Flapping-foil energy harvesters: Principles, performance, and prospects

Flapping-foil energy harvesters (FFEHs) are garnering attention as a promising alternative to conventional rotary turbines. This comprehensive review synthesises four decades of research, highlighting that power extraction is primarily governed by vortex dynamics (leading-edge-vortex formation and wake capture), which are influenced by the foil’s kinematics, geometry, flexibility and control. The reported peak hydrodynamic efficiencies are substantial: ≈40% for prescribed-motion systems, exceeding 50% for flexible or actively cambered foils. Efficiencies can be further enhanced using multi-foil arrays, ducted configurations, and tandem layouts, reaching up to 64%. However, most existing studies remain confined to two-dimensional or low-Reynolds-number regimes, limiting confidence in scaling these results for real-world deployment. Key challenges remain in stabilising complex three-dimensional wakes, developing practical power-take-off systems, and ensuring robust control in turbulent and shear flow conditions. Emerging strategies such as machine-learning-assisted optimisation, reinforcement-learning controllers, and hybrid activation concepts are also promising. Ultimately, advances in high-Reynolds-number fluid–structure interaction and data-driven control will be crucial in transitioning FFEH technology from laboratory prototypes to reliable, commercially viable renewable-energy solutions.