Design of a two-bladed counterpart to the three-bladed INNWIND 20 MW offshore reference wind turbine

Abstract. It is still an ongoing discussion in the wind field whether a two-bladed or a three-bladed rotor is economically superior for large offshore wind applications. While the inherent question is clear, the answer has to tackle a multitude of challenges. One is the task of establishing an equal three- and two-bladed turbine design to fill the research gap on the engineering side. Due to the notorious difficulties of reaching an entirely equal design, the goal is to remain as similar as possible, always with the premise of reaching a fair comparability. For this purpose, the focus of the paper is on striving for the most similar aerodynamics, including an equal absolute power curve, as well as a most similar blade structure in terms of a comparable fiber composite layer stack and matching ultimate and fatigue material stresses and stability limits. The controller features the same architecture and is equipped with basic load mitigation algorithms. The gains are tuned by utilizing an objective control cost criterion. The generator is adapted to the changed torque and rotation speed, and the tower is adapted to fatigue loads. A complete load and cost comparison and further manufacturing, transportation, installation, and operation and maintenance aspects are outside of the scope herein. In turn, the paper presents a thought-through possibility of achieving a most similar two-bladed turbine based on a three-bladed reference. While this re-design could generally be performed with any detailed three-bladed horizontal-axis wind turbine, the steps are showcased on the three-bladed 20 MW INNWIND reference turbine. The final turbine models, controller, and blade models are made open source with the paper at hand.