An experimental investigation of a passively flapping foil in energy harvesting mode

Abstract

Energy extraction through flapping foils is a new concept in the domain of renewable energy, especially when the system is fully driven by incoming free-stream flow, a phenomenon known as flow-induced vibration. To investigate this concept, a water tunnel test-rig was designed and fabricated, where a flat plate foil made from plexiglass performs two-degrees of freedom pitch and plunge motion under the influence of incoming water flow. For this study a power-takeoff system was not introduced, hence energy harvesting performance was evaluated through real-time force and motion measurements with the help of sensors. The energy harvester performed self-sustained flapping motions when the free-stream velocity reached a threshold value, known as the cut-off velocity, which for this test-rig is 0.40 m/s (without sensors) and 0.50 m/s (with sensors). To support these self-sustained flapping motions, inertial mass blocks were placed to provide the necessary inertia especially when the flat plate foil performed the pitching or stroke reversal action. Different inertial mass units (m(ib) = 0.45, 0.90 & 1.35 kg/block) were tested to analyze their effect on the flat plate foil kinematics and its energy harvesting performance. Other parameters such as pitching amplitude (theta(o) = 30 degrees, 43 degrees & 60 degrees) and free-stream velocity (U-infinity = 0.57 m/s, 0.65 m/s and 0.78 m/s) were varied at fixed pivot location (x(p) = 0.65 chords (c)) to augment the varying inertial mass unit study. In the first section at fixed m(ib) of 0.45 kg/block and x(p) = 0.65c from leading edge, energy harvesting performance ((C) over bar (p) & eta) was observed to increase with increase in pitching amplitude, while it degraded as the free-stream velocity increased. Best energy harvesting performance of eta = 52.5% and (C) over bar (p) = 1.124 was achieved with mil, = 0.45 kg/block, theta(o) = 60 degrees and U infinity = 0.57 m/s. Varying mil, also had a considerable effect on the energy harvesting performance of the test-rig, where the m(ib) = 0.90 kg/block case showed a 36.5% and 21.13% decline in performance compared to the m(ib) = 0.45 and 1.35 kg/block cases, respectively at theta(o) = 60 degrees and U= 0.57 m/s. This shows that the energy harvester is sensitive to changes in inertial loads, affecting the forcemotion synchronization which eventually affects its performance.