An Exergy-Based “Degree of Sustainability”: Definition, Derivation, and Examples of Application

The work presented in this paper is a contribution to the practical implementation of the “sustainability” concept, which is tightly connected with “resource thriftiness”, i.e., with reduction in the anthropic extraction of the irreplaceable supplies of fossil materials—ores and fuels—contained in the Earth’s crust. The saving is tied with “environmental conservation”, which is another concept embedded in the definition of sustainability. This paper starts from the assumption that the best measure of “resource consumption” is the total equivalent primary exergy extracted from the biosphere. The question is, then, while it is evidently correct to include social, ethical, and monetary matters into the definition of “sustainability”, what about the required resource consumption? To answer this question, the dynamic balances of a society represented as a thermodynamic system were examined to show that a “sustainable state” can be reached under two necessary conditions: first, the supply must consist only of renewable resources; and, second, the rate of such a supply must be higher than a certain threshold that can be attributed a physical significance. The procedure outlined in this paper leads to a rigorous definition of a society’s “thermodynamical degree of sustainability”, which is based solely on the primary renewable and non-renewable exergy inputs, as well as on the final exergy consumption. Some examples of applications to industrialized and non-industrialized countries are also presented and discussed.