Congratulations to Phil Chiu for successfully defending his thesis!

Title: Aerodynamics and design of biplane wind turbine blades

Abstract: As their wind turbine blades grow longer to meet the ever increasing demands of new wind energy systems, the design of the inboard region (near the blade root) becomes a trade-off between competing structural and aerodynamic requirements. State-of-the-art monoplane blades require thick airfoils to support high inboard loading but these thick airfoils have inherently poor aerodynamic performance. New designs are required to circumvent this design compromise. One such design is the “biplane blade”, in which the thick airfoils in the inboard region are replaced with thinner airfoils in a biplane configuration. This design was shown previously to have significantly increased structural performance over conventional blades. In addition, the biplane airfoils can provide improved lift and aerodynamic efficiency compared to the thick monoplane inboard airfoils, indicating a potential for increased power extraction.

This work investigates the fundamental aerodynamic aspects, aerodynamic design and performance, and structural design of the biplane blade. Each of these efforts reveals important results to characterize biplane blade design and to assess its challenges and benefits. First of all, most biplane designs use thin airfoils but for large wind turbines we must understand the unique behavior of biplane with relatively thick airfoils. Two-dimensional aerodynamic analysis of thick biplane airfoils shows unique phenomena which arise as a result of airfoil thickness. Next, the aerodynamic design of the full biplane blade is considered. Two biplane blades are designed for optimal aerodynamic loading, and their aerodynamic performance quantified. The results of this analysis show that the biplane blades can be designed with significantly less chord than conventional designs, a characteristic which enables decreased critical aerodynamic loading and improved transportability for larger blade designs. Secondly, by considering practical chord distributions and the drag of the mid-blade joint, it is found that biplane blades have comparable power output to conventional monoplane designs. The aerodynamic loads on the biplane blades are shown to be increased in gust conditions and decreased under extreme conditions. Finally, considering these aerodynamic loads, the blade mass reductions achievable by biplane blades are quantified. The internal structure of the biplane blades is designed using a multi-disciplinary optimization which seeks to minimize mass, subject to constraints which represent realistic design requirements.

This work shows that biplane blades can be built more than 45% lighter than conventional monoplane blades. As blade length is increased, these mass reductions are shown to be even more significant. These large mass reductions indicate the potential for significant cost of electricity reductions for rotors with biplane blades. Taken together, these results show that biplane blades are a concept which can enable the next generation of large wind turbine rotors.