Wind Energy Extraction

Wind is a large scale flow of gases, so it consists of the bulk movement of air which carries large amount of energy.  Energy has become very important matter with scarcity of nonrenewable energy sources. Fossil fuel will be over in near future, so world must have alternative energy sources to fulfill energy demand of fossil fuel. Wind becomes significant as a renewable energy source for extracting energy. Wind energy is extracted by using wind turbines to produce electrical power, windmills for mechanical power, wind pumps for water pumping or drainage, or sails to propel ships. In here important thing is power generation.There are large wind farms to generate electricity. A wind turbine is a device that converts kinetic energy from the wind into electrical power. A wind turbine used for charging batteries may be referred to as a wind charger .Generally two types of wind turbines are being used in wind farm, they are horizontal axis wind turbine and vertical axis wind turbine. 

Vertical axis wind turbine. 

Horizontal axis wind turbine 

Vertical-axis wind turbines (VAWTs) are the type of wind turbine where the main rotor shaft is set vertically and the main components have been  installed  at the base of the turbine. This arrangement allows the generator and gearbox to be located close to the ground, facilitating service and repair. VAWTs do not need to be pointed into the wind, which removes the need for wind-sensing and orientation mechanisms

Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.

Horizontal axis wind turbines are widely used than vertical axis wind turbine. Lift force is used to rotate horizontal axis wind turbines and drag force is used to rotate vertical axis wind turbines. There are some advantages and also disadvantages for both types of turbines.

Advantages and disadvantages of vertical axis wind turbine 

Advantages

• They can produce electricity in any wind direction
• Strong supporting tower in not needed because generator, gearbox and other components are placed on the ground
• Low production cost as compared to horizontal axis wind turbine
• As there is no need of pointing turbine in wind direction to be efficient so yaw drive and pitch mechanism is not needed
• Easy installation as compared to other wind turbine
• Easy to transport from one place to other
• Less maintenance cost
• They can be installed in urban area
• Low risk for human and birds because blades moves at relatively low speed
• They are particularly suitable for areas with extreme weather conditions, like in the mountains where they can supply electricity to mountain huts.

Disadvantages

• As only one blade of wind turbine work at a time so efficiency is very low
• They need an initial push to start, this action use few of its own produce electricity
• When compared to horizontal axis wind turbine they are very less efficient with respect to  them.  this is because they have an additional drag when their blades rotates.
• They have relative high vibration because the air flow near the ground creates turbulent flow
• Because of vibration bearing wear increase which result in the increase of maintenance cost
• They create noise pollution

Advantages and disadvantages of horizontal axis wind turbine

Advantages

• Variable blade pitch, which gives the turbine blades the optimum angle of attack. Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.
• 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 moves 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.

Disadvantages

• Taller masts and blades are more difficult to transport and install. Transportation and installation can now cost 20% of equipment costs.
• Stronger tower construction is required to support the heavy blades, gearbox, and generator.
• Reflections from tall Horizontal Axis Wind Turbine (HAWT)s may affect side lobes of radar installations creating signal clutter, although filtering can suppress it.
• Mast height can make 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).
• They require an additional yaw control mechanism to turn the blades toward the wind.

Wind turbines are operated on a simple principle. Same process is used to generate electricity from both turbine types. For horizontal axis turbine, the energy in the wind turns two or three propeller-like blades around a rotor. The rotor is connected to the main shaft, which spins a generator to create electricity. Wind turbines are mounted on a tower to capture the most energy. At 100 feet (30 meters) or more above ground, they can take advantage of faster and less turbulent wind. Wind turbines can be used to produce electricity for a single home or building, or they can be connected to an electricity grid (shown here) for more widespread electricity distribution. The process can be demonstrated as following manner. 

Process involved with conversion of kinetic energy of wind into electricity

When considering power demand wind energy is very important and reliable renewable energy source for world. Usage of this sources minimize air pollution , green house effect, environmental pollution etc .