Chapter 7
Wind Energy
7.1 Wind as Source of Energy
The wind power is basically nothing but another form of solar energy. Approximately 1% of the total solar energy absorbed by the Earth is converted to kinetic energy in the atmosphere, in the form of wind. Since early recorded history, people have realized the potential of wind energy and utilized it for various applications. It was used to propel boats along the Nile River as early as 5000 BC, and it was used to pump water and grind grain between 500 and 900 BC. By the 11th century, windmills were used in food production in the Middle East. The windmills were further improved by the Dutch and others and were adapted for industrial applications such as sawing wood, making paper and draining lakes and marshes. In the late 19th century, the wind power was used in windmills to pump water for farms and ranches. However, due to industrialization and rural electrification, in the 20th century, there was gradual decline in the use of windmills for mechanical applications.
First large-sized automatically operating wind turbine for generation of electricity was built by Charles Brush of the United States in 1888. From 1920 to 1940, propeller-type horizontal-axis wind turbines (HAWTs) with two or three blades were used to supply electricity in rural areas where supply of electricity from the grid was not available. However, the use of wind turbines to generate electricity at a commercial scale started in the 1970s as a result of technical advances in the field of turbine and mainly due to escalating oil prices because of OPEC crisis in 1971. During the last few years, the three-blade upwind horizontal-axis large turbines on monopole tower have become the standard. In terms of total number of wind turbines in use at present, they are of small capacity of the order of 10 kW or less. But in terms of total generating capacity, the turbines that are used in large wind farms have capacity in the range of 1.5–5 MW.
7.1.1 Origin of Wind
The unequal heating of Earth's surface due to its tilt, rotation and difference in insolation results in the wide distribution of pressure over the Earth's surface. Difference in temperature causes pressure difference which in turn results in air movement called wind. The regions which receive higher amount of heat cause heating of air which rises in the atmosphere. The upward movement of hot air induces cooler air from the surroundings to rush in. Thus, horizontal air currents and wind patterns are created on the Earth's surface. The Earth's rotation, geographic features and temperature gradients affect the location and nature of the resulting winds [1]. Two major forces that determine the speed and direction of wind on a global basis are the differential heating between the equator and the poles and the rotation of the Earth. The primary force for global winds is developed because there is a difference in heating of the Earth at equatorial and polar regions. Polar regions have lower temperatures compared to the tropical regions. Due to this difference in temperature, the heat is transported by the winds from tropical regions to polar regions. About 30% of the total global heat transfer takes place due to atmospheric currents along with ocean currents. But due to the spinning of the Earth about its own axis, a force is produced which is responsible for deviation of air currents towards the west due to Coriolis force. The westerly motion causes the north-east winds in the northern hemisphere and south-east winds in the southern hemisphere.
Apart from these global currents, local winds are created due to localized uneven heating of small area of the Earth. Land masses are heated by the Sun more quickly than sea during daytime. The hot air from the land rises and flows towards the sea, creating a low pressure at the ground level. The cold air rushes from the sea towards the land. This is called sea breeze. That is why wind is more pronounced in coastal areas during daytime. At night, the direction of wind is reversed, but as the difference in temperatures of the Earth and sea is not very large, this wind speed is low.
The local winds are also produced in mountain regions due to difference in heating of mountain slope and neighbouring air of the ground. As the density of air due to heating of the slope decreases, the air moves towards the top following the slope. At night, the air flows in the reverse direction as the mountains cool down faster than the low land. As a result, the production of electricity using wind is highly sensitive to local wind conditions and the ability of wind turbines to reliably extract energy from the wind.
7.1.2 Wind Power Potential
Approximately 1% of the total solar energy absorbed by the Earth is converted to kinetic energy in the atmosphere, in the form of wind which is ultimately dissipated by friction at the Earth's surface [2]. If this energy is assumed to be dissipated uniformly over the entire surface area of the Earth, it means that there is an average power source of 3.4 × 1014 W for the total land area of the Earth. This power is equivalent to an annual supply of energy of 10,800 EJ (1 EJ is 1018 J), which is 22 times the present global annual consumption of commercial energy. But the wind energy is not distributed uniformly over the Earth. Regional patterns of dissipation depend not only on the wind source available in the free troposphere but also on the frictional properties of the underlying surface. Estimates of global wind energy potential range from a low of 70 EJ/year (19,400 TWh/year) (onshore only) to a high of 450 EJ/year (125,000 TWh/year) (onshore and near-shore).
Wind power is used to generate mechanical power by rotating turbines and the generators to produce electricity. This turbine power can also be used for doing some mechanical work (such as grinding grain or pumping water). Wind energy is a clean energy which can reduce greenhouse gas (GHG) emissions. A number of different wind energy technologies are available for a wide range of applications. However, the primary use of wind energy mitigating climate change is to generate electricity from larger, grid-connected wind turbines. These turbines may be deployed either “onshore” or “offshore”. At present, a number of wind energy installations mostly onshore are working in various countries. The wind power capacity installed by the end of 2009 was roughly 1.8% of the total electricity demand, and if the present trend of deployment continues, it is likely to grow to more than 20% by 2050. At the end of 2013, the wind farms installed in more than 85 countries had a combined generating capacity of 318,000 MW. New data from the Global Wind Energy Council shows that new wind generating capacity of about 35,000 MW was added worldwide in 2013 (Fig. 7.1).
Figure 7.1 Cumulative installed wind power capacity 1980–2013.
Power generation from onshore wind energy is already being integrated into electricity supply system without any problem. Although average wind speeds vary considerably by location, it is possible to extract significant amount of wind energy in most regions of the world. In some areas with good wind resources, the cost of wind energy is already competitive with the current energy market prices, even without considering relative environmental impacts. Nonetheless, in most regions of the world, continued advances in onshore and offshore wind energy technologies are required for further reducing the cost of wind energy and improving GHG emission reduction potential.
7.2 Power and Energy in Wind
The wind power depends on the mass of airflow rate (density) m, velocity of air v, passing through an area of interest A [3, 4].
The kinetic energy by definition is given by
here m is given by
where ρ = density of air.
Power in wind is kinetic energy per unit time,
From the aforementioned expression, it is clear that the power available in wind is proportional to the cube of wind speed. The wind turbines are used to convert this kinetic energy of moving air into mechanical energy. The energy that a wind turbine will produce depends on both its wind speed–power curve and the wind speed frequency distribution at the site. Histogram showing the number of hours for which the wind blows at different wind speeds during a given period of time is used to determine the wind speed frequency distribution. From the histogram, operating range of the wind speed of the turbine (the speed between shutdown speed and cut-in speed) is obtained. The energy produced by the turbine at a particular speed is obtained by multiplying the number of hours of its duration by the turbine power at that speed. This data is used to plot wind energy distribution curve as shown in Fig. 7.2. The total energy produced in a given period of time can be calculated by adding the energy produced at all the wind speeds within the operating range of the turbine. In order to obtain the wind speed–power curve, maps giving estimates of the mean wind speeds are available for various locations for most of the countries.
Figure 7.2 Wind energy distribution curve.
7.3 Aerodynamics of Wind Turbines
A wind turbine extracts kinetic energy from the wind and converts it into mechanical energy. The turbine is then used to drive a generator for the production of electricity [5, 6]. Since the wind turbine power production depends on the interaction between the rotor and the wind, the major aspects of wind turbine performance such as power output and loads are determined by the aerodynamic forces generated by the wind. It is therefore desirable to use a simple model to know the power developed by an ideal turbine rotor and the effect of the rotor operation on the local wind field. The simplest model of a wind turbine, considered by Betz, is the so-called actuator disc model where the turbine is theoretically replaced by a circular disc through which the airstream flows with a velocity V and across which there is a pressure drop from P1 to P2. An actuator disc concept, representing a turbine rotor with an infinite number of blades, is used to derive the one-dimensional momentum equation. It was originally intended to provide an analytical means for evaluating ship propellers.
7.3.1 Momentum
Momentum theory assumes the following:
- The airflow is homogeneous and incompressible.
- There is no frictional drag.
- It is a steady-state time-invariant fluid flow.
- The thrust over the disk is uniform.
- The static pressure far upstream and far downstream of the disk (rotor) is equal to the undisturbed ambient static pressure.
The analysis assumes a control volume as shown in Fig. 7.3, in which the control volume boundaries are surface of the stream tube and two cross sections of the stream tube. According to the basic momentum theory, when fluid flows through an obstacle such as an actuator disc, it produces an axial thrust which is a consequence of the pressure drop in the fluid at sections adjacent immediately before and after the disc, in the flow stream. Similarly, since the energy is given up by the fluid as it moves from high pressure to low pressure, its speed also decreases.
Figure 7.3 Actuator disk model of wind turbine.
Applying the conservation of linear momentum to control volume enclosing the whole system, the air mass flow rate must remain same throughout the stream tube. Therefore,
The thrust T is equal and opposite to change in momentum of air stream.
Thus,
or
The thrust can also be calculated from pressure conditions as
Since the thrust is positive, V4 is less than V1. No work is done on either side of the rotor. The Bernoulli's equation therefore can be used in the two control volumes on either side of the disk. In upstream flow of the wind turbine, the Bernoulli's equation is
and in the downstream of wind turbine, it is
where it is assumed that 
and 
.
Using the value of 
from Eqs (7.8) and (7.9) in Eq. (7.4), the following expression is obtained for thrust.
From Eqs (7.4) and (7.10), one gets
This means that the velocity of the wind at the rotor plane is the average speed of upstream and downstream wind.
Now if the axial interference (induction) factor a is defined as the fractional decrease in the wind velocity between the free stream and rotor plane, then a is given by
or
and
From Eq. (7.14), it can be concluded that if the rotor absorbs all the wind energy, that is, 
, and 
. The limit 
is wind turbine state of propeller operation. The power output P is equal to thrust times the velocity at the disk; the power P is
or
here the control volume area is replaced by A, the rotor area, and free stream velocity V1 is replaced by V.
The maximum power will be obtained when
or
or
since a cannot be greater than 1/2, or 
. Substituting the value of 
in Eq. (7.18)
The factor 
is known as Betz's coefficient or Betz Limit Cp.
Power coefficient, Cp, is defined as the ratio of power extracted by the turbine to the total contained in the wind resource
Turbine power output
Or 59% efficiency is the BEST a conventional wind turbine can do in extracting power from the wind.
This result 
indicates that if an ideal rotor is designed and operated in such manner that the wind speed at the rotor is 2/3 of the free stream wind speed, then the turbine will be operating at maximum power generation point.
Similarly, the maximum axial thrust is given by
The variation of power coefficient Cp with interference factor a is shown graphically in Fig. 7.4. As shown in Fig. 7.4, when there is no load on the turbine, the blades just freewheel. As there is no reduction in speed, the value of 
, also 
because the turbine is not generating any power. The maximum value of power occurs at 
when 
and when 
is zero.
Figure 7.4 Variation of 
with a.
7.4 Types of Wind Turbines
Wind turbines can be classified into two general types: horizontal axis and vertical axis. A horizontal-axis turbine has its blades rotating on an axis parallel to the ground as shown in the Fig. 7.5. A vertical-axis machine has its blades rotating on an axis perpendicular to the ground. There are a number of available designs for both types of turbine, and each type has certain advantages and disadvantages. However, the horizontal-axis type machines are the most common, and very few vertical-axis machines are running commercially.
Figure 7.5 Horizontal-axis wind turbine.
7.4.1 Horizontal-Axis Wind Turbines
HAWTs have the main rotor shaft and electrical generator at the top of a tower. HAWT should be pointed towards the wind to capture the maximum power. It has blades that resemble a propeller that spin on the horizontal axis. The dominant driving force in this turbine is the lift. Depending on the different relative position of the rotor and tower, the HAWT can be divided into upwind wind turbine and downwind wind turbine. If the rotor is in front of the tower, it is known as the upwind turbine, while if the rotor is installed behind the tower, it is called the downwind wind turbine. Upwind wind turbine requires steering installation (yaw mechanism) to ensure that the rotor faces the wind during working which is a disadvantage.
Since electrical generators require speed higher than the speed of turbine, most large wind turbines have a gearbox, which turns the slow rotation of the rotor into a faster rotation. The basic advantage of upwind designs is that one avoids the wind shade behind the tower. Although there is some wind shade in front of the tower, that is, the wind starts bending away from the tower before it reaches the tower itself. By far, the vast majority of wind turbines have this design. The basic drawback of upwind designs is that the rotor has to be rather inflexible and placed at some distance from the tower.
The downwind wind turbine does not require steering mechanism as it can face the wind automatically. However, in the downwind wind turbine, because a part of wind blows to the blades across the tower, the tower disturbs the wind flow which should go across the blades. In this way, it causes the tower shadow effect, causing a drop in power each time a blade passed behind the mast, decreasing the efficiency. An important advantage of this type of turbine is that rotor can be made flexible. Thus, at higher speeds, the blades can bend, thus reducing the load on tower. Downwind turbines are generally noisier (additional aerodynamic noise), and the blades are subject to more forces than those of upwind turbines.
The HAWT can also be divided into the lift-type wind turbine and the resistance-type or drag-type wind turbine. For the drag design, the wind literally pushes the blades out of the way. Drag-powered wind turbines are characterized by slower rotational speeds and high torque capabilities. They are useful for pumping, sawing or grinding work. The lift type has a high rotating speed. Most of the HAWTs are downwind type that have the steering device and can rotate with the wind. For the small-sized wind turbine, the steering device employs the tail vane, while for the large-sized wind turbine, it often adopts the gear consisting of wind sensors and servomotor. Most commonly used HAWTs are upwind, lift-type, three-blade machines as shown in Fig. 7.5. The advantages and disadvantages of HAWT are as follows:
- Advantages
- High efficiency since the blades are always perpendicular to wind.
- Higher stability because blades are to the side of the turbines' centre of gravity.
- Ability to wing warp, which gives the turbine blades the best angle of attack.
- Variable blade pitch minimizes damage due to high wind.
- Tall tower allows access to stronger wind.
- Tall tower allows placement on uneven land or in offshore locations.
- Good starting performance.
- Disadvantages
- Difficult to transport because of taller tower and blades (20% of equipment costs)
- Difficult to install (require tall cranes and skilled operators)
- Effect radar in proximity
- Difficulty in maintenance
7.4.1.1 Horizontal-Axis Wind Turbines with Wake Rotation
In Section 7.3, it is assumed that there is no rotation of the flow. However, in any wind turbine, the rotor is not always stationary but rotates when the wind flows through it. When the rotor rotates, it generates angular momentum which affects the rotor torque. The wind imparts a torque on the wind turbine, and a thrust is created. The airflow behind the rotor is in the opposite direction to the rotor because the blades exert a torque on the wind. Thus, the flow in the wake has two components, axial and tangential. This tangential flow is referred to as wake rotation. Due to rotation, there is loss of kinetic energy of the wind rotor. The loss is high for low-speed rotors and low for high-speed rotors.
For the analysis of momentum with the wake rotation, an annular stream tube model as shown in Fig. 7.6 is used. The blade wake rotates with an angular velocity ω, and the blades rotate with an angular velocity of Ω. In this analysis, it is assumed that the angular velocity imparted to flow stream ω is small compared to the angular velocity 
of the wind turbine rotor. It is also assumed that the pressure in the far wake is equal to the pressure in free stream.
Figure 7.6 Steam tube model for rotating wake.
If the ring radius of the annular stream tube is 
and thickness is dr, then the cross-sectional area of the ring is 
.
Applying the energy equation before and after the blade, we get
or
The angular velocity of the air relative to the blade increases from Ω to 
, whereas the axial velocity remains constant. Therefore, the thrust on the annular element of the rotor is
If 
, and 
, the thrust becomes
From the conservation of momentum, the torque exerted must be equal to the angular momentum 
of the wake.
The power output from the rotor is
Tip Speed Ratio
The Tip speed ratio (TSR) is used by wind turbine designers to properly match and optimize a blade set to a particular generator. TSR is defined as the speed of the blade at its tip divided by the speed of the wind. TSR is important in designing a wind energy conversion system (WECS). For a particular generator, if the blade set spins too slowly, then most of the wind will pass by the rotor without being captured by the blades. If the blades spin too fast, then the blades will always be cutting the turbulent wind.
This is because the blades will always be travelling through a location that the blade in front of it just travelled through (and used up all the wind in that location). In short, if the blades are too slow, they are not capturing all the wind they could, and if they are too fast, then the blades are spinning through used/turbulent wind. For this reason, TSRs are employed when designing wind turbines so that the maximum amount of energy can be extracted from the wind using a particular generator.
Calculation of TSR
Find the speed of the blade at its tip.
If r is the length of blade, the distance travelled by the tip of the blade in one revolution = 2πr m. In addition, if the tip of the blade is rotating at a speed of n rpm, the distance travelled by the tip in 1 min is 2πnr m.
And TSR λ = 2πnr/60v, where v is the wind speed in m/s.
Optimal Tip Speed Ratio
The optimal tip speed ratio for maximum power extraction is obtained by relating the time taken for the disturbed wind to re-establish itself 
to the time taken for a rotor blade of rotational frequency ω to move into the position occupied by its predecessor 
. For a turbine having n blades rotating at angular speed ω
If the length of the strongly disturbed air stream upwind and downwind of the rotor is s, then the time period for the wind to return to normal is given by
If 
> 
, then some wind is unaffected. If 
, then some wind is not allowed to flow through the rotor. The maximum power extraction occurs when 
The tip speed ratio for maximum power extraction is
From practical observation 
7.4.2 Vertical-Axis Wind Turbines
Currently, HAWTs dominate the wind energy market due to their large size and high power generation characteristics. However, vertical-axis wind turbines (VAWTs) are capable of producing a lot of power and have many advantages. The main advantage of it is that it can receive wind from any direction; therefore, there is no need of a steering device to change the direction of the rotor to face the wind. This is a big advantage on sites where the wind direction is highly variable or has turbulence. Since there is no need of the steering device, the structure of the vertical wind turbine is simple. It has another advantage that the gearbox and the generator can be directly coupled to the axis on the ground. The wind turbine itself is also near the ground, unlike the horizontal-axis turbine where everything is to be placed on a tower. Similarly to HAWTs, the VAWTs are also of two types: lift-based and drag-based. Lift-based designs are generally much more efficient than drag or “paddle” designs.
The first aerodynamic VAWT was developed by Georges Darrieus in France and was first patented in 1927. It has a rotor with two or three thin curved blades with an airfoil section. Its principle of operation depends on the fact that its blade speed is a multiple of the wind speed.
The most common type of lift-type turbine is the Darrieus-type wind turbine. It has many different models, such as the Φ structure, Δ structure, Y structure and H structure. But as its wind area is small and the starting wind speed is high, it is not available on large scale. The maximum torque in this turbine occurs when blades are moving across the wind at a speed greater than wind speed. The popular type of turbine is the H structure type shown in Fig. 7.7. During working, the rotor drives the generator to send power to the controller and output the required power to the electrical equipment.
Figure 7.7 Vertical-axis wind turbine.
The drag-based type uses aerodynamic resistance of the wind, and the most typical structure is S-type rotor, which consists of two semi-cylindrical blades whose axes are staggered. A drag-type wind turbine cannot rotate at speed higher than the wind speed. The main advantage of this type is the high starting torque, but the shortcoming is the asymmetrical gas flow around the rotor, which forms lateral thrust. As for the relatively large-sized wind turbine, it is difficult to employ this structure. The ratio of utilization of the wind is lower than that of the high-speed VAWT or HAWT.
Due to the reason that the gas flow of the vertical axis is more complex than that of the horizontal axis, VAWTs were developed at a later stage, and the theory is not yet mature. But the structure is simple. It can start smoothly with low wind speed and low noise. The disadvantages of the VAWT, on the other hand, are as follows:
- Most of them are only half as efficient as HAWTs due to the dragging force.
- Airflow near the ground and other objects can create a turbulent flow, introducing issues of vibration.
- VAWTs may need guy wires to hold them up (guy wires are impractical and heavy and cannot be used in farmed areas).
7.4.3 Main Components of Wind Turbine
There are four main components in a wind turbine. These are turbine blades, nacelle, tower and control system.
Turbine blades: Turbine blades of HAWTs are made of high density wood, PVC, aluminium alloy or glass fibre. These blades have airfoil type of cross section. The blades are also twisted from the outer tip to the rotor to reduce the chances of stalling. Larger rotors with longer blades sweep a greater area, increasing energy capture. But longer blades are heavier and incur greater structural load. Special designs with advanced materials such as carbon fibres can be used to reduce the weight and load. However, the advanced materials are expensive and are used only for large power turbines.
Unlike the HAWTs where the blades exert a constant torque about the shaft as they rotate, a VAWT rotates perpendicular to the flow, causing the blades to produce an oscillation in the torque about the axis of rotation. VAWT blades are designed such that they exhibit good aerodynamic performance throughout an entire rotation at the various angles of attack they experience, leading to a high time-averaged torque. The blades are therefore curved and thin which resemble an egg beater. The pitch of the blades cannot be changed. The diameter of the rotor is less than the tower height.
Nacelle: Nacelle is the name given to a streamlined enclosure of an aircraft. In wind energy systems, the nacelle holds all the turbine machinery. Since it is required to rotate so that it always follows the wind direction, it is connected to the tower via bearings. Turbine is the most important part for a wind turbine system. It consists of more than 10 components including the generator, rotor, gear, shaft, bearing and others. The most important part in the turbine is its rotor. The trend is to build large rotors so that a large amount of wind is swept for the same or lower loads. Wind turbines are available in a variety of sizes and power ratings. Depending on the number of blades, wind speed and type of turbine, HAWT or VAWT, rotors have been developed in various shapes and sizes.
Towers: In HAWT, the towers support the nacelle and rotor hub at its top. These are made from tubular steel, concrete or steel lattice. Large wind machine towers are usually made of steel, and most of these are of the tubular or conical type. Some towers have been built out of reinforced concrete sections. Lattice or truss towers, common in the early days, are rarely used except for very small machines in the range of 100 kW and below. Guyed pole towers are used for small wind machines. Towers must be designed to resist the full thrust produced by an operating windmill or a stationary wind machine in a storm.
Height of the tower is an important parameter in the design of HWAT. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity. Generally, output power of the wind system increases with increase in height and also reduces the turbulence in wind.
The tower of VAWT has a hollow vertical rotor shaft which can rotate freely supported by bearings on top and bottom. Since all mechanical components are located at the ground, there is no load on the top of the tower. Thus, in VAWT, a very strong tower is not required.
Control system: Control of wind turbine is based on many factors [7]. These are as follows:
- To extract as much energy from the wind as possible
- Speed regulation so that noise is within limits
- Safety of turbine
The control system includes the turbine controller and inverter control of the generator. A turbine can be 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. Inverter control is used to control the active power supplied by the generator. These are described later.
7.4.3.1 Drive Train
The mechanical power received from the rotor is transmitted to the electrical generator by means of mechanical transmission which contains all the moving parts known as drive train. In HAWT, this equipment is on top of the tower. The main criterion for the design of this equipment is therefore low maintenance. A typical HAWT drive train consists of a rotor shaft assembly (low speed shaft), a speed-increasing gearbox (not required in a direct drive system), a generator drive shaft, a rotor brake, an electrical generator and control equipment. The rotor shaft connects the rotor to the gearbox. The braking used in wind turbines is either aerodynamic braking or mechanical braking.
The coupling between the low-speed shaft and the generator shaft can be either rigid or flexible. At present, three most dominant drive train technologies used are as follows:
- High-speed drive train having overall speed ratio of 100:1
- Medium speed drive train having a speed ratio of 10:1 to 40:1
- Direct drive
The VAWT has its drive train on the ground. The blades of VWAT also do not require orientation towards the wind.
7.5 Dynamics and Control of Wind Turbines
Normally, large wind turbines are placed in group in wind farms. The monitoring and control of wind turbines separately as well as overall control of the wind farm are necessary for proper operation [8]. Control of HAWT is dependent on the type of machine: whether it is upwind or down wind or a fixed pitch or variable pitch type. Moreover, a wind turbine can be of fixed speed or variable speed. A large wind turbine connected to the grid has generally four levels of controls:
- Wind farm control
- Supervisory control
- Operational control
- Subsystem control
Wind farm control is used for coordinated control of numerous wind turbines in a farm. It communicates with supervisory control of each turbine.
Supervisory control is at the top of hierarchy which decides when to start and stop the turbine depending on the wind speed and also monitors the condition of the turbine. The operational control is used in variable-speed turbines. Its main purpose is to run the machine at speed between 6 m/s and 11.7 m/s which is the region of maximum power coefficient. The purpose of this control is to operate the turbine at a constant tip speed ratio, corresponding to the maximum power coefficient and use pitch control to operate the turbine at maximum power output point. In this region, the turbine rotor runs at variable speed. For speeds above 11.7 m/s and less than about 25 m/s, the turbine is operated by using pitch control to keep it running at a constant speed.
Operational control determines how the turbine achieves its control objectives. The subsystem controller is used to control the generator, associated power electronics, yaw drive and pitch drive to achieve desirable performance.
7.5.1 Pitch Control
In case of stronger winds, it is necessary to waste part of the excess energy of the wind in order to avoid damage of the wind turbine. All wind turbines are therefore designed with some sort of power control. There are different ways of doing this safely on modern wind turbines. Pitch control, yaw and tilt control and stall control are the methods used for this purpose.
The pitch control system is one of the most widely used control techniques to regulate the output power of a wind turbine generator. The pitch of a turbine blade is controlled by rotating it from the root where it is connected to the hub. As the pitch angle is changed, the power captured by the turbine is also changed. Hydraulic actuators are used to control the pitch angle.
Pitch control can be active or passive. In active-pitch-controlled wind turbine, the turbine's electronic controller checks the power output of the turbine several times per second. When the power output becomes too high, it rotates the blade of the turbine through pitch mechanism which immediately tilts the blades slightly out of the wind. The blades are turned back into the wind whenever the wind speed becomes safe again. The pitch-controlled wind turbine will generally pitch the blades a few degrees every time the wind changes in order to keep the rotor blades at the optimum angle in order to maximize the output for all wind speeds.
In passive pitch control, the blades and their hub mountings are designed to twist to limit the load due to higher wind speeds. This scheme is not easy to implement and is generally used for stand-alone wind turbines.
7.5.2 Yaw Control
In yaw control, the entire wind turbine is rotated in the horizontal axis. Yaw control ensures that the turbine is constantly facing the wind to maximize the effective rotor area and, as a result, power. Because wind direction can vary quickly, the turbine may misalign with the oncoming wind and cause power output losses. Yaw control is used in small horizontal-axis turbines. It turns the nacelle so that the rotor always faces the wind. For small wind turbines, the same yaw control is used to control the power. The purpose of yaw control here is to yaw the turbine out of wind to limit the power during high wind.
Yaw control mechanism uses electric motors and gearboxes to orient the rotor.
7.5.3 Passive and Active Stall Power Control
The basic advantage of using stall control is that it avoids the moving parts in the rotor and a complex control system. However, stall control represents a very complex aerodynamic design problem and related design challenges in the structural dynamics of the whole wind turbine, for example, to avoid stall-induced vibrations. Around two-thirds of the wind turbines currently being installed in the world are stall-controlled machines.
The stall control of wind turbine can be active or passive.
7.5.3.1 Passive Stall Control
The blades of passive stall-controlled wind turbines are bolted onto the hub at a fixed angle. The geometry of the rotor blade profile, however, is designed aerodynamically to ensure that, when the wind speed becomes too high, it creates turbulence on the side of the rotor blade which is not facing the wind. As the actual wind speed in the area increases, the angle of attack of the rotor blade will increase, until at some point it starts to stall. Rotor blades for a stall-controlled wind turbine are twisted slightly in 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.
7.5.3.2 Active Stall Control
An increasing number of larger wind turbines (with 1 MW and more capacity) are equipped with active stall power control mechanisms. The active stall machines are pitch-controlled machines, since they have pitch-controlling blades. At low wind speeds, the machines will usually be programmed to pitch their blades much as a pitch-controlled machine. However, when the machine reaches its rated power, and the generator is about to be overloaded, the machine will pitch its blades in the opposite direction from what a pitch-controlled machine is supposed to do. Thus, it will increase the angle of attack of the rotor blades in order to make the blades go into a deeper stall, thus wasting the excess energy in the wind.
One of the advantages of active stall is that one can control the power output more accurately than with passive stall, so as to avoid overshooting the rated power of the machine at the beginning of a gust of wind. Another advantage is that the machine can be run almost exactly at rated power at all high wind speeds. A normal passive stall-controlled wind turbine will usually have a drop in the electrical power output for higher wind speeds, as the rotor blades go into deeper stall.
7.6 Wind Turbine Condition Monitoring
There is a constant need for the reduction of operational and maintenance costs of WECSs through the adoption of reliable and cost-effective condition monitoring techniques that allow for the early detection of any degeneration in the system components. Wind turbines are different from traditional rotating machines as they usually operate in remote locations, rotate at low and variable speed and work under constantly varying loads. For these reasons, a reliable condition monitoring system (CMS) for wind turbines is essential to avoid catastrophic failures and to minimize the requirement of costly corrective maintenance.
Among different techniques available for condition monitoring, vibration analysis and oil monitoring are the most predominantly used for wind turbines. Vibration analysis can pinpoint the crack locations, and oil monitoring can detect lubricant deterioration. Gearbox fault in the drive train is considered to be most common fault occurring in wind turbines.
CMS is generally composed of electronic data acquisition module, and further for vibration-based condition monitoring, there is a number of accelerometers and other transducer installed on the turbine drive train, main shaft, generator and main bearing. The standard CMS can be divided into three main components; this includes data gathering or data collection followed by the data acquisition and finally fault diagnostic algorithm or fault diagnosis. In the first phase, data is collected and then converted into digital format for further processing. In the next stage, the data acquisition receives data from the data collection module for calculating the statistical function and values in frequency and time domains. This block further applies different techniques for analyzing of data pattern for further processing. Fast Fourier transformation is generally used to convert a time-domain signal into frequency-domain one. Vibration-based condition monitoring is one of the most effective techniques that can detect gearbox failure of wind turbine especially on the high speed side rather than on the low speed side. This is attributed to the inherent sensitivity of the accelerometers to high-frequency vibration.
The third block which is more critical and complex, because of working with diagnosing fault, due to its functionality and importance in machine fault diagnosis, different methods and techniques are developed, and still a major work is under development. Newly developed analysis techniques such as wavelet analysis, fuzzy logics, case-based reasoning and neural networks for vibration or fault pattern recognition are being successfully applied and examined.
Vibrations may not be evident while faults are developing, but analysis of the oil can provide early warnings. Oil analysis is mainly applied to the gearbox, as it is the only oil-lubricated component in the drive train. The objective of oil monitoring is to detect oil contamination and degradation. Oil analysis is mostly executed off-line by taking samples, although on-line sensors are available and used rarely. The typical parameters sought in an oil sample analysis include particle counts, water content, total acid number, viscosity and particle element identification.
It is possible to detect wind turbine drive train faults through the terminals of the associated generator [9]. Since wind power generation systems mostly use induction generators, the techniques and tools available, to monitor the condition of induction machines, can be used. Some of the technologies used for monitoring include sensors, which may measure the speed, output torque, vibrations, temperature, flux densities and so on. These sensors are together coupled with algorithms and architectures, which allow for efficient monitoring of the machine condition.
7.7 Wind Energy Conversion Systems (WECS)
WECSs convert wind energy into usable form of electrical energy using power generation systems. There are a number of ways by which WECSs can be classified [10].
7.7.1 Based on Capacity of Power Generation
These are classified as small (less than 2 kW), medium (2–100 kW) and large (more than 100 kW). Second classification is based on the speed of the turbine which may be fixed speed or variable speed. Third method is based on whether it is grid-connected which may be in the form of microgrid or isolated system feeding only local loads. Fourth method of classification is based on the type of electric generator along with its control used. Several types of generators are available for power generation using wind turbines. The overall wind power conversion technologies can be divided into three categories.
- Systems without power electronics
- Systems with partially rated power electronics (converter rating less than the total power rating of the system)
- Systems with full-scale power electronics
7.7.2 Systems without Power Electronics
Systems without power electronics are fixed-speed power generation systems using squirrel-cage induction generator (SCIG). The generator shaft is driven by the wind turbine, and its stator is connected to the grid in a grid-connected system or to the load in an isolated system. In the grid-connected system, the speed of the generator and therefore that of turbine are fixed by the grid frequency. The excitation is provided by the power supply from the grid. A capacitor bank may be connected across the stator of the induction generator to limit the reactive power absorption from the grid. In stand-alone systems, the capacitors are connected to provide excitation.
Initially, the induction machine runs as an induction motor till its speed is equal to the synchronous speed. At synchronous speed, the machine neither delivers power nor takes power from the grid except the losses. When the speed of the machine driven by wind turbine exceeds the synchronous speed, it works as a generator and starts supplying power to the grid or load as the case may be. The maximum power supplied by the generator occurs at a slip of 1–2%. This is why it is known as fixed-speed WECS. SCIGs are preferred because they are simple, require low maintenance, are robust and stable. Their main drawback is that in order to get more active power production, more reactive power is required. SCIGs used in fixed-speed turbines can cause local voltage collapse due to fault. During a fault, grid voltage is reduced which results in less power delivery. The machine therefore accelerates due to the imbalance between the mechanical power from the wind and the electrical power that can be supplied to the grid. When the fault is cleared, these machines absorb reactive power, further decreasing the network voltage. If the voltage does not recover quickly, the wind turbines continue to accelerate and to consume larger amounts of reactive power. This eventually leads to voltage and rotor speed instability.
Systems with partially rated power electronics and systems with full-scale power electronics are variable-speed power generation systems. These systems use pitch control in the turbine to extract wind power. In partially rated power electronic system, the performance is improved by using slip ring induction machine with power electronic control. Figure 7.8 shows a wound rotor induction generator with power electronic control of rotor resistance. This dynamic slip controller can provide a speed control in the range of 2–10%. The power converter requirement for control of rotor resistance is high current at a low voltage. It is also possible using this control to keep the output power fixed at higher speeds.
Figure 7.8 Wound rotor induction generator with rotor resistance control.
Another solution with partial power electronic control is by using doubly fed induction generator (DFIG). Here power electronic converter is connected in the rotor circuit to extract the slip energy from rotor and supply it to the grid. As shown in Fig. 7.8, the stator is directly connected to the grid and the rotor windings are connected to the grid through PE converter. When the machine is running at super-synchronous speed, the power is delivered by both the rotor and the stator to the grid. If the machine is running at sub-synchronous speed, still the power can be supplied to the grid through rotor. A speed variation of ±30% around the synchronous speed can be obtained by the converter having only 30% of total capacity of the machine.
The converter in the rotor circuit can provide control of both active and reactive power which provides better grid performance. This method of control is slightly more expensive than rotor resistance control, but it can save the money spent on reactive power controller, and it can extract more wind energy compared to the rotor resistance control scheme. Since the power rating of the converter is only about 30% of total power rating, the overall cost is less compared to systems with full-scale power converter.
Full-scale converters are rated equal to the total capacity of the system and therefore are expensive and produce more losses but have advantage in technical performance. These systems employ a DFIG, conventional synchronous generator or permanent magnet synchronous machine. A DFIG wind energy system is shown in Fig. 7.9. At very low wind speeds, the rotational speed of the system will be fixed at maximum allowable slip to prevent over-voltage of generator. For speeds where power production is below the maximum power, the wind turbine will vary the rotational speed proportional to the wind speed and keep the pitch angle fixed. When the turbine power is above the nominal power, a pitch angle controller is used to limit the power by suitably rotating the blades. The total electrical power of the WECS is regulated by controlling the DFIG through rotor-side converter. The grid-side converter is used simply to keep the dc-link voltage fixed.
Figure 7.9 Wind energy conversion system with DFIG.
The types of synchronous generators used in the wind turbine industry are as follows:
- 1. Wound rotor synchronous generator (WRSG)
- 2. Permanent magnet synchronous generator (PMSG).
The synchronous generator with a large number of poles can be used for direct-drive applications without any gearbox. PMSGs do not require external excitation current, require no slip rings, require less maintenance and have lower losses.
A multi-pole PMSG with wind turbine control is shown in Fig. 7.10. An advantage of this WECS is that the dc link provides decoupling between the turbine and the grid. The dc link can also be used to connect it to energy storage systems (if present) for better power control to the grid. As shown in Fig. 7.10, the generated active power is controlled by the generator-side converter, and the reactive power is controlled by the grid-side converter. A dc chopper is normally introduced to prevent overvoltage of dc link in case of grid faults.
Figure 7.10 WECS with PM generator.
7.8 Offshore Wind Energy
Offshore wind speeds are generally higher and more consistent than onshore winds. It is therefore possible to extract about 20–30% more energy using offshore wind installations. Offshore wind turbines are generally built with larger capacity because there are fewer constraints on the component and assembly equipment transportation.
The first offshore wind farm was built in Denmark in 1991. First large-scale offshore wind farm was also built by Denmark in 2002 with a capacity of 80.2 MW. Offshore wind technology consists of five key components and processes: the wind turbine, the foundation, the electrical connection, their installation and their operation and maintenance. The technologies for offshore wind farms are not developed especially for this purpose, but the known technologies already developed for other industries [11] are used.
7.8.1 Offshore Wind Turbines
Offshore wind turbines have, to date, largely use modified versions of the largest onshore wind designs that are more suitable for high-capacity factors [12]. Offshore turbines require technical modifications and substantial system upgrades for adaptation to the marine environment. These modifications include strengthening the tower to cope with loading forces from waves or ice flows, pressurizing the nacelles to keep corrosive seawater from critical electrical components and adding brightly coloured access platforms for navigation safety and maintenance access. Change in design is required to obtain several other benefits. Technology is already being developed for direct-drive, gearless nacelles, improved generators (e.g. fixed magnets, increased generator coils) and to increase condition monitoring.
Offshore turbines have fewer constraints than onshore turbines in terms of visual impact and noise (particularly in regard to planning), but there are greater costs associated with reliability and servicing. For this reason, offshore turbines may have automatic greasing systems to lubricate bearings and blades as well as heating and cooling systems to maintain gear oil temperature within a specified range.
7.8.2 Foundation
The offshore foundation system depends on the water depth. Most of the projects installed so far have been in water less than 22 m deep, with a demonstration project in Scotland at a depth of 45 m. Shallow water technology at present uses monopile foundations for about 20 m depth. These are tried and tested technologies used in marine construction. Basically, these are simple steel tubes, hammered into the seabed. The industry is also considering the use of concrete gravity-based structures, adaptations of monopiles (tripods and tripiles) and jacket structures (as used for oil and gas platforms). For depths beyond 60 m, floating foundations are being developed. However, floating foundations still need reliable solutions, including advanced control systems to deal with wind, ocean waves, tides, ice formation and water currents simultaneously.
7.8.3 Electrical Connection and Installation
Installation of foundations and turbines is currently achieved with the use of standard jack-up barges and some custom-built vessels. Typically, these have 4–6 legs that extend into the seabed and lift the vessel completely out of the water. Normal HVAC subsea cables are used if the distance is not large. Cable installation uses a “cable plough” that digs a shallow trench in the seabed and buries the cables. Jack-up vessels can currently install in depths up to 35 m. Much beyond this, special floating installation vessels with hydraulics and jet thrusts might be needed. In future, when the wind farms will be located far away from the shore, HVDC will be used.
7.8.4 Operation and Maintenance
In the area of operation and maintenance, the priority is to increase the reliability of wind turbines and therefore minimize unscheduled repairs. Remote condition monitoring may reduce the need for repairs.
7.9 Advantages of Offshore Wind Energy Systems
Offshore wind has three inherent cost advantages over onshore wind. These are as follows:
- 1. Larger capacity: Offshore wind farms could be as large as 1.5 GW or even more due to the availability of almost unlimited space allowing a higher number of turbines. In addition, turbines can be larger in size as transportation by sea is not a major problem.
- 2. Higher wind speed: Offshore wind speeds are generally higher and more consistent than onshore winds.
- 3. No requirement of land: These can be located in the coasts near load centres.
7.10 Environmental Impact of Wind Energy Systems
There are many advantages of using wind power for the generation of electricity in terms of emission of carbon dioxide or other pollutants, but there are certain negative aspects which also must be looked into [13].These are as follows:
- Noise
- Electromagnetic interference
- Effect on wildlife
- Effect on ecosystem
7.10.1 Impact of Noise
As with any moving machinery, wind turbines also generate noise during operation. There are two main sources of noise in wind turbines. Mechanical noise caused by gearbox, bearings, generator and pitch control mechanism. The other noise known as aerodynamic noise is created due to interaction of airflow with the blades of turbine. The sound thus produced can be described as a swishing sound. This noise tends to increase with the speed of rotation, that is, it is more at higher wind speeds. Although the noise at higher speeds in more dominant, usually it is not noticeable because of the background noise produced by the winds. The nuisance due to noise is therefore more at low wind speeds which may look strange.
7.10.2 Electromagnetic Interference
The presence of tall wind turbine towers near radio, television or microwave towers can sometime reflect some of EM radiation in such a way that the reflected wave may interfere with the original signal at the receiving end. This can cause the received signal to be distorted significantly. The electromagnetic scattering properties of wind turbines are not simple to describe. The extent of electromagnetic interference due to wind turbines depends mainly on the materials used to make the blades and on the shape of the tower.
7.11 Combining the Wind Power Generation System with Energy Storage
Most of the renewable energy systems including wind energy are intermittent in nature. Since it is difficult to predict and control the output of wind generation, its potential impacts on the electric grid are different from the traditional energy sources. At a high penetration level, an extra fast-response reserve capacity is needed to cover the shortfall of generation when a sudden deficit of wind takes place. Various storage systems such as pumped hydro and battery are possible to integrate the wind energy to the present network. The pumped hydro is difficult to be constructed because of the geographic restrictions and environmental regulation. The batteries are not suitable to stabilize the output of the large wind energy farm because of their cost. WTES, which employs low-cost thermal energy storage system and light and low-cost heat generator, could be a better solution than the combination of wind power and thermal plant [14, 15].
7.12 Summary
Wind energy systems are now considered one of the important renewable energy sources. They now contribute to the energy mix in more than 70 countries of the world, with Denmark and Germany taking the lead. Wind turbines are classified into two general types: horizontal axis and vertical axis. A horizontal-axis turbine has its blades rotating on an axis parallel to the ground, whereas a vertical-axis machine has its blades rotating on an axis perpendicular to the ground. However, the horizontal-axis type machines are the most common, and very few vertical-axis machines are running commercially.
The future trend of WECSs is the increase in the power capacity of wind turbines and generators so that the cost of generated electricity may be reduced.
Offshore WECSs are also being considered and installed in a few countries.
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