Optimization of a counter-rotating wind turbine using the blade element and momentum theory

Abstract

A counter-rotating wind turbine has a front rotor and a rear rotor which rotate in opposite directions on the same axis. Compared to a single rotor, the flow field of the counter-rotating wind turbine is complicated due to the interactions between the front rotor and the rear rotor. The wake induced by the front rotor works on the inflow of the rear rotor and is essentially an unsteady flow state. In order to estimate the performance of a counter-rotating wind turbine, it is necessary to consider more variables than the case of a single-rotor wind turbine and to prepare an estimation system capable of performing delicate predictions. For the optimization of a counter-rotating wind turbine, the pitch angles, radius ratios, and rotation speeds of two rotors are chosen as the design values and variations of the power coefficients and thrust coefficients can be observed in this study. The torque balance of the two rotors due to the kinematic coupling of the generator is considered. Modeling by means of the blade element and momentum theory for the optimization of a counter-rotating wind turbine is developed to predict the front rotor flow and the wake flow generated by the front rotor. The wake flow is then applied for the inflow of the rear rotor. A vehicle test is carried out to validate the prediction. The optimized solution is found using a multi-island genetic algorithm.