Hybrid-Lambda: a low-specific-rating rotor concept for offshore wind turbines

Abstract

Abstract. We introduce an aerodynamic rotor concept for an offshore wind turbine which is tailored for an increased power feed-in at low wind speeds by a substantial increase in the rotor diameter while maintaining the rated power. The main objective of the conceptual design is to limit the steady-inflow loads (blade flapwise root bending moment (RBM) and thrust) to the maximum values of a reference turbine. The outer part of the blade (i.e. outer 30 % span) is designed for a higher design tip speed ratio (TSR) and a lower axial induction than the inner part. By operating at the high TSR in light winds, the slender outer part fully contributes to the increased power capture. In stronger winds the TSR is reduced and the torque generation is shifted to the inner section of the rotor. Moreover, the blade design efficiently reduces the power losses when the flapwise RBM is limited through peak shaving, below rated wind speed. This is of high importance, given the wind speed distribution at offshore sites. The characteristics of the rotor are first investigated with stationary blade element momentum simulations and further analysed with aeroelastic simulations, considering the flexibility of blades and tower to show that a structural design is feasible even for a blade of this size and complexity. The economic revenue and the cost of valued energy of the turbine are estimated and compared to the IEA 15 MW offshore reference turbine, considering a fictitious wind-speed-dependent feed-in price. Our results for the turbine concept with an increase in rotor diameter by 36 % show that the revenue can be increased by 30 % and the cost of valued energy can be reduced by 16 % compared to the reference turbine.

Power production and large-scale wake effects of offshore wind turbines with low specific rating

“…This enables the turbines to produce more power at lower wind speeds and also reach higher up in the atmosphere, where a higher energy availability is present. However, as shown in steady state blade element momentum simulations, the increase in power obtained from a single turbine only amounts to 11 % [3]. Thus, a second effect is assumed to contribute to the higher AEP.…”

“…The investment cost (CAPEX) of these new turbine concepts will be higher compared to classical concepts, due to higher material costs, as the the tower is taller and the blades longer. However, previous studies suggested that the increased CAPEX may be balanced out by their increased value for the energy system as a whole, as this new concept is able to produce more power at lower wind speeds, when the demand is highest [3].…”

“…To counteract this detrimental effect, more system-friendly turbine concepts with comparatively low specific ratings are desirable. At onshore sites such turbines have demonstrated to provide a higher power production at lower wind speeds, to operate more efficient in waked inflow situations [3] and to deliver a temporarily more balanced power output [4].…”

“…Recently a concept for a low specific rating turbine dedicated to offshore wind conditions, i.e. the so-called "Hybrid-Lambda Rotor" [3] was proposed. The design suggests a large rotor diameter of 326 m, while at the same time keeping the rated power of 15 MW.…”