Waste-heat-driven absorption intercooling of an onboard LNG boil-off gas re-liquefaction plant: A 4E analysis

Re-liquefying boil-off gas consumes 2.24 MW aboard a liquefied natural gas (LNG) carrier, and the nitrogen reverse Brayton cycle (N 2 RBC) used is limited by 32 °C seawater. No unified energy, exergy, economic and environmental (4E) assessment exists for an exhaust-driven ammonia-water absorption refrigeration cycle (ARC) coupled to such a plant, nor any marine costing basis. The two cycles are solved as one steady-state model against the unmodified seawater-intercooled plant, with three least-dependent criteria optimized simultaneously and TOPSIS, LINMAP and entropy weighting compared. Chilling the intercooler water to 5.3 °C lowers the compressor inlet from 37 to 10.3 °C and cuts specific energy consumption 8.8 percent, from 0.691 to 0.630 kWh per kilogram. Net auxiliary demand falls 198 kW and second-law efficiency rises from 30.70 to 33.68 percent, the absorption chiller yield cooling power 1.30 MW with two main engines 272 °C exhaust gas. Irreversibility in the nitrogen loop falls 12.6 percent, but the absorption subsystem adds more than it removes; that destruction is drawn from exhaust the baseline vents, carrying no fuel penalty. Moreover, land-based costs corrected for shipboard material, geometry, and installation yield absorption-plant capital from 1.37 million USD, against 1.50 million USD total incremental capital. For the economic viability, Discounted payback is 9.97 years at 8 percent over 20 years, internal rate of return 13.81 percent, and 869 tonnes of carbon dioxide are avoided annually. Last but not least, assessed for tropical service at 0.8 USD per kilogram of fuel over 7,224 operating hours per year, the retrofit loses its positive net present value if capital exceeds the estimate by more than 47 percent or fuel falls below 0.55 USD per kilogram.