Off-design and flexibility analyses of combined cooling and power based liquified natural gas (LNG) cold energy utilization system under fluctuating regasification rates

Abstract

The Liquified Natural Gas (LNG) regasification rate tends to fluctuate during the diurnal period due to the varying demand of natural gas. This fluctuation leads to the energy mismatch between LNG cold energy utilization system and users. Consequently, the actual LNG cold energy utilization efficiency deteriorates significantly. In this study, a novel cold storage-based combined cooling and power system has been designed to improve the efficiency and versatility of LNG cold energy utilization under fluctuating regasification rates. The system incorporates a CO2 based Rankine cycle, a direct expansion unit, and cooling recovery units. The waste heat (90 °C) harnessed from gas power plants instead of seawater (15 °C) is employed as the heat source to improve the Rankine cycle performance. Further, cold energy storage tanks are adopted to decouple the power and cooling outputs from the fluctuating regasification rates. Off-design performance of the system is investigated under the different regasification rates. Key results have revealed that the power output profile from the conventional system is monotonous and the valley-to-peak ratio of power is as low as 0.05, resulting in potential downtime for downstream users. In contrast, the proposed system implements a versatile power and cooling strategy to satisfy different users’ demands. Results from two case studies demonstrate that the valley-to-peak ratio of power can be markedly improved to values spanning 0.4 to 1. The daily cold recovery rate, exergy efficiency, and reducing CO2 emission approach 53.45%, 25.26%, and 645.13 tons/day, respectively, for a 3 megatons per annum regasification LNG plant. The payback period for the proposed system is 0.67 years, which is almost half of the conventional system’s period.