Wind Energy

Introduction to wind What is wind? Wind is simply air in motion. It is produced by the uneven heating of the Earth’s surface by energy from the sun. Since the Earth’s surface is made of different types of land and water, it absorbs the sun’s radiant energy at different rates. Much of this energy is converted into heat as it is absorbed by land areas, bodies of water, and the air over these formations.

Introduction to wind On the coast, for example, the land heats up more quickly than the water. The warm air over the land expands and rises, and the heavier, cooler air over the water rushes in to take its place, creating winds. In the same way, the large atmospheric winds that circle the Earth are produced because the Earth’s surface near the equator receives more of the sun’s energy than the surface near the North and South Poles.

Introduction to wind  All renewable energy (except tidal and geothermal power), ultimately comes from the sun  The earth receives 1.74 x watts of power (per hour) from the sun  About one or 2 percent of this energy is converted to wind energy (which is about times more than the energy converted to biomass by all plants on earth  Differential heating of the earth’s surface and atmosphere induces vertical and horizontal air currents that are affected by the earth’s rotation and contours of the land  WIND. ~ e.g.: Land Sea Breeze Cycle

Wind is called a renewable energy source because wind will continually be produced as long as the sun shines on the Earth. Today, wind energy is mainly used to generate electricity. Introduction to wind A weather vane, or wind vane, is a device used to monitor the direction of the wind. It is usually a rotating, arrow-shaped instrument mounted on a shaft high in the air. It is designed to point in the direction of the source of the wind. Wind direction is reported as the direction from which the wind blows, not the direction toward which the wind moves.

Wind velocity - measurement Wind speed can be measured with wind gauges and anemometers. One type of anemometer is a device with three arms that spin on top of a shaft. Each arm has a cup on its end. The cups catch the wind and spin the shaft. The harder the wind blows, the faster the shaft spins. A device inside counts the number of rotations per minute and converts that figure into miles per hour (mph). A display on the anemometer shows the speed of the wind.

Sample Power Curve of a Wind Turbine It is important to know how fast the wind is blowing. Wind speed is important because the amount of electricity that wind turbines can generate is determined in large part by wind speed, or velocity. A doubling of wind velocity from the low range to optimal range of a turbine can result in eight times the amount of power produced

Modern Wind Machines Today, wind is harnessed and converted into electricity using turbines called wind turbines. The amount of electricity that a turbine produces depends on  Size of the turbine  Speed of the wind All wind turbines have the same basic parts:  blades,  a tower, and  a gearbox. These parts work together to convert the wind’s kinetic energy into mechanical energy that generates electricity.

Modern Wind Machines 1. The moving air spins the turbine blades. 2. The blades are connected to a low- speed shaft. When the blades spin, the shaft turns. 3. The low-speed shaft is connected to a gearbox. Inside, a large slow-moving gear turns a small gear quickly. 4. The small gear turns another shaft at high speed. 5. The high-speed shaft is connected to a generator. As the shaft turns the generator, it produces electricity. 6. The electric current is sent through cables down the turbine tower to a transformer that changes the voltage of the current before it is sent out on transmission lines.

Classification of Wind Turbines A wind turbine operates by slowing down the wind and extracting a part of its energy in the process. Depending on the orientation of the axis of rotation of their rotors, Wind turbines can be classified into  Horizontal-axis turbines (HAWT): Rotor is kept horizontal and aligned parallel in the direction of the wind stream  Vertical-axis turbines (VAWT): Rotor is kept perpendicular to the direction of the wind stream

Classification of Wind Turbines Horizontal-axis turbine Vertical-axis turbine

HAWT advantages  The tall tower base allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten meters up the wind speed can increase by 20% and the 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 proposed airborne wind turbine designs, involve various types of reciprocating actions, requiring airfoil surfaces to backtrack against the wind for part of the cycle. Backtracking against the wind leads to inherently lower efficiency.

HAWT disadvantages  Massive tower construction is required to support the heavy blades, gearbox, and generator.  Components of a horizontal axis wind turbine (gearbox, rotor shaft and brake assembly) being lifted into position.  Their height makes them obtrusively visible across large areas, disrupting the appearance of the landscape and sometimes creating local opposition.  Downwind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower's wind shadow (for this reason, the majority of HAWTs use an upwind design, with the rotor facing the wind in front of the tower).  HAWTs require an additional yaw control mechanism to turn the blades toward the wind.  HAWTs generally require a braking or yawing device in high winds to stop the turbine from spinning and destroying or damaging itself.

VAWT advantages  No yaw mechanisms is needed.  A VAWT can be located nearer the ground, making it easier to maintain the moving parts.  VAWTs have lower wind startup speeds than the typical the HAWTs.  VAWTs may be built at locations where taller structures are prohibited.  VAWTs situated close to the ground can take advantage of locations where rooftops, mesas, hilltops, ridgelines, and passes funnel the wind and increase wind velocity.

VAWT disadvantages  Most VAWTs have a average decreased efficiency from a common HAWT, mainly because of the additional drag that they have as their blades rotate into the wind. Versions that reduce drag produce more energy, especially those that funnel wind into the collector area.  Having rotors located close to the ground where wind speeds are lower due and do not take advantage of higher wind speeds above.  Because VAWTs are not commonly deployed mainly due to the serious disadvantages mentioned above.

Classification of Wind Turbines Depending on the aerodynamic principle used, Wind turbines can be classified into  Lift type turbines : Wind rotate the rotor of a wind turbine by lifting the blades. High speed turbines utilize lift forces to move the blades.  Drag type turbines : Wind rotate the rotor of a wind turbine by simply passing against the blades. Slow speed turbines are generally drag type.

Types of rotors Different types of rotors used in wind turbines are  Multiblade type  Propeller type  Savonious type  Darrieus type

Multi-blade type rotor  It is fabricated from curved sheet metals blades.  The width of the blade increases outwards from the centre.  Blades are fixed at their inner ends on a circular rim  They are also welded to a circular rim at outer edge for stable support  Number of blades ranges from 12 to 18

Propeller type rotor  It has three (some times two) aerodynamic blades made from strong but light weight material eg:- fibre glass reinforced plastic  The diameter of rotor ranges from 2 to 25 m

Savonious type rotor A Savonius is a drag type turbine, they are commonly used in cases of high reliability in many things such as ventilation and anemometers. Because they are a drag type turbine they are less efficient than the common HAWT. Savonius are excellent in areas of turbulent wind and self starting.

Darrieus type rotor Darrieus wind turbines are commonly called "Eggbeater" turbines, because they look like a giant eggbeater. They have good efficiency, but produce large torque ripple and cyclic stress on the tower, which contributes to poor reliability. Also, they generally require some external power source, or an additional Savonius rotor, to start turning, because the starting torque is very low. The torque ripple is reduced by using three or more blades which results in a higher solidity for the rotor. Solidity is measured by blade area over the rotor area.