Wind energy conversion systems
Introduction
Wind energy can provide suitable solutions to the global climate change and energy crisis. The
utilization of wind power essentially eliminates emissions of CO 2, SO2, NOx and other harmful
wastes as in traditional coal-fuel power plants or radioactive wastes in nuclear power plants. By
further diversifying The energy supply, wind energy dramatically reduces the dependence on fossil
fuels that are subject to price and supply instability, thus strengthening global energy security. During
the recent three decades, tremendous growth in wind power has been seen all over the world. In 2009,
the global annual installed wind generation capacity reached a record-breaking37GW, bringing the
world total wind capacity to 158GW.As the most promising renewable, clean, and reliable energy
source, wind power is highly expected to take a much higher portion in power generation in the
comingdecades.
Wind generation
Wind energy is basically the kinetic energy of air moving over the earth’s surface. When the Sun
strikes the earth, it heats the soil near the surface that in turn warms the air lying above it. Warm air is
less dense than the cool air, therefore warm air rises inthe atmosphere and cool air flows in to take its
place and is itself heated. The rising warm air eventually cools and falls back to earth. The larger the
atmospheric pressure gradient, the higher the wind speed and thus, the greater the wind power that can
be captured from the wind by means of wind energy-converting machinery.
The wind is a by-product of solar energy. Approximately 2% of the sun's energy reaching the earth is converted into
wind energy.
It has been estimated that out of the total solar power received by the earth 2% is converted
windenergy. The available wind power that can be converted into other forms of energy is
approximately 1.26 x 109MW. Because this value represents 20 times the rate of the present global
energy consumption, wind energy in principle could meet entire energy needs of the world.
The generation and movement of wind are complicated due to a number of factors. Among them ,the
most important factors are uneven solar heating, the Coriolis effect due to the earth’s self-rotation, and
local geographicalconditions.
History of wind energy applications
The first known use of wind dates back 5,000 years to Egypt, where boats used sails to travel from shore to shore.
The first true windmill, a machine with vanes attached to an axis to produce circular motion, may have been
built as early as 2000 B.C.
, In ancient Babylon. By the 10th century A.D., windmills with wind-catching surfaces having 16 feet length
and 30 feet height were grinding grain in the areas in eastern Iran and Afghanistan.
The earliest written references to working wind machines in western world date from the 12th century.
These too were used for milling grain. It was not until a few hundred years later that windmills were modified
to pump water and reclaim much of Holland from the sea.
The first automatically operated wind turbine in the world was designed and built by Charles Brush in 1888. This
wind turbine was equipped with 144 cedar blades having a rotating diameter of 17 m. It generated a peak power
of 12 kW.
As a pioneering design for modern wind turbines, the Gedser wind turbine was built in Denmark in
the mid 1950s. Today, modern wind turbines in wind farms have typically three blades, operating at
relative high wind speeds for the power output up to several megawatts.
How wind turbines work
A wind turbine is a machine that converts kinetic energy of the moving air into mechanical energy which in
turns is converted into electrical energy by a generator. Wind passes over the blades, generating lift and
exerting a turning force. The rotating blades turn a shaft inside the nacelle. It extracts the energy from the
wind by transferring the thrusting force of the air passing through the turbine rotor into the rotor blades. All
the wind turbines work on two (or three) physical principles by which energy is extracted from the wind.
These principles are either: (i) Drag force or (ii) Lift force or
(iii) Combination of the two forces.
Drag Principle
Drag is a mechanical force. It is generated by the interaction and contact of a solid body with a fluid (liquid
or gas). It is the component of aerodynamic force parallel to the relative wind. One of the sources of drag is
the skin friction between the molecules of the air and the solid surface of the blade. Because the skin friction
is an interaction between a solid and a gas, the magnitude of the skin friction depends on properties of both
shape of the blade and air flow.
Drag devices are simple wind machines that use flat, curved or cup-shaped (unlike aerodynamic shapes of the
lift devices) blades to run the rotor. When the wind blows and drag devices comes in its way, the wind pushes
away the angularly curved blades out of its way to force its way through the blades and forces the blades and
the horizontal shaft to rotate.
Lift Principle
The Wind power plant (WPP) blades operate on the principle of lift. The longer path theory is based on the
Bernoulli’s principle states that as the speed of a fluid flow increase, its pressure decreases. Lift devices are
generally more efficient than drag devices. Unlike the drag devices, the WPP blades are shaped similar to the
aerofoil wings of an aeroplane. The rotor consists of the rotor blades and the hub placed upwar d of the tower
and the nacelle in most of the wind turbines. This is primarily done because the air current behind the tower is
, very irregular (turbulent). The rotor blades are aerofoils that act on the principle of lift similar to the wings of
an aircraft. The top surface of a blade aerofoil is more curved than the bottom surface. A cross section of a rotor
wing is shown in Figure.
When the air particles approach the leading edge of the blade, it travels either over or under the blade. Hence,
nearby particles split up at the leading edge, and then come back together at the trailing edge of the blade. The
air sliding along the upper surface of the wing moves faster than on the lower surface. Since the particle
travelling over the top goes a longer distance in the same amount of time, it must be travelling faster. This
means that the pressure will be lowest on the upper surface. This differential pressure creates a thrust force.
This lifting force is perpendicular to the direction of the wind and is converted into a mechanical torque. This
torque in turn makes the turbine rotor to rotate. It is then transformed to mechanical and electrical forms.
Therefore, the shape of the aerodynamic profile of the blade is the most crucial component of the WPP
performance.
Introduction
Wind energy can provide suitable solutions to the global climate change and energy crisis. The
utilization of wind power essentially eliminates emissions of CO 2, SO2, NOx and other harmful
wastes as in traditional coal-fuel power plants or radioactive wastes in nuclear power plants. By
further diversifying The energy supply, wind energy dramatically reduces the dependence on fossil
fuels that are subject to price and supply instability, thus strengthening global energy security. During
the recent three decades, tremendous growth in wind power has been seen all over the world. In 2009,
the global annual installed wind generation capacity reached a record-breaking37GW, bringing the
world total wind capacity to 158GW.As the most promising renewable, clean, and reliable energy
source, wind power is highly expected to take a much higher portion in power generation in the
comingdecades.
Wind generation
Wind energy is basically the kinetic energy of air moving over the earth’s surface. When the Sun
strikes the earth, it heats the soil near the surface that in turn warms the air lying above it. Warm air is
less dense than the cool air, therefore warm air rises inthe atmosphere and cool air flows in to take its
place and is itself heated. The rising warm air eventually cools and falls back to earth. The larger the
atmospheric pressure gradient, the higher the wind speed and thus, the greater the wind power that can
be captured from the wind by means of wind energy-converting machinery.
The wind is a by-product of solar energy. Approximately 2% of the sun's energy reaching the earth is converted into
wind energy.
It has been estimated that out of the total solar power received by the earth 2% is converted
windenergy. The available wind power that can be converted into other forms of energy is
approximately 1.26 x 109MW. Because this value represents 20 times the rate of the present global
energy consumption, wind energy in principle could meet entire energy needs of the world.
The generation and movement of wind are complicated due to a number of factors. Among them ,the
most important factors are uneven solar heating, the Coriolis effect due to the earth’s self-rotation, and
local geographicalconditions.
History of wind energy applications
The first known use of wind dates back 5,000 years to Egypt, where boats used sails to travel from shore to shore.
The first true windmill, a machine with vanes attached to an axis to produce circular motion, may have been
built as early as 2000 B.C.
, In ancient Babylon. By the 10th century A.D., windmills with wind-catching surfaces having 16 feet length
and 30 feet height were grinding grain in the areas in eastern Iran and Afghanistan.
The earliest written references to working wind machines in western world date from the 12th century.
These too were used for milling grain. It was not until a few hundred years later that windmills were modified
to pump water and reclaim much of Holland from the sea.
The first automatically operated wind turbine in the world was designed and built by Charles Brush in 1888. This
wind turbine was equipped with 144 cedar blades having a rotating diameter of 17 m. It generated a peak power
of 12 kW.
As a pioneering design for modern wind turbines, the Gedser wind turbine was built in Denmark in
the mid 1950s. Today, modern wind turbines in wind farms have typically three blades, operating at
relative high wind speeds for the power output up to several megawatts.
How wind turbines work
A wind turbine is a machine that converts kinetic energy of the moving air into mechanical energy which in
turns is converted into electrical energy by a generator. Wind passes over the blades, generating lift and
exerting a turning force. The rotating blades turn a shaft inside the nacelle. It extracts the energy from the
wind by transferring the thrusting force of the air passing through the turbine rotor into the rotor blades. All
the wind turbines work on two (or three) physical principles by which energy is extracted from the wind.
These principles are either: (i) Drag force or (ii) Lift force or
(iii) Combination of the two forces.
Drag Principle
Drag is a mechanical force. It is generated by the interaction and contact of a solid body with a fluid (liquid
or gas). It is the component of aerodynamic force parallel to the relative wind. One of the sources of drag is
the skin friction between the molecules of the air and the solid surface of the blade. Because the skin friction
is an interaction between a solid and a gas, the magnitude of the skin friction depends on properties of both
shape of the blade and air flow.
Drag devices are simple wind machines that use flat, curved or cup-shaped (unlike aerodynamic shapes of the
lift devices) blades to run the rotor. When the wind blows and drag devices comes in its way, the wind pushes
away the angularly curved blades out of its way to force its way through the blades and forces the blades and
the horizontal shaft to rotate.
Lift Principle
The Wind power plant (WPP) blades operate on the principle of lift. The longer path theory is based on the
Bernoulli’s principle states that as the speed of a fluid flow increase, its pressure decreases. Lift devices are
generally more efficient than drag devices. Unlike the drag devices, the WPP blades are shaped similar to the
aerofoil wings of an aeroplane. The rotor consists of the rotor blades and the hub placed upwar d of the tower
and the nacelle in most of the wind turbines. This is primarily done because the air current behind the tower is
, very irregular (turbulent). The rotor blades are aerofoils that act on the principle of lift similar to the wings of
an aircraft. The top surface of a blade aerofoil is more curved than the bottom surface. A cross section of a rotor
wing is shown in Figure.
When the air particles approach the leading edge of the blade, it travels either over or under the blade. Hence,
nearby particles split up at the leading edge, and then come back together at the trailing edge of the blade. The
air sliding along the upper surface of the wing moves faster than on the lower surface. Since the particle
travelling over the top goes a longer distance in the same amount of time, it must be travelling faster. This
means that the pressure will be lowest on the upper surface. This differential pressure creates a thrust force.
This lifting force is perpendicular to the direction of the wind and is converted into a mechanical torque. This
torque in turn makes the turbine rotor to rotate. It is then transformed to mechanical and electrical forms.
Therefore, the shape of the aerodynamic profile of the blade is the most crucial component of the WPP
performance.
