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J3: Exergy

A section of Energies (ISSN 1996-1073).

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According to many textbooks and papers, the exergy of a system is defined as the maximum theoretical, useful work (shaft work or electrical work) that is obtainable as the system is brought into complete thermodynamic equilibrium with the environment, under a specified condition. This condition requires that the system could interact only with a known reference environment (RE). The exergy definition implies the following main consequences:

  • Exergy is not a property of the system alone, but of the couple made up by the system itself and the RE.
  • When measured by their exergy, all different kinds of energy (chemical, mechanical, thermal, etc.) can be compared and summed without any inconsistency.
  • As energy is conserved but continuously spread through the system and the environment (i.e., entropy is increasing, at a global level), every energy conversion process implies an exergy destruction, at a global level.

The last two consequences are the reason why exergy is generally regarded as a powerful tool for real efficiency calculation of processes and engines, resource consumption evaluation of plants and countries, potential work conservation and so on. This is true in a wide variety of fields, from power plant design to ecosystem health evaluations to energy performance improvement of buildings. Nevertheless, or exactly for these reasons, the debate about exergy is still ongoing and very active. Various contributions may be found in the literature, claiming new, more general definitions of exergy, or the identification of improved methodologies for studying energy systems, where exergy plays a crucial role. Some of the topic questions under discussion may be summarized as follows:

  • The definition of the RE, including the alternative between a universal RE, which allows easier comparison among different processes and products, and a case-specific RE, which better describes the actual constraints affecting the process in hand.
  • The effect of time-variable parameters of the RE—in particular, temperature and composition. In fact, this is the real condition experimented by the biosphere, and the exergy content of natural, or artificial, energy storage may be affected by these variations on different time scales.
  • The effect of confinement when a system interacts only with a local environment that is permanently kept separated by the actual RE.
  • Properly accounting for indirect exergy consumption for producing goods and services, from the points of view of the algebraic formulation, of the general assumptions, of the economic database exploitation, and so on.
  • The consistent exergy evaluation of natural resources, such as raw minerals, but also the products of the ecosystem, which cannot be regarded simply as different kinds of fuels.

The Exergy Section aims to provide a platform for showcasing the latest research progress about all, but not limited to, these topics. The Section considers full-length, short communications, perspective, and review articles about both methodological aspects and practical applications, ranging in the vast fields of application of exergy, from thermodynamics to ecology, biology and engineering.

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