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

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.