10-12-2012, 01:35 PM
Wind Power and Wind Turbines
Wind Power.ppt (Size: 2.01 MB / Downloads: 48)
Wind Turbine
Wind energy is created when the atmosphere is heated unevenly by the Sun, some patches of air become warmer than others. These warm patches of air rise, other air rushes in to replace them – thus, wind blows.
A wind turbine extracts energy from moving air by slowing the wind down, and transferring this energy into a spinning shaft, which usually turns a generator to produce electricity. The power in the wind that’s available for harvest depends on both the wind speed and the area that’s swept by the turbine blades.
Wind Turbine Design
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.
Power Generated by HWind Turbine
How much power a wind turbine with 50 meters long blade can generate with a wind speed of 12 m/s? The site of the installation is about 1000 feet above sea level.
Wind Turbine – Blade Design
Number of Blades
Increasing the number of blades from one to two yields a 6% increase in aerodynamic efficiency, whereas increasing the blade count from two to three yields only an additional 3%.
Further increasing the blade count yields minimal improvements in efficiency and sacrifices too much in blade stiffness (thinner blades) and cost.
Turbine with higher number of blades produces higher torque (rotational force) at low wind speed and that is the reason it is commonly used for doing mechanical work (e.g., water pumping), not for electric power generation.
Wind Turbine – Blade Design (Shape)
If the tip of the rotor blade moves through the air with a tip speed = 64 m/s, the speed at the centre of the hub is zero. 1/4 out from the hub, the speed will then be ~16 m/s.
The yellow ribbons close to the hub will be blown more towards the back of the turbine than the red ribbons at the tips of the blades.
Wind Turbine Operation
Yaw Control
Yaw control turns the turbine to face the wind.
It can be done passively for lower power turbines.
Yaw error means less captured power but too sensitive yawing causes mechanical wear with small power gain.
Some yaw brake slippage is desirable so when the turbine is in shut-down mode, turbine will be yawed passively by the wind force.
Low power turbines can yaw away from the wind as a way to shed excessive wind power.