One of the major contributors to loads and fatigue on wind turbines is atmospheric turbulence, or gusts. The Makani AWT has a unique way of handling the problem.
On a typical wind turbine, the airfoil on the blade sees an apparent wind which is a combination of the actual wind, and the speed at which the blade is moving. While the blades are able to pitch, they are not able to do so quickly enough to respond to gusts as they are sensed. The fundamental reason for this is the high mass of the turbine blade, and the fact that the entire blade load is placed on the pivoting bearing at the blade base. To overcome this large inertia and bearing load, large servos with high torque and low speeds are required.
Fortunately, the situation is much simpler on AWTs. The Makani AWT is designed with a very low pitch inertia and high pitch stability due to the passively stabilizing tail surface. When a gust hits, the tail surface begins to lift, tipping the wing into the wind and thereby relieving load. This action happens passively in a fraction of a second, meaning no active control is needed to reject the largest impact of significant gusts. One way to think of it is that we use aerodynamic controls, taking the load that was in a servo and replacing it with a rigid structure.
Wind turbine blades are quite heavy, meaning that when large gust deflects a blade, it then oscillates for quite some time before aerodynamic damping returns the blade to a steady state. The same effects are in play on AWTs, but because the AWT is so light, aerodynamic damping removes gust disturbances from the system much more quickly. While a wind turbine blade might flex back and forth numerous times after a sharp gust, an AWT settles back to a steady state without further oscillations. This means little or no reversing stress on the blades, reducing the number of fatiguing load cycles and increasing the life of the structure.
