Gibbs energy chemical exergy

For the determination of a compound s chemical exergy value we need to define a reference environment. This reference environment is a reflection of our natural environment, the earth, and consists of components of the atmosphere, the oceans, and the earth s crust. If, at P0 and T0, the substances present in the atmosphere, the oceans, and the upper part of the crust of our earth are allowed to react with each other to the most stable state, the Gibbs energy of this whole system will have decreased to a minimum value. We can then define the value of the Gibbs energy for a subsystem, the "reference environment"—at sea level, at rest, and without other force fields present than the gravity field—to be zero as well as for each of the phases present under these conditions. It is a logical extension of these assumptions to... 

Table 7.2 gives the standard chemical exergy of H20 and 02, respectively. Calculate with the help of Equation 7.13 the standard chemical exergy of H2. The standard Gibbs energy of formation for H20 can be found in [1],... 

Nevertheless the criterion of the heat exchanger has been described by the term of temperature, whereas that of the chemical reaction system by the Gibbs free energy. The above fact suggests that the same criterion may be applied to both cases when systematic analysis of a process system is performed based on the concept of exergy transformation. This point will be discussed more quantitatively in the later section. 

However, the energy of the chemical input and output is not so easily defined. At a simple level we could say that it is the chemical energy of the H2, O2, and H2O that is in question. The problem is that chemical energy is not simply defined - and terms such as enthalpy, Helmholtz function, and Gibbs free energy are used. In recent years the useful term exergy has become quite widely used, and the concept is particularly useful in high-temperature fuel cells. There are also older (but still useful) terms such as calorific value. 

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