Exergy factor

The above equation is a generalization of the Carnot relation. The ratio between the exergy and the heat Ex/q is called the exergy factor. When T < T0, there is a lack of energy in the system the value of Ex/q greatly increases for... 

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. 

This is the definition of the physical exergy of the effluent stream The computation of the terms will yield the physical component of the stream, and the combination with the chemical and mixing components will allow for the computation of the efficiency. The question now remains Why did the computation of the efficiency based on the Carnot factor give the correct number The answer is that since the temperature of the effluent gases is fixed, it mimics an infinite heat reservoir, and therefore n[AH - TqAS] simplifies to nAH[ 1 - T0(AS/AH)] = nAH[ 1 - (T0/T)], since AG = AH - TAS = 0 at equilibrium. 

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. 

With Equations 13.14 through 13.16, and with the abundance factors for oil, coal, and solar energy as determined above, it is possible to determine the parameter for sustainable resource utilization for a process that uses solely one or more of these three resources. Table 13.5 lists the relevant data for several such processes, each extracting different percentages of the total required exergy from oil, coal, and solar energy. 

Exergy Source Depletion Time [years] Abundance Factor [-]... 

First of all, Table 13.5 lists three processes that each extract their exergy entirely from one of the three resources (processes 1, 2, and 3). In these situations, there is only one relevant abundance factor, and its value then solely determines the values of the average and minimum abundance factor. Also, the sustainability parameter is then simply this value squared. The results show that the process using only oil (process 1) is the least sustainable in its utilization of resources, the process using only coal (process 2) is more sustainable, and the process extracting all exergy from solar energy (process 3) is practically entirely sustainable in terms of resource utilization. 

We wish to conclude this section by referring to some related work. Our former student Wassenaar [37] has defined the fossil load factor as the percentage of fossil exergy input of the exergy of the final product. He calculated 9% for fresh potatoes from "ecological" agriculture and 13% from conventional agriculture, 64% for fresh French fries, and 80% for frozen French fries. 

In 2002, Wassenaar [67] made an analysis of the fossil fuel requirements of the Dutch potato industry and its main products. The products are of course, in terms of their "energy" that is, available work or exergy, of solar origin. Therefore, he introduced the concept of "fossil load factor," defined as the number of fossil fuel exergy units per exergy unit of potato-based useful... 

According to this definition, the quality of mechanical or electrical energy is equal to unity and that of thermal energy at a temperature, T, is equal to the Carnot factor, 1 - TQ/T. For chemical reactions, the exergy ratio (a-) represents that fraction of the delivered energy that could be converted to thermodynamic work by a reversible process and has a value most often (but not always) between zero and unity. 

When the concept of the exergy increase A is applied, the process vector on the thermodynamic compass shown in Fig. 1 (b) may be decomposed into two vectors Q and W. The vector Q on the diagonal line is equivalent to the heat sink at the reference temperature To, of which direction factor is unity. On the other hand, the vector W on the abscissa is equivalent to the work sink and has the magnitude of the exergy increase of the process, A . 

Chemical reaction. To perform detailed exergy analyses of chemical reactions, information about composition of both reactants and products is required. Then the changes in enthalpy, entropy, exergy, and the direction factor for the reaction process can be calculated. For primary dicussions for the reaction system synthesis, however, the standard direction factor D° defined by the following equation may be utilized. 

On the other hand, Fig. 9 shows the vectors for exergonic reactions. They often plays the role of donating exergy to other processes and their reactants especially with negative or small positive values for the direction factor such as ATP shown in Fig. 

Then the exergy destruction for any process system may be obtained as the shaded area on the energy - direction factor diagram shown in Fig. A2. When the number of subprocesses is increased, the width of each AHha is decreased, resulting in continuous change in Dha 1" and Dhd ", Of course the exergy destruction obtained in this method is the same as that in the text (jL), hut Eq. (29) becomes now the sufficient condition for a process system to hold. 

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,... 

Ishida, M., and Kawamura, K., "Energy and Exergy Analysis of a Chemical Process System with Distributed Parameters Based on the Enthalph-Directed Factor Diagram," Ind. Eng. Chem. Process Des. Dev., 21, 690 (1982). 

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