Wind Power By

Dr Adil Sarwar

Layout of Lecture

• Wind potential of the world

• Global wind installed capacity

• India: Wind installed Potential

• India: Wind installed Capacity

• Wind Energy basics

• Wind turbines classifications

Wind Potential 80 m=748 GW

Wind Energy basics

• Wind is air in motion.

• It is produced by the uneven heating of the earth's surface by the sun.

• Since the earth's surface is made of various land and water formations, it absorbs the sun's radiation unevenly.

• Hot air rises and cool air sinks. This brings about spatial differences in atmospheric pressure, caused by uneven heating.

Contd…

• Moderate to high speed winds, typically from 5 m/s to about 25 m/s are considered favourable for most wind turbine.

• The global potential in wind energy for large scale grid connected power generation has been estimated as 9000TWh/year.

Flow of Wind

Factors affecting flow of wind

• On the planetary level, great mountain masses influence the circulation of air current.

• Surface roughness or friction, owning to the resistance that different elements of the earth’s surface offer to air circulation affects the nature of wind.

• Climatic disturbances such as downdraught from thunderclouds and precipitation also affect the local winds.

• Winds speed also increases while passing through narrow mountain gaps, where it gets channelled.

Earlier Use

Wind Mill Vs Wind Turbine

Brief History

• The idea of harnessing wind is not new.

• Windmills were used in Babylon and China around 2000 to 1700 BC to pump water and grind grains.

• It was used traditionally been used worldwide for ship propulsion until start of industrialisation era.

• Europeans were the first to introduce the horizontal axis windmill around the 12th century, and by 1750, Holland had 8000 wind mills and England had 10000.

Present Scenario

• Several demonstration and commercial plants of different sizes, from few MW are in operation in different parts of the world.

• Improved turbine design and plant utilization have contributed to large scale reduction of wind energy generation cost from Rs 17.00 kWh in 1980 to about Rs 2.50 per kWh at present, at favourable location.

• The installation cost has come down to level comparable to that of a conventional thermal power plant i.e. to 4.00 crore Rs/MW.

• It is fastest growing energy source among all renewable resources in recent years.

How wind turbine works

Energy estimation of wind

• If uo is the speed of free wind in unperturbed state, the volume of air column passing through an area A per unit time is given by Auo .

• If ρ is the density of air column ,the air-mass flow rate, through area A, is given as ρAuo .

• Thus power(Po) available in wind, is equal to kinetic energy associated with the mass of moving air, i.e.,

Po=½ (ρAuo)uo² or Po=½(ρA)(uo)³ (1)

• Power available in wind per unit area is

Po/A = ½ρ(uo)³ , (2)

this indicates that power available in wind is proportional to the cube of wind speed.

• If a typical value of wind density ,ρ at 15°C at sea level is 1.2kg/m³, then power available in moderate wind of 10m/s is 600W/m³.

Power Extraction from wind

• A wind turbine is used to harness useful mechanical power from wind.

• For a simple analysis an unperturbed smooth laminar flow is assumed. A horizontal wind turbine which is most commonly used , is considered.

• The rotor may be considered as an actuator disk across which there is decrease in pressure as energy is extracted.

• As air mass flow rate must be same everywhere within the stream tube , the speed must decrease as air expands.

• The stream tube model, also known as Betz model of expanding air stream tube is shown as

uo Ao uo

(a)Unperturbed wind stream tube in absence of turbine

uo u1 u2

Upstream Downstream zo z2

(b)Wind stream tube in presence of turbine

Fig. Betz model

• As per the Betz model, the stream tube area of constant air mass is Ao upstream, which expands to A1 while passing through the rotor and becomes A2 downstream.

• The wind speed is uo upstream, which reduces to u1 while passing through the rotor and becomes u2 downstream.

• The air-mass flow rate remains constant throughout. Therefore,

m̊= ρAouo = ρA1u1 =ρA2u2 .......(3)

The force/thrust on rotor is equal to the reduction in momentum per unit time from the air mass flow rate m̊:

F= m̊uo-m̊u2 …….(4)

This force is applied by the air at uniform air flow speed of u1, passing through the turbine. The power extracted is

PT= Fu1 = m̊ (uo-u2)u1 ……….(5)

• The wind power extracted from is also equal to loss in KE per unit time. Thus,

PW= ½m̊(uo²-u2²) ……(6)

Equating eqn. 5 & 6, we have

u1= (uo+u2)/2 ……(7)

An interference factor, a is defined as fractional wind speed decrease at the turbine thus:

a= (uo-u1)/uo ………(8)

or u1=(1-a)/uo

or a=(uo-u2)/(2uo) ……...(9)

a is also known as the Induction factor.

• Using eqns. 3, 5, 7 & 8, power extracted by the turbine may be written as:

PT=4a(1-a²)(½ρA1uo³) (10)

comparing with eqn.2 :

PT=CpPo (11)

where Cp is the fraction of available power in the wind that can be extracted and is known as power coefficient. Cp is given as:

Cp =4a(1-a²) (12)

• The variation of power coefficient with the interference factor a is shown graphically in the figure below

0.7

0.6

0.5

0.4

0.3

0.1

0 0.2 0.4 0.6 0.8 1.0

a

Fig. CP versus a

• When no load is coupled to the turbine, the blades just freewheel. There is no reduction of wind speed at the turbine, therefore, u1=uo and the value of a is zero. The turbine does not generate any power.

• Now as load is applied, power is extracted, so Cp increase as u1 decreases. Maximum value of Cp (i.e.,Cpmax=16/27=.593) occurs at a=1/3.At this condition, u1=2uo/3 and u2=uo/3.

That means at maximum power extraction condition, the upstream wind speed is reduced to two-third at the turbine at the turbine and further reduced to one third downstream.

• The criterion for maximum power extraction, i.e., Cp=16/27 is called the Betz criterion. This applies to an ideal case. For a commercial wind turbine, however, maximum power coefficient is less than the ideal value.

• When u2=0, a=0.5 and the simple model breaks down as no wind is predicted to be leaving downstream. In practice, this is equivalent to the onset of a turbulence downstream.

• Power extraction decrease due to mismatch of rotational frequency and wind speed and partial stalling begins. The turbine blades will still be turning, causing extensive turbulence in the air stream, leading to more losses.

• When the speed reduces to zero, a becomes unity and no power is extracted. This state is known as (complete) stall state of blades.

• Betz Criterion:

In practice, all of the kinetic energy in the wind cannot be converted to shaft power since the air must be able to flow away the rotor area. The Betz criteria, derived using the principles of conservation of momentum and conservation of energy, suggests a maximum possible turbine efficiency, (or power coefficient) of 59%. In practice, power coefficients of 20-30% are more common.

Types of Wind Turbine

• Horizontal Axis Wind Turbine (HAWT)

Dutch windmills

Multi-blade water pumping windmills

High speed propeller type wind machines

• Vertical Axis Wind Turbine (VAWT)

Darrieus

Savonius

HAWT

Axis of rotation is horizontal w.r.t Ground.

HAWT’s have emerged as the most widely

used turbines.

These are being used for commercial energy

generation in many parts of the world.

Their theoretical basis is well researched and

sufficient field experience is available with

them.

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Main Components

Turbine Blades

Hub

Nacelle

Yaw-control Mechanism

Tower 32

Turbine Blades

These are made of high density wood or glass fiber

and epoxy composites.

They have an air foil type of cross section.

The blades are slightly twisted from the outer tip to

the root to reduce the tendency to stall.

In addition to centrifugal force and fatigue due to

continuous vibrations, there are many extraneous

forces arising from wind turbulence, gust,

gravitational forces, and directional change in the

wind.

All these factors are to be taken care off at the

designing stage. 33

App. Dia. of MW range, modern rotor is 100 m.

Modern wind turbines have two or three blades.

Two/three blades rotor HAWT are also known as

propeller-type wind turbines owing to their similarity

with propeller of old aeroplanes.

However, the rotor rpm in case is case of a wind

turbine is very low as compared to that for propeller.

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Relative merits and demerits of two and three blade rotors:

Compared to the two-blade, the three blades machine has

smoother power output and balanced gyroscopic force.

There is no need to teeter (to the discussed later in this

section) the rotor allowing the use of a simple rigid hub. The

blades may be cross –linked for greater rigidity.

Adding a third blade increase the power output by about 5%

only, while the weight and cost of a rotor increases by 50%,

thus giving a diminished rate of return for additional 50%

weight and cost.

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The two-blades rotor is also simpler to erect, since it

can be assembled on the ground and lifted to the shaft

without complicated maneuvers during the lift.

Three blades are more common in Europe and other

developing countries including India. The American

practice, however, is in favour of two blades.

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HUB

The Center solid portion of the rotor wheel is known

as hub.

All blades are attached to the hub.

The mechanism for pitch angle control is also

provided inside the hub.

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NACELLE

The term nacelle is derived from the name for housing

containing the engine of an aircraft.

The rotor is attached to the nacelle, and mounted at the

top of a tower.

It contain rotor brakes, gearbox, generator and electric

switchgear and control.

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Brakes are used to stop rotor when power generator is

not desired.

The gearbox steps up the shaft rpm to suit the

generator.

Protection and control functions are provided by

switchgear and control block.

The generator electrical power is conducted to ground

terminals through a cable.

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Yaw-control Mechanism

Adjusting the nacelle about the vertical axis to bring

the rotor facing the wind is known as yaw control.

The yaw-control system continuously orients the rotor

in the direction of wind.

For localities with prevailing wind in one direction

only, the rotor can be in a fixed orientation. Such a

machine is said to be yaw fixed.

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Most wind turbines however, are yaw active.

In small wind turbines, a tail vane is used for passive

yaw control.

In large turbines however, an active yaw control with

power steering and wind direction sensor is used to

maintain the orientation.

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Control mechanism in wind turbine

Types of Rotors

Depending on the number of blade, wind speed and

nature of application rotors have been developed in

various types of shapes and size.

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The type of rotors shown in (a) to (e) are relatively

high speed ones, suitable for application such as

electric power generation.

Large HAWTs have been manufactured with two and

three blades.

A single blade rotor, with a balancing counterweight

is economical, has simple controls but it is noisier and

produces unbalanced forces.

It is used for low-power applications.

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Those given in (f) and (g) are low-power rotor and

most suited for water-lifting applications, which

require a high starting torque.

They can capture power even from very slow winds.

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Teetering of Rotor

As wind speed rises with height, the axial force on

blade when upper position is significantly higher as

compared to that when it is at a lower positions.

For one-and two blade rotors, this causes cyclic

(sinusoidal) load on a rigid hub leading to fatigue.

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This is greatly relieved by

providing a teeter hinge (a pivot

within the hub) that allows a

see-saw motion to take place out

of the plane of rotation (i.e.,

vertical plane).

The rotor leans backwards to

accommodate the extra force.

This also reduces blade loads near the root by

approximately 40%.

The use of a third blade has approximately the same

effect as a teeter hinge on the hub moments since the

polar symmetry of the rotor averages out the applied

sinusoidal loads.

Therefore, teetering is not required when the number

of blades is three or more.

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Upwind and Downwind Machines

In an upwind machines, the rotor is located upwind (in front) of the tower whereas in a downwind machine, the rotor is located of (behind).

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Both types have certain benefits.

A downwind machine allow the use of a free yaw

system (in low rating machines).

It also allows the blades to deflect away from the

tower when loaded.

However, it suffers from wind shadow effects of the

tower on the blades as they pass through the tower’s

wake, in a region of separated flow.

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For a high-solidity tower with limited rotor overhang,

the wind speed might be effectively reduced to zero

causing a severe impulsive load of periodic nature.

This may be very dangerous as it may excite any

nature mode of the systems if that lies near a rotor

harmonics.

An upwind machine, on the other hand, produces

higher power as it eliminates the tower shadow on the

blades.

This also results in lower noise, low blade fatigue and

smoother power output.

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Pitch Control System

The pitch of a blade is controlled by rotating it from its

root, where it connected to the hub as shown in fig.

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The pitch controlled mechanism is provided through the

hub using a hydraulic jack in the nacelle.

The control system continuously adjusts the pitch to

obtain optimal performance.

In modern machines, pitch control is incorporated by

controlling only the outer 20% length of the blade (i.e.,

tip), keeping the remaining part of the blade as fixed.

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Vertical Axis Wind Turbine

A wind turbine, in which axis of rotation is perpendicular

to the air stream (i.e. vertical), is known as Vertical Axis

Wind Turbine (VAWT).

Vertical axis wind turbines are in development stage and

many models are undergoing field trial.

The size of rotor and its speed depends on the rating

of the turbine.

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The main attractions of a VAWT are:-

(1) It can accept wind from any direction, eliminating the need

of yaw control.

(2) The gearbox, generator etc. are located at the ground, thus

eliminating the heavy nacelle at the top of the tower, thus

simplifying the design and installation of the whole structure,

including the tower.

(3) The inspection and maintenance also gets easier, and

(4) It also reduces the overall cost.

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56 Fig .1 Vertical Axis Wind (Darrieus) Turbine

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CONSTRUCTION:-

A. MAIN COMPONENTS-

The constructional details of a vertical axis wind turbine

(Darrieus-type rotor) are shown in fig.1. The main

components of VAWT are as follows:

Tower (or Rotor Shaft):- The tower is a hollow vertical

rotor shaft, which rotates freely about the vertical axis

between the top and bottom bearings. It is installed above

a support structure . The upper part of the tower is

supported by guy ropes. The height of a large turbine is

around 100 m .

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Blades:- It has two or three thin, curved blades shaped like

an eggbeater that minimizes the bending stress caused by

centrifugal forces. The blades have an airfoil cross section

with constant chord length. The first large (3.8 MW),

Darrieus type, Canadian machine has a rotor height as 94m

and the diameter as 65 m with a chord of 2.4 m.

Support Structure:- The support structure is provided at

the ground to support the weight of the rotor. Gearbox,

brakes, electrical switchgear and controls are housed within

this structure

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B. TYPES OF ROTORS-

Various types of rotors for Vertical Axis Wind Turbine

(VAWT) are:

Cup type rotor

Savonious rotor

Darrieus rotor

Musgrove rotor

Evans rotor

60 Various Types of Rotors for VAWT

Cup Type Rotor

• It consists of a three or four-cup structure attached symmetrically to a vertical shaft.

• The drag force on the concave surface is more than that on the convex surface. As a result, the structure starts rotating.

• The main characteristics of this rotor is that its rotational frequency is linearly related to wind speed. Therefore, it is used as a transducer for measuring the wind speed and the apparatus is known as Cup Anemometer .

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Savonious Rotor

• The Savonious or S- rotor consists of two half

cylinders attached to a vertical axis and facing in

opposite directions to form a two vaned rotor.

• It has high starting torque, low speed and low

efficiency.

• It can extract power even from very slow wind,

making it working most of the time.

• These are use for low power applications.

• A high starting torque particularly makes it

suitable for pumping applications.

Darrieus Rotor :-

• It is used for large scale power generation.

• Its power coefficient is better than that of an S-rotor.

• It runs at a large tip-speed ratio.

• One of the drawbacks of this rotor is that it is usually not self-starting. Movement may be initiated by using electrical generator as motor.

• As the pitch of the blade cannot change, the rotor frequency and thus the output power cannot be controlled.

• Rotor frequency increases with wind speed and power output keeps on increasing till the blades stall.

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Musgrove Rotor

• Musgrove or H- shaped rotor consists of fixed

pitch blades, attached vertically to a horizontal

cross arm.

• Power control is achieved by controlled

folding of the blades.

• Inclining the blades to the vertical provides an

effective means of altering the blades angle of

attack and hence controlling the power output.

Evans Rotor

• The Evans rotor, also known as Gyromill is an

improvement over the H-shaped rotor.

• In this type of rotor, blades remain straight but

the blade pitch is varied cyclically to regulate

the power output.

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Lift and Drag -Type Machines Wind turbines make use of either lift force or drag

force predominantly to cause motion and accordingly

are known as lift or drag-type machines.

• In lift devices, the ratio of lift to drag forces may be

as high as 30:1. Lift devices are more efficient and

turn faster than wind. They are able to benefit from

high-power densities available in strong winds.

• In drag devices, the wind literally pushes the blades

out the way. Drag devices are less efficient and turn

slower than wind. It produce high torque and thus are

suitable for pumping applications. They do not

benefit from high energy density available in wind. 66

Drag-based wind turbine

Lift-based Wind Turbines

Horizontal Axis Versus Vertical-

Axis Turbines

Most wind turbines used at present are of

horizontal axis type. They have been well

researched and have gone through extensive

field trial. As a result, well established

technology is available for HAWTs.

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Advantages of VAWT

It can accept wind from any direction without adjustment which avoids the cost and complexity of a yaw-orientation system.

Gearing and generators are located at ground level, which simplifies the design of tower, the installation and subsequent inspection and maintenance, and

Also they are less costly as compared to HAWTs.

Disadvantage VAWT

• Many vertical axis machines have suffered

from fatigue arising from numerous natural

resonances in the structure.

• Rotational torque from the wind varies

periodically within each cycle, and thus

unwanted power periodicities appear at the

output.

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• It normally requires guy ropes attached to the top

for support, which could limit its applications

particularly for offshore sites.

• It is noisier than HAWT.

• As wind speed increases significantly with

height, for the same tower height HAWT

captures more power than VAWT, and

• The technology is under development stage and

far less is known about them as compared to

HAWTs.

Speed Control Strategies for Wind

Turbine Various options are available for speed control of a

turbine. Small machines use simple, low-cost

methods while large machines use more sophisticated

methods incorporating pitch control along with power

electronic circuit. These methods may be grouped in

the following categories:

i. No speed control at all. Various components of the

entire system are designed to withstand extreme

speed under gusty winds.

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ii. Yaw and tilt control, in which the rotor axis shifted out

of wind direction, either by yaw control or by tilting the

rotor plane with respect to normal vertical plane when

the wind exceeds the design limit.

iii. Pitch control, in which the pitch of the rotor blades

is controlled to regulate the speed.

iv. Stall control, in which the blades are shifted to a

position such that they stall when wind speed exceeds

the safe limit.

Wind Turbine Operation and Power

Versus Wind Speed Characteristics The power-speed characteristics of a wind turbine

have four separate regions:

(1) Low-speed Region(Zero to Cut-in speed) : In this

region, the turbine is kept in braked position till

minimum wind speed (about 5m/s), known as cut-in

speed becomes available. Below this speed, the

operation is not efficient.

(2) Maximum Power-coefficient Region : In this region

rotor speed is varied with wind speed so as to

operate it at constant tip-speed ratio, corresponding

to maximum power coefficient, Cpmax. 75

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(3) Constant Power Region(Constant-turbine-speed Region):

During high-speed winds (above 12 m/s), the rotor speed is

limited to an upper permissible value based on the design

limits of the system components . In this region, the power

coefficient is lower than Cpmax.

(4) Furling speed Region (Cut-out Speed and Above) :

Beyond a certain maximum value of wind speed (around 25

m/s ), the rotor is shut down and power generation is stopped

to protect the blades, generator and other components of the

system.

Axial Thrust on Turbine, FA

• With no energy extraction, Bernoulli’s eqn. for upstream and downstream,(refer Betz model) may written as

po/ρo + gzo +uo²/2 = p2/ρ2 + gz2 +u2²/2

As z0=z2 and variation in air density is negligible compares to other terms, considering ρ as average air density, the static pressure difference across the turbine may be written as

∆p=po-p2 =(uo²-u2²)ρ/2

• The maximum value of static pressure difference occurs when u2 approaches zero. Thus,

∆pmax= ρuo² and maximum thrust on disk is

Fmax= A1ρuo²/2 (13)

This axial thrust must me equal to loss of momentum. Therefore,

FA = m̊(uo-u2)

Using eqn 1, 7 & 8 we can write

FA=4a(1-a)(A1)ρuo²/2 (14)

or FA=CFFAmax (15)

where CF = 4a(1-a) (16)

• Maximum axial thrust occurs when CF=1, which is achieved when a=0.5, equivalent to u2 =0. Maximum power extraction by the Betz criterion occurs when a=1/3, corresponding to CF=8/9.

Torque Developed by the Turbine, T

• The maximum conceivable torque TM on an ideal rotor would occur if maximum circumferential force acts at the tip of the blade with radius R. Thus,

TM=FcirmaxR (17)

TM=PoR/uo (18)

Now, if λ is the tip speed then TM can be written as

TM=Poλ/ω (19)

Thus, the shaft torque, Tsh is given as

Tsh=CTTM

• As the product of shaft torque and angular speed equals power developed by the turbine,

Tshω=PT

or CTTMω=CPPo

Substituting for TM from eqn 19, we have,

CTPoλ=CPPo

or CT=CP/λ (20)

Both CT & CP are functions of λ. As per Betz criterion, the maximum value of CP can be 0.593, therefore,

CTmax= CPmax/λ (21)

Thus, machines with higher speeds have low value of CTmax or low starting torque.

References

• B. H. Khan, “ Non conventional Energy Resources”, 3rd ed, Tata Mc graw Hill publisher.