Educating undergraduate mechanical engineering students about exergy analysis

Abstract

The concept of availability and exergy analysis is one of the most abstract ideas in thermodynamics and undergraduate mechanical engineering students have a difficult time to understand it. Unfortunately usually existing textbooks cannot help students much when it comes to exergy. This paper presents the approach that the author has developed to teach the concept of exergy (and not just mathematical formulations). The first law of thermodynamics deals with amount of energy transfer regardless of its form and conditions. This approach, however, is not appropriate for some applications. For instance, some forms of energy are more valuable than others. For example, work is more valuable than heat because the efficiency of generating work from heat is always lower than unity, with the maximum efficiency in the Carnot cycle. Similarly, the thermal energy at a high temperature is more valuable than the same amount of thermal energy at a low temperature because the thermal energy at the higher temperature can be converted to work at a higher efficiency in a thermal engine (e.g. a Carnot cycle) than that of the thermal energy at the lower temperature. Based on the first law of thermodynamics, 1 kJ of electricity, chemical energy of fuel, thermal energy at 500°C, and thermal energy at 50°C are basically equal. But in reality their economic values are not the same. Therefore, it is vital to find a methodology to evaluate thermal systems not only based on the amount of energy conversion but also the actual value of energy. This approach is called exergy.