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Experimental study on drag characteristics of a horizontal axis double rotor wind turbine

Double rotor wind turbines are studied for improving wind energy harvesting. The location, size and number of blades of the second rotor are important factors which affect performance of the double rotor wind turbines. These and other blade parameters may influence the drag and output power characteristics of the wind turbine. In the present work, the drag forces acting on two double rotor wind turbine configurations are experimentally investigated using wind tunnel testing. The two configurations are cocurrent and counter current double rotor wind turbines. A single rotor wind turbine is used as a comparison reference to compare with the two double rotor wind turbine configurations. Models of the three horizontal axis wind turbines were produced using 3D printing technology and were tested in the wind tunnel while wind power augmentation was also evaluated. The experimental results revealed an increase in the value of drag coefficient when a second rotor is added. The increase on the drag coefficient depends on the configuration, the size and location of the second rotor. The drag coefficient for the counter current rotation double rotor is close to the single rotor wind turbine; however, an increase of about 25% on drag coefficient is observed for the case of cocurrent double rotor wind turbine.