Novel Graphical Expression Method of Thermodynamic Process Parameters: Methodology and Case Study

Abstract

Graphical methods are crucial for quantitatively describing energy conversion processes and improving efficiency. For a transient system or those involving multi-energy conversion processes, it's challenging to comprehensively describe the energy conversion process using existing thermodynamic state parameters. To address this, this study proposes a new graphical method called an Energy-Energy (E-E) diagram, which employs different energy forms as the axes and is capable of representing dynamic performance variations through the geometric parameters. To demonstrate its applicability, the dynamic performance of an organic Rankine cycle (ORC) system under a finite heat capacity boundary condition is investigated on the E-E diagram. Results show that the E-E diagram 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. Furthermore, the E-E diagram intuitively reveals performance differences between systems with varying boundary conditions. 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