Equivalent Circuits for Exergy Flow in Thermodynamic Systems

Abstract

The second law of thermodynamics explains the nature of all spontaneous processes, and it imposes a limit on the performance of all technologies, from heat engines to refrigerators. These limits are well described as early as Sadi Carnot's 1824 publication that established the field of thermodynamics; researchers later developed the concept of exergy, or the available work, that a thermodynamic system can produce when interacting with a specified environment. In this work, we describe a resistance analogy for thermodynamic systems, in which the need to remove entropy forces some amount of energy to leave the system as heat rejection. Specifically, it is the inverse temperature of the heat sink that resists energy flowing out of the system as heat rejection. An equivalent circuit can be drawn for any thermodynamic system, with energy flowing through different branches of the circuit. The different paths correspond to different energy and exergy flows, including the energy that must flow out of the system as heat rejection and, therefore, cannot contribute to the exergy content of the system. After establishing this equivalent circuit, it is applied to a natural gas combined cycle example problem, a desalination example problem, and a transient heating problem.