Thermodynamic investigation and optimization of two novel combined power-refrigeration cycles using cryogenic LNG energy

The present study aimed to introduce two novel combined power-refrigeration cycles into optimal usage of liquefied natural gas (LNG) cryogenic energy and reduce exergy destruction due to high-temperature difference in the heat transfer process. The combined cycles include a compression-ejector refrigeration cycle and two low and high-pressure Rankine cycles in which the power required to drive the compression-ejector refrigeration cycle compressor is provided by the power generated by the two low and high-pressure Rankine cycle turbines. Increasing the utilizable cooling energy of LNG is considered as the benefits of the new combined cycles compared to direct LNG evaporation. A comprehensive thermodynamic analysis, along with optimizing both novel combined power-refrigeration cycles, was performed through the first and second thermodynamics laws and the constant area model assumption for the ejector. Analyzing the design parameters demonstrated that the maximum energy efficiency, exergy efficiency, and the highest refrigeration increasing ratio (RIR) increase in both novel combined power refrigeration cycles as increasing the pump discharge pressure and decreasing turbine outlet pressure of the low-pressure Rankine cycle. The maximum thermal efficiency and exergy efficiency were 77.3% and 23.7% in cycle I and 87.5% and 23.9% in cycle II, respectively, through performing optimization in the boundary set for the design parameters. Finally, the highest obtainable cooling energy to direct the evaporation of LNG ratio in the two cycles I and II was 62.6% and 73.9%, respectively. (c) 2020 Elsevier Ltd and IIR. All rights reserved.

Automated summary

This study introduced two novel combined power-refrigeration cycles to optimize the utilization of liquefied natural gas (LNG) cryogenic energy and reduce exergy destruction. By analyzing design parameters and performing optimization, the cycles showed improved energy efficiency, exergy efficiency, and refrigeration increasing ratio compared to direct LNG evaporation. The maximum obtainable cooling energy ratios in the cycles I and II were 62.6% and 73.9%, respectively.