Symmetry of reversible thermodynamics

Abstract

<p indent=0mm>Symmetry is very important in physics. However, most of the current thermodynamic literature focuses on asymmetrical concepts. For example, the second law of thermodynamics states that even in a reversible thermodynamic cycle, the heat input to a system cannot be entirely transformed to work. Therefore, generally work is considered to be a higher grade of energy than heat. Another example is that thermodynamic cycles are evaluated based on the heat efficiency as the performance criterion, while reversed cycles use the coefficient of performance (COP) as their performance criterion. That is, the second law of thermodynamics and its related principles, theorems, laws, performance criteria, and parameters are all asymmetric. Meanwhile, according to the concept of symmetry, there could be another set of asymmetric principles, laws, performance criteria and parameters, which are corresponding to the second law of thermodynamics. These new concepts, new physical quantities and new understandings include the work entropy as the core physical quantity for a reversed cycle. The work entropy is actually the system volume, but with a new physical meaning corresponding to heat entropy. A second concept is that the volume work quantity has a grade, i.e. the pressure. A higher pressure of work can yield more heat through a reversed cycle driven by the same amount of volume work comparing to the volume work with a lower pressure. A third concept is that the heat quantity and the work quantity have the same grade for reversible conditions. The reasons are stated in two points. On one hand, not all the heat in a reversible thermodynamic cycle can be transformed to work because the ambient temperature cannot reach absolute zero, hence there must be a certain amount of heat flows to the heat source with a lower temperature. On the other hand, in reversible reversed cycles, not all work input can be transformed to heat, neither. That is because the ambient pressure cannot reach absolute vacuum (zero pressure). Through this comparison it can be seen that the volume work and heat have the same grade in reversible thermodynamics. A fourth concept is that thermodynamic cycles can also have a coefficient of performance (COP_w), and it can be used as a performance criterion. The applications of these theoretical results could provide a deeper understanding of some thermodynamic quantities. First, entropy (thermal entropy) and exergy (thermal exergy) could be used to describe the ability of a heat engine to produce work, while the volume (work entropy) and work exergy could be used to describe the ability of a heat pump to produce heat from work. Second, since the grade of volume work in a reversible reversed cycle is the pressure, the work at high pressure into a system will improve the coefficient of performance (COP_h) of reversible reversed cycles. Third, new thermodynamic cycles produce work from pressure sources with coefficient of performance (COP_w) greater than 1 could be possibly proposed, and the work generated by these cycles could be greater than the work generated from a Carnot cycle for the same temperature limits.