A Strategy for Sizing and Optimizing the Energy System on Long-Range AUVs

Abstract

Autonomous underwater vehicles (AUVs) are utilized in a variety of unmanned missions, such as environmental surveys or guarding coastal waters. Currently, the most commonly used energy storage is a secondary battery pack. A potential solution for increasing the range even further on larger AUVs is to utilize hybrid fuel cell/battery systems. Fuel cell systems can have a significantly increased specific energy compared to batteries and are now starting to become well-developed technologies. However, in an underwater environment, both hydrogen and oxygen need to be stored for the fuel cell. This study considers the sizing of a hybrid fuel cell/battery system with respect to operational power profiles, i.e., how to select the best combination of fuel cell stack, battery pack, and reactant storage, and compares its volume and weight with conventional battery systems. This article presents a sizing strategy developed for this purpose, where the optimum combination of fuel cell output power and battery size is analyzed. The strategy is implemented on power profiles from real AUV missions. The sizing strategy will outline the techniques to reach the optimum hybrid configuration capable of meeting the power demand at any time of the mission. Results show that for longer missions, the volume of the hybrid system becomes significantly lower than for the battery system, meaning that the endurance can be increased thanks to the use of fuel cells.

Including Heat Balance When Designing the Energy System of Fuel Cell-Powered AUVs

“…For each mission considered, the same procedure as used for the 50 h mission is implemented, and the score of each alternative is calculated using both the entropy and the CRITIC weighting methods, see Figures 4 and 5. For both methods, and for all alternatives, the score changes when increasing the length of mission from 1 to 100 h, while a plateau is reached when duration of mission exceeds 100 h. This trend is due to the fact that the energy density and specific energy of the hybrid systems reach an asymptote when the mission duration increases, as observed in [4]. Furthermore, the best-ranked alternatives remain the same (A 8 , A 7 and A 2 ) for both methods when the mission becomes longer than 20 h, while the worst-ranked alternative is always the combination of Comp H 2 and H 2 O 2 (A 3 ).…”

“…The use of a hydrogen fuel cell can increase significantly the energy content in the AUV, and therefore its endurance [1][2][3]. It was demonstrated that for missions longer than a few hours, volume-optimized hybrid fuel cell/battery systems are more relevant for AUVs than full battery systems, as they combine the high energy density of fuel cells and the power flexibility of batteries [4]. Since the late 90s, several projects have demonstrated the capabilities of low temperature fuel cell-powered AUVs using various hydrogen and oxygen storage units, fuel cells and batteries [5][6][7][8].…”

“…Different storage solutions have different materials, chemistries and heat properties; therefore, the choice of storage has a direct impact not only on the final volume and weight of the energy system, but also on the heat management. Strategies and guidelines about the energy-system design of fuel cell-powered AUVs are found in the literature [1,4,11,12]. However, they focus on the energy content of the hybrid system, while thermal properties of the components and the total heat balance of the energy system are not considered.…”

“…However, in the case of an AUV, the available volume and weight are limited, which makes it complicated to add subcomponents, such as heat exchangers or heat pumps, in order to reuse the heat. For the AUV, the final buoyancy is also a crucial parameter that must be considered when designing and selecting components for the energy system [4,11].…”

Research on Energy Supply System Applied to Autonomous Underwater Observation Vehicles

“…With the improvement of people's understanding of the ocean, the exploration and development of Marine environments covering 70% of the surface has received extensive attention. Autonomous underwater observation vehicle is an autonomous underwater equipment suitable for various environments, characterized by long working hours and wide range of activities [ 1 – 6 ]. Therefore, the energy supply requirements of this equipment are relatively high.…”