Page 237 - Materials Chemistry, Second Edition

4.5 Impact Categories, Impact Indicators and Characterisation Factors 221

The quantification of CED has been thoroughly discussed in the chapter on
inventory (Section 3.2.2).
There is a relation between CED and the cumulative exergy demand (CExD). 108)
Whereas CED designates the overall primary energy per functional unit of a
product system, exergy 109) designates the available amount of energy and is thus
related to the ‘free energy’ or ‘free enthalpy’ of physical chemistry. The laws of
thermodynamics imply that even though the total energy of a system cannot be
lost (first principle), heat may be produced or lost during the transformation of one
form of energy into the other (e.g. frictional heat) that can no longer be employed
for work in a physical sense. Exergy quantifies that part of the total energy that is
available for work. As such it is the opposite of entropy, which quantifies a tendency
of the system to be transformed into a non-ordered type and is not usable for work,
(second principle, see Equation 4.7). A small variation of enthalpy dH (energy
at constant volume), for example, within a chemical reaction is composed of a
variation of free enthalpy (dG) at temperature T plus a further amount of energy
T
TdS where dS signifies the variation of entropy:
dH =(dG) + TdS (4.7)
T
H (J) enthalpy
G (J) free enthalpy
−1
S (J K ) entropy
T (K) temperature
With each energy conversion, the free energy, respectively, the free enthalpy
constitutes the maximum that can be converted into work. In technology this
thermodynamic figure is called exergy and can also be applied to non-energetic
resources, above all, minerals and ores. 110) Thus, a loss of resources by dispersion
without actual consumption can be integrated into a uniform figure. Exergy
can be assigned to all raw materials that can prove their applicability in LCA
and databases. 111) Like most thermodynamic figures, exergy cannot be computed
absolutely but needs a reference compound, which usually corresponds to the lowest
state of energy of the element, for each material. The exergy then corresponds to
the work necessary for the formation of the desired substance – mineral, fresh
water, and so on, – or the maximum work generated in the case of the reverse
reaction. Reference compounds and – energies obtain an exergy value of zero. Such
assignments cannot be made without certain arbitrariness, and therefore, require
some convention to be followed. It can be ‘de facto’ provided in the form of a large
table, which is inserted into a database. 112) Furthermore, assumptions concerning
their composition must be made for ores and something similar is also valid for
chemical substances, which do not represent pure compounds, but mixtures.

108) Finnveden and ¨ Ostlund (1997), Dewulf and Van Langenhove (2002), De Meester et al. (2006),
B¨ osch et al. (2007), Koroneos, Rovas and Dompros (2011) and Koroneos et al. (2011).
109) Szargut, Morris and Steward (1988) and Szargut (2005).
110) Finnveden and ¨ Ostlund (1997), Szargut (2005) and B¨ osch et al. (2007).
111) B¨ osch et al. (2007).
112) Szargut (2005), B¨ osch et al. (2007) and Koroneos et al. (2011).