The concept of available energy, as defined by Gibbs is revisited. Being more general, this concept of available energy differs from that referred to commonly by the same name, or as “exergy” or “availability.” He gave representations of available energy for two circumstances. The first was the available energy of a “body,” for the case when a body, alone, is in a nonequilibrium condition and therefore has energy available. In turn, he presented the available energy of “the body and medium,” for the energy that is available because a body is not in equilibrium with some arbitrarily specified medium or “reference environment.” Gibbs’ did not present formulas to represent available energy. His representations were verbal descriptions regarding surfaces, curves and lines. Although his verbiage was augmented by some graphics, visualization of the geometrical entities he described depended largely on the imagination of the reader. In Part I, we take advantage of modern graphics software to illustrate more vividly not only the available energy he described verbally but also his interesting concepts of “available vacuum” and “capacity for entropy.” We argue that all of these concepts are equivalent. Since Gibbs, representations with formulas have been developed and are common for the “available energy of body and medium.” Gaggioli has developed formulas which are more general, to represent “the available energy of the body (alone)” and to assign an exergy to subsystems of the body as a measure of each subsystem’s contribution to the available energy. In contrast to the available energy, exergy is an additive property, so that balance equations can be written. This exergy is independent of any “reference environment,” which is important both theoretically and practically because of its relevance to proper selection of “the dead state.” In those special cases when the dead state is one in equilibrium with a “reference environment,” this more generalized exergy encompasses that concept called (today) exergy in textbooks and journals.

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