Exergy
Description
Simon Michaux:
" Exergy is uniquely suited to use as a global, strategic indicator of the sustainability of mineral resources as it allows direct comparison between all metals, minerals, and fuels. Exergy is the application of thermodynamics to the accounting of natural resources and material fluxes. It examines the real energy costs, that is, the replacement costs, relative to a standard reference environment (RE). Therefore, one can compare in the same units the costs of different industrial operations in context of natural resources: Exergy (in Joules, J). In thermodynamics, the exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir, reaching maximum entropy (Rant 1956). The maximum fraction of an energy form which (in a reversible process) can be transformed into work is called exergy. The remaining part is called anergy, and this corresponds to the waste heat (Honerkamp 2002). Using an exergy standard states makes it possible to express these enthalpy and entropy data as Exergy by using for example the methodology and standard states expressed in Szargut (2005).
When the surroundings are the reservoir, exergy is the potential of a system to cause a change as it achieves equilibrium with its environment. Exergy is the energy that is available to be used. After the system and surroundings reach equilibrium, the exergy is zero. Determining exergy was also the first goal of thermodynamics. The term “exergy” was coined in 1956 by Zoran Rant (1904–1972) by using the Greek ex and ergon meaning “from work” (Rant 1956 and Grubbström 2007). Energy is neither created nor destroyed during a physical process, but changes from one form to another (as per the 1st Law of Thermodynamics). In contrast, exergy is always destroyed when a process is irreversible, for example loss of heat to the environment (As per the 2nd Law of Thermodynamics).
This destruction is proportional to the entropy increase of the system together with its surroundings. The destroyed exergy has been called anergy (Honerkamp 2002). "