Entropy variation is always related to the evolution of the state of any system, between its initial and its final states . There are many different approaches to describe irreversibility in any process , but entropy is the physical quantity which quantifies just the irreversibility . Classical thermodynamics is developed by using a small number of state variables , and, in non-equilibrium thermodynamics, the system is described by considering its subsystems , under the assumption that each of them is in local equilibrium . This last hypothesis is required in order to introduce the temperature as a measurable quantity . So, T σ = i J i X i > 0 represents a measure of the dissipation, where T is the absolute thermodynamic temperature, σ is the entropy production density, i.e. the time rate of the entropy density, generated by an irreversible process, J i is the i-th heat or mass flow, and X i is the i-th generalized driving force for vector transport processes or for chemical reactions . Moreover, recently, an increasing interest is growing in the analysis of the fundamental role of fluxes in thermodynamics, and natural systems .