June 14, 2015
June 14, 2015
June 17, 2015
Energy Conversion and Conservation
26.570.1 - 26.570.12
Educating Undergraduate Mechanical Engineering Students about Exergy AnalysisThe concept of availability and exergy analysis is one of the most abstract ideas inthermodynamics and undergraduate mechanical engineering students have a difficult time tounderstand it. Unfortunately usually existing textbooks cannot help students much when it comesto exergy. This paper presents the approach that the author has developed to teach concept ofexergy (not just mathematical formulation).The first law of thermodynamics deals with amount of energy transfer regardless of its form andconditions. This approach, however, is not appropriate for some applications. For instance, someforms of energy are more valuable than others. For example, work is more valuable than heatbecause work can be completely converted to heat but the efficiency of generating work fromheat is always lower than unity, with the maximum efficiency in the Carnot cycle. Similarly, thesame amount of thermal energy at high temperature is more valuable than thermal energy at lowtemperature because the thermal energy at higher temperature can be converted to work at ahigher efficiency in a thermal engine (e.g. Carnot cycle) than that of thermal energy at lowtemperature. Based on the first law of thermodynamics, 1 kJ of electricity, chemical energy offuel, thermal energy at 400°C, and thermal energy at 50°C are basically equal. But in reality theireconomic values are not the same. Electricity can be converted to mechanical work with veryhigh efficiency (higher than 90%) and fuel can be used to produce work at relatively highefficiency (up to 60% in modern power plants) but for thermal energy at 400°C and 50°C,potential to generate work is much limited and is bounded by the efficiency of the Carnot cycle(for a heat sink temperature of 27°C, this maximum theoretical efficiency is equal to 55.4% and7.1%, respectively). Therefore, it is vital to find a methodology to evaluate thermal systems notonly based on amount of energy conversion but also the actual value of energy.
Zabihian, F. (2015, June), Educating Undergraduate Mechanical Engineering Students About Exergy Analysis Paper presented at 2015 ASEE Annual Conference & Exposition, Seattle, Washington. 10.18260/p.23908
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