Thermodynamic analysis and optimization of a multi-stage Rankine cycle power system combining with hydrate energy storage for liquefied natural gas cold energy utilization

Natural gas (NG) as a primary energy source plays an important in industrial systems. Natural gas generally needs to be converted into liquified natural gas (LNG) during transportation and stored at a low temperature (around-162 °C), which contains a large amount of cold energy that can be used. The discharge of cold energy into the ocean or atmosphere will cause not only energy waste but also environmental pollution. In order to recover LNG cold energy more efficiently, a novel LNG cold energy utilization system integrating Organic Rankine cycle (ORC), transcritical CO₂ cycle (TRCC), CO₂ energy storage and seawater ice-making cycle and combustion power system is proposed and investigated in this study. Energy analysis and exergy analysis were studied in details. The exergy efficiency and net electric generation (NEG) were analyzed and optimized using the optimizer in Aspen HYSYS. The results reveal that, exergy loss ratio of combustion power system and CO₂ energy storage system are relatively larger than that of other subsystems. In addition, The ORC performance of six working fluids was analyzed. The NEG of R290 and R1270 as working fluid of ORC is significantly higher than that of the other four working fluids. Under the condition of basic process simulation, the NEG of ORC and TRCC is 223.7 kW and 216.4 kW, respectively. The total exergy efficiency reaches 39.01 %. After parameter optimization in the optimizer, the total NEG of ORC and TRCC increases by 6.9 % from 440.1 kW to 470.6 kW and the exergy efficiency can reach 39.8 %.