Entransy analysis of reversible thermodynamic cycles based on the conservation of entransy equation and its application

Abstract

Entropy analyses and entropy generation analyses have been widely used to analyze and optimize thermal processes. For instance, the entropy generation minimization principle has been used to optimize heat transfer processes, heat transfer systems and thermodynamic cycles. However, some studies have reported that the entropy generation minimization principle cannot derive the optimal results for pure heat transfer processes and heat transfer systems without heat-work conversion processes. The concept of entransy and the principle of the least entransy dissipation-based thermal resistance were proposed for these problems, and they have been widely applied to optimize pure heat transfer processes and systems. Besides, the concept of work entransy was developed for irreversible thermodynamic cycles involving heat-work conversion processes, and it is defined as the product of the work and the temperature. This concept was then used as the entransy analysis approach to optimize thermodynamic cycles. However, previous studies applying the work entransy concept mainly focused on (1) the relation between the work entransy loss and the endoreversible thermodynamic cycle efficiency, and (2) the optimization of practical thermodynamic cycles involving irreversible processes. There have been few such studies of reversible thermodynamic cycles or combined cycles. This article analyzes reversible thermodynamic cycles and combined thermodynamic cycles using the entransy analysis method with a derivation of the conservation of entransy equation. First, the entransy conservation equation for a reversible thermodynamic process is derived, which introduces the concept of heat entransy and heat-work entransy flow. They are both process dependent physical quantities. The corresponding entransy conservation equation is then derived for a reversible thermodynamic cycle. The equation shows that there are two different conservation equations for reversible thermodynamic cycles, i.e. the energy conservation equation (1st law of thermodynamics) and the entransy conservation equation. Meanwhile, according to thermomass theory, the entransy is nothing else but the relative energy. The entransy conservation equation can be used to analyze the performance of reversible thermodynamic cycles. On the other hand, since the entropy returns to the initial value in a reversible thermodynamic cycle, an entropy analysis cannot evaluate the performance of reversible thermodynamic cycles. Second, a combined heat engine-heat pump cycle is taken as an example application of entransy analysis. In the philosophy of entransy analyses, the performance indicator of the combined cycle is its net heat entransy flow. For the properly chosen parameters, the combined cycle can be regarded as an ideal voltage transformer. Therefore, this kind of combined cycle can be referred to as a temperature transformer, which is different from the absorption temperature transformer. A diagram is introduced to show the total effect of the combined cycle to help understand the combined cycle. Finally, two practical cases, i.e. combined cycles for heating and refrigeration, are chosen to show applications of this entransy analysis approach. The concept of entransy efficiency of combined cycles is introduced to evaluate the performance of the combined cycles.