Exergy analysis

Specific exergy ex based on a unit mass is given by 

31 kPa. Sometimes, the standard state values are the average values of the ambient temperature and pressure of a location where the process under consideration takes place. 

The environment is composed of large numbers of common species within the Earth s atmosphere, ocean, and crust. The species exist naturally. They are in their stable forms and do not take part in any chemical or physical work interactions between different parts of the environment. We mainly assume that the intensive properties of the environment are unchanging, while the extensive properties can change because of interactions with other systems. Coordinates in the environment are at rest with respect to each other, and relative to these coordinates, we estimate kinetic and potential energies. 

In the natural environment, however, there are components of states differing in their composition or thermal parameters from thermodynamic equilibrium state. These components can undergo thermal and chemical processes. Therefore, they are natural resources with positive exergy. Only for commonly appearing components can a zero value of exergy be accepted. A correct definition of the reference level is essential for the calculation of external exergy losses. The most probable chemical interaction between the waste products and the environment occurs with the common components of the environment. 

The decrease of exergy of a system during a process can be expressed as 

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Horlock, J.H., Manfrida, G. and Young, J.B. (2000), Exergy analysis of modem fossil-fuel power plants, ASME J. Engng Gas Turbines Power 122, 1-17. 

El-Masri, M.A. (1987), Exergy analysis of combined cycles. Part 1. Air-cooled Bray ton-cycle gas turbines, ASME J. Engng Power Gas Turbines 109, 228-235. 

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. 

Consequently, exergy analysis can likely assist in efforts to optimize the design of CTES systems and their components, and to identify appropriate applications and optimal configurations for CTES in general engineering systems. 

Bjurstrom, H., and B. Carlsson, 1985. An exergy analysis of sensible and latent heat storage, Heat Recovery Syst., 5, 233-250. 

Keywords Latent heat storage exergy analysis, thermoeconomics, economic analysis... 

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... 

Ramayya, A.V., and Ramesh, K.N., 1998, Exergy analysis of latent heat storage system with sensible heating and subcooling of PCM, Int. J. Energy Res. 22 411—426. 

Ozgener O., Hepbasli A. (2007). A review on the energy and exergy analysis of solar assisted heat pump systems. Renewable Sustainable Energy Reviews, 11(3), 482-496. 

M. V. Sussman, Availability (Exergy) Analysis (Lexington, MA Mulliken House, 1981). 

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. 

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. 

Source Szargut, J. et alv Exergy Analysis of Thermal, Chemical, and Metallurgical Process, Hemisphere Publishing Corp., New York, 1988. 

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... 

Chapters 9 through 12 demonstrate thermodynamic, or exergy analysis of industrial processes. First, Chapter 9 deals with the most common energy conversion processes. Then, Chapter 10 presents this analysis for an important industrial separation process, that of propane and propylene. Finally, Chapter 11 analyzes two industrial chemical processes the production of polyethylene. Chapter 12 is included to discuss life cycle analysis in particular its extension into exergetic life cycle analysis, which includes the "fate" or history of the quality of energy. 

In any case, the principles of exergy analysis or lost work analysis are important. 

We wish to alert the reader that in the analyses presented above, the results were essentially independent of the type of fuel used. From an efficiency point of view, this may be true, but from a sustainability point of view, it is not. In general, gas is a much cleaner burning fuel than coal and requires less pre- and posttreatment. Even though the standard power generation plants can be made more efficient using thermodynamic analysis (lost work, availability, or exergy analysis), we note that power generation based on fossil fuels is not sustainable since the combustion of these fuels leads to increased... 

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