Towards optimal layout design of vertical-axis wind-turbine farms: Double rotor arrangements

Designing an optimal wind farm layout requires fundamental knowledge of the interaction of wind turbines in an arrangement. In this paper, extensive high-fidelity CFD simulations are performed to investigate the influence of relative spacing, i.e., distance (R) and angle (Phi), in double rotor arrangements of co-rotating Darrieus H-type vertical axis wind turbines (VAWTs) on their aerodynamic performance. The relative spacing varies within 1.25d <= R <= 10d (d: turbine diameter) and -90 degrees <= Phi <= +90 degrees. The turbines operate at their optimal tip speed ratio. The analysis is focused on the individual and overall power performance of the turbines and their aerodynamics. Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, validated with experiments, are employed. It is found that an optimal region exists in which a higher overall power coefficient (C-P(overall)) compared to the C-P of an isolated solo rotor (C-P(Solo)) can be achieved. This region corresponds to compact rotor arrangements, i.e. R/d < 3d with Phi >= +45 degrees and Phi <= -45 degrees, yielding a maximum 1.8% increment in C-P(overall)/C-P(Solo) at R/d = 1.25 and Phi = +75 degrees. Detailed flow analysis reveals that in the optimal spacing, a narrow passage between the two rotors is formed within which the flow accelerates, forming a high-velocity region. The downstream turbine benefits from its blade(s) passing through this region and consequently yields higher C-P values. The findings highlight the high potential for compact VAWT farms with high power density and support the optimal layout design of VAWT farms.