Thermodynamic exergy

Any spontaneous change of substances that occurs in the natural environment advances with a decrease in exergy of the substances this is the law of exergy decrease in spontaneous processes in analogy to the law of affinity decrease in spontaneous processes. In contrast to energy which is always conserved in any processes due to the first law of thermodynamics, exergy is exempt from the law of conservation and so is the affinity. 

Thermodynamic exergy analyses of gas-turbine cycles show that the major losses occur neither during the compression of air nor during the expansion of hot combustion products, but rather during the combustion reactions. Main reasons for these losses stem from the peak temperature limitations imposed by the materials of construction, coupled with the very high thermodynamic quality of the fuel source. 

THERMOECONOMICS is the branch of thermal sciences that combines a thermodynamic (exergy) analysis with economic principles to provide the designer or operator of an energy-conversion system with information which is not available through conventional thermodynamic analysis and economic evaluation but is cmcial to the design and operation of a cost-effective system. Thermoeconomics rests on the notion that exergy (available... 

In contrast, an efficiency based on the second law of thermodynamics could make more sense as it takes energy quahty into account. Exergy analysis is the method developed for industrial applications based on the second law of thermodynamics. Exergy could be done in reality, but the effort in data requirement could be prohibitive. More importantly, the second law of thermodynamics is a difficult concept to grasp for many process engineers, which is not common for appUcations in the process industry. 

Since these were preliminary conclusions, further explanations of the.se disadvantages are given using the second law of thermodynamics in this chapter. The ideas of reversibility, irreversibility, and the thermodynamic properties steady-flow availability and exergy are also developed. 

In both cases heat is taken from the exhaust gases to feed the reaction process, enhancing the heating value of the resulting modified fuel, which is then fed to the combustion chamber. But the main thermodynamic feature is that the exergy loss in the final exhaust gas is thus reduced and the efficiency increased. 

Thermodynamic principles can be used to assess, design and improve energy and other systems, and to better understand environmental impact and sustainability issues. For the broadest understanding, all thermodynamic principles must be used, not just those pertaining to energy. Thus, many researchers feel that an understanding and appreciation of exergy is essential to discussions of sustainable development. 

The second law of thermodynamics is instrumental in providing insights into environmental impact. The most appropriate link between the second law and environmental impact has been suggested to be exergy, in part because the magnitude of the exergy of a system depends on the states of both the system and the environment and because exergy is a measure of the departure between these states. This departure is zero only when the system is in equilibrium with its environment. The authors have discussed this concept extensively previously [1, 2, 9, 14-16],... 

Assessments of the sustainability of processes and systems, and efforts to improve sustainability, should be based in part upon thermodynamic principles, and especially the insights revealed through exergy analysis. 

Combined principles of thermodynamics are widely utilized in assessing the performances of heat storage systems. Thermoeconomics further combines the thermodynamic principles with engineering economics to estimate the cost of exergy, and optimize the cost under various constraints. Although, Valero et al. (1989) tried to unify the thermoeconomic theories, the concepts and procedures may vary, and create ambiguity in practical applications (Szargut, 1990 Tsataronis, 1993 Erlach et al., 1999 Sciubba, 2003). 

Accessible work potential is called the exergy that is the maximum amount of work that may be performed theoretically by bringing a resource into equilibrium with its surrounding through a reversible process. Exergy analysis is essentially a TA, and utilizes the combined laws of thermodynamics to account the loss of available energy. Exergy is always destroyed by irreversibilities in a system, and expressed by... 

The HPF must be supplied reversibly with the work uvHPFrev equalising the exergy epFC of the fuel with the thermodynamic state of the fuel cell and the heat f/npFrev from the environment... 

Grassmann diagram for the Linde liquefaction process of methane. One thousand exergy units of compression energy result in 53 exergy units of liquid methane. The thermodynamic efficiency of this process is 5.3%. The arrowed curves, bent to the right, show the losses in the various process steps. 

Part II, "Thermodynamic Analysis of Processes" (Chapters 6 through 8), discusses the thermodynamic efficiency of a process and how efficiency can be established and interpreted. A very useful thermodynamic property, called exergy or available work, is identified that makes it relatively easy to perform and integrate the environment into such an analysis. Some simple examples are given to illustrate the concept and its application in the thermodynamic or exergy analysis of chemical and nonchemical processes. 

In this chapter, we make preparations for performing a thermodynamic analysis of a process. The principles of such an analysis are defined first. From the calculation of the minimum, also called the ideal amount of work to perform a certain task, the convenience, not the necessity, of defining the concept of exergy is made plausible. Exergy can have a physical and a chemical component. The quality of the Joule is another convenient concept for a clear analysis and for conclusions on process performance. 

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. 

Suppose we deal with a process in which iron, Fe, has to be used as a reactant, for example, in a reduction reaction. The standard chemical exergy of Fe is 376.4 kj/mol. If we wish to carry out a thermodynamic or exergy analysis of this process, this value is not appropriate. After all, to put the exergy cost of the product, for which Fe was needed as a reactant, in proper perspective, we need to consider all the exergetic costs incurred in order to produce this product all the way from the original natural resources— iron ore and fossil fuel in this example. The production of iron from, for example, the iron ore hematite and coal has a thermodynamic efficiency of about 30% [1], and therefore it is not 376.4 kj/mol Fe that we need to consider... 

If the thermodynamic efficiency of a process step is calculated, the chemical exergies should be excluded from the calculation if the process step does not include chemical conversions. If it does, it may be appropriate to distinguish between the physical and the chemical efficiency, itphys and T chem, °f the process step. 

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