Exergy Concept

Dincer, I. and Y.A. Cengel, 2001. Energy, entropy and exergy concepts and their roles in thermal engineering, Entropy, 3, 116-149. 

We would now like to illustrate essentials of such an analysis and the role of the exergy concept with a simple example. We borrow this example from Sussman [2] because we can hardly think of a nicer and clearer illustration. Figure 6.5 illustrates how a stream of 1 kg/s of liquid water at 0°C is adia-batically mixed with a second stream of 1 kg/s of liquid water at 100°C to produce a stream of 2 kg/s of liquid water. The task at hand is to provide a thermodynamic analysis or exergy analysis of this process. The temperature of the environment is 25°C. 

Equation 6.33 provides the definition of exergy if state 1 is chosen as the state at ambient condition, namely, P, = P0 and = T0 the minimum amount of work required to transfer the system from environmental conditions to those at P2 and T2. At these conditions, this is the maximum amount of work available for the reverse process. That is the valuable idea behind the exergy concept to be able to assign to any process stream a value, its exergy, that expresses the confined work available in the stream. For the general change in state from P0r T0 to P, T, we can write the net energy input as... 

The convenience of the exergy concept houses the possibility to discuss energy issues on a clear and quantitative basis, in particular if the exergy values assigned to process streams are combined with exergy losses incurred in the processes in which these streams participate, either actively or passively, as the next chapter illustrates. 

Finally, although the exergy concept is not strictly necessary for the calculation of the available work lost in the process, it is an extremely handy tool to calculate losses and efficiencies and for making a quick assessment of process options. Chapter 8 gives some simple illustrations, whereas Part III, "Case Studies" presents the results of integrated studies in the world of energy and chemical technology. 

In this chapter, we explore how the exergy concept can be used in the analysis of energy conversion processes. We provide a brief overview of commonly used technologies and analyze the thermodynamic efficiency of (1) coal and gas combustion, (2) a simple steam power plant, (3) gas turbine, and (4) combined cycle and cogeneration. At the end of this chapter, we summarize our findings with some concluding remarks. 

Fratzscher, W., and Eckert, F., "Experience Gained by the Introduction of the Exergy Concept for the Standardized Evaluation of Power and Technological Processes in an Industrial Chemical Complex," Proc. 9th World Energy Conf., l, 85 (1974). 

This equation shows that the rate of exergy transferred into the control volume must exceed the rate of exergy transferred out, and the difference is the exergy destroyed due to irreversibilities. Exergy concepts for some steady-state processes are ... 

Use of Thermo-Economic Analysis Based on Exergy Concepts to Evaluate the Cost of Electricity From Sugar Cane Bagasse in the Brazilian Sugar Cane Sector... 

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