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The variable temperature characteristics of isobaric phase transition process for zeotropic mixture can effectively reduce the heat transfer irreversibility between working fluids and heat sources or heat sinks for thermodynamic cycle systems. One of the key problems in the application of zeotropic mixtures for power generation systems is how to determine their optimal components and compositions. The traditional method is mainly based on the exhaustive method, which means that every considered component and their compositions in the mixture are optimized one by one and mainly caused by the independence and discontinuity of components. This process will consume a large amount of computing resources, which limits the optimization of zeotropic mixtures. In order to describe the discrete components and optimize them, the selective coefficient representing every component alternative is introduced and this paper recommends a simultaneous approach to achieve the optimal components and compositions of zeotropic mixture at the same time. Two case studies with different heat sources and sinks as well as system layouts are illustrated to show the proposed approach. Besides, it also proves the systems with zeotropic mixture have better performance than that of systems with pure 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]