Thermal and mechanical aspect of entropy-exergy relationship

Abstract

The mechanical aspect of entropy-exergy relationship, together with the thermal aspect usually considered, leads to a formulation of physical exergy based on both useful work and useful heat that are the outcomes of available energy of a thermodynamic system with respect to a reservoir. This approach suggests that a mechanical entropy contribution can be defined, in addition to the already used thermal entropy contribution, with respect to work interaction due to pressure and volume variations. The mechanical entropy is related to energy transfer by means of work interaction and it is complementary to the thermal entropy that accounts energy transfer by means of heat interaction. Furthermore, the study proposes a definition of exergy based on Carnot cycle that is reconsidered in the case the inverse cycle is adopted and, as a consequence, the concept that work depends on pressure similarly as heat depends on temperature, is pointed out. Then, the logical sequence to get mechanical exergy expression to evaluate useful work withdrawn from available energy is demonstrated. Based on mechanical exergy expression, the mechanical entropy set forth is deduced in a general form valid for any process. Finally, the formulation of physical exergy is proposed that summarizes the contribution of either heat or work interactions and related thermal exergy as well as mechanical exergy that both result as the outcome from the available energy of the composite of the system interacting with a reservoir. This formulation contains an additional term that takes into account the volume and, consequently, the pressure that allow to evaluate exergy with respect to the reservoir characterized by constant pressure other than constant temperature. The basis and related conclusions of this paper are not in contrast with principles and theoretical framework of thermodynamics and highlight a more extended approach to exergy definitions already reported in literature that remain the reference ground of present analysis.