Wind energy is a well proven and cost-effective technology and expected to be a promising technology in which industry responds to the environmental targets—so becoming an important source of power generation in years to come. This paper focuses on the current status of wind energy and more advanced subjects needed to understand the current technology in the wind power engineering.
As green energy industry emerges from initial stage caused by the global economic downturn, it is a new stage of rapid change of power science and technology. The worldwide demand for electricity is expected to triple by 2050, when fossil fuels account for no more than 60% of all energy consumed, compared with 80% of the energy consumed today. Traditional fossil resources such as oil, natural gas, and coal are nonrenewable and generate air pollution by releasing a huge amount of pollutants into the atmosphere, thereby damaging the environment, from acid rain to climate change. In order to help combat these problems, 158 countries have ratified of 197 parties to the Convention the Paris Agreement put into force on Nov. 4, 2016. The first session of the parties serving as the Meeting of the Parties to the Paris Agreement (CMA 1) was held in Morocco during Nov. 15–18, 2016 for controlling the environmental problems .
Wind power has recently become the world's fastest-growing source of renewables. According to the U.S. Department of Energy, it is expected that wind energy contributes to a significant portion of the U.S. electricity supply in two decades. For this reason, there has been a keen interest in wind energy because they are emission-free and the wind is cost-free renewable. Nevertheless, the amount of electricity generated and obtained by wind energy conversion systems is still unsteady, relatively expensive, and difficult for integrating into conventional electrical systems because of the variation in wind source and unresolved energy storage issues. On a large-scale, spatial variability articulates the situation that there are various climatic regions throughout the regions, some much windier than others. These areas are mostly illustrated by the latitude, which affects the amount of insulation. Within any one climatic region, there is lot of variation on a smaller scale, largely dictated by physical geographies such as the land, lake, sea, the size of land masses, and the presence of mountains and hills or plains. The wind energy resource map in the U.S.  indicates that the wind resource data are based on geographical wind data, coastal ocean area data, and higher-air data, wherever applicable. In wind-sparse regions, three fundamental indicators of wind velocity or power levels are employed: topographic/meteorological indicators (e.g., valleys, mountains, and chasm), wind-deformed ravine, and eolian landforms (e.g., plains and dunes). The data were analyzed at a regional level to produce 12 regional wind resource categories; the local groups are thus incorporated into the national wind resource assessment.
In the U.S., while the wind makes up only 2% of total power supply, it is one of the highest sources of wind electricity in the country, second only to natural gas generation regarding new capacity built each year since 2005. During the last few years, wind power in the U.S. has been increasing sharply. Thousands of wind turbine units larger than 1.8 megawatts (MW) are now in operation in the U.S. Larger turbines are demanded due to economic benefits: a taller turbine with a larger blade radius produces higher power at a modest cost per kilowatt-hour. The rapid quest for more electrical power, moving from 2 MW turbines in 2009 to 6 MW turbines sited in early 2012 has shown the tower head mass went from 140 to 360 tonne, with more variations in structural loading and fatigue. Stabilization at the current power level will improve opportunities for consistent design and manufacture of the structures within a farm.
As shown in Fig. 1, the wind capacity was 24 GW in 2001, but it monotonically grew up to 490 GW in 2016. Total wind capacity in the U.S. reached 82 GW by the end of 2017. Wind power accounted for 35% of the country's new power-production capacity from 2007 to 2016. According to AWEA, Texas is the number one state with the most installed wind power with 21 GW, with Iowa being a leader in wind generation with 3675 MW installed, while California and Minnesota harvest significant amounts of the wind with 4322 MW and 2733 MW, respectively  (see Fig. 2).1
Although the installation mentioned earlier is a significant growth for wind power, this is still a minor portion of the entire energy resources in the U.S. electricity supply. The Department of Energy released the target of 20% by wind by 2013. Therefore it could provide a significant increase in the current economy. Factors pushing for growth in the U.S. wind power include the high cost of fossil fuels and concern over national energy security. As a result, policymakers consider a broad range of legislation that would support and enhance wind power growth.
The progressive public policy has been the main ingredient both for encouraging wind energy technology development and assisting in determining what forms that growth will take. Future growth will likely come from commercial-scale wind farms, which are typically vast arrays of turbines owned and operated by large corporations.
There are now over a quarter million wind turbines operating with a total capacity of 282,482 MW as of end 2015  worldwide. The U.S. developed wind farms and led the world in installed capacity in the 1980s and into the 1990s. In 1997, German installed capacity exceeded the U.S. and led until once again overtaken by the U.S. in 2008.
The size of the turbines becomes larger every year. The longest wind turbine blade is 88.4 m of an offshore turbine manufactured by the Adwen AD-180, 10% longer than even those of the MHI Vestas V164. Wind power is expected to reach 8% by 2018 . As of 2017, 90 countries in the world will be using wind power on a commercial basis .
As shown in Fig. 1, the size of the wind turbines is getting larger and larger for off-shore wind, and a large size of modern turbines, such as MHI Vestas or Adwen AD-180, can go as large as most of the commercial airplanes, like Hughes H-4 (Fig. 3). The largest wind turbines are built-in Denmark and then shipped across the world.
The diameter of the turbine blade alone is considerably greater than the London Eye, which is a giant Ferris wheel situated on the South Bank of the Thames River in London. The wind turbine blade size has grown over the last two decades. It went from 17 m to over 90 m in 2015 with the capacity went up from 50 kW to 8 MW.
Why Do We Need Wind Energy
The earth is becoming hotter by 1 °C in land area, which is attributed to the human activities, particularly the emission of greenhouse CO2 . The energy sector is by far the largest source of greenhouse carbon dioxide emissions. Thus, it is clear that current power generation technologies need to move away from the limited amount of fossil fuel resources to more sustainable and renewable sources of energy. As well as being good for the earth, this step from fossil to renewable sources is also good for the earth's resources as it decreases the dependency on fossil-fuel imports while also increasing energy supply security. The U.S. has set a target for 20% of the U.S.'s electricity to come from renewable sources by 2030 .
Windpower, one of the major renewables, can play a significant role in meeting U.S.'s high demand for electricity, 20% wind energy by 2030. Increasing Wind Energy's Contribution to U.S. electricity supply was prepared by the U.S. Department of Energy, the American Wind Energy Association, Black & Veatch, and others from the energy sector. In increasing wind power capability, more wind power installations can increase to more than 16,000 MW per year by 2020 and continue at that rate through 2030. Wind energy siting costs and performance are expected to drop slowly over the next two decades.
During the next decades, the U.S. wind industry could support more than 500,000 jobs with an annual average of more than 170,000 workers employed by the wind manufacturers. It also supports more than 150,000 jobs in associated industries, supports more than 250,000 jobs through economic expansion, and increases annual property tax revenues to as high as $1.5 billion in 20 years.
Owing to the rise in energy prices and the demand, there are now hundredths of wind farms developed around the world. Currently, major electric power companies are going green and actively proclaiming it too from rooftops.
Current Status of Wind Power Technology
Over the two decades, average wind turbine ratings have increased linearly ten times as large as the 2000s. Wind turbine designers have estimated that their machines and capacity are greater than ever be. However, with each new generation of wind turbines, the size has increased along the linear curve and has achieved reductions in the life-cycle cost of energy. The scope of developing larger turbines stems from a desire to take advantage of wind shear by placing rotors in the higher, stronger powered winds at a greater elevation above ground (wind speed increases with height above the ground). For this reason, the capacity factor of wind turbines grew. However, the continued growth in larger sizes has some constraints since it results in higher costs to build a larger turbine. The primary rule for a size limit for wind turbines is that the power is proportional to the area of the turbine blade area. That is, as a diameter of wind turbine rotor blade increases in size, its energy output goes up as the rotor-swept area increases. At the same time, the volume of material also increases, and because of that, its volume and price increase as the cube of the diameter. In other words, at some size the cost for a larger turbine will grow faster than the resulting energy output gains, making scaling a losing economic game. Manufacturers have successfully applied this law by increasing size and by using material more efficiently to reduce weight and cost. According to the wind blade scaling law, the size of wind turbine blade mass is approximately an exponent of 2.3 as opposed to the expected 3.
Comparison of Vertical Axis Wind Turbine and Horizontal Axis Wind Turbine
Vertical axis wind turbines (VAWTs) have emerged as a potential unit to the need, after being shelved by wind turbine companies in the late 1980s as a result of the greater success of horizontal axis wind turbines (HAWTs). VAWTs present several advantages over HAWTs, however, which is especially pertinent in the built environment. There are, however, also significant challenges that have prevented their widespread adoption. These challenges must overcome if VAWTs significantly contribute to meeting the 2030 wind power goal of the Department of Energy.
Vertical axis wind turbines require the equipment that needs maintenance near the ground. VAWTs are used for smaller capacity of the electric generator and are used only in small applications such as a residential use and office buildings. VAWT is used for powerless than 50 kW. There are no commercial-sized VAWTs because they do not make a profit, but if they exist, they would spin at speeds from 1.6 km/h to 200's of km/h. VAWTs would, of course, fly apart as soon as the wind went over about 30 km/h, which is why they are not in commercial operation, as centripetal forces may cause the blade damage. There are many types of VAWTs, because they are all under development stage. VAWT's cost less in total dollars, but significantly more in dollars per kilowatt generated because they do not make much power for their cost.
Drawback of VAWT.
While VAWT has several advantages over HAWT such as the low levels of noise and the independence from the wind direction, they are not used for the electric supplier as a replacement of fossil energy power plants. One of the disadvantages that VAWT possesses is that they must be installed close to the ground. Since the wind blows at a higher speed and evenly at greater heights, an installation that is not on a mast loses lots of efficiencies. If with this type of setup, the generator is installed in a machine room on the ground, and thus, maintenance is more straightforward and less expensive. Despite this arrangement, it is doubtful that the lower yield because of the weaker winds close to the ground would be balanced out by the money saved in maintenance costs. It remains to be seen whether plans to use existing tall structures to mount planned megawatt-level installations with vertical spindles can be realized. It will probably not be easy to find buildings or structures that would be able to handle the static and dynamic loads from a large wind power installation with a vertical axis. And it goes without saying that these structures should be in regions where the wind velocities are of interest. Another point against the current conceptions of larger VAWTs is the greater material expenditure per square meter of the surface covered in comparison to installations with a horizontal shaft. This feature mentioned here might result in a substantial additional cost factor that can hardly be compensated for by the theoretically better possible exploitation of stronger winds or gusts.
The main disadvantage of VAWTs is their low efficiency relative to HAWTs, which is a result of the variable torque produced by dynamic stall caused by each blade as it passes around the azimuth. The blades of a HAWT, on the other hand, generate constant torque around the azimuth. A further disadvantage is an inability to VAWTs for self-starting. By improving the torque, and, therefore, power produced by VAWTs, and enabling self-start, the two main roadblocks to their adoption would be removed. This treatment would enable the widespread adoption of VAWTs for low-wattage power generation in the residential wind power markets. This will increase renewable energy production in the locations where most power is consumed, reducing the losses and cost associated with transmission. Furthermore, it will reduce reliance on fossil fuel-based electricity and relieve consumers from the price variations that result from this reliance.
Dossena et al.  reported that the experimental thrust and power curves of the H-type VAWT, developed from basic measurements, exhibit the expected trends with a peak power coefficient of about 0.28 at a tip-speed ratio with 2.5. Wind velocity measurements for several tip-speed ratio demonstrate the full three-dimensional character of the wake, especially, in the tip region where a skew-symmetrical wake and tip vortex are observed. The aerodynamic loading on the wake unsteadiness shows the time-dependent character of the tip vortex and the onset of the dynamic stall for tip-speed ratio lower than 2.
Several benefits of VAWTs over HAWTs are:
As VAWTs operate with a smaller tip speed than HAWTs, they cause less noise. VAWTs also have a better esthetic due to their three-dimensional nature, making them popular with architects. HAWTs are sensitive to yaw and skew, experiencing decrease in torque and power due to the aerodynamic asymmetry on the rotor disk under such flow conditions.
Due to the complexity of airflow in the urban environment, the wind direction is not perpendicular to the vertical, making this issue a significant problem in this environment. VAWTs, however, are less sensitive to both yaw and skew .
Vertical axis wind turbines have the feature of the simplicity of the mechanical design and maintainability of the turbine system.
Vertical axis wind turbines can be designed with all significant components located at ground level, except a single bearing. This advantage is of particular interest to the residential power generation market, in which ease of repair is critical.
Of the various configurations of VAWT that exist, the simplest is the H-Darrieus VAWT, illustrated in Fig. 4. One of its major advantages is its ease of manufacture since the blades can be extruded. It makes the H-Darrieus VAWT, in particular, less expensive to build. Even the curved and helical blades of the more common “egg-beater” and helical VAWT designs are economical to manufacture than the tapered blades of HAWTs. By reducing the manufacturing cost of wind turbines for the home market, the installation cost is reduced, as is the return time on investment. This time is one of the main barriers to most renewable energy system in the home power market. On the other hand, the situation with smaller installations with a nominal output up to approximately 10 kW can be considered to be substantially different. At this level of the production, there are very many applications which up until now could only be insufficiently covered with horizontal systems. In particular, horizontal installations come up against their limits when located in high mountain areas, in regions with extremely strong or gusty winds, or in urban areas. But also in areas with relatively constant winds, that is, where the conditions are ideal for systems with a horizontal axis, a VAWT can have its advantages, at minimum if the neighbors complain about the annoyance of the noise. There have already been reports of enraged neighbors who have settled the acoustical problems with firearms.
Wind Turbine Blade Design
Research on implementation of structural re-enforcement of mechanism in the polymer matrix composite materials has been increasing . Matt et al.  reported a study toward development of a feasible method to supply the healing agent throughout woven fiberglass reinforced epoxy composite. Huang et al.  investigated the influence of embedded circular hollow vascules on the structural performance of a fiber-reinforced polymer (FRP) composite laminate. The presence of these off-axis vascules caused resin-rich regions in the FRP laminates. In-plane and out-of-plane fiber alignments were changed due to the inclusion of these vascules. A proportional correlation between the cross-sectional diameter of vascules to that of resin-rich region area, pocket length, and fiber disturbance height was found in their experimental study. The study revealed that the compressive strength of the FRP composites decreases with the inclusion of vascules and further decreases with the increase in cross-sectional vascular diameter.
Motuku et al.  used hollow glass fibers along with borosilicate glass microcapillary pipettes (1.15 mm outer diameter), flint glass Pasteur pipettes, copper tubing, and aluminum tubing in addition to plain weave glass 2 fiber fabrics in polymer composites to study and choose an optimal material to supply the healing agent for low velocity impact. Borosilicate glass microcapillary tubes were selected as the best material to provide the healing agent with equal impact strength to that of conventional composite material. This concept is supported by the study made by Matt et al.  on hollow borosilicate tubes (1 mm outer diameter) embedded in glass fiber-reinforced-polymer composites. Equivalent tensile strength was observed in the samples with and without borosilicate tubes in their study. The microtubes are used both as reinforcements and to supply the healing agent (see Fig. 5). This study is a continuation of study made by Matt et al. [11,14] using smaller, hollow borosilicate tubes of 500 μm outer diameter and 250 μm inner diameter to supply the healing agent. Microscopic analysis of fiberglass reinforced polymer composite with smaller circular tubes is made in this study along with the effect of these embedded microtubes on the tensile and flexural strength of the composite. It is anticipated that such healing technology might advance the structural problem that is facing to an enormous size wind turbines.
Franco et al.  present a technique for optimizing wind turbine blade designs in smart rotors. The objective was to maximize power regardless of wind conditions. An extensive analysis of what is known as “smart blades” from aeronautical solutions and helicopter rotors is provided. The authors show that the analysis of the primary components such as sensors, mechanisms of actuation, and materials is included. Advance research in this technology is presented as a potential solution for more efficient blade designs, and methods for reducing aerodynamic loads are discussed.
Many different wind turbine blades are modeled by designers, scientists, and researchers such as slotted turbine blade, which is a biomimetic model of layered bird wind, that increases power efficiencies [16,17], a tubercle wind turbine blade, similar to whale fin,  for controlling turbulence wake and suppress drag power reductions (Fig. 6).
Wind farm (Fig. 7) efficiency is a function of many variables including atmospheric conditions, geographic terrain, wind turbine design, turbine spacing, and electrical transmission. Understanding the behavior of turbulence generated from wind turbines and wind turbine wake dynamics can lead to more robust wind turbine designs and aid engineers in wind farm layout and lead to increased wind farm efficiency.
Periodically, researchers have summarized advances and state-of-the-art approaches to wind turbine modeling or wake simulations [19–22]. Several of these reviews are noteworthy in that they are comprehensive, relevant, and detailed. In 1999, Crespo et al.  provided an overview of the different modeling methods used to predict velocity deficit in the wake of single and multiple wind turbines. Their review included discussions on kinematic wake models, field models, terrain effects, and wind farm modeling. Kinematic models express the velocity deficit by an analytical expression developed from theoretical work on coflowing jets and experimental data. Field models are much like today's computational fluid dynamics models in that they calculate the velocity at every point of the flow field and rely on a numerical solution of turbulent momentum and continuity equations. Early kinematic and field models are still incorporated into software used for wind farm analysis. Vermeer et al.  followed in 2003 with an overview of computational methods relative to horizontal axis wind turbines and included further discussion of kinematic and field models. The unique aspect of the Vermeer paper was their segregation of experimental and analytical research based on near and far wake studies. They also included a thorough review of experiments that had been performed on a variety of wind turbines. Hansen et al.  studied computational methods for wind turbine analysis including blade element momentum methods, panel methods, vortex methods, and actuator disk methods. Aeroelastic methods for predicting the dynamic response of the turbine blades from time-dependent aerodynamic loads were also presented. In 2011, Sanderse et al.  provided a state-of-the-art review of computational fluid dynamics methods for simulating wind turbines. They classified different numerical methods used and distinguished between models specific for simulating the rotor versus affecting the wake.
Two turbine integration on a complex terrain was studied by Hyvärinen and Segalini that is aligned in the streamwise direction  as shown in Fig. 8. This phenomenon was more prominent at homogeneous inflow conditions than regular conditions, while with a turbulent boundary layer inflow the diffusion of the front-row turbine wake decreased this effect.
The dependency of the atmospheric boundary layer characteristics on the boundary layer height is investigated by using large-eddy simulations [22,28,29]. These researchers investigated the impacts of atmospheric boundary layer's height on the wind turbine power production by simulating two subsequent wind turbines using the actuator line method .
Special Treatment on Wind Turbines
Lens Wind Turbines.
The researchers at Kyushu University, Fukuoka, Japan, developed a lens type wind turbines . The blades are installed in a shroud, which is known as diffuser-augmented wind turbines, which can considerably increase the performance of the rotor. In wind tunnel experiments, the power output and aerodynamics characteristics diffuser-augmented wind turbines installed, in side-by-side arrangements, demonstrated an increase of up to 12% in total power output. The results can be explained by observing the bluff body flow phenomena in the wake interference around the multiple circular disks. An airfoil section of the turbine blade gives the best performance in a low-tip to wind speed ratio range. Since the shroud suppresses vortices generated from the turbine blades within the diffuser shroud, the aerodynamic performance is effectively enhanced, and noise is reduced (see Fig. 9).
Floating Wind Turbines.
Offshore wind power has a keen interest in the development of next-generation alternative energy. This interest stems primarily from several advantages that offshore wind energy has over traditional onshore technology. Specifically, the marine environment provides substantial, steady, and relatively uniform wind conditions that constitute a rich and robust energy resource. Current off-shore turbine technology focuses on designs for shallow water scenarios. Wind turbines sited in shallow water are fixed to the bedrock with sturdy foundations. Although these models are proven the technology, there has been a recent interest in the wind energy community to move further offshore to exploit stronger winds . However, at present deep water designs remain purely conceptual and in prototype development.
Offshore wind needs consideration by taking into account the experience of offshore wind farms in current operation, a new design based on dimensionless wave height parameter is proposed by Esteban et al. The authors presented improving a preliminary design of scour protection systems taking into account climatic parameters as the wave height or the wave period.
The U.S. National Renewable Energy Laboratory has provided concepts of deep water designs that consist of the tower and rotor mounted on a floating platform, which is attached to a mooring/tension line station-keeping system . Despite the progress made in these conceptual designs, there still exist many engineering challenges associated with the employment and commercialization of these floating offshore wind turbines. For instance, even though station-keeping systems are used to keep wind turbines from drifting away, floating offshore wind turbines are free to move about six degrees-of-freedom. It is anticipated that the off-shore environment will actively force surge and pitch motions.
The Swedish company Hexicon (Stockholm, Sweden) has developed an innovative floating wind farm (see Fig. 10).2 These types may soon supply Malta with 9% of its electricity needs via one of these floating wind farms.
The hexagonal structure of floating wind turbine platform made of concrete carries lens turbines developed by Kyushu University. The floating structure consists of six-cylinder floating bodies that are constructed by truss members. The cylinder floating bodies and the truss members are made of prestressed concrete with high durability [35,36] (see Fig. 11).
Magenn Power Air Rotor System is developing a “lighter-than-air” helium-lifted wind turbine, which may serve as a future land- and sea-based windmills. Magenn Power proposes an air rotor that operates with Magnus effect. The company is on schedule to put the first units into production shortly. This type of floating wind turbines can generate roughly 4 kW residential systems, costing just USD 10,000. They claim that the floating turbine operates either straight lift from the helium or the Magnus effect as wind speed increases; as this turbine uses the Magnus effect of rotation, lift increases, drag can be minimized because of reduced leaning, and stability increases. The future versions will dwarf even some legendary airships (Fig. 12).
Wind turbines operating in cold areas or at high altitudes often face icing conditions during winter operation (Fig. 13). At the same time, the best sites for wind farm installation are located at higher elevations, as wind speed increases by 1 m/s per 1 km of altitude for the first 1 km. In regions with northern climate, available wind power is approximately 10% higher than other areas due to increased air density at lower temperatures. Under icing conditions, thermal management is one of the treatments for wind energy systems, regarding de-icing and anti-icing of the turbine blades in cold climates and cooling of the heat-generating components. Nowadays, large wind farms or wind power projects are more often implemented in cold climates and at higher altitudes mainly due to their attracting wind energy potential and helpful wind power resources [36,37]. Ice accretion is detrimental to turbine performance, durability, and the safety of those in the vicinity of operating iced turbines. For example, the ice buildup on the turbine blades, even for slight icing, disturbs the flows which attribute to the decrease in the power generation, overloads stall-regulated turbines, and deteriorates the performance. Besides, the added ice mass increases the loads on all the turbine components and causes a mass imbalance among turbine blades, which might cause mechanical failure and thus the turbine lifetime. Therefore, icing mitigation techniques are essential for the operation of wind turbines in cold climates. Icing reduction techniques include anti-icing and de-icing methods. Anti-icing prevents ice to accrete on the blade while de-icing removes the accreted ice layer from the rotor blade surface. Ice reduction techniques can also be divided into passive and active techniques. Passive methods take advantage of the physical properties of the blade surface to eliminate or prevent ice, including ice-phobic coatings, or blades sprayed with a chemical or painted while effective methods use external energy such as thermal, mechanical, chemical reacting, or pneumatic energies  (Fig. 14).2
The use of wind power was an old practice since thousands of years ago. However, this technology has revived due to the shortage of fuels and the environmental problems generated by the traditional energy resources. During the last decade, there is a rapid increase in wind turbine generated electricity worldwide and is widely recognized with an extensive industry manufacturing and installing a large amount of electric power of new capacity every year. Despite the exciting new technology implements arose, particularly for large wind turbines, and many challenges remain, there is a considerable rate of established knowledge concerning the science and technology of wind turbines. This book is intended to present some of this knowledge and to present it in a form suitable for use by students and by those involved in the design, manufacture, or operation of wind turbines.
The research on this project was funded by the U.S. National Science Foundation (NSF) under CBET 1236312 and CBET 1539857.