Experimental study of two low tip-speed ratio micro-scale wind turbines operating in tandem

This paper presents a parametric experimental study of a dual-rotor wind turbine system operating at low tip-speed ratios of the order of 1. The flow-field in the wake of such a multibladed-high solidity micro-scale wind turbine is measured and characterized at distances up to 15Rt, with Rt=70 mm being the tip radius of the turbine. At the best operating point of a single rotor, the near wake presents, on the one hand, an axial velocity deficit of the order of 40% of the infinite upstream velocity and, on the other hand, an average tangential velocity of the order of 30% of it. The far wake recovery is almost achieved at 15Rt, but tangential velocity of the order of 10% remains. The performances of two identical corotating and counter-rotating wind turbines separated by distances in the range 3Rt to 12Rt are then experimentally studied. The counter-rotating configuration gives 6% more power than the corotating configuration at the shortest distance 3Rt. However, the velocity field analysis reveals notable interference between the front and rear rotors, degrading the performance of the first turbine. This study provides an experimental foundation that guides in designing an optimized system in terms of tip-speed ratios of the rotors and the distance between them.