Downwind Pre-Aligned Rotor for a 13.2 MW Wind Turbine

Abstract

To alleviate the mass-scaling issues associated with conventional upwind rotors of extreme-scale turbines, a downwind rotor concept is considered that uses coning and curvature to align the non-circumferential loads for a given steady-state condition. This alignment can be pre-set to eliminate downwind blade moments for a given steady-state condition near rated wind speed and to minimize them for other conditions. The alleviation in downwind cantilever loads may enable a reduced structural blade mass as compared with a conventional upwind rotor. Previous quasi-steady scaling analysis indicates that this cantilever load alleviation becomes significant for extreme-scale systems (10-20 MW). To examine the potential impact of this design, Finite Element Analysis (FEA) was conducted for a 13.2 MW rated turbine at steady-state conditions for two rotor configurations with similar power outputs: 1) a conventional upwind rotor with three blades and 2) a downwind pre-aligned rotor (DPAR) with two blades. Based on previous work, the pre-aligned rotor configuration was set based on steady-state loads at a wind speed equal to 1.25 times the rated wind speed. By keeping the blade mass about the same between these two configurations, the rotor mass was reduced by approximately one third for the DPAR configuration. In addition, the average stresses on the blades for a variety of steady-state wind speeds was reduced for the DPAR configuration. However, these results can only be considered to be qualitative in terms of impact on turbine mass and cost. In particular, simulations at non-ideal, extreme and unsteady conditions are needed to determine the viability of this concept.