bats
Birds can be killed by colliding with the rotating propellers of a wind turbine or they can suffer lethal injuries because of the collision with wind turbine towers, nacelles, or other structures in a wind farm such as guy cables, power lines, and meteorological masts [5].
Various factors influence wind-turbine-induced bird mortality, such as the wind turbine design and layout, bird species, and climatic variables. Examining the wind turbine design and layout, Orloff and Flannery [6] reported that the bird mortality rate was higher for three blade lattice turbines than for other turbine types. Smallwood and Thelander [7] indicated that the end of turbine strings (rows or columns of turbines on a wind turbine farm), the edge of the gap in turbine strings, and the edge of turbine clusters were the most dangerous sections of wind farms for flying birds. Sovacool [8] demonstrated that the bird mortality rate increased in regions where turbines were located on ridges or upwind slopes, or close to the bird migration routes. Holmstrom et al. [9] showed a significant correlation between the collision probability and the approaching angle between the bird flight path and the turbine orientation surface. Bird mortality is also associated with bird species [10]. Orloff and Flannery [6] observed that golden eagles, red-tailed hawks, and American kestrels were killed by wind turbines more often than turkey vultures and ravens. Langston and Pullan [11] suggested that one should consider diurnal and nocturnal species (different behaviors of birds during days and nights) when characterizing the sensitivity of the bird–wind turbine collisions. For the climate variables, Smallwood and Thelander [7] found that wind turbines killed more birds during winter and summer.
In addition to the mortality rates, another negative impact of wind turbines on birds is disturbance, including habitat destructions, barrier effects, and impacts on the bird breading and feeding behaviors. For example, the construction of wind turbines and associated infrastructures may cause the destruction of local birds’ habitat [11]. Some wind turbines can create physical barriers that deny birds’ access to their natural feeding grounds and roosting locations. Noises and turbulent air currents produced by operations of wind turbines may scare birds away and narrow their territories, which can affect birds’ foraging behaviors. Christensen et al. [12] studied the behavior of birds using radar tracks and concluded that 14%–22% of the observed birds increased their flying altitude to pass through the studied wind farm. Another research [13] found, through 10 years of collecting data on 47 eagle territories at western Norway, that coastal wind farms affected the breeding success rate of white-tail eagles.
A high mortality rate close to wind farms has been observed for bats as well. Wind turbines are associated with the mortalities of nearly a quarter of all bat species in the United States and Canada [14]. There are different opinions among researchers on the reasons for these bat mortalities [15,16]. Early studies observed barotrauma-related internal hemorrhaging in over 50% of the dead bats near wind turbines, therefore, some researchers believed that the barotrauma and internal hemorrhaging caused by the sudden pressure drop near turbine edges were the main reason for the wind-turbine-induced bat mortalities [17]. More recent studies found that the impact trauma was actually responsible for the majority of the turbine-associated bat deaths [18]. Concerning the reasons why bats are attracted by wind turbines, Arnett et al. [19] explained that bats were attracted by ultrasound emissions and lights of wind turbines. Other researchers [20,21] believed that bats treated wind turbines as trees and tried to access them as potential roosting sites or the large amount of insects attracted by the high heat radiation of wind turbine nacelles caused hunting bats to aggregate around wind turbines. Regarding the factors related to wind-turbine-induced bat mortalities, Kunz et al. [22] found that there were almost twice as many dead bats in grassland areas than in agricultural landscapes or forested ridge tops. Marsh [23] indicated that wind farms on forested ridges were more dangerous for bats and many bats were killed during the 2-hour period after sunset during their autumn migration. A study by Barclay et al. [24] showed that the height of wind turbine towers was associated with the death toll of bats. A comprehensive bibliography associated with the wind-farm-induced bat moralities up to 2008 could be found in Arnett et al. [25].
To reduce these bird and bat fatalities and disturbances, several mitigation strategies have been suggested, including restricting construction activities, improving structural designs, and optimizing site selections. Restricting construction activities to nonbreeding periods could be effective in helping to reduce the negative effects of bird disturbances [26]. Structural design improvements were effective in reducing bird mortalities as well [27]. For example, McIsaac [28] found that the pattern-painted wind turbine blades could increase the visual acuity of raptors. Marsh [23] proposed that wind turbine blades with night illuminations could be more visible to birds. However, Langston and Pullan [11] believed that the night illumination on wind turbines may attract birds, especially in bad weather conditions, and increase the chance of collision. Arnett et al. [19,29] found that there was no difference in bird or bat fatality rates for wind turbines with or without lighting. In addition to construction activity restrictions and structural designs improvements, the site selection of a wind farm is also important [8]. For example, Foote [30] proposed that bird flight activities in a zone of 200–500 m surrounding the planned wind farm location should be recorded and analyzed systematically, including flight heights, directions, species, and behaviors of birds. Carrete et al. [31] recommended assessments of the spatial distribution and aggregation activities of vulnerable bird species before starting wind farm constructions in order to minimize bird disturbances. Busch et al. [32] pointed out that, in addition to the technical improvements, an international cooperative effort is also important to reduce the environmental impacts of global wind farm projects. A detailed discussion on mitigation methodologies for the environmental impacts of wind turbines can be found by Northrup and Wittemyer [33].
22.2.1.2 Marine Species
Offshore wind turbines may impact marine species. Wind turbines and their scour protection may change the nearby fish distributions and wind farm constructions may create an artificial reef, which impacts the biodiversity of marine species. The construction of wind turbine foundations and the on-site erection of wind turbine towers may make seawater turbid and introduce additional objects on the seabed (blocking effect), which can cause damages to the benthic fauna and flora and block sunshine into the water. Berkenhagen et al. [34] indicated that the offshore wind farm constructions would induce a substantial effect on fisheries. In particular, the opportunities to catch valuable species would be considerably reduced. In addition to the blocking effect, the noise and the electromagnetic fields around operating wind turbines may lead to negative effects on fishes as well [35]. Marine mammals such as porpoises and seals may also react to wind farm noise and electromagnetic fields, especially during the construction phase [36,37]. The maintenance activities of wind turbines, such as part replacements or lubrications, may also impact on marine species by leaking oils or wastes into the surrounding seawater and polluting marine species living environments. With the increasing height of wind turbine towers and the increasing size of offshore wind farms, the environmental impacts of wind farms on fishes and marine mammals are becoming more evident.
22.2.2 Noise Problems and Possible Solutions
Noise is one of the major environmental hindrances for the development of the wind power industry and can induce sleep disturbances and hearing losses in humans. Exposure to high-frequency noises can trigger headaches, irritability, and fatigue, as well as constrict arteries and weaken immune systems. The public also can become annoyed or dissatisfied by the disturbing noises [38]. According to Van den Berg [39], during quiet nights, people reacted strongly to wind turbine noises in the range of 500 m surrounding a wind farm and experienced annoyance in the range of 1900 m surrounding a wind farm. The wind-turbine-induced visual and aesthetic impacts on the landscape could increase the public’s annoyance [40]. However, due to the paucity of literature and the fact that annoyance can be caused by many other factors, the clear association between annoyance and wind turbine noises still needs more rigorous studies. Wind turbine noises could be categorized into tonal and broadband noises based on the noise frequencies and aerodynamic and mechanical noises based on the noise sources. The total noise, measured by the sound pressure level, is a combination of mechanical and aerodynamic noises. The low-frequency noises (10–200 Hz) are considered as the substantial part of the noises with the larger modern turbines [41]. Many factors affect the wind-turbine-induced noise propagation and attenuation, such as air temperature, humidity, barriers, reflections, and ground surface materials. Background noise is another important factor. For example, noises can be perceived differently at night. The whooshing (amplitude-modulated noise from wind turbines) can be perceived as being more intense than during the daytime and even be heard as thumping because of the low human-made background noises and the stable atmosphere [1].
To control the noise level, governments and medical institutions have recommended minimum separation distances between wind farms and habitations or upper limit noise levels of dBA values that can be heard at the closest inhabited dwellings. Different criteria on these separation distances and noise dBA values have been provided by different countries and regions. Suitable criteria should be followed with a comprehensive consideration of specific local conditions for wind farm developments. To reduce the noise from wind turbines, improved blade design is a key issue. The application of upwind turbines is useful to reduce low-frequency noises [42]. The insulations inside the turbine towers can effectively mitigate the mechanical noise during the course of operation [43]. In addition to these technical measures, building wind farms close to noisy areas is another way to reduce the noise-induced problems [34].
22.2.3 Visual Impacts and Mitigation
A wind turbine blade may cast a shadow in the sunshine on its neighbor area. This shadow may induce an undesirable visual impact or even a disturbing flicker when a rotating blade casts a moving shadow on landscapes and houses [44]. In addition to the shadow and flicker issues, the negative visual impact of wind farms on landscapes is another factor that creates a negative opinion of the wind energy industry, in people’s minds [45]. However, this problem is subjective. People’s positive or negative attitude may depend on their perception of the unity of the environment, their personal feeling toward the effects of wind turbines on landscape, and their general attitude about the wind energy industry [46]. Evaluation of the visual impact of a wind farm is a difficult task. A survey study by Krohn and Damborg [47] showed that the public usually supports wind power and the renewable energy industry. However, the local residents may oppose building a new wind farm close to them, even though they know it will benefit the society. This is so called Not-In-My-Backyard syndrome (NIMBY).
Factors influencing the visual impact intensity of wind turbines include background nature, local landscapes, and landscapes between viewers and turbines. For example, a wind turbine located on a hill may induce direct visual impact, but the intensity of the impact can be weakened when viewing from a higher position [48]. Therefore, when selecting wind farm sites, areas that are considered visually pleasing, especially on the coast, should be avoided. A simulation study conducted by Bishop and Miller [49] showed that in all weather and light conditions, the visual impact intensity of wind turbines decreases with increasing distances. The number of blades and the blade rotating directions of a wind turbine can influence its visual impact intensity as well. According to Sun et al. [50], a wind turbine with three blades is more acceptable than the one with two blades for people who are sensitive to visual impacts. Wind turbines with anticlockwise rotating blades generated stronger negative reactions from viewers [50]. The layouts of wind turbines in a farm, which can be categorized into regular and irregular layouts, can also affect their visual impact intensity. Generally, the regular layout created a better sense of visual regularity and consistency than the irregular layout, which may lead to a sense of chaos.
Four considerations were suggested to limit the wind farm visual impacts on landscapes during the design phase: (1) whether it is acceptable to change the landscape; (2) how visually dominant are the wind turbines on the landscape; (3) what is the relationship between aesthetics and the wind energy development; and (4) how important is the impact. To promote positive attitudes of local communities toward wind farms, public participations in the early stages of the planning and implementation of wind power projects are recommended, such as working together to seek solutions to the visual impact issues [46]. Design improvements can be used to mitigate the visual impact intensity of wind turbines. For example, the shadow flicker issue of wind turbines can also be predicted and avoided with an appropriate sitting design of a wind farm. Layout design of wind frames and aesthetic design of wind turbines can also be helpful. The fewer the number of wind turbines and the simpler the layout, the easier it is to create a visually balanced, simple, and consistent image. Selecting an appropriate color for a turbine is important to mitigate its visual impact. Rather than painting turbines a color to camouflage them against their background, it is more suitable to choose a color to engage the turbines to suite the background at different views and in different weather conditions.
22.2.4 Climate Change and Considerations
Different studies have shown that wind turbines can impact local weather and regional climate such as the temperature and wind speed. Zhou et al. [51] studied 8-year satellite data in a region of west-central Texas equipped with 2358 wind turbines and reported a temperature increase of 0.724°C in the area. Barthelmie et al. [52] studied the recovery rate of the wind speed after it passed through a wind farm and reported a decreasing curve. Roy and Traitor [53] believed that large wind farms induced cooling effects during daytime and warming effects at night as a result of the vertical air mixture near the ground surface. These cooling and warming effects altered the regional climate, and the change can induce a long-term impact on wildlife and regional weather patterns. In contrast, some other studies reported that wind farms were able to alleviate adverse climates such as sand storms, even though the effect was very limited [54].
Different analytical methods and models, such as the blade element momentum model, the vortex wake method, the computational fluid dynamics method, have been suggested for the wind farm climate studies to mitigate the meteorological impacts of wind farms [55]. For example, Keith et al. [54] proposed that, through improved rotor and blade designs and a proper design of turbine spacing and pattern, the rotor-generated turbulence of wind turbines can be mitigated and the hydro-meteorological impact of wind farms can be reduced.
22.3 Structural Safety Studies for Wind Turbine Towers
Along with the fast development of wind energy industry, today’s wind turbines are much taller than their predecessors. Additionally more and more wind farms are located in strong earthquake zones and/or strong wind-prone regions. In recent years, many structural failures of wind turbines caused by extreme winds or strong earthquakes have been reported based on qualitative observations and phenomenological assessments. This section summarizes studies related to wind turbine tower structural safety problems.
22.3.1 Wind Turbine Tower Structural Performances Under Wind and Seismic Loads
Many structural failures of wind turbines caused by extreme environmental loadings have been reported and studied in recent years. For example, Ishihara et al. [56] studied the collapse of two wind turbine towers in Japan caused by the 2003 typhoon Maemi. Chou and Tu [57] analyzed the tower collapse and the rotor blade damage of a wind turbine in Taiwan caused by the 2008 typhoon Jangmi. Chen et al. [58] conducted a forensic engineering study on failures of several wind turbine towers in a coastal wind farm after a typhoon event.
The tower structures for modern megawatt wind turbines are generally over 60 m high. Therefore, wind turbine towers belong to tall structures, which are typically flexible and vulnerable to vibration fatigue damages caused by wind loads. Therefore, the structural design of a wind turbine tower is generally controlled by wind loads [59–61]. Design specifications such as Risø, GL, IEC [59–61] provide guidance for the design of wind turbine towers. For example, IEC provides 22 design loading cases, including eight turbine operation conditions. During the design procedure, time history analysis can be adopted to accurately consider tower–blade coupling [62]. Design parameters for wind turbine towers are typically provided by manufacturers who designed the blades and turbines [63,64]. Blade rotation should be considered as an important parameter. Murtagh et al. [62] found that the vibration amplitude at the top of wind turbine towers under wind loads were underestimated if blade rotation was not considered. The reasons may be because the blade rotation can induce the centrifuging stiffening effect [62] and the aeroelastic damping effect [65] to the wind turbine structure and influence its vibration performances.
In addition to the extreme wind loads, the seismic performance of wind turbine towers is getting more attention of structural engineers and researchers because more wind farms are located in earthquake-prone areas. Previously, researchers [64,66] believed that the earthquake loads would not control the design of small wind turbine towers. However, recent studies [67] pointed out that the bending moment at the bottom of a wind turbine tower sometimes is controlled by earthquake loads. Therefore earthquake design should be conducted for wind turbine towers. For the earthquake design procedure, the response spectrum analysis and the time history analysis are the two main analysis methods. The response spectrum analysis method has been widely used to design all different types of structures due to its simplicity. However, when applying this method on the design of wind turbine towers, Nuta et al. [68] indicated some uncertainties and suggested that various factors need to be considered due to the low-damped feature of the wind turbine towers. The time history analysis method is more suitable for flexible (slender/tall) wind turbine tower structures. When selecting earthquake time history records for this analysis method, the effects of ground motion durations and soil characteristics should be considered [69]. The effects of the near-fault earthquakes (which have obvious pulse and directionality effects) [70] and the vertical earthquake motions [66,71] should also be included in the analysis if necessary. The advantage of the time history analysis method for wind turbine towers is that it can properly consider the structural nonlinearities and damping characteristics. For example, IEC [61] specified a 1% structural damping ratio for a parked wind turbine tower structure. The aerodynamic damping characteristics could also be involved in the analysis if necessary [71]. The aerodynamic damping is an extra damping induced by aerodynamic behaviors of blades at the operational conditions of wind turbines [72]. Some detailed earthquake design requirements could be found in different wind turbine design guidelines. For example, Risø [59] provided a single-degree-of-freedom model with a lumped mass for the earthquake analysis of wind turbine towers. The lumped mass is a concentrated mass point at the top of a turbine tower, which includes the masses of the blades, the turbine engine, and the top one-fourth of the tower. GL [60] specified a return period of 475 years for earthquake analysis and required that at least the first three modes and six ground motions should be studied. IEC [61] explicitly stipulated that the participating mass in the seismic design should be more than 85% of the total mass of a wind turbine tower. Shaking table tests can help designers understand the seismic performances of a wind turbine tower structure. A typical shaking table test study of a wind turbine tower can be found by Prowell et al. [73], which verified the low-damped feature of the wind turbine tower structures.
22.3.2 Health Monitoring and Vibration Control of Wind Turbine Towers
In addition to the wind and earthquake studies, health monitoring and vibration control are also important for the structural safety of wind turbine towers. To monitor the health and control the vibration of a wind turbine tower structure, the modal parameters of the tower structure are needed. Modal testing is a commonly used procedure to identify the vibration modal parameters of a wind turbine tower. There are two excitation methods for the modal testing procedure: active excitation method and ambient excitation method. There are limited examples of using the active excitation method on wind turbine tower structures [74]. On the other hand, the ambient excitation method is feasible to identify the dynamic parameters of a wind turbine tower due to its large-scale feature. Velazquez et al. [75] estimated the dysfunction probability of a wind turbine tower based on ambient vibration measurements. Kusiak et al. [76] analyzed the collected ambient vibration data of a wind turbine tower and obtained a parametric model of the structure by using a genetic evolution algorithm. There are two hypotheses for using the ambient vibration testing: (1) the measured structure must be a linear time-invariant system [77,78] and (2) the input excitation must be white noise [77]. It seems that these two assumptions were not perfectly satisfied for wind turbine tower structures, especially when the turbine is operating. To solve this problem, Allen et al. [79] and Malcolm [80] proposed a method to simplify the wind turbine towers into a linear cycle time-variant structure and investigated the feasibility of applying the Coleman conversion method for this analysis.
Structural health monitoring of wind turbine towers can be applied to help assess their current structural performances, identify their structural damages, estimate their lifetime, and improve their structural reliabilities [81,82]. Structural health monitoring has been successfully utilized for wind turbine towers to identify structural damages, such as loose of high-tension bolts, resonance problems [81], and cracking [83,84]. Several structural health monitoring equipment has been developed to collect operational, environmental, and structural parameters of wind turbine towers [78,84–86]. Example case studies on structural health monitoring of wind turbine towers can be found in Pavlov [87], Simarsly et al. [86], Rohrmann et al. [65], and Benedetti et al. [84]. For the analysis procedure of structural health monitoring, the change of vibration frequency method was popularly used to identify structural damages. However, for a large-scale wind turbine tower structure, the sensitivity of this change of vibration frequency method may not be good enough to effectively identify its structural damages due to modeling uncertainties, data noises, data processing errors, environment-induced changes, and blade rotation effects [82]. Kim et al. [88] reported that the sensitivity of vibration mode shapes typically is better than that of vibration frequencies. Therefore, the change of vibration mode shape method could be considered to identify structural damages of wind turbine towers. However, whether this method works or not is still unknown because noises and measurement errors always exist in actual data. In addition to the change of vibration frequency method, several advanced analysis methods have also been proposed, such as the wavelet method [89], the HHT [90], the genetic algorithm based on time domain measurements, the neural network method [91], and the statistical method [92].
The aforementioned structural health monitoring can provide information for timely maintenances of wind turbine tower structures. Structural vibration control technologies, on the other hand, can effectively depress structural vibrations of wind turbine towers, therefore reducing fatigue damages and preventing structural failures under extreme loadings. The selection of vibration control devices for wind turbine towers, unlike that of conventional dampers, should consider the effects of limited installation spaces inside the tubular tower, multidirection vibrations, and broadband vibration frequencies. Passive energy dissipation devices are the most popular used vibration control devices for wind turbine towers, which include tuning mass dampers (TMD), tuning liquid dampers (TLD), tuning liquid column dampers (TLCD), and circular tuning liquid column dampers (CTLCD). Many applications of these passive energy dissipation devices for structural vibration control of wind turbine towers have been reported along with the increasing height of modern wind turbine towers. For example, Murtagh et al. [93] designed a TLD based on the blade–tower coupling theory and installed it at the top of a wind turbine tower to reduce the abnormal vibration displacements of the tower; Karimi et al. [94] reported positive conclusions of a TLCD application on vibration controls of wind turbine towers. In addition to the passive energy dissipation devices, some conventional dampers specifically designed for tall and slender structures [95,96] and oil dampers [97] may also be applied for wind turbine tower structures. However, most of the aforementioned dampers and devices were only effective under a tuned condition, namely a certain frequency, usually the natural frequency of the first vibration mode. It is desirable that multiple frequencies of wind turbines, e.g., the fundamental frequency of the tower and the frequencies of the blade rotation, can be controlled. Therefore, some researchers proposed to use particle dampers with broadband working frequencies and high durability features to control the wind turbine tower vibrations [98]. The particle dampers are based on the mechanism of friction and impact effects between particles, which is insensitive to temperature changes and flexible for deployment locations. Some other researchers are working on developing structural vibration control devices and technologies suitable for wind turbine towers under harsh operating environments.
22.4 Summary
Renewable energy, a solution for global energy problems, can also impact the socioeconomy positively, such as diversifying energy supplies, increasing regional and rural development opportunities, and creating additional domestic industries and employment opportunities. However, renewable energy also induces some environmental and structural safety issues. These environmental and structural safety issues of wind turbines are controversial topics and should not be ignored. Minor issues today may cause disastrous effects in the future when wind energy becomes a major energy source.
This chapter summarized the environmental impacts of wind power industry and potential mitigation methods. As shown in this review study, it is recommended that the wind energy exploitation including related infrastructure constructions and facility operations should be evaluated for the economic, social, environmental, biological, and ecological influences. Suitable measures should be implemented to mitigate those negative impacts. Developers, planners, and government officials need to gather and communicate full information with the public to ensure that the projects are developed in a way that avoids or minimizes those negative impacts.
As the supporting structures of wind turbines, the safety of wind turbine towers is an important topic for researchers and engineers, which includes structure stabilities and fatigue resistances of wind turbine towers under operational vibrations, extreme wind loads, and earthquake effects. Tests and numerical simulations are the two basic research and design/analysis methods for wind turbine towers. For the numerical simulation, response spectrum and time history analysis are the two main procedures for calculating the resistances of wind turbine towers. The response spectrum procedure is more suitable for design/analysis because it is easy to apply and the time history analysis procedure is more suitable for research because it could get more data. For current research of wind turbine towers, the pulsating wind action for wind resistance studies and the tower coupling action for earthquake studies are the two popular topics. Health monitoring and vibration control of wind turbine towers is another significant research topic. Health monitoring identifies structural damages and vibration control mitigates structural vibrations. Effective health monitoring and vibration control technologies can help construct taller wind turbine towers under lower budgets.
Acknowledgments
The authors would like to acknowledge supports from State Key Laboratory of Disaster Reduction in Civil Engineering (SLDRCE14-B-02), State Key Laboratory for Geomechanics and Deep Underground Engineering (SKLGDUEK1514), State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (SKLGP2016K006), and Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Key Laboratory of Energy Engineering Safety and Disaster Mechanics (Sichuan University) (EES201603), Ministry of Education, and International Collaboration Program of Science and Technology Commission of Shanghai Municipality (16510711300).