Land-based wind turbines with flexible rail transportable blades – Part I: Conceptual design and aeroservoelastic performance

Abstract

Abstract. This work investigates the conceptual design and the aeroservoelastic performance of land-based wind turbines whose blades can be transported on rail via controlled bending. The turbines have a nameplate power of 5 MW and a rotor diameter of 206 m, and they aim to represent the next generation of land-based machines. Three upwind designs and two downwind designs are presented, combining different design goals together with conventional glass and pultruded carbon fiber laminates in the spar caps. The results show that controlled flexing requires a reduction in the flapwise stiffness of the blades, but it represents a promising pathway to increase the size of land-based wind turbine rotors. Given the required stiffness, the rotor can be designed either downwind with standard rotor preconing and nacelle uptilt angles or upwind with higher-than-usual angles. A downwind-specific controller is also presented, featuring a cut-out wind speed reduced to 19 m per second and a pitch-to-stall shutdown strategy to minimize blade-tip deflections toward the tower. The flexible upwind and downwind rotor designs equipped with pultruded carbon fiber spar caps are found to generate the lowest levelized cost of energy, 2.9 % and 1.3 %, respectively, less than the segmented design. The paper concludes with several recommendations for future work in the area of large flexible wind turbine rotors.

Development of an open-source segmented blade design tool

“…To keep the aerodynamics the same as the baseline, the outer shape of the blade is not changed: the joint must fit within the given blade cross section. This analysis begins with a joint located at 70% of the blade span, as used in the prior study [3], and M36 bolts. Blade mass is used as the merit figure.…”

“…The baseline designs used in this study were developed by Bortolotti et al [3]. The turbines are class IIIA, with U avg = 7.5 m/s; each has different blade types but all have 100-m blade lengths.…”

“…However, the sensitivity analysis presented in Section 4 reveals negligible AEP sensitivity to changes in joint parameters, so this is not a concern. Although its smaller, less expensive joint yields a lower blade mass and cost than the original BAR-USC, the BAR-USC II 70-m joint design still has heavier, more expensive blades than the rail-transportable monolithic designs from Bortolotti et al [3], as tabulated in table 1. The higher blade costs are partly from more material, and partly from added manufacturing and assembly costs.…”

“…Smaller blade segments can be manufactured and transported separately, then assembled on-site at the destination wind farm, overcoming the aforementioned issues with large monolithic blades. Other solutions, such as large monolithic rail-transportable blades, are also being researched [3].…”

“…The performance and economics of large segmented blades must be compared to other large monolithic blade types, such as upwind and downwind flexible blades, to determine their scaling potential. Bortolotti et al [3] performed such research with the open-source tools OpenFAST [11] and the Wind-Plant Integrated System Design and Engineering Model (WISDEM ® ) [12], creating detailed designs of 100-m monolithic and segmented blades and comparing their performance under a variety of conditions. They found that flexible, rail transportable blades can be lighter and less expensive than segmented blades.…”