A vertical axis hydrodynamic turbine with flexible foils, passive pitching, and low tip speed ratio achieves near constant RPM

A full-scale vertical axis turbine with flexible foil blades and a passive spring-loaded pitching mechanism was tested in the Glomma River in Norway, demonstrating a maximum turbine efficiency of 37% (0.79 m/s, 3.7 RPM), 25% (1.18 m/s, 4.2 RPM), and 20% (1.55 m/s, 4.7 RPM). The turbine rotational speed showed a limited revolutions per minute (RPM) variation (increasing by only a factor of 1.3) with a doubling of inflow speed. The turbine RPM and load variations were controlled using five wings, double-cambered morphing blades with a relatively high blade lift/drag ratio over a wide range of angle of attacks, and passive spring-controlled blade pitching. The inherent passive blade pitching demonstrated a distinct flopping motion around the nominal (non-pitched) position at ∼0° azimuth angle and a less distinct motion at ∼180° for each revolution around the axis. The turbine's high solidity design, morphing blades, and Knoller-Betz effect limited the RPM variations over a wide range of water velocities. The flex foil turbine had higher power coefficients at lower tip speed ratios compared to typical lower solidity turbines with fixed blades, higher tip speed ratios, and no blade pitching and/or flopping.