Experimental analysis of dual coaxial turbines in skew

Abstract Ocean currents are a potentially reliable source of renewable energy, but the complications associated with deploying current energy conversion (CEC) devices in deep water make harvesting that energy a challenge. One promising approach is to use tethered axial-flow CECs composed of one or more pairs of coaxial counter-rotating turbines. However, a dual-rotor system moored in unsteady water by a flexible tether is likely to experience a condition, called skew, where the axis of rotation is not aligned with the direction of flow. A lab-scale turbine was constructed to investigate the effect of skew on fluid power conversion of a coaxial CEC. A semi-empirical model of the power conversion was developed for comparing the results to a recently published analytical model. It was found that the analytical model represents the data better than a simple ad hoc modification to previous models that is often used to estimate the power dynamics. Additionally, the results support the existence of a physical phenomenon – captured by the recent model but not represented in the ad hoc modification – in which the downstream rotor of a coaxial pair is partially within the wake and partially out of the wake of the upstream rotor.

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