Performance characteristics of an experimental cross-flow turbine array at high confinement*

Cross-flow turbines show great promise for extracting power from river and tidal currents due to their ability to achieve high blockage. As a turbine occupies more of the channel cross-sectional area, its efficiency and structural loading increase since both kinetic and potential energy in the freestream are converted to mechanical power. Here, we characterize the performance of a laboratory-scale two-turbine array at various levels of confinement in a recirculating water channel. The array blockage ratio is varied from 30% to 60%—the upper end of what might be realizable in a natural channel—while other important non-dimensional parameters are held constant. The cross-flow turbines were tested across a range of tip-speed ratios under a counter-rotating coordinated constant speed control scheme, wherein the turbines rotate at the same speed but in opposite directions, and with a constant angular phase offset between their cycles. At the higher end of confinement, we observe performance coefficients exceeding unity and force coefficients substantially higher than in conventional array designs. This has substantial implications for the design and control of full-scale arrays at high confinement.