Volume 52

A Novel Graphical Expression Method for Thermodynamic Systems under Dynamic Boundary Conditions Kunteng Huang, Ruihua Chen, Weicong Xu, Shuai Deng, Li Zhao

Abstract

Graphical methods are among the techniques for quantitatively describing energy conversion processes, which also serve as an important technical pathway to improve energy conversion efficiency. For a thermodynamic system with dynamic boundary conditions, it is challenging to comprehensively describe the energy conversion process using existing graphical methods. To fill this gap, this study proposes a new graphical method called an Energy-Energy (E-E) diagram, which employs different energy forms (thermodynamic process parameters) as the axes and is capable of representing dynamic performance variations through the geometric parameters. The applicability of the E-E diagram is demonstrated through a case study of an organic Rankine cycle (ORC) system under finite heat capacity boundary conditions. The results show that the E-E diagram can not only present the system performance parameters related to both the 1st and 2nd Laws of Thermodynamics but also illustrate the global and local performance of energy conversion process. A significant advantage in describing system thermal performance variation and irreversible losses distribution can be obtained when combined both E-E diagram and T-s diagram. Moreover, the performance differences between two systems with different boundary conditions can be revealed intuitively through the E-E diagram. The E-E diagram proposed in this study pioneers the application of process quantities as thermodynamic coordinates, providing a novel graphical method for characterizing the performance of dynamic systems.

Keywords graphical method, energy conversion, thermodynamic process parameter, finite heat capacity;