Exergy vector

Fig. 11.8. Schematic exergy vector diagrams for exergy transfer between an exergy-absorbing process and an exergy-releasing process (a) the combined process is not feasible, (b) the combined process is feasible.

We notice in Fig. 11.9 that no vector of thermal processes can occupy the separating and mixing regimes in the exergy vector diagram. 

Fig. 11.9. Exergy vectors for heating and cooling processes at constant temperature.

In contrast to isentropy, the process of isothermal compression-expansion, which is accompanied by heat release or heat absorption, is represented by an exergy vector with the slope of < 1 in the regimes of heat absorption and heat release as shown in Fig. 11.10(a). 

Fig. 11.10. Exergy vectors for gaseous compression and expansion processes.

Fig. 11.14. Exergy vector diagram for an iron oxide reduction, FeO - Fe + 0.5O2, coupled with a heating process at 2800 K [Ref. 15.].

We examine, as an example, the exergy vector diagram for methanol synthesis to estimate the minimum exergy loss thermodynamically required for the synthesis reaction of methanol from methane [Ref. 16.]. First, we consider a direct (single step) synthesis of methanol from methane through a coupled-and-coupling reaction consisting of the oxidation of methane (objective reaction) and the dissociation of water molecule (coupled reaction) shown, respectively, as follows ... 

We now examine the exergy vectors of electrochemical reactions for water electrolysis and hydrogen-oxygen fuel cells at the atmospheric temperature. The electrochemical reaction of water electrolysis is expressed as follows ... 

Fig. 11.20. Exergy vector diagram of water electrolysis reaction at room temperature.

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