Experimental validation and extension of a blade element momentum model for counter-rotation dual-rotor wind turbine with double rotational armature design

Small wind turbines face significant challenges in achieving commercial viability due to lower efficiency and higher energy costs compared to utility-scale systems and competing renewable technologies. Counter-rotating dual-rotor wind turbines (CR-DRWTs) with dual-rotational armature configurations offer a potential pathway for efficiency improvements through doubled direct-drive power and minimal mechanical complexity, suitable for urban applications. This study presents a detailed experimental investigation of a 1.6 m diameter CR-DRWT through wind tunnel testing at the Centre Scientifique et Technique du Bâtiment (CSTB) in Nantes, France, conducted at wind speeds between 4 and 15 m s−1. An improved turbine design is tested with enhanced instrumentation, including independent measurements of rotor rotational speed and blade pitch angle, enabling a detailed characterization of rotor interaction, operating behaviour, and power performance. The turbine achieved a maximum electrical power output of 1014 W and a peak measured power coefficient of 0.33 while demonstrating reliable self-starting at wind speeds as low as 3.5 m s−1. To interpret and generalize the experimental findings, an existing blade element momentum (BEM) model for dual-rotor systems is extended to explicitly represent torque balance and power production in a dual-rotational armature configuration. The extended model shows good agreement with experimental trends and is further applied in a sweep optimization. The optimized configuration predicts a theoretical maximum power coefficient of 0.56, highlighting substantial remaining performance potential. By combining wind tunnel measurements with a validated BEM model extension, this work provides unique reference data and supports the development of mechanically simple CR-DRWTs for future small-scale wind energy systems.