Exergy difference

For the calculation of the exergy value at P, T of a mixture, of a given composition, with respect to the exergy values of the pure components at P and T, the exergy difference is defined as... 

In accord with the classical procedure of Gibbs, a chemical reaction linking a resource res to its reference species is chosen. The chemical exergy difference A6ch between the two states is calculated as A(>ch =... 

In the preceding discussion, we learned how to calculate the maximum available work (i.e., the exergy) and lost work of a system that goes from an initial state, state 1, to the dead state, state 0. However, often we are interested a system that undergoes a process from state 1, to some final state, state 2, that is not yet at the dead state. In such a case, the magnitude of the lost work is given by the exergy difference between the states ... 

Lost work, EW, is the irreversible loss in exergy that occurs because a process operates with driving forces or mixes material at different temperatures or compositions. 

Unlike the conservation guaranteed by the first law, the second law states that every operation involves some loss of work potential, or exergy. The second law is a very powerful tool for process analysis, because this law tells what is theoretically possible, and pinpoints the quantitative loss in work potential at different points in a process. 

Different ways of formulating exergy efficiency (second law efficiency) are considered. The exergetic COP (i.e., efficiency ratio) used here is as follows ... 

One known disadvantage of direct TES is the fact, that they have to be at a higher (or lower) temperature as the ambience. Due to this temperature difference (their exergy content) the are able to operate as heat (or cold) storage. A thermal insulation is necessary to avoid losses over the storage period. 

The first term on the right-hand side of this equation expresses the amount of work available due to differences in pressure and temperature with the environment. The second term, the chemical exergy, expresses the amount of work available due to the differences in composition with respect to the environment. The superscript in Ex, expresses that the chemical exergy is considered at ambient conditions. 

Exergy is a convenient concept if one wishes to assign a quantitative quality mark to a stream or a product. This quality mark expresses the maximum available work or potential to perform work because of its possible differences in pressure, temperature, and composition with the prevailing environment. The physical exergy, Exphys, only accounts for the differences in pressure and temperature the standard chemical exergy, Ex/hf.rn, accounts for the difference in composition with the environment at the environment s pressure and temperature. Thus... 

Recall that exergy values reflect the extent to which a compound or mixture is out of equilibrium with our environment. Examples are differences in pressure and temperature with the environment. Differences in temperature lead to heat transfer, while differences in pressure lead to mass flow. Chapter 6 shows that the physical exergy represents the maximum amount of work that can be obtained from a system by converting a system s pressure and temperature to those of our environment. 

The heating value of methane (see Chapter 6) is almost equal to its exergy value [11,14]. For the sake of simplicity in the ensuing analysis, we will set this exergy value of the gas to be equal to the energy value or, equivalently, the value of the heat of reaction. In Table 9.2, we have tabulated the exergy of methane at a number of different conditions. 

Standard Exergy Value of Methane at Different Conditions... 

Note that the ExQ1 and ExQ2 terms differ for the reversible and irreversible cases. The reversible case has the minimum heat input Qmm at temperature level Tb since heat is transferred reversibly without a temperature gradient. Similarly, it is rejected at Tt. It is in these two terms that the reversible and irreversible exergy balances differ. Now, first consider the reversible exergy balance, where Exlost is, by definition, equal to zero. From Equation 10.19, for the reversible case, we obtain... 

The third advantage, according to Ayres, is the possibility to accomplish year-to-year environmental auditing for large firms, industries, or even nations. The current approach is highly unsatisfactory, with a built-in incompetence to compare flows of different nature "apples" and "oranges." Exergy takes away this important imperfection. 

In an earlier work by De Wulf et al. [36], we showed that different aspects of process sustainability can be quantified by using thermodynamic principles. Indeed, the thermodynamic concept of exergy is used as the basis for the construction of sustainability parameters, which conveniently express particular aspects of process sustainability on a scale of zero to one. Elements of this work were used to analyze the sustainability of several industrial processes, and new insights have led to some meaningful improvements. 

Consider the following hypothetical process. Suppose a process can be driven by three different sources of exergy oil, coal, and solar energy. Using their current consumption rate, regeneration rate, and extent of natural deposits, the depletion times of oil and coal are calculated (Equation 13.13) to be 150 and 1000 years, respectively. For solar energy, the depletion time equals the lifetime of the sun, which is approximately 5 billion years (Table 13.4). 

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. 

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. 

Biomass differs from conventional fossil fuels in the variability of fuel characteristics, higher moisture contents, and low nitrogen and sulfur contents of biomass fuels. The moisture content of biomass has a large influence on the combustion process and on the resulting efficiencies due to the lower combustion temperatures. It has been estimated that the adiabatic flame temperature of green wood is approximately 1000°C, while it is 1350°C for dry wood [41]. The chemical exergies for wood depend heavily on the type of wood used, but certain estimates can be obtained in the literature [42]. The thermodynamic efficiency of wood combustors can then be computed using the methods described in Chapter 9. 

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