Development of a Graph-based Modeling Framework for Transient Exergy Analysis

Electrification of vehicle systems is a technological megatrend, and managing the heat generated by these systems is increasingly challenging. Moreover, the nature of the heat loads being generated is increasingly transient and uncertain, thereby further complicating the thermal management problem. Therefore, tools that enable dynamic analysis of such systems for the purposes of design and control are critical to solving this problem.Here we derive a dynamic graph-based framework for modeling the governing second law dynamics of thermodynamic systems. Vertices of the graph are used to represent the stored entropy of a control volume and edges represent paths for flow entropy. Source and sink vertices capture the transfer of entropy by heat and entropy generation. We show that a combination of the first and second law graph-based models results in a system of differential equations which are equivalent to an exergy balance of each control volume within a larger system but whose mathematical structure is amenable to transient analysis and simulation. The model can be used to quantify and analyze the rate of exergy destruction in specific control volumes, the interactions between control volumes in a subsystem, and even the interaction between systems. Through a series of case studies, we demonstrate the utility of the proposed modeling framework for both analysis and selection of candidate system designs.