Finite-Time Thermodynamics

Traditional thermodynamics evolved from Carnot’s introduction of the concept of the ideal reversible process, a process that would proceed infi- nitely slowly. Precisely because of that constraint, such a process would incur none of the losses of friction or other kinds of dissipation that result from real-time operation. The concepts of thermodynamic potentials such as the free energies provide limits of performance based on exactly those reversible processes as the standards of comparison with real processes. Onsager and then others showed that one can say useful things about irreversible processes, especially those near equilibrium. An entire field of engineering thermodynamics grew out of the concept of ‘local thermal equilibrium’ or LTE, in which one can describe a large system such as a flow process in terms of how the system changes as it moves, in effect, through a succession of steps which may be near or quite far from true thermodynamic equilibrium but that can be assigned local temperatures.