The aerodynamic coupling design and wind tunnel test of contra-rotating wind turbines

The performance of contra-rotating wind turbines (CRWTs) is highly dependent on aerodynamic design. However, flow interaction between two rotors hasn't been fully understood. In this paper, flow interaction was considered in theoretical model based on actuator disc theory with wake rotation. As a consequence, a coupling design method was formulated. Simultaneous blade design of both rotors was originally expressed by a global optimization problem. The input variables include axial and tangential velocity induction factors of both rotors. The output is energy efficiency of CRWT at the annular disc. As a result, chord length and blade section pitch distribution were determined synchronously. This method was applied on the 500W cases of a CRWT and a single rotor wind turbine (SRWT) with identical radius. Wind tunnel test were carried out on the prototypes, with rotating speed and pitch of both rotors controlled independently under various wind velocity. Result validated the effectiveness of design method. At rated wind velocity, the maximum power coefficients of CRWT and SRWT were 0.426 and 0.373. Despite a slightly high start-up wind speed, CRWT demonstrated a higher power coefficient than single turbine at wind velocity 8–14 m/s, with power coefficient relative increment 5.3–28.9%.