Theoretical and experimental investigation on bidirectional duct and turbine system for tidal energy conversion

Tidal kinetic energy is characterized by its bidirectional flow nature, which raises unique challenges for efficient power conversion. Conventional tidal current energy systems commonly employ horizontal-axis open-propeller turbines, often requiring additional mechanisms to accommodate changes in flow direction. This study presents the development of a theoretical analysis model aimed at improving the performance of a bidirectional ducted tidal turbine. A systematic design methodology for a bidirectional turbine rotor is proposed, emphasizing simplicity, mechanical reliability, and obedience to fundamental turbomachinery principles. The theoretical framework incorporates actuator disk theory to establish a performance reference, resulting in a maximum power coefficient of 0.385 based on the maximum duct area. This value is proposed as the ducted Betz limit for ducted tidal turbines. A model-scale experimental validation of the bidirectional duct–turbine system was conducted at a blockage ratio of 0.105. The experimental results yielded a maximum power coefficient of 0.093, which, although significantly lower than the theoretical limit, confirms the applicability of the proposed ducted Betz limit as a meaningful performance benchmark. The rotor design approach is based on Euler’s turbomachinery equation, ensuring consistency with angular momentum conservation principles. The integration of a duct allows for a reduction in turbine diameter for a given power output, potentially enhancing structural durability and reducing system costs. Overall, the proposed analytical framework and design methodology provide a structured basis for evaluating and optimizing the performance of bidirectional ducted tidal turbines.