A preliminary approach to the 3D construction of thermodynamic cycle based on zeotropic working fluids

Abstract

Thermodynamic cycle is one of the important ways to convert energy. Improving the efficiency of thermodynamic cycle is still an important way to promote the efficient use of energy at the present stage, and it is also the efficient way to solve the energy crisis. Organic Rankine cycle (ORC), with advantages of simple structure, less maintenance and possibility of small scales, has been widely applied for various types of source in recent years. In exiting researches, the working fluid, as the blood of ORC, is commonly considered to play a vital role in the cycle: (1) relative to ideal cycle, the 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. The thermos-physical properties parameters, transport parameters and other parameters of working fluid could directly affect the efficiency, safety, stability and economy of ORC. With the increasing requirements on working fluids, it is difficult to find a pure working fluid not only with satisfied thermodynamic performance but also permissible environmental protection and safety. In contrast, zeotropic working fluid, which is mixed with two or more pure working fluids, is easier to meet the requirements of thermos-physical properties, environmental and safety. But at the present stage, the application of zeotropic working fluids in ORC still adapts or employs the methodology originated from what we learn from pure working fluids, which leads to the fact that the efficiency of the ORC using zeotropic working fluids is still low. This paper takes the working fluid as the starting point and uses the characteristics of the component shift to break the limitation of the thermodynamic cycle performance of the single circulating working fluids by adding the mixing and separation processes of zeotropic working fluids. Firstly, the 3D construction method of ORC is proposed by adding the dimensionality of the zeotropic component on the basis of the transmission temperature entropy analysis. Subsequently, the 3D ORC is compared with the traditional 2D ORC, and the advantages of 3D construction of thermodynamic cycle are clearly defined: (a) analysis of variable mass system; (b) reveal the thermodynamic process of composition regulation; (c) explore new ways to reduce the loss of exergy. At last, in view of the utilization of waste heat of internal combustion engine, a component adjustable ORC is proposed, and the application and function of the 3D construction method are expounded. The preliminary exploration results are given for the advantages of the thermodynamic cycle in 3D space relative to the traditional 2D construction method.