Analysis of bi-directional ducted tidal turbine performance

Abstract

Several commercial tidal turbine designs feature axial flow rotors within bi-directional ducts. Such devices are typically intended to increase power extraction through a flow-concentrating effect, operating on flood and ebb tides without a yawing mechanism. Research focused on such devices has been limited so far, with available results indicating poor performance relative to bare rotors. This study further investigates the relative performance of bi-directional ducted tidal turbines in confined flow.

Several duct profiles are evaluated relative to unducted rotors using the Reynolds-averaged Navier–Stokes solver ANSYS Fluent. The rotor is represented as an actuator disc, which mimics the streamwise thrust of a real device but does not reproduce its swirl or additional turbulence generation. This idealised model achieves optimal energy extraction and enables fair comparison of duct geometries. Device power is reported relative to total frontal area, reflecting the fact that the overall dimension of the device will be limited by water depth. Comparisons based on rotor area show how the absolute power is increased by a duct, but that this is attributable to an increase in blockage.

The fundamental effect of a duct on a rotor, as well as the secondary effects of duct camber and thickness, are identified by analysing streamwise distributions of velocity, pressure and cross-sectional area along the rotor streamtube. Ducts are found to limit the expansion of the downstream flow, in turn restricting the pressure reduction immediately behind the rotor. This effect, in combination with the reduced volumetric flux through a ducted rotor relative to a bare rotor, results in reduced power extraction.

The effects of duct curvature and thickness on turbine performance are also examined. Where a ducted rotor is desirable, e.g. for the protection of rotor blades, a thick profile with slight curvature performs best.