Wind is atmospheric air in motion. It is omnipresent and one of the basic physical elements of the Earth’s environment. Depending on the speed of the moving air, wind can feel light and ethereal, being silent and invisible to the naked eye, or it can be a strong and destructive force, loud and visible as a result of the heavy debris it carries along. The velocity of the air motion defines the strength of wind and is directly related to the quantity of energy in the wind, i.e., its kinetic energy. The source of this energy, however, is solar radiation. The electro-magnetic radiation from the sun unevenly heats the earth surface, stronger in the tropics and weaker in the high latitudes. Also, as a result of a differential absorption of sun light by soil, rock, water and vegetation, air in different regions warms up at different rate. This uneven heating is converted through convective processes to air motion, which is adjusted by the rotation of the Earth. The convective processes are disturbances of the hydrostatic balance whereby otherwise stagnant air masses are displaced and move in reaction to forces induced by changes in air density and buoyancy because of the temperature differences. Air is pushed from high to low pressure regions, balancing friction and inertial forces because of the rotation of the Earth.
The patterns of differential Earth surface heating as well as other thermal processes such as evaporation, precipitation, clouds, shade and variations of surface radiation absorption appear on different space and time scales. These are coupled with dynamical forces because of Earth rotation and flow momentum redistribution to drive a variety of wind generation processes, leading to the existence of a large variety of wind phenomena. These winds can be categorized based on their spatial scale and physical generation mechanisms.
Wind systems span a wide range of spatial scales, from global circulation on the planetary scale, through synoptic scale weather systems, to mesoscale regional and microscale local winds. Example of planetary circulations are sustained zonal flows such as the jet stream, trade winds, and polar jets. Mesoscale winds include orographic and thermally induced circulations. On the microscale wind systems include flow channeling by urban topography as well as sub-mesoscale convective wind storm phenomena as an example.
Meteorology is the scientific field involved in the study and explanation of all the wind phenomena. It enables both a theoretical understanding and the practical forecasting capabilities of wind. Statistics of observed wind occurrences define wind climates in different regions. Mathematical and computer models are used for theoretical simulation, exploratory resource assessment and operational forecasting of winds.
The total quantity of economically extractable power available from the wind is very high. Axel Kleidon of the Max Planck Institute in Germany carried out a ‘top down’ calculation on how much wind energy there is, starting with the incoming solar radiation which drives the winds by creating temperature differences in the atmosphere. He concluded that somewhere between 18 TW (terra watt which is one trillion watt) and 68 TW can be extracted. Cristina Archer and Mark Z. Jacobson presented a ‘bottom-up’ estimate based on actual measurements of wind speeds. As per this estimate there is 1700 TW of wind power available at an altitude of 100 meters (m) over land and sea. Out of this available power, between 72 TW and 170 TW can be extracted in a practical and cost competitive manner. They later estimated it to be 80 TW. However, studies at Harvard university estimate 1 Watt/sq-km on an average and 2 MW/sq-km to 10 MW/sq-km capacities for large scale wind farms, suggesting that these estimates of total global wind resources are too high by a factor of around 4.
Wind is one of the most important sources of green and renewable energy. Both the terms ‘wind energy’ and ‘wind power’ refer to the process of using the wind to generate mechanical or electrical power. Wind is caused by the uneven heating of the atmosphere by the sun, variations in the Earth’s surface, and rotation of the Earth. Mountains, bodies of water and vegetation influence wind flow patterns. Wind speeds vary based on geography, topography, and season. As a result, there are some locations better suited for wind energy generation.
The fundamental equation of wind power answers the most basic quantitative question which is how much energy is in the wind. First, one is to distinguish between concepts of power and energy. Power is the time-rate of energy. For example, one need to know how much energy can be generated by a wind turbine per unit time. On a more homely front, the power of the wind is the rate of wind energy flow through an open window (normally in cylindrical shape). Wind energy depends on (i) quantity of air (the volume of air in consideration), (ii) speed of air (the magnitude of its velocity), and (iii) mass of air (related to its volume through density).
Wind power quantifies the amount of wind energy flowing through an area of interest per unit time. In other words, wind power is the flux of wind energy through an area of interest. Flux is a fundamental concept in fluid mechanics, measuring the rate of flow of any quantity carried with the moving fluid, by definition normalized per unit area.
Wind energy by definition is the energy content of air flow because of its motion. This type of energy is called the kinetic energy and is a function of its mass and velocity. Wind power is the rate of kinetic energy flow. It is the quantity of kinetic energy flowing per unit time through a given area. It is customary to normalize ambient wind power dividing by the area of interest, i.e., in terms of specific power flow. This leads to the definition of kinetic wind energy flux, known as the wind power density (WDP). Similar to the definitions of flux and flow rate definitions above, wind energy flux is wind energy flow rate per unit area. Wind power density is used to compare wind resources independent of wind turbine size and is the quantitative basis for the standard classification of wind resource.
The kinetic energy in the wind is a promising source of renewable energy. The energy which can be captured by wind turbines is highly dependent on the local average wind speed. Regions which normally present the most attractive potential are located near coasts, inland areas with open terrain or on the edge of bodies of water. Some mountainous areas also have good potential.
Wind energy is a special form of kinetic energy in air as it flows. Wind energy can be either converted into electrical energy by power converting machines or directly used for pumping water, sailing ships, or grinding gain. Mechanical power can also be utilized directly for specific tasks such as pumping water. The mechanism used to convert air motion into electricity is referred to as a turbine. The power in the wind is extracted by allowing it to blow past moving blades which exert torque on a rotor. The rotor turns the drive shaft, which turns an electric generator. The quantity of power transferred is dependent on the rotor size and the wind speed.
Wind energy is a clean, low-cost energy resource which offers the security of a renewable energy supply. Harnessed wind energy can be used to generate electricity. A wind turbine is a rotating machine which converts the wind’s kinetic energy to mechanical energy. This mechanical energy is then converted into electricity by a generator, without the emission of pollutants or green-house gases. For centuries windmills have been used to harness wind energy, normally to power a piece of machinery such as a grinder for grain, and wind turbines are the modern equivalent.
A wind farm is a group of wind turbines in the same location used for the production of electric power. A large wind farm can consist of several hundred individual wind turbines distributed over an extended area, but the land between the turbines can be used for agricultural or other purposes. A wind farm can also be located offshore. Almost all large wind turbines have the same design normally a horizontal axis wind turbine having an upwind rotor with three blades, attached to a nacelle on top of a tall tubular tower. In a wind farm, individual turbines are interconnected with a medium voltage (frequently 33 kilovolts, kV), power collection system and communications network. At a substation, this medium-voltage electric current is increased in voltage with a transformer for connection to the high voltage electric power transmission system.
For centuries windmills have been used to harness wind energy, normally to power a piece of machinery such as a grinder for grain. The first windmill used for the production of electricity was built in Scotland in July 1887 by Prof. James Blyth of Anderson’s College, Glasgow. Blyth’s 10 m high, cloth-sailed wind turbine was installed in the garden of his holiday cottage at Marykirk in Kincardineshire and was used to charge accumulators developed by the Frenchman Camille Alphonse Faure, to power the lighting in the cottage, hence, making it the first house in the world to have its electricity supplied by wind power. Blyth offered the surplus electricity to the people of Marykirk for lighting the main street, however, they turned down the offer as they thought electricity was ‘the work of the devil’. Although he later built a wind turbine to supply emergency power to the local ‘Lunatic Asylum, Infirmary and Dispensary of Montrose’, the invention never really caught on as the technology was not considered to be economically viable.
Across the Atlantic, in Cleveland, Ohio a larger and heavily engineered machine was designed and constructed in the winter of 1887-1888 by Charles F. Brush, this was built by his engineering company at his home and operated from 1886 until 1900.The Brush wind turbine had a rotor 56 feet (17 m) in diameter and was mounted on a 60 feet (18.2 m) tower. Although large by today’s standards, the machine was only rated at 12 kW. The connected dynamo was used either to charge a bank of batteries or to operate up to 100 incandescent light bulbs, three arc lamps, and different motors in Brush’s laboratory.
As a pioneering design for modern wind turbines, the Gedser wind turbine was built in Denmark in the mid-1950s. Today, modern wind turbines in wind farms have typically three blades, operating at relative high wind speeds for the power output up to several megawatts.
In recent decades wind power generation and wind turbine design has made remarkable advances because of modern technological developments. It has been estimated that advances in aerodynamics, structural dynamics, and micro-meteorology can contribute to a 5 % annual increase in the energy yield of wind turbines. Different wind turbine concepts have been developed and built for maximizing the wind energy output, minimizing the turbine cost, and increasing the turbine efficiency and reliability.
Wind varies with the geographical locations, time of day, season, and height above the earth’s surface, weather, and local landforms. The understanding of the wind characteristics helps in optimizing the wind turbine design, develop wind measuring techniques, and select wind farm sites.
Wind speed – Wind speed is one of the most critical characteristics in wind power generation. In fact, wind speed varies in both time and space, determined by several factors such as geographic and weather conditions. Since wind speed is a random parameter, measured wind speed data are normally dealt with using statistical methods. The daytime variations of average wind speeds are frequently described by sine waves. In a study, based on the wind speed data for the period 1970 to 2003 from up to 66 onshore sites around United Kingdom, it has been concluded that monthly average wind speed is inversely propositional to the monthly average temperature, i.e., it is higher in the winter and lower in the summer.
The year-to-year variation of yearly mean wind speeds depends highly on selected locations and hence there is no common correlation to predict it. The variation in wind speed at a particular site can be best described using the Weibull distribution function, which illustrates the probability of different mean wind speeds occurring at the site during a period of time (Fig 1). It has been reported that Weibull distribution can give good fits to observed wind speed data.
Fig 1 Characteristics of wind and wind energy
Wind turbulence – Wind turbulence is the fluctuation in wind speed in short time scales, especially for the horizontal velocity component. The wind speed at any instant time can be considered as having two components namely (i) the mean wind speed, and (ii) the instantaneous speed fluctuation. Wind turbulence has a strong impact on the power output fluctuation of wind turbine. Heavy turbulence can generate large dynamic fatigue loads acting on the turbine and hence reduce the expected turbine life-time or result in turbine failure. In selection of wind farm locations, the knowledge of wind turbulence intensity is crucial for the stability of wind power production.
Wind gust – Wind gust refers to a phenomenon which a wind blasts with a sudden increase in wind speed in a relatively small interval of time. In case of sudden turbulent gusts, the wind speed, turbulence, and wind shear can change drastically. Reducing rotor imbalance while maintaining the power output of wind turbine generator constant during such sudden turbulent gusts calls for relatively rapid changes of the pitch angle of the blades. However, there is typically a time lag between the occurrence of a turbulent gust and the actual pitching of the blades based upon dynamics of the pitch control actuator and the large inertia of the mechanical components. As a result, load imbalances and generator speed, and hence oscillations in the turbine components can increase considerably during such turbulent gusts, and can exceed the maximum prescribed power output level. Moreover, sudden turbulent gusts can also considerably increase tower fore-aft and side-to-side bending moments because of the increase in the effect of wind shear. To ensure safe operation of wind farms, wind gust predictions are highly desired.
Several different gust prediction methods have been proposed. Contrary to the majority of the techniques used in operational weather forecasting, Brasseur developed a new wind gust prediction method based on physical consideration. In another study, it is reported that using a gust factor, which is defined as peak gust over the mean wind speed, can well forecast wind gust speeds.
Wind direction – Wind direction is one of the important wind characteristics. Statistical data of wind directions over a long period of time is very important in the selection of the location of the wind farm and the layout of wind turbines in the wind farm. The wind rose diagram is a useful tool of analyzing wind data which are related to wind directions at a particular location over a specific time period (year, season, month, week, etc.). This circular diagram displays the relative frequency of wind directions in 8 or 16 principal directions. An example of it is shown in Fig 1, where there are 16 radial lines in the wind rose diagram, with 22.5-degree apart from each other. The length of each line is proportional to the frequency of wind direction. The frequency of calm or near calm air is given as a number in the central circle. Some wind rose diagrams can also contain the information of wind speeds.
Wind shear – Wind shear is a meteorological phenomenon in which wind increases with the height above the ground. The effect of height on the wind speed is mainly because of the roughness on the Earth’s surface and can be estimated using the Hellmann power equation which relates wind speeds at two different heights. The wind shear coefficient is normally lower in day-time and higher at night. Empirical results indicate that wind shear frequently follows the ‘1/7 power law’. Since the power output of wind turbine strongly depends on the wind speed at the hub height, modern wind turbines are built at the height higher than 80 m, for capturing more wind energy and lowering cost per unit power output.
Wind energy characteristics
Wind energy is a special form of kinetic energy in air as it flows. Wind energy can be either converted into electrical energy by power converting machines or directly used for pumping water, sailing ships, or grinding gain.
Wind power – in order to achieve a higher wind power, it needs a higher wind speed, a longer length of blades for gaining a larger swept area, and a higher air density. Since the wind power output is proportional to the cubic power of the mean wind speed, a small variation in wind speed can result in a large change in wind power.
Blade swept area – The blade swept area can be calculated from the equation A = Pi (3.1416) [L (L + 2r)], where ‘L’ is the length of wind blades and ‘r’ is the radius of the hub. Hence, by doubling the length of wind blades, the swept area can be increased by the factor up to 4.
Air density – Air density is another important parameter which directly affects the wind power generation. Air density can be calculated from the equation of state p = p/RT, where p is the local air pressure, R is the gas constant (287 J/kg-K for air), and T is the local air temperature in Kelvin (K). The density of air decreases non-linearly with the height above the sea level.
Wind power density – Wind power density is a comprehensive index in evaluating the wind resource at a particular location. It is the available wind power in air-flow through a perpendicular cross-sectional unit area in a unit time period. The classes of wind power density at two standard wind measurement heights are listed in Tab 1. Some of wind resource assessments utilize 50 m towers with sensors installed at intermediate levels (10 m, 20 m, etc.). For large-scale wind plants, class rating of 4 or higher is preferred.
|Tab 1 Classes of wind power density|
|Wind power class||10 metres height||50 metres height|
|Wind power density||Mean wind velocity||Wind power density||Mean wind velocity|
|1||Less than 100||Less than 4.4||More than 200||Less than 5.6|
|2||100 – 150||4.4 – 5.1||200 -300||5.6 – 6.4|
|3||150 – 200||5.1 – 5.6||300 – 400||6.4 – 7.0|
|4||200 -250||5.6 – 6.0||400 – 500||7.0 – 7.5|
|5||250 – 300||6.0 – 6.4||500 – 600||7.5 – 8.0|
|6||300 – 400||6.4 – 7.0||600 – 800||8.0 – 8.8|
|7||More than 400||More than 7.0||More than 800||More than 8.8|
Wind power coefficient – The conversion of wind energy to electrical energy involves primarily two stages. In the first stage, kinetic energy in wind is converted into mechanical energy to drive the shaft of a wind generator. The power coefficient which deals with the converting efficiency in the first stage, is defined as the ratio of the actually captured mechanical power by blades to the available power in wind. Since there are different aero-dynamic losses in wind turbine systems, for example, blade-tip, blade-root, profile, and wake rotation losses, etc., the real power coefficient is much lower than its theoretical limit, normally ranging from 30 % to 45 %.
In the second stage, mechanical energy captured by wind blades is further converted into electrical energy through wind generators. In this stage, the converting efficiency is determined by several parameters namely (i) gear-box efficiency, (ii) generator efficiency, and (iii) electrical efficiency.
The power losses in a gear-box can be classified as load-dependent and no-load power losses. The load-dependent losses consist of gear tooth friction and bearing losses and no-load losses consist of oil churning, windage, and shaft seal losses. The planetary gear-boxes, which are widely used in wind turbines, have higher power transmission efficiencies over traditional gear-boxes. The generator efficiency is related to all electrical and mechanical losses in a wind generator, such as copper, iron, load, windage, friction, and other miscellaneous losses. Electrical efficiency encompasses all combined electric power losses in the converter, switches, controls, and cables. Hence, the total power conversion efficiency from wind to electricity is the product of these three efficiencies.
Lanchester–Betz limit – The Lanchester-Betz limit is the theoretical maximum efficiency for a wind turbine, estimated by German physicist Albert Betz in 1919. Betz concluded that this value is 59.3 %, meaning that at the most only 59.3 % of the kinetic energy from wind can be used to spin the turbine and generate electric power. In reality, turbines cannot reach the Betz limit, and common efficiencies are in the range of 35 % to 45 %. If a wind turbine is to be 100 % efficient, then all of the wind has to stop completely upon contact with the turbine which is not practically possible.
Wind speed – power curve – Wind speed largely determines the quantity of electric power generated by a turbine. Higher wind speeds generate more power since stronger winds allow the blades to rotate faster. Faster rotation translates to more mechanical power and more electrical power from the generator. Turbines are designed to operate within a specific range of wind speeds. The limits of the range are known as the cut-in speed and cut-out speed. The cut-in speed is the point at which the wind turbine is able to generate power. Between the cut-in speed and the rated speed, where the maximum output is reached, the power output increases cubically with wind speed. As an example, if wind speed doubles, the power output increases 8 times. This cubic relationship is what makes wind speed such an important factor for wind power. This cubic dependence does cut out at the rated wind speed. This leads to the relatively flat part of the curve in Fig 2, so the cubic dependence is during the speeds below 15 meter per second (m/s). The cut-out speed is the point at which the turbine is to be shut down to avoid damage to the equipment. The cut-in and cut-out speeds are related to the turbine design and size and are decided on prior to construction. The relationship between wind speed and power for a typical wind turbine is shown in Fig 2.
Fig 2 Wind power curve
Tip speed ratio – The ‘tip speed ratio’ (frequently known as the TSR) is of vital importance in the design of wind turbine generators. If the rotor of the wind turbine turns too slowly, majority of the wind passes undisturbed through the gap between the rotor blades. Instead, if the rotor turns too quickly, the blurring blades appear like a solid wall to the wind. Hence, wind turbines are designed with optimal tip speed ratios to extract as much power out of the wind as possible. The tip speed ratio is given by dividing the speed of the tips of the turbine blades by the speed of the wind. As an example, if a 6 m/s wind is blowing on a wind turbine and the tips of its blades are rotating at 24 m/s, then the tip speed ratio is 24/6 = 4.
Force on a wind turbine – Air-flow over a surface creates two types of aero-dynamic forces namely (i) drag forces in the direction of the air-flow, and (ii) lift forces perpendicular to the air-flow. Either or both of these can be used to generate the forces needed to rotate the blades of a wind turbine.
Drag-based wind turbine – In drag-based wind turbines, the force of the wind pushes against a surface, like an open sail. In fact, the earliest wind turbines, dating back to ancient Persia, used this approach. The Savonius rotor is a simple drag-based windmill. It works since the drag of the open, or concave, face of the air cylinder is greater than the drag on the closed or convex section.
Lift-based wind turbines – More energy can be extracted from wind using lift rather than drag, but this needs specially shaped air-foil surfaces, like those used on air-plane wings. The air-foil shape is designed to create a differential pressure between the upper and lower surfaces, leading to a net force in the direction perpendicular to the wind direction. Rotors of this type are to be carefully oriented (the orientation is referred to as the rotor pitch), to maintain their ability to harness the power of the wind as wind speed changes.
Types of wind turbines
Wind turbines can be classified according to the turbine generator configuration, air-flow path relatively to the turbine rotor, turbine capacity, the generator-driving pattern, the power supply mode, and the location of turbine installation.
One of the classifications of wind turbines is based on the quantity of power which they can generate, and are directly correlated to the physical size of the turbine (larger wind turbines produce more energy). The three types of turbines in terms of power output levels are utility‐scale, industrial‐scale, and residential‐scale wind turbines. Utility‐scale turbines produce the highest quantity of energy which is normally sold back to the grid, while residential‐scale turbines produce only 2 % to 28 % of the energy of a Utility‐scale wind turbine and are used for single buildings. Industrial‐scale wind turbines lie between the range of a utility‐scale and residential‐scale turbine, and can either be applied for single buildings or communities or sold to the grid.
Wind turbines can be categorized into two basic types determined by which way the turbine spins. Wind turbines which rotate around a horizontal axis are more common (like a wind mill), while vertical axis wind turbines are less frequently used (Savonius and Darrieus are the most common in the group).
Based on the configuration of the wind rotor with respect to the wind flowing direction, the horizontal-axis wind turbines can be further classified as upwind and downwind wind turbines. The majority of horizontal-axis wind turbines being used today are upwind turbines, in which the wind rotors face the wind. The main advantage of upwind designs is to avoid the distortion of the flow field as the wind passes though the wind tower and nacelle. For a downwind turbine, wind blows first through the nacelle and tower and then the rotor blades. This configuration enables the rotor blades to be made more flexible without considering tower strike. However, because of the influence of the distorted unstable wakes behind the tower and nacelle, the wind power output generated from a downwind turbine fluctuates greatly. In addition, the unstable flow field can result in more aerodynamic losses and introduce more fatigue loads on the turbine. Furthermore, the blades in a downwind wind turbine can produce higher impulsive or thumping noise. Fig 3 gives comparison of the configurations the horizontal and vertical types of the wind turbines.
Fig 3 Wind turbine configurations
Horizontal axis wind turbines (HAWT) are the common type. A HAWT has a similar design to a windmill, it has blades which look like a propeller which spin on the horizontal axis. Horizontal axis wind turbines have the main rotor shaft and electrical generator at the top of a tower, and they are to be pointed into the wind. Small turbines are pointed by a simple wind vane placed square with the rotor (blades), while large turbines normally use a wind sensor coupled with a servo-motor to turn the turbine into the wind.
Majority of the large wind turbines have a gear-box, which turns the slow rotation of the rotor into a faster rotation which is more suitable to drive an electrical generator. Since a tower produces turbulence behind it, the turbine is normally pointed upwind of the tower. Wind turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted up by a small quantity.
Downwind turbines have been built, despite the problem of turbulence, since they do not need an additional mechanism for keeping them in line with the wind. Additionally, in high winds the blades can be allowed to bend which reduces their swept area and hence their wind resistance. Since turbulence leads to fatigue failures and reliability is so important, majority of the HAWTs are upwind machines. Fig 4 gives the components of horizontal axis wind turbine.
Fig 4 Components of horizontal axis wind turbine
Important point to remember regarding HAWT are (i) lift is the main force, (ii) the cyclic stress is much lower, (iii) 95 % of the existing wind turbines are HAWTs, (iv) nacelle is placed at the top of the tower, and (v) yaw mechanism is needed.
The advantages of the HAWT are (i) the tall tower base allows access to stronger wind in locations with wind shear (in some wind shear locations, every 10 m up, the wind speed can increase by 20 % and the power output by 34 %, and (ii) high efficiency, since the blades always move perpendicular to the wind, receiving power through the whole rotation.
The dis-advantages of the HAWT are (i) massive tower construction is needed to support the heavy blades, gear-box, and generator, (ii) components of HAWT (gear-box, rotor shaft and brake assembly) being lifted into position, (iii) their height makes them obtrusively visible across large areas, disrupting the appearance of the landscape and sometimes creating local opposition, (iv) downwind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower‘s wind shadow (for this reason, the majority of HAWTs use an upwind design, with the rotor facing the wind in front of the tower), (v) HAWTs need an additional yaw control mechanism to turn the blades toward the wind, (vi) HAWTs normally need a braking or yawing device in high winds to stop the turbine from spinning and destroying or damaging itself.
Vertical axis wind turbines (VAWT) have the main rotor shaft arranged vertically. The main advantage of this arrangement is that the wind turbine does not need to be pointed into the wind. This makes them suitable in places where the wind direction is highly variable or has turbulent winds. With a vertical axis, the generator and other primary components can be placed near the ground, so the tower does not need to support it, also makes maintenance easier.
The main drawback of a VAWT is that, it normally creates drag when rotating into the wind. It is difficult to mount vertical-axis turbines on towers, meaning they are frequently installed nearer to the base on which they rest, such as the ground or a building roof-top. The wind speed is slower at a lower altitude, so less wind energy is available for a given size turbine. Air-flow near the ground and other objects can create turbulent flow, which can introduce issues of vibration, including noise and bearing wear which can increase the maintenance or shorten its service life. However, when a turbine is mounted on a roof-top, the building normally redirects wind over the roof and this can double the wind speed at the turbine. If the height of the rooftop mounted turbine tower is around 50 % of the building height, this is near the optimum for maximum wind energy and minimum wind turbulence. Fig 5 shows different types of vertical axis wind turbines.
Fig 5 Types of vertical axis wind turbines
Important aspects for VAWTs are (i) nacelle is placed at the bottom, (ii) drag is the main force , (iii) yaw mechanism is not needed, (iv) lower starting torque, (v) difficulty in mounting the turbine, and (vi) unwanted fluctuations in the power output.
The advantages of the VAWTs are (i) since no yaw mechanisms is needed, the turbine can be located nearer the ground, making it easier to maintain the moving parts, (iii) VAWTs have lower wind start-up speeds than the typical HAWTs, (iv) VAWTs can be built at locations where taller structures are prohibited, (v) VAWTs situated close to the ground can take advantage of locations where roof-tops, means hill-tops, ridge-lines, and passes funnel the wind and increase wind velocity.
The dis-advantages of the VAWT are (i) in contrast to HAWT, all vertical axis wind turbines, and the majority of the air-borne wind turbine designs, involve various types of reciprocating actions, needing air-foil surfaces to the wind leads to inherently lower efficiency, (ii) majorities of VAWTs have an average decreased efficiency as compared to a common HAWT, mainly because of the additional drag that they have as their blades rotate into the wind (versions which reduce drag produce more energy, especially those which funnel wind into the collector area), and (iii) having rotors located close to the ground where wind speeds are lower and do not take advantage of higher wind speeds above. Since VAWTs are not normally deployed mainly because of the serious disadvantage, they appear novel to those who are not familiar with the wind industry. This has frequently made them the subject of wild claims.
Wind turbines can be divided into a number of broad categories in view of their rated capacities. These are micro, small, medium, large, and ultra-large wind turbines. Though a restricted definition of micro wind turbines is not available, it is accepted that a turbine with the rated power less than several kilowatts can be categorized as micro wind turbine. Micro wind turbines are especially suitable in locations where the electrical grid is not available. They can be used on a per-structure basis, such as street lighting, water pumping, and residents at remote areas. Since micro wind turbines need relatively low cut-in speeds at start-up and operate in moderate wind speeds, they can be extensively installed in the majority of the areas around the world for fully utilizing wind resources and greatly improving the wind power generation availability.
Small wind turbines are normally referred to the turbines with the output power less than 100 kW. These wind turbines have been extensively used at residential houses, farms, and other individual remote applications such as water pumping stations, and telecom sites, etc., in the rural regions. Distributed small wind turbines can increase electricity supply in the regions while delaying or avoiding the need to increase the capacity of transmission lines.
The most common wind turbines have medium sizes with power ratings from 100 kW to 1 MW. This type of wind turbines can be used either on-grid or off-grid systems for village power, hybrid systems, distributed power, and wind power plants, etc.
Megawatt wind turbines up to 10 MW can be classified as large wind turbines. In recent years, multi-megawatt wind turbines have become the mainstream with respect to the use wind power. Majority of the wind farms presently use megawatt wind turbines, especially in offshore wind farms. Ultra-large wind turbines are referred to wind turbines with the capacity more than 10 MW. This type of wind turbine is still in the earlier stages of research and development.
As per the drive train condition in a wind generator system, wind turbines can be classified as either direct drive or geared drive groups. To increase the generator rotor rotating speed to gain a higher power output, a regular geared drive wind turbine typically uses a multi-stage gear-box to take the rotational speed from the low-speed shaft of the blade rotor and transform it into a fast rotation on the high-speed shaft of the generator rotor. The advantages of geared generator systems include lower cost and smaller size and weight. However, utilization of a gear-box can considerably lower wind turbine reliability and increase turbine noise level and mechanical losses. By eliminating the multi-stage gearbox from a generator system, the generator shaft is directly connected to the blade rotor. Hence, the direct-drive concept is more superior in terms of energy efficiency, reliability, and design simplicity.
Wind turbines can be used for either on-grid or off-grid applications. Majority of the medium-size and almost all large-size wind turbines are used in grid tied applications. One of the obvious advantages for on-grid wind turbine plants is that there is no energy storage issue. As a contrast, majority of small wind turbines are off-grid for residential homes, farms, telecommunications, and other applications. However, as an intermittent power source, wind power produced from off-grid wind turbines can change dramatically over a short period of time with little warning. Hence, off-grid wind turbines are normally used in conjuntion with batteries, diesel generators, and photovoltaic systems for improving the stability of wind power supply.
Onshore wind turbines have a long history on its development. There are a number of advantages of onshore turbines, including lower cost of foundations, easier integration with the electrical-grid network, lower cost in tower building and turbine installation, and more convenient access for operation and maintenance. Offshore wind turbines have developed faster than onshore since the 1990s because of the excellent offshore wind resource, in terms of wind power intensity and continuity. A wind turbine installed offshore can make higher power output and operate more hours each year compared with the same turbine installed onshore. In addition, environmental restrictions are more laxed at offshore sites than at onshore sites. For example, turbine noise is no longer an issue for offshore wind turbines.
Mill turbine components
There are an estimated 8,000 parts. These parts can be simplified four categories namely the rotor, the nacelle and controls, the generator and power electronics, and the turbine tower. The rotor includes the blades, which harness wind energy and convert it into mechanical energy, as well as a hub, which supports the blades. The generator and power electronics system includes the drive-train, a generator – which converts mechanical energy into electricity, a yaw drive to rotate the nacelle, and electronic controls.
Wind turbines are massive machines which typically weigh between 200 tons and 400 tons. A wide range of materials are used for wind turbine construction. Steel is one of the most important materials because of its strength and durability. Turbines are primarily made of steel, which accounts for 90 % of the machine by weight. A single 1 MW utility scale wind turbine tower is constructed from an estimated 100 tons of steel, and larger turbines use a considerably higher quantity of steel. The rotor is made from around 45 % steel, with the hub being made of 100 % steel, and the blades being made up of 2 % steel and a combination of fiber-glass (78 %) and adhesive (15 %). Steel accounts for between 87 % and 92 % of nacelle components.
The tower is the physical structure which holds the wind turbine. It supports the rotor, nacelle, blades, and other wind turbine equipment. Typical commercial wind towers are normally 50 m to 120 m long and they are made from steel or reinforced concrete.
Blades are physical structures, which are aero-dynamically optimized to help capture the maximum power from the wind in normal operation with a wind speed in the range of around 3 m/s to 15 m/s. Each blade is normally 20 m or more in length, depending on the power level.
The nacelle is the enclosure of the wind turbine generator, gear-box, and internal equipment. It protects the internal components of the turbine from the surrounding environment.
The rotor is the rotating part of the wind turbine. It transfers the energy in the wind to the shaft. The rotor hub holds the wind turbine blades while connected to the gear-box through the low-speed shaft. Pitch is the mechanism of adjusting the angle of attack of the rotor blades. Blades are turned in their longitudinal axis to change the angle of attack according to the wind directions.
The shaft is divided into two types namely low speed and high speed. The low-speed shaft transfers mechanical energy from the rotor to the gear-box, while the high-speed shaft transfers mechanical energy from gear-box to the generator.
Yaw is the horizontal moving part of the turbine. It turns clockwise or anti-clockwise to face the wind. The yaw has two main parts namely the yaw motor and the yaw drive. The yaw drive keeps the rotor facing the wind when the wind direction varies. The yaw motor is used to move the yaw.
The brake is a mechanical part connected to the high-speed shaft in order to reduce the rotational speed or stop the wind turbine over speeding or during emergency conditions.
Gear-box is a mechanical component which is used to increase or decrease the rotational speed. In wind turbines, the gear-box is used to control the rotational speed of the generator. Large bearings are used in gear-boxes. Defective gear-boxes are the main cause of wind turbine failure, and hence these components need to be manufactured to the highest quality.
The generator is the component which converts the mechanical energy from the rotor to electrical energy. The most common electrical generators used in wind turbines are induction generators (IGs), doubly fed induction generators (DFIGs), and permanent magnet synchronous generators (PMSGs).
The controller is the brain of the wind turbine. It monitors constantly the condition of the wind turbine and controls the pitch and yaw systems to extract optimum power from the wind. Anemometer is a type of sensor which is used to measure the wind speed. The wind speed information is necessary for maximum power tracking and protection in emergency cases.
The wind vane is a type of sensor which is used to measure the wind direction. The wind direction information is important for the yaw control system to operate.
Wind power plant
A wind power plant (WPP) is simply a collection of wind turbines in one area. There are several different types of wind power plants. The following classification is based on their construction, size and usage. Fig 6 shows schematics of wind energy-based power plant.
Fig 6 Schematics of wind energy-based power plant
Remote wind power plants – Areas which are remote but have good wind speeds and frequency need a wind turbine which is maintenance free or low-maintenance for long periods of time. This means that the turbines need to have the capability of standing against all odds of climate even if they are relatively smaller in size than their conventional counterparts. These types of turbines are known as remote wind power turbines and are specifically designed with these objectives in view.
Hybrid wind power plant – Wind is not fully reliable so one cannot depend on wind alone for generation of power. The best option is to combine a wind power plant with some other renewable source of energy, like solar power plant. When there is a lot of heat, the solar generators generate power and when the sky is overcast and winds are blowing, the wind power plant takes over. Such an arrangement is known as hybrid arrangement and is useful in regions where there is a lot of heat and wind.
Grid connected wind power plants – The wind power plant is used in conjunction with a main grid which supplies most of the power. The main purpose of the wind turbines is to supplement the energy supply for the grid.
Wind farms – As the name itself suggests, a wind farm is a collection of wind turbines which collectively power a given area or utility harnessing the wind force in a collective manner thereby amplifying the effect of a single unit. These configurations are used at various locations depending on the conditions of the region and the presence of other sources of electrical supply. An optimum mix consists of an ingenious combination of the various sources in the best possible manner.
Working of wind power plants – Wind (moving air which contains kinetic energy) blows toward the turbine’s rotor blades. The rotors spin around, capturing some of the kinetic energy from the wind, and turning the central drive shaft which supports them. Although the outer edges of the rotor blades move very fast, the central axle (drive shaft) turns quite slowly.
In the majority of the large modern turbines, the rotor blades can swivel on the hub at the front so they meet the wind at the best angle (or ‘pitch’) for harvesting energy. This is called the pitch control mechanism. On big turbines, small electric motors or hydraulic rams swivel the blades back and forth under precise electronic control. On smaller turbines, the pitch control is frequently completely mechanical. However, several turbines have fixed rotors and no pitch control at all.
Inside the nacelle (the main body of the turbine sitting on top of the tower and behind the blades), the gear-box converts the low-speed rotation of the drive shaft (perhaps, 16 revolutions per minute, rpm) into high-speed (perhaps, 1,600 rpm) rotation fast enough to drive the generator efficiently. The generator, immediately behind the gear box, takes kinetic energy from the spinning drive shaft and turns it into electrical energy. Running at maximum capacity, a typical 2 MW turbine generator produces 2 million watts of power at around 700 volts.
Anemometers (automatic speed measuring devices) and wind vanes on the back of the nacelle provide measurements of the wind speed and direction. Using these measurements, the entire top part of the turbine (the rotors and nacelle) can be rotated by a yaw motor, mounted between the nacelle and the tower, so that it faces directly into the oncoming wind and captures the maximum quantity of energy. If it is too windy or turbulent, brakes are applied to stop the rotors from turning (for safety reasons). The brakes are also applied during routine maintenance.
The electric current produced by the generator flows through a cable running down through the inside of the turbine tower. A step-up transformer converts the electricity to around 50 times higher voltage so it can be transmitted efficiently to the power grid (or to nearby buildings or communities). If the electricity is flowing to the grid, it is converted to an even higher voltage by a sub-station nearby.
The consumer gets clean, green energy since the turbine has produced no green-house gas emissions or pollution as it operates. Wind carries on blowing past the turbine, but with less speed and energy and more turbulence, since the turbine has disrupted its flow.
Pitch control and yaw control – Different control methods are used either to optimize or limit power output. A turbine is controlled by controlling the generator speed, blade angle adjustment, and rotation of the entire wind turbine. Blade angle adjustment and turbine rotation are also known as pitch and yaw control, respectively. The purpose of pitch control is to maintain the optimum blade angle to achieve certain rotor speeds or power output. One can use pitch adjustment to stall and furl, two methods of pitch control.
By stalling a wind turbine, the angle of attack is increased, which causes the flat side of the blade to face further into the wind. Furling decreases the angle of attack, causing the edge of the blade to face the oncoming wind. Pitch angle adjustment is the most effective way to limit output power by changing aero-dynamic force on the blade at high wind speeds. This maintains the safety of the turbine in the event of high winds, loss of electrical load, or other catastrophic events.
Yaw refers to the rotation of the entire wind turbine in the horizontal axis. Yaw control ensures that the turbine is constantly facing into the wind to maximize the effective rotor area and, as a result, power. Since the wind direction can vary quickly, the turbine can misalign with the oncoming wind and cause power output losses.
In case of stall control, passive stall-controlled wind turbines have the rotor blades bolted onto the hub at a fixed angle. The geometry of the rotor blade profile, however, has been aero-dynamically designed to ensure that the moment the wind speed becomes too high, it creates turbulence on the side of the rotor blade which is not facing the wind. This stall prevents the lifting force of the rotor blade from acting on the rotor. In other words, as the actual wind speed in the area increases, the angle of attack of the rotor blade increases, until at some point it starts to stall. If one looks closely at a rotor blade for a stall-controlled wind turbine, one notices that the blade is twisted slightly as one move along its longitudinal axis. This is partly done in order to ensure that the rotor blade stalls gradually rather than abruptly when the wind speed reaches its critical value.
The basic advantage of stall control is that it avoids moving parts in the rotor itself, and a complex control system. On the other hand, stall control represents a very complex aero-dynamic design issue, and related design challenges in the structural dynamics of the whole wind turbine. Around two thirds of the wind turbines presently being installed in the world are stall-controlled machines.
Locations of wind power plants – The power available in the wind increases rapidly with the speed, hence wind energy conversion machines are to be located preferable in areas where the winds are strong and persistent. Although daily winds at a given site can be highly variable, the monthly and especially annual average are remarkably constant from year to year. The major contribution to the wind power available at a given site is actually made by winds with speeds above the average. However, the majority of the suitable locations for wind turbines are to be found in areas where the annual average wind speeds are known to be moderately high or high.
The location choice for a single or a spatial array of the wind energy conversion system (WECS) is an important matter when wind electrics is looked at from the systems point of view of aero-turbine generators feeding power into a conventional electric grid. If the WECS locations are wrongly or poorly chosen, the net wind electrics generated energy per year can be sub-optimal with resulting high capital cost for the WECS machine, high costs for wind generated electric energy, and low ‘returns on investment’ (ROI). Even if the WECS is to be a small generator not tied to the electric grid, the location is to be carefully chosen if inordinately long break even times are to be avoided.
Technical, economic, environmental, social, and other factors are to be examined before a decision is made to put up a generating plant on a specific location. Some of the main location selection considerations are (i) high annual average wind speed, (ii) availability of anemometry data, (iii) availability of wind power curve at the proposed location, (iv) wind structure at the proposed location, (v) altitude of the proposed location, (vi) terrain and its aerodynamic, (vii) local ecology, (viii) distance to road or railways, (ix) nearness of the location to local sub-station / users, (x) nature of ground, and (xi) favourable land cost.
Since the small increases in velocity markedly affect the power in the wind (doubling the velocity, increases power by a factor of 8), it is obviously desirable to select a location for WECS with high wind velocity. Hence, a high average wind velocity is the principle fundamental parameter of concern in initially appraising WECS location. For more detailed estimate value, it is necessary to have the average of the velocity cubed. Availability of anemometry data is another factor for the selection of WECS. The anemometry data is to be available over some time period at the precise spot where any proposed WECS is to be built. Availability of wind power curve at the proposed location is important since this curve determines the maximum energy in the wind and hence is the principle initially controlling factor in predicting the electrical output and hence revenue return of the WECS machines. It is desirable to have average wind speed higher than 4 m/s, which is around the lower limit at which present large scale WECS generators cut in, i.e., start turning.
The wind power curve also determines the reliability of the delivered WECS generator power, for if the curve goes to zero there be no generated power during that time. If there are long periods of calm the WECS reliability is lower than if the calm periods are short. In making such reliability estimates, it is desirable to have measured wind power curve over around a 5-year period for the highest confidence level in the reliability estimate.
The ideal location for the WECS is such a location where the wind power curve is flat, i.e., a smooth steady wind which blows all the time; but a typical location is always less than ideal. Wind specially near the ground is turbulent and gusty, and changes rapidly in direction and in velocity. This departure from homogeneous flow is collectively referred to as ‘the structure of the wind’.
The altitude of the WECS location affects the air density and hence the power in the wind and the useful electric power output of the WECS. Also, as is well known, the wind tends to have higher velocities at higher altitudes. One is to carefully distinguish altitude from height above ground. They are not the same except for a sea level WECS location.
Terrain and its aero-dynamic is important for the WECS location. It is necessary to know about terrain of the location. If the WECS is to be placed near the top but not on the top of a not too blunt hill facing the prevailing wind, then it is possible to obtain a ‘speed-up‘ of the wind velocity over what it otherwise is. Also, the wind here does not flow horizontal making it necessary to tip the axis of the rotor so that the aero-turbine is always perpendicular to the actual wind flow. It can be possible to make use of hills or mountains which channel the prevailing wind into a pass region, hence getting higher wind power.
Local ecology of location for WECS is important. If the surface is base rock, it means lower hub height hence lower structure cost. If trees or grass or vegetation are present, all of these tend to de-structure the wind, and hence the higher hub heights are be needed which results in large system costs than the bare ground case.
Distance to road or railways is another factor which the system engineer is to consider since heavy machinery, structure, materials, blades and other apparatus are to be moved into the chosen WECS location. Nearness of site to local sub-station / users is an obvious criterion, since it minimizes transmission line length and hence losses and cost.
After applying all these string criteria, one narrows the choice for WECS locations to one or two which are relatively near to the local sub-station of the generated electric energy.
Ground condition is to be such that the foundations for a WECS are secured. Ground surface is to be stable. Erosion issue is not concrete o be there, since it can possibly later wash out the foundation of a WECS, destroying the whole system.
Land cost is to be reasonable, since land cost along with other location related costs, enters into the total WECS system cost. Other issues such as icing problem, salt spray, or blowing dust are not to be present at the site, since as these affect aero-turbine blades or environment which becomes normally adverse to machinery and electrical equipments.
Grid integration issues of wind power plants – The electrical grid is the electrical power system network comprised of the generating plant, the transmission lines, the substation, transformers, the distribution lines, and the consumer. Ideally the electric grid is aimed to operate at constant voltage and frequency. However, the grid can take some fluctuation in voltage and electrical equipment is designed for maximum and minimum allowable voltage levels, normally around + / – 10 %. Wind power generation varies depending on how wind fluctuates. However, the variations in output are smoothed when several wind power plants are aggregated over an area in a power system. To deal with uncertainty, wind power output can be forecasted minutes, hours, and even days ahead. Aggregating wind power plants over a wider geographic area improves the forecast accuracy at all time frames.
Wind power as a generation source has specific characteristics, including variability, geographical distribution, favourable economics. Large-scale integration of both on-shore and off-shore wind raises challenges for the different stakeholders involved, ranging from generation, transmission and distribution to power trading and consumers. In order to integrate wind power successfully, a number of issues need to be addressed in several areas as given below.
Variability – Power plants which run on fuel (along with some hydro and geothermal plants) can be ramped up and down on command. They are, in the jargon, ‘dispatchable’. But ‘variable renewable energy’ (VRE) plants produce power only when the wind is blowing or the sun is shining. Grid operators do not control VRE, they accommodate it, which needs some agility.
Uncertainty – The output of VRE plants cannot be predicted with perfect accuracy in day-ahead and day-of forecasts, so grid operators have to keep excess reserve running just in case there is high fluctuations in generated power.
Location-specificity – Sun and wind are stronger (and hence more economical) in some places than in others and it is not always in places which have the necessary transmission infrastructure to get the power to where it is needed.
Non-synchronous generation – Conventional generators provide voltage support and frequency control to the grid. VRE generators can too, potentially, but it is an additional capital investment.
Low-capacity factor – VRE plants only run when sun or wind cooperates. According to the Energy Information Administration, in 2014, the average capacity factor (production relative to potential) for utility-scale wind power plant was 34 % (By way of comparison, the average capacity factor of solar PV plant was 28 %, nuclear power was 92 %, and fuel-based plants are almost always producing power). Because of the low-capacity factor of VRE, conventional plants are needed to take up the slack, but because of the high output of VRE in peak hours, conventional plants sometimes do not get to run as frequently as needed to recover costs.
Grid integration of wind farms and power quality issues – The issue of power quality is of great importance for the wind turbines. The critical power quality issues related to integration of wind farms are described below.
There is the issue of voltage variation. If a large proportion of the grid load is supplied by wind turbines, the output variations because of the wind speed changes can cause voltage variation, flicker effects in normal operation. The voltage variation can occur in specific situation, as a result of load changes, and power produce from turbine.
Another issue is the issue of voltage dips. It is a sudden reduction in the voltage to a value between 1 % and 90 % of the nominal value after a short period of time, conventionally 1 milli-second to 1 minute. This issue is considered in the power quality and wind turbine generating system operation and computed according to the rule given in ‘IEC 61400-3-7 standard, Assessment of emission limit for fluctuating load’.
Switching operation of wind turbine on the grid is an important issue. Switching operations of wind turbine generating system can cause voltage fluctuations and hence voltage sag, voltage swell which can cause considerable voltage variation. The acceptances of switching operation depend not only on grid voltage but also on how frequently this can occur. The maximum number of above specified switching operation within 10 minute period and 2 hours period are defined in IEC 61400-3-7 Standard.
Harmonics are an issue. The harmonics voltage and current are to be limited to acceptable level at the point of wind turbine connection in the system. This fact has leads to more stringent requirements regarding power quality, such as Standard IEC 61000-3-2 or IEEE-519.
Another issue is flickers. Flicker is the one of the important power quality aspects in wind turbine generating system. Flicker has widely been considered as a serious draw-back and can limit for the maximum quantity of wind power generation which can be connected to the grid. Flicker is induced by voltage fluctuations, which are caused by load flow changes in the grid. The flicker emission produced by grid-connected variable-speed wind turbines with full-scale back-to-back converters during continuous operation are mainly caused by fluctuations in the output power because of the wind speed variations, the wind shear, and the tower shadow effects.
Reactive power is an issue. Traditional wind turbines are equipped with induction generators. Induction generator is preferred since they are inexpensive, rugged, and needs little maintenance. Unfortunately, induction generators need reactive power from the grid to operate. The interactions between wind turbine and power system network are important aspect of wind generation system.
Location of wind turbine has an effect on the grid integration. The way of connecting wind turbine into the electric power system highly influences the impact of the wind turbine generating system on the power quality. As a rule, the impact on power quality at the terminal of the consumer for the wind turbine generating system (WTGS) located close to the load is higher than WTGS connected away, that is connected to high voltage (HV) or extra high voltage (EHV) system.
Low voltage ride through capability is an issue. The impact of the wind generation on the power system no longer is negligible if high penetration levels are going to be reached. The extent to which wind power can be integrated into the power system without affecting the overall stable operation depends on the technology available to mitigate the possible negative impacts such as loss of generation for frequency support, voltage flicker, voltage and power variation due to the variable speed of the wind and the risk of instability due to lower degree of controllability.
For consistent and replicable documentation of power quality characteristic of wind turbine, the International Electro-technical Commission IEC 61400-21 has been developed, and today, mojority of the large wind turbine manufactures provide power quality characteristic data accordingly. IEC 61400-21 describe the procedures for determine the power quality characteristics of wind turbines. The allowable limit as per the grid code for wind farms are (i) voltage rise – less than 2 %, (ii) voltage dips – less than or equal to 3 % (iii) flicker – less than or equal to 0.4, for average time of 2 hours, and (iv) grid frequency – 47.5 Hertz to 51.5 Hertz.