Study on a flapping foil based flow energy harvester

Abstract

Energy shortage has attracted great attention in past few decades. To achieve sustainable economic and social development, finding renewable energy from various sources is a vital solution. Inspired by natural flyers and swimmers, a novel energy harvesting concept has been proposed recently, where a flapping wing is utilized to extract energy from air/water flows. Compared to conventional turbines based technologies, this concept has great advantages, including lower cut-in speeds, better filling factors, better scalability and environmental friendly. Previous investigations have revealed that the power efficiency of this concept is comparable to rotary turbines. However, these investigations were majorly focused on fully prescribed or semi-passive systems, and barely touched fully passive systems, in which the flapping wing is purely driven by the incoming flow. To fill this gap, recently we designed and developed a device that is able to extract flow energy using a fully passive flapping foil, and investigated its performance under various geometrical and flow conditions. The power extraction efficiency can be up to 32%. As follow-up work, the present study aims to further improve the system performance through understanding the effects of several other key factors and investigating detailed fluid-structure interactions (FSI). First, a group of optimal parameters are tired to be studied by the experimental method. To achieved it, some unexplored parameters are investigated at start. The results show that with the increasing of heaving limiters distance dh, the net power extraction becomes larger, but due to the faster increase of swept distance, the efficiency of the system decreases linearly. As the distance between heaving limiter and the pivot axis sweep plane dx becomes larger, the system performance changes very smoothly. By further studied the system performance at different pitching amplitudes and free stream velocity, the largest energy extraction efficiency (η≈40%) is achieved. Additionally, with the pitching amplitudes becoming larger at smaller heaving limiters distance, the pure heaving stage may disappear. Second, a theoretical model is developed to help further improve the system. It was found that the model can predict the system performance well, especially for the lower pitching amplitude and smaller heaving limiter distance cases. Moreover, the model also confirms that there may be a suitable heaving mass and inertia moment, which can achieve the best system performance. Lastly, we found that for this fully passive flapping system with a rigid plate foil, the system can work constantly as the pivot location xp is after around c/3 and not after the mid-chord point. Third, the effect of the foil's flexibility on the system performance is studied. The flexibility is introduced by attaching a small head/tail plate to the main foil through a pair of torsional springs. The water-tunnel test results reveal that as a tail installed, an optimal stiffness exists for power extraction when the set pitching amplitude is not too small. Conversely, if a head plate is attached, its passive deflection can enhance the system performance at lower pitching amplitudes, and there is also an optimal stiffness and length where the maximum efficiency can be attained. Last, the system is extended to a tandem configuration where two flapping foils are arranged in array. The effects of two key parameters are studied, i.e., the gap and phase difference between the two foils. Results show that the dynamics of the downstream foil is affected by the center-to-center distance, but independent of the initial conditions. The relation between center-to-center distance and phase difference can be expressed by a simple equation. Moreover, the interaction between the wake and downstream systems only can enhance the system performance with a limited center-to-center-distance. This research enriches our understanding in the FSI and energy extraction of the fully passive flow energy harvester, which is essential for further improving and maturing this novel "green" concept.