Downwind pre-aligned rotors for extreme-scale wind turbines

Downwind force angles are small for current turbines systems (1–5 MW) such that they may be readily accommodated by conventional upwind configurations. However, analysis indicates that extreme-scale systems (10–20 MW) will have larger angles that may benefit from downwind-aligned configurations. To examine potential rotor mass reduction, the pre-alignment concept was investigated a two-bladed configuration by keeping the structural and aerodynamic characteristics of each blade fixed (to avoids a complete blade re-design). Simulations for a 13.2 MW rated rotor at steady-state conditions show that this concept-level two-bladed design may yield 25% rotor mass savings while also reducing average blade stress over all wind speeds. These results employed a pre-alignment on the basis of a wind speed of 1.25 times the rated wind speed. The downwind pre-aligned concept may also reduce damage equivalent loads on the blades by 60% for steady rated wind conditions. Even higher mass and damage equivalent load savings (relative to conventional upwind designs) may be possible for larger systems (15–20 MW) for which load-alignment angles become even larger. However, much more work is needed to determine whether this concept can be translated into a practical design that must meet a wide myriad of other criteria.