Exergy-economic performance comparison of three organic flash cycles integrated with an LNG subsystem to produce electricity and sub-zero cooling

Abstract

The performances of three organic cycles based on the flash process combined with a liquefied natural gas (LNG) subsystem are assessed and compared from thermodynamic and economic standpoints. The thermodynamic analysis of the systems comprises energy and exergy analyses, and the specific exergy costing technique was employed to conduct the economic analysis of the systems. The topping cycle is an organic flash cycle (OFC), an organic flash regenerative cycle (OFRC), or a double-expansion organic flash cycle (DEOFC). Enjoying three advantages, the LNG subsystem is utilized in the configurations. First, it improves the topping cycle performance by reducing the condensation temperature and pressure. Second, the cryogenic exergy of the natural gas in the bottoming system is utilized in two stages to produce sub-zero cooling. Third, a high amount of the LNG exergy can be converted into power through a turbine. The optimized performance of the three configurations revealed the superiority of the DEOFC-based system over the two other systems with an exergy efficiency of 43.4%, a total cost rate of 223.7 $h-1, and a specific cost of cogeneration of 29.69 $GJ-1. Moreover, the net output power and the total cooling rate produced by the DEOFC-based system are higher than the other systems. A comparison between the performances of the topping cycles and those of a study in the literature indicates a significant improvement in the exergy efficiency and the specific work produced by the topping cycles due to the use of the LNG subsystem. The exergy efficiency of the OFC, OFRC, and DEOFC is improved by 26.4% points, 26.5% points, and 36.8% points, respectively. Moreover, the present setups outperform many previously-introduced configurations, as evidenced by comparing the performances of the three systems and those proposed in the literature.