Balance exergy

To evaluate the energy utilization, energetic and exergetic process analyses are used. Since exergetic/anergetic flowcharts show local internal and external irreversibilities, the locations and quantities of heat losses may be detected, leading to thermodynamic optimization from the consideration of process energy improvements. 

According to Fig. 1-7 the exergy balance equation, for a steady-state open system, under isobaric conditions, is 

More information on energetic and exergetic analysis can be found in the literature [1.6-1.9]. The exergetic analysis of a rectification unit is given as an example in [1.10]. 

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An exergy balance for a CTES undergoing a complete cycle of charging, storing and discharging can be written as... 

Because chemical transformation does not occur, we do not need to include the chemical exergy in the exergy flows. We can now write expressions for the terms in the exergy balance based on 1 mol feed entering the system for both the reversible and real separation processes. 

Note that the ExQ1 and ExQ2 terms differ for the reversible and irreversible cases. The reversible case has the minimum heat input Qmm at temperature level Tb since heat is transferred reversibly without a temperature gradient. Similarly, it is rejected at Tt. It is in these two terms that the reversible and irreversible exergy balances differ. Now, first consider the reversible exergy balance, where Exlost is, by definition, equal to zero. From Equation 10.19, for the reversible case, we obtain... 

Fig. 11.3. Schematic band diagram of exergy balance for a simple process from a raw material and heat to a product and waste.

Figure 11.3 shows in a schematic way the exergy balance in an irreversible process in the form of a band diagram, where the bandwidth is proportional to the exergy value. For complex physicochemical processes operating in industrial manufacturing plants, the exergy... 

Exergy Balances. Writing a steady-state balance for exergy is just like writing a steady-state energy balance except for one major difference. While energy is conserved, exergy can be annihilated (not lost, but actually consumed), and so the balance must contain a destruction term ... 

In actuality, however, there will always be a consumption of some exergy to drive the various transformation and/or transfer processes. Furthermore, there may be effluent losses of exergy. Then, for real operation, an exergy balance says... 

Now, having evaluated the relevant transport terms, two important consumptions of exergy within the boiler may be evaluated, by applying an exergy balance to the hypothetical "combustor" and one to the "heat exchanger"... 

Summary. In illustrating the role played by exergy analysis in design optimization, one of two methods was demonstrated. The boiler problem used exergy balances to obtain all the pertinent... 

The detailed mass, energy and exergy balance for this complex thermodynamic system was performed with the aid of a set of computer programs named THESIS (Thermodynamic and Economic Simulation System), which was developed at the LehrstuET ftlr Technische Thermo-dynamik of the Technical University Aachen for the ther-... 

Figure 5. Energy and Exergy balance of the hydrogasification plant.

An exergetic analysis of coal gasification has already been performed for individual processes (1, 2). This showed that high exergetic losses may occur in the waste heat section of such a plant. In this article, an attempt is made to determine the influence of the chosen concept and the gasification parameters on the exergy balance by systematic investigation. 

Figure 5 shows the boiler feedwater requirement for the waste heat boiler according to alternative I, and for alternative II, the quench water requirement, i.e. the amount of water which evaporates in the quench. The partial pressure of the water, which increases as a function of the total pressure, results in a corresponding increase in the steam concentration, which has a favourable effect on the exergy balance. 

There seems to be an agreement to name equation (5) "Exergy". The lost work (called also dissipation),. in a system or a subsystem may also be computed from an "exergy balance" according to equation (6). 

The exergy flow diagram, Figure 2, shows the results of the exergy balances on the various zones of the system. The reference state of zero exergy was taken as P0=l atm, TQ=528 R, mass fraction water"0.3, mass fraction sodium palmitate=0.7. 

Two Objectives. The objective function, f-, is calculated by performing an exergy balance of the system shown in Figure 1. 

Use exergy balance or exergy loss calculations to evaluate the utilization of energy and as a guide for process... 

The exergy balance consists of internal exeigy losses. Irreversible processes may cause the distribution of exergy losses within the volume, and the partition of exergy losses may help in understanding the thermodynamic performance of the system. 

The exergy balance of a closed system (Figure 4.16) between states 1 and 2 is... 

The exergy balance for a steady-state system shows the exergy loss... 

Exergy balance can also be expressed in exergy rate form... 

The exergy balance based on specified hot and cold sources yields... 

The exergy balance for each unit with m = lkg/s working fluid is ... 

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