Page 207 - Materials Chemistry, Second Edition

EXERGY ANALYSIS AND ITS CONNECTION TO LIFE CYCLE ASSESSMENT 193

zero, because nothing is accomplished although exergy-containing resources
are consumed (Rosen and Dincer, 2001).
Knowledge of the relations between exergy and the environment can help
in appreciating the linkages between exergy and LCA and thus in address-
ing environmental damage. Three main connections between exergy and envi-
ronmental impact (Rosen and Dincer, 1997), which generally occur during all
phases of the life cycle, are discussed below:

• Waste exergy emissions: The exergy associated with waste
emissions can be viewed as a potential for environmental damage
in that this exergy represents a potential to cause change due to
its not being in equilibrium with the environment,. When emit-
ted to environment, this exergy represents a potential change the
environment. Usually, emitted exergy causes a change which is
damaging to the environment, such as damage to buildings and
the health impacts to flora and fauna. Further, exergy emissions
to the environment can interfere with the net input of exergy
via solar radiation to the earth (e.g., emissions of C0 2 and other
greenhouse gases appear to cause changes to the atmospheric
C0 2 concentration, affecting the receiving and re-radiating of
solar radiation by the earth).
• Resource degradation: Resources found in nature have exergy as
a consequence of their being out of equilibrium with the environ-
ment, and the degradation of natural resources is a form of envi-
ronmental damage. The reactivity is valued for some resources
(e.g., natural gas), which represents their potential to drive
processes. The composition is valued for other resources (e.g.,
gold), and processes exist to increase the value of such resources
by purifying them (i.e., increasing their exergy), at the expense
of consuming at least an equivalent amount of exergy elsewhere
(e.g., burning natural gas to produce process heat).
• Order destruction and chaos creation: More fundamentally,
exergy a measure of order and entropy of chaos, and the
destruction of order, or the creation of chaos, is a form of envi-
ronmental damage. The exergy of an ordered system is greater
than that of a chaotic one, relative to the same environment,
while a system of high entropy is more chaotic or disordered
than one of low entropy. For example, a landscape with garbage
scattered about has lower exergy and higher entropy than when
the garbage is collected and contained. The exergy difference
of the two systems is a measure of (i) the exergy (and order)
destroyed when the garbage is scattered and (ii) the minimum
work required to convert the chaotic system to the ordered one
(i.e., to collect the scattered garbage). In reality, more than this
minimum work, which only applies if a reversible clean-up
process is employed, is required. On a more abstract level, the