An Analysis of Extreme-Scale, Load-Aligned WindTurbines

Abstract

Over time the cost of wind energy has dropped and the size of wind turbines has grown. This trend has been consistent over the last several decades, but will the trend continue? Wind turbine designers are reaching technical barriers that limit the size of wind turbines. One such barrier is the out of plane bending loads the blades must support. A 100m long rotor blade experiences loads of a magnitude that no other aerospace structure has ever been designed to support. The blade must both be strong enough to withstand the ultimate and fatigue loads, and stiff enough not to deflect backwards to strike the tower. Satisfying these concerns has become cost prohibitive at extreme-scales (>10MW). One clever solution is the load-aligned rotor, which offsets the thrust bending loads with the centrifugal bending load resulting from downwind coning. The first challenge of the load-aligned rotor is that it must operate in the downwind configuration, and therefore must pass through the wake of the tower. The effect of the tower shadow must be fully understood as it may undo the benefits experienced by load-alignment. The second challenge is that the effects of coning as a method of load-reduction are not fully understood. This dissertation explores these two areas (tower shadow and load-alignment) in extensive detail. Some key findings were that tower shadow significantly effects wind turbine aerodynamics. The tower shadow can excite oscillatory modes for small-scale wind turbines, which typically have relatively stiff blades and fast rotational rates, but can be mitigated with a tower fairing. For large scale wind turbines, which typically have relatively flexible blades and slower rotational rates, operating under field-conditions tower shadow has relatively minimal effect, and no tower fairing is required. Downwind coning was shown to reduce loads sufficiently that a load-aligned rotor could meet structural and power constraints using 26% less material that conventional upwind rotors. Load-alignment via longer blades with a variable coning hinge was shown to increase power production by 12% without significantly affecting ultimate loads or fatigue damage. Load-alignment has been shown to be an effective way to decrease cost of wind energy, which may bring us one step closer to a more resource responsible world.