Available Energy—Part II: Gibbs Extended

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. Moreover, the formulas are independent from any “medium,” which is important both theoretically and practically—because of its relevance to proper selection of “the dead state.” In Part II, these issues are discussed and extended. In the context of Gibbs’ “available energy of the body,” Gaggioli’s development of exergy for subsystems of the body without any reference to a “medium” are reviewed. It is illustrated that the concept of “constraints” underlies available energy, equilibrium and stability, and thermostatic property relations. Furthermore, it is argued that the “availability” and “capacity” concepts of Gibbs are all equivalent to each other. In turn, because of interconvertability, it is shown that available energy is something more fundamental than “maximum useful work.”