As you may know, harnessing the energy of the wind is not some new discovery. Wind mills have been in use for thousands of years for a wide variety of purposes, as have boats and ships that use sails.
Nowadays we have large wind turbines and wind farms that generate sustainable and clean electricity on a massive scale. We’ve built on the wind mills of old and created a new and vastly more efficient model.
The primary difference between the wind mills of old and today’s wind turbines, is the deeper understanding and application of aerodynamics. Namely, the concepts of drag and lift around an airfoil. Modern advances in engineering and science enable more complex blade designs, incorporating curves and twists to achieve greater aerodynamic efficiency.
The Difference Between a Fan and a Windmill
It might seem logical that a fan and a windmill use the same mechanical principles, with only the wind going in opposite directions. Although conceptually this is true, there’s a little more to it than that. Fans are engineered and built to provide comfort at a comparatively low cost, and have very small wind-output requirements.
Blade design, therefore, is simple and economic to produce. More complex aerodynamic principles are unnecessary due to the small scale of the machine and its function. Fan blades are generally little more than flat, light, paddle-like objects that generate wind by “pushing air out of the way” as they spin.
Turbine rotors on the other hand, have to be designed much more delicately in order to be effective. For one, the required torque is incomparable to a commercial fan, and two, they have significant strength requirements due to their sheer size and weight, as well as the wind forces they will encounter.
And being that the wind is the acting force on the rotor blades, which in turn generate the torque that drives the turbines, the rotor blades themselves must be designed to absorb the maximum amount of energy from the wind and convert it to angular momentum.
Aerodynamics and the Airfoil
To do this, one must understand and apply the principles of aerodynamics. The most efficient shape for an object moving through a fluid (air or water) is what we refer to as an airfoil, the cross-section of which is round in the front and sharp in the back.
This is the same general shape of airplane wings, propellers, rotors, etc. The angle the airfoil makes with the oncoming wind is called the angle of attack (AOA), and is what determines how much lift and drag there is. Simply put, lift is created as wind moves around and past the airfoil nose resulting in higher pressure beneath the airfoil and lower pressure above.
Up to about 15 degrees, lift and drag increase more or less equally with an increase of AOA, after which lift decreases and drag increases sharply. Of course, a wind turbine is different than an airplane wing or propeller in that the former is being moved by the wind as opposed to the latter being propelled into it by a separate energy source (engine).
Planes use a combination of lift and thrust to achieve flight, while a turbine rotor will use these same forces to generate angular momentum. And just as a larger and/or heavier plane generally requires a larger wingspan (for equal thrust), so an efficient wind turbine needs large rotor blades to adequately harness the wind.
Generally speaking, the power generation capabilities of a wind turbine are directly related to the size and length of the rotor blades. The larger the area of the blade the more wind-energy it can capture and subsequently convert to angular momentum. This goes without saying that the turbine and generator specifications must then also correspond to the rotor blade size for maximum efficiency.
Topography and wind patterns for a given area are also important factors that determine what kind of wind turbine should be used — if at all. Rotor speed must also be controlled during periods of high wind to avoid damage to the turbine. This is done primarily by adjusting the blade-pitch, or angle of attack.
- Stall and Furl
Stalling refers to the increasing of the angle of attack to the point where lift is cancelled out by drag. Furling refers to the decreasing of the angle of attack to where the edge of the blade faces the oncoming wind, resulting in a cancellation of angular forces.
- Pitch and Yaw Control
Although minimizing or otherwise controlling the amount of wind-energy that is absorbed by the turbine during periods of high wind is important, ensuring optimal absorption of wind-energy during regular winds is equally as important. This is done by pitch and yaw control.
Pitch control is the adjustment of the airfoil’s angle of attack. The 2 extremes of pitch control result in stall and furl as explained above. Yaw control is the rotational adjustment of the entire propeller head on the vertical axis to ensure the turbine is facing the oncoming wind.