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to evaluate useful work withdrawn from available energy is demonstrated.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.
