Design and thermodynamic evaluation of onboard NH3 BOG re-liquefaction systems for ocean-going NH3 Carriers

Abstract

This study stems from the burgeoning interest in ammonia (NH₃) as a green energy source, particularly for maritime applications where conventional refrigeration cycles pose both environmental and economic challenges, specifically focusing on an 88,000 m³ class Very Large Ammonia Carrier (VLAC). Two distinct refrigeration cycle concepts were evaluated for the re-liquefaction system. The optimization technique used in the study was a hybrid method that combined the SQP and BOX algorithms to optimize the system. Key process variables were set to the final compression and expansion pressures of the refrigeration cycle, which were optimized to minimize the specific energy consumption (SEC) of the systems. An economic analysis was conducted to assess the costs associated with the equipment used in both systems. The first optimized re-liquefaction system employs a vapor-compression refrigeration cycle using NH₃ as the refrigerant. The thermodynamic analysis indicated energy consumption, SEC, and exergy efficiency of 112.44 kW, 0.1898 kWh/kg, and 38.31 %, respectively. The second system utilizing the Linde–Hampson refrigeration cycle demonstrated energy consumption, SEC, and exergy efficiency of 102.35 kW, 0.1728 kWh/kg, and 43.03 %, respectively. Exergy destruction within these systems was predominantly observed in the heat exchangers, accounting for 43.00 % and 51.80 % of the total exergy destruction, respectively. Economic analysis revealed that the life cycle cost (LCC) and sensitivity analysis of the re-liquefaction system using the Linde-Hampson refrigeration cycle are approximately 2.0 million USD lower than the system using the vapor compression refrigeration cycle. In conclusion, the Linde-Hampson re-liquefaction system is energy efficient and economical.