Book: Wind energy explained
Chapter 1: Introduction: Modern wind energy and its origins
Today, the most common design of wind turbine is the horizontal axis wind turbine (HAWT).
HAWT rotors are usually classified according to:
The rotor orientation: upwind or downwind of
the tower
Hub design: rigid or teetering
Rotor control: pitch vs. stall
Number of blades: usually two or three blades
Alignment with the wind: free yaw or active yaw
The principal subsystems of a typical (land-based) horizontal axis wind turbine include:
The rotor = consists of the hub and blades of the wind
turbine.
The drive train = consists of the other rotating parts of the
wind turbine downstream of the motor.
The nacelle and main frame = the main frame provides the
mounting and proper alignment of the drive train
components. The nacelle cover protects the contents from
the weather.
The tower and the foundation = tower height is typically 1
to 1.5 times the rotor diameter, but at least 20 m.
The machine controls
o Sensors
o Controllers
o Power amplifiers
o Actuators
o Intelligence
The balance of electrical system
The main option in wind turbine design and construction:
Number of blades
Rotor orientation
Blade material
Hub design
Power control via aerodynamic control or
variable-pitch blades
Fixed or variable rotor speed
Orientation by self-aligning action, or direct control
Synchronous or induction generator
Gearbox or direct drive generator
Power output prediction = power output varies with wind speed and every wind turbine has a
characteristic power performance curve, which makes prediction possible.
1
,The performance of a given wind turbine generator can be related to 3 key points on the velocity
scale:
Cut-in speed = the minimum wind at which the machine will deliver useful power
Rated wind speed = the wind speed at which the rated power (generally the maximum
power output of the electrical generator) is reached
Cut-out speed = the maximum wind speed at which the turbine is allowed to deliver power
(usually limited by engineering design and safety constraints.
3 basic rules:
The speed of the blade tips is ideally proportional to the speed of wind
The maximum torque is proportional to the speed of wind squared
The maximum power is proportional to the speed of wind cubed
2
,Chapter 2: Wind characteristics and resources
Overall global patterns:
Global winds are caused by pressure differences across the earth’s surface due to the uneven
heating of the earth by solar radiation.
The spatial variations in heat transfer to the earth’s atmosphere create variations in the
atmospheric pressure field that cause air to move from high to low pressure. There is a
pressure gradient force in the vertical direction, but this is usually cancelled by the
downward gravitational force.
Thus, the winds blow predominately in the horizontal plane, responding to horizontal
pressure gradients.
At the same time, there are forces that strive to mix the different temperature and pressure
air masses distributed across the earth’s surface.
In addition to the pressure gradient and gravitational forces, inertia of the air, the earth’s
rotation, and friction with the earth’s surface, affect the atmospheric winds.
Mechanics of atmosphere’s wind motion (simple model):
Pressure forces
Coriolis force = caused by the rotation of the earth
Inertial forces = due to large-scale circular motion
Frictional forces = at the earth’s surface
Geostrophic wind = resultant of pressure forces and Coriolis force. Tends to be parallel to isobars.
This is an idealized case since the presence of areas high and low pressure causes the isobars to be
curved.
Gradient wind = resulting wind from further force on the wind, a centrifugal force.
Different surfaces of the Earth can affect the flow of air due to variations in pressure fields, the
absorption of solar radiation, and the amount of moisture available.
Smaller scale atmospheric circulation can be divided into:
Secondary circulation = occurs if the centers of high or low pressure are caused by heating or
cooling of the lower atmosphere.
o Hurricanes
o Monsoon circulation
o Extratropical cyclones
Tertiary circulation = small-scale, local circulations characterized by local winds.
o Land and sea breezes
o Valley and mountain winds
o Monsoon-like flow
o Foehn winds
o Thunderstorms
3
, o Tornadoes
Atmospheric motions vary in both time and space. Space variations are generally dependent on
height above the ground and global and local geographical conditions.
Variations in time
Inter-annual = occur over time scales greater than one year.
Annual = significant variations in seasonal or monthly averaged wind speeds are common
over most of the world.
Diurnal = time of the day. Due to differential heating of the earth’s surface during the daily
radiation cycle. A typical diurnal variation is an increase in wind speed during the day with
the wind speeds lowest during the hours from midnight to sunrise. Furthermore, the diurnal
variation in wind speed may vary with location and altitude above sea level.
Short-term (including gusts and turbulence) = usually mean variations over time intervals of
ten minutes or less. Turbulence can be thought of as random wind speed fluctuations
imposed on the mean wind speed. These fluctuations occur in 3 directions:
o Longitudinal = in the direction of the wind
o Lateral = perpendicular to the average wind
o Vertical
A gust is a discrete even within a turbulent wind field, and characterized by:
o Amplitude
o Rise time
o Maximum gust variation
o Lapse time
Variations due to location = wind speed is also very dependent on local topographical and ground
cover variations.
Variations in wind direction = wind direction also varies over the same scales over which wind
speeds vary. Short-term direction variation are the result of the turbulent nature of the wind.
Short-term variations in wind direction and the associated motion affect the fatigue life of
components such as blades and yaw drives.
Wind power per unit area, or wind power density:
The wind power density is proportional to the density of air.
Power from the wind is proportional to the area swept by the roto.
The wind power density is proportional to the cube of the wind velocity.
In practice, a maximum of about 45% of the available wind power is harvested by the best modern
horizontal axis wind turbines.
The maximum power-producing potential that can be theoretically realized from the kinetic energy
contained in the wind is 60% of the available power.
4
Chapter 1: Introduction: Modern wind energy and its origins
Today, the most common design of wind turbine is the horizontal axis wind turbine (HAWT).
HAWT rotors are usually classified according to:
The rotor orientation: upwind or downwind of
the tower
Hub design: rigid or teetering
Rotor control: pitch vs. stall
Number of blades: usually two or three blades
Alignment with the wind: free yaw or active yaw
The principal subsystems of a typical (land-based) horizontal axis wind turbine include:
The rotor = consists of the hub and blades of the wind
turbine.
The drive train = consists of the other rotating parts of the
wind turbine downstream of the motor.
The nacelle and main frame = the main frame provides the
mounting and proper alignment of the drive train
components. The nacelle cover protects the contents from
the weather.
The tower and the foundation = tower height is typically 1
to 1.5 times the rotor diameter, but at least 20 m.
The machine controls
o Sensors
o Controllers
o Power amplifiers
o Actuators
o Intelligence
The balance of electrical system
The main option in wind turbine design and construction:
Number of blades
Rotor orientation
Blade material
Hub design
Power control via aerodynamic control or
variable-pitch blades
Fixed or variable rotor speed
Orientation by self-aligning action, or direct control
Synchronous or induction generator
Gearbox or direct drive generator
Power output prediction = power output varies with wind speed and every wind turbine has a
characteristic power performance curve, which makes prediction possible.
1
,The performance of a given wind turbine generator can be related to 3 key points on the velocity
scale:
Cut-in speed = the minimum wind at which the machine will deliver useful power
Rated wind speed = the wind speed at which the rated power (generally the maximum
power output of the electrical generator) is reached
Cut-out speed = the maximum wind speed at which the turbine is allowed to deliver power
(usually limited by engineering design and safety constraints.
3 basic rules:
The speed of the blade tips is ideally proportional to the speed of wind
The maximum torque is proportional to the speed of wind squared
The maximum power is proportional to the speed of wind cubed
2
,Chapter 2: Wind characteristics and resources
Overall global patterns:
Global winds are caused by pressure differences across the earth’s surface due to the uneven
heating of the earth by solar radiation.
The spatial variations in heat transfer to the earth’s atmosphere create variations in the
atmospheric pressure field that cause air to move from high to low pressure. There is a
pressure gradient force in the vertical direction, but this is usually cancelled by the
downward gravitational force.
Thus, the winds blow predominately in the horizontal plane, responding to horizontal
pressure gradients.
At the same time, there are forces that strive to mix the different temperature and pressure
air masses distributed across the earth’s surface.
In addition to the pressure gradient and gravitational forces, inertia of the air, the earth’s
rotation, and friction with the earth’s surface, affect the atmospheric winds.
Mechanics of atmosphere’s wind motion (simple model):
Pressure forces
Coriolis force = caused by the rotation of the earth
Inertial forces = due to large-scale circular motion
Frictional forces = at the earth’s surface
Geostrophic wind = resultant of pressure forces and Coriolis force. Tends to be parallel to isobars.
This is an idealized case since the presence of areas high and low pressure causes the isobars to be
curved.
Gradient wind = resulting wind from further force on the wind, a centrifugal force.
Different surfaces of the Earth can affect the flow of air due to variations in pressure fields, the
absorption of solar radiation, and the amount of moisture available.
Smaller scale atmospheric circulation can be divided into:
Secondary circulation = occurs if the centers of high or low pressure are caused by heating or
cooling of the lower atmosphere.
o Hurricanes
o Monsoon circulation
o Extratropical cyclones
Tertiary circulation = small-scale, local circulations characterized by local winds.
o Land and sea breezes
o Valley and mountain winds
o Monsoon-like flow
o Foehn winds
o Thunderstorms
3
, o Tornadoes
Atmospheric motions vary in both time and space. Space variations are generally dependent on
height above the ground and global and local geographical conditions.
Variations in time
Inter-annual = occur over time scales greater than one year.
Annual = significant variations in seasonal or monthly averaged wind speeds are common
over most of the world.
Diurnal = time of the day. Due to differential heating of the earth’s surface during the daily
radiation cycle. A typical diurnal variation is an increase in wind speed during the day with
the wind speeds lowest during the hours from midnight to sunrise. Furthermore, the diurnal
variation in wind speed may vary with location and altitude above sea level.
Short-term (including gusts and turbulence) = usually mean variations over time intervals of
ten minutes or less. Turbulence can be thought of as random wind speed fluctuations
imposed on the mean wind speed. These fluctuations occur in 3 directions:
o Longitudinal = in the direction of the wind
o Lateral = perpendicular to the average wind
o Vertical
A gust is a discrete even within a turbulent wind field, and characterized by:
o Amplitude
o Rise time
o Maximum gust variation
o Lapse time
Variations due to location = wind speed is also very dependent on local topographical and ground
cover variations.
Variations in wind direction = wind direction also varies over the same scales over which wind
speeds vary. Short-term direction variation are the result of the turbulent nature of the wind.
Short-term variations in wind direction and the associated motion affect the fatigue life of
components such as blades and yaw drives.
Wind power per unit area, or wind power density:
The wind power density is proportional to the density of air.
Power from the wind is proportional to the area swept by the roto.
The wind power density is proportional to the cube of the wind velocity.
In practice, a maximum of about 45% of the available wind power is harvested by the best modern
horizontal axis wind turbines.
The maximum power-producing potential that can be theoretically realized from the kinetic energy
contained in the wind is 60% of the available power.
4
