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EXERGY AND ANERGY

Physics. Research in physics.

Doctor of technical sciences, prof., Etkin VA

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The equation of the energy balance of the system (2) does not take into account the difference in the quantity and quality of energy. Therefore, it is not sufficient to assess the perfection of energy converters. In technical systems, such an indicator could be the ability of the system to perform work in conditions of lack of balance with the environment. The corresponding terms were proposed in 1955 by the Yugoslavian scientist Z. Rant, who divided the energy of the system into exergy (technically usable, transformable energy) and anergy (a technically unsuitable, inescapable part of it) [6]. In the technical literature, the term "exergy" is very common. However, the exergy calculations are rather complicated, and they require an agreement on the environmental parameters taken as the origin of the exergy. In particular, the exergy of a heat source is determined by the product of its quantity Q by the thermal efficiency of an ideal Carnot machine operating from this source. This efficiency, as is known, depends on the temperature of the environment, which is different at different points of the globe. This applies even more to the exergy of various substances whose concentration in the environment differs by many orders of magnitude. In view of this heterogeneity of the environment, the choice of the origin of the exergy test presents considerable difficulties, which hinders the extension of this term and the exergic analysis of processes as a whole.

Further, the concept of exergy is not unambiguous and complete. Exergy is determined by the possibility of interaction of the system in question with the environment and depends on the parameters of this environment. Therefore, unlike energy, it is not a function of the state of the system itself and is not part of internal energy. This is especially evident when the work is done not only at the expense of the internal energy of the system, but also through the supply of heat from outside. Another difficulty in using exergy arises when systems with a lower temperature than the ambient temperature are considered. Then the exergy of the heat source becomes negative, and the performance by the system of work is accompanied by an increase in this exergy. In general, it is also erroneous to claim that exergy is part of the energy that can be turned into any other form of energy. For example, the exergy of elemental carbon is higher than its calorific value. Consequently, if a certain amount of exergy is used to extract pure carbon from the carbon dioxide of the atmosphere, the resulting increase in chemical energy will be less than the expended exergy [6]. Finally, the division of energy into exergy and energy can not reflect the tendency of isolated systems to equilibrium, since the exergy of such a system is zero because of the lack of an environment.

Thus, none of the attempts described above to reflect the quantitative and qualitative characteristics of energy is satisfactory or can not be considered exhaustive.

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Author: Doctor of technical sciences, prof., Etkin VA
PS The material is protected.
Date of publication on August 15, 2004