Exergy flows

Eq. (2.22) may be interpreted in terms of exergy flows, work output and work potential (Fig. 2.5). The equation may be rewritten as... 

The analysis requires the calculation of three exergy flow rates, at 0°C, 50°C, and 100°C. As no heat or work is transferred between the considered system and its environment, the first law, Equation 6.1, yields that the overall enthalpy change is zero ... 

Introducing this relation into Equation 6.22 gives us the temperature T3 of the flow leaving the mixer T3 = 323.15 K. The exergy flow rates can now be calculated from... 

The diagram clearly shows the "value" of each flow in terms of its ability, or potential, to perform work. The diagram also shows that a considerable amount of the exergy flowing into the system is dissipated as a result of the... 

Exergy flow diagram or Grassmann diagram for the example of Figure 6.5. 

The exergy flowing out of the system is then calculated from... 

Consider lkg/s of coal that is combusted with an adequate amount of air (approximately zero exergy contribution). The rate at which exergy flows into the system is therefore 23,583 kW. The combustion releases heat, namely, at a rate of 21,860 kW at a temperature T. Since we have created a heat source at temperature T, it is straightforward to compute the work potential (exergy) of this heat source. All we need to do is multiply the heat release rate (21,860 kW) by the Carnot factor 1 - (T0/T). This means that if the combustion takes place at temperature T = 1200 K for a fluidized bed reactor (Table 9.1), the efficiency of the combustion alone is combustion = (21,860/23,583) [1 - (T0/T)] = 0.93 [1 - (T0/T)] = 0.93 [1 - (298.15/1200)] = 0.7 This means that already 30% of the maximum work has been lost We summarize this simplified analysis in Figure 9.15. 

The steam production allows for the computation of the exergy flow at various points (Table 9.7). For example, the exergy flow at point (1) is simply 2.73 x 10-4 x 1402.8 = 0.38kj/s. The exergy value of the flue gas stream (5) is computed by using the first law efficiency and realizing that the flue gas stream is a heat stream being discarded ... 

Schematic representation of the exergy flow in a separation process.

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. 

It is clear that ultimately our only limitation to sustainable production is obtaining the exergy to run the production processes and to drive closed material cycles. In view of this, exergy can be considered the ultimate scarce resource in our technological processes, and exergy flows to or from these... 

The sustainability parameter a is based on the average abundance factor aaveras , which considers all the abundance factors a, and exergy flows Ex of the individual resources used in the process. 

Equation 13.15 uses the exergy flows of the resources because these values express the minimum amount of work required to produce these resources from compounds that are thermodynamically in equilibrium with the natural environment. As, ultimately, only exergy is scarce, the exergy flows of resources are better indicators of relative importance than their mass or volume flows. 

Another special situation arises when one resource has its origin in two different processes. For instance, when a process uses the electricity provided by an energy company, it is possible that this electricity is generated partly by burning coal and partly by burning natural gas. In this case, the process should be considered to use two different types of electricity electricity from coal and electricity from natural gas. Both types of electricity then have their own derived depletion time and, based on how much they contribute to the total amount of electricity supplied, their own exergy flow. 

Equation 13.17 explicitly mentions the useful exergy flows coming out of the process because exergy can be lost in two different ways. First, exergy is lost in any real process as a result of irreversibility in the process itself, and such losses are called internal exergy losses. Second, exergy can be lost via waste streams that are not yet at equilibrium with the natural environment. 

Now, assume that there are other exergy flows into the system, which are required to make the process work. These amount to 410kj/mol of electricity, and comes from the combustion of coal (C is 410 kj/mol). How do the answers to a and b change What if the efficiency of electricity generation is 25% ... 

Fig. 11.4. Flows of enthalpy and exergy through an open system in which two processes 1 and 2 are advancing at constant temperature H = enthalpy flow, E = exergy flow, AH = enthalpy change, AE = exergy change in the system.

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 ... 

How the tools are organized into a methodology for process evaluation via exergy is illustrated in Reference 13 with a coal-fired boiler. It will be used to demonstrate the calculation of exergy flows, losses and consumptions. 

Figure 3. Exergy flow diagram for coal-fired boiler.

On the other hand, the exergy flow diagrams show the true picture and can be used to gain useful insight into our overall energy problems. 

The references referred to in Tables I and II usually contain more or less elaborate exergy flow diagrams. 

Consider the unit costs of the exergy flows between major component groups shown in parentheses on Figure 10. Notice the inordinately high costs of steam, shaft power, electricity and oxygen — especially considering the inexpensive cost of fuel to... 

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