Non-Equilibrium and Classical Thermodynamics for Practical Systems: Today Closer Together Than Ever Before

Year 1998, Volume: 1 Issue: 1-4, 45 - 59, 01.06.1998

Stanislaw Sieniutycz

Abstract

We deal with applications of thermodynamics and availability theory to practical systems where a certain external control is possible in order to achieve improved performance. In particular, results of optimization of endoreversible processes which yield mechanical work are discussed. Equations of dynamics which follow from energy balance and transfer equations are difference constraints for optimizing work. Irreversibilities caused by the energy transport are essential. A model system is developed which incorporates finite heat resistances for an energy conversion process, and may be extended to take into account friction, heat leakage, mixing and other effects decreasing the thermodynamic efficiency. Deviation of efficiencies from their limiting Carnot values are analyzed in terms of the finite heat flux. The variational calculus and optimal control theories are shown to be the basic tools when formulating and solving problems with maximizing work. For a finite-time passage of a resource body between two given temperatures, optimality of an irreversible process manifests itself as a connection between the process duration and an optimal intensity. Extremal performance functions which describe extremal work are found in terms of final states and process duration measured in terms of the number of the heat transfer units. An extended exergy that has an irreversible component and simplifies to the classical thermal exergy in the limit of infinite duration is discussed. With this exergy performance criteria and bounds are defined for real processes occurring in a finite time. Enhanced bounds for the work released from an engine system or added to a heat-pump system are evaluated. A comparison between the optimization in thermodynamics (with exergy) and in economics (with costs) is made. Examples of exergy analysis to seek the best adjustable parameters of solar collectors, separation processes (distillation) and a chemical process with catalyst deactivation are discussed.

Keywords

exergy, thermodynamic limits, finite resources, heat transfer