Aero-structural design investigations for biplane wind turbine blades

Perry Roth-Johnson Phillip Chiu Edward Lin Richard Wirz

Large wind turbine blades are being developed at lengths of 85-125 meters, in order to improve energy capture and reduce the cost of wind energy. Bending loads in the inboard region of the blade make large blade development challenging. The "biplane blade" design was proposed to use a biplane inboard region to improve the design of the inboard region and improve overall performance of large blades. This work uses a "structures-first" approach with aero-structural analyses to (1) examine the feasibility of the biplane blade, (2) determine how the dimensions of the biplane inboard region affect performance, and (3) compare the aero-structural performance of a 100-meter biplane blade to the Sandia SNL100-00 reference blade. Two-dimensional CFD simulations were used to compare the aerodynamic performance of a biplane with a thick monoplane. The lift-to-drag ratio and the maximum lift coefficient is significantly greater for the biplane than the thick monoplane for angles of attack of 0-15 degrees. Analytical methods and beam finite elements with cross-sectional analysis were both used to examine the performance of biplane blade structures. These structures varied in complexity from isotropic spars to composite spars to composite full blades. In each case, biplane blade structures were compared to monoplane blade structures of the same length, mass, and complexity. Simple load cases were applied to each structure and their displacements, bending moments, axial forces, and stresses were compared. Similar performance trends are identified with both the analytical and computational models. Parametric analyses show that gap-to-chord ratios bewteen 1.0-1.2 and joint length-to-span ratios of about 0.5 give good aero-structural performance. At the tip, the biplane blade increases flapwise structural efficiency by 20-40%, depending on the load. Edgewise structural efficiency was decreased by 27-35% at the tip. The benefits for the inboard region could lead to mass reductions in wind turbine blades. Innovations that create lighter blades can make large blades a reality, suggesting that the biplane blade is an attractive design for large (100-meter) blades.