ENERGY-EXERGY diagrams for states and energy and exergy balance equations representation

V.A.F. Costa

Energy, 2021, vol. 218, issue C

Abstract: This work proposes energy-exergy diagrams that clarify the relationships between energy and exergy, relevant for both the exergy teaching/learning process and for energy and exergy analyses. It proposes the energy-exergy diagram for states representation, where are represented the non-flow and flow energy-exergy for ideal gases of constant specific heats. After that is proposed the diagram for the energy and exergy balance equations representation, including graphical representations of their terms of different natures. It is detailed how the energy and exergy balance equations of systems for mechanical work production or for forcing (fluid or heat) flows are represented in the energy-exergy diagram. It is also seen how the reversible limits emerge from the graphical representations of the real (irreversible) situations, isentropic efficiencies being trigonometric relationships, and how the Gouy-Stodola relation is also included in the graphical representations. The energy and exergy balance equations for turbines, compressors and pumps, throttling devices, thermal engines, refrigerators and heat pumps, and heat exchangers are represented in the proposed energy-exergy diagram. It is discussed how the energy and exergy efficiencies can be evaluated from the graphical representations in the energy-exergy diagram, and set the general relationship between the energy and exergy efficiencies.

Keywords: Energy-exergy diagram; States representation; Balance equations representation; Energy and exergy analyses; Energy and exergy efficiencies (search for similar items in EconPapers)
Date: 2021
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Persistent link: https://EconPapers.repec.org/RePEc:eee:energy:v:218:y:2021:i:c:s036054422032613x

DOI: 10.1016/j.energy.2020.119506

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