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WIND TECHNOLOGYLecture#09

Submitted By: Aitazaz Ahsan 10-ME-04

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Wind Turbine

• It is a device used to convert kinetic energy of air into mechanical energy which in turn produce electrical energy by using a generator coupled with rotor.

• A wind turbine obtains its power input by converting the force of the wind into a torque (turning force) acting on the rotor blades.

• The amount of energy which the wind transfers to the rotor depends on the density of the air, the rotor area, and the wind speed.

• The kinetic energy of a moving body is proportional to its mass (or weight). The kinetic energy in the wind thus depends on the density of the air, i.e. its mass per unit of volume.In other words, the "heavier" the air, the more energy is received by the turbine.

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Inside View of a Wind Turbine

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• A wind turbine extracts energy from moving air by slowing the wind down, and transferringthis energy into a spinning shaft, which usually turns a generator to produce electricity. Thepower in the wind that’s available for harvest depends on both the wind speed and the areathat’s swept by the turbine blades.

• Wind energy is the kinetic energy of air in motion, also called wind.

• Total wind energy flowing through an imaginary area A during the time t is:

• E =1/2 M v 2 = ½ (A v t ρ) v 2

• = ½ (A t ρ) v 3

• where ρ is the air density;

• v is the wind speed;

• Avt is the volume of air passing through A

• Avtρ is therefore the mass m passing per unit time. Note that ½ ρv2 is the kinetic energy of the moving air per unit volume.

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• Power is energy per unit time, so the wind power incident on A (e.g. equal to the rotor area of a wind turbine) is:

• P= E/t = ½ (A ρ) v 3

Rotor

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• Blades are connected to a hub, which is connected to a shaft

• Rotational speed will depend on blade geometry, number of blades, and wind speed (40 to400 revolutions per minute typical speed range)

• Gear box needed to increase speed to 1200-1800 RPM for generator.

• *The power in wind is proportional to the cubic wind speed ( v^3 ).

• ~ Kinetic energy of an air mass is proportional to v^2

• ~ Amount of air mass moving past a given point is proportional to wind velocity (v)

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TYPES OF WIND TURBINE

• Two types of turbine design: Horizontal axis and Vertical axis.

• Horizontal axis turbines: Can reach higher altitude wind but requires a substantial tower structure. Used in most modern wind turbine designs.

• Vertical axis turbines: No need to turn into wind (yaw), easier construction and maintenance (generator and gear box are on the ground) level, lower efficiency.

Horizontal axis Turbine

Vertical axis Turbine

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PARTS DESCRIPTION

• •The nacelle contains the key components of the wind turbine, including the gearbox, and the electrical generator.

• •The tower of the wind turbine carries the nacelle and the rotor. Generally, it is an advantage to have a high tower, since wind speeds increase farther away from the ground.

• •The rotor blades capture wind energy and transfer its power to the rotor hub.

• •The generator converts the mechanical energy of the rotating shaft to electrical energy

• •The gearbox increases the rotational speed of the shaft for the generator

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Betz Limit & Power Coefficient

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Lift & Drag Forces

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• Drag Design

• The wind literally pushes the blades out of the way.

• Slower rotational speeds and high torque capabilities. Useful for providing mechanical work (water pumping e.g.).

• Lift Design

• Blade is essentially an airfoil (like wings of airplanes).

• When air flows past the blade, a wind speed and pressure differential is created between the upper and lower blade surfaces. The pressure at the lower surface is greater and thus acts to "lift" the blade.

• The lift force is translated into rotational motion.

• Lift design generally has higher efficiency and is used in most modern turbines.

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Cp and Betz Limit

• The power in the wind is Pwind = ½ (ρ)(A)(V)3. The amount of power that can be captured by a turbine is only 35% to 45% of that amount (i.e., Cp = 0.35 ~ 0.45).

• The theoretical maximum for Cp is 0.593, i.e., the theoretical maximum efficiency of a turbine is 59.3%. This maximum efficiency is called Betz Limit.

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• Blade Angle

• The angle between the chord line of the blade and the wind direction (called angle of attack) has a large effect on the lift force (see figure below). Typically, maximum lift force is achieved with 1.0 to 15.0 degrees angle of attack.

• Relative Wind direction

• Wind direction relative to blade depends on wind speed and rotor speed.

Angle of

Attack

Wind

Lift

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Pitch Angle

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