Development & performance enhancement of an AUV wave-charging system

Abstract

Adoption of autonomous underwater vehicle (AUV) technology has recently experienced rapid growth, fueled by possibilities enabled by technological advances. AUVs are particularly useful as unmanned survey platforms, and typically have an array of on-board sensors to collect data for a variety of commercial and military applications. AUVs are autonomous and untethered systems and require a power source, typically batteries, to be carried onboard. An increase in available energy by even a small amount can be game-changing for AUV applications with benefits including longer mission durations, higher sampling rate, more sensing capability, and improved communication capability. This can be accomplished through some self-recharging capability within the AUV, allowing the AUV to extract energy from its surrounding environment, and eliminating the need to recover the vehicle until the mission is complete.  This work presents a Wave Power Capture System (WPCS) that can be integrated into an AUV, allowing it to operate for significantly longer periods of time without the need for recovery. This concept utilizes two rotary power take-off (PTO) units that are driven by two independent tendons, located axially along the length of the body. The two tendons are connected to a retractable reaction plate that can be stowed against the body of the AUV when not in use and deployed autonomously when the AUV needs to surface and recharge. This arrangement allows both pitch and heave motion to be primary contributors to relative (power generating) motion. Additional motion in surge, sway and yaw will also result in some secondary power generation. This work focuses on the hydrodynamics and design of the reaction plate so that power capture and quality are enhanced. The geometry of the reaction plate will be constrained to a baseline semicircular shape, allowing the AUV to operate normally when the plate is stowed. Based on previous work, which has indicated that increasing the reaction plate added mass improves WEC power capture, the current work will thus look at different modifications to the reaction plate that effectively enhance its size when deployed, for example, incorporating multiple ‘nested’ reaction plates that mate together when stowed. The light weight of the reaction plate relative to its size means that there may be a tendency for the tendons to go slack and subsequently experience snap loading in cases where the reaction plate does not fall as fast as the AUV body. This work will further investigate the incorporation of dynamically adaptable geometries that reduce the reaction plate’s hydrodynamic resistance in the downward direction, for example, a structure that contains multiple flaps that hinge open during downward travel. A series of experiments will be presented in which the different reaction plate concepts are sinusoidally forced in a quiescent basin to characterize the translational and rotational hydrodynamic coefficients over a range of representative frequencies and amplitudes. Finally, a time-domain model developed using ProteusDS software, informed by the hydrodynamic coefficients measured experimentally, will be used to calculate AUV power performance in different sea states and evaluate the effect of the different reaction plate modifications.