Chapter 5 –Wind Energy

Chapter 5 –Wind Energy

Solar radiations are absorbed by earth and atmosphere. A temperature difference is created in air layers which produce air motion or wind. Weather conditions and topography of the place affects motion of air and wind production.

5.1 Fundamentals of Wind Energy

Wind is consisted of air. Moving air has mass and velocity, hence wind has kinetic energy. When a rotor is placed in area of wind the kinetic energy of wind is transferred to the rotor. The efficiency of wind energy harvesting depends on the fact that how efficiently rotor interacts with wind.

5.1.1 Power available from wind

The kinetic energy of wind is

K = ½ mv²,        m= mass of air,  v= speed of air                           

Therefore power (energy/time) is

P = K/t

   = ½ mv²/t

   = ½ rVv²/t,    r=density of air,  V=volume of air in contact of rotor

   = ½ r(Al)v²/t,  A= area of rotor perpendicular to wind flow, l= thickness of air layer

   = ½ rAv³, where l/t=v

Hence power available from wind depends on

·       Density of air (r)

·       Surface area of rotor perpendicular to wind flow (A)/diameter of rotor

·       Speed of air (v)

         Speed of air is most effective in determining the power as it occurs with cube in the equation. Density of air depends on

·          Temperature

·          Atmospheric pressure

·           Elevation

·          Air constituents

Speed of wind increases with height.

5.1.2 Theoretical and available efficiency

The maximum theoretical efficiency h_max is the ratio of maximum output power to total power available in the wind. It is also known as Power coefficient, C_p , which is equal to 0.593. The factor 0.593 is known as the Bitz limit. The theoretical efficiency is further limited by mechanical efficiency of the various parts of wind mill. If the efficiency of wind mill is 60%, including all the parts, the available efficiency will be 0.6 X 0.593 = 35.5%.

5.1.3 Capacity Factor (CF)

It is defined as average power output during a period/rated power output. It is also known as Wind Turbine Capacity Factor (WTCF).

5.2 Aerodynamic Operation involved in Wind Turbines

Aerodynamics deals with the movement of solid bodies through air. The rotor of wind turbine is made up of blades. The cross-section of blades is called airfoil. One (top) surface of blade has curvature than other. As the air passes through the upper side of blade, it covers longer distance than lower part. Hence a pressure difference is created between upper and lower surface. Air pressure is low on upper surface than lower surface. It helps the blade to lift in upward direction. This causes the rotor to rotate about an axis. Aerodynamics is consisted of two operations, drag and lift. Drag is the resistance experienced by an object when a fluid moves over it. The force exerted by fluid on the object in the direction of motion of fluid is known as drag force. Hence drag force is a negative factor during the motion of body in fluid. However, in some cases drag force is necessary for safety point of view. For example, brakes applied in automobiles and safe landing with a parachute.

The force applied by fluid on any object in a direction perpendicular to fluid motion is the lift force. It causes the body to lift in upward direction. In case of wind turbine, blade of rotor is struck by wind at an angle, experiencing both lift and drag force. Streamline object experiences less drag force.

5.3 Types of Land for Wind Energy

On the basis of wind climate the earth can be divided into several categories.

5.3.1 Regions

·       The Tropic: Tropical regions are at 30° North and South of the Equator. These are high pressure belts.

·       The Equator: This is low pressure region due to high temperature.

5.3.2 Areas

·       Open seas: It has high wind potential (offshore).

·       Coastal areas: Coastal area experiences stronger wind than other land area.

·       Hills: Rounded hills and ridges have higher acceleration of wind, depending on the height and its slope profile.

·       Valley: For deep valley, ridge is better.

·       Terrace: Aerodynamically it is an ideal place for wind turbine generators.

·       Saddle: Shallow dip between two mountains is called saddle. Ideal place of wind is where the slope just starts.

5.3.3 Khals (Low Depressions)

Low depression saddles with water divides or river valleys have good potentials for wind energy. These types of khals are found in Garhwal Himalayas.

5.4 Wind Turbines

5.4.1 Classification of wind turbines

Depending on the direction of axis about which rotor rotates, wind turbines are classified in two categories.

·       Horizontal-axis turbine

·       Vertical-axis turbine

5.4.2 Types of rotors

·       Multiblade rotor (horizontal axis)

It is comprised of 12 to 18 blades, made up of curved metal sheets. The width of blades increases outwards from the centre.

·       Propeller rotor (horizontal axis)

It has 2 to 3 aerodynamic blades, made up of strong and light weight materials. It has diameter of 2m to 25 m.

·       Savonious rotor (Vertical axis)

It has two identical hollow semi-cylinders fixed to a vertical axis.

·       Darrieus rotor (Vertical axis)

This rotor has 2 to 3 thin curved blades of flexible metal strips. Both the ends of blades are attached with vertical shaft.

5.4.3 Parts of a horizontal-axis wind turbine generator

Due to wind energy, blades of rotor rotates, which makes the shaft to rotate. This shaft is attached to generator by gear and coupling mechanism, kept inside a nacelle. An assembly is provided, which links the tower with the nacelle to permit its rotation about vertical axis to keep the rotor in wind direction. The whole system has following parts:

·       Blades: Blades are fabricated by lightweight strong material like glass fibre reinforced polyester in shape to fulfil aerodynamic process.

·       Nacelle: It is a box containing shaft, gear box hydraulic system, generator and yawing mechanism. Nacelle is placed at the top of the tower and is linked with the rotor.

·       Power transmission system: Mechanical power generated by rotor blades is transferred to the generator by two-stage gear box. From the gear box energy is transmitted to shaft which passes to generator. Gear box is provided to increase the generator speed to 1500 rpm.

·       Generator: This is used to produce electricity. Depending on the application small, medium or large generators are used.

·       Yaw control: Yawing is done by two yawing motors. Yaw control continuously tracks and keeps the rotor axis in the wind direction. During high speed wind, more than the cut-out speed, the machine is stopped by turning the rotor axis at right angles to the wind direction.

·       Brakes: It is used for an emergency stop by activating hydraulic disc brakes.

·       Controllers: The whole system is monitored by a micro-processor based control unit.

·       Tower: The whole wind turbine generator is installed on tower. The height of tower is decided by available wind speed.

5.4.4 Limitation of wind speeds

There are three types of wind speeds, which are characteristics for a given wind turbine generator.

·          Cut-in-speed (V_in): It is the minimum wind speed (4 m/s) at which turbine output begins.

·          Rated speed (V_full): It is the wind speed at which turbine is designed to give rated power.

·          Cut-out-speed (V_out): For safety of turbine, at higher speed (25 m/s) generator is stopped to produce power. The upper limit of wind speed is known as cut-out-speed.

5.4.5 Regulating system for rotor

As the direction and speed of wind changes with time, a regulating system is needed to ensure maximum production of power by wind turbine. This is done by adopting two different methods for wind electric generators (WEGs). First is stall regulated and second is pitch regulated.

·          Stall regulated: In this case blades are fixed to the rotor at an optimum pitch angle with suitably designed blade profile and thickness. Pitch angle remains constant at all wind speed. At larger wind speed, shaft experiences less torque and therefore less power is produced. The rotors are stopped mechanically or hydraulically in stall regulated system at high wind speed.

·          Pitch regulated: In this case, blades can be rotated about the length of the blade axis. Thus the angle (pitch angle) made between blade chord and plane of the rotation of blade may be changed. In pitch regulated system, output power of WEG remains constant with wind speed.

5.5 Modes of Wind Power Generation

The wind power can be generated in dispersed plant and may be used in different way.  As it is not a steady source of power, it has to be combined with some other energy options for backup power. On the basis of applications wind power generation is categorized in three options.

5.5.1 Standalone mode

            This type of wind generator is established in an area where conventional electricity transmission is not available. This type of wind generators are installed to feed local community. The power from wind electric generator is used for charging battery, which is attached to an inverter to supply power for domestic, commercial, hospital, telephone exchange or any other purpose. Battery may supply power for few more hours even in no wind condition. These are small power plants of capacity about 5 kW.

5.5.2 Backup mode like wind-diesel

For continuous supply of electricity, a backup production is needed together with wind electric generator, as the wind flow is not continuous. Diesel generator may be added with wind generators to provide electricity in no wind condition.

5.5.3 Grid connected wind turbine generators

            If large plant of wind electric generators is installed, the remaining power after being utilized in local community may be fed to electric grid (11 kV) using step-up transformer.

5.6 Interconnection between wind turbine generator and grid

With changing scenario of global world energy, two emerging features are; production of electricity through renewable energy sources and feeding of this electricity to the national electricity grid. For utilization of maximum energy produced by renewable resources, the feeding process to grid should also be efficient. As the voltage and frequency of electricity produced by any dispersed plant may be different from the grid, it should be processed before feeding to the grid. This is known as interfacing.

5.6.1 Wind Farm

In wind farm, many WEGs are interconnected to give high power output. In a typical 10 MW wind energy plant 50 WEGs are installed in many rows. Installation of individual WEG is done in such a manner that distance between two turbines in a row is 5 times of the rotor diameter and distance between two rows is 10 times of the rotor diameter.

5.6.2 Interface issues between WEGs and grid

            Wind turbine generators may produce variable output in terms of voltage and frequency due to variation in wind speed. This output cannot be interfaced with grid directly, as its parameters are different from the grid. While connecting the WEGs output with grid, following parameters must be regulated:

·       Reactive Power Compensation: In wind farm many WEGs are installed. All these are induction type generators, which need reactive power for magnetising. For reactive power requirements, shunt capacitors are provided. When connected with grid, WEGs draw reactive power with grid.

·       Voltage Regulation: The voltage at interface point of WEGs and grid should be same.

·       Frequency control: Frequency of output may change from 50 Hz due to gusting wind and it also affects the output power.

5.6.3 Power electronic interface: To adopt wind energy as powerful source of renewable energy option, its production at larger level will become effective only when output power will be available at grid. For high efficiency of whole system Power electronic interface is used. If WEGs are connected directly to the grid, the fluctuation of voltage and frequency in output of WEGs will be transferred to grid and vice versa. To remove this problem a control system is needed. With the advancements in semiconductor devices this is achieved by power electronic interface system. It controls frequency, voltage, active and reactive power in WEGs.

5.6.4 Grid connection topologies

            For interfacing WEGs with grid, two broad classifications are; fixed speed turbines and variable speed turbines.

·       Fixed speed turbines: In this case WEG can be linked to grid directly. Reactive power compensator (shunt capacitor) and step-up transformer together with gear box and soft start are required in this case. The generator required in this case may be;

i.                  Squirrel cage/wound rotor induction machine.

ii.               Wound rotor induction machine with rectifier control in the rotor.

·       Variable speed turbines: These WEGs cannot be connected with grid directly. They are interfaced with power electronic system. Because of variation in frequency, output power is first changed into DC and then using inverter it is again changed into AC. After that step-up transformer is used before it gets connected with grid. In this whole system power electronic is used. In this case generators required are;

i.                  Wound rotor induction machine with thyristor/IGBT bridge on the rotor for reduced converter size.

ii.               Permanent magnet synchronous machine with thyristor bridge and permanent magnet/wound rotor machine with IGBT bridge on the stator for full converter size.

5.6.5 Microprocessor-based control system for wind farms

             Large wind farms need accurate and fast control over many parameters simultaneously. Hence it require more powerful controller i.e., microprocessor. It is computer based control system, which interface between generator and grid through control over power (active and reactive), voltage, frequency and load. Simultaneously it also controls speed, yaw, pitch and brake in turbine through sensing wind speed. Hence it offers an optimum output to end users.