Epilogue - Thanatia: The Destiny of the Earth's Mineral Resources - page 499

hypothetical indicating an “end”, it is not an expression of impending catastrophe
but rather a consistent thermodynamic theory supported by the Crepuscular Earth
Model, which is in the fact, the quantitative representation of Thanatia. Using this
model as a reference, any regional or global planetary state can be assessed as an
exergy departure from Thanatia, as long as one knows the physicochemical para-
meters which characterise the two systems. This allows for the exergy calculation
of any resource. Thus abiotic resources, which were previously considered finite
but countless, are converted, at least theoretically, into finite and countable. Their
thermodynamic value is a measure of their exergy distance from depletion. In fact,
with this approach debatable problems such as which R.E. to choose (Stewart and
Weidema, 2005) become irrelevant and exergy values can easily be obtained using
computer programmes, once the intensive properties of the material are specified 3 .
17.2.4 Exergy costs inform as to the physical irreversibility
of production processes
Once any two states of a system are characterised it is possible to calculate the
actual exergy investment needed to reach a final state from an initial one. This is
referred to as either exergy cost, embodied exergy or cumulative exergy consumption
and can be assessed through the Thermoeconomics theory. If the process is a real
one, the exergy cost is calculated by measuring the whole exergy consumed from
the initial to the final state. In such a case, embodied exergy and exergy cost are
equivalent. However, if the process is hypothetical, the exergy cost is not truly
embodied exergy and is assessed with the best available technology.
Hence, in contrast to exergy, exergy cost calculations are based on current tech-
nology. And, as technology is far from reversible, exergy cost and exergy differ
greatly, in many cases by several orders of magnitude. Displayed over time their
difference is an objective and unique measure of technological achievement. Usually,
exergy costs decrease over time, as the production chain becomes more e cient.
This is because the exergy cost of a product reflects the exergy losses (irreversibili-
ties) associated with the production process.
Exergy is thus a more appropriate indicator for purely scientific purposes,
whereas exergy cost can support social interpretations. Both are equally valid from
a thermodynamic point of view.
Finally, considering that the link between entropy generation and irreversibility
is established by the Gouy-Stodola law, (I = T 0 ), entropy with exergy and exergy
cost concepts can be easily bridged just in accounting for the exergy losses in pro-
cesses. Therefore, entropy (generation) might no longer be used as a metaphor but
as a quantitative parameter. Note that other indicators related to the exergy cost
such as embodied energy, do not necessarily fulfill these thermodynamic conditions,
even if they can be used as proxy values of the exergy cost.
3 Such as those in the webpage: www.exergoecology.com. Accessed Jan. 2014.