Can a Conventional Propulsion System Match the Efficiency of an Underwater Glider Buoyancy Engine?

Abstract

Autonomous underwater gliders are a family of autonomous vehicles used for a long-range, long-term observation of oceanic environments. To do this, they leverage changes in net buoyancy and the resulting vertical motion to generate forward locomotion via hydrodynamic surfaces. To function for extended periods, these systems operate in a low-speed, low-drag regime. Conventionally propelled underwater vehicles typically operate at speeds in excess of those achievable by gliders and therefore require more energy in order to compensate for losses due to hydrodynamic drag. An interesting question arises when considering the operational efficiencies of conventionally propelled systems when they operate at speeds typical of underwater gliders. A first-principles energy-based approach to glider operations was derived and verified using real-world data. The energy usage for buoyancy-driven propulsion was compared to conventional propulsion types. The results from these calculations indicate that a conventionally propelled autonomous underwater vehicle can compete with a buoyancy-driven system given the proper propulsive efficiency.

Research on Sailing Efficiency of Hybrid-Driven Underwater Glider at Zero Angle of Attack

“…Furthermore, in cases that require a horizontal translation only, the energy needed to propel a traditional neutrally-buoyant AUV will be less than the energy consumed by the variable buoyancy system for the glider with a zigzag course to travel the same voyage [23]. Taking Slocum glider as an example, Hockley and Butka [24] compared the energy consumption of the glider driven by the variable buoyancy system with the propulsion system to achieve the same profile. The result shows that the energy consumption of the glider driven by the propulsion system is only 82% of that driven by the variable buoyancy system.…”

Developing the next generation of Autonomous Underwater Gliders

“…Given the expansion of propulsion modes, a new set of optimization problems has emerged regarding the most energy efficient form of locomotion for the hybrid glider. Both Hockley et al [61] and Duguid et al [21] explored the energy consumption of the buoyancy engine as a function of depth and reached similar conclusions. In water depths shallower than 200 m, the transport cost of using the buoyancy engine to travel a certain horizontal speed is comparable, and in some cases greater than the transport cost incurred for continuous thruster-driven travel.…”

“…The previous work of Hockley et al [61] and Duguid et al [21] explored whether an AUG thruster could match the efficiency of a buoyancy engine in certain use cases. For the purpose of reaching an empirical conclusion, Hockley et al [61] chose a 200 m rated Slocum glider with a 250 cm 3 buoyancy engine and the standard physical properties of the vehicle listed within [12].…”

“…The previous work of Hockley et al [61] and Duguid et al [21] explored whether an AUG thruster could match the efficiency of a buoyancy engine in certain use cases. For the purpose of reaching an empirical conclusion, Hockley et al [61] chose a 200 m rated Slocum glider with a 250 cm 3 buoyancy engine and the standard physical properties of the vehicle listed within [12]. Steady-state glide data was taken from Graver et al [71]; per the data, the vehicle was reached a 24°glide path angle on ascent and descent (a pitch angle of 26 ∘ was commanded) with a pumped ballast of 240 cm 3 .…”

“…Multiplying the thrust force value over the horizontal glide distance for one glide cycle and assuming a thruster efficiency of 41% yields an energy consumption 23% less than that of the buoyancy engine. Hockley et al [61] chose a force-based comparison of propulsion efficiency; however, a more direct comparison of propulsion efficiency exists for a speed-based comparison. Simply translating the magnitude of the buoyant force in the horizontal plane to an equivalent thrust force does not equate to an identical vehicle speed in the horizontal plane.…”