Comparison of entropic and exergetic methods of quantification of loss of power due to irreversibilities in real processes using the Gouy–Stodola theorem

Abstract

Thermodynamics offers two methods of quantification of irreversibilities in real processes, namely entropy generation method and exergy destruction method. The engineering students in different disciplines are generally taught only the entropy analysis of processes in their second, advanced level, course in thermodynamics. The exergy analysis of quantification of irreversibilities is rarely covered adequately in the undergraduate courses in thermodynamics, especially when chemical effects are involved where the concept of chemical exergy plays the key role. In this article, the entropy and exergy methods of quantification of irreversibilities are reviewed and are applied to two real processes: steady-state de-mixing of a binary gas mixture into pure components and steady-state combustion of carbon monoxide gas. According to the entropy generation method, the loss of power due to irreversibilities is proportional to the rate of total entropy production (internally and externally). According to the exergy destruction method, the loss of power is equal to the rate of total exergy destruction (internally and externally). Although the calculation procedures involved in the two methods are quite different, the two methods yield the same results in terms of the loss of power due to irreversibilities in the real processes considered in this article. Thus, the detailed calculations carried out in this work confirm that the two methods of quantification of irreversibilities are equivalent. The exergetic method has the advantage that only the knowledge of the exergies of the flowing streams at the inlet and outlet conditions is required in order to calculate the loss of power due to irreversibilities, whereas the entropic method requires a stepwise calculation scheme in going from the inlet conditions to the outlet conditions of the process flow streams. As the material presented in this article involves advanced level concepts in thermodynamics, the appropriate place for the introduction of this material to the engineering students is the second, advanced level, course in thermodynamics.