Explaining exergy - A cycle approach

The exergy of a system at a given state traditionally is defined as the maximum potential useful work available from the system as it reaches equilibrium with its surroundings or a specified state (dead state). Boettner, et al. [1] demonstrate consideration of work required to restore the system to its original state is inherent in the definition of exergy. They provide a visual interpretation for the concept of exergy of a closed system whose temperature and pressure are above those of the dead state: closed system exergy corresponds to the sum of net work associated with a power cycle and a heat pump cycle with both cycles incorporating the system state and the dead state. On further investigation, the second cycle is not limited to a heat pump cycle and can be modeled as either a power cycle or a refrigeration/heat pump cycle. This paper simplifies the analysis such that one can immediately graph on a pressure-volume diagram and a temperature-entropy diagram a cycle whose enclosed area represents the exergy of a closed system at state i interacting with its surroundings (dead state). This paper also examines the case in which the closed system temperature and pressure are below those of the dead state. [1] Boettner, D.D., Bluman, J., Rowland, M., Bodenhamer, J., and Arnas, A.O., 2011, "Graphical Interpretation of Exergy," Proceedings of the ASME 2011 International Mechanical Engineering Congress and Exposition, Denver, CO, Nov 11-17, IMECE2011-63099.