wind energy

1

Wind Energy

Instructor: Waqas Khalid

Contact: [email protected]

Phone: 6076

mailto:[email protected]

Wind energy is another form of renewable energy which is available in abundant and is

renewable. So, there will always be constant supply of it.

The initial cost to set up wind energy is very high and requires huge initial investment, but

once it starts working, it is one of the cheapest form of electricity to maintain.

Wind energy is a form of solar energy which is caused by sun's heating of the

earth's surface. The use of wind energy is growing rapidly because it is abundant,

cheap, clean, widely available and environment friendly.

The wind turbines effectiveness depends on the location where they are placed.

Ideally, wind turbines should be located where there are constant flow of wind

throughout the year.

U.S, Germany, India, France, U.K, Spain, China, Italy, Denmark and Portugal

are the top ten nations where wind turbines are used most efficiently.

Wind energy uses the kinetic energy of the wind and turns into electrical energy.

Initial cost are quite steep but in the long run they prove to be much cost

effective.

Facts of Wind Energy

In the year 2005 wind energy accounted for less than 1% of the total

electricity production in the world. In 2008, it was estimated about 1-2%

of the world 's energy supply comes from wind energy source

Wind energy approximately cost $1 million per megawatt to install and one

megawatt of wind energy.

Wind turbines are relatively quiet. Improved engineering and appropriate

setback from homes mitigates noise issues.

At the appropriate setback from homes, a wind turbine makes the sound

equivalent of a common kitchen refrigerator.

The opposition usually met from a proposed wind farm development comes

down to the "NIMBY" factor (not in my backyard).


Wind energy is the only power generation technology that can deliver the necessary

cuts in CO2 in the critical period up to 2020, when greenhouse cases must peak and

begin to decline to avoid dangerous climate change.

The 120.8 GW of global wind capacity will produce 260 TWh and save 158 million

tons of CO2 every year.

The wind industry also creates many new jobs: over 400,000 people are now employed

in this industry, and that number is expected to be in the millions in the near future.

The cost of electricity from the wind has dropped from about 25 cents/kWh in 1981 to

averaging near 4 cents/kWh in 2008, with 50 % of projects in the range of 3.3 to 5.2

cents/kWh

Though wind turbine prices have increased some since 2005, in areas with the best

resources, wind power is cost competitive with new generation from coal and natural

gas plants.


Wind power is a clean renewable energy source. There are, however some

environmental considerations to keep in mind when planning a wind power

scheme.

They include the following:

Electromagnetic interference - some television frequency bands are

susceptible to interference from wind generators.

Noise - wind rotors, gearboxes and generators create acoustic noise when

functioning; this needs to be considered when siting a machine.

Visual impact - modern wind machines are large objects and have a

significant visual impact on their surroundings. Some argue that it is a

positive visual impact, others to the contrary.

Environmental Concerns

Worldwide Installed Wind Power Capacity

Europe & Eurasia

44%

North America

22%

Asia Pacific

31%

Rest of the world

3%

Share of world Installed wind energy

Share of world Installed wind energy

Over the past ten years, global wind

power capacity has continued to grow

at an average cumulative rate of

over 30%,

And 2008-09 was another record year

with more than 27 GW of new

installations, bringing the total up to

over 120 GW.

The United States passed Germany

to become the number one market

in wind power,

And China’s total capacity doubled

for the fourth year in a row.

Cost for per unit of electricity generation From Wind

Winds are caused by uneven heating of the atmosphere by the sun, the roughness of the

Earth surface and Earth's rotation.

The term wind power describes the process by which wind is used to make the

mechanical energy or electricity.

Wind turbines convert the kinetic energy of wind energy into mechanical power.

This mechanical power may be used for certain activities (such as milling of rice or

pumping water) or a generator can convert mechanical energy to electrical energy at

home, business, schools and other facilities.

A group of wind turbines can make electricity for the utility grid. The electricity is sent

through transmission and distribution lines to homes, businesses, schools, and so on.

Generally, average annual wind speeds of at least 4.0-4.5 m/s are needed for a small

wind turbine to produce enough electricity to be cost-effective.

Wind Energy Basics

Wind turbines convert the kinetic energy of wind energy into mechanical power.

If the mechanical energy is used to produce electricity, the device may be called a wind

generator

If the mechanical energy is used to drive machinery, such as for grinding grain or

pumping water, the device is called a windmill or wind pump.

There are two main families of windmills/wind generators:

Vertical axis machines (VAWT)

Horizontal Axis Wind Turbines (HAWT)

The smallest turbines are used for applications such as battery charging or auxiliary

power on sailing boats;

while large grid-connected arrays of turbines are becoming an increasingly large source

of commercial electric power.

Wind Turbines

Wind Turbines

Principles of wind energy conversion

There are two primary physical principles by which energy can be

extracted from the wind;

These are through the creation of either drag or lift force (or through a

combination of the two).

Classification of Wind turbines

Drag Force

Hold a piece of paper by the top edges, as shown in left side of the figure below.

Blow directly at the paper.

The bottom of the paper should flip up as shown.

This is the type of force used on drag-based wind turbines like the Savonius rotor

Drag And Lift Force

Lift Force

Hold a piece of paper by the edges, as shown in left side of the figure below.

The edge that you’re holding should be parallel to the ground, while the unsupported

edge should be hanging down.

Blow directly above the edge of the paper that you’re holding.

The paper should lift. The higher velocity on the top creates a reduced pressure

(Bernoulli’s effect). This is the type of force used on lift based wind turbines.

Drag And Lift Force

A type of wind turbine in which the axis of

rotation is perpendicular to the wind stream

and the ground.

VAWTs work somewhat like a classical

water wheel in which water arrives at a

right angle (perpendicular) to the rotational

axis (shaft) of the water wheel.

Vertical-axis wind turbines fall into two

major categories:

i. Darrieus turbines and

ii. Savonius turbines.



Savnoius type Wind Turbines Darrieus type Wind Turbines

For the Darrieus the driving wind

forces are lift,

Rotates by drag force

Horizontal axis wind turbines, also shortened to HAWT, are the common style that most

of us think of when we think of a wind turbine.

A HAWT has a similar design to a windmill, it has blades that look like a propeller that

spin on the horizontal axis. The dominant driving force is lift

Two- and three-bladed rotors are common for electricity generation.

The three-bladed rotors operate more smoothly and, generally, more quietly than two-

bladed

Horizontal Axis Wind Turbines

Most horizontal axis turbines built today are

two- or three-bladed, although some have

fewer or more blades.

Horizontal-axis wind turbines (HAWT) have

the main rotor shaft and electrical generator

at the top of a tower, and must be pointed

into the wind.

The purpose of the rotor is to convert the

linear motion of the wind into rotational

energy that can be used to drive a generator.

Small turbines are pointed by a simple wind

vane, while large turbines generally use a

wind sensor coupled with a servo motor.


2. The generator component, which is

approximately 34% of the wind turbine

cost, includes the electrical generator, the

control electronics, and most likely a

gearbox, component for converting the

low speed incoming rotation to high

speed rotation suitable for generating

electricity.

3. The structural support component, which

is approximately 15% of the wind turbine

cost, includes the tower and rotor yaw

mechanism

Wind turbines convert wind energy to electricity for distribution. Conventional

horizontal axis turbines can be divided into three components:

1. The rotor component, which is approximately 20% of the wind turbine cost,

includes the blades for converting wind energy to low speed rotational energy.


A 1.5 MW wind turbine of a type frequently seen in the United States has

a tower 80 metres (260 ft) high.

The rotor assembly (blades and hub) weighs 48,000 pounds (22,000 kg).

The nacelle, which contains the generator component, weighs 115,000

pounds (52,000 kg).

The concrete base for the tower is constructed using 58,000 pounds

(26,000 kg) of reinforcing steel and contains 250 cubic yards (190 m3) of

concrete.

The base is 50 ft (15 m) in diameter and 8 ft (2.4 m) thick near the center.


Vertical axis wind turbine is a type of wind turbine that has its rotor shaft installed

vertically and can therefore work even if the turbine is not directly pointed to the wind.

This ensures wind power even in areas where there is low wind speed.

Vertical Axis Wind Turbines, more commonly referred to as VAWTs have the generator

and gearbox assembled near the ground instead of these parts being supported by a

tower. This proves to be advantageous especially considering maintenance purposes.

Part of the disadvantage of using a wind turbine is the difficulty of setting up a tower to

put it up. With these turbines, there is no need for tower structures.

They also have parts which are easier to maintain and are less likely to break down

and collapse during high winds.

There are areas where horizontal axis wind turbines may be prohibited. In these areas,

Vertical Axis Wind Turbines may be the alternatives.

Vertical axis machines (VAWT) Advantages

Since VAWT are mounted closer to the ground they are more bird friendly and

down not destroy the wildlife.

VAWT quiet, efficient, economical and perfect for residential energy

production, especially in urban environments.

Vertical axis machines (VAWT) Advantages

First, most of the turbines of this type only has an energy-producing capacity

which is fifty percent less efficient than those produced by horizontal axis wind

turbines.

Because there is no tower structure required, they cannot take full advantage of

the higher wind speeds that are available on higher, elevated locations.

They also require energy to start the turning of blades due to their low starting

torque.

They will have parts which are difficult to change without disassembling the

entire turbine should it not be assembled properly.

Another disadvantage of the Vertical Axis Wind Turbines is that they require

wires to hold the structure in place.

This puts stress on the bearing because all the weight of the rotor rests upon it.

Vertical axis machines (VAWT) Disadvantages

The area through which the rotor

blades of a wind turbine spin, as

seen when directly facing the

center of the rotor blades.

The power output of a wind

turbine is directly related to the

swept area of its blades.

The larger the diameter of its

blades, the more power it is

capable of extracting from the

wind


Energy from the Wind

In physics it can be shown that

And, in fluid dynamics, given a mass flow rate (m ) of air with density (ρ) through a

surface of area (A), the available fluid power becomes

These equations characterize the dynamic

power is available in any fluid.

Therefore, they also describe the wind

power incident a turbine with a rotor

swept area (Ar) and a given air density and

air speed.

While this calculation does provide a baseline for comparing two competing sites for

installing a turbine or building a wind farm, it is more useful to determine the power

captured by the wind turbine.

This value is a function of the difference between the upstream (Vi) and downstream

(Vo) air velocities, and is given by

The mass flow rate through the turbine

can be approximated as


The maximum power that can be extracted from the wind in terms of

upstream wind velocity alone


Where Cp is the coefficient of power,

Power developed by the wind machine is mainly affected by:

i. Wind speed,

ii. Area swept by the rotor

iii. Density of air

iv. Radius of the rotor, R

CALCULATIONS WITH GIVEN DATA

We are given the following data:

Blade length, l = 52 m

Wind speed, v = 12 m/sec

Air density, ρ = 1.23 kg/m3

Power Coefficient, Cp = 0.4

Pmax= 3.6 MW

Example

The proportion of the power in the wind that the rotor can extract is termed the

coefficient of performance (or power coefficient or efficiency; symbol Cp)

It is physically impossible to extract all the energy from the wind, without

bringing the air behind the rotor to a standstill.

Consequently there is a maximum value of

Cp of 59.3% (known as the Betz limit),

although in practice real wind rotors have

maximum Cp values in the range of 25%-

45%.

The coefficient of performance is not a

constant, but varies with the wind

speed, the rotational speed of the

turbine, and turbine blade parameters

like angle of attack and pitch angle

Coefficient Of Performance (Cp)

Power Coefficient (Cp) with downstream

to upstream air velocities (Vo/V)

Maximum value of Cp is 16/27 or

0.59 according to the Betz Limit.

Therefore, because the most

efficient, high speed, two- and

three-blade turbines have a power

coefficient of just less than 0.50

due to in efficiencies and certain

losses attributed to different

configurations ,rotor blades and

turbine design


“V” is the upstream velocity

Because the most efficient, high speed, two- and three-blade turbines

have a power coefficient of just less than 0.50, rotor power has an

effective limit given below


Tip speed ratio ()

The power coefficient is not a static value as defined before it varies with the

tip speed ratio of the turbine.

The Tip Speed Ratio (often 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, most of the wind will pass

undisturbed through the gap between the rotor blades.

Alternatively if the rotor turns too quickly, the blurring blades will appear like a

solid wall to the wind.

Therefore, wind turbines are designed with optimal tip speed ratios to extract

as much power out of the wind as possible.

Tip Speed Ratio

In reference to a wind energy conversion device's blades, the ratio between the

rotational speed of the tip of the blade and the actual velocity of the wind.

High efficiency 3-blade-turbines have tip speed ratios of 5-6.

On the whole, a high tip speed ratio is better, but not to the point where the

machine becomes noisy and highly stressed.

The tip speed ratio determines

how fast the wind turbine will

turn

Tip Speed Ratio

Most have of the wind turbines 3 blades because of bending moments

caused by having an even number of blades--for example 2 blades.

With 2 blades, when one blade is at the top of the cycle the opposite

blade is at the bottom of the cycle.

The top blade is receiving the greatest force of the wind and the bottom

blade is in the shadow of the tower, which shelters it from the wind.

This sets up a bending torque on the blades which wears out the

bearings and also causes undue stresses

No of blades of Wind Turbines

Two- and one-bladed machines require a more complex design with a

hinged (teetering hub) rotor as shown in the picture, i.e. the rotor has to

be able to tilt in order to avoid too heavy shocks to the turbine when a

rotor blades passes the tower

No of blades of Wind Turbines

Drag devices always have tip-speed ratios less than one and hence turn

slowly,

Whereas lift devices can have high tip-speed ratios and hence turn

quickly relative to the wind.

Tip Speed Ratio

( )Speed of the rotor tip

T ip speed ratiowind speed

v wr

V V

l =

= =

V = Wind Speed m/s

v = Velocity of the rotor tip

r = Radius of the rotor

w = Angular velocity

How To Find The Tip Speed

i. Measure the rotor radius (length of

one blade)

ii. Speed = distance divided by time.

iii. The distance travelled is the

circumference (2r)

tan( / )

dis ceT ip speed of theblades m s

time=

Tip Speed Ratio

Why is TSR Important???

Knowing the tip speed ratio of your turbine will help in maximizing the power output

and efficiency of the wind turbine.

Remember that if the rotor spins too slowly, a lot of wind will pass through the gaps

between the blades rather than giving energy to the turbine.

But if the blades spin too quickly, they could create too much turbulent air or act as a

solid wall against the wind.

So, to maximize the turbine’s efficiency, we’ve got to calculate the perfect Tip Speed

Ratio

For the optimum TSR for maximum power output, this formula has been empirically

proven:

λ (𝑚𝑎𝑥 𝑝𝑜𝑤𝑒𝑟) =4𝜋

𝑛 (n = number of blades)

For n = 2, the optimal TSR is calculated to be 6.28, while it is 4.19 for three-

bladed rotor, and it reduces to 3.14 for a four-bladed rotor.

With proper airfoil design, the optimal TSR values may be approximately 25 –

30 percent above these values.

Using this assumption, the optimal TSR for a three-bladed rotor would be in the

range of 5.24 – 5.45

Poorly designed rotor blades that yield too low of a TSR would cause the wind

turbine to exhibit a tendency to slow and stall.

On the other hand, if the TSR is too high, the turbine will rotate very rapidly,

and will experience larger stresses, which may lead to catastrophic failure in

highly-turbulent wind conditions

Tip Speed Ratio

Typical Wind Turbine Operation at different wind Speeds

0 ~ 4.5 m/s Wind speed is too low for generating power. Turbine is not

operational. Rotor is locked.

4.5 ~ 11 m/s 10 mph is the minimum operational speed. It is called “Cut-

in speed”. In 10 ~ 25 mph wind, generated power increases

with the wind speed.

11 ~ 23 m/s Typical wind turbines reach the rated power (maximum

operating power) at wind speed of 25mph (called Rated wind

speed). Further increase in wind speed will not result in

substantially higher generated power by design. This is

accomplished by, for example, pitching the blade angle to

reduce the turbine efficiency.

23 m/s Turbine is shut down when wind speed is higher than 50mph

(called “Cut-out” speed) to prevent structure failure.

Cut-out speed

The highest wind speed at which a wind turbine stops producing power. and is

usually around 25 meters per second.

Cut-in speed

The lowest wind speed at which a wind turbine begins producing usable power

and is typically between 3 and 4 metres per second.

Wind turbine swept area

The area through which the rotor blades of a wind turbine spin, as seen when

directly facing the center of the rotor blades. The power output of a wind turbine is

directly related to the swept area of its blades. The larger the diameter of its blades, the

more power it is capable of extracting from the wind

Rated output power and rate output wind speed

Typically somewhere between 12 and 17 m/s, the power output reaches the

limit that the electrical generator is capable of.

This limit to the generator output is called the rated power output and the wind

speed at which it is reached is called the rated output wind speed


With increasingly competitive prices, growing environmental concerns, and the call to

reduce dependence on foreign energy sources, a strong future for wind power seems

certain.

The Global Wind Energy Council projects global wind capacity will reach 332 GW by

2013, almost triple its current size, with growth especially concentrated in the United

States and China.

In 2013 alone, new installations could reach 56 GW, more than double the current

annual global market

Turbines are getting larger and more sophisticated, with land-based turbines now

commonly in the 1-2 MW range, and offshore turbines in the 3-5 MW range.

The next frontiers for the wind industry are deepwater offshore, and land-based

systems capable of operating at lower wind speeds. Both technological advances will

provide large areas for new development

The Future of Wind Power

Wind gradient

To calculate the wind speed at the height of the hub, it is necessary to take care

that the wind speed varies with height due to the friction against the structure of

the ground, which slows the wind.

This phenomenon is named wind gradient or wind profile

If a “z” height is considered, the average of the wind

speed at this height is described by

vz0 = wind speed at the reference height z0 [m/s];

z0 = reference height [m];

α = value depending on the roughness class of the

terrain, as shown in the following table;

𝒗𝒛 = 𝒗𝒛𝒐

𝒛

𝒛𝒐

∝

Wind gradient

Wind speed at the height of 10 m is 5.5 m/s. Considering the ground

surface coefficient () 0.4 what will be the wind speed of the wind at the

hub height of the wind turbine. The hub height of the wind turbine is

100m.

Wind speed at the height of 10 m=V1 = 5.5 m/s

Wind speed at the height of 100 m= V2=?

Ground surface coefficient==0.4

h1=10m

h2=100m

V2 =13.8 m/s (wind speed at 100 m height)

Problem 1

𝒗𝒛 = 𝒗𝒛𝒐

𝒛

𝒛𝒐

∝

A wind turbine has the rotor diameter of 42 m and hub height of 41.5m.

What will be the rotor efficiency at upstream velocity of 9 m/s and

downstream velocity of wind is 6.5 m/s.

The density of air can be assumed 1.2 kg/m3.

How much energy will be produced from this turbine annually?

Also calculate the tip speed ratio of the wind turbine?

Problem 2

Rotor dia = D=42 m

Hub height=41.5m

Upstream wind speed=V=9 m/s

Down stream wind speed=Vo=6.5 m/s

Vo/V =6.5/9=0.7222

Rotor swept area=/4 *D2

=1385.44m2

Air density==1.2 kg/m3 Rotor efficiency=?

Cp =0.412 or 41.2%

Or

Problem 2

Power extracted by turbine= ½ x.412 x1.2 x 1385 x 93

=249.67 kW

Annual energy production= 249.67 x (365*24) = 2186401.2KWh

Tip speed ratio ()

Problem 2

A home-made turbine has blades that are 0.25 meters long. They are

spinning rapidly at 600 RPM. What is the tip speed?

600 RPM 10 rev per second

Distance covered in one rev = 2r

=2*3.14*0.25=1.57 m

10 rev per sec

t an

1.5715.7 /

1 / 10

dis ceT ip speed of theblades

time

m s

=

= =

Problem 3

Wind turbine have rotor blade radius of 10 m, rotating at 1 rotation per

second find out the tip speed ratio at a wind speed of 15 m/sec.

Also calculate the power it can produced at this wind speed?

( )Speed of the rotor tip

T ip speed ratiowind speed

v wr

V V

l =

= =

1sec

2 2 / sec

* 2 * 10 62.8 /

rotationf

f rad

v r m s

w p p

w p

é ùê ú=ê úë û

= =

= = =

62.8( ) 4.2

15T ip speed ratio l = =

Problem 4

An offshore wind turbine with three 60 meter blades rotates at a leisurely

12 RPM.

The wind is whipping along at 18 meters per second.

What is the tip speed ratio for this turbine? How does this compare to the

“optimal” tip speed ratio for this turbine?

12 RPM 0.2 rev per second or 5 second for one rev

Distance covered in one rev = 2r

=2*3.14*60=376.8 m or 377 m

Problem 5

tan

37775.4 /

5

dis ceT ip speed of theblades

time

m s

=

= =

75.4TSR 4.2

18

T ip speed of theblades

W inds peed= = =

4 4Opt imal TSR 4.2 ( )

3blades are movingslow

n

p p= = =

Problem 5

A wind turbine with three blades, each 4 meters long, how much distance

does the tip of each blade travel in one full revolution?

i. And if this turbine is rotating at a rate of 42 Revolutions per Minute

(RPM), how long does it take to make one full revolution?

Problem 6

i. Calculate how fast the tips of this wind

turbine are moving through the air?

ii. If the wind is blowing at 6 meters per

second, what is the tip speed ratio of this

turbine?

iii. According to the “optimal” tip speed

ratio for this three-bladed turbine, are

these blades moving too fast or too slow?

The length of one blade is equal to the radius.

The distance travelled is equal to the circumference of the circle created

by the blades.

Circumference of a circle = 2Πr. So, (2 x Π x 4) = 25.13 meters

Turbine is rotating at a rate of 42 Revolutions per Minute (RPM),

How long does it take to make one full revolution?

Problem 6

Calculate how fast the tips of this wind turbine are moving through the

air?

Tip speed of the blades = distance /time

= 17.6 m/s

If the wind is blowing at 6 meters per second, what is the tip speed

ratio of this turbine?

17.6TSR 2.93

6

T ip speed of theblades

W inds peed= = =

4 4Opt imal TSR 4.2 ( )

3blades are movingslow

n

p p= = =

Problem 6

wind energy

Documents

wind energy source wind

megawatt of wind energy

wind energy instructor

use of wind energy

wind generators

wind industry

wind power scheme

wind turbines effectiveness