In this post, you’ll learn what is wind turbine and how it works, and two different types of wind turbine.
Types of Wind Turbine
A wind turbine is a mechanical machine that converts the kinetic energy of the fast-moving winds into electrical energy.
On the basis of the axis of rotation of the blades.
Following are the two different types of wind turbine.
- Horizontal axis wind turbine (HAWT)
- Vertical axis wind turbine (VAWT).
The small turbines are used for applications such as battery charging for auxiliary power for boats or caravans or to power traffic warning signs. Slightly larger turbines can be used to contribute to the domestic electricity supply while selling unused power back to the utility supplier through the power grid.
Arrays of large turbines known as wind farms are becoming an important source of intermittent renewable energy. These are used by several countries as part of a strategy to reduce dependence on fossil fuels.
1. Horizontal Axis Wind Turbine Generator
In these types of a wind turbine the axis of rotation is horizontal and the aero turbine plane is vertically facing the wind. A common type of wind turbine with the horizontal axis is as shown in the figure.
Construction of Horizontal Axis Wind Turbine
Horizontal axis windmills have the rotor shaft and electric generator at the top of the tower and it is pointed in or out of the wind. Smaller turbines are indicated by a simple wind vane, while larger turbines use wind sensors coupled with an auxiliary motor. Most have a gearbox, which transforms the slow rotation of the blade into a faster rotation that is more suitable for driving an electric generator.
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Parts of Wind Turbine
Following are the parts of the wind turbine:
- The rotor
- A gearbox and coupling (transmission system)
- Electrical generator
- Supporting structure.
Lifting style wind turbine blades. These are designed most efficiently, to capture the energy of strong and fast winds. Some European companies manufacture single blade turbines.
2. The Rotor
The rotor is aerodynamically designed to occupy the maximum surface area of the wind to spin the most ergonomically. The blades are lightweight, durable and corrosion-resistant material. The best materials are composites of fibreglass and reinforced plastic.
A housing which contains all the components which are essential to operate the turbine efficiently is called a nacelle. It is mounted on top of a tower and includes gearboxes, low and high-speed shafts, generators, controllers and brakes. Wind vane and wind speed anemometer are mounted on the nacelle.
Nacelle provides housing for:
- Low-speed shaft
- High-speed shaft
- Wind vane.
4. A Gearbox and Coupling (Transmission System)
A gearbox magnifies or amplifies the energy output of the rotor. The gearbox is located between the rotor and the generator. A rotor rotates the generator as guided by the tail pane.
Aeroturbine converts wind energy into rotary mechanical energy.
The controller senses wind direction, wind speed power output of the generator rotor and other performance quantities of the system. And initiates proper control signals to take suitable corrective action.
7. Electrical Generator
This unit produces electricity from the rotation of the rotor. The generator comes in various sizes with respect to the output. This generator converts mechanical energy into electrical power. The output of the generator coupled to the load or system grid.
8. Supporting Structure
This is the heavy structure set up with a proper foundation and carries all the components of the windmill. It should be properly designed with a proper factor of safety to withstand a dead load of all components and wind force.
Working of Horizontal Axis Wind Turbine
As the wind blows, a wind turbine converts the kinetic energy of the wind’s motion into mechanical energy by the rotation of the rotor and this mechanical energy is transmitted by the shaft to the generator through the gear train. The generator, further converts this mechanical energy into electrical energy, thereby generating the electricity.
The wind passes over both surfaces of the airfoil-shaped blade but passes more rapidly over the longer (upper) side of the airfoil, thus creating a lower pressure area above the airfoil. The pressure differential between the top and bottom surfaces results in the aerodynamic lift.
As the blades of a wind turbine are constrained to move in a plane with the hub as the centre, the lift force causes rotation about the hub. In addition to the lift force, a drag force perpendicular to the lift force prevents rotor rotation.
The main objective in wind turbine design is to have a high lift-to-drag ratio for the blades. This ratio can vary with the length of the blade to optimize the energy output of the turbine at different wind speeds. The main rotor shaft and generator are placed in the box at the top of the array and everything is placed on top of a high column.
It is suitable for use in places where the wind factor is high because it is strong and manufactured to withstand strong forces. Disadvantages of horizontal axis wind turbines are that they require heavy construction and are often considered more unpleasant.
Types of Rotors for Horizontal Axis Wind Turbine
Following are the different types of rotors are used for horizontal axis wind turbines:
- Multiblade type rotor
- Sail type rotor
- Propeller type rotor
- Four blade type rotor.
Following figures show the type of blades used for HAWT.
Both multiblade and sail type mills run at speeds of 60 to 80 rpm. The propeller type has two or three aerofoil blades and runs at a speed of 300 to 400 rpm. These rotors have to face the direction of the wind in order to generate wind power. The four-blade type rotor is shown in the figure. This motor runs at low speed in the range of 100 to 150 rpm.
Advantages of Horizontal Axis Wind Turbine
Following are the advantages of a Horizontal Axis Wind Turbine:
- The tall tower base allows access to the stronger wind in sites with wind shear. In some wind shear sites, wind speeds can increase by 20% every ten meters and power output by 34%.
- High efficiency, since the blades always move perpendicularly to the wind, receiving power through the whole rotation. In contrast, all vertical axis wind turbines, and most advanced airborne and turbine designs involve a wide variety of reciprocating actions, requiring the airfoil surfaces to backtrack against the wind for part of the cycle. The Backtracking against the air naturally leads to lower efficiency.
Disadvantages of Horizontal Axis Wind Turbine
Following are the disadvantages of a Horizontal Axis Wind Turbine:
- Massive tower construction is required to support the heavy blades, generator and gearbox.
- Components of a horizontal axis wind turbine (gearbox, rotor shaft and brake assembly) being lifted into position.
- Their height makes them visible over large areas, disrupting the landscape’s appearance and sometimes causing local opposition.
- The downward variant suffers from fatigue and structural failure caused by turbulence when a blade passes through the wind shadows of the towers (for this reason, most of the HAWTs use an inverted design, with the rotor facing the tower in front of the wind).
- HAWT’s require an additional yaw control mechanism to turn the blades towards the wind.
- HAWTs require a braking or yawing device in high winds that prevent the turbine from spinning and destroying or damaging itself.
2. Vertical Axis Wind Turbine Generator
In these types of a wind turbine the axis of rotation is vertical. The sails or blades may also be vertical. Vertical axis wind turbines are a type of wind turbine where the main rotor shaft is set transverse to the wind (but not necessarily vertically) while the main components are located at the base of the turbine. This arrangement allows the generator and gearbox to be located close to the ground, facilitating service and repair.
Vertical axis wind turbine need to be pointed into the wind, which removes the need for wind sensing and orientation mechanisms.
The major disadvantage of early designs involved notable torque difference or ripple during each revolution and large bending moments on the blade. Later designs directed the torque ripple issue by sweeping the blades helically.
A vertical axis wind turbine works sideways, tipped sideways along the axis perpendicular to the wind streamlines. A more common term that includes this option is transverse axis wind turbine or cross-flow wind turbine.
Construction of Vertical Axis Wind Turbine
Mechanical and electrical components:
- Rotor (blades and connection items)
- Braking system
- Control system
1. Rotor (blades and connection items)
Its function is to produce aerodynamic efficiency. It is constituted of light but resistant material and it is provided with a protective layer.
2. Braking system
Its function is to stop the rotor motion in adverse weather conditions. It is constituted of disc brakes similar to those of cars and of friction brakes which function in case of failure of the electrical grid.
3. Control system
It regulates the function of the aerogenerator according to the different working conditions. It optimizes the function of the aerogenerator. It ensures safety and controls any further devices (anemometer).
It has to withstand the action of wind and the weight of the blades. It is generally tabular or trellis and the base can be either superfield or deep.
Working of Vertical Axis Wind Turbine
In a vertical axis wind turbine, the blades are aerofoil shape and it can take the wind from any directions. The rotor is not self-starting. The rotor is supported by a system of guy cables. The base of the wind turbine rotor is coupled to a generator by a drive flexible coupling. Here the blades rotate around a vertical axis.
Wind turbine captures the high-pressure wind and it causes the kinetic energy, this kinetic energy converted into mechanical energy and mechanical energy converted into electric power.
This generated electrical power is taken by the static frequency converter from the generator at a variable frequency (0 – 20 Hz) and supplies a constant frequency output (60 Hz) and feeds to the grid.
The static frequency converter is controlled by a microprocessor-based control system.
Types of Rotors for Vertical Axis Wind Turbine
Following are the different types of rotors are used for vertical axis wind turbines :
- Savonius type rotor
- Darrieus type rotor.
1. Savonius Type Rotor
The Savonius windmill consists of a hollow circular cylinder sliced in half, two halves being fixed to a vertical axis with a gap in between. Torque is produced by the pressure difference between the two sides of the half facing the wind. This design is efficient but requires a large surface area.
2. Darrieus Type Rotor
In contrast, the Darrieus type requires much less surface area. It is shaped like an egg beater and has two or three blades shaped like aerofoils. Both the Savonius and Darrieus types run independently of the direction of the wind because they rotate about the vertical axis. On the other hand, the horizontal axis machines have to face the direction of the wind in order to generate power.
Advantages of Vertical Wind Turbines
Following are the advantages of vertical wind turbines:
- More of these types of turbines can be installed on the same amount of land because as the wind passes through them not as much turbulence is created.
- Due to the orientation of these devices, the entire pole or shaft can be turned which makes it possible to mount the generator and other mechanisms at ground level. This makes maintenance work quite easier.
- VAWT’s don’t have to point into the wind to start up.
- Devices with this type of design are more strong than horizontally oriented units. Because they do not have to go in the air, they do not need a tail section to keep the rotor blade assembly on the airside. During gusty wind conditions, these units are less likely to sustain damage than traditionally designed units.
- They don’t need as much wind to generate power which means they can be installed closer to the ground.
- The vertical orientation of the blades often results in these devices being quieter than standard turbines.
- Since tall towers aren’t necessary, installation and maintenance costs are reduced.
Disadvantages of Vertical Wind Turbine
Following are the disadvantages of vertical wind turbines:
- While this design is less likely to be damaged by gusty wind conditions, they are more likely to stall out and stop spinning.
- These types of turbines aren’t typically well suited for use in areas of high wind speeds.
- Due to the vertically oriented blade design, the blades flex and turn faster and faster as the rotor assembly. The centrifugal force generated by the spinning blade has been reported to cause stress and fatigue on some blade designs which sometimes results in them breaking.
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