Analysis of wind turbine blade aerodynamic optimization strategies considering surface degradation

Wind turbines suffer with the degradation of blades during their working life as a result of weathering or adhesion of dirt on their surface. This decreases efficiency in the energy production, requiring in some cases the exchange, repair or cleaning of the blades surfaces, which causes a decrease in the capacity factor of the wind farm due to the machinery shutdown and the increase in the operation cost of the farm. In this paper, we analyse different strategies of how to deal with this issue in the wind turbine design phase. The genetic and simplex optimization algorithms were employed to design the geometry of the blades of two different turbines, one which operates with fixed speed and another which operates with variable speed. The objective function was to maximize the annual energy production (AEP). For each of the turbines, we have designed two different blades: one which is optimal for smooth blade surface and another which is optimal for rough blade surface. The four designs were then tested (using a blade element momentum code) considering smooth and rough surface conditions; in order to assess the difference in performance, the turbines would have taken into account their entire working life. The comparisons were made using an index we have developed for this work: the ratio of the average AEP of the turbine with the blades designed for smooth surface condition and the average AEP of the turbine with the blades designed for rough surface condition. This index was calculated for 0 to 100% of the turbine life working with rough blade surface, for both turbines (fixed and variable speed). The conclusion was that, for the turbine with fixed speed, designing for rough blades is better when the turbine operates with rough blade surface for 34.95% or more of its lifespan. On the other hand, for the turbine with variable speed, designing for rough blades is better only if the turbine works with rough blade surface for 50.59% or more of its lifespan.