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Computational Physics
Computational Physics

Irreversible Thermodynamics

Since its early stages as a science thermodynamics provides bounds and limits for processes exchanging work and heat. Usually these limits are obtained from the study of reversible processes. Irreversibilities due to finite time or finite process rates are neglected and processes are effectively treated as if they would proceed without loss and at infinite slow speed. However, this contradicts the dynamics of the world we are living in. Although the performance limits of reversible processes provide upper bounds for real, irreversible processes, they may not be guides useful enough for the evaluation of real processes. Existing heat engines, for example, seldom attain more than a fraction of the reversible Carnot efficiency. A number of approaches - Finite-Time Thermodynamics and Endoreversible Thermodynamics - have been developed to overcome that shortcoming and to provide better limits and bounds.

Are there theoretical bounds for the performance of thermodynamic processes under the constraints of finite process times or rates?

If such bounds exist, what are the optimal operating conditions necessary to reach these bounds?

This kind of questions is indeed important for the future development of technologies and has thus inspired researchers to conduct a wide range of scientific inquiries leading to new non-equilibrium theories in thermodynamics. A variety of novel theoretical techniques have been developed to model the performance of for instance automobile engines, refrigerators, heat pumps, chemical processes and solar cells. The aim of these investigations is to identify the main thermodynamic features of the system in order to make a model as simple as possible. The idea is to reduce the mathematical description and computational effort and yet to find more realistic optima and bounds for the operation of a thermodynamical system. The concept of `endoreversibility' has proven to be a powerful tool for the construction of models with the desired qualities. Endoreversible systems basically are composed of internally reversible subsystems with (irreversible) interactions between them. The losses due to the finite times or rates of processes are located in the interactions alone. The hypothesis of endoreversibility simplifies the expenditure for the analysis essentially. Nontheless a general theory describing the non-equilibrium flows of energy, matter and other extensities between systems and its accompanying entropy production remains an open challenge.

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