Experimental study of the effect of the duct on dual co-axial horizontal axis wind turbines and the effect of rotors diameter ratio and distance on increasing power coefficient

Abstract

Due to the recent growing interest in wind energy in urban industry, the present research investigates the increase in power of a micro-wind system using a duct and an auxiliary rotor. The system has the capacity to be employed separately to generate electricity or to be used as part of other systems such as INVELOXes. A duct increases the air flow rate through the turbine, while an auxiliary rotor allows the extraction of the wake kinetic energy. For this purpose, the effect of the duct on increasing wind speed is first studied, and then increasing wind turbine power is investigated. The built duct increases the wind speed up to 42% (from 5 m/s to 7.1 m/s); consequently, the wind turbine power increases over two times. Next, in order to achieve more power, a rotor is placed inside a duct within the front rotor wake. The effect of rotor diameters and the rotors' distance have been investigated by evaluating different configurations to obtain the highest system power. The results show that in the case of adding an auxiliary rotor with the same diameter in a duct, the power coefficient of the rotary part increases to 16%. Moreover, if the two rotors have different diameters (the front rotor has a smaller diameter), the power coefficient will increase up to 13% compared to two rotors with the same diameter. Importantly, as the system is designed to test rotors with independent rotation speed, using the same TSR for both rotors increases the power coefficient of the rotary part by up to 23% compared to the case where two rotors operate at the same rotational speed. Considering the frontal area of the system instead of the rotor area, the power coefficient of the system for ducted wind turbines can also be computed. Accordingly, it is found that its value will be significantly reduced.