Multi-objective optimization of a novel combined parallel power generation system using CO2 and N2 for cascade recovery of LNG cryogenic energy

Liquefied natural gas (LNG) is a kind of clean fuel with high energy density, and a large amount of cold energy will be released during gasification, which should be recycled for energy conservation. A novel combined parallel power generation system respectively using CO2 and N2 as working fluid is proposed for cascade utilization for LNG cryogenic energy. The solar heating system is introduced to improve the overall performance, and the obstacle of discontinuous operation of the solar system is solved by using two serial turbines with different inlet parameters. Thermodynamic analysis and multi-objective optimization are conducted for the novel system, and performance comparison has been made with combined N2 and LNG system with or without regenerator, combined CO2 and LNG system, LNG direct expansion system. Results show that the novel parallel power generation could realize the cascade utilization of LNG cold energy with less exergy loss and better overall performance compared to the other four systems, and it is more efficient than most ORC systems with less complex configurations. The optimal average net power of the novel system is 12.4816 MW, and the energy efficiency and exergy efficiency are 17.25% and 29.90%, respectively.