Abstract
<jats:p>The increasing demand for sustainable and renewable energy has intensified research on hydrokinetic energy systems capable of harnessing power from tidal and coastal currents. Conventional vertical-axis turbines are generally constrained by poor self-starting capability in lift-based designs or low hydrodynamic efficiency in drag-based configurations. To address these limitations, this study presents the design, development, numerical evaluation, and experimental validation of a hybrid vertical-axis tidal turbine consisting of a three-bladed vertical spherical turbine circumscribing a two-bladed helical Savonius rotor. The proposed configuration combines a lift-based spherical rotor utilizing the NACA 0012 hydrofoil profile with a drag-based helical Savonius rotor to enhance startup characteristics and maintain stable rotational performance under low-flow conditions. Three-dimensional Computational Fluid Dynamics (CFD) simulations were performed using ANSYS Mechanical 2023 R2 at a free-stream velocity of 1.8 m/s. Mesh independence, time-step independence, and domain-size independence analyses were conducted to ensure numerical reliability and solution accuracy. Simulation results showed that the turbine achieved an average angular velocity of 14.34 rad/s (approximately 137 rpm), an initial angular acceleration of 1.55 rad/s², a mean tip speed ratio of 1.99, a torque coefficient of 0.001137, a power coefficient of 0.002265, and an average mechanical power output of 1.33 W, demonstrating stable rotational behavior through the combined lift and drag mechanisms. A physical prototype was subsequently fabricated and experimentally tested at Gubat Coastal Beach, Gubat, Sorsogon, Philippines. Field testing confirmed the turbine’s self-starting capability and continuous rotational operation under estimated current velocities ranging from 0.24 to 0.58 m/s, with corresponding rotational speeds of 48–116 rpm. The prototype also generated measurable electrical output of 0.01–0.02 V using a permanent magnet DC generator. The agreement between the numerical and experimental results validates the proposed hybrid turbine concept and demonstrates its technical feasibility for low-velocity hydrokinetic energy harvesting, providing a foundation for future optimization and development of hybrid tidal turbine technologies.</jats:p>