Making Watts from Wind

Friday, September 09, 2012
Avinash Jha
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“Wind is free, but it doesn’t imply that generating electricity from wind is free as well.” To understand how electricity is generated from wind, it is necessary to briefly consider some basic underlying mechanism and technological inputs. Wind turbine turns kinetic energy of the wind into electrical power. The rotor blades of the wind turbine are similar to the propeller blades of an airplane.

The rotor blades generate lift from the passing wind, causing them to rotate the hub of the turbine. The gearbox connects the low speed shaft to the high speed shaft and increases the rotational speed. This rotating action then turns the generator on, creating an electromagnetic field, thus generating electricity. This power is then either fed into an electric grid or stored in batteries for use on-site. In off-grid, very small systems, the DC appliances can be directly operated off the batteries. And for other standard appliances that require conventional AC, an inverter is used to convert DC to AC. For grid-connected systems, only power-conditioning unit (i.e. inverter) is enough for making power output compatible with the utility grid- no batteries are needed.

Turbine power output is controlled by rotating the blade around long axis to change their angle of attack with respect to the relative wind as the blades spins around the rotor hub. This mechanism is called blade pitch control. The turbine is pointed into the wind by rotating the nacelle around the tower. This is called yaw control. In modern wind turbines wind sensors on the nacelle tell the yaw controller in which direction to point the turbine, to extract maximum available power from the wind. Generally, wind turbines starts producing power at wind speed of 3 m/sec (cut-in speed). The turbine delivers maximum power near 12-14 m/sec (rated speed) and stops delivering power at 26 m/sec (cut-out speed).

The power available in the wind is proportional to the cube of its speed. That means 10% increase in wind speed, increases output power by 33%. One easy way to get higher wind speed is simply by going up. Wind higher above the ground is stronger and steadier than wind near the ground.

Modern wind turbines come in two varieties: horizontal axis and vertical axis. In horizontal axis turbines blades spins on the axis that is parallel to the ground, while in vertical axis turbine it is perpendicular to the ground. Most utility-scale turbines have upwind configurations, meaning their blades operate with the upwind of the tower to avoid blockage created by the tower.

While the wind speed is important, so is the size of the rotor blade. Increasing the size of rotor blade increases the swept area and thus collects more wind, which is why blade size has been growing dramatically in the recent years. Since transportation of blades to wind farm development sites is impacted by the size of the blades, it can be a major logistical constraint. Nevertheless, blade size is not expected to grow in this ratio in future, and many turbine manufacturing company have limited their blade diameter to about 100 m.

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