Exergy consumption

Exergy input — [Exergy recovered + Exergy loss] — Exergy consumption = Exergy accumulation (9)... 

External Supplied Heat and its Exergy Consumption in Methylcyclohexane Dehydrogenation under Superheated Liquid-Film Conditions... 

In the last chapter, the concepts of exergy and physical exergy, in particular, were introduced. This chapter deals with three other important concepts, namely, exergy of mixing, chemical exergy, and cumulative exergy consumption, and their numerical evaluation. 

The concept of cumulative chemical exergy consumption is very useful and accounts for the fact that when a compound (e.g., ammonia) is introduced into a process, its chemical exergy has to be corrected for the exergy consumption accumulated since this compound was manufactured from its natural constituents (air and natural gas in the case of ammonia). 

Preheat the polymer to its melting point, thus reducing the exergy consumption of the extruder... 

Berthiaume et al. [39] introduce the so-called renewability indicator relating the work produced from solar energy to the work required to restore the degraded products from nonrenewable origin. Based on their analysis and making use of concepts such as the thermodynamic cycle, exergy, and exergy consumption, they conclude that the process to produce ethanol from com is not sustainable as it requires more work of restoration than is produced. 

In Chapter 18 we show that a chain of process steps, starting from natural gas, will result in the production of liquid H2 with an overall thermodynamic efficiency of 57%. Calculate the cumulative exergy consumption of this H2. 

N2 and H2 and the production of H2 from natural gas. Assume physical steps to have efficiencies of 10% and chemical steps of 60%. What is the cumulative exergy consumption of urea Separations take place with the help of compressors with an efficiency of 75%, running on electricity from an advanced natural gas-fired power station with a thermodynamic efficiency of 55%. 

Open system (Chemical plant) [Exergy consumption] — Chemical reaction Pressure change Heat transfer Mixing... 

For the exergy flow and the exergy consumption, however, the law of exergy consumption or entropy creation (the second law of thermodynamics) yields Eq. 11.21 ... 

The law of energy conservation makes the composite vector lie on the y-axis where AH= 0. Furthermore, the law of exergy consumption (entropy creation) for irreversible processes makes the composite vector of the two reactions emerge on the lower side of the y axis where AE < 0. The composite vector therefore indicates the amount of exergy consumed (exergy loss) AE during the combined process of the coupling and coupled reactions. 

There is another mode of gaseous expansion called the Joule-Thomson expansion, in which the change in gas volume occurs at constant enthalpy AHW = 0 without any change in energy. The vector of the isenthalpic expansion then stands perpendicular to the abscissa on the ordinate and points in the negative direction (exergy consumption) as is shown in Fig. 11.10(b). 

Fig. 11.17. Exergy diagram for a direct synthesis of methanol showing the theoretical minimum exergy consumption [Ref. 16.].

Now that all the transports of exergy across the system boundary have been evaluated, the exergy consumption term can be determined ... 

Figure 4. Exergy (and energy) flow diagram for a conventional fossil-fired steam power plant (negative numbers are exergy consumptions). ...

Furnaces, which are considered to be very efficient, are very inefficient — as shown by Table II, which also shows that the primary cause is the large exergy consumption in heat transfer (which is a consequence of the large temperature difference between combustion products and the heated medium). 

Efficient use of both energy and resource in an evaporation system was studied based on multi-objective analysis. The exergy consumption and the total investment cost were used to measure energy and resource conservation, respectively. The trade-off curve between the two objectives shows the change in the optimal solution as the unit cost of exergy is changed. 

Overall Evaluation. From the point of view of economic optimization the cost of energy and resource are but one of the important factors. The operating cost is estimated based on the exergy consumption, fl, multiplied by the unit cost of exergy (lO). The unit cost of exergy is a macroscopic index which shows the energy situation at the time. Since the annual investment cost has been used to measure resource conservation in this paper, the annual total cost is represented as follows,... 

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