Design and Analysis of Large-Scale Contra-Rotating Offshore Vertical-Axis Turbines

In this article, we describe numerical simulations of an innovative design of close-packed contra-rotating vertical-axis turbines (VATs), which enable assessment of power performance, vorticity distribution, and wake deficit. The design comprises a large diameter rotor without traditional supporting arms, stable in pitch and roll. Close-packing reduces leakage between rotors and yields a high blockage fraction, which significantly enhances the performance of offshore wind and tidal VATs. Rotor rings that support blades at both ends help reduce bending moments, and ease the task of the bearings required to achieve variable-pitch. The contra-rotation concept is based on two opposite-signed vortices cancelling each other out and conditions the flow though the turbine, lowering turbulent kinetic energy in the wake. Flow downstream of the turbines then experiences less stream-wise variation, reducing mixing losses thus enhancing energy extraction. Two types of VATs are considered. The first derives from experimental two-bladed H-type wind turbines, whereby the VATs comprise four close-packed contra-rotating wind turbines. The second is based on a 1:6 scale UNH-RM2 VAT that is extended to an array of ten closely packed 36-bladed hydrokinetic turbines positioned in two rows. It is found that high blockage, contra-rotating, vertical-axis rotors could facilitate higher potential power generation and appear to be a promising near-term technology for sustainable energy. The findings should prove useful in future assessments of the commercial feasibility of multiple cross-flow turbine configurations for both offshore wind and tidal stream power generation.