Flapping foil power generation: Review and potential in pico-hydro application

Flapping foils are introduced as an alternative to rotary turbines in small-scale water-flow power generation applications. The role of a number of parameters are reviewed, including foil kinematics, foil and system geometry, and flow physics effects. Details of the kinematics are found to have the single largest influence on power output and efficiency (measured as the ratio of power output to that available and accessible in the fluid stream). The highest levels of power and efficiency are associated with very large foil pitch angles (upwards of 70 degrees) and angles of attack (30 to 40 degrees), such that the flow is massively separated for much of the flapping cycle, in contrast to rotary turbines which rely on attached flow over as much of the rotor disk as possible. Flapping foils maintain high efficiency at low flow speeds and small sizes, making them of interest for pico-hydro installations.