12 Components and Types of Wind Turbines

Dr. Dhanya M.S M.S

1. Introduction

The wind energy converted to mechanical or electrical energy to meet the various energy demands are done by wind energy conversion system. The most important component of WECS is wind turbine. This was synonym to the earlier term wind mill. Wind turbine system is essential to harness the wind energy exists in any location.

2. Basic Components of Wind Energy Conversion System

The main components of a wind energy conversion system for electricity (Fig 1) are

Aeroturbine Gearing

Coupling

Electrical generator Controller

The wind turbine converts energy in the wind to rotary mechanical energy. This is capable with the help of pitch control and yaw control for proper operation. The step up gear and a coupling constitutes mechanical interface. This helps in transmission of rotary mechanical energy to an electric generator and power output is connected to the load or power grid.

Yaw control is fixed in wind turbines in the areas where there is change in wind direction. A motor rotates the turbine slowly about the vertical axis so as to face the blades into the wind. The controller helps in sensing different parameters like wind speed, wind direction, shafts speed and torques at various points, power generated and temperature in the generator. It also senses the control signals for comparing electrical output to the wind energy input. This also protects the wind turbine system from extreme conditions (like strong winds, electrical faults, etc.).

3. Wind turbine

A wind turbine is a system that converts the kinetic energy available in the windinto mechanical or electrical energy.

Parts of a wind turbine system:

Foundation Tower

Nacelle Hub

Rotor

Drive -train Gearbox

Generator

Electronics & Controls Yaw

Pitch

Braking Cooling

In general the parts of a wind turbine system are grouped into

Rotor: The two types of rotors are Horizontal axis rotor and Vertical axis rotor. The vertical axis machines operate in all wind directions and no need of yaw adjustment.

Wind mill head or turbine consists of rotor, housing of the rotor bearings. It houses control mechanism incorporated like changing the pitch of the blades for safety devices and tail vane to orient the rotor to face the wind.

Transmission and control: The rate of rotation of large wind turbine generators operating at rated capacity or below is controlled by varying pitch of rotor blades. The transmission options include mechanical systems involving fixed ratio gears, belts and chains singly or in combinations or hydraulic systems involving fluid pumps and motors. Fixed ratio gears are recommended for top mounted equipment.

Supporting structures

The Fig 2 gives the detailed diagram of different parts of wind turbine.

Fig. 2 Diagram of different parts of wind turbine

Source http://pluslink.blogspot.in/2011/11/wind-turbine-design-construction.html

3.1 Rotor: The component of wind turbine that is rotating and helps in conversion of kinetic energy to mechanical energy. It consists of blades and hubs.

3.2 Hub: The function of rotor hub is to connect rotor blades to the rotor shaft. This component controls the power generation of the wind turbine.

3.3 Blades: This is a rotating component of the system. This component is based on the principle of lift and drag (principle of aerodynamics). It converts kinetic energy first to mechanical energy and then transferred through shaft to generator for converting into electrical energy.

Two or three blades are common in the wind turbines. The wind blown over the blades lift the blades and rotate it. The two bladed wind turbines have lighter hub and so the whole structure is lighter. But three bladed wind turbines are aerodynamically efficient and have low noise..

The length of the blade is the important parameter for estimation of wind power generation potential of a wind turbine. The torque increases with more number of blades.

The blades are commonly made from composites mainly fiberglass or glass reinforced plastics (GRP) or carbon fiber reinforced plastics (CFRP). Wooden or epoxy laminates are also used for making blades.

Solidity is defined as the ratio of the projected area of the rotor blades on the rotor plane to the swept area of the rotor. High- solidity rotors use force for rotation and these rotor turn slower. Low- solidity rotors have slender aerofoil and these rotors use lift force for rotation and turn faster.

Chord: It is the width of the blade which is across distance from one edge of the blade to the other edge.

3.4 Drive train: The drive train consists of low –speed shaft, gear box and a high speed shaft. It also has support bearings, couplings, brake and rotating parts of generator.

3.5 Nacelle: The covered part of the wind turbine system over the top of tower is nacelle. It houses gear box, low speed shaft and high-speed shaft, generator, controller, and brake. It has an important role in protection of components of wind turbine from the various weather conditions. It also helps in reduction of noise produced from the rotation of wind turbine.

3.6 Low-speed shaft: This shaft transfers torque from rotor to the drive train. It is a rotating part present on the rotor side of the turbine. The connection of this low speed shaft increase rotations per minute. The rotor weight is also supported by this shaft.

3.7 Gear box: The function of gear box is to step up the speed as per needed by the electric generator. The low speed shaft is connected to the high speed shaft with gears. It increases the rotational speed that is required for the generator to generate electricity. The increase in rotational speeds is of the order 30-60 rpm to 1000-1800 rpm. This part is very costly. Some of the types of gear boxes are Planetary Gear Boxes, Parallel shaft gear.

3.8 High-speed shaft: The speed and torque produced from the gear box are transmitted for driving the generator with high speed shaft. This is present on the generator side of the turbine.

3.9 Brake: This part is meant to stop the running of wind turbines during extreme weather conditions. The various types of brakes are Mechanical brake (Disc brake, clutch brake), Aerodynamic brake (Tip brake and spoilers)

3.10 Generator: The conversion of rotational energy to electrical energy is carried out by generator. In general the wind driven electric generator produces 50-cycle AC electricity. The types of generators are

• – Synchronous generator (Electrically excited, permanent magnet),

• – Asynchronous generator (SQIG -Squirrel cage induction generators, Slip ring)

3.11 Controller: The grid quality electric current is controlled by the controller of the turbine system. The controller starts up the machine at cut-in wind speed (generally 3 m/s) and shuts off the machine at cut-out wind speed (generally 25 m/s) as per the design requirement. The controller measures and controls parameters like Voltage, current, frequency, Temperature inside nacelle, Wind direction, Wind speed, The direction of yawing, shaft speed, Over-heating of the generator, Hydraulic pressure level, Correct valve function, Vibration level, Twisting of the power cable, Emergency brake circuit, Overheating of small electric motors for the yawing, hydraulic pumps, Brake caliper adjustment etc.

3.12 Anemometer: Anemometer is an instrument that measures wind speed. It gives the input to the controller for power regulation and braking beyond the cut out & survival wind speed. Generally the anemometer is fixed on top of wind turbine.

3.13 Pitch: The electricity production is controlled by pitch under different wind intensities. Blades are turned or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.

3.14 Tower: This helps to use the wind energy at sufficient heights above ground. This helps to absorb and securely discharge static and dynamic stress exerted on the rotor, the power train and the nacelle into the ground. The major types of towers used in wind turbine are Lattice tower, Tubular tower, Guyed tower, Hybrid Tower

The characteristics of different towers are given in Table 1.

The tubular towers are more popular among modern turbines because of their lower airflow interference and downstream turbulence creation apart from its more aesthetic acceptability.

The tubular steel tower is costly. The transportation and fabrication of lattice tower,three legged tower and guy wired tower are easy. But guy wired tower is suitable for small wind turbines.

Generally tower height is 1 to 1.5 times the rotor diameter and it is normally at 20m. The site characteristics have a great influence on tower selection. Tower stiffness is important factor in wind turbine system because of possibility of coupled vibrations between rotor and tower. Tower shadow is the wake created by air flow around a tower. There is an effect of tower shadow on turbine dynamics, power fluctuations and noise generation especially on downwind wind turbines.

3.15 Foundation: The foundation type of foundation has been chosen based on soil conditions and water table location present at the planned site of a wind turbine.

Onshore Foundation Types: Slab Foundation (preferred when the top soil is strong), Pile Foundation (Preferred when the top soil is of a softer quality) Offshore Foundation Types: Monopile, Gravity base, Tripod

3.16 Wind vane: This measures the wind direction. It provides information to the controller for orienting the turbine properly (yawing) with respect to the wind direction.

3.17 Yaw drive: The function of this is to keep the turbine aligned to the wind. It turns the nacelle with rotor as per the wind direction using a rotary actuator engaging on a gear ring beneath the nacelle. Two types of yaw drive systems are active yaw or free yaw systems. Active yaw drive mechanism is with yaw motors and is controlled by automatic yaw control system with its wind direction sensor mounted on nacelle in position when it is not in yawing. It is used commonly with upwind wind turbines. Free yaw systems can self-aligned with the wind and commonly used on downwind wind turbine systems.

3.18 Yaw motor: The yaw drive is powered by the yaw motor.

4. Classification of wind turbines

There are different types of turbine technology

Based on axis orientation:

• – Horizontal
• – Vertical

Based on Number of Blades

• – One
• – Two
• – Three
• – Multi-bladed. Based on Location
• – on-shore
• – Off-shore

Based on Power control

• – Stall
• – Variable Pitch
• – Controllable Aerodynamic
• Based on Surfaces / Yaw Control Yaw Orientation
• – Driven Yaw
• – Free Yaw
• – Fixed Yaw

Based on Rotor Position

• – Upwind
• – Downwind

Based on Transmission

-with gear

– without gear

Based on Type of Hub

• – Rigid
• – Teetered
• – Hinged blades
• – Gimbaled

Based on Generator Speed

• – Constant
• – Variable

The common classification is based on axis type.

Horizontal axis Wind turbine (HAWT) Vertical axis Wind turbine (VAWT)

4.1 Horizontal axis Wind turbine (HAWT)

The most common type of wind turbine is Horizontal axis Wind turbine (HAWT). The axis of the rotation of this wind turbine is parallel to the ground. In HAWT, the axis of blade rotation is also parallel to the wind flow. Commonly found horizontal axis wind mills are aero-turbine mill with 35% efficiency and farm mills with 15% efficiency. The main components in HAWT are.

Rotor with blades for the conversion of wind energy to rotational energy. It costs 20% of the wind turbine cost.

Generator component consists of electrical generator, the control systems and gearbox. It converts low speed rotational energy to high speed rotational energy needed for electricity generation. It contributes to 34 % of wind turbine cost.

Structural support includes tower and yaw mechanism and it costs 15% of the turbine cost. The fig 3 represents the main components of a HAWT.

Fig 3 Main components of Horizontal axis wind turbine system

The    HAWT    works     on    principles     of    aerodynamic     lift     of    wind    turbines     (Fig     4).

Source:https://colortheearthturquoise.files.wordpress.com/2012/03/horizontal-and-vertical-axis-wind-turbines-1.pdf

4.1.1 Classification of HAWT are based on

Rotor orientation

• – Upwind of the tower
• – Downwind of the tower Hub design
• – Rigid
• -Teetering
• – Pitch -Stall
• – Two blades -Three blades
• – Free yaw -Active yaw

The advantages of HAWT are

More table due to position of blades of the HAWT to the side of the turbine’s center of gravity.

Efficiency is more because of maximizing collection of wind energy by turbine by adjusting angle of attack .During storm rotor blades are able to pitch and reduce the damage Accessibility to strong wind is possible in HAWT (tall tower)

It is suitable for areas with wind shear and placement on uneven land or in offshore locations But it has disadvantages like

Facing difficulty in operating near the ground

Difficulty in transporting heavy and tall towers and long blades Requires a special installation procedure

It creates navigation problem at offshore sites

4.2 Vertical Axis Wind Turbine (VAWT):

The axis is parallel to the ground. The VAWT do not take advantage of the higher wind speeds at higher elevations above the ground. So it is less common than HAWT. The basic vertical axis designs are the

• – Darrieus wind turbine (Fig 7), which has curved blades and efficiency of 35%,
• – Giromill wind turbine (Fig 8), which has straight blades, and efficiency of 35%, and
• – Savonius wind turbine (Fig 9), which uses scoops to catch the wind and the efficiency of 30%. The fig .7 of Darrieus wind turbine explains how the VAWT works.

The advantages of VAWT are

• – No need ofwind turbine to orient with respect to wind direction.
• – Lighter weight
• – Easy maintenance because the shaft is vertical, the transmission and generator can be mounted at ground level.
• – Lower cost tower.

But VAWT is not as efficient at collecting energy from the wind in comparison to HAWT.

5. Parameters influencing wind energy conversion

The wind energy conversion depends on

optimum tower height, control systems,

number of blades and blade shape

6. Designing of wind turbine

The design of wind turbine are based on

• – number of blades
• – rotor orientation
• – blade material, construction method
• – hub design
• – power control
• – rotor speed (fixed or variable)
• – orientation of turbine
• – type of generator
• – gear box or direct drive generator

 The different parameters of wind turbine like solidity, blade tip speed ratio, rotational speed are also important.

Solidity = Projected area of blades/swept area

Example: A wind mill has rotor of 6m with 30 blades. Each blade has width of 0.39. Calculate solidity?

Solution:

Solidity = Projected area of blades/swept area

For a particular wind speed, there exists an optimum turbine tip speed for producing maximum output. The slow running multi bladed windturbine rotors operate with tip speed ratios in the range of 1-4 and fast runners use 5-7 as tip speed ratios. So it is important to match the rotation of wind turbine to the corresponding wind speed.

Example: Estimate λ if the rotational speed of the turbine is 20 rpm and diameter of 6m and wind speed of 4m/s?

Solution

7. Effect of Wind Turbine on Grid

The variable wind speed fluctuate the output voltage and power. It leads to

• – flicker effects,
• – voltage asymmetry,

This affects the power quality of the network or grid system.

Fixed-speed turbines produce a power pulsation emanating from the wind share over height. So variable-wind speed turbines are helpful as they are able to absorb short-term power fluctuations by using immediate storage of energy on the rotating drive train.

8. Controlling parameters for power generation of wind turbine

The process of controlling the velocity of the wind turbine is known as pitching. The speed of wind turbine is controlled by

• – changing the orientation of wind turbine,
• – pitch of the blades

This helps to modify the aerodynamics and efficiency of wind turbine.

Conclusion

The wind turbine technology is very advanced today for the extraction of energy from wind. This helps to provide a huge amount of power especially in areas with high wind speed. The designing of the wind turbine is to be done carefully in accordance with the required power output to improve the efficiency.