Understanding the 3D construction method of thermodynamic cycle: Insights from limiting performance of pure working fluid

The 2D temperature-entropy diagram, which is commonly applied to construct and analyze conventional thermodynamic cycles, cannot clearly express novel thermodynamic cycles with multi-working fluids. The 3D construction method provides a methodological guidance to the construction and analysis of novel thermodynamic cycles. In order to understand the mechanism of the 3D construction method, in this study, the limiting performance of 3D thermodynamic cycle with pure working fluid is analyzed and compared with the limiting performance of organic Rankine cycle. Based on the diagram of the 3D coordinates in terms of temperature, entropy, and mass flow rate, the 3D limiting thermodynamic cycle is defined and decoupled into two limiting organic Rankine cycle first. Then, equations of limiting thermal efficiency and thermodynamic perfection are derived based on pure working fluid. Three parameters affecting limiting performance are revealed, namely the latent heat, the slope of the saturated liquid line in the temperature-entropy diagram, and the separation quality. Lastly, effects of key operating parameters on the limiting efficiency of the 3D cycle are analyzed. With the increase of high temperature or middle temperature, the limiting thermal efficiency increases, and thermodynamic perfection increases first and then declines. With the increase of separation quality, the limiting thermal efficiency shows a downward trend. However, the trend of thermodynamic perfection with separation quality depends on values of high temperature and middle temperature. Besides, the limiting performance between the 3D cycle and conventional organic Rankine cycle is compared. The limiting thermodynamic perfection of 3D cycle using R245fa is 7.3%-16.42% higher than that of conventional organic Rankine cycle.