Academics Jingru Xing and Liang Yang from the Division of Energy and Sustainability at Cranfield University have recently published a paper which explores the concept of using wave energy for propulsion.
The paper, available in “Renewable and Sustainable Energy Reviews”, provides “a comprehensive review of flapping foils for Wave Devouring Propulsion (WDP)”. The flapping foil, inspired by the fin of a whale, and first proposed back in the 1950s, can utilise wave energy to generate thrust and has a number of applications, including in small, unmanned surface vessels (USVs) and hybrid propulsion systems for manned vessels.
As the paper explains, the flapping foils used in Wave Devouring Propulsion Systems (WDPSs) have three modes of motion: heave (the vertical movement of the foil), pitch (rotation around a pivot point), and flap (the combination of heave and pitch). The movement of the foils can also be constrained in three ways: passive (where the flapping motion is flow-induced, and the foils have springs to restore equilibrium); semi-passive/semi-active (where one of the pitch or heave motions is forced into a trajectory while the other is flow-induced); and active (where both pitch and heave are forced). Perhaps unsurprisingly, passive foils may be least efficient - but on the other hand, cause less disruption to the wave flow. WDPSs therefore provide a range of options, depending on the requirements of the specific application.
The paper also conducts a comprehensive review of the industrial development of WDP devices, outlining existing applications, products, and prototypes of foils and ships. These include the Wave Glider by Liquid Robotics, Autonomous Surface Vehicles by AutoNaut, and the M/F Teistin by Wavefoil, a vessel equipped with retractable bow foils.
Finally, the paper provides inspiration for future research, including exploring the foil-ship hull-wave interaction, the optimal number and placement of foils (see, for example, the tanker ship with two foils in the image above (Bøckmann et al)), optimizing ship and foil size, submergence depth and so on, and analyzing the economic aspects of WDP.
The ability to provide propulsion via wave energy, reducing fuel costs and the carbon footprint of shipping energy, is intriguing, and no doubt this new research will provide a springboard for future innovation.
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