Comparative analysis: Exergetic and economic assessment of LNG cold energy power generation systems with different cold utilization methods

Abstract

Harnessing the cold energy inherent in LNG transportation processes can significantly mitigate energy wastage. Employing an innovative incremental analysis methodology, this study scrutinizes six LNG cold energy power generation systems, featuring a newly proposed parallel and cascade combined cycle (PAC) system. A novel approach that setting the minimum pressure within the systems higher than atmospheric levels has been adopted, for the optimal working fluid selection. The net power output (Wnet) of the direct expansion (DC) system registers at −129.51 kW, while both the Single-Stage Organic Rankine Cycle (SORC) and Combined Cycle (CC) systems yield Wnet of 2868.46 kW and 3081.46 kW with R32 as working fluid. Despite the lower available exergy extraction of the working fluid, the CC system outperforms due to its superior efficiency in converting pressure exergy into power output. Sensitivity analysis suggests the optimal Tcon in CC is limited by the normal boiling point temperature (NBPT) of working fluid, while that in SORC remains unrestricted. The maximum Wnet of two-stage Parallel Combined Cycle (PCC) and Cascade Combined Cycle (CCC) can reach 3291.65 kW and 4268.78 with the optimal working fluid combinations R32 + propane, and ethane + propane. The reason Wnet of CCC outperforms is the cascade utilization of LNG cold exergy enables the working fluid in its second stage obtains significantly 9.49 times higher amount of exergy compared to PCC. Through sensitivity analysis, while Tcon1 mostly predominates the performance of PCC, both Tcon1 and Tcon2 exert substantial influence on CCC. For the three-stage PAC, its Wnet can reach the highest 4700.82 kW with ethane + propane + propane. It is because the PAC not only utilizes LNG’s cold exergy in a cascaded manner, but also obtains the exergy from working fluid. According to economic analysis, the PAC system exhibits an advantage with the highest annul total net income (ATNI). And the CC emerges as a cost-effective choice if the irreversible losses and Wnet are not considered. According to innovatively explores about the impact of direct expansion and changes in NG outlet pressure on working fluid selection and economic feasibility, it shows the optimal combinations remain the same, and the systems incorporating a direct expansion component with lower NG outlet pressure demonstrate a more economically advantageous solution.