Optimization and analysis of reliquefaction system utilizing hydrogen as refrigerant for liquid hydrogen carriers

Abstract

In this study, a reliquefaction system utilizing a reverse Brayton cycle with hydrogen as a refrigerant to handle boil-off hydrogen (BOH) generated within liquid hydrogen carriers operating in oceanic routes from a thermodynamic perspective is proposed. Furthermore, optimization targeting specific energy consumption (SEC) as the fitness function was performed using a genetic algorithm. Subsequently, an economic analysis was conducted at the equipment unit level for the reliquefaction system. Thermodynamic analysis of the optimized system revealed a SEC of 5.028 kWh/kg and an exergy efficiency of 44.48 %. During optimization, the circulation rates of BOH and refrigerant decreased, leading to a significant reduction in the exergy destruction within the heat exchangers and an improvement in the system efficiency. Notably, the optimization results indicated the significant influence of the heat exchange conditions in the heat exchangers on the overall system efficiency. It is also worth noting that the optimal BOH compression pressure was approximately 25 bara, which is similar to the pressure of the raw material hydrogen used as a feed gas in onshore hydrogen liquefaction systems. This suggests that the design experience gained from manufacturing equipment for onshore hydrogen liquefaction systems can provide significant benefits when producing equipment for onboard BOH reliquefaction systems. Economic analysis revealed that in the optimized system, it costs USD 1.09 to reliquefy 1 kg of BOH.