You don’t necessarily need strong winds to get useful energy production from your wind-electric system. You might only need marginally stronger winds. So, how can you access those stronger winds? Time for another analogy.
Imagine floating down a river in a canoe, but rather than sitting back and enjoying yourself, you pay very close attention to what’s happening on the water. The first thing you notice when you put your canoe in is that there is slow flow near the bank. As you paddle out into the river, however, flow picks up somewhat. By the time you get to the center of the river, the farthest you can be from either bank, you notice that the flow is fastest.
Near the bank of the river, the flowing water is slowed by its interaction with the riverbank’s friction. Like water, air is a fluid. And just like water, its flow is inhibited by its contact with what’s on the ground: vegetation, land forms, and buildings. The farther from these sources of friction, the faster the wind.
The illustration above shows a wind profile, the length of the arrows representing wind speed—longer arrows mean higher speeds. As you move away from the surface of the earth and its ground cover, wind speed increases. To access stronger winds, you need to reduce friction by getting your wind turbine rotor higher up in the wind profile.
Different locations will have different wind profiles depending on the amount of friction presented to the air mass (see graph in sidebar). The friction that ground cover poses is known as ground drag. Different ground covers are akin to different grades of sandpaper. Smooth ground cover—such as a hay field—doesn’t present much drag to a moving air mass, whereas densely scattered trees and buildings present a lot of drag. The rougher the ground cover, the greater the drag and the more the air mass is slowed.
Wind turbine blades are airfoils, similar to airplane wings. Both operate on the same principle of lift— the force that allows planes to fly, and wind turbine blades to rotate to extract energy out of the wind. Airfoils need laminar airflow—constant and smooth flowing winds—over them to maximize the lift they can generate—which will maximize the kinetic energy they are able to extract from the wind.
Turbulence, which is caused when the wind tumbles over obstacles (trees and buildings, for example), is chaotic airflow. The greater the ground drag due to taller or more obstacles, the more turbulence that is created. Wind breaks, farmyard wind barriers, and snow fencing are often used to create turbulence to disrupt strong winds.
Turbulence changes laminar airflow into a chaotic, tumbling, churning mess. This wreaks havoc on lift devices that depend on laminar winds. Remember the last time you flew in an aircraft that hit a pocket of turbulence? The plane was tossed about and lost altitude because of decreased lift on the airfoils (wings). The same thing happens with a wind turbine: It is buffeted by turbulence as is evidenced by the way it changes direction, trying to follow the chaotic wind, and spins erratically without generating much electricity, since there is little usable energy in turbulent winds. Unfortunately, the “bubble” of turbulence around a house or on a farm can be of considerable distance, height, and width.