Prototype testing of a hydrokinetic turbine based on oscillating hydrofoils

A new concept of hydrokinetic turbine using oscillating hydrofoils to extract energy from water currents (tidal or gravitational) is presented and tested in this investigation. This technology is particularly well suited for riverbeds and shallow waters near the coasts due to its rectangular extraction plane. Very encouraging hydrodynamic efficiencies are demonstrated in this study through field tests which showed good agreement with the theoretical predictions obtained in the design phase. Following detailed analysis based on CFD modeling, an experimental 2 kW prototype has been designed, built and tested. The turbine includes two rectangular oscillating hydrofoils of aspect ratio 7 in a tandem spatial configuration. The pitching motion of each hydrofoil is coupled to their cyclic heaving motion through four-link mechanisms which effectively yield a one-degree-of-freedom system driving a rotating shaft. The rotating shaft is connected to a speed-controlled electric generator which, in the present setup, is used to charge a battery bank. In order to facilitate testing at different water flow velocities, the turbine has been mounted on a custom-made pontoon boat and dragged on a lake. Very good flow conditions and repeatability have thus been obtained. Instantaneous extracted power was measured and cycle-averaged for several water flow velocities and hydrofoil oscillation frequencies. The heaving and pitching amplitudes of the hydrofoils were kept constant for all runs at values of 1 chord and 75° respectively. Results are demonstrated to be self-consistent and show optimal performances at a reduced frequency of about 0.11 at which condition the measured power extraction efficiency reaches 40% once the overall losses in the mechanical system are taken into account. The 40% hydrodynamic efficiency of this first prototype exceeds expectation and reaches levels comparable to the best performances achievable with modern rotor blades turbines. It thus demonstrates the promising potential of the oscillating hydrofoils technology to efficiently extract power from an incoming water flow.