Thermodynamic Analysis and Optimization of Shipborne Cascade ORC/TCO2 Power System Utilizing LNG Cold Energy

Liquefied natural gas (LNG)-powered ships possess available utilizable energy resources, including LNG vaporization cold energy and main engine flue gas waste heat. To improve the ship energy utilization efficiency, this paper takes a 215,000-ton Very Large Crude Carrier (VLCC) as the research object, and couples the organic Rankine cycle (ORC) and transcritical carbon dioxide (TCO2) Rankine cycle to achieve the combined recovery of ship cold energy and waste heat. To systematically investigate the thermal performance of the integrated system, a simulation model is established by using Aspen HYSYS to acquire basic thermal parameters. Advanced exergy analysis is adopted as the core research method for the first time to evaluate the thermal performance of the system, and the obtained results are compared with those of the conventional exergy analysis. Conventional exergy analysis reveals that Heat Exchanger 1 is the core component restricting the performance optimization of the whole system. Further results from advanced exergy analysis show that the proportions of avoidable exergy destruction and endogenous exergy destruction of the overall system are 40.46% and 58.72%, respectively. Key optimization links are further identified based on the advanced exergy analysis results, and the genetic algorithm (GA) is applied for targeted system optimization. After optimization, the system output power reaches 1772.06 kW and the exergy efficiency is 48.32%, which are increased by 5.56% and 15.54% correspondingly. Economic evaluation indicates the annual net profit reaches 3.8336 million CNY, with a static payback period of 5.49 years.