Research Progress of Archimedes Spiral Hydrokinetic Turbines in Free-Flow Conditions: A Comprehensive Review

Ocean current energy is abundant, yet its exploitation is severely constrained by the low-velocity conditions typical of most marine environments, where conventional lift-type turbines exhibit poor self-starting capability and low efficiency. This review provides the first comprehensive synthesis of research on free-stream Archimedes spiral hydrokinetic turbines (ASHTs), a class of drag-dominated rotors developed specifically for low-velocity kinetic energy harvesting. A unified classification is introduced, dividing ASHTs into single-blade long-axis (SL-ASHT) and three-blade short-axis (TS-ASHT) configurations. The energy conversion mechanisms, governed by pressure difference and hydrodynamic force synergy within helical passages, are elucidated, and the influence of critical geometric parameters is assessed. For SL-ASHTs, the analysis highlights exceptional self-starting capability (cut-in velocity: 0.1 m/s), a starting torque coefficient of 0.52, a maximum power coefficient of 0.51, and passive yaw adaptability that limits efficiency variation to below 2% over yaw angles of 0–40°. TS-ASHTs feature a compact architecture and higher rotational speed, facilitating direct generator coupling. With variable blade-angle distributions, thin airfoils, and non-uniform gap ratios, the power coefficient reaches 0.312. Performance-enhancement measures, including multi-parameter optimization, ducts, and winglets, deliver power gains of up to 35%, 122%, and 12%, respectively. This review further identifies critical barriers to engineering deployment: sediment erosion, cyclic fatigue, performance degradation under large yaw angles, and wake interactions. Future priorities include multi-objective optimization, advanced materials and flow control, full-scale sea trials, multiphysics coupling, array layout optimization, and hybrid energy system integration. By establishing a coherent classification and performance-evaluation framework, this work demonstrates that ASHTs offer strong potential as core devices for large-scale utilization of low-velocity ocean current and river hydrokinetic energy.