Page 204 - Materials Chemistry, Second Edition

190 LIFE CYCLE ASSESSMENT HANDBOOK

8.2.4 Life Cycle Interpretation (Improvement Analysis)
Life cycle interpretation is the final LCA phase, and integrates the LCI and
LCIA results to develop conclusions and recommendations that relate to the
goal and scope of the study. Life cycle interpretation can help decision mak-
ers make improvements by identifying and choosing the most environmental
benign alternative, bearing in mind that the decision process is also affected by
technical, economic, social and other factors.

8.3 Exergy and Exergy Analysis

Exergy is a quantity that stems from the second law of thermodynamics, and
helps in analyzing energy and other systems and processes. The exergy of a
system is defined as the maximum shaft work that can be attained when it
is in a reference environment that is assumed to be infinite, in equilibrium,
and to enclose all other systems. Exergy is not a thermodynamic property, but
rather is a property of both a system and the reference environment (Dincer
and Rosen, 2007). The reference environment is typically defined by specifying
its temperature, pressure and chemical composition.
Exergy is conserved only when all processes occurring in a system and its
surroundings are reversible. Exergy is destroyed whenever an irreversible pro-
cess occurs. When an exergy analysis is performed on a process such as power
generation or chemical processing, the thermodynamic imperfections can be
quantified as exergy destructions, which represent losses in energy quality or
usefulness (e.g. wasted shaft work). Like energy, exergy can also be transferred
across the boundary of a system. For each type of energy transfer there is a cor-
responding exergy transfer. Exergy analysis reflects quantitatively the differ-
ent thermodynamic values of different energy forms, e.g., work and heat. The
exergy transfer associated with shaft work is equal to the shaft work, while
the exergy transfer associated with heat transfer depends on the temperature
at which it occurs in relation to the temperature of the reference environment
(Dincer and Rosen, 2007).

8.3.1 Characteristics of Exergy

Some important characteristics of exergy are described and illustrated below
(Dincer and Rosen, 2007):

• A system in complete equilibrium with its environment has no
exergy.
• The exergy of a system increases the more it deviates from the
environment.
• Exergy is the part of energy which is useful and therefore has
economic value and is worth managing carefully. When energy