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Aiming at the problem that the irreversible loss of the condensation process for the traditional LNG power generation system is too large, this paper presents four different two-stage condensation Rankine cycles based on different arrangements of pumps and turbines. The effects of the operating conditions (the first-stage and second-stage condensation temperatures) and various working fluids on the performance of the four cycles with different structures are investigated. The results show that when the arrangement of the turbines is the same, the arrangement of the pumps (series or parallel) has little effect on the net output power. However, when the arrangement for the pumps is the same, the arrangement for the turbines has a significant effect on the net output power. When the working fluid is n-pentane and the turbines are of series arrangement, the maximum net output power is increased by about 17% compared with the condition that the turbines are of parallel arrangement. In addition, for the same cycle, whether the isentropic efficiency of turbine is a constant value or not has a significant effect on the maximum net output power, the optimal operating conditions, and the optimal working fluid. [ABSTRACT FROM AUTHOR]

For the low efficiency of the traditional power generation system with liquefied natural gas (LNG) cold energy utilization, by improving the heat transfer characteristic between the working fluid and LNG, this paper has proposed a two-stage condensation Rankine cycle (TCRC) system. Using propane as working fluid, compared with the combined cycle in the conventional LNG cold energy power generation method, the net power output, thermal efficiency and exergy efficiency of the TCRC system are respectively increased by 45.27%, 42.91% and 52.31%. Meanwhile, the effects of the first-stage and second-stage condensation temperature and LNG vaporization pressure on the performance and cost index of the TCRC system (net power output, thermal efficiency, exergy efficiency and UA) are analyzed. Finally, using the net power output as the objective function, with 14 organic fluids (such as propane, butane etc.) as working fluids, the first-stage and second-stage condensation temperature at different LNG vaporization pressures are optimized. The results show that there exists a first-stage and second-stage condensation temperature making the performance of the TCRC system optimal. When LNG vaporization pressure is supercritical pressure, R116 has the best economy among all the investigated working fluids, and while R150 and R23 are better when the vaporization pressure of LNG is subcritical. [ABSTRACT FROM AUTHOR]