Wind Tunnel Experiments on Interaction Between Three Vertical-Axis Wind Turbines Arranged in Tandem

Wind tunnel experiments on the interaction among three closely spaced vertical-axis wind turbines arranged in tandem were conducted. The rotor spacing between an upwind turbine and a downwind turbine was kept constant at 7D (D is the turbine diameter) in the experiments. There are four experimental cases depending on the combinations of the rotational directions of the three turbines. Monotonic increases in the rotational speed and the rotor power of an upwind turbine and a middle turbine, with the increase in g1, are demonstrated for all the cases tested. Here, g1 is the rotor gap (spacing between the surfaces of these turbines. On the other hand, monotonic decreases in the rotational speed and the rotor power of a downwind turbine, with increasing g1, are found for all the cases. There is optimal gap g1, opt, in which the averaged normalized power of the three turbines Pnorm, ave takes maximum value, for each experimental case. The results demonstrate that a trio of turbines including an inversely-rotating pair expands the range of g1, opt in the comparison with the case only containing co-rotating turbines.