Most people, particularly in this region, are well aware that the wind can be harnessed for energy, but they may not know exactly how it happens.
It all begins with the kinetic energy of wind, which turbines convert into mechanical power that runs a generator to produce electricity.
Propeller-like blades are mounted on a shaft to form the rotor of the wind turbine. When the wind blows, a pocket of low-pressure air forms on the downwind side of the blade, which acts like an airplane wing. The low-pressure pocket of air then pulls the blade toward it, causing the rotor to turn.
That pulling is called lift. The force of the lift is much stronger than the wind’s force against the front of the blade, which is called drag. The combination of lift and drag causes the rotor to spin like a propeller.
The pitch of the blades can be adjusted to accommodate varying wind speeds. Changing the pitch can increase efficiency in low winds and decrease the efficiency in winds higher than 28 mph in order to protect the inner workings of the turbine. According to Aaron Thompson, lead technician for AES Wind Generation’s Lake Benton facility, their turbines reach maximum output at 1,400 RPM, which occurs at 28 mph.
Thompson said the generators in the turbines he and his crew maintain are not allowed to exceed 1,400 RPM. If they do, the turbines shut down automatically, using an inner brake operated by a hydraulic unit within the nacelle, which is the fiberglass structure that houses the gearbox and generator near the top of the tower. The towers typically shut down, he said, in winds higher than 65 mph.
Each tower contains an inducer that monitors the RPM of the generator and sends a signal to what Thompson referred to as the card cage – the computer that acts as the brain of the turbine. This card cage automatically adjusts the pitch of the blades depending on wind speed.
The turbines also automatically adjust the yaw, or direction the rotor faces, in order to face directly into the prevailing wind. This is done by the yaw drive and yaw motor housed below the nacelle.
Under normal circumstances, the turning of the rotor turns the low-speed shaft within the gearbox. The low-speed shaft then turns a high-speed shaft, which is connected to the generator. This, Thompson said, is done by a series of gears with a gear ratio of between 40-and 80-to-1, depending on the turbine. The towers owned and operated by AES northwest of Lake Benton have a 40-to-1 gear ratio. That means for every one turn of the rotor blades, there is 40 turns of the high-speed shaft.
After the generator produces the power, it is sent through cables inside the tube of the tower, through a controller unit at the bottom of the tower and to the transformer outside. Thompson said the AES towers then send the power directly to an Xcel Energy substation southeast of Lake Benton. From there, it is sent out into the power grid.
Size and energy output can vary by turbine, Thompson said, but the AES towers are 175 feet tall and each blade is about 73 feet long. Most turbines are mounted at 100 or more feet aboveground in order to take advantage of faster, less turbulent winds.
Each of AES’s 141 towers has a maximum output of 107 megawatts per hour. Each turbine, he said, produces enough energy each year to power roughly 250 homes.
