(EN-423) RENEWABLE

ENERGY RESOURCES
Lecture 8 – Wind Energy
(Wind Energy Technology)

Course Instructor: Dr. Muddassir Ali

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 CONTENTS
 Introduction
 Harvesting Energy from Wind
 Wind Resource Map
 Land Area Requirement
 Energy and Power from Wind
 Capacity Factor for a Wind Turbine
 Energy Production
 Turbine Types
 Comparison Between Turbines
 Cost of Electricity from Wind Energy
 Effect of Capacity Factor
 Basic Principles of Wind Resource Evaluation
 Wind Farm
 Low Frequency Noise form Wind Turbines
 Wind Energy and Intermittency
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 TURBINE TYPES
Horizontal Axis Wind Turbines (HAWTs)
 The blades of horizontal-axis wind turbines spin in a vertical plane.

 During rotation, blades move more rapidly over one side, creating a low pressure
area behind the blades and a high pressure area in front of it.

 The difference between these two pressures creates a force which causes blades
to spin.

 The HAWTs have the main rotor shaft and electrical generator at the top of a
tower, and are pointed into the wind.

 Small turbines are pointed by a simple wind vane, while large turbines generally
use a wind sensor coupled with a servo motor.

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 TURBINE TYPES
 Most have a gearbox, which turns
the slow rotation of the blades into a
quicker rotation that is more
suitable for generating electricity.

 The basic structure of a HAWT is

shown in Fig. 1.23.

 There are several advantages of

horizontal wind turbines.

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 TURBINE TYPES
Horizontal Axis Wind Turbines Advantages
 The design and location of blades provide a better stability of the structure.

 The ability to pitch the rotor blades in a storm minimizes the damage.

 The use of a tall tower allows access to stronger wind in sites with wind sheer and
placement on uneven land.

 The manufacturing cost can be less because of higher production volume, larger
sizes and, in general, higher capacity factors and efficiencies.

Horizontal Axis Wind Turbines Disadvantages

 tall towers and long blades (up to 180 ft long) are difficult to transport,

 higher install costs, and

 higher maintenance costs.

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 TURBINE TYPES
 Horizontal axis wind turbines are most
widely used for commercial power
generation.

 Currently three blade rotor systems are

preferred; however, in the past both
one blade and two blade wind turbines
have been designed and tested (see
Fig. 1.24).

 One-blade and two-blade wind

turbines generate 15% and 5%less
power than three blade wind turbines,
respectively.

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 TURBINE TYPES
 However, the main issue for using one-blade or two-blade systems is the stability
of the turbine.

 More sophisticated and costly control mechanisms are necessary to make one-
blade or two-blade turbines stable during rotation.

 Wind turbines with more than three blades (multi-blade) have also been explored,
but no significant gain in costs or stability of multi-blade systems over three-blade
turbines was achieved.

 Various multi-blade horizontal axis wind turbines and other proposed designs are
shown in Fig. 1.25.

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 TURBINE TYPES

Wind Turbine Components

 The basic components of a wind turbine are as follows:
 Nacelle
 Rotor blades
 Hub
 Low speed shaft

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 TURBINE TYPES
Wind Turbine Components
 Gearbox
 High speed shaft with its mechanical brake
 Electrical generator
 Yaw mechanism
 Electronic controller
 Tower
 Anemometer
 Wind vane

 A diagram of a wind turbine with its components is shown in Fig. 1.26.

 A brief description of these components is given below.

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 TURBINE TYPES
Wind Turbine Components
Nacelle
 The nacelle contains the key
components of a wind turbine,
including the gearbox, and the
electrical generator.

Rotor Blades
 The rotor blades capture the wind and
transfer its power to the rotor hub.

 A 1,000 kWe wind turbine has rotor

blades that are about 27m (80 ft) in
length.

 The blades or “rotors” catch the wind

and cause the movement of the
blades that turns the shaft.
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 TURBINE TYPES
Wind Turbine Components
Rotor Blades
 The generator then turns this
movement into electricity.

 Blades come in many sizes; as

shown in Fig. 1.22, the longest
blades in use today are about
62m long (rotor diameter is
124m).

Hub
 The hub of the rotor is attached to
the low speed shaft of a wind
turbine.

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 TURBINE TYPES
Wind Turbine Components
Low Speed Shaft
 The low speed shaft of a wind
turbine connects the rotor hub to
the gearbox.

 The rotor rotates at about 19–30

rotation per minute (rpm) in a
1,000 kWe wind turbine.

 The shaft contains pipes for the

hydraulics system to enable the
aerodynamic brakes to operate.

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 TURBINE TYPES
Wind Turbine Components
Gearbox
 Gears connect the low-speed
shaft to the high-speed shaft and
increase the rotational speeds
from about 19 to 30 rotations per
minute (rpm) to about 1,000–
1,800 rpm, which is required by
most generators to produce
electricity.

 The recent design uses “direct-

drive” generators that operate at
lower rotational speeds and do
not need gear boxes.

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 TURBINE TYPES
Wind Turbine Components
High Speed Shaft with Its
Mechanical Brake
 This drives the generator and
employs a disc brake, which can
be applied mechanically,
electrically, or hydraulically to
stop the rotor in emergencies.

Electrical Generator
 The generator converts the
mechanical energy of the rotating
shaft into electrical energy.

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 TURBINE TYPES
Wind Turbine Components
Yaw Mechanism
 The turbines must face upwind
for power production.

 The yaw drive is used to keep the

rotor facing into the wind as the
wind direction changes.

 Downwind turbines don’t require

a yaw drive, since the wind blows
the rotor downwind.

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 TURBINE TYPES
Wind Turbine Components
Electronic Controller
 The controller starts the machine
at the specified wind speed which
is generally between 8 and 16
miles per hour (mph) (3.58 and
7.15 m/s) and shuts off the
machine at about 55 mph (24.6
m/s).

 Turbines do not operate at wind

speeds above about 55 mph
(24.6m/s) because they might get
damaged above this wind speed.

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 TURBINE TYPES
Wind Turbine Components
Tower
 The tower is a high stationary support
structure for the wind turbine, so that
consistent wind speed can be
sustained for the operation of the
turbine.

Anemometer
 It measures the wind speed and
transmits wind speed data to the
controller.

Wind Vane
 It measures wind direction and directs
the yaw drive to appropriate
orientation so that the turbine is
properly aligned with respect to the
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 TURBINE TYPES
Vertical Axis Wind Turbines
(VAWTs)
 The blades of vertical-axis wind
turbines spin in a horizontal plane.

 VAWTs have the main rotor shaft

running vertically.

 Various components of a VAWT are

shown in Fig. 1.27.

 An advantage of this arrangement is

that the generator and/or gearbox can
be placed at the bottom, near the
ground; therefore, a tower is not
needed to support the turbine.
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 TURBINE TYPES
Vertical Axis Wind Turbines (VAWTs)
 Also, the turbine does not need to be pointed into the wind.

 The disadvantages are usually the pulsating torque that is produced during each
revolution and the drag created when the blade rotates into the wind.

 The vertical axis turbines on towers need lower and more turbulent air flow near
the ground.

 This type of condition is difficult to sustain resulting in a lower energy extraction

efficiency.

 A variety of designs for VAWTs have been proposed and are described below.

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 TURBINE TYPES
Vertical Axis Wind Turbines (VAWTs)
Darrieus Wind Turbine
 The most common type of VAWT is the Darrieus
wind turbine.

 The design of these types of turbines looks like

an eggbeater (Fig. 1.28).

 Generally, an external power source is required

to start the rotation.

 The starting torque is very low.

 In the newer design, three or more blades are

used which results in a higher solidity for the
rotor.

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 TURBINE TYPES
Vertical Axis Wind Turbines (VAWTs)
Darrieus Wind Turbine
 Solidity is measured by blade area over the rotor area.

 New Darrieus type turbines are not held up by guy

wires, but have an external structure connected to the
top bearing.

Savonius Wind Turbine

 The Savonius turbine consists of two half-cylinders
mounted on a vertical shaft that has an S-shape
appearance when viewed from the top.

 A schematic diagram of the Savonius rotor is shown in

Fig. 1.29.

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 TURBINE TYPES
Vertical Axis Wind Turbines (VAWTs)
Savonius Wind Turbine
 This drag-type VAWT turns relatively slowly, but yields a high torque.

 Because of the curvature, the scoops experience less drag when moving against the wind.

 The differential drag causes the Savonius turbine to spin.

 Most of the swept area of a Savonius turbine is near the ground, therefore, the overall
energy extraction efficiency is lower.

 However, Savonius turbines are cheap and reliable.

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 COMPARISON BETWEEN
TURBINES
 Wind turbines may be compared against
each other by comparing their coefficient
of performance ( 𝐶𝐶𝑝𝑝 ) against tip speed
ratio ( 𝜆𝜆).

 Various forces working on a turbine are

shown in Fig. 1.31.

 The coefficient of performance, also

known as the power coefficients, is
defined by Eq. 1.
𝑃𝑃
𝐶𝐶𝑝𝑝 = (1)
𝑃𝑃0
where,𝑃𝑃 and 𝑃𝑃0 are defined as
𝜌𝜌𝑎𝑎
𝑃𝑃 = 𝑣𝑣1 2 − 𝑣𝑣2 2 𝑣𝑣1 + 𝑣𝑣2 𝐴𝐴 𝑇𝑇
4
𝜌𝜌𝑎𝑎
𝑃𝑃0 = 𝑣𝑣1 3 𝐴𝐴 𝑇𝑇
2
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 COMPARISON BETWEEN
TURBINES
 Using the swept area and substituting the value of 𝑃𝑃0 ; 𝑃𝑃 may be expressed as
𝑃𝑃 = 𝑃𝑃0 𝐶𝐶𝑝𝑝
1
𝑃𝑃 = 𝜌𝜌𝑎𝑎 𝜋𝜋𝑟𝑟 2 𝑣𝑣 3 𝐶𝐶𝑝𝑝 2
2

 The performance of the wind turbine depends on the wind speed and the rate of
rotation of the rotor.

 The Tip Speed Ratio 𝜆𝜆 refers to the ratio between the wind speed and the speed
of the tip of the wind turbine blades and is expressed as:
𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝑜𝑜𝑜𝑜 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝑡𝑡𝑡𝑡𝑡𝑡 𝑣𝑣𝑟𝑟 𝜔𝜔𝑟𝑟
𝑇𝑇𝑇𝑇𝑇𝑇 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅, 𝜆𝜆 = = =
𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝑣𝑣 𝑣𝑣
where,
𝑣𝑣 =the wind speed (m/s)
𝑣𝑣𝑟𝑟 =velocity of rotor tip (m/s)
𝑟𝑟 =rotor radius (m)
𝜔𝜔 =angular velocity (radian/s) and is given by 𝜔𝜔 = 2𝜋𝜋𝜋𝜋
𝑓𝑓 =frequency of rotation (Hz), sec-1.
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 COMPARISON BETWEEN
TURBINES
 The tip speed ratio is an important factor in designing the wind turbine.

 The rotor must rotate at an optimum speed to maximize its efficiency.

 If the turbine rotates slowly, it will not catch any wind.

 The wind will simply pass unperturbed through the gap between the blades.

 If the turbine rotates at a very high speed, it will behave like a solid wall to the wind
passage.

 Therefore, the turbine design must be optimized.

 Both the design of the blades (rotor airfoil profile) and number of blades play a critical
role in optimization of a turbine’s performance.

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 COMPARISON BETWEEN
TURBINES
 In general, a high tip speed ratio is desirable since it results in a high shaft
rotational speed, which in turn can increase the efficiency of the electrical
generator.

 The optimum value of tip speed ratio (𝜆𝜆𝑜𝑜𝑜𝑜𝑜𝑜 )can be approximated by the following
expression
𝜔𝜔𝑜𝑜𝑜𝑜𝑜𝑜 𝑟𝑟 2𝜋𝜋 𝑟𝑟
𝜆𝜆𝑜𝑜𝑜𝑜𝑜𝑜 ≈ ≈ (3)
𝑣𝑣 𝑛𝑛 𝐴𝐴 𝑇𝑇
𝑟𝑟
where 𝑛𝑛 is the number of blades. The ratio is generally 2, therefore,
𝐴𝐴𝑇𝑇

4𝜋𝜋
𝜆𝜆𝑜𝑜𝑜𝑜𝑜𝑜 ≈ (4)
𝑛𝑛

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