Evaluation of the performance of twin-rotor vertical axis wind turbines employing large-eddy simulations

The current strong global consensus on reducing carbon emissions is a motivation to develop more efficient means of harnessing sustainable sources of energy. Accordingly, research efforts toward the development of more efficient wind turbine designs are desirable. With this motivation, we present a set of numerical studies on flows past vertical axis wind turbines (VAWT). We perform large eddy simulations (LES) of flows past several VAWT configurations. The influence of turbine blades on the flow field is modelled using the actuator line method (ALM). Our focus is on a twin-rotor configuration wherein the rotors are placed close enough, so that the separation between the centres of the two rotors is less than the diameter of the two individual turbines (the overlapping configuration). We demonstrate that such a configuration indeed results in (a) enhanced power coefficient (ratio of power extracted by the turbine configuration to the power available in the free stream) and (b) better power density (power extracted by a turbine configuration per unit ground area occupied by the VAWT) compared to a single rotor VAWT configuration. Based on our findings, we conclude that the overlapping twin-rotor arrangement can prove to be the preferred configuration for large-scale VAWT-based wind farms.