On the Performance of Long-Range Autonomous Underwater Vehicles
Enhancing the Endurance of AUVs
Time: Wed 2022-06-15 09.00
Location: Auditorium, Kristineberg Center, 450 34 Fiskebäcksil
Subject area: Vehicle and Maritime Engineering
Doctoral student: Clemens Deutsch , Marina system, Lättkonstruktioner, marina system, flyg- och rymdteknik, rörelsemekanik, Centre for Naval Architecture
Opponent: Professor Martin Ludvigsen, Norwegian University of Science and Technology
Supervisor: Professor Jakob Kuttenkeuler, Marina system, Flygteknik, Farkost- och flygteknik, Lättkonstruktioner, Farkostteknik och Solidmekanik; Roger Berg, Lättkonstruktioner, marina system, flyg- och rymdteknik, rörelsemekanik
Autonomous underwater vehicles (AUVs) are robotic platforms that are commonly used to gather environmental data, provide bathymetric images, and perform manipulation tasks. These robots are used not only for scientific, but also for industrial and military purposes. Climate change, political instabilities, and the increasing demand for both renewable and fossil energy sources have created a need for high-performance AUVs and particularly long-range AUVs.
The performance of long-range AUVs is characterised by several parameters, such as autonomous decision making, accurate navigation, system reliability, and vehicle endurance. The vehicle’s endurance is the key capability enabling long-range missions and is determined by the energy capacity and power consumption. By cruising at optimum speed, the vehicle endurance can be utilised most efficiently, resulting in the longest achievable vehicle range. The range of AUVs can be extended by maximising the available energy capacity and by minimising the overall power consumption. This thesis shows how the choices of propulsion system and power source can help improving the range of AUVs.
The power consumption comprises the hotel load and propulsive power. While the hotel load is largely depending on the payload sensors, the propulsive power can be minimised by choosing the right propulsion system. As a part of this thesis, the transit performance of underwater gliders is analysed using an analytical approach. The analysis yields a glide metric for the assessment of the energy efficiency of underwater gliding and allows for comparison to other conventional propulsion systems.
The most common energy systems for AUVs are primary and secondary electrochemical cells, in particular lithium-ion batteries. Alternative energy systems such as fuel cell (FC) systems can potentially improve the range of AUVs. Through a conceptual design study using off-the-shelf components, it is shown how FC systems can increase the energy capacity of AUVs. FC systems are typically implemented as hybrid systems paired with a small capacity battery system. Energy management strategies (EMS) are required to coordinate these two power sources. In this thesis, deterministic and optimisation-based strategies have been tested in simulations and evaluated against realistic AUV power consumption data from field trials. The results suggest that the complexity of the EMS needs to grow with mission complexity. While deterministic methods can yield the lowest energy consumption for standard missions (e.g. bathymetric imaging), optimisation-based methods provide best load-following behavior, making these methods better suited for retaining power reliability through maintaining battery state of charge.