Abstract
Cross-flow turbines show great promise for extracting power from river and tidal channels due in part to their ability to achieve high blockage ratios, defined as the ratio between the turbine projected area and the channel cross-sectional area. In these high blockage environments, resistance to flow through the turbine combined with confinement from the channel boundaries accelerates the flow through the rotor, which increases turbine efficiency. A row of turbines distributed along the width of a channel is an effective arrangement to achieve higher blockage ratios, and thus higher efficiencies. In such an array, adjacent turbines may be coordinated with each other, which can influence performance. At lower blockage ratios, both rotation scheme (i.e., whether the rotors rotate in the same direction or opposite direction) and the phase offset between the azimuthal positions of adjacent turbines have been shown to influence array performance. Additionally, for individual rotors, intracycle control strategies—wherein the angular velocity of a turbine oscillates as a function of phase throughout a rotational cycle—have been shown to enhance efficiency at lower blockage ratios. However, the benefits of such advanced control strategies have not been evaluated at the upper end of blockage ratios achievable in a realistic channel (i.e., 30% to 60%).
Here, we experimentally investigate how different array control strategies influence the performance of a laboratory-scale dual-rotor cross-flow turbine array at blockage ratios ranging from 35% to 55%. We consider counter-rotating intracycle velocity control schemes, several co-rotating and counter-rotating constant speed control strategies, and a counter-rotating torque control scheme with no imposed coordination. In contrast to prior work at low blockage, neither variations in rotation direction nor phase significantly affect array performance at higher blockage ratios. Although certain combinations of rotation scheme and phase offsets perform slightly better than others, the variations are minor relative to the overall influence of blockage and the tip-speed ratio. Notably, under uncoordinated torque control, the time-average array efficiency is comparable to that under constant speed control, and the rotors are observed to naturally synchronize at zero phase offset. In contrast to low blockage, intracycle control is unexpectedly found to degrade array efficiency. This is attributed to the less-steady resistance to flow provided by the array when the angular velocity fluctuates, which undermines the beneficial hydrodynamics of confinement. The results show limited benefits to several advanced control strategies at high blockage, and suggest that other system design pathways (e.g., rotor geometry optimization) are more effective for addressing system design constraints and reducing levelized cost of energy.