Life Cycle and Technical Evaluation of Fuel Cell–Battery and Diesel–Battery Hybrid Propulsion Systems for USV Application

The transition to sustainable and self-suffcient fuel alternatives remains an unexplored area for the Swedish Navy. To address this challenge, a comparative study has been conducted between a serial fuel cell–battery (FC/BE) hybrid system, a parallel battery–diesel (BE/Diesel) hybrid system, and a conventional diesel propulsion system. The three propulsion systems are evaluated through a Life Cycle Assessment (LCA) conducted in GaBi, focusing on four impact categories: Global Warming Potential (GWP), Acidifcation Potential (AP), Eutrophication Potential (EP), and Photochemical Ozone Creation Potential (POCP). In addition, the propulsion systems are evaluated based on technical parameters such as weight and volume. The results show that the Fuel Cell/Battery hybrid system has a signifcantly lower GWP, nearly one-tenth compared to the Battery/Diesel hybrid and the Diesel system. However, for AP, the FC/BE system has about 50% higher impact than the other two. Regarding EP, the Diesel system performs best, with only about one-third the impact compared to the two hybrid systems. As for POCP, the values are similar across all three systems. When comparing weight and volume, the Diesel system is the most compact and lightweight. The Fuel Cell/Battery hybrid is the largest and heaviest, which may introduce integration challenges. It is evident that the FC/BE hybrid offers the greatest potential from an environmental perspective. Although it generates higher emissions of SO2-eq and P hosphate-eq compared to the other systems, its CO2-eq emissions are signifcantly lower. In terms of weight, the diesel system is the lightest, followed by the BE/Diesel hybrid and then the FC/BE hybrid. The diesel system is 42% lighter than the FC/BE hybrid. However, the difference in volume is even more notable: the diesel system requires 62% less volume than the FC/BE hybrid, while the BE/Diesel hybrid is 45% smaller in volume.