Exergy and industrial ecology. Part 2: A non-dimensional analysis of means to reduce resource depletion

The thermodynamic interpretation of ecosystem evolution introduced in Part 1 of this series provides a basis for quantitative analysis of strategies for reducing resource depletion. In Part 2, we express resource depletion rate as a product of consumption rate and the Depletion number (Dp)—a non-dimensional indicator of depletion per unit consumption. We introduce two generalized models of resource use that incorporate a choice between virgin resource extraction and post-consumption resource recycling. These models are used as a basis for expressing Dp as a function of non-dimensional indicators for three resource conservation strategies: exergy cycling fraction (ψ) for resource recycling, exergy efficiency (φ) for process efficiency gains, and renewed exergy fraction ($\text{Ω}$) for extent of renewed resource use. We use the resulting expressions to fully characterize strategy interaction, strategy limitations, and the roles that these strategies play in allowing resource consumption to occur with decreasing levels of resource depletion. We also show how the derived framework may be incorporated into an economic analysis to identify least-cost approaches to depletion avoidance in a toluene production and cycling system.