Novel graphical expression method of thermodynamic process parameters: Methodology and case study

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 systems under dynamic boundary conditions, traditional thermodynamic state parameters often prove insufficient in comprehensively characterizing the energy conversion process. To fill this gap, this study proposes a novel graphical method called the Energy-Energy (E-E) diagram, which employs different energy forms (thermodynamic process parameters) as coordinate axes while drawing an analogy between energy conversion to rotational motion of a circle. By representing the radius length (r) and rotation angle (θ) to the total system energy and energy conversion efficiency respectively, the E-E diagram can characterize the energy conservation (1st Laws of Thermodynamics) and global system performance (2nd Laws of Thermodynamics). Local system performance variations during energy conversion processes are revealed through slope (k) changes along the heat source capacity curve. When applied to an organic Rankine cycle system under finite heat capacity boundary conditions, the combination analysis of E-E diagram and Temperature-entropy diagram (at instantaneous states) provides comprehensive insights into both system performance variations and pinch point location variations. The E-E diagram enables intuitive visualization of performance differences among systems with varying operating conditions, establishing a novel framework for characterizing dynamic system performance.