How to approach Carnot cycle via zeotropic working fluid: Research methodology and case study

Abstract A great amount of researches on thermodynamic cycles have been active in recent years, such as ORC (organic Rankine cycle), Kalina cycle, et al. However, the ultimate aim of such researches, which could even be traced back to more than one century ago, has not changed with a tireless pursuing to Carnot cycle. In exiting researches, the working fluid, as a medium for energy conversion, is commonly considered to play an important role in the thermodynamic cycle: (1) relative to ideal cycle, most of actual power cycles in the engineering field cannot operate without working fluid; (2) energy efficiency, considering the analysis of second-law efficiency, of actual cycle has a significant decrease due to the introduction of working fluid. Thus, working fluid is a hot spot in the research of thermodynamic cycle in recent years. Zeotropic mixture, which commonly consists of two or more pure working fluids, has flexibility in thermos-physical properties with a possible potential to enhance the cycle performance. The effects of thermos-physical properties of zeotropic mixture should be considered when determining the cycle structure and the design of components. This paper presents a novel construction method of thermodynamic cycle based on the zeotropic mixture. By adding the thermodynamic coordinate of working fluid, a 3D cycle diagram based on the traditional temperature and entropy cycle diagram is applied for performance analysis of cycle. According the proposed construction method, a baseline cycle, composed by ORC sub-system and compositions regulating sub-system, is put forward and available compositions regulating techniques for such cycle are discussed as well. Finally, a representative case is described briefly and the features are summarized. This work provides a new methodology view to guide researchers in energy-efficient design of thermodynamic cycles.